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A spectroscopic study of the charge-transfer complex of anthracene and sym-trinitrobenzene 1960

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A SPECTROSCOPIC STUDY OF THE CHARGE-TRANSFER COMPLEX OF ANTHRACENE AND SYM-TRIKITROBENZENE by STEPHEN KENT LOWER B.A., University of California, Berkeley, 1955 a thesis submitted in partial fulfilment of the requirements for the degree of M.Sc. in the Department of CHEMISTRY We accept this Thesis as conforming to the required standard: THE UNIVERSITY OF BRITISH COLUMBIA August, I960 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree th a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood tha t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n permission. Stephen K. Lower Department of Chemistry The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date 22 September, I960 i i ABSTRACT Complexes of aromatic hydrocarbons (donors) with electron acceptors such as 1,3,5-trinitrobenzene (THB) are stabilized principally by resonance between a dative and a "no-bond" wave function: $ N = a fe (DA) • b </f ( d V ) . By means of second-order perturbation theory i t can be shown that, providing certain symmetry requirements are met, the energy corresponding to $>M w i l l be less than that of the free separated donor and acceptor. The resulting molecular complexes have a stab- i l i t y of 2-10 kcal, and usually possess a colour which i s associated with a transition between the ground state and an excited state V„ These charge-transfer spectra have previously been studied only in solution, and have been useful sources of thermodynamic data. In order to learn more of the transition i t s e l f , i t i s necessary to study the spectrum of the crystal, where the molecules are held in fixed and (sometimes) known positions. The procedure by which the polarized crystal spectrum of the anthracene-THB complex was obtained i s briefly described. Compar- ison of the observed spectrum with the partially-known crystal structure has shown i r : 1) The supposition that the transition dipole moment is perpendicular to the planes of the aromatic rings is supported; 2) Vibrational structure can exist i n a charge-transfer band; this i s the f i r s t reported observation of i t ; 3) The band i s s p l i t into two oppositely-polarized components, differing in energy by about 200/cm. An attempt i s made to explain this splitting in terms of Davydov's "weak-coupling" model of a crystal, in which degenerate molecular states are presumed to become crystal states whose degeneracy depends on the symmetry of the associated unit-cell wave functions. Some preliminary steps are described which should eventually lead to the detailed calculation of this effect in the present case. Abstract Approved: i i i TABLE OF CONTENTS INTRODUCTION .1 THEORY OF THE CHARGE-TRANSFER COMPLEX 3 PROPERTIES OF THE ANTHRACENE-TNB COMPLEX.IN SOLUTION . 9 THE POLARIZED CRYSTAL SPECTRUM OF ANTHRACENE-TNB . . . 17 General Principles 1? Experimental Procedure 19 Results 22 Discussion 28 BIBLIOGRAPHY 35 APPENDIX I 37 APPENDIX II 39 FIGURES Fig. 1 Correlation Diagram for the Anthracene-TNB Complex . . . . . . 7 Fig. 2 Probable Orientation in the Complex 7 Fig. 3 Absorption of Anthracene-TNB in Solution . . 13 Fig. 4 Absorption of Anthracene-TNB in Solution . . 13 Fig. 5 Effect of Contact-Charge-Transfer 14 Fig. 6 AH plots for Anthracene-TNB in Various Solvents 14 Fig. 7 Optical System for Polarized Crystal S pectra 21 Fig. 8 Anthracene-TNB, -180°C. 23 Fig. 9 Anthracene-TNB, 23°C. . . . . . . . . . . . . 24 Fig. 10 Anthracene-TNT, -180°C 26 Fig. 11 9,10-Dimethylanthracene-TNB, -180°C. . . . . 27 Fig. 12 The Space Group C2/c 30 Fig. 13 Projection of Unit Cell in (001). 31 iv Fig. 14 Orientation of Transition Dipole Moments . . 32 Fig. 15 Transition Moments along (110); Projection on (100) 33 Fig. 16 Projections of Transition Moments in Unit Cell; Two Possibilities . . . . . . . . . . . . 34- Fig. 17 Graphical Determination of Equilibrium Constant of Anthracene-TNB . . . 3d V ACKNOWLEDGEMENT "Res est arduissima vincere naturam. - my attempt to merely observe i t would not have been possible without the valuable guidance of Dr. R.M. Hochstrasser, to whom I wish to express my gratitude and thanks* I am also indebted to Dr. C. Reid for stimulating my interest in this f i e l d , and to the National Research Council for summer financial assistance. 