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The primary photoprocesses of chromium (III) complexes Chen, Schoen-Nan 1970

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THE  PRIMARY  PHOTOPROCESSES  OF C H R O M I U M ( I I I ) C O M P L E X E  by SCHOEN-NAN B.Sc,  A  Engineering, National M.Sc, N a t i o n a l Taiwan  THESIS THE  SUBMITTED  Taiwan U n i v e r s i t y , U n i v e r s i t y , 1964  IN PARTIAL  REQUIREMENTS  FOR  DOCTOR OF  in  CHEN  F U L F I L M E N T OF  THE DEGREE  OF  PHILOSOPHY  t h e Department  I  of CHEMISTRY  We  accept  required  THE  this  thesis  as c o n f o r m i n g  to the  standard  UNIVERSITY  OF  August,  BRITISH 1970  COLUMBIA  1962  In  presenting  this  an a d v a n c e d d e g r e e the I  Library  further  for  agree  in  at  University  the  make  that  it  partial  freely  this  written  representatives. thesis  for  It  financial  gain  Department The  University  Date  8,  of  British  Columbia  Canada  Svfj,  3,  of  Columbia,  British for  by  the  tfnO  shall  not  the  requirements  reference copying of  Head o f  understood that  permission.  Vancouver  of  extensive  granted  is  fulfilment  available  permission for  s c h o l a r l y p u r p o s e s may be  by h i s of  shall  thesis  I agree and  be a l l o w e d  that  Study.  this  thesis  my D e p a r t m e n t  c o p y i n g or  for  or  publication  without  my  ABSTRACT  Energy  transfer  between  hexacyanochromate(III) quenching  o f donor  Reineckate  i o n (donor)  i o n (acceptor) has been  phosphorescence-  s e n s i t i z a t i o n of acceptor phosphorescence.  all  measurements  f i tthe expected  Stern-Volmer 5  • with  a quenching  constant  =  The  pre-exponential factor  6.6  x  10"^ M  constant, rather 'ectly and  '"sec  1  H  a quenching  involved  acceptor.  (collisional)  are the Energy  x  Kcal/mol  kg / i s attributed  than  7.2  M  intensity) from  relationship -1  sec  energy  at  -65°C.  of  respectively.  entirely'to  through  Results  -1  10  and a c t i v a t i o n  a n d 4.8  studied  ( l i f e t i m e and  and  and  an energy  are The transfer  process. 2  The e l e c t r o n i c s t a t e s 2 (and/or i g ) states of both  E^  donor  T  transfer  process.  d i r -  i s a  diffusion-controlled  Hexacyanochromate(III)  ion i s itself  quenched i n the presence o f R e i n e c k a t e i o n . The q u e n c h i n g c o n s t a n t , k ' , w h i c h may b e a t t r i b u t e d t o b a c k e n e r g y transfer QH from  a c c e p t o r t o donor,  12 10  -1 M  of 2 x  -1 sec  In been  has a p r e - e x p o n e n t i a l f a c t o r  and an a c t i v a t i o n  t h e same  studied  system,  a t -65°C.  energy  quenching  o f 7.6  Kcal/mol.  of photoaquation has  The p h o t o a q u a t i o n  quantum  yield  also  of  -2 Reineckate of  i o n i s 1.02  x  hexacyanochromate(III)  phorescence  of Reineckate  10  It.is  reduced  i o n , b u t n o t as much i o n i s reduced.  oo  unquenchable  part,  ^ ^ g ^ occurs  v i a the  The 4  i n the presence as the  phos-  limiting  Tp^ s t a t e ,  while  the  quenchable  part  intermediate. posed  i s back  state,  which The  path  f o r the  quenchable  through  the  of  2  of  E  idea  crossing.  2  the  state  g  temperature  some C r ( I I I )  the  from  state  E  as  part  to  an pro4  the  aquation.  processes  supports  intersystem  the  undergoes  lifetimes  evidence  through  -inter-system c r o s s i n g  primary .  2  occur  actual  then  investigated escence  The  must  dependence  complexes.  o f .the  According  molecules  have  of  phosphor-  A l l the  available  thermally activated to  this  been  mechanism,  back  the  origins  4 of  the  far  states  lower  i n energy  Assuming the  application  intersystem The ions  values are  also  ternal  of  of  energy  §j_  been  t o be  excitation, represents  a  back  and  rise  new  which  a  the  <|>^  and  , .has  0.35,  population of  ,  has  crossing,  been  of  demonstrated,  respectively.  for these  suggests  T  are  hexacyanochromate (III)  that  phosphorescence  parameter,  crossing  determination  strong temperature  of  by  intersystem  to  yield,  0.52  reached  expected.  -with- t e m p e r a t u r e  measured,  the  of  Reineckate  r  c o n v e r s i o n has From  been  transfer  quantum  f°  sc  cj>^  has  occurrence  crossing of  Cr (III)complexes  than  the  estimated  variation has  of  complexes  i n general, i n dependence.  with  been  Cr(III)  The  time  after  obtained  pulse  which  the  p h o s p h o r e s c i n g s t a t e and i s 4 2 b e l i e v e d t o be t h e l i f e t i m e o f t h e T ( o r less likely ig) state. E f f o r t s h a v e b e e n made t o c o n f i r m a n d i d e n t i f y this T  parameter. ^  Studies  of  x  x  have  been  carried  out  as  a  function  iv  of  temperature.  .Mechanisms'based  assignments  of  All  processes, except  primary  t i o n s , seem different  T  are proposed  to consist  pathways  and  and  on  different  their  implications  the i n t r i n s i c  of at least  two  are d i f f e r e n t  tentative  radiative  components,  functions  of  examine transi-  which  take  temperature  T A B L E OF  CONTENTS Page  Abstract Table  i  of Contents  List  of Tables  List  of Figures  v v i i i i x  Acknowledgements CHAPTER  I.  i  x i i  INTRODUCTION  1  1.  Thermal Reactions  2  2.  Electronic  2  3.  Spectral  4.  Primary  5.  Photochemical  6.  A Brief  CHAPTER I I .  States  Properties  •  Photophysical Processes Processes  Description  7  . . . .  of this  6  10  Work  12  G E N E R A L E X P E R I M E N T A L PART  14  1.  E m i s s i o n Measurements  14  2.  Lifetime  19  3.  Photolysis  Measurements  21  4.  A b s o r p t i o n Measurements  24  5.  Deoxygenation  24  6.  Chemicals  CHAPTER  Measurements  25  I I I . ENERGY  Experimental  Techniques  T R A N S F E R AND  Section  QUENCHING  STUDIES . . .  28 29  Results  30  Discussion  43  vi Page  CHAPTER  IV.  PHOTOCHEMICAL  STUDIES  51  General Principles  52  Experimental  54  and R e s u l t s .  Discussion  55  Kinetic  59  Treatment  C H A P T E R V.  TEMPERATURE  DEPENDENCE  OF THE  PHOSPHOR-  ESCENCE L I F E T I M E S  63  Results  66  Discussion.  71  A Further  79  Comment  CHAPTER V I .  QUANTUM Y I E L D OF I N T E R S Y S T E M C R O S S I N G AS  A F U N C T I O N OF T E M P E R A T U R E  81  Experimental  82  and R e s u l t s  Discussion  89  CHAPTER V I I .  THE L I F E T I M E OF THE  4  Results  T„ OR 2g  2 T, S T A T E l g  . .  95 98  Discussion  103  CHAPTER V I I I .  THE P R I M A R Y P R O C E S S E S  Mechanism  I l l  I  117  Mechanism  II  121  Mechanism  III  124  r  vii Page  CHAPTER I X .  130  SOME FINAL REMARKS  1.  The O r i g i n  2.  T u n n e l i n g M e c h a n i s m and T e l l e r  3.  Primary  4.  Suggestions  o f t h e Lowest Q u a r t e t  Processes  and L i g a n d  f o r F u r t h e r v-Iork.  130  State  Crossing  Field  . . . . 133  Strength  . . 134 135  BIBLIOGRAPHY  139  APPENDIX  145  viii  L I S T OF  TABLES  TABLE  I.  II.  III.  IV.  V.  VI.  VII.  VIII.  Page  P h o t o a q u a t i o n Quantum Y i e l d s as Donor  of  [Cr(NH ) 3  2  (NCS)^] 55  F r e q u e n c y f a c t o r s and a c t i v a t i o n e n e r g i e s o f t h e t e m p e r a t u r e - d e p e n d e n t p r o c e s s e s o f t h e 2pg s t a t e .  70  I n t r i n s i c p h o s p h o r e s c e n c e r a t e c o n s t a n t s and l o w - t e m p e r a t u r e l i m i t i n g p h o s p h o r e s c e n c e quantum yields  83  R a t e c o n s t a n t s ( s e c "'") i n l u m i n e s c e n c e d e c a y C r ( I I I ) complexes  99  Fluorescence lifetimes at l i q u i d temperature .  nitrogen  The A r r h e n i u s p a r a m e t e r s Mechanism I and I I  based  of k i  s  c  109  on 116  4 The p r e d i c t e d l o c a t i o n s o f t h e T2g s t a t e s a c c o r d i n g t o Mechanism I ana The A r r h e n i u s p a r a m e t e r s  of  of 1 / T  9  and T2g II. . . .  .  .  120 127  ix L I S T OF  FIGURES  FIGURE 1. 2. 3.  Page M o l e c u l a r o r b i t a l diagram of t r a n s i t i o n complex o f 0^ symmetry  metal  S i m p l i f i e d energy l e v e l diagram h e d r a l d3 c o n f i g u r a t i o n  octa-  Primary processes i n v o l v i n g states  3  f o r the  5  the lowest  excited 9  4.  Schematic o f the s e t u p f o r e m i s s i o n measurements.  15  5.  Schematic of the s e t u p f o r l i f e t i m e measurements.  19  6.  Schematic of the setup f o r p h o t o l y s i s ments  22  7. 8.  measure-  The a b s o r p t i o n s p e c t r a o f t h e [Cr (CN) ,. ] ~3 trans-[Cr"(NH3) 2 (NCS) 4] - s y s t e m . . .  and  The e m i s s i o n s p e c t r a o f t h e [ C r ( C N ) , ] ~ 3 and t r a n s - [ C r ( N H 3 ) ( N C S ) 4 ] " system.  33  Stern-Voliner quenching of donor i n t e n s i t y a t -65°C  34  2  9. 10. 11.  12.  13.  31  phosphorescence . .  S e n s i t i z a t i o n of acceptor phosphorescence m o n i t o r e d a t 806 nm and a t 825 nm  35  The d o n o r p h o s p h o r e s c e n c e d e c a y various acceptor concentrations temperature .  rate constants at as f u n c t i o n s o f 36  S t e r n - V o l m e r cruenching o f d o n o r l i f e t i m e a t -65°C  phosphorescence .  Arrhenius plot  o f k,_„  37 39  OH  14.  Quenching of the phosphorescence l i f e t i m e of [Cr(CN)g]-3 i the p r e s e n c e of [ C r ( N H ) (NCS)4]~.  40  15.  Arrhenius plot  41  16.  The r i s e and d e c a y of the a c c e p t o r  n  3  2  of t r a c e of the  phosphorescence 42  X  FIGURE  17.  Page  Non-exponential decay o f the trans-[Cr(NH )2(NCS)4]~ i n the energy t r a n s f e r system . 49 3  :  18.  19.  S t e r n - V o l m e r p l o t s f o r I ^ / I and; f o r T"/TD . . . . . . . . . ? o D  The l i f e t i m e s o f t h e fer ( N H ) (NCS)4]~ and temperature 3  20.  21.  22.  23.  24.  25.  states of trans[ C r ( e n ) ] + 3 as f u n c t i o n s  2  lifetimes  of the  and  [Cr(CN)g]~3  as  2  67  E  a  of Cr(acac)  states  functions  3  of  a  [Cr(NCS) ]  of temperature of  Phosphorescence temperature  functions  yields  as  - 3  g  The p h o s p h o r e s c e n c e quantum y i e l d i o n as a f u n c t i o n o f t e m p e r a t u r e . quantum  as  68  . . .  Reineckate 84 of  The p h o s p h o r e s c e n c e quantum y i e l d o f C r t a c a c ) ^ a function of temperature . . . . .  as 86  Quantum, y i e l d s o f i n t e r s v s t e m c r o s s i n g f o r [Cr(NCS) ]-3, trans-[Cr(NH3) (NCS)4]and [Cr(CN)-]-3 . • 2  87  Quantum y i e l d s o f i n t e r s v s t e m c r o s s i n g f o r (1) [ C r ( e n ) 3 ] + 3 - (2) C r ( a c a c ) 3 ; (3) t r a n s [Cr(NH ) (NCS) 4]-;  (4) C r ( C N ) ] "  27.  Plots  -l)  28.  Oscilloscope traces of luminescence (A) C r ( a c a c ) , (B) [ C r ( C N ) ] -  3  2  of log(l/(j)  i s c  88  3  6  vs  1/T  94 decay: 96  3  3  29.  g  x as f u n c t i o n s o f t e m p e r a t u r e f o r t r a n s f e r ( N H ) (NCS) ] - , a n d C r ( a c a c ) . . . 3  30.  69  85  6  26.  of  3  T h e l i f e t i m e o f t h e "'Eg s t a t e function of temperature The  57  T  x  2  4  as a f u n c t i o n  100  3  of temperature  for [Cr(en) ] 3  .  101  of temperature  .  102  +  3  _3 31.  T  x  of  [Cr(NCS),]  as a f u n c t i o n  xi  FIGURE 32.  Page Luminescence s p e c t r a o b t a i n e d from decay curves for [Cr(CN)g]i n r i g i d g l a s s s o l u t i o n a t 77°K 3  33.  34.  35.  Intersystem crossing rate constant as a f u n c t i o n o f t e m p e r a t u r e  of  Intersystem crossing rate constant as a f u n c t i o n o f t e m p e r a t u r e  of  Intersystem  of  [Cr(NCS)g]~ 36.  crossing 3  as  a  rate  function  Intersystem crossing f e r ( N H ) 2 ( N C S ) 4 ] ~ as  constant of  3  106  Cr(acac)3 112 [Cr(en)3]  + 3  113  temperature  rate constant a f u n c t i o n of  .  . . . .  of t r a n s temperature.  114 .  37.  Schematic  of Mechanism  I.  38.  Schematic  of  II  123  39.  Schematic  of Mechanism  III  125  Mechanism  . .  115 118  4  40.  The o r i g i n o f t h e T _ s t a t e of C r ( I I I ) as f u n c t i o n o f l i g a n a f i e l d s t r e n g t h 2  complexes 132  ACKNOWLEDGEMENTS  I wish for  t o express  inspiration,  throughout  the course of t h i s  A. P f e i l  thanks  t o D r . G.B. P o r t e r  e n c o u r a g e m e n t , a d v i c e , and c r i t i c i s m  I would a l s o Mr.  sincere  like  work.  t o thank  Dr. J.S.E.  f o r indispensable technical  M c i n t o s h and  aids  and h e l p f u l  discussions. I am i n d e b t e d t o my w i f e f o r a s s i s t a n c e the n u m e r i c a l  i n some o f  treatments.  Financial  s u p p o r t from  o f Canada i s g r a t e f u l l y  the N a t i o n a l Research C o u n c i l  acknowledged.  CHAPTER I INTRODUCTION  Stimulated of  the  by  c h e m i c a l and  general,  the  the  great  s t r i d e s taken i n the  p h y s i c a l nature of  excited molecules  photochemistry of c o o r d i n a t i o n  c e n t l y quickened  i t s p a c e and  studies  emerged as  an  in  compounds has interesting  renew  branch of p h o t o c h e m i s t r y .  Numerous r e v i e w s have b e e n  published  in  and  new  the  field  last  few  i s now  years,"'"  being  any  other  and  r e s u l t s of the  are  only  lated. istic  field  "^  gradually  the  importance of  acknowledged.  i n i t s infancy,  most o f  photochemistry of  p h e n o m e n o l o g i c a l and  are  the  this  However, as  e x i s t i n g data  coordination  rather  in  compounds  scattered  and  I t i s t h u s e s s e n t i a l t o have more s y s t e m a t i c  iso-  mechan-  studies. The  family  of chromium(III) complexes  most s u i t a b l e s y s t e m s  f o r both  photochemical i n v e s t i g a t i o n . plexes  with widely varying  prepared,  their  satisfactory, have b e e n w e l l properties  of  intensive A  their  ligand f i e l d  the  extensive  are  strength usually  ground s t a t e c h e m i c a l  characterized the  and  of  l a r g e number o f C r ( I I I )  thermal s t a b i l i t i e s  and  i s one  and  studied."'"'"'"'''  electronic states  are  com-  have b e e n moderately  reactions The  relatively  bonding clear 12 — 18  and  the  Unlike  spectroscopic Co(III)  bands have b e e n w e l l  complexes, C r ( I I I )  assigned.  complexes o n l y  undergo  2  photosubstitution, this few  with  no  known p h o t o r e d o x r e a c t i o n s ,  l a r g e l y s i m p l i f i e s the Cr(III)  problem.  In a d d i t i o n , q u i t e  complexes p h o s p h o r e s c e i n f l u i d  temperatures,  hence  o r e v e n a t room t e m p e r a t u r e ,  a  solutions at for  low  example,  +3 [Cr(en)^] adjunct  .  Emission  s t u d i e s where p o s s i b l e  to photochemical In the  following  f a c t s which are  1.  in this  sections  important  therefore  slowly.  to  the  t h e o r e t i c a l and  empirical  f o r t h i s work a r e can  be  found  briefly  i n the  outlined.  references  T h e r m a l Reactions'^' ^ c o m p l e x e s have h a l f - f i l l e d considered  exchange r e a c t i o n s  and  essential  chapter.  Cr(III) are  an  studies.  More d e t a i l e d d e s c r i p t i o n s cited  are  They c o n s i s t o f  racemization. 17  i n the  Their  t o be  t~ orbitals 2g  and  substitutionally inert.  absence of aquation,  light  take place  anation,  a c t i v a t i o n energies  Ligand only  isomerization, range  from  15  Kcal/mol. 12-18  2.  Electronic The  States  molecular o r b i t a l  complex o f 0^  symmetry  diagram f o r a t r a n s i t i o n metal  i s shown i n F i g u r e  1.  The  d  electrons  o f an o c t a h e d r a l complex i o n o c c u p y t h e o r b i t a l s t . ^ g' w h i c h a r e n o n - b o n d i n g o r ir a n t i b o n d i n g ( t . ) and a a n t i * 3 b o n d i n g ( g) • t h e d. s y s t e m , t o w h i c h chromium ( I I I ) a n <  e  2 q  2 q  e  belongs,  the  I  n  ground  state o r b i t a l  configuration  is t  2  ^.  The  3  s p l i t t i n g , lODq, of the two from the anti-bonding i t may  sets of o r b i t a l s a r i s e s p r i m a r i l y  c h a r a c t e r of the eg o r b i t a l s .  However,  a l s o be a f f e c t e d by TT bonding of the t g o r b i t a l s .  Thus  2  a l a r g e value of lODq f o r a g i v e n complex means t h a t a complex  4 i n the  2 2g^  T  t  g c^ e  2  s t a t e i s expected  to have c o n s i d e r a b l y  l a r g e r chromium-ligand s e p a r a t i o n s than the ground s t a t e molecule because of the decrease  i n the o v e r a l l  (a and TT) bonding  f o r c e when a t^^ e l e c t r o n i s promoted to an anti-bonding orbital.  In other words, i t i s expected  (obtained from the a b s o r p t i o n maximum of  t h a t the value of lODq 4  T_  4  A„  2g corresponding  eg  and  thus  2g  to the v i b r o n i c t r a n s i t i o n with maximum Franck-  Condon overlap) w i l l be determined not only by the s e p a r a t i o n of the o r i g i n s of the e l e c t r o n i c s t a t e s themselves, but a l s o by the change i n the e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n . phenomenologically, maximum and  the Stokes  s h i f t between the  1  Therefore,  absorption  the f l u o r e s c e n c e maximum should i n c r e a s e markedly  as lODq i n c r e a s e s f o r a s e r i e s of complexes. The  energy l e v e l diagram from the s t r o n g - f i e l d  calcula-  t i o n s of Tanabe and Sugano i n the s i m p l i f i e d form i s shown i n 3  F i g u r e 2.  A l s o a r i s i n g from the c o n f i g u r a t i o n t  doublet s t a t e s  2  2  E^ and  2  E^,  2 T  ig'  a n <  ^  T  2  2g*  T h e  2  are  three  degeneracy of the  s t a t e s i s removed by s p m - o r b i t a l c o u p l i n g and  d e v i a t i o n of the l i g a n d f i e l d from o c t a h e d r a l symmetry.  by  5  lODq  Ficrare  2.  Simplified  3 d  configuration.  energy  (kK)  level  diagram  f o r the  octahedral  The  next  lowest  the  quartet  e l e c t r o n i c c o n f i g u r a t i o n , t„ zg 4 4  states  and  T ^(F).  The  1  2  e  g  other  , gives  rise  to  quartet  state  4 (P) t_  e  g 3 . 2  v/hich  complexes sharp E  g ible of  shown  in  configuration. g S p e c t r a l Properties"*'^ The  2  i s not  main are:  lines A_  features in  of  4 + +  <? region  low  and  2  two  relatively  the  low  of  the  A  spectrum, and/or  observed: T_ 2g  ->  tween  a n c  sometimes  usually of  transfer  emission  i s the  served  in Cr(III)  the  nm)  intense  a  Cr(III)  set  of  transition  transitions;  in  the  vis-  of  order  of  i n the  50  M  ^ cm  ^2q  «-  4  UV  transitions and  bands  near  which  UV are  region  4 A  ^ ) , and  2 g  of  the  intraligand  bands. two 2  electronic states  phosphorescence  820  of  s t r u c t u r e l e s s bands  E  g  types ->  fluorescence  i s of  spectra  max  spectra,  phosphorescence  the  broad  (e  L  Here  from  three)  forbidden  phosphorescence *  4 A„ . 2g two  *  Laporte  to to  spin-forbidden  intensity  number  charge In  4  a  the  (700  assigned  u  representing 4 4 T^ 2g'  absorption  region  intensity  (or  figure arises  ^  red  other  the  of  the  complexes,  same  been  radiation transition  be-  spin multiplicity,  while  which  there  have  fluorescence, '  spin multiplicities.  emission but  transitions  A„ , and 2g  i s the  different only  4  of  are  can some  be  Usually,  readily  cases  ob-  for  T h e 0^ s y m m e t r y d e s i g n a t i o n i s u s e d t h r o u g h o u t t h i s t h e s i s f o r s i m p l i c i t y , e v e n t h o u g h some c o m p l e x e s a r e n o t octahedral, f o r e x a m p l e , t r a n s - [ C r (NH3) 2 (NCS) 4] ~ i s o f symmetry.  7  which  fluorescence emission  t h e p h o s p h o r e s c e n c e and  i s comparable i n i n t e n s i t y  still  others  w h i c h show o n l y  with  fluo-  19 rescence.  Which o f t h e s e  the  energies  relative  o c c u r has b e e n c o r r e l a t e d w i t h 2 4 20 the E and T„ states. g 2g  of  Phosphorescence s p e c t r a , which are u s u a l l y the images o f t h e  corresponding  consist  of a s e t of  i n both  absorption  t h e band o r t h e the 2  E  on  g  sharp and  spin-forbidden absorption  lines,  emission  zero-zero  equilibrium nuclear  one and  of which occurs represents  transition.  This  the  spectra, distinctly  origin  of  indicates that 4  c o n f i g u r a t i o n s of the  " 2g A  a n <  ^  ^  e  s t a t e s must be the  nearly i d e n t i c a l . The f l u o r e s c e n c e s p e c t r a , -' hand, a r e i n v a r i a b l y b r o a d and s t r u c t u r e l e s s .  other  There i s m i r r o r  image r e l a t i o n s h i p 4  with  the  Stokes'  corresponding shift  equilibrium  nuclear  Primary The  cmite  A  shown i n F i g u r e are d e s c r i b e d  T„ , 2g 3.  fluorescence  band  ^ s o r p t i o n , but This  the  indicates that 4  A~ and 2g  the  the 4  T_ 2g  different.  2  T„ , 2g  For  i n 0^  2g  large.  Processes  J a b l o n s k i diagram 4  the  c o n f i g u r a t i o n s of the ^  Photophysical  i n v o l v i n g the ^  of  4 ^2q  is relatively  s t a t e s must be 4.  mirror  2  T, , lg  2  E  , and cr  simplicity,  symmetry 2  f o r chromium(III)  the  f o r a l l the  4  A  0  2g  complexes states i s  electronic complexes  states studied  2 T, and E s t a t e s , because of s m a l l ig g e n e r g y s e p a r a t i o n between, them, a r e assumed t o be i n r a p i d in  t h i s work, and  the  8 equilibrium otherwise  and  a r e c o n s i d e r e d an e q u i l i b r a t e d  state  unless  specified.  It  i s generally believed  Frank-Condon l e v e l s the v i b r a t i o n a l higher excited  that  of the e x c i t e d  relaxation states  and  after  electronic  internal  are very  excitation  fast,  to  states,  conversions  the molecule  the  because  between  must  reach  sec.  The  -12 its  lowest e x c i t e d  electronic  s t a t e s w i t h i n 10  4  lifetime  of the  strength  and  sec.  T„ 2g  s t a t e has  been e s t i m a t e d  f l u o r e s c e n c e quantum y i e l d  However, t h e o b s e r v e d  values  t o be  from  oscillator  less  than  f o r complexes which  10  _7  fluo-  —6  r e s c e a r e i n t h e o r d e r o f 10 system  crossing,  processes, crossing,  and  k„,completes f a v o r a b l y w i t h  internal  of photochemical  and  far  unity.  Primarv studied.  conversion, k  k g ) , s h o u l d be  s m a l l e r than The  sec or a l i t t l e  significant  2  as  less.  the  Inter-  other  (and/or i n t e r s y s t e m judged  from  the  sum  e m i s s i o n quantum y i e l d s w h i c h i s u s u a l l y  processes  from  phosphorescence  have b e e n a p p r o x i m a t e l y  2  the  E  s t a t e have b e e n b e t t e r g r a d i a t i v e r a t e c o n s t a n t s , k^,  evaluated through  a simple  spin-  18 o r b i t - c o u p l i n g model. are r e a d i l y yields  detectable at l i q u i d  are u s u a l l y  crossing  less  than  t o the ground s t a t e ,  pathway a t low system  Although  temperatures,  crossing,  k.,  may  be  phosphorescence  n i t r o g e n temperature,  a few kg, and  emissions  percent.  Intersystem  i s the dominant  degrading  thermally activated  important  the  at higher  inter-  temperatures.  9  2g  \ *7 4  N  \k_  \  ^  4  \  2g Figure  3.  states.  Primary k^,  processes  intrinsic  i n v o l v i n g the  fluorescence  lowest  emission;  excited  k ,  internal  2  A  conversion; k^,  intersystem  intersystem kg,  k^,  r e a c t i o n from  crossing;  c r o s s i n g ; k^,  intersystem  chemical  photochemical  thermally intrinsic  crossing to  r e a c t i o n from  the  2  the E  q  the  ^pg  activated  state.  s t a t e ; k^,  ate;  reverse  phosphorescence  ground  s 1 :  emission;  photo-  10  5.  Photochemical Aquation  studied  is  i s the p r i n c i p a l  f o r chromium(III)  photoaquation 0.5.  Processes  The  complexes  quantum y i e l d s  range  low quantum y i e l d s  e v i d e n t from  [Cr(H20)g]  the  by  + 3  photoreaction that  studies  are of  not  i n aqueous from due  less  has  been  solution. than  0.1  The  to  t o cage e f f e c t s  the p h o t o s u b s t i t u t i o n  oxygen-18 e n r i c h e d w a t e r — a l t h o u g h  as  of  the s o l v e n t 21  cage i s one All  of  the reactants,  investigations  present  from nor  c o m p l e x e s w i t h o n l y one  r e p o r t the quantum y i e l d  wavelength preted  of  the quantum y i e l d  of e x c i t a t i o n .  as m e a n i n g  level  other than  the  reached  can  does not  i n the  lowest excited  ligand  independent  These r e s u l t s  that photoaquation  the v i b r o n i c from  t o be  i s s t i l l  be  type  of  best  occur  low.  the inter-  directly  absorption process state  o f one  multi-  plicity . The tions  immediate  precursor to the photochemical  reac-  of Cr(III)  complexes has been s o u g h t f o r y e a r s . It 4 has b e e n s u g g e s t e d t o be t h e ^2g s t a t e , b a s e d o n t h e b o n d 22 i n g c h a r a c t e r and e l e c t r o n i c s t r u c t u r e . However, i t has a l s o been h e l d t h a t the l i f e t i m e of the lowest e x c i t e d quartet,  4 T  2 '  ^  q  s  brief  to enable  decav  processes.  is  relatively  n  o  greater than  10  -7  sec, which  a q u a t i o n t o compete e f f e c t i v e l y 23  The  long-lived  lowest excited so  is  too  with  other  doublet state,  c h e m i c a l r e a c t i o n may  2  well 22  pete  w i t h i n t e r s y s t e m c r o s s i n g s and  phosphorescence."  E  , g com-  24 '  11  Considerable of  the  effort  quantum y i e l d  sharp a b s o r p t i o n  on  has  been d i r e c t e d a t  irradiation  bands, i n o r d e r  i n the  measurement  region  to p o p u l a t e the  of  the  doublet 25  states d i r e c t l y The  idea  ponsible  rather  i s to bypass the  T  2g  s  t  a  te  s  that  o  on  the  to photoaquation,  grounds t h a t  the  intersystem  However, t h e s e e x p e r i m e n t s a r e  carry  .So f a r t h e r e  out. are  any  be  res-  might  i s not  difficult  firm evidence that  d i f f e r e n t f o r e x c i t a t i o n i n the  be 100%  to  the  region  quantum of  the  bands. I t has  may  i s no  yield  crossing  efficient.  doublet  i f i t were  p h o t o c h e m i c a l r e a c t i o n , t h e y i e l d w o u l d be 2 I f , on t h e o t h e r hand, t h e E s t a t e were t h e g  immediate p r e c u r s o r  yields  ?8  crossing.  f o r the  decreased.  larger,  than through i n t e r s y s t e m 4  a l s o been p r o p o s e d  a vibrationally excited  that  or hot  the  reacting  species  ground-state  molecule  which i s formed through  i s o e n e r g e t i c t r a n s i t i o n s from t h e 26 electronically excited states. However, t h e r e a c t i o n p a t t e r n s and a c t i v a t i o n p a r a m e t e r s a r e q u i t e d i f f e r e n t between 27 p h o t o c h e m i c a l and t h e r m a l r e a c t i o n s . And r e c e n t e v i d e n c e has shown t h a t w h i l e p h o t o s u b s t i t u t i o n does n o t depend on 29 the  solvent  composition thermal s u b s t i t u t i o n does.  over, c o n s i d e r i n g equilibrated, short  highly -12  (about 10  favored.  the  fact  that  the  l i f e t i m e s of  the  e x c i t e d v i b r a t i o n a l l e v e l s are  ' sec),  this  supposition  Morenonextremely  i s generally  not  12  6.  A  Brief The  D e s c r i p t i o n of main  rationale  this of  this  4 role of  played  Cr(III)  state  by  transfer  and  tion  energy  energy The  the  step  about  The  studies primary  of  transfer  of  investigate  i n the  populating  more  of  as  3  the  photoaquation the  efficient  to  of  of  f o r the  systems, an  doublet  energy  transfer  a  i s not  i s considered  only  primary  energy  transfer  excited  i n Chapter stage  that  alone.  processes are needed. In o r d e r t o i d e n t i f y the  applicaalso  on  to  the  studies  i n the  of  energy  IV. definitive  s t a t e s cannot  photochemistry  the  in  process.  2  this  on  s t u d i e s but  trans-[Cr(NH^) (NCS)^]~  at  energy 2  photochemical as  suitable  trans-[Cr(NH^) (NCS)^]~  acceptor,  there  i s described  clear  the  search  the  itself  application  role  of  states  the  to  emphasis  transfer  was  the  was  [Cr(CN)g]  system  It  E  Instead  One  photochemistry  transfer  and  excitation,  first  III.  the  to  used.  Chapter of  T_  systems.  donor  w o r k was  2  complexes.  was  The  a  the  direct  technique  as  by  Work  be  obtained  Studies  quenchable  information  of  part  from  the  of  other  the  photo-  2 aquatxon,  the  primary  vestigated  as  to  that  confirm  k_4/  processes  described the  from  i n Chapter  thermally  the  V.  activated  E^  The  state  main  were i n -  effort  was  intersystem crossing,  occurs. In  determine transfer  Chapter  VI,  the  possible methods'available  i n t e r s y s t e m c r o s s i n g quantum and  quenching  are  considered.  yields  from  Intersystem  to energy crossing  13  quantum y i e l d s were measured as f u n c t i o n s the  hope  4  T~ 2g  that  this  would  provide  of temperature i n  some i n f o r m a t i o n  about t h e  state.  In Chapter V I I , a t r a n s i e n t species or s t a t e , b e l i e v e d 4 2 t o b e e i t h e r o f t h e T„ s t a t e o r o f t h e T, s t a t e , was 2g l g characterized with  by t h e r i s e  of the phosphorescence  intensity  time a f t e r pulse e x c i t a t i o n . In Chapter V I I I , assuming  t h e newly  observed  lifetime  4 to  represent  that  of, first,  the  T^  state  and then t h a t o f  2 the  state, three Finally,  the and  4  mechanisms  i n Chapter  are proposed  and  IX, the spectroscopic  examined.  origin of  T„ state i s studied. Some s p e c u l a t i o n s a r e d e s c r i b e d 2g suggestions f o r f u r t h e r i n v e s t i g a t i o n s a r e made.  CHAPTER I I GENERAL EXPERIMENTAL PART  Experimental is  outlined  setup and  i n this  i n f o r m a t i o n common t o t h e e n t i r e chapter.  This includes details  of equipment, e x p e r i m e n t a l  purification  i n f o r m a t i o n and  procedures  techniques,  of the  chemicals  minor m o d i f i c a t i o n s l i m i t e d  investigation will  and  be m e n t i o n e d i n t h e  thesis of  the  used.  the  sources Specific  only to a  certain  c h a p t e r where i t i s  described.  1.  Emission  Measurements  A block diagram of the  setup  f o r the  steady-state  e m i s s i o n measurements i s shown i n F i g u r e 4 . were c a r r i e d  out  a r r a n g e m e n t was Fortunately is  i n a dark adopted  chamber.  mainly  f o r a l l the  right  angle  optical  to avoid s c a t t e r e d l i g h t .  systems s t u d i e d the  inner f i l t e r  effect  negligible.  1.1  Excitation The  arc  lamp  (PEK  light  (PEK  Light  F.L.)  110)  was  Source  s o u r c e was  model 4 0 1 ) .  27 mm  a 100-watt, p o i n t - s o u r c e m e r c u r y  powered by  housing  was  similar  beam was  sample w i t h  light  from  t o t h a t d e s c r i b e d by so f o c u s e d  DC  quartz  p l a c e d r i g h t b e f o r e the  t h e maximal amount o f  light  a stabilized  A short focal-length  collect  The  The  A l l measurements  power lens  supply (1" d i a m .  lamp i n o r d e r the  arc.  The  C a l v e r t and  t h a t i t passed  nearly constant c r o s s - s e c t i o n .  to  through  lamp  Pitts. the  30  15  M  CRS& PMT  PSD  PA  Figure  4.  Schematic of the setup f o r emission  LS = l i g h t F = glass  s o u r c e ; M = monochromator; S = filter;  shutter,  L = l e n s ; BS = beam s p l i t t e r ; CRS  c r y o s t a t ; C = sample c u v e t t e ;  PT = p h o t o t u b e ; CHP =  c h o p p e r f o r t h e l o c k - i n a m p l i f i e d ; PMT  =  recorder.  = light  photomultiplier  t u b e ; PA = p r e a m p l i f i e r ; PSD = p h a s e s e n s i t i v e R =  measurements  detector;  16  Each o f the mercury isolated  from  the total  lines  a t 4 5 6 , 4 3 6 , o r 366  output o f t h e a r c by a monochromator,  was e m p l o y e d  as t h e monochromatic e x c i t a t i o n  546  a yellow  nm  line,  eliminate  the second  u s e d was a B a u s c h visible  grating  exit  times the  relative  1. 2  and  and  drift  through  stability  The  cylindrical  The could tubing  blaze.  light  1%, someTherefore  was  monitored (RCA 9 3 5 ) ,  f o r i n thec a l c u l a t i o n s .  s a m p l e c r y o s t a t was b u i l t t h i c k Styrofoam.  of Plexiglass  Three  w i n d o w s w e r e made f o r t h e e x c i t i n g ,  streams  mw.  i n the excitation  e m i t t i n g . l i g h t beams r e s p e c t i v e l y .  when  a  entrance  12  a phototube  The f l u c t u a t i o n  insulated thermally with  ting  The  10 h o u r s .  of the excitation  corrected  Cryostat  with  nm/mm.  was b e t t e r t h a n  a beam s p l i t t e r ,  Sample  blanketed  The monochromator  (33-86-25) w i t h  a s m u c h a s 5% o v e r  intensity  quartz  3-74) was a d d e d t o  o f t h e l a m p a t 5 4 6 nm w a s a b o u t  was p r o p e r l y  layered  For the  5.36 a n d 3.00 mm r e s p e c t i v e l y .  a 10-mv r e c o r d e r .  light  uv r a d i a t i o n .  d i s p e r s i o n i s 6.4  i t s short-term  continuously and  were  output  i tcould  (Corning  light.  ( 3 3 - 8 6 - 0 2 ) o f 1 3 5 0 g r o o v e / m m a n d 5 0 0 nm  and  Although  filter  & Lomb M o n o c h r o m a t o r  reciprocal linear  The  glass order  The  slits  nm,  The o u t e r  o f warm a i r t o p r e v e n t  double-  transmitting windows  them  from  were  fros-  necessary. sample c e l l - - a  1-cm q u a r t z  be p o s i t i o n e d r e p r o d u c i b l y connected  to the c e l l  luminescence  on a copper b l o c k .  and p r o t r u d i n g  cuvette-A  glass  out of the cryo-  17  stat  served  as  the  conduit  to introduce  sample, t o c a r r y a t h e r m o c o u p l e tinuous  flow  length  quartz  to c o l l e c t The evenly  of n i t r o g e n  the  1.3  cell  w h i c h was  solution. t o be  stirred,  Temperature C o n t r o l  flow  r a t e of  heater  i n a 50  t u r e was  junction  The filter,  the  of  order  spread inlet  cell  i n the  cell  base. a  to  stir  too  becomes i n s e n s i t i v e t o critically  required.  Measurement  gas,  controlling  evaporated  liquid  the  from  current  nitrogen.  a g l a s s - c l a d copper-constantan  out  in  the  i n t o the  to  the  Temperathermo-  s o l u t i o n with  excitation light.  a Rubicon  liquid  The  its emf  potentiometer.  System  detecting  lenses,  on  c o n t r o l l e d m a n u a l l y by  measured w i t h  Detecting  cell  s o l u t i o n becomes  i s not  immersed d i r e c t l y  c l o s e to but  d e v e l o p e d was  placed  1 Dewar c o n t a i n i n g  c o u p l e w h i c h was  focal-  sample was  holes  i n t u r n c o n t r o l l e d by  probed with  con-  homogeneous t e m p e r a t u r e ,  emission  cold nitrogen  t h a t was  the  when t h e  and  short  to e n c i r c l e the  was  thus s t i r r i n g  A  a  emission.  extended  the  the  to maintain  c l o s e t o the  through  Fortunately,  temperature,  nitrogen,  1.4  very  to ensure a reasonably  withdraw  solution.  used to c o o l the  walls  T e m p e r a t u r e was the  placed  T e f l o n - c l a d magnetic bar  viscous the  over the  cold nitrogen  onto the  In o r d e r  l e a d , and  t h e maximal amount o f  copper tubing  tiny  l e n s was  and  system c o n s i s t e d of  a chopper f o r the  a sharp-cut  red  l o c k - i n a m p l i f i e r , an  ana-  18  l y z i n g monochromator, and  photomultiplier.  2-58)  was  used  the  scattered excitation light.  The  analyzing  was  a J a r r e l l - A s h 0.25  nm  gratings  of  respectively.  e n t r a n c e and ning  s p e e d was  meter E b e r t 1180  The  exit  slits  25  the  focused,  image o f to f a l l  chromator, the  The  emission,  entirely  emission  l i g h t was  c h o p p e r w h i c h was  operated  the  of  entrance s l i t The  CVP  red  the  100  liquid  nitrogen.  voltage  supply.  was  1.3  kv,  the  load r e s i s t o r  again  nm/mm;  been  Before  The The  laid  Hz  and  a PAR  placed  scan-  on i t s to  carefully  reaching  c h o p p e d by  t h e monoModel  right  BZ-1  before  I t was The  of  powered by  voltage  optimal  i n a c r y o s t a t and  a p p l i e d to the  value.  the  a Kepco ABC  The  as  the  1500  signal voltage  The  The  the  then recorded linearity  of  on  with  DC  high  across  a m p l i f i e d by  the  lock-in detection.  s y s t e m has  a  amplified signal  s i g n a l from the  a L e e d s and  150  photomultiplier  p h o t o m u l t i p l i e r was  reference  cooled  chopper  f e d i n t o a B r o o k d e a l Phase S e n s i t i v e D e t e c t o r / M e t e r  s i g n a l was  600  monochromator.  placed  PM322 w h i c h c a r r i e d o u t  der.  and  h o r i z o n t a l i n order  B r o o k d e a l LA350 Low-Noise A m p l i f i e r . f r o m i t as w e l l  82-400)  s e n s i t i v e ( S - l s p e c t r a l response) P h i l l i p s  p h o t o m u l t i p l i e r was  the  at  300  microns.  w h i c h has  onto i t .  (Model  at  monochromator was  entrance s l i t  the  100  further  monochromator  blazed  d i s p e r s i o n i s 33  u s e d were b o t h  nm/min.  to e l i m i n a t e  monochromator  groove/mm and  linear  s i d e w a l l t o make t h e allow  CS  CVP  red  two  (Corning  150  The  with  filter  a Phillips  The  Northrup  s a t i s f a c t o r y v/ith a s e r i e s o f n e u t r a l d e n s i t y  Unit  resulting  10-mv  been checked  was  to  filters.  recorbe  19  Figure  5.  Schematic  of the setup  OS = o s c i l l o s c o p e ; PMT  = photomultiplier  M = monochromator; F = g l a s s filter;  F2 = K C r 0 y 2  cuvette;  2.  is  FL = f l a s h  Lifetime The  2  block  The Operated  l a m p ; PT =  tube;  CRS =  F l = CuSO^  solution filter;  diagram 5.  45° t o t h e e x c i t a t i o n  Flash  filter;  measurements. cryostat;  solution  L = lens;  C = sample  phototube.  Measurements  shown i n F i g u r e  2 .1  forlifetime  of the setup  The e m i s s i o n  for lifetime  was d e t e c t e d  measurements  a t an  angle  light.  Lamp nitrogen  flash  l a m p w a s made  i n this  a t 5 a t m a n d 15 k v , i t h a s a r i s e  time  laboratory. o f 60 n s e c a n d  14 half-height width details  o f 20 0 n s e c w i t h  of i t sconstruction  5 x 10  photons/pulse.  and c h a r a c t e r i s t i c s w i l l  The  be pub-  31 lished  by P f e i l  a 5-cm  saturated  and P o r t e r .  The f l a s h  lamp was f i l t e r e d  CuSO^ s o l u t i o n t o p r o v i d e  f r o m a b o u t 3 0 0 t o 5 0 0 nm. the sample c u v e t t e .  I t was f o c u s e d  the excitation to a small  spot  with light onto  20  2 .2  Sample The  The  inner  with end cell  cryostat one  cold of  Cryostat  a  2 mm  outer  cans,  ing.  The  small  quartz-clad  cuvette  immersed the  flash quartz the  windows  exciting  control the  and  and  and  became  apparently  2 .3  mator, CS  the  The  sample  the  the to  space was  to  be  about  between  the  evacuated however  the  other  The  point  on  the  similar  to  Temperature  inner  before  only  coola  the  cap  point  of  of  focus  There  outer  were  the two  cylinder  for  Temperature described  changed  after  and  junction  those was  cap-  i t allowed  respectively.  taken  1 ml  i n s e r t e d through  temperature.  lights were  holder.  with  solution.  true  each  cell  the  in  very  temperature  System  a  system  consisted  photomultiplier. a  lower  steady.  and  and  the  situated at  emitting  detecting  eliminate  2-cm  saturated  scattered  m o n o c h r o m a t o r . was an  45°  The  2-64)  with  was  measurements were  Detecting  to  capped,  measurements.  slowly  attached  tightly  measurements  emission  block  was  monitor at  circulated  was,  the  cans.  or  cuvette  into  cylindrical  nitrogen  cuvette  The  thermocouple  two  liquid  as  Pyrex  of  the  directly  to  served  length.  thermocouple light  with copper  blown  path  where  A  can  specially  and  of  filled  gas.  cooling  acity  and  be  nitrogen  this  was  can  consisted mainly  infrared  a  red  filters,  glass  I<2Cr207  filter  The  & Lomb M o n o c h r o m a t o r  (33-86-03)  of  a  monochro(Corning  s o l u t i o n were  excitation light.  Bausch  grating  A  of  675  used  to  analyzing (33-86-25)  groove/mm  and  1.0  21  micron blaze. The  entrance  tively. that  The r e c i p r o c a l l i n e a r and e x i t  However,  the exit  The p h o t o m u l t i p l i e r , same t y p e  the  bleeder  for  f a s t response  were  as an e n t r a n c e i t scooling  as t h o s e used  network  system  and power  of the photomultiplier  applications.  was w i r e d  The v o l t a g e  measured  a 1,000-ohm l o a d  a Tektronix  Oscilloscope. and  sensitivity  nally  Type L P l u g - I n  resistor,  or externally with  The  and d i s p l a y e d  The c o m b i n e d u n i t h a s a r i s e o f 5 mv/cm.  applied  was  a way an  specially to the  signal,  on a Type o f 15  either  a Du M o n t O s c i l l o g r a p h  Record  Camera  Type  302 a n d P o l a r o i d  tained  were  then enlarged with  Type  410 f i l m .  543B nsec  inter-  The  recorded with  were  except  s i g n a l from a phototube.  curves were  exit.  preamplified  time  I t was t r i g g e r e d  respec-  supply  i n e m i s s i o n measurements  w a s b e t w e e n 0.9 a n d 1.5 k v .  with  i n such  and t h e e n t r a n c e  photomultiplier across  i s 1 2 . 8 nm/mm.  s e t a t 3.4 a n d 6 mm  t h e m o n o c h r o m a t o r was u s e d  served  the  slits  dispersion  decay  The p i c t u r e s  a Delineascope  ob-  (American  Optical Co.).  3.  Photolysis A block  shown i n F i g u r e that light cell.  described  Measurements diagram 6.  of the setup  The l i g h t  f o rphotolysis  s o u r c e was e x a c t l y  i n t h e e m i s s i o n measurements  beam w a s s o f o c u s e d  that  i tf i l l e d  studies i s  t h e same a s  except  that the  most o f t h e p h o t o l y s i s  22  Figure LS  6.  Schematic of  = light  filter; guard  source;  H = heater; troller;  setup  f o r p h o t o l y s i s measurements.  M = monochromator; S = s h u t t e r ; F =  L = l e n s ; BS  t u b e ; CRS  the  = beam s p l i t t e r ;  PT = phototube; GT  = c r y o s t a t ; C = sample c u v e t t e ;  T =  and  integrator.  =  thermistor;  HET = heat-exchange tube; TC = temperature  R & I = recorder  glass  con-  3 .1  Photolysis The  cuvette.  photolysis  long g l a s s  tube which  samples,  photolysis  3.2  was a 1-cm c y l i n d r i c a l a t b o t h ends w i t h l o n g ,  cell  Temperature  to place  a thermocouple,  above t h e s o l u t i o n  and t o m a i n t a i n a  during photolysis.  Control  outside i t .  and Measurements  low t e m p e r a t u r e i s o t h e r m a l b a t h was b u i l t  t a i n i n g methanol passing  cold  changer  immersed  insulated with thick  as t h e c o o l a n t .  S t y r o f o a m , and c o n -  C o o l i n g was a c h i e v e d by  i n the coolant.  Temperature  was k e p t  t e m p e r a t u r e by a h e a t e r w h i c h was  by a C o l e Parmer V e r s a t h e r m E l e c t r o n i c  a low t e m p e r a t u r e t h e r m i s t o r p r o b e .  before photolyzing.  Temperature  constantan thermocouple  Photon C o u n t i n g  the part  intensity  beam s p l i t t i n g  within con-  C o n t r o l l e r and  The s o l u t i o n  introduced  was a l l o w e d t o r e a c h e q u i l i b r i u m w i t h t h e o u t s i d e  The  of a  n i t r o g e n gas through a copper t u b i n g h e a t ex-  0.2°C o f t h e d e s i r e d  3.3  The  was immersed i n a low t e m p e r a t u r e b a t h w i t h  square copper v e s s e l  trolled  evacuated  s e r v e d as t h e c o n d u i t t o i n t r o d u c e and  guard tubes p r o t r u d e d p a r t l y  The  quartz  The o p e n i n g o f t h e c u v e t t e was c o n n e c t e d t o a  flow of n i t r o g e n  the  cell  I t was e x t e n d e d  guard tubes.  withdraw  Cell  coolant  was m e a s u r e d w i t h a c o p p e r -  immersed d i r e c t l y  into  the s o l u t i o n .  System of the i n c i d e n t  light  was m e a s u r e d w i t h  t e c h n i q u e as shown i n F i g u r e  o f t h e . r a d i a t i o n was r e f l e c t e d  and d e t e c t e d by a RCA 935 p h o t o t u b e .  6.  