1 I. INTRODUCTION Molecular compounds or complexes between aromatic hydrocarbons and 1 ,3,5-trinitrobenzene have been known for many years (3,34), and along with the more widely known picric acid adducts have played an important part in the purification and characterization of a wide variety of aromatic compounds. It i s only in recent years that much interest has been shown in understanding their electronic structure, and although the past decade has seen considerable progress in this direction, there i s s t i l l much that i s not understood about the spectroscopic properties of these complexes. The study of such donor-acceptor complexes i s important not only for i t s intrinsic interest, but also for i t s possible application to the general problem of inter- and intramolecular energy transfer, which i s becoming of special concern to the biologist. There i s i n - creasing evidence that donor-acceptor complexes may play an important role in certain biochemical processes. Two recent examples of speculation along these lines deal with two of the most important energy-transfer steps in the l i f e process, namely DPN-DPNH hydrogen transfer and photosynthesis. It i s known (36) that the transfer of hydrogen from substrate to DPN occurs directly at the 4-position of the nucleotide ring, and Chance (6) has given evidence for the existence of DPNH-enzyme and DPN-substrate complexes. Kosower (19) has found that the course of substitution of the 1-methylpyridinium ion i s influenced by the complexing ability of the donor, and he discusses the significance 2 of this result in the light of Chance's findings. Piatt (35) has suggested a new photosynthetic pathway involving a complex of the type DONQR--(carotene) —ACCEPTOR as the "energy sink", the completing being responsible for placing the energy of the carotene below that of the chlorophyll. He suggests various ways in which this or other similar complexes might act as active intermediates in photosynthesis. The present investigation i s a prelude to what is hoped w i l l be a f a i r l y detailed study of the polarized absorption and emission spectra of several polyacene-THB* complexes. The immediate need for this study arises from the question of the origin of the emission spectrum of the anthracene-TNB complex. The earlier idea (38) was that the emission i s from the triplet level of the hydrocarbon, and i s thus really donor phosphorescence, the T-*S forbiddeness rule being broken down by the strong f i e l d of the TNB. Later work (9a) suggested that the emission consists of a fluorescence component > (reverse of C-T absorption) and a superimposed polyacene phosphor- ; escence. The current status of the problem, as summarized by McGlynn (20, 20a) is s t i l l one of some uncertainty. The f i r s t part of the present investigation consists of a study of the properties of the anthracene-TNB complex in solution. The major part deals with the absorption spectrum of the anthracene-TNB crystal. *Abbreviations used here are as follows: TNB = 1,3,5-trinitrobenzene; C-T = charge-transfer; D= donor; A = acceptor; T= triplet; S = singlet. 3 I I . THEORY OF THE CHARGE-TRANSFER COMPLEX Molecular complex formation has long been thought due to the combination of an electron donor (D) with an acceptor (A), Weitz (42,43) and Weiss (40) suggested an ionic structure D+A~ and predicted that a low ionization potential for the Lewis base D and a high electron affinity for the Lewis acid A would lead to a stable complex. Brackmann (3) came much closer to the present concept by ' proposing resonance between a "no-bond" and a bonded structure in which the color of the complex (assuming colorless components) i s due to the complex as a whole, and not localized to any one component. The presently accepted ideas on the nature of the donor-acceptor complex were set forth by Mulliken in a series of papers (22-30) beginning in 1950. His original concern was with complexes of the benzene-iodine type (22), but he later (25) generalized his theory to include a l l charge-transfer complexes of the weak type.' The - theory was applied specifically to the TNB-polyacene complexes by McGlynn and Boggus (20). The following theoretical treatment applies specifically to the TNB-anthracene system. The wave function for a molecular compound DA in i t s ground state can be written a ft, (DA) + b ft (D+A") + c - r i ( D V ) , (1) where ro i s the "no-bond" wave function and includes the various classical intermolecular forces such as dipole-dipole and dipole- induced dipole interaction, hydrogen bonding, etc. The dative-bond wave functions ft and h. correspond to electron transfer from one member to another, resulting in the establishment of a weak 4 covalent bond involving the odd electron remaining in D and A* In the present treatment, a»b, and £ can be taken as zero. Cor- responding to ^ , there i s also postulated an excited-state wave function given by § £ = a » ^ + b»$£ (2) where a ' ~ a, b'~ b. Starting with the ground-state wave function written as = a Hf + b K , (3) we define S12 a jVfc > H12 = /#V*4 <4'5) The energy is given by ^ E JW¥77  (6> Substituting with (3) and simplifying, we find or E(A . * S \ , - » - 2 a t S ( i + fc*S„.) = a? H<if 2 a k f U + b lWn . ( 7 a) We wish to find the coefficients which minimize the energy* Taking f i r s t the derivative with respect to a, and setting i t equal to zero, we obtain 3 * v ' . . . ( 8 ) and E is a minimum when F ( 2 4 S „ t 2 l S , z ) a 2 a H a + 2 . k H i x . ( 9 ) Similarly, minimizing E with respect to Jb, we have E ( 4 S a + t 5 i t ) = a H u + l H » i . . (10) 5 Equations (9) and (10) can be rewritten A ( H « - S „ E ) + b ( H i x - S a C ) - O (Hx, - S l t E) + \> (Hti - S l t E ) = O which are satisfied when the coefficients a and b are such that (9a) (10a) = O (11) In the complex, the hamiltonian operator ~H° for the separated molecules i s perturbed by their interaction, giving rise to a new operator (12) and we seek the corresponding energies, given by * §i » *i $i • (13) The secular equation (11) now becomes H : - ( E > F , * ) HU-es — o H a - & s et-e". <i4) where E has been replaced by in the second diagonal element, and H^2 has been dropped ( i t i s small compared to E^ - fig )• The determinant i s evaluated by multiplying the second row by (H« 2 - ES)/(Eg - E x) and subtracting i t from' the f i r s t : Hi, - A £ - = o (15) ( H i - Ef») = O 6 and since the energies are non-degenerate, Following the usual convention, we replace by WQ, the energy of the structure DA, and Eg by Ŵ , the energy of D+A~. The ground- and excited-state energies are Wn and WQ, respectively. The relations between these quantities are shown in the simplified correlation diagram of Fig. 1. Equation (16) now becomes o • (Hot - V A ^ S ) * " - E-Ejo = W „ - W 0n= Wo w , - ^ c ' (17) Similarly, for the excited state deriving from eq. (2), we have The ratio of the coefficients i s easily found. From (14), or, again since E^W^ i s close to tfQ, i i ( w t - W o ) -»-b ( H ' ^ - l A / o ' S ) - O ( 2 0 ) whence " ~ ' (21) In the above equations, WQ includes the energies of the separated donor and acceptor molecules as well as a l l the various attractive and repulsive intermolecular forces. In addition, also includes an attraction energy of covalent and ionic bonding. The difference between W, and W i s the sum of the donor ionization l o potential and acceptor electron affinity. The ground-state resonance 5 0 0 0 0 D + , A" - W, 3 0 0 0 0 E N E R G Y , C M " 1 10 0 0 0 WN D.A Wo CORRELATION DIAGRAM FOR THE ANTHRACENE-TNB COMPLEX FIG. I O m O PROBABLE ORIENTATIONS IN THE COMPLEX FiG.2 8 energy i s the difference between tfQ and Ŵ , and w i l l be-large i f p ( H Q 1 - SWQ) is large and - WQ i s small* In the case of r e l - atively weak complexes of the type under discussion, WQ - should be 0-10 kcal (20b). It i s seen that $ Nhas mostly no-bond char- acter while $ 6 is mostly ionic; the transition connecting the two ' states thus corresponds to the partial transfer of an electron from the donor (hydrocarbon) to the acceptor (TNB). If we assume that the resonance energy (W G - W^) is the primary stabilizing force in the complex, then HQ and S must be non-zero, requiring that $ 0 and $»be of the same symmetry species (in the point group of the complex). TNB most probably belongs to the point group D^, and i t s ground state (considering only the tt -electrons) i s ...... (.aipZW)\aQ2ien)\ 1AV while that of the anion TNB" i s probably ... ( a j p ^ e ' O ^ C a j p ^ e " ^ ^ ) 1 . 2B n. Using the perimeter method of Mofitt (21), the symmetry species of the ground state of p the anthracene cation has been assigned as Bgg (°2h^ (20a)• T n e molecular complex, having only one plane of symmetry, belongs to C£ n or (both usually called C g; see Fig. 2). Inspection of the appropriate character tables shows that Bgg transforms like A" and p E" like A" + A', the latter being the only two representations of G . s Thus T$o r*. T$( = A'A'A' = A ' A ^ = A', and the transition i s allowed by symmetry. 9 III. PROPERTIES OF THE ANTHRACENE-TNB-COMPLEX IN SOLUTION Previous studies of molecular complexes have been carried out in solution, and since thermodynamic data so obtained are of import- ance in understanding charge-transfer bonding, some consideration of the TNB-anthracene complex in solution seems pertinent here. It has been studied by Bier (2a) and Briegleb (5) in carbon tetrachloride and benzene. Their work has been repeated here using these solvents and 1,3-dioxane and ethanol in addition. The method of Benesi and Hildebrand (2) was used to determine the equilibrium constants. This assumes the relation (for a 1:1 complex) to hold, where Xg i s the mole fraction of complex, and X^ and X^ are the i n i t i a l mole fractions of donor and acceptor, respectively. From the Beer-Lambert law, where D i s the optical density, (C) i s the concentration of complex; and € the extinction coefficient of the complex at the observed wavelength. This method i s generally applicable to a 1:1 complex AB where an absorption band exists which i s characteristic of the complex and outside the regions where A and B absorb separately. Substituting concentrations for mole fractions and making B>> A, (22) becomes K = x G ) ( x A . x c ) (22) D = log ( l 0 / D = ( O i e (23) K = C (24) (B - C)(A) or C = KAB - KAC (24a) 10 Inserting (23)» D/16 + (D/16 )KA = KAB (25) which can be written I Te * e = T * ( 2 5 a ) If BL/D is now plotted as a function of 1/A, there should result a straight line of slope 1/K6 and intercept 1/6 . The TNB used was Eastman 639 "white label" and was recrystallized from alcohol and water. Neither a solid melt nor a 0.1M solution in O chloroform (1 cm cell) showed any absorption above 4400A. Because of i t s higher solubility and lower cost, i t was (in most cases) used as "B" in eq. 25a. The reagent grade anthracene was also recrystall- ized from alcohol. The carbon tetrachloride, benzene, 95% ethanol,• and dioxane were a l l redistilled prior to use. The dioxane was additionally purified by the procedure given in~Tfeissberger.