f r o m t h e beam The s i g n a l  A  small  splitter from t h e  24  p h o t o t u b e was r e c o r d e d der  and i n t e g r a t e d w i t h  (Model 143x58) e q u i p p e d  (Model 2 0 1 ) . cell  The r a t i o  with  a Brown  a Disc Chart  o f photons r e a c h i n g  t o the i n t e g r a t o r counts  (MH) R e c o r -  Integrator  the p h o t o l y s i s  was d e t e r m i n e d  with  a  Reineckate  28 actinometer 4.  i n t h e same p h o t o l y s i s  Absorption  Measurements  Absorption a Cary 5.  s p e c t r a and a b s o r b a n c e s were m e a s u r e d  14 S p e c t r o p h o t o m e t e r a t room  Deoxygenation All  cell.  temperature.  Techniques  s o l u t i o n p r e p a r a t i o n s were c a r r i e d  gen  box.  gen  viaa fritted  with  out i n a n i t r o -  S o l u t i o n s were d e o x y g e n a t e d by b u b b l i n g g l a s s gas d i s p e n s e r  through  pure  them.  L i q u i d A i r Co. L g r a d e n i t r o g e n  (oxygen c o n t e n t  was washed s u c c e s s i v e l y t h r o u g h  two b o t t l e s  nitro-  Canadian  20 ppm  max.)  o f vanadous  solu-  32 tion  and one o f d i l u t e NaOH s o l u t i o n b e f o r e u s e .  p o n e n t s o l v e n t s had b e e n p u r g e d w i t h (days) After  before adding  purged w i t h ment.  nitrogen f o r a long  time  t h e y were u s e d t o make, up t h e m i x e d s o l v e n t . the s o l u t e , the r e s u l t i n g  nitrogen f o r at least  s o l u t i o n was  again  t e n minutes before  measure-  A l l s o l u t i o n s u n d e r measurement, i f open t o t h e  a t m o s p h e r e , were c o v e r e d  under a continuous  to  The s o l u t i o n s f o r l i f e t i m e  keep o f f t h e o x y g e n .  ments were p r e p a r e d  and i n t r o d u c e d i n t o  t h e n i t r o g e n box and were t i g h t l y ported  The com-  t o t h e sample c r y o s t a t .  flow of n i t r o g e n measure-  t h e sample c u v e t t e i n  capped b e f o r e  being  trans-  The s o l u t i o n s f o r e m i s s i o n  25  and  photolysis  syringe which  6.  measurements were t r a n s p o r t e d w i t h a 10-ml  with a long f l e x i b l e  were a l r e a d y under  Teflon  needle  to their  cells,  flows of nitrogen gas.  Chemicals  6.1  Potassium  Hexathiocyanatochromate(III)  Anhydrous potassium h e x a t h i o c y a n a t o c h r o m a t e ( I I I ) , [Cr(NCS)g], tallized  o b t a i n e d from  more  evacuation. and  stored  than  P~0  tion  of the free  6.2  Potassium  I n o r g a n i c s I n c . , was  three times  from  cold  then dried  over  P ° 5 -*-  I t was over  Alfa  C  95% a l c o h o l n  under  drying  a  2  i n a vacuum d e s i c c a t o r .  thiocyanate ion i s less  pistol  The m o l a r  than  0.5%.  grade  ootassium  c h r o m i c v a n i d e , K_. [ C r (CN) -  City  Chemical  Co. was  recrystallized  washed w i t h e t h a n o l and then w i t h e t h e r . P Ot2  i n a vacuum d e s i c c a t o r .  cyanide 6.3  frac-  Hexacyanochromate(III)  Electronic from  recrys-  i o n i s less  Potassium  than  The m o l a r  3  t w i c e from water I t was  fraction  ] ,  D  and  stored  over  of the  free  0.5%.  Tetrathiocyanatodiamminechromate(III)  Potassium  tetrathiocyanatodiamminechromate(III),  K[Cr(NH^)2(NCS)^],  was  prepared  from  Fisher  Certified  trans-  grade  28 Reinecke's  salt  as d e s c r i b e d by Wegner  s a m p l e was  further  recrystallized  over  P 2 ° 5 ""-  free  thiocyanate i o n i s less  n  a  v  a  c  u  u  m  desiccator. than  from  and Adamson. cold  The m o l a r 0.3%.  alcohol fraction  The and  dried  of the  26 6.4  Tris(ethylenediamine)chromate(III)  Perchlorate  Tris(ethylenediamine)chromate(III) prepared  from i t s c h l o r i d e s a l t ,  w h i c h was a v a i l a b l e tallized in  twice  from A l f a  [Cr (HpNCR^CHpNHp) 3 ] C I * 3$ R^O,  Inorganics  f r o m w a t e r and t h e n  dried  Chromium(III)  obtained times  from A l f a  Inorganics  from benzene under All  and s t o r e d o v e r  &2 - 5 (  >  desiccator  Cr(CH3COCHCOCH3)3,  I n c . , was r e c r y s t a l l i z e d  three  evacuation.  t h e above compounds  f o r e were p u r i f i e d  are l i g h t  under dim r e d l i g h t  sensitive,  and t h e r e -  and s t o r e d i n a vacuum  i n the dark.  Methanol Eastman Kodak S p e c t r o  Spectroanalyzed  grade o r F i s h e r C e r t i f i e d  m e t h a n o l was u s e d w i t h o u t  A.C.S.  further purification.  Ethylene G l y c o l Eastman Kodak r e a g e n t  ethylene  6.8  I t was r e c r y s -  Acetylacetonate  Chromium(III) a c e t y l a c e t o n a t e ,  6.7  Inc.  a vacuum d e s i c c a t o r .  6.5  6.6  p e r c h l o r a t e was  grade or F i s h e r C e r t i f i e d  g l y c o l was u s e d w i t h o u t  further purification.  Water Water u s e d was d i s t i l l e d  water.  grade  27  6.9  The M i x e d The  tions  Solvent  solvent  consisted  u s e d t o make up t h e C r ( I I I )  o f two p a r t s  of ethylene  glycol,  glass  about -100°C.  The up  below  solu-  ( i n volume) o f m e t h a n o l , one p a r t  and one p a r t  o f water.  The g l a s s  s o l u t i o n remains c l e a r whether slowly.  complex  cracks  I t forms  clear  a t about  -150°C.  i t i s cooled  down o r warmed  CHAPTER I I I  ENERGY  In the  order  TRANSFER  AND  to elucidate  QUENCHING  the kinetics  p h o t o p h y s i c a l and p h o t o c h e m i c a l  important bypass  t o be a b l e  a certain  photochemistry, most  efficient For  energy  state  Cr(III)  populate,  has been  technique  complexes,  state  technical been  has o f t e n  difficulties  directed  recently  Quenching  been  attempted,  and d i v e r s e  of organic  aspect.  3  3  ,  3  ^'  3  to the  28 '  b u t owing  to  more a t t e n t i o n  energy  triplets  t o be t h e  excitation  results,  to excitation  or  In organic  i n this  the direct  i t i s  depopulate,  proven  25 doublet  processes,  of a molecule.  transfer  and v e r s a t i l e  and mechanisms o f  primary  to selectively  excited  STUDIES  ha  transfer.  by C r f a c a c ) ^  has been  36 observed, was  b u t i t was  involved.  Only  organic  molecules  clearly  through  Unfortunately, of  clearer  has energy  that  energy  transfer  transfer  from  excited  complexes been demonstrated 37 38 emission and p h o t o a q u a t i o n .  i s s t i l l  acceptor  some  reached.  ambiguity  i f a system  reasons.  i s so chosen  and  s p i n - f o r b i d d e n bands  and  higher  those  about  the state  I t i s imperative  and a c c e p t i n g e l e c t r o n i c  and p r a c t i c a l  than  then  to Cr(III)  there  the donating  theoretical  recently  sensitized  the inoraanic  tify  not certain  states  The s i t u a t i o n  that  of the donor  of the acceptor.  the lowest  to iden-  f o r both will  be  spin-allowed  are, respectively, Accordingly  lowe  [Cr(CN)g]  is  t h e b e s t among t h e C r ( I I I )  fact,  energy  transfer  from  complexes  a series  as an a c c e p t o r .  of Cr(III)  double  In  salts  _3 containing  [Cr(CN)g]  i n the c r y s t a l l i n e  s t a t e has been  39 studied. effect  However, t h e r e i s e v i d e n c e  i s n o t caused  by e n e r g y  that  the observed  t r a n s f e r b u t by a c r y s t a l p e r -  turbation. The  energy  t r a n s f e r between t h e p o t a s s i u m  Reineckate  salts  of the  i o n , t r a n s - [ C r ( N H ^ ) ( N C S ) ^ ] ~ , and t h e h e x a c y a n o _3 chromate(III) i o n , [Cr(CN) ] , was i n v e s t i g a t e d i n t h i s work. Experimental Section 2  g  The  concentration of trans-[Cr(NH^)^(NCS)^]  was k e p t  _3 c o n s t a n t a t 0.05 M; w h i l e t h a t to  0.07 M.  tion  o f [Cr(CN) .] o  varied  /  A l l t h e s o l u t i o n s were d e o x y g e n a t e d .  s p e c t r a were m e a s u r e d i n 1 mm  cells  a t room  Quantum y i e l d measurements were made w i t h ation  a t -6 5°C.  absorbed  both  ions.  scanned. tive  The a b s o r p temperature. 546 nm  radi-  S i n c e t h e a b s o r p t i o n s p e c t r a o f t h e two  ions are s u f f i c i e n t l y d i f f e r e n t is  from 0  (see F i g u r e 7 ) , t h i s  o n l y by t h e R e i n e c k a t e Both  ion i n solutions containing  d o n o r and a c c e p t o r e m i s s i o n s p e c t r a  Since the phosphorescence  t o temperature,  the average  wavelength  intensity  i s very  v a l u e was t a k e n  from  were sensiat least  t h r e e s e p a r a t e measurements. L i f e t i m e measurements were made f r o m The vide was  -110° t o -40°C.  f l a s h l a m p was f i l t e r e d w i t h a y e l l o w g l a s s radiation excited.  near  500 nm s o t h a t  The p h o s p h o r e s c e n c e  filter  t o pro-  again only Reineckate i o n decays  o f t h e d o n o r and  30  acceptor  were measured  separately  at  751  and  840  nm  respec-  tively.  Results The ions,  as  absorption  shown  spectrum  i n Figure  7,  of  the  solution containing  is exactly  the  sum  of  the  both  separate  -3 absorption  spectra  solutions. not  of  the  Therefore,  interact  with  [Cr(CN)g]  the  each  _  two  other  ions  and  [Cr(NH^) (NCS)^] 2  i n the  noticeably  same  i n the  solution sense  do  of  ion-  nm  radi-  pairing. Excitation ation of  i n the  of  presence  phosphorescence  phosphorescence emission fact  [Cr(NH3)2(NCS)4]~  of  at  the  Therefore, [Cr(CN)g]~  on  the  as  3  spectra The  of  former.  from  of  emission  whole  acceptor are  the  the  in this  i n the  system. as  p h o s p h o r e s c e n c e maxima,  that  is  3  acts The  as  two  at 9  detectable. donor  and  phosphore-  and  8.  sensiti-  various  acceptor  and  10.  The  were  taken  emissions  i s , 751  the  of  i n Figure  quenching  i n Figures  acceptor  the  observed  absence  shown  measurements  and  of  the  6  phosphorescence  appearance  i s e s t a b l i s h e d by  4  separable  presented  donor  That  [Cr(CN) ]~  2  quantum y i e l d are  but  from  546  decrement  [Cr(NH3) (NCS) ]~  easily  r e s u l t s of  concentrations  the  and  the  with  r e s u l t s i n the  - 3  latter  same c o n d i t i o n s ,  2  tensities  the  intensity  [Cr(NH3) (NCS)4]no  zation  [Cr(CN)g]  i s s e n s i t i z e d phosphorescence  that  scence  of  only  nm  for  inat  [Cr(NH3) NCS)4]~ 2  650  600  550  500  X  Figure  7.  The  trans-[Cr(NH ) (NCS) ]~ 0.05  2  M  400  350  (nm)  absorption  3  450  spectra of  the  [Cr(CN)g]  and  3  system.  4  [Cr(CN) ]"  1 mm  3  g  path-length;  0.05  M  [Cr ( N H ) ( N C S ) ] "  0.05  M  [Cr(CN) ]  1 mm  path-length.  3  2  1 mm  4  - 3  6  1 mm  +  0.05  path-length; M  [Cr(NH ) (NCS) 3  2  and  806  and  825  nm  acceptor emission  for  [Cr(CN)g]  _3  .  The  (see F i g u r e 8 ) .  the donor, a c c o r d i n g t o Stern-Volmer 1°  and  1^  1  +  k  are the  quenching  N  „ T  [ A ]  0  c o n s t a n t , the  absence o f the  acceptor,  and  of  [Cr(CN)g]~3  the  yields  to k ^ T ^ , i s estimated  y s e c , k^,^  i s 7.7  (J ri  For the  x 10  5  M  -1  sensitized  [A]  and  are, respectively,  unthe  l i f e t i m e of the donor i n the  9 shows t h a t t h e p l o t o f  tration equal  (3.1)  T Q , and  H  Figure  of  mechanism,  i n t e n s i t i e s o f t h e quenched  quenched e m i s s i o n , k g , total  For quenching  D =  D  the  a t b o t h w a v e l e n g t h s have b e e n c o r r e c t e d f o r  the s m a l l donor e m i s s i o n  where I  i n t e n s i t i e s of  c o n c e n t r a t i o n of the (IQ/I ) D  a g a i n s t the  a straight t o be  24.3  line. M  A  .  acceptor,  concen-  Its slope, Since  T  0  is  33  -1  sec  emission of the acceptor,  i t can  be  shown t h a t QH l  '  A  +  K k , et  (3.2) K  k  e t  T  c ™  A where I  i s the i n t e n s i t y o f the  k ^ the energy  transfer  proportionality the  r a t e c o n s t a n t , and  constant.  reciprocal  intensity,  F i g u r e 10 1/I ,  c e n t r a t i o n of acceptor y i e l d s seen  that  sensitized  A  acceptor K an  emission,  experimental  shows t h a t t h e p l o t  a g a i n s t the r e c i p r o c a l  a straight  line.  I t can  of  conbe  i s equal t o the r a t i o of the i n t e r c e p t to the QH o ^ ^ s l o p e o f t h e l i n e o f E q u a t i o n 3.2. The v a l u e s e s t i m a t e d f o r k__  T T  T  D  33  ^QH o T  a  r  20.7  e  sequently, For decay ious  rate  k^  21.8  i s 6.8  H  the  at and  lifetime  constant  acceptor  presented  and  806 7.2  x  825  10^  or  D  1/T ,  concentrations 11.  of  D  as  nm  M  measurements,  k ,  i n Figure  and  respectively. sec  the  from  these  data.  phosphorescence  [Cr(NH^)^(NCS)^]  functions  Con-  of  at  temperature  It i s straightforward  to derive  varis the  expression  K  Figure  D  12  acceptor  =  1 / T  shows  D  =  that  1 / T  D  + k  the p l o t  concentration  yield  Q R  [A]  of k a  D  (3.3)  at  straight  -65°C line.  against The  the  slope  of  J  I  [Cr(CN) ] g  gure  9..  Stern-Volmer  3  x  100  quenching  1  I  (M) of donor  phosphorescence  intensity  at  -65°C.  —I  4.4  1  1  1  1  4.6  4.8  5.0  5.2  1,000/T Figure various  11.  . The  acceptor  donor  (°K  phosphorescence  concentrations  as  _ 1  1  l _  5.4  5.6  )  decay  functions  rate of  constants  at  temperature.  38 the  line  sec  ^,  yield  gives d i r e c t l y  i n good  value  agreement w i t h  measurements.  log(k  the  Figure  those  13  ) against reciprocal  of  k-„  as  found  shows  the  temperature  7.2  from  x  10  the  a  -1  M  quantum  Arrhenius as  5  plot  straight  of  line.  QH  The  frequency  are  6.6  x  factor  10^  M  and  sec  activation  ^ and  4.8  energy  obtained  Kcal/mol  for  k_„  respectively.  _3 The in  the  energy  Firstly, was  results  the  found  of  the. l i f e t i m e m e a s u r e m e n t s  transfer  system  phosphorescence  to  be  shortened  are  especially  lifetime  of  of  [Cr(CN)g]  interesting.  the  acceptor,  c o n s i d e r a b l y i n the  x  presence  ,  of -3  [Cr(NH ) (NCS) ]~ 3  is  2  quenched  sufficient tionship,  as  4  by  shown  [Cr(NH^)^(NCS)^]~  experimental the  i n Figure  data  quenching  to  14.  too. check  constant,  k'  Apparently  Although  [Cr(CN) ] g  there  are  the  Stern-Volmer  has  been  not  rela-  estimated  from  On  two  points.  The  Arrhenius  plot  of  k'  against  On  reciprocal  t e m p e r a t u r e i s shown i n F i g u r e 15. The f r e q u e n c y f a c t o r and i 1 2 — 1 —1 a c t i v a t i o n e n e r g y o b t a i n e d f o r k' a r e 2.2 x 10 M sec •• QH and  7.6  Kcal/mol  perimental  respectively,  uncertainty i n these  although  there  is a  large  ex-  values.  _3 Secondly, was  followed with  showed by  when  a  an  initial  longer decay,  an  the  oscilloscope,  as  shown  the  dependence  the  and  acceptor  of  the  donor  only  of  [Cr(CN)g]  the  can  on be  of  i n Figure  of  with  time  16.  As  phosphorescence  time,  after  expressed  as:  at  traces  increase i n intensity  Appendix, donor  emission  an  the and  840  emission followed  derived in  intensities  ideal  pulse  nm  the of  excitation  39  4.2  4.6  5.0 1,000/T  Figure  14.  [Cr(CN),]""  Quenching 3  5.4 ( K 0  )  _ 1  of the phosphorescence  i n the presence of  [ C r (NH,)  5.8  0  lifetime  (NCS) .] ~ .  of  41  Time Figure  16.  The r i s e  of the acceptor. detected  and d e c a y t r a c e o f t h e p h o s p h o r e s c e n c e  Donor, 0.05 M and a c c e p t o r ,  a t 840 nm, a t - 7 0 . 7 ° C .  Time s c a l e :  0.05 M, 50 y s e c p e r  division.  I  D  I  A  where  (3.4)  =  K  xp(-t/T )  =  K .[exp(-t/T )  D  i e  A  2  and K  stantaneous  2  ~exp(-t/x )]  are adjustable  intensity  paraments.  and t h a t t h e s l i t s  c h r o m a t o r u s e d were l a r g e a t t h e s e t t i n g  fore,  of the Reineckate emission the t o t a l  Because t h e i n -  o f t h e donor e m i s s i o n  than t h a t o f the acceptor  part  (3.5)  D  intensity  was s t i l l  i s much  greater  o f t h e mono-  o f 8 40  nm a s m a l l  detectable.  has t h e f o l l o w i n g  form:  There-  43  ^ o t a l  =  l  = where  peratures  in  nm  K  3  l  D  values  11. from  electronic  2  lifetime,  the intensity  The v a l u e s  3.7  This  T  D  )  ,at various  so obtained  clearly  agree w i t h  decay  of  indicates  of the Reineckate  tem-  traces of emission  a r e shown as s t a r r e d  the phosphorescence  state  6  factor.  to Equation  t h e same s y s t e m .  1  '  (3.7)  D  3  of t h e donor from  3  (K K +K )exp(-t/T )  +  A  2  obtained  Figure  (  K exp(-t/x )  according  directly in  +  i s a weighting The  840  A  at  points  those  found  [Cr(NH^) (NCS)^]~ 2  that the donating  i o n i s the  phosphorescent  2 state,  E . g  Discussion 4 With ion  5 4 6 nm  i s populated 2  to  the  E  on  quenching,  radiation,  directly,  state which  the  state  followed,by  emits  sensitization,  of the  Reineckate  intersystem crossing  phosphorescence. and e s p e c i a l l y  41  The  lifetime  results measure-  2 ments e s t a b l i s h eckate  unequivocally that the  i o n i s the donating However,  state,  i ti s conceivable  F^ s t a t e  or the state that,  of the Rein-  being  quenched.  i n s t e a d o f the phos-  2 phorescent  state,  E  , some t h e r m a l l y a c t i v a t e d  electronic  g excited with 1 .  state  the  chmg.  42  2  E  9J E  (S)" , o f t h e Reineckate state,  ion i n equilibrium  c a n be t h e immediate p r e c u r s o r  t (S)' g -? (S) • + a c c e p t o r ->  t o quen-  (in equilibrium) 4  A» + acceptor 2g  k^ QH IT  It  i s kmetically  i n d i s t i n g u i s h a b l e whether  the  2E ^ o r some  2 other doublet volved.  state,  e.g., the  However, based  state  on t h e f o l l o w i n g  i s directly  in-  arguments,  i t c a n be  estimated that the thermally activated state, (S) , 4 4 c a n n o t b e t h e T» state. F i r s t l y , t h e T_ state 2g 2g many r e l a x a t i o n p r o c e s s e s a n d h a s a d e c a v c o n s t a n t ,  i f any, undergoes ^ 1/k.j., a t  ?  least  an o r d e r o f magnitude b i g a e r  than  that  2  of the  E  4 (see  Chapter V I I ) . 2  librium with  the  Therefore, E  state.  the  state  In fact,  state -  g  i s not i n equi-  the thermally  activated  4 T» i s the rate-determinina step o f deg 2g 2 4 gradmcf t h e e x c i t a t i o n energy o f t h e E s t a t e v i a t h e T_ 9 2g s t a t e ( s e e C h a p t e r V) . E v e n i f t h e ^ T s t a t e i s quenched, 2 ^ the E s t a t e w i l l n o t be a f f e c t e d . S e c o n d l v , t h e T„ state, g 2g --1 2 . . transition  2 E  »•  2 c f  lying is  about  much t o o h i g h  the  Q H  ,  are  i n energy  values  from both  [Cr ( N H ) 2 (NCS)^] 3  [Cr(CN) ]~ . 3  g  state  crossing  i n energy  about  f o r the total  experimental  (see Chapter V ) ,  1,700 cm  quenching  and t h e l i f e t i m e errors.  transfer—  This  quantum y i e l d  . constant,  measurements  indicates  of t h e donor,  , i s n o t changed by t h e p r e s e n c e o f  In fact,  of t h e donor  system.  i s only  t h e quantum y i e l d  the intersystem  E^ s t a t e  t o be i n v o l v e d  of  obtained  t h e same w i t h i n  that  fer  above t h e  a c t i v a t i o n energy The  k  3 , 0 0 0 cm  i ti s highly  i s not perturbed  probable  that  the  4  T  a t a l l i n the energy  2  g  trans-  There  a r e two modes f o r q u e n c h i n g  2  o f t h e donor  E  g  state.  Sg  (  ( E ) 2  ( A  D  4  g  +  2  g  )  first  tionless with  fer,  transition  transfer.  tensity only  +  ( E  (3 8  g  or  T  2  g  ) "  (3.9)  t o an i n t r a m o l e c u l a r o r enhanced by  the second  c a n b e made  t h e sum o f t h e i r  their  strated  intermolecula  importance o f energy  QH  individual  trans  quantitative  t h e d a t a on p h o s p h o r e s c e n c e i n -  E q u a t i o n s 3 . 1 , 3 . 2 , a n d 3.3  incorporate  rate constants:  =  k  q  +  k  e t  (  values or their  participation  by t h e s e n s i t i z e d  quenching  o f energy  3  '  1  0  )  ratio.  transfer  i s clearly  emission of the acceptor.  demon-  I f  a s i n E q u a t i o n 3.8 o c c u r r e d , w i t h o u t t h e i n v o l v e m e n t  any e l e c t r o n i c a l l y  there  from  radia-  interaction  p r o c e s s i s an  As t o t h e r e l a t i v e  and l i f e t i m e s .  The  in  )  facilitated while  k  of  ->  )A  E q u a t i o n 3.9 a n d q u e n c h i n g , E q u a t i o n 3 . 8 , n o  assessment  not  2 g  process i s equivalent  the quencher;  energy  Sg  A  ( A The  )D + (  excited  i s no r e a s o n t o e x p e c t  quenching  than t h e donor  states  [Cr(CN)g] itself.  does n o t o c c u r , as i s demonstrated phosphorescence  lifetime  of the acceptor molecule, t o be any  Y e t such by t h e f a c t  different  self-quenching that the  of the Reineckate i o n i s independent  46  of  i t s concentration.  tant k  ,, et'  reaction  Therefore,  and  quenching  as m e a s u r e d ,  c a n n o t be  represent  energy  an  impor-  transfer,  only. z  2 Although energy t r a n s f e r donor t o e i t h e r o f  2  the  E  43 spin-allowed, the  the  donor, p r o b a b l y too  energy t r a n s f e r . only  state  v i a the  Then t h e  4  or  9  of  high  from the T_  the  E  s t a t e of the g of the a c c e p t o r  states  2g  acceptor  i s , like  i n e n e r g y t o be  I t i s assumed t h a t  lowest doublet s t a t e s  involved  energy t r a n s f e r  o f d o n o r and  energy t r a n s f e r e f f i c i e n c y ,  that  are  of  in occurs  acceptor.  k  , / ( k . + k ) can be et et q following equations. k . [A] „ „ pk x (3.11) U/T) + (k + k ) [A] ° ° 4 et 7  estimated i A sp  from the .D *isc  v  p  <!> p  =  p  isc  (iA°) From E q u a t i o n s transfer  efficiency  k  k  3.10,  . + k et q  the  (k  and  q  intrinsic  e t  3.12,  [A]  one  can  derive  the  energy  as  x  and  k )  +  1  phosphorescence, the and  3.11,  et  where cf)^, " ^ g ^  +  (3.12)  k^  k  k_„[A] o QH  k  A  are  the  P  *so D  p  T  quantum y i e l d  quantum y i e l d  radiative rate  (3.13)  (}) p  A  of  of  the  intersystem  quenched  crossing,  constant respective,  of  the  A a n d <b P  donor;  and k  phosphorescence the  A P  a r e t h e quantum y i e l d  and t h e i n t r i n s i c  -65°C.and  [A] = 0.07 M,  1.6--evaluated by comparing  constant of  s e n s i t i z e d phosphorescence  S-l  photomultiplier  and  k  energy  transfer  about  the  1.1.  