(41) . Absorption measurements were made on a Gary model 14 recording spectrophotometer, in 1- or 5-cm cells, with the hydrocarbon component of the complex in the reference beam at the same concentration as in the complex in the sample c e l l . Runs were made at three or four temperatures, held constant (within 0.5° C by means of thermostatted c e l l holders. In most cases five concentrations of TNB were used, as well as a blank containing only hydrocarbon. The scanning was begun at a point well above the t a i l of the CT absorption, and was continued at 20A/sec through the peak and into the aromatic bands. The absorption readings were made to three significant figures, but only the f i r s t two are really dependable. Details of the calculations are presented in tabular form in the appendix, along with the graphic a l l analysis of a typical run'. As well be discussed below, there i s some uncertainty in the value of £ , and this quantity was taken as constant 11 for a given complex-solvent combination. The equilibrium constants obtained in this way were used to calculate AH values, by means of the familiar relations -AP = ST In K (26) / 9JLK\ (27) X _ AH JLO/r) ' * (28a) Before considering the above results, i t i s perhaps pertinent to note some of the recent discussion (1, 20b) regarding the validity and meaning of data obtained by the Benessi-Hildebrand method. Especially noteworthy is the contribution of Orgel and Mulliken (32) in which they discuss the effects of formation of more than one 1:1 complex with different orientations, and of "contact" CT spectra. 1 They show f i r s t that the conventional Benessi-Hildebrand method applied to measurements made at One temperature does not permit one to distinguish between a system in which one well-defined complex exists, and one in which several (differently oriented) complexes: are present. The observed properties of complexes are sta t i s t i c a l averages over a l l possible configurations in thermal: equilibrium, and especially in relatively loose complexes like these, there i s a strong likelyhood that the observed equilibrium constant K* i s made , up of several terms: K8 s S K . i 1 12 One cheek on this is to plot values of log K* against 1/T. The resulting graph should be a straight line only i f one complex i s present (or i f a l l the are identical). Another indication of multiple configurational complexes would be temperature dependence of the effective oscillator strength. The present results do not clearly show such an effect of any magnitude, although the small number of points taken, their scatter due to experimental error, and the relatively narrow temperature range would obscure a small effect. There i s some indication (in two independent runs) that the apparent £ H of anthracene-TNB in carbon tetrachloride i s higher at lower temperatures, but further discussion of the matter should await more detailed experimental work. One of the greatest drawbacks of the B-H method i s that values of e so determined for weakly absorbing complexes are-subject to considerable uncertainty. In a given system, considerable scatter of results (showing no trends) was noted for observations at different temperatures, and i t was decided to take an "average" or "most prob- able" value of € for a given complex-solvent system. There i s really no reason why i t should be constant with temperature and certainly not with the solvent, but the present technique does not provide the accuracy required to discuss these effects even qualitatively. The second point made by Orgel and Mulliken i s that intermol- ecular charge-transfer spectra can occur as the result of specific interactions between donor and acceptor molecules through mere physical proximity, and that such interactions can exist- even i f the equilibrium constant for complex formation approaches zero. 13 4.8 4.0 3.2 L O G € 2.4 - 1 8 0 ° D A T A OF C Z E K A L L A 16 24 32 40 48 FIG. 3 C M " ' x I 0 3 ABSORPTION OF ANTHRACENE-TNB IN SOLUTION looo i 8 0 0 6 0 0 400 2 0 0 FIG. 4 14 i A A C T U A L O B S E R V E D D U E T O C O N T A C T E F F E C T OF C O N T A C T „ H A R G E - T R A N S F E R F I G . 5 3-0 25 20 O C C l E T H A N O L D I O X A N E 3-6 &-H P L O T S F O R A N T H R A C E N E - T N B IN V A R I O U S S O L V E N T S F I G . 6 15 Mulliken explains (30) contact charge-transfer spectra on the basis that the acceptor orbital of the anion may exceed the van der Waals size of the neutral acceptor molecule, permitting a donor molecule to interact with i t while s t i l l outside the van der Waals distance of approach. Orgel and Mulliken discuss the effects of such contact charge-transfer complexing in terms of a simple st a t i s t i c a l model. The most significant result i s that such behavior would result in a serious overestimation of 6 ; i f the equilibrium constant approaches zero and a l l absorption i s due to contact charge-transfer, the 1/ 6 intercept occurs at zero and the results lose their meaning, as shown in Fig. 5* page 14. These points are mentioned here to bring out the fact that the interpretation of charge-transfer spectra in solution i s even more uncertain than in the case of "conventional" electronic spectra, and i t i s correspondingly more necessary to be cautious in applying results of such studies to .other problems. A good example of this i s the effect of the solvent on the heat of formation, as determined from the equilibrium constant' data and the graph of Fig. 6. In carbon tetrachloride, which should presumably show the least interaction with the complex, i t i s 4.4 kcal. In the other three solvents the value i s between 2-3 kcal, and since i t i s known that TNB forms complexes with benzene (16,17), ethanol (20a), (and no doubt with dioxane), i t i s obvious that we are not looking at anything as simple as the 1:1 TNB-anthracene complex alone. The same effect was noted in a similar series of experiments with the benzpyrene complex, in which the heat of formation was' found to be 6.2 kcal in carbon tetrachloride and 3*3 kcal in benzene. Other 16 authors have noted similar effects, and Kosower (19) finds a correl- ation between the heat of formation of the charge-transfer complex and the "Y-value" of the solvent. 17 IV. THE POLARIZED CRYSTAL SPECTRUM OF TNB-ANTHRACENE The study of crystal spectra affords one the opportunity of examining molecules which are held in fixed positions, and in favor- able cases where the erystal structure i s known, the use of polarized light often permits one to find the direction of the transition moment, t and to make important decisions regarding the assignment of symmetry species to the electronic states involved. However, the very forces which hold the molecules in position give rise themselves to effects which radically change the appearance of the spectra from the vapor or liquid states. Besides a considerable shift (to the red) and change in intensity, other effects arise, such as mixing of nearby strong and weak transitions, and splitting of a f a i r l y intense transition into two components, differing in intensity and polariz- ation (Davydov splitting). To understand these phenomena i t is necessary to consider the entire crystal as a molecule, and to replace "molecular states" by "crystal states", where the symmetry properties of the unit c e l l assume primary importance'. This view was f i r s t applied to ionic crystals by Frenkel (14,15) with much success, and was later applied to molecular crystals by Davydov (12,13; see Craig, 8). The simplest view of a crystal which Davydov proposes i s the "oriented gas model" in which there i s no interaction between the molecules. More practical for a l l but the weakest transitions i s the "weak coupling" model in which a small interaction occurs, which however i s not strong enough to perturb the electronic structure of the individual molecules. Thus the crystal ground state can be 18 written as a product of the individual molecular wave functions - <P, <2a. <Pj •'" ?N (29) while in the excited state, any one of the molecules can he excited: = <?, *et • <pw (30) Interaction of these results in crystal states (unit c e l l wave funct- ions) composed of linear combinations of the : <§* * vfe" ( 4t f + #3 + " ') in which the combinations of molecular wave functions must transform like representations of the unit c e l l symmetry group i f the transition i s allowed. Davydov considers the electrostatic interaction energy between two molecules (a) and (b) which arises from the coulombic interaction between the electrons (e) and the nuclei (n). and which can be roughly expressed as V l , 2 = n l e 2 ' V l + 8 l e l + n l a 2 ( 3 2 ) Further elaboration of the theory (8) results in the summation of these terms over a l l atoms in the molecule, the derivation of a crystal wave function, and finally in an expression for the shift in peak position from the gas to crystal state, and the splitting 19 between oppositely polarized components of corresponding peaks, which can be written d E ° M M * = (constant) + H I j p • + (33) where the 1 ^ are interaction integrals between the jth and kth molecules, p runs over a l l molecules translationally equivalent to it and the q, r, etc, run over a l l molecules related to 3 by the other symmetry operations of the unit c e l l group* The summed interaction integrals are taken in linear combinations corresponding to those of eq* 31 which are allowed. The integrals I ^ are given by (34) I j k = " r 3"" l M l 2 c o s e j l c o s Q j 2 ~ c o s e j 2 C 0 S 9k2 " c o s Q j 3 c o s ®k3 °2 in which M i s the molecular transition moment (in A ) and r., i s the distance between their centers. The angles 6^, 9^, etc., are angles made by the transition moment with an orthogonal set of axes erected at i t s center, with 6^ referring to an axis along the line connecting the centers of the two transition moments* 1 Experimental. Both the anthracene and' TNB were Eastman reagent grade* The former was recrystalllzed twice from alcohol, and the latter from water. The complex was prepared by combining a hot alcoholic solution of anthracene with a hot concentrated solution of TNB, and allowing the mixture to crystallize in the dark; The orange crystals were removed by fi l t r a t i o n and recrystalllzed from concentrated alcoholic TNB. 20 The