shown t h a t  <t>".  spectra. flat  T  indicates  2g state  the energy  = 0.1) w i l l  t h e quenched and  The s e n s i t i v i t y i n this region.  respectively.  previously A  the  so, then  p  efficiency obtained This  conclusion  A  i s relatively  a r e 2 0 0 a n d 16 s e c  A  t h e r a t i o <f> /<f>p i s e q u a l t o  the areas under  the  if  radiative rate  acceptor. At  is  of the sensitized  that  t o Equation  3.13  tT  c a n n o t be t h e a c c e p t i n g  be 11 w h i c h  of the  k^ = k . , and agrees v/ith QH et ^ From t h i s i t c a n a l s o be  made.  transfer  k^  The v a l u e  according  of  state  efficiency obtained  because,  (taking  i s not possible.  ISC  Energy  transfer  does n o t o c c u r  a t temperatures  -130°C w h e r e t h e s o l v e n t  becomes a r i g i d  inates  transfer,  long-range  trivial  process  energy  of reabsorption.  k^„  i s very  close  to that  and  t h e m a g n i t u d e o f k^.^ i s c l o s e  glass.  complex  This  below elim-  formation, and  The a c t i v a t i o n e n e r g y o f  of solvent  fluidity,  5 ± 0.5  to the estimated  Kcal/mol,  rate  of  yn  6 diffusion solvent, facts fusion  controlled estimated  suggest  that  controlled  process,  about  4 x 10  -1 M  from v i s c o s i t y measurements. the energy process.  transfer  —  sec  1 for this  A l l these  i s essentially a  dif-  48 Figure  14  a l s o quenched two  modes o f  shows t h a t  i n the  presence of  quenching  radiationless  2  the  are  transition  E  s t a t e of  g  Reineckate  p o s s i b l e : the  k  and  the  [Cr(CN),] 6 ion.  k  t  .  Again  self-quenching  is  Similarly,  enhanced  intramolecular  intermolecular  energy ^-  q transfer  -3  transfer i s favored. Since the _3 [Cr(CN)g] i s n o t n o t i c e a b l e as i s e v i d e n c e d  of  energy  i  from Figure should  not  14, be  the  operative.  [Cr(NH ) (NCS) ]~ 3  2  and  This  to  be  due  In  emission  4  non-exponential may  process k  , similar fact,  at  740  the nm  delayed  self-quenching,  decay  (see  decays more s l o w l y the  to  Figure  i n the  energy  t r a n s f e r from  of  the  [Cr(NH^) (NCS) ] .  E  s t a t e of  intensity curve  Unfortunately,  -  2  of  at  the  4  delayed  that part  In  the  too  phosphorescence  noisy  ( E  g  )  +  D  to  f o l l o w i n g energy k  2  ( V  i<i  )  A  .  17)  later  the becomes  part.  c a u s e d by the -3 [Cr(CN).,] to c  <? that  of  phosphorescence 2  back  curve  allow  the  instantaneous  i s too  rigorous  small  and  the  analysis.  transfers  OH *  (  i  4  A ) 2g  ( E ) a  D  ?  2  +  A  QH k^  H  i s assumed, t o be  constant  given  by  equal  Debye  to  the  diffusion-controlled rate  equation  8RT  k „ -  =•  n  3000 n  • where A E ^ It  i s the  s  -  exp(-AE r j  "  /RT)  (3.14)  .  44  a c t i v a t i o n energy  i s f u r t h e r assumed  which  =  i s endothermic,  that  the  requires  of  reverse an  the  solvent  energv  Arrhenius  fluidity.  transfer,  k',  a c t i v a t i o n energy  49  Time  Figure  17.  in  energy  the  acceptor per  Non-exponential transfer  system.  concentration:  division.  decay of the  0.05  trans-[Cr(NH^)^(NCS)^]  Donor c o n c e n t r a t i o n : M,  indicates  a t ~38°C.  the  ideal  0.0 5  Time s c a l e :  exponential  1  M; ysec  decay  trace.  equal  t o the  enerqv d i f f e r e n c e ,  A E , between t h e  E  states of g  45 the  d o n o r and k  0H  •  =  acceptor. s  = The  activation  e x 0 H  s  0 H  P(-  Therefore,  A E f l  /  exp[-(AE  enerqv  p T  F L  o f k.  TT  )  e x  P(  - A E  /  R T  )  + AE)/RT] obtained  (3.15) does agree w i t h  Qn  (4.8 7.6  vs  5 Kcal/mol).  Kcal/mol,  i s about  The 2.6  activation Kcal/mol  enercry o f k'„ more t h a n  AE-,.  AE_, f l  obtained, The  50  difference  is  equal  to  AE.  The v a l u e  of  AE e v a l u a t e d  from  -3 the is  emission 870 cm  methods  1  are  spectra (2.5 in  of  [Cr(CN)g]  Kcal/mol).  agreement  with  The two each  and  [Cr(NH^)^ (NCS)^]  values other.  from  different  CHAPTER I V PHOTOCHEMICAL STUDIES  Energy  t r a n s f e r has b e e n u s e d s u c c e s s f u l l y i n o r g a n i c  photochemistry  t o i n v e s t i g a t e the r o l e played 33  and  triplet  states.  '  '  However t h e a p p l i c a t i o n o f t h i s  of inorganic photoreactions  i n a few c a s e s .  has  only  And a l m o s t a l l o f t h e i n v e s -  t i g a t i o n s were made on t h e p h o t o s e n s i t i z e d c o m p l e x e s by o r g a n i c  singlet  34 35  technique t o the study been r e p o r t e d  by  donors, p a r t i c u l a r l y  reactions biacetyl.  of metal Vogler  46 and  Adamson  have s t u d i e d  some C o ( I I I )  ammines.  the photosensitized reduction of 47 Porter has s t u d i e d t h e p h o t o s e n s i -  t i z e d a q u a t i o n o f c o l b a t i c y a n i d e i o n ; and S a s t r i and L a n g f o r d t h e t e t r a c h l o r o p l a t i n a t e ( I I I ) i o n . As f o r C r ( I I I ) c o m p l e x e s , Adamson, M a r t i n , photosensitized  and C a m e s s e i aquation  38  48  have i n v e s t i g a t e d t h e  of [Cr(NH ) (NCS)] , +2  3  5  [Cr(NH ) (NCS) ]" 3  2  4  -3 and  aquation  R e c e n t l y , some work on t h e p h o t o s e n s i t i z e d +3 of [Cr(en)^] by b i a c e t y l has b e e n r e p o r t e d by 49  Balzani,  Ballardini,  the  [Cr(NCS)g]  accepting  conclusion states  .  Gandolfi,  and M o g g i .  s t a t e s were n o t known  on t h e r e a c t i v i t y  is still  f o r sure,  of the various  f a r from c e r t a i n .  However, b e c a u s e a clear-cut  excited electronic  Furthermore, there  c o m p l i c a t i o n t h a t t h e e x c i t e d b i a c e t y 49 l may w i t h t h e l i g a n d s o f t h e complex i o n s .  i s the  react directly .  52  General  Principles The  energy  general principles  transfer  photoreactive systems the  donor  under it and  of  those  study  have  been  i n which  as  be  an  used  t o be  made  as  either  at  all.  the  of  crossing  tatively.  phosphorescent photochemical tration yield  from  portion  the  A  of  i d  donor  the  of  or  state, quantum  an  states  easy  state.  as  the maximal  the  former. t o be  of  the intersystem reactivity evaluate  photochemical  of  or  the  quanti-  quantitative  i n the  re-  quenching  quencher  photochemical while  or  uncertainties  reduction high  a  occurs  of  to  If  reactive  By  state,  of  That i s ,  a donor.  the  i  state  allow  the  to obtain  at verv  a measure  state  of  the  difficult  phosphorescent to  and  a  acceptor.  photoreaction s t i l l  donor,  i s used  yield  the  because  the  i s very  of  e  photoreactive molecule  determined  efficiency  yield  as  be  usually  molecule  i s ecmal  fluorescent  whether  state  i s taken  35,50  i t s fluorescent  can  It is relatively  i f the  a  reactivity  state  transfer  quantum  phosphorescent  sults,  on  the  However,  energy  3 3  phosphorescent  involved.  use  information concerning  described. the  of making  acceptor, populating i t s phosphorescent  phosphorescent depending  criteria  obtain  only  acceptor are  may  to  simultaneously bypassing  inert not  states  and  i s used  study the  are  measurements  and  the  overall concenquantum  the u n a f f e c t e d  quantum  yield  from  the  53 The if  back  intersystem  Cr(III) k_^, of  above s t a t e m e n t s h a v e t o be  complexes,  has E  lation  state  g  2  of the  tation)  t o be  E^  state  by  the  T^  than that  active. obtain  As from  observing  photosensitized  for  that  from  chable  part  the  In this  state  or both  from  phosphorescent state  be  state  used  is re-  part  Careful  quenching  to  conclude  directly  quantum quen-  from  the  a n a l y s e s h a v e t o be  of photoreaction state  or i s  t r a n s f e r between  As  photo-  However, the maximal  the phosphorescent  I I I energy  merely  o f .the  i s the photochemical  state.  to  i s reactive.  i n o v e r a l l p h o t o c h e m i c a l quantum  In Chapter  POPU-  sensitizationis  cannot  the quantum y i e l d  now.  from  the decrease  depletion  case, i f the  a q u a l i t a t i v e r e s u l t of  s t i l l  n o t be  determine whether  crossing  for  (or d i r e c t e x c i -  the fluorescent  reaction  fluorescent  may  phosphorescent  for  state.  the phosphorescent  c h e m i c a l quantum y i e l d  to  transfer  the quenching method, the unquenchable  yield  crossing,  out q u a n t i t a t i v e data are d i f f i c u l t  sensitization,  unambiguously  intersystem  from d i r e c t p h o t o l y s i s , the  pointed  For  I f so, then d i r e c t  obtained  i s reactive; i f smaller,  modified  involved.  the main pathway  energy 4  p h o t o c h e m i c a l quantum y i e l d  state  the back  (see C h a p t e r V ) .  does not bypass  greater  i s appreciably  f o r example,  been considered  2  the  crossing  considerably  made  intersystem responsible yield.  [Cr(NH^) (NCS)^]" 2  _3 and  [Cr(CN)g]  doublet of  states  has  b e e n shown c l e a r l y  of each  the photoaquation of  ion.  In this  to take place  chapter quenching  [Cr (NH3) (NCS)4]~ a r e 2  via studies  described.  54  Experimental  and  Results  Photolysis -65.0  -  0.2°C  water,  and  nm  lasted  and  solution to  keep  of  i n the  ethylene  was  the  deoxygenated glycol  tynically kept  Reineckate  o f f oxygen.  a  The  solution  (2:1:1).  one  under  i o n was  hour.  flow  of  pure  out  was  at  irradiation  n i t r o g e n gas  concentration of  at  methanol,  Irradiation During  of  carried  the  546  the  in  order  Reineckate  _3 ion  was  0.07 by  0.03  M.  the  For  of  except  a l l the  that  the  same  ion.  by  the  The the  amount  irradiation  of  [Cr(CN)g]  solutions  according to  analyzed that  while  Reineckate  measured tion  M,  the  radiation  photochemical  and  through  the  as  the  technique  of  Adamson  added  from  was  quantum  with  the  same  blank.  0  only  to  absorbed  yield  thiocyanate i o n produced.  served  e t h a n o l was  varied  was  A  solu-  operations  T h i o c y a n a t e i o n was 28 Wegner, except  and  [(CH^J^NjCl to  precipitate  _3 [Cr(CN)g] filtered stand and  as  well  solution  was  overnight before  measuring  plex.  The  the  molar  Quantum  yields  perature  aquated  the  Reineckate  made  adding  alkaline  of  the  was  releases  allowed  The to  solution  ferric-thiocyanate  4.30  x  10  taken  for  the  M  sec  - -.  3  ^  by r e f e r e n c e t o 28 taking  four  photoaquation  several  and  ion.  perchlorate  coefficient  actinometer, '  The  Reineckate  ferric  determined  analysis  ion at  then  complex  were  complex.  unreacted  extinction  Reineckate  that  the  absorbance  ferric-thiocyanate  fact  as  the  account  1  concentrations of  yields  28  room  of  thiocyanate ions quantum  com-  the per  of  [Cr(CN),]  tem-  the 3  55  are  listed  escence value  i n Table  quantum  at  each  I,  yield  point  together with of  the  same  i s averaged  ion  from  Table Photoaquation  Quantum  the  Yields  relative  phosphor-  f o r comparison. at  least  three  The runs.  I of  [Cr(NH3)2(NCS) 4 ] ~  as  Donor  Concentration  M./1  d> , x 10^ chem (donor) T  Donor  Acceptor  D I (relative)  ,<» d> , (donor)  T  —-  0  1.02  +  0.02  1  0.03  0.79  +  0.04  0.58  0. 49  X  lO"  0.03  0 .05  0.76  + 0.01  0.45  0.55  X  10  0.03  0.07  0.69  + 0.01  0.37  0 .51  X  lO"  0.03 0.03  The  :•  acceptor  is  [Cr(CN) ]  2  -2 2  _3  g  Discussion It the but of  Reineckate less the  extent  the  so  of  from  than  the  of  the  equation:  that  i n the  "E  can  be  g  state  the  photoaquation  presence Since  quenched  2  phosphorescence,  photoaquation  following  data  phosphorescence.  i o n i s not  quenching of  these  i o n i s quenched  Reineckate  quenching of  is clear  the  in this can  the maximal calculated  be  of  of  [Cr(CN) ]  -3  g  4  T„ 2g system,  estimated  guenchable  accordingly to  state and  the  from portion the  ,  (  *chem - *chem  ^chem ^  s  t  ^  =  }  (<f,  lifting  l e  concentration of  chem " ^chem <  ° o  ) ( l  D  D  )  ( 4  p h o t o a q u a t i o n quantum y i e l d  [Cr(CN)g]  -3  '  1 }  at high •'  when t h e p h o s p h o r e s c e n t  '  state  _3 is  totally  quenched-  The  solubilitv  of  [Cr(CN)_]  precludes  b  a d i r e c t measurement o f ^ h e m ' for  i t f r o m E q u a t i o n 4.1,  however, t h e v a l u e s o b t a i n e d  as  listed  i n Table I, are  consistent  -3 and have an a v e r a g e Equation  4.1  v a l u e o f 5.2  and m a k i n g u s e  l o w i n g e q u a t i o n s c a n be ^chem  ^chem  x 10  .  of Equations  o  of  the  intensity,  3.3  the  fol-  D ,., [A]  „,  r  k_„T  (4.2)  1  photoaquations  and  and  , . . 1 +  =  Plots  3.1  derived. o  *chem " * c h e m  By r e a r r a n g i n g  quantum  phosphorescence  yield,  lifetime  phosphorescence  against  the  concen-  _3 tration in  of  [Cr(CN)g]  F i g u r e 18.  The  according to Equation  fact  that  a l l the  lines  4.2  are  are  shown  practically  c o i n c i d e n t d e m o n s t r a t e s t h a t p a r t o f p h o t o a q u a t i o n , criven by (cj)°, - (j) , ) , and t h e p h o s p h o r e s c e n c e o c c u r v i a t h e same cbem cnem 00  state. Two (1) 10  -3  half  c o n c l u s i o n s can  of  ) i s not  the  total  quenched  excited  molecules  doublet  states,  drawn  photoaquation by  that  and  be  (2)  [Cr(CN)g]  have the  not rest  -3  been of  from at  these  -65°C  and. t h u s through the  experiments:  ( w i t h <> j = occurs  5  1  x  from  equilibrated  photoaquation,  again  with via  <f> =  5 x  either  reachable  way.  lowest  , c a n be q u e n c h e d  the phosphorescent by thermal  The ing  10  results After  excited  excitation quartet  occurs  the  from  rapid  excited  other  this  manifold  must  degradation  level  of the  results  i n a  i n competition  state  state  i n the follow-  vibrational  manifold  4  state,  degrade  state  the phosphorescent  state,  t o the ground  occur  quartet molecule.  crossing to the doublet  conversion  doublet  from  to a high  state,  aquation  internal  o r some  are therefore interpreted  equilibrated  intersystem  state  activation  vibrationally then  and t h e r e f o r e must  Photowith  and  Molecules  t o the lowest  i n  doublet  2 state, at  E  65°C.  , as t h e phosphorescence Such m o l e c u l e s  phosphorescence all  o f which  two  paths  tion  lowest to  and i n t e r s y s t e m  c a n be quenched  t o be c o n s i d e r e d  reaction:  doublet  direct  state  by water,  excited  quartet  the energy  aquation  undergo  explanation  for this  state,  33  with  quartet  transfer.  latter  part  state-  o f t h e aquai n the  crossing to the  an a c t i v a t i o n  energy  t h e two s t a t e s ,  lifetime.  equal  followed  molecule.  the studies of the  d e p e n d e n c e .of t h e p h o s p h o r e s c e n c e  with  There a r e  o f t h e complex  state  ysec  i n competition  intersystem  between  from  i s s t i l l  c r o s s i n g t o the ground  by energy  o r back  resulting  i s favored  aquation  substitution  difference  o f .the  lifetime  The  by  second  temperature  This  means  that  2 the  Eg  detailed  state  i s essentially  arguments  substitutionally  are presented  i n Chapter  V.  inert.  The  However, a t  -65°C, back i n t e r s y s t e m c r o s s i n g of t h e R e i n e c k a t e with  an e f f i c i e n c y  crossing  the  through  the  v i a the  required i n two  paths  the experimental  i t i s necessary  be  The 4 D ( A2g  +  still  treatment  made and  interthe  valid.  I  hv  D  4  (  T  (4.3)  2g>  D( A 4  2 g  )  +  hv  (4.4) (4.5)  4  2g  based  c o m p l e t e mechanism i s  4  4  results  t o demonstrate t h a t even i f back  drawn a r e  D ( T 2g  2g  in  i n c l u d e back i n t e r s y s t e m c r o s -  system c r o s s i n g i s i n c l u d e d the data conclusions  those  c o n v e n t i o n a l analyses which are i n f a c t  a mechanism w h i c h does n o t  D( T  be  above c o n c l u s i o n s , as w e l l as  I I I , were d e d u c e d f r o m  D( T  intersystem  VI).  Chapter  sing,  occurs  Treatment Since  on  would then  a q u a t i o n quantum y i e l d s  (see C h a p t e r  Kinetic  A forward  o f a b o u t 52%  efficiency  order t h a t the equal  o f a b o u t 90%.  ion  k ->  (4.6)  Product  k ->  z  D( T 4  2g  (4.7)  k_  (4.8)  ->  k  r  D( A 4  2 g  )  +  hv'  (4.9)  60  D( E )  +  D( E )  ->  2  q  2  D( E 2  The  9  the in  k ^  ) + A( A~) 4  reverse  omitted  °(  6  g  2g  Q  f o r t h e sake  4  g  (4.11)  )  2  g  energy  transferi s  In thetransient studies,  o f t h edonor c a n be expressed,  = k  D  (4.12)  + A( E J  2g  4.12, i . e . , r e v e r s e  and Chapter  = k  2  o f convenience.  lifetime  the Appendix  4 A  D( A_)  H  of process  apparent  (4.10)  Product  7  V, a s  + k  5  a s shown  + k  6  (1 - a) k _  +  y  4  + k  Q H  [A]  (4.13)  D  T  = 1/T° +  k  + k  ±  and  .  D o  T  mation  + k  3  k  + k  5  6  + k  ?  4.14 i s t h e r e f o r e  In quantum  (4.15)  + k  2  4  1  =  Equation  + (1 - a ) k _ exactly  t h ephosphorescence  yield  measurements  i sjustified,  Reineckate  ions  D  ( E )°  g ss  t h e same a s E q u a t i o n  where  the E  and  4  T.  4g  k .1 4  and  approxi-  concentrations  states  3.3.  photoaquation  thesteady-state  thesteady-state  2  m  (4.16)  4  intensity  g  2  (4.14)  QH  4  a =  where  k n N ^  O  t  of  the  c a nbe d e r i v e d as  a  = (  k  l  +  k  2  +  k  3  +  V  (k  -4  +  k  5  +  k  6  +  k  7  +  k  QH  [ A  &" 4 -4 k  k  (4.17)  61 (  ( T  )  4  - 4  k  =  D  (k  1 +  k +k 2  3 +  V V V  Q H a — k ) (k_ +k +k +k +k +  4  c a n be e x p r e s s e d  n  O  (k +k +k  S  •  k  3  (  k  +k  -4  +  +  k  6  ^chem (k +k +k +k ) ]  2  From E q u a t i o n  1° ° 1°  =  3  4  +  k  7  +  k  OH  C  A  P°v, D  h  This  4  5  state.  )  +  6  D  h  = 1 +  s  o  k  s  4.22  i s also  from Equation  0 H  Lim [ A 3 * °° ^ c h e m  k  4  4  5  4 7 K  (4.20) Q H  [A])-k k_ 4  4  that  Q H [A] +  k  6  +  k  7  (4.21) +  (  l-a)k_  4  x°[A] t h e same a s E q u a t i o n  (4.22) 3.1.  4.19 3  k, + k„ + k., + k  t o t h e quantum y i e l d I t can f u r t h e r  k k_  (4.19)  ?  exactly  shows t h e u n q u e n c h a b l e p a r t  equal  ]  k  o  (4.18) [A])-  Q H  [A])-k k  : (k_ +k +k +k +k  k  ^chem  7  k  = 1 + k  Now,  6  4.18, i t c a n be shown  P  Thus, E q u a t i o n  I  4 5 1_5 +k +k +k +k  )(k  5  k  5  ]  as  = p  A  o f t h e phosphorescence and p h o t o -  k  A  C  4  T h e r e f o r e quantum y i e l d s aquation  k  (4.23)  of the photoaquation i s s t i l l  of photoaquation from the q u a r t e t  be shown  that  62  *chem " *chem _2£^JB £11^. ^chem ^chern  k  = i  This can  i s again  exactly  [  A  ]  (4.24)  1 +  k  6  Q H  7  -4  x°[A]  (4.25)  t h e same as E q u a t i o n  conclude that Equations  4.2.  We  therefore  3 . 1 , 3 . 3 , and 4 . 2 a r e g e n e r a l l y  v a l i d whether back i n t e r s y s t e m mechanism o r n o t .  H  + 5  =  °  crossing  i s included  i n the  CHAPTER V TEMPERATURE-DEPENDENCE  OF THE  PHOSPHORESCENCE  LIFETIMES  It  h a s l o n g been  times o f C r ( I I I ) perature.  known t h a t  complex  ions  the phosphorescence  are strongly  A few i n v e s t i g a t i o n s  dependent  have b e e n made  life-  on tem-  i n the attempt 51  to  clarify  the  t h e mechanisms.  phosphorescence  decay  T a r g o s and F o r s t e r constants consisted  ture-independent, a s l i g h t l y strongly well  found of a  tempera-  temperature-dependent,  temperature-dependent  term.  that  and a  The s e c o n d t e r m , as  as p a r t  o f t h e f i r s t , was a s s i g n e d t o i n t e r s y s t e m 2 4 from t h e E t o the ground A state. The t h i r d  sing  g  which tures,  crosterm,  2 g  i s overwhelmingly dominant although not d e f i n i t e  at relatively  high  t h e n , was p o s t u l a t e d  tempera-  to represent 2  the  thermally activated 4 back t o t h e T„ state. 2g  intersystem Such  crossing 2 crossing, E g  from t h e E 4 • T„ , c e r 2g g;  +3  tainly  does  o c c u r i n those complexes,  e.g. [Cr(urea)^]  , 2  w h i c h have r a t h e r s m a l l e n e r g y s e p a r a t i o n between t h e E and 4 52 53 the T states. ' However, t h e f i r s t d e f i n i t e e v i d e n c e g  2 a  for  t h e p r o c e s s came f r o m s t u d i e s  temperatures.  In both c r y s t a l s  o f r u b y and e m e r a l d a t h i g h  the c a l c u l a t e d  energy of the temperature-dependent  activated  component a g r e e d r e a s o n a b l y  w e l l w i t h t h e e n e r g y d i f f e r e n c e between t h e z e r o - v i b r a t i o n a l levels  of the  4 2 T„ and E states, 2a a  and e m i s s i o n o f d e l a y e d  fluorescence  was  also  observed."*" '^ 8  Recent  4  work  of Camassei  55 and  Forster  have  unambiguously  the involvement  4  2 of  demonstrated  Eg  •  crystalline  T  i n a group  2  hosts  that  of Cr(III)  fluoresce  only  complex  ions  i n various  at sufficiently  high  temperature. Unfortunately, a l l the above-mentioned complexes have s m a l l lODq v a l u e s and a c c o r d i n g l y , p r o b a b l y have s m a l l 4 2 20 T ( v = 0) E ( V = 0) s e p a r a t i o n s . F o r those complexes which c t  2 c r  have  large  lODq  fluorescence,  values  et  where  56 aJL.  whether  delayed the thermal  4 E  *• T „ g  tions  do n o t e x h i b i t  i t i s interesting to inquire  2 activated  and g e n e r a l l y  s t i l l  prevails  (especiallv i n solu-  2g  ^  photoreactions . studied  have  phosphorescence  may  be c o m p e t i t i v e ) .  the temperature  intensity of Cr(III)  Schlafer  dependence complex  of the  ions  i n solu-  57 tions.  Zander  has e x t e n s i v e l y  effect  on t h e p h o s p h o r e s c e n c e  plexes  i n both  additional  crystalline  support,  thermal  quenching  thermal  activated  were  they  +  the temperature  l i f e t i m e s o f many  state  simply  Cr(III)  and s o l u t i o n . concluded  that  In fact,  and t h e r e f o r e  f o r the Cr(III)  their  the  data  complexes  com-  Without  o f t h e p h o s p h o r e s c e n t s t a t e was 2 4 E >• T_ . However, t h e i r g 2g  not de-oxygenated,  reliable. +  investigated  strong  due t o t h e solutions are not  with  a  large  O x y g e n q u e n c h i n g o f t h e p h o s p h o r e s c e n t s t a t e o f some C r ( I I I ) complexes i n s o l u t i o n s has been demonstrated i n t h i s laborat o r y . 