samples were prepared by melting a small quantity of the solid complex between two s i l i c a discs (or a s i l i c a disc and a micro- scope cover glass) and allowing the melt to cool under pressure* In order to obtain large uniform areas, the sample was usually partially remelted and cooled several times, the unmelted areas being used to seed the remelted ones. Examination through the polarizing micro- scope located an area of sufficient size which showed uniformity and complete extinction. The sample disc was then oriented on a brass > disc so that the pinhole (# 60 d r i l l ) in the latter corresponded with the area to be examined. The brass disc and sample were then placed in a Oewar c e l l in such a manner that the direction of extinction corresponded to either the horizontal or vertical axis of the optical system. ' The spectrograph was* a Hilger medium quartz prism instrument, equipped with a Hilger motor-driven scanning attachment containing a 1P28 photomultiplier which was operated at 800 volts from*a Gintel stabilized supply. The detector was a Keithly Model 200 B electro- meter equipped with a decade shunt. The output of the meter was connected to a standard Leeds & Northrup chart recorder. The photo- —8 tube currents varied from full-scale meter readings of 2x10* to 2x10"^ amperes. Ranges were changed by altering the shunt resistance. The optical system i s shown diagramatieally in Fig. 7. The light source was a six-volt, 18 ampere air-cooled projection lamp, operated from a stabilized ac supply. A converging lens focussed a spot of light onto the pinhole covering the sample. The emerging light was collected and made parallel by a second lens, then passed through a Wollaston prism. The resulting two beams were then 21 i t TUNGSTEN L A M p DEWAR W O L L A S T O N SPECTRO- SAMPLE CELL PRISM GRAPH SLIT OPTICAL SYSTEM FOR POLARIZED CRYSTAL SPECTRA FIG. 7 22 focussed on the entrance s l i t of the spectrograph; The polarization to be used was selected by rotating the Wo1laston prism so that only the desired beam f e l l on the s l i t . Before running a spectrum, the optical system was adjusted so as to give equal intensities in each polarization at a wavelength well separated from the C-T absorption. This adjustment was found to be very c r i t i c a l , and after making i t care was taken to avoid disturbing any part of the optical system except the Wollaston prism. After recording the two spectra, the sample was replaced by a simple aperture and the spectrum of the light source and phot©multiplier response were determined. Through- out the process, care was taken to avoid diffraction patterns and spurious effects due to chromatic aberrations of the simple lenses used. In interpreting the results, i t was assumed that the trans- mitted light at a wavelength outside the region of absorption was the same for both sample and reference beams. The readings of sample beam intensity were then corrected to zero optical density at this wavelength, and log I Q A calculated. Results. Several spectra were run, and the results are in substantial agreement. As expected (see7 following section on crystal structure), the absorption i s more intense along the long axis. The absorption commences near 19000/cm and rises sharply to form a broad band, the most interesting feature of which is the presence of a 1400/cm vibrational interval, probably corresponding to an anthracene ring breathing vibration. This is the f i r s t reported observation of vibrational structure in a charge-transfer band. A second point of 23 M I L L I M I C R O N S 600 550 500 450 4 0 0 360 2 7 5 , 1 f 1 , _ , H 2-50 2-25 2 0 0 DENSITY ANTHRACENE- TNB. -180° FIG. 8 24 M I L L I M I C R O N S tOPTICAL DENSITY C M " 1 x 1 0 - 3 ANTHRACENE-TNB. 23* c. FIG. 9 25 interest i s that the peaks in the short-axis polarized absorption are located 100-200 /cm to the red of the corresponding peaks of the other component. These shifts, though not much greater than the accuracy and reproducibility of the method ( 100/cm), are always seen. They appear to be quite sensitive to the nature of the crystal; preliminary experiments with a crystal of anthracene containing a small amount of complex show a reversal of the shift. The spectrum shown in Fig. 8 i s most representative. The apparent absorption to the red of 16000/cm i s attributed to reflection losses, which are expected to change with wavelength. Thus the apparent optical density should be corrected by extrapolation of the reflection loss. Making such a rough extrapolation out to the center band (21700/cm) and subtracting the reflection from the optical densities of the two peaks, at this wavelength, we arrive at a very approximate optical density ratio (not polarization ratio) of 3:1. Fig. 9 shows the spectrum of the same sample at room temperature. The expected broadening obliterates the detail of the peaks, although some structure remains in the strongly-absorbed polarization. The optical density ratio i s approximately the same. Preliminary studies have been made on the 9,10-dimethylanthracene- TNB complex, and of anthracene-TNT (2,4-,6-trinitrotoluene), and their spectra are shown in Figs. 10-11. The