58  65  lODq  value,  the  i d e n t i f i c a t i o n of  the  strongly  temperature-  2  dependent  depleting  2  vated as  a  component  of  E  with  the  thermal  acti-  4  E  y  T„  g  t r a n s i t i o n remained  thus,  at  best,  merely  2g  plausible Aside  intersystem  speculation.  from  the  interest  crossing  clarifying  the  itself,  in  thermal  there  speculation.  are  Firstly,  activated  important at  room  back  reasons  for  temperature,  2  the  relaxation  the  strongly  firming  or  of  the  E^  state  proceeds  temperature-dependent  ruling  out  the  almost  component.  proposed  entirely Without  mechanism,  studies  by  conof  the  2  reactivity  of  the  E  depleting  i t by  hardly  interpreted  posed  be  mechanism  energies  state,  g  direct  is  even.by  excitation  populating  energy  transfer,  unambiguously.  confirmed  i t offers  or  solely  us  a  way  by to  Secondly,  making  use  estimate  of  the  or could  i f the  the  pro-  activation  zero-vibrational  4  l e v e l of the ^ 2q from s p e c t r o s c o p i c the tant  s  t  which i s , i n general, not available studies. Knowledge of the l o c a t i o n of a  t  4  zero-vibrational for  the  e  level  of  the  understanding  of  the  T^^  state  primary  i s very  impor-  photoprocesses  ori-  4  ginating  from  the  Presented for time  the of  T^^ in  temperature some  state  this  thermal  chapter  dependence  Cr(III)  complexes  deoxygenated. s o l u t i o n s the  and  activated  as  well  of  of are  intersystem  the  the  with as  i t s geometrical  experimental  phosphorescence  large  the  distortion,  lODq  values  arguments  crossing.  The  to  results lifein  well  support  other  photo-  66 processes o r i g i n a t i n g here.  from t h e  S t u d i e s extended  4  'T„ state w i l l 2g  2  E  state  g  are also  to the z e r o - v i b r a t i o n a l  be d i s c u s s e d  discussed  level  of the  i n C h a p t e r s V I I I and IX.  Results The [Cr(en) ] 3  lifetimes + 3  ,  o f t h e complex i o n s  [Cr (NCS)  ] ~ , [Cr (NCS) 3  have b e e n m e a s u r e d as f u n c t i o n s t o -140°C and  ethylene g l y c o l  complex The  i n the deoxygenated  (NH ) ]  _  4  3  1  2  ,  maxima.  The r e s u l t s ,  ciprocal  lifetime  3  from about  0°  water,  The c o n c e n t r a t i o n s o f a l l t h e  a t t h e known  plotted  was  0.01  M.  phosphorescence  as t h e l o g a r i t h m  vs the r e c i p r o c a l  a r e shown i n F i g u r e s  ,  [Cr(acac) ]  of methanol,  i o n s were 0.05 M e x c e p t C r ( a c a c ) ^ w h i c h  e m i s s i o n was m o n i t o r e d  -3  and  of temperature  solution  (2:1:1).  [Cr(CN)g]  of the r e -  absolute temperature,  19, 20 and 21. 51  According time, o r decay k  D  =  1 / T  t o Targos  rate D  and F o r s t e r  the r e c i p r o c a l  life-  c o n s t a n t , c a n be e x p r e s s e d as =  k  0  b ~ a -Eh k + s e RT + s e ^ T 0 a b +  k  a  +  k  b  +  k  E  =  where k  0  rt  bability constant.  includes  n  the r a d i a t i v e  2 E  g  ->  4A_  2g  and t h e t e m p e r a t u r e - i n d e p e n d e n t However, t h e f a c t  become q u i t e  straight  each dominates  that  5  >  2  )  transition pro-  nonradiative  rate  a l l the experimental curves  a t b o t h ends may  not important over t h i s  (  K  temperature  indicate  range  t h a t kg i s  and t h a t k^ and k^  a t one end o f t h e t e m p e r a t u r e  range  (in this  6.0  2.0  I  "  4.0  1 6.0  5.0 1,000/T  Figure  21.  The  lifetimes  of  the  2  E_ y  CK  - 1  )  states  _3 and  [Cr(CN),]  as  functions  of  » 7.  temperature.  of  [Cr(NCS),] 6  - r>  70 work). into  The  two  decay  rate  c o n s t a n t s were t h e r e f o r e decomposed  temperature-dependent -Eg. k  =  D  The  s e "RT a  terms o n l y :  -E  b  + s ^ " ^  (5.3)  estimated values of s  , E a  marized  P  i n T a b l e I I where some o f T a r g o s  Zander's curves  , s, , and a  data are a l s o  i n F i g u r e s 17,  Equation  5.3  included 18,  and  w i t h parameters  19 from  factors  and  Table  The  and  smooth  according to  II.  II  activation  e n e r g i e s of the 2 p r o c e s s e s of the E^ s t a t e  temperature-dependent  s (sec  Complex  are p l o t t e d  sum-  Forster's  f o r comparison.  Table Frequency  and  E. a r e p  sj-, Ei-, (Kcal/mol) (sec ) (Kcal/mol E  a  )  a  x  7.2xl0  2  0 .26  8.7X10  1 1  8 .2  4.6xl0  2  0 .19  9 .2  trans-[Cr(NH ) (NCS) ]  4.3xl0  3  0 .076  [Cr (en)  1.6xl0  4  2.5xl0  3  0 .08  1.8xl0  3  0 .06  6.2xl0  3  0 . 18  2.1xl0 13 7.2xlO 13 1.5xlO 12 9 xl0 8.5X10 14 6.9x10 1? 6 xlO  [Cr(CN) ]  3  c  6  [Cr(NCS)g]  : -3 3  ]  3  [Cr(en) ] 3  +  2  3  + 3  a  [Cr(en) ] (C10 ) 3  Cr ( a c a c )  4  4  3  c  3  Cr ( a c a c ) : A l ( a c a c ) 3 3  a.  b  6  xlO  0 .4  X J  9 .0  X J  10 .2  x z  1 1  z  9 .9 9 .2 7 .7 8 .0  From Z a n d e r ' s d a t a , i n w a t e r and g l y c e r i n ( 1 : 1 ) w i t h o u t deoxygenation. [Cr (en) ^ l " " was known n o t t o be q u e n c h e d by oxygen.5 8 From T a r g o s and F o r s t e r ' s d a t a . ^ C a l c u l a t e d from Z a n d e r ' s d a t a i n s o l i d s t a t e . The phosphorescence decay i s e x p o n e n t i a l i n s o l i d s t a t e f o r t h i s complex. E s t i m a t e d from t h r e e p o i n t s . 1  b. c.  0 .10  1 4  71  Discussion An  o v e r a l l examination of the curves reveals  onset temperature, drastically,  a t which the l i f e t i m e  i s a strong  function  begins  to  of the solute.  that the shorten  This  i s not  22 compatible with temperature tion  the proposed explanation  corresponds  that  t o the temperature a t which the s o l u -  starts to fluidify  or s o l i d i f y .  C e r t a i n l y , the environ-  ment does a f f e c t t h e r e l a x a t i o n p r o c e s s e s , ently able  that  the i n f l u e n c e  i s rather  over the temperature  the l i f e t i m e  but i t i s appar-  continuous  and d i f f e r e n t i -  range.  On t h e w h o l e , i t i s common r a t h e r that  the onset  depends v e r y  than  exceptional  much on t e m p e r a t u r e  i n solu59  tion. be  I n some o r g a n i c  compounds, t h i s  h a s b e e n shown  due t o t h e d i f f u s i o n c o n t r o l l e d q u e n c h i n g by  since  the a c t i v a t i o n energy of t h i s  different  solutes  was p r a c t i c a l l y  energy o f the s o l v e n t plex  ions,  there  fluidity.  depleting  to  impurities  process f o r  t h e same as t h e a c t i v a t i o n I n t h e cas e o f C r ( I I I ) 0  com-  i s much e v i d e n c e t o r u l e o u t t h e i n t e r m o l e c u l a r 58  mechanisms.  Q u e n c h i n g by o x y g e n  complex  v i a energy  well  ions  studied.  transfer  and by t h e o t h e r (see C h a p t e r  III)  E v e n i f we assume t h e p r e s e n c e o f a  amount o f o x y g e n o r some o t h e r  possible  quenching  effect i s s t i l l  f a r too small  accounted  f o r , not t o mention that  v/ell deoxygenated  and t h e C r ( I I I )  fied.  i n the organic  Impurities  complex  Cr(III) has been noticeable  ions, the  t o be a d e q u a t e l y  t h e s o l u t i o n s have complexes  solvents  been  carefully puri-  are not  impossible.  However, thanks ties  would  to quench this E  a  be  to the  it.  E, s o  different  facts  controlled  Furthermore,  from  clearly  rule  quenching. that  althoucrh  There s^,  51  are w e l l  activation  E ,  E^  in different  i n the methanol:water:ethylene  This  i s also  line  h o s t Aliacac)^ The  true  diffusion to  (1:1)  f o r Cr (acac)^  support intra-  comes f r o m  change w i t h environment,  remain  A l l  are a l lmainly  [Cr(en)^1  the w a t e r : g l y c e r i n  o f any  some e v i d e n c e  and  solvent  used.  i s also  E, r e s p e c t i v e l y p  at  and a l l  of the  solvent  and  mixture,  important  energy  example, E  a + 3  conditions  spread  the p o s s i b i l i t y  a  impuri-  the a c t i v a t i o n energies,  Conclusive evidence  s„ a n d s, a D invariant  are p r a c t i c a l l y  organic  i s not  f o r the mixed  out  s^,  molecular parameters. that  the  5 Kcal/mol  the s u p p o s i t i o n  fact  state,  t o meet the n e c e s s a r y  i n t h e same s o l v e n t  1  fluidity--about these  E  S e l f - q u e n c h i n g , i f any, 7  are quite  2  lying  very u n l i k e l y  c o n c e n t r a t i o n . ~*  's a n d  E^  low  E  the and  a  environments.  the  same f o r  glycol  m i x t u r e , and  i n solution  For  and  (2:1:1)  in solid in  state.  crystal-  (see T a b l e I I ) .  possible  intramolecular  pathways  for depopulation  2 of the e x c i t e d phorescence E^ state  2  E  is  g  -*  state, &2o f  as  s h o w n i n F i g u r e 3, a r e : (1) p h o s i n t e r s y s t e m c r o s s i n g t o the ground  , (3) p h o t o c h e m i c a l r e a c t i o n s , a n d (4) t h e r m a l 2g' . 2 4 intersystem crossing E >- T_ . T h e r a d i a t i v e g 2g 4  g activated E  > A  E^ ->-  "A,, 2g  included  0  transition, i n k^  assumed t o be  i n E q u a t i o n 5.2.  temperature-independent, ^  Phosphorescence  quantum  73 yields  f o r t h e complex a r e u s u a l l y v e r y s m a l l and t h u s n o t  an i m p o r t a n t c o m p e t i t i v e p r o c e s s i n t h i s Since E tation  a  2  from  E  g  temperature  i s t o o s m a l l t o be due t o t h e t h e r m a l to  2  T_ , k h a s b e e n a s s i g n e d t o a d i r e c t 2g a 3  55  transition  p r o b a b i l i t y k,- may be t e m p e r a t u r e  g  to  4  from  that  E  A~ .  Although the r a d i a t i v e  2g  i ti s vibronically  k,_ c a n n o t be j u s t i f i e d quantum y i e l d  exci-  4  transition  the f a c t  range.  induced, to i d e n t i f y  on t h e g r o u n d s  i s generally  dependent  that  f a r too small.  due t o k  with  g  the phosphorescence Therefore, k  is  2E t o 4A„ and a s s i g n e d t o t h e i n t e r s y s t e m c r o s s i n g from g 2g ~-  ~  a  t h u s i d e n t i f i e d w i t h k,. o  Modern t h e o r i e s and F r o s c h ,  6 ( )  of radiationless  Siebrand,^  and L i n ^  1  may be b u t a s m a l l t e m p e r a t u r e deactivations.  t r a n s i t i o n by  a l l indicate  2  an e x p l i c i t  effect  on t r i p l e t - s t a t e  that there  dependence f o r r a d i a t i o n l e s s  L i n and B e r s o h n have d e r i v e d  theory  Robinson  equation r e l a t i n g  from  Lin's  t o the temperature  lifetimes.^  A t s u f f i c i e n t l y low  temperature 1/T  -  1/T + q  where T i s t h e l i f e t i m e o  a/T  Q  e~  a t T=0°K.  e  /  (5.4)  T  I n most a r o m a t i c compounds _  studied, Lin  the v a l u e of 8 ranges  suggested that  vibrations temperature II,  s  a  from a b o u t  300 t o 800 cm  some o f t h e l o w e r - f r e q u e n c y  i s generally  less  For Cr(III) t h a n 300 cm . x  complexes,  into  '.  f o r the from. T a b l e  T a k i n g t h e lower  f r e q u e n c i e s of the m e t a l - l i g a n d v i b r a t i o n s  g3  intramolecular  ( o u t - o f - p l a n e bends) may be r e s p o n s i b l e 63 dependence.  T  account,  these values For the  are reasonably  the  second  satisfactory.  t e m p e r a t u r e - d e p e n d e n t component  last  one  t o be  crossing  kg,  (2) p h o t o c h e m i c a l  activated t h a t k^  identified,  4  comes m a i n l y  In s t u d i e s of the energy  [Cr(NH ) (NCS) ]" 3  2  4  (see C h a p t e r  and  [Cr(CN)g]  - 3  ,  k^,  from  q u e n c h e d by  the  same s y s t e m a t -65°C i n d i c a t e d  the  latter.  well  demonstrated  (see C h a p t e r  e x t e n t than  was  2  i s a process the  first  of the  Further photochemical  but  out  confirming  photochemical  i t was  of t r a n s - [ C r ( N H ^ ) ( N C S ) ^ ] ~  to a less  thermally  t r a n s f e r between t r a n s -  photoaquation  rules  (3)  I I I ) t h a t the phosphorescence s t a t e  was  t h a t k^  (1) i n t e r s y s t e m  or  Evidence  4  k_  a g a i n be  reaction  i n t e r s y s t e m c r o s s i n g k_ .  is actually  studies.  i t can  phosphorescence.  former  s t u d i e s of  IV)  that  quenched  too,  This requires  leading to photochemical p o s s i b l e case,  k^,  reaction  i . e . , k, = ' b  .  and  Wegner  6  28 and  Adamson  [Cr (NH ) 3  direct found tum  s t u d i e d the photoaquation  (NCS) ] " and  2  4  excitation  [Cr(NR" )g]  into  the  within experimental  yield  was  + 3  3  essentially  lowest error the  of t r a n s -  a t room t e m p e r a t u r e doublet excited  by  state  t h a t the photoaquation  same—about  complexes—whether  t h e e x c i t a t i o n was  quartet  states.  or doublet  yields  0.3  for  i n t o the  and quan-  both  excited  S i n c e the d e p l e t i o n p r o c e s s  of  2 the  E  caused  state  a t room t e m p e r a t u r e  by p r o c e s s  k^,  a q u a t i o n quantum y i e l d  i f k^  = kg  s h o u l d be  is practicallv o r k^ 0 or  = k^, 1,  the  entirely photo-  respectively,  upon  75 direct not  e x c i t a t i o n i n t o the  the observation.  possible out.  cases,  2 E  state.  g  Therefore,  i.e.,  = k  I f the possible  cases  both the f i r s t  listed  true is  that  last  possible  a l l the experimental r e s u l t s .  the thermally  not a photoreactive  k, = k „ i m p l i e s b -4  activated process  2 the E  that  and s e c o n d  are exhaustive,  by a s s u m i n g k^ = k_^, i t becomes p o s s i b l e  consistently  t h i s was  and k^ = k^ have t o be r u l e d  g  must be i d e n t i f i e d v / i t h k_^ — t h e fact,  Apparently,  state  g  case.  In  to explain  It i s certainly  intersystem  in itself,  t h e n k^  crossing  k_^  and t o a c c e p t  i s substantially y  inert,  o  However, p r o c e s s k up  t o the  as w e l l  4 T„ 2g  as t h e o t h e r p r i m a r y p r o c e s s e s  cyclings of is  4 T_ 2g  clear that  And  •  2 E  4 »• T„ ) o c c u r s . 2g  g  k_^ i s a p r o c e s s  the  thermally  2 E  g  state  leading  possible r e Therefore, i t '  to photoaquation.  2 E  c a u s e d by  a c t u a l l y r e s u l t s from a quenching o f  activated  intersystem  same mechanism, i t f o l l o w s the  (with  t h e d e c r e a s e i n p h o t o a q u a t i o n quantum y i e l d  quenching the  . a c t i v a t e s complex i o n s i n t h e E state -4 g s t a t e and f r o m t h i s s t a t e photosubstitution  that  crossing  k ^.  t h e complex i o n s  From t h e formed i n  s t a t e v i a d i r e c t e x c i t a t i o n a t room t e m p e r a t u r e  g entirely  be a c t i v a t e d  relaxations  proceed--a  t o the  4 T_ 2g  state  f r o m where  will  further  s i t u a t i o n , as f a r as p h o t o a q u a t i o n i s  c o n c e r n e d , n o t d i f f e r e n t from d i r e c t e x c i t a t i o n i n t o t h e quartet  bands.  Therefore,  the f a c t that  the photoaquation  quantum y i e l d , was t h e same w h e t h e r e x c i t a t i o n was i n t o t h e 2 E  a  state  or the excited  quartet  states  i s self-evident.  76  The above arguments and i n t e r p r e t a t i o n s may cized  be  on t h e g r o u n d s t h a t due a t t e n t i o n has n o t b e e n p a i d t o  t h e p o s s i b l e a l t e r n a t i v e mechanisms o t h e r t h a n Phenomenologically,  k^ may  to the photochemical transition. k^ s, 1  criti-  From  consist  reaction  =  o f two r o u t e s :  the s t r a i g h t n e s s of the Arrhenius  A non-radiative t r a n s i t i o n with i s hard  to understand  Hammond has i l l u s t r a t e d  i n the case  from  plots  of  undergo the  same r a t e - d e t e r m i n i n g s t e p o r have q u i t e c l o s e  v a t i o n energy  leads  and t h e o t h e r t o a n o n - r a d i a t i v e  i t r e q u i r e s t h a t t h e two r o u t e s e i t h e r  energies.  one  activation  such  a large  theories.  acti-  However,  of the photoisomerization  64 of o l e f i n i c the lent  compounds  internal case  " that the photochemical  conversion are v i r t u a l l y  process  t h e same.  An  and  equiva-  i n the p h o t o s u b s t i t u t i o n o f C r ( I I I ) complexes can  41 be,  as s u g g e s t e d  i n r i g i d m e d i a by C h a t t e r j e e and F o r s t e r ,  t h a t k^ i s a p r o c e s s recombination  involving  bond b r e a k i n g  and  immediate  c a n be o p e r a t i v e b e c a u s e o f a c a g e  effect. 27 A c t u a l l y , C r ( I I I ) p h o t o c h e m i s t r y has been thought t o be e s s e n t i a l l y t h a t o f cage r e a c t i o n s . _3 T a k i n g [CrAg] as an example, [CrA H 0] 5  [Cr.7\  ] ~  6  3  (  2  E  )  g  -  J  •*•  ( C r A r • • -A) "  3  J  ^^^^  10 • Q. • S •  ^  J  interpretation,  *  ^[CrA ]~ ( A 6  In t h i s  (5.5)  2  2  t h e bond b r e a k i n g  ky---is t h e r a t e - d e t e r m i n i n g s t e p .  4  2 g  )(5.6)  process—virtually  I f i t i s f o l l o w e d by  77  diffusion reaction  away o f t h e  leaving  ition it  a large can  studied  energy  state  then the photoaquation  occurs.  a  Appar-  t h i s mechanism. quantum y i e l d  d e p e n d e n t on t h e c a g e s t r u c t u r e . of  then  f o r the n o n - r a d i a t i v e t r a n s -  r a t i o n a l i z e d with  the s o l v o l y s i s  chemical  recombination,  t o the ground  activation  t h u s be  i s true,  strongly  then a net  o c c u r s ; i f i t i s f o l l o w e d by  net i n t e r s y s t e m c r o s s i n g ently,  ligand,  [Cr(NCS)g]  But i f  should  Langford  be  has  in acetonitrile-water  ?9 mixtures tion,  found  that,  the photoaquation  solvent be  and  composition.  important  i n contrast  quantum y i e l d  i s independent  This indicates  + +  i n the photoaquation  t h e r e f o r e makes t h i s mechanism The  to the thermal  aquaof  cage e f f e c t may  of C r ( I I I )  complexes  the not and  improbable.  other i n t e r p r e t a t i o n  i s t h a t k,  is essentiallv  p  kg, b u t a f t e r i s o e n e r g e t i c i n t e r s y s t e m c r o s s i n g s t a t e , v i b r a t i o n a l r e l a x a t i o n and a q u a t i o n f r o m  t o the ground the h i g h l y  4 excited vibrational titive.  Taking  levels  [CrAg]  -3  of the  as an  A_ state 2g  c a n be  example, .[CrA,H  [CrA ] fi 6  -32 ( E g  .  ) r  ^6 -34 v[CrA,] ( A_ d s - - ' J  6  r  a  5  cotnpe-  2  g  ' ;V=n) \_ ^  [CrA  (thermal  -3 c  O]"  2  ]  equilibrated)  I t has t o be n o t e d t h a t l a r g e r p a r t o f t h e p h o t o a q u a t i o n may come f r o m t h e l o w e s t e x c i t e d q u a r t e t s t a t e s and t h e r e s u l t s may l a r g e l y r e f l e c t t h e p r o p e r t i e s o f t h o s e s t a t e s , However,, i f caqe e f f e c t i s n o t i m p o r t a n t f o r t h e l o w e s t e x c i t e d q u a r t e t s t a t e s , t h e r e a r e no r e a s o n s why i t s h o u l d be i m p o r t a n t f o r t h e l o n g e r - l i v e d l o w e s t e x c i t e d d o u b l e t states.  I n t h i s mechanism, t h e radiative for  transition  kg  the photochemical  t h a t of the thermal thought  t o be  large activation i s hard  energy  to e x p l a i n .  r o u t e , t h e mechanism  based  on  the s h o r t ,  excited  ferent  characteristics  r e a c t i o n p a t t e r n s and  27 photochemical It  i s not  reactions.  But  the  ition  large  states  i s again hard  lifetime and  on  energy  energies nearly far  independent  consist  of  t o the ground  that  complexes  state.  the  rule  seems activation  studied  seems  the i d e n t i f i c a t i o n  arguments, i t i s c l e a r  of the s t r o n g l y  term k^ w i t h t h e t h e r m a l l y a c t i v a t e d k  independent  possible.  From t h e above e v i d e n c e and that  thermal  p r o c e s s e s a r e t h e same o r  t h e same f o r a l l t h e C r ( I I I )  t o o c a s u a l t o be  dif-  Unfortunately, to  + +  However, t h e f a c t  f o r t h o s e two  the  f o r the n o n - r a d i a t i v e t r a n s -  to r a t i o n a l i z e .  difficult.  the  between  out the i n t e r p r e t a t i o n w i t h e x p e r i m e n t a l evidence extremely  of  '  intersystem crossing  activation  l o n g been  29  i m p o s s i b l e t h a t k^ may  c h e m i c a l p r o c e s s and  Moreover,  + +  a c c e l e r a t e d m o d e l w h i c h has  improbable  non-  is essentially  non-equilibrated vibrational  and  f o r the  temperature-dependent  intersystem crossing .  2 4 . from t h e F to T s t a t e i s overwhelmincr.lv f a v o r e d . -4 g 2g - A d d i t i o n a l s u p p o r t comes from t h e l o c a t i o n s o f t h e n  4  zero-vibrational  level  of the  T_ s t a t e , which can 2g  be  ++ Intersystem c r o s s i n g s through the i n t e r s e c t i o n p o i n t s of t h e p o t e n t i a l e n e r g y s u r f a c e s ( T e l l e r c r o s s i n g ) may account f o r a l a r g e a c t i v a t i o n .energy. However, t o t h e a u t h o r ' s k n o w l e d g e , no p r e c e d e n t examples w i t h s u c h a l a r g e a c t i v a t i o n e n e r g y have been r e p o r t e d .  estimated  bv  adding  E  ' s to the  2  known  D  all  the C r ( I I I ) complexes  tected fore  n e a r and  the  E (v=0) g  states.  s t u d i e d , f l u o r e s c e n c e has  For  been  o n l y n e a r t h e p h o s p h o r e s c e n c e maxima.  f l u o r e s c e n c e maximum s h o u l d  t h e p h o s p h o r e s c e n c e maximum  l i e fairly  (see C h a p t e r V I ) .  de-  There-  closely It is  to  found  4  that  the  estimated  T„  (v=0)  levels  a l l l i e about i n the  central  2g  r e g i o n between t h e  absorption  and  f l u o r e s c e n c e maxima.  Secondly,  4  studies  of the  intersvstem  excited  doublet  states  c r o s s i n g from the  (see C h a p t e r V I I ) 4  activated process  assigned  to  T_  s t a t e to  revealed a  the  thermally  2  *" 2 g "  ^2q  T  t  *  i e  c  o  r  r  e  s  P  o  n  ~  4  ding a c t i v a t i o n  energies  are  added t o t h e  T^{v=0)  levels 2  respectively,  the  energy  agree v e r y w e l l w i t h Chapter V I I I ) .  levels  those  so o b t a i n e d  predicted theoretically  These a l l s u p p o r t  T^^  f o r the  indirectly  the  state  (see  thermally  2  a c t i v a t e back 4  intersvstem  c r o s s i n g from t h e  E  state to  the  g T„  2g  state.  A Further  Comment  Since d e t e r m i n e d by no  the phosphorescence decay r a t e c o n s t a n t the  c o n v e n t i o n a l method w h i c h t a c i t l y  a p p r e c i a b l e back i n t e r s y s t e m  to i n v e s t i g a t e the validity  of the  limitations  escence  an  ideal  intensity  the Appendix)  as  of  the d a t a  as  was  assumes analysis  treatments  and  arguments i s i n d i s p e n s i b l e when t h e b a c k  s y s t e m c r o s s i n g i s known t o be After  crossing, a careful  k^  the inter-  greatly involved.  instantaneous  excitation,  a f u n c t i o n of time can  be  the  phosphor-  expressed  (see  80 -k I  =  A( e  t  -k t )  - e  a  (5.7)  15  i  where  k  k  _ a  6  1 *• ( k  *  =  (  E  k  E  +  + k )  - * [ ( k  T  V  *  +  [  (  k  T  "  - k  T  E  k  )  2  )  E  +  4  z  k  +  4k k_ ] = 4  4 -4 k  ]  4  1  and  In  k  E  =  k  5  + k  g  k  T  =  k  x  + k  2  the Cr(III)  greater faster  than than  constant well  k  k  +  +  ?  +  3  complexes  k_ k  the f i r s t  4  studied  term.  determined  t h e maximum,  Because from  k^ = k D a as  Assuming certainly k a  <J>j_  Chaoter  VI),  meters  generally  R  decreases  the apparent  much  decay  nart  rate  of the curve  t (5.8)  part  o f the decav  curve  should  be  observed. k  m  i s sufficientlv over  dependent  as  a  c  over  k„ which i s range,  then  (see t h e Appendix) (l-<f>. )k . isc -4 T  (5.9)  0 a n d 1, a n d i t i s n o t v e r y  the studied  temperature  (1 - d>. ) i s essentiallv isc varying k_ »  and Eb e s t i m a t e d  h a s t o be c o r r e c t e d  temperature  k, + k + k_ + 5 6 7  =  the factor  s , s]-), E ,  greater than  the studied  to the greatly a  term  k  a  can only be between  sc  comparing  Ae  , and t h i s  the case  temperature  sb  =  can be approximated - • k^ = k D a  Since  work,  the decaying  -k  exponential  i n this  i n practice  I  Therefore  4  , t h e r e f o r e the second  k ^ was  past  + k  bv a f a c t o r  4  Therefore  are justified of  (1 -6. ). isc  range  (see  constant the paraexcept  that  CHAPTER  QUANTUM  YIELD  OF  INTERSYSTEM OF  Except kinetic  of  few  quantum  yield  doublet  of  the  most  Cr(III)  sing  from  of  observations  complexes,  the  appreciable  is  to  extent.  is generally that  ^- ^-  that  fluoresce,  the  s  a  e  a  r  e  crossing  or  the  quantum  yield  of  phosphorescence  i t i s evident  that  emission  intersystem  in  cros-  E  g  state  Forster  et  experimentally"^ '  substantially less  internal  takes  place  i  However, and  certainly  conversion  a l . have 4 x  '  than  from  to  an  r  the  demonstrated  that unity 4 T„ 2g  i t s quantum  and to  therefore 4 A state 2g 0  important. The  pressed  quantum  yield  of  intersystem  =  ,  is interesting  fied  can  be  ex-  occurs,  throuqh  to  note  Equation  Equation  4.19  (6.1)  P  5  crossing  crossing  as cj).  It  FUNCTION  2 T„ 2g  theoretically  concluded  ^2q  complexes  intersystem  4  yield  A  formation.  From  both  4  AS  TEMPERATURE  Cr(III)  the  CROSSING  generally inacces2 The o n l y p r o b e o b t a i n a b l e f r o m 'E parameters that can g ^ 4 t o p r o v i d e some i n f o r m a t i o n a b o u t t h e T„ state is 2g  used  the  a  parameters  sible. be  for  VI  that 6.1 and  p e v e n when b a c k s t i l l the  holds.  intersystem  This  definition  can  be  veri-  82 k  ^isc  ~  k  4  + k  1  The p h o s p h o r e s c e n c e as  functions  Further,  o f some C r ( I T I )  functions  determine  intersystem  that  2 )  complexes  will  the temperature  f o r many  quantum  V.  Cr(III)  k,- i s t e m p e r a t u r e  of phosphorescence  of temperature  *  have been d i s c u s s e d i n Chapter  I f we a s s u m e  4  then measurements  invari-  yields  as  e n a b l e u s , t h r o u g h E q u a t i o n 6.1, effect  on t h e quantum  yield  of  crossing.  Experimental  and R e s u l t s  Except complex  ( 6 4  k,- h a s b e e n e s t i m a t e d t h e o r e t i c a l l y 1  to  + k  3  lifetimes  of temperature  complexes. **' "'" ant,  + k  2  the Cr(acac)^  solutions  used  solution  were  which  w a s 0 . 0 1 M,  0.05 M a n d a b s o r b e d  the  a l l the  excitation -3  light  totally.  The i r r a d i a t i o n  [Cr(NH ) (NCS) ]~, 3  for  2  [ C r ( C N ) ] ~ , and C r ( a c a c > ; 3  4  [Cr(en).] 3  +3  6  •and [Cr(CN)-] 6  photochemical reaction, possible. the  3  -3  e m i s s i o n maximum was u s e d  was s c a n n e d tribution  quantum y i e l d .  intermittently  i n this  work.  the  phosphorescence  low  temperature  f o r [Cr(NCS)g] a n d a t 436  ,  nm  In order to minimize the t i m e was k e p t  complex,  as s h o r t as  only the intensity  at  as a measure o f t h e r e l a t i v e However,  t o ensure  that  was r e a s o n a b l y c o n s t a n t o v e r  No m e a s u r e m e n t s attempted  .  irradiating  T h e r e f o r e , f o r each  phosphorescence  w a s a t 5 4 6 nm  the whole  spectrum  the spectral  the temperature  o f t h e a b s o l u t e quantum  yield  A c c o r d i n g t o Chatter-|ee and  quantum y i e l d  reaches  and t h e v a l u e remains  dis-  a limiting  t h e same i n  range.  were Forster  41  value at  different  83 solvents. this  The  absolute phosphorescence  c h a p t e r were e s t i m a t e d by  limiting  intensities  Forster  fixing  quantum  f o r k_ a n d 5  at the values obtained  limitina  used i n  the low-temperature  f o r the absolute phosphorescence  values  yields  by C h a t t e r j e e  quantum  and  yields.  l o w - t e m p e r a t u r e <f> a r e l i s t e d P  The i n  Table I I I . Table I I I Intrinsic  Phosphorescence  Limiting  Rate  C o n s t a n t s and Low-Temperature  Phosphorescence  Complex  k  (sec  - 1  5 x  Quantum  3  0.20  [Cr(NCS) ]~  0.23  2  4  6  [Cr ( C N ) ] ~  3  .  , lim <P P  10 )  [Cr(NH ) (NCS) ]3  Yields  0.011 0.23  + +  0 .016  0.0042  3  0.15  0.0090  [Cr(acac) ]  0.13  0.021  3  6  [ C r (en)  3  ]  +  3  Value taken from phosphorescence nitrogen temperature.  The p h o s p h o r e s c e n c e of  yields  temperature f o r these f i v e  Figures were of  quantum  22, 23, 24.  then evaluated  decay  were p l o t t e d  Cr(III)  The quantum  25,26.  complexes,  yields  v i a E q u a t i o n 6.1  temperature i n Figure  constant at  as as  liquid  functions shown.in  of intersystem  and a r e shown  as  crossing functions  0  86  0.7  89  Discussion On (JK  s c  ,  t h e w h o l e , t h e quantum y i e l d  does not  complexes 0.1  for  studied.  i n the curves  k  the  temperature Since  of  but,  5  c  mated  and  3  probably  steady  be  are not very  + 3  .  i n both  o f cb , t h e p  [A]=0,  i t can  =  <J>.  small to  oscil-  experimen-  curves  at  the  real.  the e v a l u a t i o n s  However, 6. isc  esti-  for  photochemical  to other C r ( I I I )  be  from  absolute values  from  extended  Cr(III) range  sc  The  c o n s i d e r e d t o be  been e s t i m a t e d  same method c a n be  f o r 4>^  attributed  reliable.  r  has  f o r the  d e c r e a s i n g of the  the d e t e r m i n a t i o n s  A s s u m i n g k^ = 0 and 4.20  can  [Cr(en) ]  there are u n c e r t a i n t i e s  f o r d>. isc  the  calculated  for  ends must be  [Cr(NH^)2(NCS)^] and  temperature  values  t o 0.7  3  6  errors,  high  The  [Cr(CN) ]~  lations tal  change much w i t h  of i n t e r s y s t e m c r o s s i n g  shown t h r o u g h  studie  complexes  Equation  that  6 °° 1 -  *chem  1  k  where  cb. isc  <\>.  i s not  studies  ( 1  (1  -  readilv  of the  lifetimes. ~  k  ^isc isc  )k  . + -4  C  1  8  C  -4 k  available,  c  5  +  but  (6.4)  k 6 r  i t c a n be  temperature-dependence of  From r e s u l t s  cJ). Y  =  (6.3) 8  i n Chapter  -4  ) k  . +  -4  k  5  c  +  k,  6  „  from  phosphorescence  V, k  =  estimated  b  k_ D  (6.5)  90 —  For  oo  a t -65°C,  [Cr(NH ) (NCS) ] 3  2  4  = 0.51 from Chanter Equations  2  c h e m  The quantum y i e l d estimated  throuqh  In t h e e n e r g y [Cr(CN)g]  -3  t h e s t u d i e s on t h e s e n s i t i z e d  state  = ' *? 1  S  (see C h a p t e r  (  k  C  k  Q  ^  H  n  D  n H  phosphorescence.  II). ^  k  and  i s u n i t y and  E  g  T h e mechanism p r e d i c t s : )  (3.12)  A  A  (—* )  isc  2  QH  kp T  )(  ]  + k [A]  1  =  2  transfer efficiency  0  P  c r o s s i n g c a n a l s o be  3  S P  <f>  crossing i n  .  of intersystem  , t h e energy  A  0.0052/0.0102  t r a n s f e r system o f [ C r ( N H ) ( N C S ) ^ ] ~  the accepting *  =  t o give a value o f  of intersystem  a n d 0.9 4 f o r ^  4  V  /4>  6.3, 6.4, and 6.5 c a n be s o l v e d  [Cr (NH ) (NCS) ] " " ,  is  c h e i T  I V and k^/k^ = 0.89 f r o m C h a p t e r V.  0.52 f o r t h e quantum y i e l d 3  <!>  (6.6)  ,A k  From t h e above two e q u a t i o n s  *  i  s  =  c  ( *  )  p  7A"  )<{,  k + k °  *sp At  -65°C  5.9  4  = 0.52.  obtains  4>  D  + k  O  C  A  =0.35  values  greater  A  III),  p  =  and  into Equation  6.7, one  for [Cr(CN),]~ . 3  6  of intersystem  [Cr (NH-.) (NCS) . ] ~ and [ C r ( C N ) , ] " " 3 Z 4 6 times  ( F i g u r e 2 3 ) , cf>  r A ] ) = 0.63 ( C h a D t e r  S u b s t i t u t i n g these  isc  4  yn  The quantum y i e l d s  three  S  A  x 1 0 ~ , k^„[A]/(k sc  [A] ^  1  and [A] = 0.07 M, 4> = 6.3 x 1 0 ~  Qri  <})?  (6.7)  D  D  than  those  3  crossing for  obtained  estimated  i n t h i s wav a r e  f r o m E q u a t i o n 6.1  (0.52 are  and  0.35  vs  0.17  comparatively  from the  a  0.11).  certain,  systematic  absolute  and  error  this  either  determinations  Since  constant i n the  of  lifetime  <f> .  factor  measurements  could  evaluations  However,  of  arise kg  applying  or  in  this  + 3 factor  to  [Crfen)^]  system  crossing  would  far greater  acceptable.  However,  notoriously  difficult  values  become  may  be  All  the  are  cally  and  below  the  must  and  quantum that,  emission  yields  of  of  quantum  i t is  yield  of  course,  interis  yields  likely  -70°C.  intersystem  This  not  are  that  or  temperature  range.  at  higher  of  be  kg  only  these  with  from  obtained  internal 4  the  ^2g  state  slightly  temperature  However,  thev  temperatures.  considered:  crossing  indicates that  crossing yields  independent  first  variation  unity  to measure  intersystem  in this  decrease  factors  absolute  quantum  temperature  pendent  than  in a  in error.  constant  conversion  result  For  this,  photochemical  temperature.  do  de-  monotoni-  two  other  reaction  Photochemistry,  and  as 4  established state, tum  and  previous  its yield  yield  cause  i n the  rarely  the  yield  appreciably.  Change  in  and  ruby  (213 2  transition that  kg  does  increases  exceeds  quantum  244  chapters,  of  0.1  with  below  -30°C.  intersystem  kg  with  -  1  at  and  the but  i t s quan-  It therefore  crossing  temperature 77°  from  temperature,  of  sec  occurs  has  300°K).  6 5  to  cannot  decrease  been  noticed  Because  the  4 E  «-  g have  T  i s v i b r o n i c a l l y induced, i t is possible 2g a temperature dependence. In f a c t , the 0  J  spectral  distribution  ~130°C.  However,  change the  [Cr(CN)g]  the spectra  appreciably  decrease  of  from  o f <f>. isc  _3  e m i s s i o n changes  below  of the emissions studied  -120° t o -30°C.  i s s t i l l  do n o t  I t i s believed  t o o larcre  that  t o be a c c o u n t e d f o r  T  by  any v a r i a t i o n From  o f k,..  E q u a t i o n 6.2, b y n e g l e c t i n g  k^,  and p h o t o c h e m i c a l r e a c t i o n ,  can  be o b t a i n e d . 1 Y  Both may  internal consist  dependent)  isc  conversion,  component  expression  2  k^,  (6.8)  and i n t e r s y s t e m  crossing,  (or only  and a temperature-dependent  the temperature-independent  a r e dominant,  process,  4 1 + —T-=  =  x  the following  of a temperature-independent  Below'-70°C, each  k  k^,  fluorescence  therefore  d>. isc  does  k^,  slightly component.  component  parts  n o t c h a n g e much  of  with  Y  temperature. come  Above  i m p o r t a n t and  unlikely  with  hiah.  rising  conversion  interpretation  gies  starts  to decrease with  decreases  against  tf>.  the temperature-dependent to decrease.  1/T w i l l  o f k^  temperature,  the fact  implies  that  i s temperature-dependent.  yield  be-  4  temperature  i s right,  parts  S i n c e k„ i s  i s c  r  internal  -70°C,  then  that at  least  o f log(l/c}K  i n the a c t i v a t i o n  a n d k^, p r o v i d e t h e t e m p e r a t u r e  s c  I f t h e above  an A r r h e n i u s p l o t  the difference  $^  range  -1)  ener-  i s sufficiently  93  The  and  Arrhenius -3 [Cr(CN) l with 6' lines  activation  temperature  end  are  has  b e e n done f o r  the  2  c o r r e c t e d d>. from F i g u r e isc  slightly  curved  as  about  5 Kcal/mol  25.  The  shown i n F i g u r e  energy d i f f e r e n c e s estimated are  [Cr(NH^) (NCS)^]  ;  r  resulting The  plot  for  from  the  27.  high-  [Cr(NH^) (NCS)^]~  and  2  -3 2 Kcal/mol for  their  for  [Cr(CN)^]  internal  respectively.  internal  conversions,  that  temperature  s h o u l d be has  not  Therefore,  conversions  Kcal/mol  the  .  They a r e  activation  g r e a t e r than  surprisingly  s i n c e the p r e s e n t  been m e a s u r e d  5 and  2 for  t h e o r i e s a l l p r e d i c t ^ " ' ^ ' ^'" transitions  However, i n t e r n a l  in detail  enercies  large values  dependence of n o n - r a d i a t i v e  s m a l l o r none a t a l l .  really  must be  the  f o r any  conversion  molecule.  Con4  firmation state.  at t h i s  point w i l l  require direct  study  of the  T  2 g  0.5  igure 27.  Plots  of  log(l/<j).  - 1) v s  1/T.  CHAPTER THE  In  the  Reineckate preceding tor  was  decay It  longer  curve  was  rise  a  of  i n the  of  I t was  III).  sort  THE  4  T  measurements  decay  process of  Chapter  OF  of  the of  ions,  donor,  an  the  find  phosphorescence  of  too,  rise  the  t o be of  accepthe  the acceptor  initial  that  between  initial  state  R e i n e c k a t e i o n , was to  transfer  recognized  curiosity,  serendipity  donor  energy  phosphorescent  of  STATE  phosphorescence  immediately  the  Out  the  OR  2g  hexacyanochromate(III)  noticed.  population (see  lifetime  and a  LIFETIME  VII  part  of  the  reinvestigated.  there  was  an  although i n a  initial  shorter  period. It an  has  initial  a,general Figure decay  been  rise  established  of phosphorescence  phenomenon  28A curve  shows, of  from  as  among a  the  after  example,  studies  a pulse  individual  typical  Cr(acac)^  subsequent  phosphorescence.  excitation  Cr(III)  the  that  complexes.  population The  is  only  and  exception  _3 observed and  is  a much  [Cr(CN)g] slower  Similar rise  to  whose  component the  case  of phosphorescence  decay  as of  curve  consists  of  shown  i n Figure  2 8B.  energy  transfer,  the  relates  to the population  a  faster  initial process  2 of the  the  E  state. The 9" population process  importance  of being  i s apparent.  able  However,  to  first  observe of  all,  97 we  have t o p r o v e  tal  factors The  that  o r from  t h e phenomenon i s n o t due  h a l f - h e i g h t width i t s scattered  of the f l a s h  n s e c and  not  d e t e c t a b l e a t the s e n s i t i v i t y conclude that  instrumen-  impurities.  200  could  to  light  lamp was  from the b l a n k used.  less  than  solution  was  From t h e s e f a c t s  t h e lamp d i d n o t i n t e r f e r e w i t h t h e  one  emis-  _ 3 sion.  In the case of  f a s t e r decay  could  [Cr(CN)g]  n o t be  this  c a u s e d by  implied  the t a i l  that  the  initial  o f t h e lamp  A more p r o b a b l e s o u r c e o f d i s t o r t i o n ,  i f any,  the  However, t h e p h o t o m u l t i -  detecting  p l i e r was fact,  and  recording  wired s p e c i a l l y  systems.  for fast  the b l e e d e r r e s i s t o r s  l o s c o p e has is  therefore  used  individually  the r i s e  curves remained RC  In f a c t  nsec r i s e rise  and  t i m e c o n s t a n t was  tortions .  curve.  the d e t e c t i n g nsec decay  In  has  The  of about  100.  and  were  to a l l e v i a t e less  oscil-  used  capacitors  k e p t t o be  a factor  t h e same a v e r a g e  150  decay  resistor  the  than And  that the  f i g u r e upon d e c r e a s i n g t h e the e x t e r n a l  system  can e a s i l y  of the f l a s h  times of the phosphorescence these r u l e  a p p r e c i a b l e change  Different  a c r o s s the load  time c o n s t a n t o r e l i m i n a t i n g  all.  All  fast.  t i m e s t u d i e d by  from  applications.  t i m e o f 15 n s e c a t t h e s e n s i t i v i t y  sufficiently  n o i s e , b u t t h e RC of  no  i n t h e p o p u l a t i o n and  a rise  come  have been changed t o a c h i e v e a •  higher bleeder current, but s t i l l t h u s been c a u s e d  response  might  output.  out the p o s s i b i l i t y  capacitor follow  lamp.  does v a r y w i t h  after  the  Moreover,  50 the  temperature.  of having instrumental d i s -  98  For i m p u r i t y t o cause a r i s e o n l y way  i n the decay  curve, the  c a n be t h o u g h t a b o u t i s t h a t t h e i m p u r i t i e s  absorb  l a r g e p a r t o f t h e e x c i t a t i o n r a d i a t i o n and t r a n s f e r i t s l o w l y to the C r ( I I I )  complex  under  s t u d y . J u d g i n g from the a b s o r -  b a n c e s , i t i s h i g h l y i m p r o b a b l e t o h a v e s o much o f i n the sample.  Of c o u r s e , i m p u r i t i e s w i t h  e m i s s i o n quantum y i e l d  ion.  f a s t d e c a y and  can cause a f a s t e r decay  before the slower phosphorescence  decay  of  impurities high  component  hexacyanochromate(ITI)  H o w e v e r , c a l c u l a t i o n s b a s e d on some r e a s o n a b l e a s s u m p t i o n s  show t h a t t h e amount o f i m p u r i t y n e e d e d  is still  too high  to  rise  the  be p r o b a b l e i n t h e compound u s e d . It initial  i s therefore b e l i e v e d that the i n i t i a l  and  f a s t component a r e i n t r i n s i c phenomena o f t h e C r ( I I I )  complexes. Results I t was  found t h a t a l l the phosphorescence  t i m e c u r v e s c o u l d be f i t t e d  satisfactorily  to the  intensityfollowing  equation I = A[exp(-t/x where x  P  p  ) +  a e x p ( - t / x )] x  i s the l i f e t i m e of the  2  E  g  (7.1)  s t a t e , x i s the ' x  lifetime  2  of a s t a t e p r e c e d i n g the next s e c t i o n . A is  s t a t e as w i l l  be shown i n t h e  B o t h A and a a r e p r o p o r t i o n a l i t y  arbitrary.  monitored.  E  The  The  v a l u e s o f a d e p e n d on t h e w a v e l e n g t h  d a t a from, l i q u i d  ments a r e c o l l e c t e d  constants.  i n Table IV.  n i t r o g e n temperature measureTemperature  dependence o f  99  T  x  has a l s o been s t u d i e d  f o r [Cr(NCS) ]  ,  g  [Cr(NHg) (NCS)^]~, 2  +3 [Cr(en)^] Figures  , and C r ( a c a c ) ^ ;  and t h e r e s u l t s  Rate C o n s t a n t s  (1/T ) ' p  Complex  xl0~ (NCS)  ?  tCr(NCS) ]"  4  ]~  3  g  [Cr(CN) ]~  3  6  Cr(acac)  3  [Cr(tn) ]  + 3  [Cr(en) )  + 3  3  3  Decay  Complexes  (1/T ) F  xlO-  k^ ( c a l c d ) xlO"  4  a  + +  4> xl0  4  f  4  3.0  13  30  -0. 38  6  0.23  12  20  -0.47  2  0.29  2.0  10  + 1.6  3  2. 8  12  10  -0.40  2  9.3  13  —  -0.47  -  8.9  23  -0.38  3  10.0  3  IV  ( s e c "*") i n L u m i n e s c e n c e  of C r ( I I I )  3  in  29, 30, a n d 31. Table  [Cr (NH )  are presented  The v a l u e s o f a depend on t h e w a v e l e n g t h m o n i t o r e d . The v a l u e s r e p o r t e d a r e a t t h e i r p h o s p h o r e s c e n c e maxima.  The noise values  level  accuracy  of the curve.  they  average value  x  x  At l i q u i d  calculated fluctuate within  peratures,  three  i n evaluating  depends v e r y nitrogen  40%, w h i l e  temperature, the at higher  c a n f l u c t u a t e as much as 100%.  of x  x  oscillograms.  a t each  much on t h e  tem-  Usually the  t e m p e r a t u r e was d e t e r m i n e d  from :  102 log  (1/T ) X  4.0  Figure 31.  5.0  T  of  6.0 -3  [Cr(NCS),]  as a f u n c t i o n of temperature.  7.0  103 Discussion Since 4  T^  states  the  c o m p l e x i s m a i n l y pumped i n t o  T a n d  e x c i t a t i o n , the p o s s i b l e known e l e c 2 4 4 2 t r o n i c s t a t e s p r e c e d i n g the E s t a t e are T, , T_ , T„ , g lg' 2g' 2g' 2 4 2 and T, states. The T and T„ s t a t e s c a n be confidently lg lg 2g g  i n the  4  the  pulse 3  n  ruled If  out  as  the  long-lived  state  (ca.  t h e y -were, t h e n e m i s s i o n s w o u l d h a v e o c c u r r e d  much s h o r t e r 2  comparatively  than  the  T,l g  and  2 E g states  fore  the  following  that  the  4  T„ 2g  excitation  has  energy  i s the  mechanism c o n t a i n i n a  If  4  then according sity  of  length  a  expressed  Equation  escence  fast. the  loner-lived precursor to  the  2 4  E  g  to  derivations  the  and  function  of  of  i n the  assumption  However,  n  to  Appendix,  time a f t e r a pulse  any  the a  long-lived  the  phosphorescence at  There-  from which  state.  loner-lived precursor '  wavelengths  2 E  g  state,  the  inten-  one  wave-  e x c i t a t i o n can  be  as I  f  =  A  I  p  .=  A  7.3  because the possible  verv  b a s e d on  i s the  fluorescence as  T  be  be  ysec).  t r a n s i t i o n between  i n s t e a d of T„ as t h e lg 2g discussed i n Chapter V I I I .  be  T„ 2g  will  is transfer 9  The  been assumed to  discussion  state  precursor w i l l  phosphorescence.  at  10  exp(-t/x )  f  [exp(-t/T  does not value  of  explanations: are  (7.2)  f  ) - exp(-t/x )]  agree with a i s not (1)  i n fact detected  (7.3)  f  the  empirical  generally  both at  -1.  There are  fluorescence the  Equation  wavelength  and  7.1, two  phosphor-  monitored,  104  therefore  the t o t a l i n t e n s i t y  I = A  thus  a =  {exp(-t/T  (A /A ) f  F  be  equal  state and  P  are positive  i s populated  = A y.j  (7.4)  f  than  proportionality  constants,  - 1 , as o b s e r v e d .  (2) The  i n p a r t through  iri p a r t through  I  ) - l]exp(-t/x )}  f  (7.5)  to or greater  Therefore,  [(A /A  - 1  p  S i n c e A. and A  ) +  i s actually  the slower  the instantaneous  [exp(-t/T  some v e r y  direct  4  intensity  ) - exp(-t/T  -y  2  2 E  g  transitions  E g  transition.  c a n be e x p r e s s e d  )] + A''exp(-t/T J_  t-J  T~ 2g  fast  a must  as  )  (7.6)  t*j  yj  A  =  (A' + A'') [exp(-t/x P  therefore  P  a =  -A'/(A'  both  P  A ' and A ' ' a r e P  between 0 and - 1 .  j (A  P  P Since  )  p  P  + A  ,, p  exp(-t/r )]  (7.7)  f  )  1  + A'')  (7.8)  P  •  positive  constants,  a c a n o n l y be  P  J  Unfortunately,  this  explanation  i s inade-  _3 quate f o r [Cr(CN)g] modified in  scheme o f t h i s  explanation w i l l  a.  A slight  be d i s c u s s e d  again  C h a p t e r IX. If  at  which has a p o s i t i v e  f l u o r e s c e n c e and p h o s p h o r e s c e n c e  are both  t h e same w a v e l e n g t h , by making u s e o f E q u a t i o n s  7.3,  the steady-state  emission  detected 7.2 and  i n t e n s i t i e s c a n be r e l a t e d  as  105 I I  V  f  I (t)dt  S S  / o  X =  Since 7.1  A  f  f  (7.9)  A  p  T  f  I p  (t)dt  (T A )/(T F  f  T , T , and . ( A / A F  A )  -  (,r »  p  A. f  f  p  T  ) c a n be o b t a i n e d  f  and 7.5, t h e r a t i o  p  x A P P  of the steady-state  )  F  (7.10)  through  Equations  i n t e n s i t y of  f l u o r e s c e n c e t o t h a t o f p h o s p h o r e s c e n c e c a n be e s t i m a t e d Equation plexes  7.10.  As a l l t h e e m i s s i o n  are readily  roughly  available,  constructed.  Figure  from  s p e c t r a o f C r ( I I I ) com-  the fluorescence  s p e c t r a c a n be  32 shows t h e f l u o r e s c e n c e  spec-  -3 trum o f [ C r ( C N ) ]  by t h i s  g  If Equation the wavelength, 4>  technique.  7.10 i s f u r t h e r i n t e g r a t e d w i t h  X,  respect to  then T  f  P  P  F  A / — — dX A  (7.11)  p  P h o s p h o r e s c e n c e quantum y i e l d s  o f C r ( I I I ) c o m p l e x e s have b e e n  18 g i v e n by F o r s t e r ,  t h e r e f o r e by m a k i n g u s e o f E q u a t i o n 7.11,  the  f l u o r e s c e n c e quantum y i e l d  The  calculated  minimum v a l u e s For occurs  values  c a n be v e r y  are l i s t e d  i n Table  roughly IV.  estimated.  They  represent  b e c a u s e t h e f l u o r e s c e n c e s p e c t r a may be  a l l t h e complexes  broader.  s t u d i e d / t h e f l u o r e s c e n c e maximum  n e a r t h e p h o s p h o r e s c e n c e maximum; and t h e r e f o r e f a r _3  awav from t h e a b s o r p t i o n maximum. the  extreme case  and  f l u o r e s c e n c e maximum w e l l o v e r  ordinary  Stokes  1  [Cr(CN)^]  represents  t o have t h e s e p a r a t i o n between t h e a b s o r p t i o n  shift  14,000 cm . x  The e x t r a -  implies that the e q u i l i b r i u m nuclear  800 Figure  32.  Luminescence  850 spectra  900  o b t a i n e d from  decay  curves  for  (nm) [Cr(CN)g]  -3  in  o rigid  glass  solution  phosphorescence  with  at  77°K.  details  The of  the  upper  curve  spectrum  i s fluorescence;  s k e t c h i n from  the  lower  steadv-state  is measurements  107 configuration that  of the  o f the ground  dence  from  4  state  state.  the studies  system  c r o s s i n g , which  levels  are very  i s indeed  This  i s also  of the thermally indicates that  very  different  supported  from  by t h e e v i -  a c t i v a t e d back  inter-  the zero-vibrational  4 f a r below  the absorption  maxima  of the  T„  2g state  (see Chapters Because  state  V and V I I I ) .  the fluorescence  measurements  f o r these  was  not detected  complexes,  i n  steady-  i t has been  widely  4 held  that  the l i f e t i m e  of the  T»  state  2g  _ Q  should  be  shorter  8  t h a n 10 s e c . From d a t a g i v e n by F o r s t e r , the i n t r i n s i c f l u o r e s c e n c e r a t e c o n s t a n t , k^, c a n be c a l c u l a t e d u s i n g t h e oscillator according  strengths,  assuming  .  x  and n = l .  IV.  where  i t should  In every  The  values  are  also  one  case  2g  ->•  4  T~  2g  v /1.5  f  i n fact  included four  be s m a l l e r ,  l  *f  =  reflects  i n Table  orders  (7.12)  2  as t h e phosphorescence o f k^ a r e i n c l u d e d i n  k^ i s l a r g e r t h a n  f o r the observed  (according  doubt  2  The c a l c u l a t e d v a l u e s  k  about  n  v t o b e a t t h e same w a v e l e n g t h  Table  is  T_  to  k  maximum  4  f, f o r the transition  k^ IV.  /  T  since  (7.13)  f  (according  to Equation  In general,  of magnitude  t o Equation  7.12).  the errors  inherent  1/T^,  the observed  smaller  7.13)  the observed  k^ .  than, t h e c a l c u l a t e d  The l a r g e  discrepancy  no  i n the application of  22 Eauation  7.12.  Adamson  has d e s c r i b e d  i n detail  the  argu-  108 ments f o r t h e  disparity  b a s e d on  the  expected d i f f e r e n c e i n 4  equilibrium Strictly  nuclear  speaking,  c o n f i g u r a t i o n s of Equation  s y s t e m s , whose t r a n s i t i o n s equation  of S t r i c k l e r  and  7.12 are  the  4  ^2q  an(  ^'  A  2g  i s applicable only sharp.  Although  a  B e r g has  been p r o p o s e d  transition  i s strongly  s  to  t  a  tes.  atomic  modified f o r broad 66  molecular  bands when t h e  there  still  is  are  no  equations  symmetry-forbidden  and  derived  f o r the  allowed,  fluorescence  has  a l a r g e Stokes'  shift.  have shown i n t h e  diphenylpolyene  series that  which  Birks  67 and  Dyson  k^(obs.)/k^(cal.) becomes l a r g e r and  p r o g r e s s i v e l y d e c r e a s e s as l a r g e r , t h a t i s , the  becomes d i s t o r t e d more and extraordinarily Cr(III)  complexes  lowest  more f r o m t h e  small value  the  Stokes'  excited  ground  seems c o n s i s t e n t w i t h  the  state  state.  for k^(obs.)/k^(cal.)  in  large  shift  The  these  Stokes'  shift. The  C r ( I I I ) c o m p l e x e s w i t h r e l a t i v e l y s m a l l lODq v a l u e 20 have been known t o f l u o r e s c e . The f l u o r e s c e n c e l i f e t i m e s 57 o f them have r e c e n t l y b e e n m e a s u r e d c o n v e n t i o n a l l y by the  results  Table  at  liquid  nitrogen  temperature  are  Zander,  reproduced  in  V. The  have n o t  fluorescence  been r e p o r t e d ,  then k ^ ( o b s . ) / k ^ ( c a l . ) magnitude  less  quantum y i e l d s but still  i f we will  be  than u n i t y , although  Stokes'  shift  complexes  assume them t o be  group of C r ( I I I ) complexes d e s c r i b e d known t h a t t h e  f o r these  one not  or  two  orders  so much l e s s  i n t h i s work.  increases  as  lODq  about  as  0.01,  of in  It is  increases,  the  109  Table Fluorescence  Lifetimes  v  at liquid  Nitrogen  Temperature  Complex  x  [Cr(atp) ]  (C10 )  3  [Cr(atp)g]  (C10 )  3  [Cr(atp)g]  I  g  3  4  4.8 (NH ) 4  therefore,  1.4  3  i t i s expected  the complexes  than  that  the  •x  -  25°K)  we  cannot  be  of  of  value  lODq  by D i n g l e  t h e 14,222  ent  (prompt  ent  (delay  cm  certain  52  4  of the  [Cr(urea)g]  +3  do g i v e  -1 from  lODq  larger  the lifetime  the studies  (below  smaller  the value  k^(obs.)/k^(cal.)  should  value.  be  This  greater certainly  observation.  , i s really  from  with  that  f o r t h e one w i t h  Although x  0.36  3  3  [CrFg]  is  6.5 6.0  4  for  sec 20  3  [Cr(urea)g](C10 ) CrCl  10  T_  that 2g  t h e new  state,  at very some  low  the  lifetime, observations  temperatures  support.  The  emission  4 region  fluorescence fluorescence  ( T  2  g  ?) o f T ^  ?) h a s a f a s t 50  ysec  ?) o f T ^ 3 0 0 - 4 0 0  and a  ysec,  decay  compon-  slow  compon-  while  emission  110 -1 from  14,196 cm  50  70  -  ysec  region  after  2 ( E  ) does not  the p u l s e .  reach  Although  no  a maximum  detailed  until  quantita2  tive  analysis  was  g i v e n , the b u i l d - u p process  seems t o c o r r e s p o n d v e r y w e l l  of  the  w i t h the prompt decay  E of  state  g  the  4 T„ 2g  state. Although  the  4  T_ 2g  Chapter  state IV).  the d e t a i l e d has  mechanism  b e e n shown t o be  From t h i s  the  lifetime -7  expected  t o be  The  longer than  internal  10  of  be  temperature  range. region results  Therefore too  the  s h o u l d be  strongly  obtained i n this  chemicallv reactive of  ^  T 2  g  state  c o n s t a n t , k^,  the q u a r t e t s t a t e  lifetime  unknoi^n, (see  would  be  sec  conversion rate  d e p e n d e n t on  remains  -1  shown, w i t h o u t u s i n g any strongly  s t i l l  of  chapter support  been  parameters,  at the higher 4 the T„ state m 2g  d e p e n d e n t on  has  temperature this  temperature.  this  to  conclusion.  The  CHAPTER THE  The evaluated  PRIMARY  intersystem according  k  VIII  PHOTOPROCESSES  crossing rate constant,  to the f o l l o w i n g 4  W  =  T  k^, c a n be  equation:  f  I f we assume t h a t t h e n e w l y o b s e r v e d  lifetime,  fluorescence  the r e s u l t s  and  lifetime, '  reported  function  as  value,  [Cr(CN),] 6  .  and 36.  The r e s u l t s a r e  I t h a s t o be n o t e d  that  due t o t h e u n c e r t a i n t i e s i n A. and ^ isc  and C r ( a c a c >  + 3  3  and  33,3 4,35,  o n l y be a c c u r a t e ,  [Cr(en) ]  f o r d>. 1 sc Y  o f t e m p e r a t u r e c a n be o b t a i n e d .  absolute can  with  f  i n C h a p t e r s V I and V I I r e s p e c t i v e l y , k^ a s a  shown i n F i g u r e s the  then  T , i sthe  3  at best,  to within  100% f o r  and w i t h i n 50% f o r [Cr(NH^)^(NCS) 4 3 ~  The c u r v e s  so o b t a i n e d  a f u n c t i o n o f temperature a r e found  f o r k. , i . e . , k. isc ' 4  t o have t h e f o l l o w i n g  form: k  The  best  values  collected are  = =  4  k + k s exp(-E /RT) + s^exp(-E /RT) c  c  c  d  i n Table  V I , where t h e A r r h e n i u s  p a r a m e t e r s f o r k^  f o r convenience.  temperature dependence o f i n t e r s y s t e m  some a r o m a t i c  (8.2) (8.3)  o f t h e parameters f o r t h e f o u r complexes a r e  also included The  d  compounds, e . g . , a n t h r a c e n e ,  crossing i n  naphthalene,  pyrene,  114  1,000/T  4.0 Figure as  a  35.  of  x  )  I  I  5.0  6.0  Intersystem  function  (°K  crossing  temperature.  rate  7.0  constant  According  to  of  [CrfNCP)^]  Mechanism  I.  u  Ul  M  •H  0O  4.0 Figure  36.  function  of  5.0 Intersystem crossing temperature.  6.0 rate  According  constant of  t o Mechanism  7.0 trans-[Cr(NH^)^(NCS)^ I.  Table The  Arrhenius  frequency  Complex  Parameters  factors  are  of  k. b a s e d on M e c h a n i s m s I and I I . isc i n s e c l and t h e a c t i v a t i o n e n e r g i e s i n  s  E  fCr(NH ) (NCS) ]~  4. 3 x l 0  3  Cr (acac)  6 .2 x l 0  3  4. 6 x 1 0  2  ?  4  3  [Cr(NCS) ]" 6  [Cr(en) ] 3  + 3  3  The Kcal/mol.  _  a  3  VI  1. 6 x 1 0  4  S  a  - b  b  E  13  0 .08  7  0 .18  6  .9xl0  X 4  0 .19  2.IxlO  1 4  0 .10  1.5xl0  1 3  .2x10 14  F  s  s  C  c  9.0  1. 2 x l 0  7.7  7. 1 x 1 0  7.9  7. 9 x l 0  3  10 .2  2. I x l O  5  5  4  d  d  3.IxlO  1 2  7.0  0 .20  2.6xl0  1 5  7.8  0 .20  1  0 .26  3.6xl0  0  12 .0x10 6  7.5 3.3  117  and  t h e i r d e r i v a t i v e s has 68  I t was  constants  found  consisted  d e p e n d e n t components. intersystem  crossing  the  lowest  to  a higher  triplet  be  assigned  vibrational  getic the to  strongly  (higher of  the  4  lowest e x c i t e d  the  T» Zq  Cr(III)  and  direct  singlet state same  to  state  the  process  the  T,  and/or  E  crossing  v i a high  following  sections,  Cr(III)  complex a r e  the  may  c  the  lowest  the  lsoener-  states.  dependent p r o c e s s  k  And  correspond  vibrational levels by  an  average of  E^)  o v e r a l l mechanistic  discussed  with  a l l the  information. I ...  In  t h i s mechanism  intersvstem  crossing  l e t us  from the  4 p r o c e s s k^  from the  the  from the  complexes,  zero-vibrational level 2 ' s t a t e to the T_ state. Zq  the  available Mechanism  ,  latter  to  i n t e r s y s t e m c r o s s i n g from 4 the T„ s t a t e d i r e c t l y to 2g 2 2  temperature  schemes i n t h e  2 c t  the  crossing  independent  assigned  than the  In  A  research  state.  l e v e l s of  intersystem  temperature  f o r m e r was  from the  l e v e l s of  vibronic  the  several  t h e i r intersystem  of both  The  i n the  to  that  state while  triplet  Similarly, can  by  73  groups. rate  been s t u d i e d  2  the  thermally  4  2  assume t h a t  process k  is  the  cl  E  g  state  activated  to  the  around  intersystem  state crossing  E • to T„ state, process k the i n t e r s v s t e m crossg 2g c • 4 2 ma from the T„ to E s t a t e , and n r o c e s s k, intersvstem 2g a ' d 4 2 ' c r o s s i n a from the T~ to T„ s t a t e (see F i g u r e 3 7 ) . All Zq  Zq  118  Franck-Condon s  \  State  r  /  k 3  \  2g  •  T  •I 2g  /  d  \ fast  •4— k \  ' \  k  b  J'  7  !H  k  4,  2g  l  Figure 3 7 .  S c h e m a t i c o f Mechanism I .  119 the  four processes  a r e assumed  t o proceed  v i a a tunnelling  mechanism. 4 Accordingly, state  the z e r o - v i b r a t i o n a l  c a n be e x p r e s s e d 4  the l o c a t i o n  2  2 g  )  W V  =  of the  v( T  1^^  of the  as  W V and  level  2  2T«  state  2g  =  v  _  0  (  0  4  +  E  b  (  8  -  4  )  c a n be e s t i m a t e d by J  T  2  g  )  +  E  (8.5)  d  2 The  zero-vibrational  levels  of the  state are mostlv g — ( 4 — 2 known. The v a l u e s e s t i m a t e d f o r v T_ ) and v ( T„ ) a r e 0-0 2g 2g l i s t e d i n Table V I I . Since the z e r o - v i b r a t i o n a l l e v e l s of rt  the  E  n  4  T_ s t a t e s o f t h e s e C r ( I I I ) c o m p l e x e s have n e v e r 2g  observed  spectroscopically,  a direct  assignment i s out of the q u e s t i o n .  confirmation of this However,  the  74 has b e e n o b s e r v e d from  18,000 cm  the v a l u e s  1  i n some C r ( I I I ) c o m p l e x e s , t o 21,000 cm .  Within  x  listed  i n Table VII f a l l  been  2 T  band  2 g  — 2 v( T  experimental  2 g  )  lies  errors,  quite w e l l within the  region. In getic  the the  of the s a t i s f a c t o r y  argument b a s e d  c o n s i d e r a t i o n s f o r t h i s mechanism,  ficulties and  spite  with  i t too.  s^, supposedly  being  Firstly,  t h e r e a r e some  the frequency  the t r a n s i t i o n  on e n e r -  factors,  difs  c  p r o b a b i l i t i e s between 2 i s o e n e r a e t i c l e v e l s of the T and E s t a t e s and o f 2g • a 4T„ and 2T„ s t a t e s , d i.f f e r by as much as s e v e n o r d e r s 2g 2g  4  n  120  of magnitude. factors  In the aromatic  compounds s t u d i e d ,  o f t h e two components o f i n t e r s y s t e m  the frequency  crossing  are of  2  comparable magnitude. closer  to the T „ 4  (v  E, ) , t h e f a c t p  the  T  state  2 g  n o t much  2  A  1  that  60  applicable  lies  ) than the E s t a t e ( c o m p a r i n g E , and U-U g ct s , i s much more e f f i c i e n t t h a n s i s not a c  n  Zg  Since  76  on t h e e n e r g y gap h y p o t h e s i s .  '  Table VII The  predicted  according  locations  t o Mechanism  Vo  Complex  trans-[Cr(NH^) (NCS) ]~ 2  [Cr(CN)g]~  4  3  [Cr(NCS) ]"  3  g  [Cr(en) ]  + 3  3  Cr ( a c a c ) ^  It states, '  2  i s considered E  a  ,  2  .  of the  T, , and la  4  T„  and  2g  T„  states  2g  I and I I . Wave number i n kK  ( 2  V  Mechanism I  Mechanism,II  Vo^V  13.33  16.5  12.38  15.2  12.85  18.9  14.0  15.6  18.2  13.0  14.98  18.6  19.7  17.4  .12.84  15.5  18.3  12.8  t o be due t o t h e f a c t 2  2  T„ , h a v e n e a r l y 2 g '  1  that  t h e same  the doublet equilibrium l  4  nuclear  configuration  i^ith  the ground  state  A  2 a  ,  while the  4  T s t a t e i s s t r o n g l y d i s t o r t e d from t h e ground s t a t e . There4 — 2 f o r e , t h e intersystem. c r o s s i n g from T„ {v ) t o t h e T, and ' 2a 0-0 lg 2 c t  n  n  2 E  g  states  i s strongly  forbidden  by t h e F r a n k - C o n d o n p r i n c i p l e ,  On t h e o t h e r hand, b e c a u s e o f t h e  Condon  state  together  (absorption  T_ 2g  state  and n u c l e a r  between them i s e x p e c t e d  and t h e F r a n k -  4  maximum) o f t h e  i n both energy  radiationless  2  T,,  state  geometry, the i n t e r a c t i o n  t o be l a r g e ,  consequently, the  t r a n s i t i o n p r o b a b i l i t y between them  Unfortunately,  are close  more s e r i o u s  difficulty  i s high.  comes f r o m t h e  other p a i r of frequency  factors, s  the  t r a n s i t i o n p r o b a b i l i t i e s of the  forward  and r e v e r s e  and s ^ , s u p p o s e d l y  c  being  i n t e r s y s t e m c r o s s i n g between t h e z e r o - v i b r a t i o n a l l e v e l o f 4 2 the T_ s t a t e and t h e i s o e n e r g e t i c l e v e l s o f t h e E state. 2g cr J  They a r e e x p e c t e d in  t o be o f t h e same m a g n i t u d e , b u t t h e y a r e  f a c t d i f f e r e n t by f r o m s e v e n t o t e n o r d e r s o f m a g n i t u d e  from each o t h e r .  A l t h o u g h r a d i a t i o n l e s s t r a n s i t i o n s between  two e l e c t r o n i c s t a t e s  which d i f f e r  nuclear  have n e v e r b e e n e x p l o r e d  configuration  or e xp er imentally, forward  the extreme l a r g e  and r e v e r s e  be p o s s i b l e  v e r y much i n e q u i l i b r i u m theoretically  d i s c r e p a n c y between t h e  t r a n s i t i o n s o b s e r v e d on t h i s mechanism  but can h a r d l y  be  may  reasonable.  Mechanism I I In Mechanism  t h i s mechanism, I , we  assume  k. a r e t h e f o r w a r d b activated  to obviate  (see F i g u r e  and r e v e r s e  intersystem  ceed v i a a t u n n e l i n g  crossing mechanism  the d i f f i c u l t i e s i n  38) t h a t  p r o c e s s e s k^ and  t r a n s i t i o n s of the and assume but instead  thermally  t h e y do n o t p r o via Teller  crossing  122 through  the i n t e r s e c t i o n boundaries  4  p o t e n t i a l energy s u r f a c e s of the course, process  k  c  system c r o s s i n g .  is still  of the  2  and  2g  multi-dimensioned E  g  state.  Of  assumed t o be t h e t u n n e l i n g i n t e r -  I t h a s t o be n o t e d  that the tunneling pro-  c e s s and T e l l e r c r o s s i n g h a v e n o t b e e n r e p o r t e d t o be competitive  i n t h e same m o l e c u l e  But t h e o r e t i c a l l y According  f o r any o t h e r compound s t u d i e d .  t h e r e i s no r e a s o n why  they  s h o u l d n o t be.  t o t h i s mechanism, t h e z e r o - v i b r a t i o n a l  level  4 of the  ^2q  s t a i : e  l S  n  o  expressed  w  V Q - C / V  =  W  The v a l u e s s o o b t a i n e d f o r v  as  E  2  g  )  „( T_  0-0  E  b  " d E  ( 8  -  6 )  ) are a l s o l i s t e d i n  4  n  +  2g  Table V I I . The o n l y d i f f i c u l t y v / i t h t h i s m e c h a n i s m i s t h a t i t r e -  4 cmires  the l o c a t i o n of the z e r o - v i b r a t i o n a l  l e v e l of the  T„  2g s t a t e t o be v e r y  low i n energy.  However, t h i s  — as a m a t t e r  4  o f f a c t , t h e l o c a t i o n o f v . „( T_  Now  f o r these  complexes.  the frequency  magnitude as'they  between t h e  T„  zq  f a c t o r s s ^ and s ^ a r e o f c o m p a r a b l e  s h o u l d be a c c o r d i n g t o t h i s  It i s interesting  4  and  ) has never been  Zq  (J-U  determined,  i s not impossible,  2  E  g  t o note  mechanism.  t h a t the energy s e p a r a t i o n  states estimated  55  from  delayed  fluorescence i s the d i f f e r e n c e i n the a c t i v a t i o n energies of :  t h e f o r v / a r d and r e v e r s e i n t e r s y s t e m c r o s s i n g . found  f o r ruby  and e m e r a l d  spectroscopically  The  agree w i t h the values  ( s e e R e f . 18).  values  obtained  I f the l i f e t i m e s of  these  intersection \  /  \  k  I  3 /  1 2g  s  k  'Eg  k  2g Figure  38.  Schematic of  Mechanism  II  124  compounds  are  thermally  activated intersystem  vation  studied,  energy  which  i s E^  4 between  the  Mechanism  Now  and  2g  E  than  the  l e t us  values  have  energy  of  uncertain  mechanism  lifetime, decompose -  for  predicted  from  an  the  T  (see , to  be  into  two  x  T  Figure the  39),  we  acti-  delayed  assume  lifetime  of  ±  E^  Since  components  x =  k  =  s  +  y  k  (8.8)  z  exp(-E  /RT)  +  s exp(-E /RT) z  two  2  7.1  7.7,  7.8).  E  g  i s known  z  only  T  and  T, states lg  E  z  are  ±  transition the  between  However,  0  is  orders  to  E  g  is  be  the  automatically  and  -1  (refer  i n order  to  explain  of  Kcal/mol.  assumed  •  g  VIII.  y  v  2  T^  mechanism  1  , s x  within  2  the  a  2  and  step,  v/ith  s  0.0 3 K c a l / m o l ,  and  rate-determining  and  (8.9)  z  50%, ±  the  T, state, lg  i n Table  the  Equation  2  , E , s , and E are c o l l e c t e d y y z z d i f f i c u l t y i n the measurement of -  is  promptly  that  the  s  the by  magnitude,  and  the  separation  J  lated  to  studies.  this  found  Because  c r o s s i n g may  higher  state  . g  1/T ' x  The  corresponding  III  In newlv  component  2 T„  fluorescence  the  to  popu•  slow  fits  Equations  the  7.6,  positive a  -3 for  [Cr(CN)g]  and  it  i s necessary  to  state  is also  buted  to  the  fact  assume  detected.  that  that  a  changes  with  phosphorescence  Unfortunately,  the  wavelength,  from  spectra  the so  2 the  phosphorescence  (see  Chapter  VII)  are  2 attri-  F i g u r e 39.  Schematic of Mechanism I I I .  126  located 2 E  n e a r o r even  phosphorescence. -  g  phorescence cence  a t l o n g e r wavelengths A large ^  i s unlikely  Stokes' s h i f t  i n the 2  (because  that  of the  E  g  2 T, Dhoslg  phosphores-  i s o n l y about  100 cm "*") b u t i s n o t i m p o s s i b l e . 2 o f t h e T.