apparently small optical < density ratio i s probably a consequence of the rather thick crystals used, and further work on these complexes i s necessary. It i s very di f f i c u l t to obtain good single crystals of either of them. 26 M I L L I M I C R O N S 2-75 2-50 2-25 2 0 0 1-73 1-50 1*00 0*75 0-50 0-25 O P T I C A L D E N S I T Y C M - 1 x | 0 ~ 3 ANTHRACENE-TNT. -ISO* FIG. 10 27 M I L L I M I C R O N S 2-75 2-50 2-25 2 0 0 1-75 1-50 1-25 1-00 0*75 0-50 0-25 O P T I C A L D E N S I T Y C M " 1 x 1 0 - 3 FIG. 9,10- DIMETHYL ANTHRACENE - T.N.B. -180* FIG. II 28 Discussion. The orange acicular prisms of anthracene-TNB complex (as obtained) by crystallization from solvents) shov a strong dichroism when examined in polarized light, the orange color being most intense when the light i s polarized along the long axis of the crystal. Similar examination of a thin section of the needle (looking down the long axis) f a i l s to reveal any dichroism. Since the C-T transition presumably occurs with a moment perpendicular to the planes of the anthracene and TNB molecules, these results suggest that the crystal is made up of such units stacked up along the long (c) axis. The relative intensities of absorption in the two polarizations also indicate such a structure. Detailed interpretation of the energy splittings cannot yet be given, as the crystal structure of the complex i s not completely known, and certain questions regarding Davydov splitting calculations and the effect of permanent dipoles (as in TNB) on the splittings are currently being studied and discussed. The calculations summarized below are to be regarded purely as preliminary steps toward the explan- ation of the observed effects. The crystal structure of the complex has been partially determined by Wallwork (39) # who states that the crystals are monoclinic, = 1331&0. and belong to the space group C2/c (Cg^)* with eight molecules (i.e., four complexes) per unit c e l l . The molecules are stacked alternately in a staggared manner in columns parallel to the c_-axis, but t i l t e d by about 6° from the position in which they would be perpendicular to that axis. The unit c e l l dimensions are a = 11.69 A, b = 16.36 A, o o and C Q = 13.23 A. The special positions are assigned as follows: 29 %%0i KM. % fc*yt%i Fig. 12 shows the general point positions and symmetry operations of the space group c2/c, and Figs. 13 and 15 show projections of the unit cells (as deduced from Wallwork's data) in the ab and be planes, respectively. Without knowing the value of "y", the exact locations of the TNB molecules cannot be known; in the present calculations i t has been assumed to be 0.17 A, which would be the distance required to t i l t the line connecting the centers of the anthracene and TNB molecules by 6°. It i s further assumed that the molecules are inclined along the b axis, and that the transition moments are perpendicular to the planes of the aromatic rings. There is some question as to just what constitutes a complex "molecule" in this crystal, and i t i s defined for the purposes of the present discussion as the pair of centers between which the charge-transfer process i s taking place. If we assume that this process w i l l occur between parallel molecules in preference to , those inclined 12° with respect to each other, then a knowledge of the relative directions of inclination of the TNB and anthracene would f i x the location of the complex "molecules" in the crystal. Thus in Fig. 16 (pg 33)» the transition moments are drawn in two different ways, using the same locations of the individual molecules. The calculations summarized in Appendix II. are based on the relation ships of Figs. 16a, 14 and 15t and on a transition moment length of 0.88A as estimated from the absorption spectrum of Fig. 4 and the Anthracene: 0,0,0; 0,0,%; TNB: 0,y,%; 0,y,%; 30 O Oi- i + O o 1 3 4 ' 4 I 3 4 ' 4 THE SPACE GROUP C 2 c - C l H FIG. 12 31 PROJECTION IN (OOI) (UNIT CELL SHADED) FIG. 13 32 ORIENTATION OF TRANSITION DIPOLE MOMENTS ( N O T T O E X A C T S C A L E ; I N C L I N A T I O N O F M O M E N T S E X A G G E R A T E D ) FIG.14 33 TRANSITION MOMENTS ALONG (110); PROJECTION ON (100) L A R G E C I R C L E S R E P R E S E N T A N T H R A C E N E L O C A T I O N S UNIT CELL SCALE FIG. 15 34 equilibrium constant data. The values of the interaction integrals between molecule "0" (Fig. 14) and the other fourteen adjacent molecules are calculated and given in Appendix II. The integrals between molecules related to each other by the same space group operations are summed in the table below: molecule "0" trans- operation XT 1^, cm~* translation + ?60 formed into no.: 3,4; 11,12. 1,7. £ screw axis - 2440 2,6,8,5; 10, 13. 14, 9. a + c / 2 s l i d e + 1 2 3 0 Further interpretation of the splitting awaits the completion of work now in progress. It should be mentioned that there i s reason to feel that the relations shown in Fig. 16b may be more likely, and i t i s hoped to confirm this shortly. Y -\ v A L - ^ t 8 / Fig. 16. PROJECTION OF TRANSITION MOMENTS IN A UNIT CELL ON (100). Two possibilities are shown; part A i s identical with Fig. 15* TNB molecules are at heads of arrows. 