^ s t a t e and f u r t h e r p o p u l a t i o n o f  Depletion 2 E ^ s t a t e a r e a c h i e v e d by p r o c e s s k  the  than those of the  a t lower  y  temperatures  4  and by p r o c e s s k „ t o t h e T„ s t a t e f o l l o w e d bv rar>id i n t e r • z 2g 2 system c r o s s i n g t o t h e E state a t higher temperatures. I f we assume p r o c e s s k, and k proceed v i a a tunneling b z mechanism, t h e n z  c  g  r  v( T, 2  ) -  lg  The  values f o r ( E  for  [ C r ( N H ) (NCS) ] ~ , 3  2  b  4  v( E 2  g  )  =  E,  - E_  p  (8.10)  z  - E ) a r e 0.4, 0.6, - 0 . 9 , and 6.6 K c a l / m o l z  [Cr (NCS) ] ~ ,  Cr(acac) ,  3  g  3  and  [Cr(en), ]  + 3  3  2 respectively. However, t h e e n e r g y s e p a r a t i o n o f t h e E and 2 74 T^ i s known t o be a b o u t 2 K c a l / m o l . The v a l u e s p r e d i c t e d J  g  g  for  [Cr(NH_)„(NCS) . ] ~ and [ C r ( N C S ) , ] 5  1  4  Cr(acac)  3  experimental e r r o r , but those f o r [Cr(en)^]  can n o t be.  I n t h i s mechanism, j u d g i n g f r o m time o f the less. the  4  mav be c o n s i d e r e d s a t -  6  +3  isfactory within and  -  4  s ^ and s , t h e l i f e z  -10  T„ s t a t e must be a t t h e o r d e r o f 10 sec or 2g I n f a c t , K i s l i u k and Moore e s t i m a t e d t h e l i f e t i m e o f  T_ m 2g  r u b v t o be l e s s  than  10  to the f l u o r e s c e n c e - o n l y complexes, -7  much s h o r t e r  (10  -3  54  sec.  But  comparing  the values speculated are  -10  v s 10  s e c ) . Since the only  apparent  127 Table  VIII  The A r r h e n i u s  Parameters  Y (sec  y (Kcal/mol)  (sec  Complex  of l / x  s  E  S  )  z  E  3.4xl0  5  0.15  1.2x10  Cr ( a c a c )  l.SxlO  5  0.14  1.5x10  5.9xl0  4  0.18  5.9x10  2.0xl0  5  0.12  1.2xl0  [Cr  2  4  3  (NCS) ]~  3  3  [Cr(en) ]  + 3  3  difference only  between  Cr(III)  crossing quartet mainly  complexes  by  from  intersystem  formation  and  i f we  assume  the l a t t e r crossing,  quantum  yield  14 17 '  t o be  8.7  14  8.6 3.6  9  fluorescence-  of  intersystem  the shortening  t o the former then  8.6  the  i s the p a r t i c i p a t i o n  i n the former, lifetime  doublet  the phosphorescence  we  would  unity.  z  (Kcal/mol)  )  [Cr(NH ) (NCS) ]" 3  x  of the  i s caused expect  the  T h e <j>. JL S C  obtained  i n Chapter  VI, according  to this  mechanism,  should  2 be  t h e quantum 1  is  yield  of the  E  state  formation.  Since  g  J  substantially less  than  6. r  unity,  we  are l e d to the  isc  conclusion  2 that  intersystem  crossing  from  the  state  to the  ground 2 state i s very e f f i c i e n t . This i s d i f f e r e n t from the E g s t a t e ( 1 0 ^ v s 1 0 s e c "*") . From c o n s i d e r a t i o n s o f n u c l e a r 2 2 c o n f i g u r a t i o n and energy l e v e l , t h e E and T states are ^ -— g lg 4  n  128  expected  to  behave s i m i l a r l y  states.  Moreover,  with  respect to  -  the  2  transition  the  2  T..  •  E  lg be r a t i o n a l i z e d , small. ++ In  order  as t h e  is  too  slow  to  g  s e p a r a t i o n between the  f o r the chemical  ground  reaction  two  states  t o compete  is  with  4 other ical is  processes in  r e a c t i o n must be  state, larger  than  the  to  the  effect. +3 [Cr(urea) ] , it  In  1  ted  o  0  above the that 4  sing  T  thus  not  2  at  state  extremely 2 •>  g  T  l  g  in  is  energy.  .  The  must be n e g l i g i b l e If  phosphorescence from  the  2  T.  the  chem-  not  sub-  state  lg it  is  expec-  intersystem 2  and t h e so,  of  difficulty  is  Therefore,  low t e m p e r a t u r e s , . .  a phosphorescence curve without the  1  solution  known t h a t  appreciably populated.  observed  in  constant  cage  ' 4 lies  rate  l O ^ sec  why a r e a c t i o n p r o c e e d i n g s o f a s t  jected  in  the  this  cros-  state would  result  a rise.  In f a c t , D i n g l e +3 [Cr(urea)g] ~ a t 25°K d i d 52  show an i n i t i a l r i s e b e f o r e d e c a y . R e c e n t l y , an e x p e r i m e n t a l u p p e r l i m i t o f 4 n s e c has 4 been determined f o r the l i f e t i m e of the T~ s t a t e of ruby. 2g 2 2 A n d i t a l s o h a s b e e n s h o w n t h a t t h e '7. and E states are lg g practically  in  thermal  equilibrium  as i s  evident  from  the  2 fact at  that  the  300°K b u t  made i n  phosphorescence of 25 not  at  77°K.  Mechanism III.  processes should not  between the  This  However, ionic  be o v e r l o o k e d .  the  T^  g  states  contradicts the  is  the  difference  in  and m o l e c u l a r C r ( I I I )  observed  assumption primary compounds  T h e e n e r g y gap h y p o t h e s i s s h o u l d be v a l i d i f t h e i n i t i a l and f i n a l s t a t e s o f a t r a n s i t i o n a r e n o t t o o d i f f e r e n t i n the e c r u i l i b r i u m n u c l e a r configuration. + +  129  From favored  the  that  undeniable  T  that  unambiguously needed Chapter  none  lifetime  of  of  this  although i t i s  the  the mechanisms  confirmed or negated. problem.  4  T„ state, 2g  proposed  Some m o r e  -These w i l l  be  s t i l l  i t is  has  been  research  is  discussed  in  IX. matter which  primary processes  tures. the  considerations,  i s the  to clarify  No the  x  above  But  as  soon  activated  complicated  behave  as  primary processes  thermally  mechanism  the start  routes  mechanistic  i s true,  quite  on  "normally"  temperature r i s e s t o change and  scheme.  the whole, a l l  result  at  low  tempera-  sufficiently,  their  courses to  the  i n an  "abnormal"  and  CHAPTER IX SOME FINAL REMARKS  The  primary photoprocesses  very d i f f e r e n t i n behavior pounds.  F o r example,  fluorescence tically, and  i n Cr(III)  states  t h e s e c a n be e x p l a i n e d equilibrium  nuclear  than p r e d i c t e d  instead  c o n s i s t e n t l y by t h e f a c t t h a t t h e  configuration  of the  investigating  roles  the possible  l e t us f i r s t  o f lODq, t h e l i g a n d  4 T^  state 4  state,  i s dis-  A„ .  the d i s t o r t i o n plays study  Before i n the  t h e d i s t o r t i o n as a  strength. The d i s t o r t i o n — 4 — 4 q u a l i t a t i v e l v m e a s u r e d by t h e q u a n t i t y v ( T_ ) - v _ ( T, ~ max Zq 0-0 ^  field  rt  The are  arguments and d i s c u s s i o n s  b a s e d more on i n t u i t i o n  i n the following  t h a n on s o l i d  they  s h o u l d be c o n s i d e r e d  9.1  The O r i g i n o f t h e L o w e s t Q u a r t e t  t o be t e n t a t i v e  evidence,  the z e r o - v i b r a t i o n a l l e v e l  therefore  State  of the T „ state: 2g 4  sections  and s p e c u l a t i v e .  T h r e e methods a r e u s e d t o e s t i m a t e r o u g h l y of  are large,  t o photochemical reactions. A l l  o f t h e ground  is  theore-  o f the phosphorescent  t o r t e d v e r y much f r o m t h a t  primary processes,  com-  complexes, t h e i n t r i n s i c  t h e quantum y i e l d s o f i n t e r n a l c o n v e r s i o n s  are d i r e c t precursors  function  complexes a r e  f r o m t h o s e known i n o r g a n i c  l i f e t i m e s a r e much l o n g e r  the fluorescent  states  i n Cr(III)  the l o c a t i o n (1)  for  131  t h o s e complexes which f l u o r e s c e , a t the  center of  absorption  necessary data are  and  the  o r i g i n i s assumed t o  fluorescence  taken from the  r e v i e w by  maxima.  be  The  Fleischauer  and  17 Fleischauer. delayed  (2)  from the  fluorescence.  The  temperature-dependence r e s u l t s are  g i v e n by  studies  Camassei  of and  55 Forster.  (3)  From t h e  a c t i v a t i o n e n e r g i e s of  activated  primary processes  shows t h e  p l o t of  (see  the  Chapter V I I I ) .  thermally  Figure  40  4 It  can  insensitive  be  to  the  o r i g i n of  seen t h a t  lODq.  And  the  the  T^  state  g  l o c a t i o n of  t o our  surprise,  against  lODq.  the  o r i g i n i s very  the  o r i g i n of  the  4 ?2q  s  r  tate  i s lower i n energy  lODq v a l u e dicates the be  than  the  excited  f o r those with  inadequacy of  i n the  states)  r e a c h e d by  nuclear  configuration  excited  the  Ligand F i e l d of  the  Molecular O r b i t a l Theories  impotent the  to p r e d i c t  excited  f i g u r a t i o n may This  the  states, be  Transitions  origins  (not  because the  i n the  But  in  stage are  to mention the  equilibrium  i s excited  from the  ground  to  case that an  state  the  well  same general, rather geometry)  nuclear  a bonding or  anti-bonding to  for  (Frank-Condon  different in different electronic  i s c e r t a i n l y true  bonding e l e c t r o n  at present  in-  maxima i n m e t a l  states  state.  This  theories  t r a n s i t i o n have t h e  ground  large  T h e o r y may  absorption  vibronic  vertical as  lODq v a l u e .  present bonding  prediction  complexes because the  of  the  smaller  electronic states.  justified  the  f o r complexes w i t h v e r y  con-  states. non-  orbital.  excited  quartet  132  14  16  18  20 lODq  22  24  26  (kK) 4  Figure  40.  The o r i g i n  of the  as f u n c t i o n o f l i g a n c l f i e l d Method 2; and O  T^^  s t a t e of C r ( I I I ) complexes  strength.  M e t h o d 3, a c c o r d i n g  •  , M e t h o d 1; A  t o Mechanism. I .  , The  57  broken c i r c l e s  are d a t a from Zanders.  even lower a c c o r d i n g represents  t o Mechanism I I o r I I I .  the l i m i t i n g  s t a t e s have i d e n t i c a l  The o r i g i n s a r e  condition  The h e a v y  that the i n i t i a l  equilibrium nuclear  and  line final  configuration.  133 states  i n Cr(III)  bonding o r b i t a l distortion  of  From F i g u r e  (t g^  t  o  a  e x c i t a t i o n of n  a  2  the  40  involve  quartet  i t can  an  nti-bonding  excited  a l s o be  e l e c t r o n from  orbital  states  seen t h a t  (e ),  the  strong  g  i s thus  the  expected.  d i s t o r t i o n of  the  4  2q  s  9.2  *- te a  increases  Tunneling The  has  not  matic  competition  compounds. route  w h i c h have t h e that  their  high  energy  crossing the  other fore  the  faces  o r do  the  surfaces  low  level  tunneling  crossing  with  are  of  quite  of the the  tunneling  configuration;  and  to the  electronic state.  p o t e n t i a l energy  temperature, T e l l e r  Teller process  d i f f e r e n t from  intersection lines  process  The  surface  go of  so  very  each  there-  t h e i r p o t e n t i a l energy  initial  more  electronic states  i n t e r s e c t at a  the  aro-  i s the  configuration  i n energy w i t h r e f e r e n c e  tunneling  Teller  i n t e r s e c t at a l l .  equilibrium nuclear  minimum i n t h e  state,.the high  not  electronic states  temperature, unless the  that  same e q u i l i b r i u m n u c l e a r  intersection lines  vibrational  and  f o r t r a n s i t i o n s between two  become c o m p e t i t i v e  l i e very  Crossing  for radiationless transitions in  It i s believed  level  i n the  Teller  \  between t u n n e l i n g  p o t e n t i a l energy  can  two  lODq i n c r e a s e s .  Mechanism and  been o b s e r v e d  efficient  if  as  sur-  zeroAt  low  r i g h t through the  initial  i s a l w a y s d o m i n a n t ; however,  crossing  gradually  i n t e r s e c t i o n boundaries, T e l l e r  stands out.  c r o s s i n g must t a k e  at At  place  134  in  a few v i b r a t i o n s .  Therefore,  the net process  i s expected  t o have an a c t i v a t i o n e n e r g y e q u a l t o t h e s e p a r a t i o n the  zero-vibrational level  t o have a p r e - e x p o n e n t i a l that  i s , about t h e r a t e For  tions  electronic has  constant  10  12  states  or T e l l e r  are a v a i l a b l e .  a l s o has a t h e r m a l l y energy.  9.3  Primary Processes this section  13  -1 sec , ;  whether  crossing,  transi-  because  But a t l e a s t T e l l e r  i n the i n t e r n a l conversion  activated  tion  t o 10  of the v i b r a t i o n .  p r o c e s s k^ and k^, we c a n n o t t e l l  t o be assumed t o o c c u r  In  and t h e i n t e r s e c t i o n l i n e s ; and f a c t o r o f about  occur v i a tunneling  between  component w i t h  a large  higher crossing which activa-  and L i g a n d F i e l d  Strength  the e f f i c i e n c i e s  of the primary p r o -  4 cesses  from t h e  state  are discussed  xn t e r m s o f e n e r g y  gaps^  and b a r r i e r w i d t h s ^ between t h e i n i t i a l  states  o f t h e t r a n s i t i o n s when t h e y t a k e p l a c e  and t h e f i n a l  7  v i a tunneling  mechanism; and i n terms o f t h e e n e r g y s e p a r a t i o n s intersections take place  and t h e o r i g i n s o f t h e i n i t i a l  v i aTeller  between t h e  states  when  they  crossing.  4 T„ s t a t e does n o t change . 2g much w i t h l i g a n d f i e l d s t r e n g t h , lODq, t h e more l i k e l y e f f e c t o f lODq on t h e p r i m a r y p r o c e s s e s i s t h e change i t c a u s e s i n Since  the  the o r i g i n of the  equilibrium  With i n c r e a s i n g  nuclear  configuration 4  lODq, t h e  more and more f r o m b o t h  T s t a t e  of the  4 T . state. 0 g  i s generally  the lowest doublet s t a t e s  distorted and t h e ,  135  ground  state.  I t can  sional potential  be  energy  seen from the  surfaces that this 4  b a r r i e r w i d t h s between t h e state and  and  the  state.  conversion  p e c t i v e l y with At sing  and  T„ 2g  s t a t e and  2  the  E 4  Therefore,  are  expected  two-dimen-  results  a narrower b a r r i e r w i d t h between the  ground  internal  simplified  intersystem to decrease  i n wider 2  and  g  T_ 2g  state  crossings and  T.. lg ,  and  increase  res-  i n c r e a s i n g lODq.  liquid  n i t r o g e n temperature,  internal  conversion  are  only  important  intersystem in depleting  crosthe  4 T^g  s t a t e , t h e quantum y i e l d  smaller; as  and  t h e quantum y i e l d  lODq i n c r e a s e s .  crossing t o be  In f a c t ,  of  internal  c l o s e to u n i t y , but are  substantially  9.4  Suggestions Although  work, t h e r e anisms of  f o r those  less  for Further  crossing w i l l  conversion  t h e quantum y i e l d s  f o r C r ( I I I ) complexes w i t h  they  than  of i n t e r s y s t e m  than  low  of  high  the primary  were e x p e c t e d  known  values,  unity.  Work  questions  introduced,  photoprocesses  before.  are  lODq  many p r o b l e m s have b e e n c l e a r e d up  a r e more new  larger  intersystem  lODq v a l u e s  with  be  Besides  are  and  f a r more  being  in  this  the  mech-  complicated  comparatively  less  4 explored,  the  ^2a  s  * - t e i s bound  on C r ( I I I ) c o m p l e x e s will  be  widely  formation  used:  a  i n the  near  t o be  future.  photochemical  quantum y i e l d ,  and  the  focus  Three  of  parameters  quantum y i e l d ,  especially  the  research  doublet  lifetime  of  the  136 4, T^g  state.  The f o l l o w i n g  a r e some s u g g e s t i o n s  f o r further  work, 4 (1)  The  cursor  T^g s t a t e  h a s b e e n p r o v e n t o be t h e i m m e d i a t e  t o the photosubstitution  Cr(IIl)  i n Reineckate  c o m p l e x e s have t o be s t u d i e d  to e s t a b l i s h the g e n e r a l i t y interesting  pre-  i o n , b u t more  (by t h e same t e c h n i q u e )  of the conclusion.  Especially  will  be t h e work e x t e n d e d t o t h o s e C r ( I I I ) com+2 p l e x e s , f o r example, [Cr (NH^) (NCS) ] ", w h i c h e x h i b i t two modes o f p h o t o s u b s t i t u t i o n and a r e t h o u g h t t o have two d i f f e r 22 27 ent  photoreactive  intermediates.  (2)  The t e m p e r a t u r e - d e p e n d e n c e  quantum y i e l d s o f t h e C r ( I I I )  ' studies  of the photochemical  complexes  i n deoxygenated  s o l u t i o n s are e s s e n t i a l to understand the r e a c t i v i t y of the 4 4 T_ state. I n t h e c a s e t h a t t h e l i f e t i m e o f t h e T„ state 2g 2g is  available,  the rate  c o n s t a n t and t h e a c t i v a t i o n e n e r g y o f .  the  photochemical reaction  c a n be e v a l u a t e d .  the  a c t i v a t i o n energy w i t h  the ligand  give  field  Correlation of strength  will 4  us an i n s i g h t i n t o t h e d e t a i l e d mechanism o f t h e  state  chemistry  and w i l l  between s t r u c t u r e In available,  some c l u e  to the r e l a t i o n s h i  and r e a c t i v i t y .  the case that only  provide  the l i f e t i m e of the  4 T  2g n  state  t h e a p p a r e n t a c t i v a t i o n e n e r g y c a n be  mated, and i t i s d i f f i c u l t However, a t s u f f i c i e n t l y  to interpret  high  T^^  i snot esti-  unambiguously.  temperature,  repopulation  of  4 the 2 Eg  T^g s t a t e state;  causes p r a c t i c a l l y complete d e p l e t i o n  there are only  two e f f i c i e n t  of the  pathways t o d e g r a d e  137  the  excitation  energy,  therefore,  k  <f>  h  C  o  o  T  h  =  n  e  k  m  3  (9.1)  +  2  k  3  k  r  (  *chem  "  X )  -T-  =  k  (These  can  also  be  derived  from  '  ( 9  2 )  3  Equation  4.20  by  assuming  the  2 E^  state  is  log (i/^hgjrj activation  substitutionally inert.) ~  1)  v  1/  s  energies  will  T  of  the  yield  The  the  Arrhenius  difference  photochemical  plot  between  reaction  and  of the  internal  conversion. (3)  The  confirmation  of  the  l i f e t i m e of  the  T_  4  state 2g  utmost The  importance  rise  studied acy  of  a,more  of  data  T  must flash  be  in  and  studies  Cr(III)  and  the  This  a more  4  complexes  systematicallv.  improved.  lamp  of  can  T« Zg  be  sensitive  state.  should  And  the  of  be  accur-  achieved  with  but  noise  less  system.  Direct is  extensively  intense  detecting  subsequent  phosphorescence  more the  for  is  observation  is possible  flash-kinetic flash-kinetic  and  of  the  primising  spectrophotometry  excited by  using  method.  state the The  whose  lifetime  submicrosecond millisecond  s t u d i e s of the C r ( I I I ) complexes have r e c e n t l y "77 78 * 5 been r e p o r t e d . ' However, from the f a c t t h a t T =10 x s e c , we k n o w t h e r e a r e p r a c t i c a l l y no d e t e c t a b l e excited  138 4  2  (or  T  ig)  state molecules remaining  after millisecond 2  delay  times.  states  Therefore  of C r ( I I I )  only  the  complexes can  be  s e c o n d f l a s h - k i n e t i c method has excited  state having  x  of  observed.  the  will  spectra The  of  E  submicro-  p o t e n t i a l to detect  eventuallv  g  l e a d us  the  to  the  x  r  identification  and  absorption  i t .  This  can  be  achieved  with  a  pulsed  laser. (4)  The  three  mechanisms m e n t i o n e d  primary processes tions  of  the  can  2  be  confirmed  2  i n Chapter VIII or  for  the  n e g a t e d once the  loca4  T. and T„ s t a t e s and t h e o r i g i n o f t h e T„ lg 2g 2g 2 s t a t e a r e known. There are s c a t t e r e d d a t a f o r the T, and g s t a t e s , b u t a more s y s t e m a t i c s t u d y i s d e s i r e d . l To look y  :.Cf  tor  the  ficult tion  origin task.  spectra  temperature,  of  the  4  T_  state  zg  spectroscopically is a d i f -  However, c a r e f u l i n v e s t i g a t i o n s o f of  these Cr(III)  say,  at  2°K,  may  c o m p l e x e s a t an r e v e a l the  the  absorp-  extremely  l o c a t i o n s of  low  the  4  origins  of the  T„  state.  2g  (5)  I t i s clear that  important plexes. solvent Since  the  r o l e i n a l l the  environmental structure plays primary processes  However, f o r a f i x e d s o l v e n t , does n o t  the  e f f e c t of v i s c o s i t y of 59  p r o c e s s e s has systematic gradually.  seem t o a f f e c t t h e  been s t r e s s e d ,  s t u d y by  varying  the  the  an  of C r ( I I I )  com-  v i s c o s i t y of  primary processes solvent  on  the  at a l l ,  primary  i t i s i n t e r e s t i n g t o have the  composition  of  the  the  a  solvent  B I B L I O G R A P H Y  140  1.  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(1970).  A P P E N D I X  146  According  to  the f o l l o w i n g m e c h a n i s t i c  scheme  (see  3) : 4 " 2g  l  k  (1)  T  k  2  (2)  2g k  2g  3  Photochemical  -y  S 2g  k  4  2  E  k  -4  k  5  k  E  6  T_  2g  7  (6)  Photochemical  ->  g  an  P  2g k  4  (5)  hv  2g  g  after  (4)  g  g 2  (3)  products  (7)  products  i d e a l i n s t a n t a n e o u s e x c i t a t i o n , suppose t h a t  state  expressed  i s immediately  populated,  the r a t e  only  equations  the  can  be  as d[  ;  Sa -f~ ]  d t  d[ E  = (VVVVf T 4  4  2  ] - k_ [ E 2  4  ]  (8)  ]  2  V  ___  d t  / 1_  .1• .1-} [ E \] r ^ k + k1 -  (W 7  -4  g  1  " V  T-I  1  V  r ?r  ( 9 )  and [ 4 T  2g  ]  =  'SgV  tE 2  ] = 0  (10)  147  The s o l u t i o n s a r e : r n  [ E i] 2  [ T 4  2 g  k  =  4  ] =  f  where  and  e  =  * (k  k  g  =  |(k  k  p  =  k  T  =  k  +  + V  E  k  E  6  +  that  n  =  K  I  f  =  K'[(k  4  -k t  e  a  +  (  V  k  p  )  e  + k  2  T  3  - k  B  t  (12)  }  + k  2  + 4k k_ ]*  (13)  + 4k k_ ]*  (14)  4  2  4  4  4  (16)  4  are p r o p o r t i o n a l  t o t h e con-  therefore,  3 )  - k  t  k  t  a  = 0, t h e n  )  species,  - k )e" a  that  E  (15)  intensities  - e  E  - k  k  7  (e  the case  i s , k_  _ ^  E  + _4  k  of the e m i t t i n g  I  In  k  .... (11)  0  + |[(kT-kE)  T  Since emission centrations  (  - H ( k  + k )  + k  ±  {  -k t , - e 8 }  k  a  5  r  " a  k  k  -k t { e a  ]  V o  [  k  2q o  T  -  (17)  (k  - k )e" B ] k  E  reverse process  (18)  t  p  4 does n o t o c c u r ,  t h e above e q u a t i o n s  c a n be  simplified  to I  p  =  K(e I  If k„,  then  - e " T ) k  f  processes  =  k_  K' ' e  4  (19)  t  - k  T^  :  do o c c u r , b u t k^ i s much g r e a t e r  (20)  than  148  k  a  =  *< E k  V  +  -  • T  - V  ( k  f  1  + f  v  v ( k  =  "  where  A(k  "  k E  + k )  E  4  k  perly  - 4  A  - rp. k . * i s c -4  =  k  + k , + k_ + 6 7  c  5  =  mechanistic well  - k ) E  (1 -  T  we  - k k_ /(k 4  4  E T  1 " -  2  k ) E  cp. )k . isc -4  k . / ( k , + k~ + k_, + 4' 1 2 3 can  scheme  k.) 4  obtain  and  to the energy  changed.  T  "  }  T  k^ E  cf>. 'isc  equally  k  >(k  =  Similarly,  This  -  T  T  k  t h e above transfer  derivations  system  with  c a n be  the terms  applied pro-  

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