35 BIBLIOGRAPHY 1. ANDREWS, L.J.: Ghent. Revs. 5^ . 713 (1954). 2 . BENESI, H.A., and J.H. HILDEBRAND: J. Am. Chem. Soc. 7JL, 2703 (1949). 3 . BRACKMAN, W.: Rec. trav. chim. 68, 14? (1949). 4. BRIEGLEB, G.: Zwischenmolekulare Krafte: G. Braun, Karlesruhe, 1949. 5. BRIEGLEB, G., and T. SGHACHOWSKOY: Z. Physik. Ch. B19., 255 (1949). 6. CHANGE, B.: Mechanism of Enzyme Action: Johns Hopkins Press, Baltimore, Md., 1954. Ppg 433-452. 7. COULSON, C.A.: Valence: Oxford, 1952. Ppg 58-6?. 8. CRAIG, D;P.: J. Chem. Soc. 1955. 539. 9 . CZEKALLA, J.: Die Naturwissenschaften 20, 467 (1956). 10. CZEKALLA, J.: Z. Elektrochemie 61, 537 (1957)* 11. CZEKALLA, J.: Z. Elektrochemie 63_, 1157 (1959). 12. DAVYDOV, A.S.: Zhurn. Eksp. Teor. Fyz. 18, 210 (1948). 13. DAVYDOV, A.S.: Theory of Molecular Excitons, trans. M. Kasha: McGraw-Hill, N.Y. (in press). 14. FRENKEL, J.: Phys. Rev. 3JL, 17 (1931). 15. FRENKEL, J.: Fyz. Zhurnal Sovjet. 9., 958 (1936). 16. HAMMICK, D., G. HILLS and J. HOWARD: J. Chem. Soc. 1932. 1530. 17. HEPP, P. Annalen 215. 344 (1882). 18. KETELAAR, J.A.A., et a l . : Rec. trav. chim. 2k* H04 (1952). 19. KOSOWER, E.M.: J. Am. Chem. Soc. 80, 3^6? (1958). 20. McGLYNN, S.P., and J.D. BOGGUS: J. Am. Chem. Soc. 80, 5096 (1958). 20a. McGLYNN, S.P..: Chem. Revs. 5 8 , 1113 (1958). 20b. McGLYNN, S.P., J.D. BOGGUS, and E. ELDER: J . Chem. Phys. 22, 357 (I960). 21. MOFITT, W. J. Chem. Phys. 22, 320 (1954-). 36 22. MULLIKEN, R.S.: J. Am. Chem. Soc. 22, 600 (1950). 23. MULLIKEN, R.S.: J. Am. Chem. Soc. £2 , 4493 (1950). 24. MULLIKEN, R.S.: J. Chem. Phys. 19., 514 (1951). 25. MULLIKEN, R.S.: J. Am. Chem. Soc. £ 4 , 811 (1952). 26. MULLIKEN, R.S.: J. Phys. Chem. £ 6 , 801 (1952). 27. MULLIKEN, R.S. Symposium on Molecular Physics. Nikko, Japan, 1953. 28. MULLIKEN, R.S.: J. chim. phys. 52, 341 (1954). 29. MULLIKEN, R.S.: J. Chem. Phys. 23., 397 (1955). 30. MULLIKEN, R.S.: Rec. trav. chim. 21* 845 (1956). 3 L NAKAM0T0, K.: J. Am. Chem. Soc. 24, 390 (1952). 32. ORGEL, L.E., and R.S. MULLIKEN: J. Am. Chem. Soc. 22, 4839 (1957). 34. PFEIFFER, P.: Organisehe Molekulverbindungen, 2nd. ed. F. Enke, Stuttgart, 1927. 35. PLATT, J.R.: Science 129. 372 (1959). 36. PULLMAN, M., A. SAN PIETRO, and S.P. COLOWICK: J. Biol. Chem. 206. 129 (1954). 37. REID, C.: Excited States in Chemistry and Biology: Butterworth's, London, 1957. Ppg 113-118. 38. REID, C.: J. Chem. Phys. 20, 1212 (1952). 39. WALLWORK, F.: Acta Cryst. £ , 648 (1954). 40. WEISS, J.: J. Chem. Soc. 1942. 245. 41 . WEISSBERGER, ed.: Technique of Organic Chemistry, Vol. IV: Organic Solvents: Interscience, Nev York, 1955. Pg* 371. 42* WEITZ-HALLE, E.: Z. Elektrochem. 3jt. 538 (1928). 43. WEITZ-HALLE, E.: Angew. Chem. 66, 658 (1954). 44. WOODWARD, R.B.: J. Am. Chem. Soc. 64, 3058 (1942). 37 APPENDIX I DETERMINATION OF THE EQUILIBRIUM CONSTANT OF THE TNB-ANTHRACENE COMPLEX BY THE METHOD OF BENESI AND HILDEBRAND. Solvent: carbon tetrachloride, Cells, 1 cm. Stock solutions: Anthracene ("B"), 0.0337 M; TNB ("A"), 0.0119 M; concentration of TNB in each sample was 0.953 mM. 296°K 2?8°K 331°K ml "A" A 1/A D B/D D B/D D B/D 10 .0135 74.0 .168 5.6? .266 3.58 12 .0162 62.7 .193 4.94 .309 3.08 .460 10.35 14 .0189 52.9 .211 4.51 .348 2.74 .533 8.94 16 .0216 46.3 .241 3.96 .392 2.43 .599 7.95 18 .0243 41.1 .270 3.53 .420 2.27 .664 .7.18 20 .0270 37.0 .294 3.24 .453 2.10 .733 6.50 22 .0297 33.7 .320 2.98 .484 1.97 .800 5.95 These results are plotted in Fig. 15 on the following page. The average intercept is taken as 1/ = (0.70 _+ .05) x 10~^t or = 1430 + 100. The equilibrium constants are calculated below. 278°K: slope = 1/K = .0485 x l o " 3 ; K = .70/.0485 = 14.5. , 296°K: K = .70/.0775 = 2*0. \ i ' 331°K: K = .70/. 119 = 5.9. s : ~ The values for l:H of TNB-Anthracene in various solvents are calculated to be the following (see Fig. 6): Benzene 2.7 kcal Ethanol 2.5 Carbon Tetrachloride 4.4 Dioxane 2.3 38 39 APPENDIX II. NOTES ON THE CALCULATION OF THE ENERGY SPLITTING The direction cosine between two lines J, and k in terms of the orthogonal coordinates (1,2,3) is given by cos JPj k = cos 8ji c o s ®iji + c o s 9 j 2 e o s 9k2 + c o s 9 j 3 c ^ 8 ®k3 * By the use of this relation we can simplify Eq. 34-: e 2 I3k = * r5*" W 3 c o s e j l c o s 6 k l " c o s *jk* ( 3 4 a ) jk Thus we must calculate the angle between the two transition moments, and the angles between each moment and the line connecting their centers. This problem is in general quite laborious, but is consid- erably simplified when we assume (as we do here) that all the trans- ition moments lie in the same plane or in parallel planes. The calculation of 1^. for the various interactions (j =0, k s 1-14; see table 14 for numbering of adjacent moments) is summarized in the following table. The transition dipole moment is estimated at 0.88A. k * cosOjk cos Otj 3ab 3ab-eos r xlO r jk 3. 11 -.232 +.232 -.161 -1.16 1090 + 190 4, 12 +.232 -.232 -.161 -1.16 1090 + 190 1. 7 +.999 +.999 +.987 +1.98 289 -1220 2* 10 -.009 -.009 -.006 -0.98 810 + 216 6, 13 +.540 +.540 -.572 -1.55 3370 + 82 8> 1* +.280 -.280 -.384 -1.36 985 + 246 5, 9 -.472 -.472 -.395 -1.37 3500 + 70 * value of k in I^k, with j = 0.


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