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Sensing of carbon monoxide via luminescence of ruthenium complexes containing a hemilabile phosphine… Thorne, Lisa Margaret 2004

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SENSING OF CARBON MONOXIDE VIA LUMINESCENCE OF RUTHENIUM COMPLEXES CONTAINING A HEMILABILE PHOSPHINE PYRENYL ETHER LIGAND by LISA M A R G A R E T THORNE B. Sc. (Honours), Queen's University, 2002 A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in T H E F A C U L T Y OF G R A D U A T E STUDIES (Department of Chemistry) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH C O L U M B I A July 2004 © Lisa Margaret Thorne, 2004 Library Authorization In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Name of Author (please print) " Date (dd/mm/yyyy) /T,tle0fTheSiS: Stxsln*.^^ D e g r e e : - M - ^ c Year: ^QCW D e p a r t m e n t o f f h ^ i J r n The University of British Columbia Vancouver, BC Canada Abstract Our laboratory previously reported a R u 2 + complex with a hemilabile phosphine ether ligand fcc-RuCl2(POC4Pyr-P,0)2 (14) (POC4Pyr = 4-{2-(diphenylphosphino)phenoxy}butylpyrene). This complex reacts rapidly with carbon monoxide to produce a significant luminescence response in which the monomer: excimer emission ratio changes dramatically. The reaction with CO is accompanied by a geometric isomerization from the initial ?ra«s-dicarbonyl product, ttt-RuCl 2(CO) 2(POC4Pyr-P) 2, to the cis-dicarbonyl product, crt-RuCl 2(CO) 2(POC4Pyr- JP) 2. However, an OFF-ON response was not achieved. Two methodologies are explored in this thesis to obtain the desirable OFF-ON response by promoting pyrene fluorescence quenching before exposure to CO. Attempts are reported to synthesize ligands with tether lengths less than four carbon atoms in length in order to probe the effect of tether length between the pyrene moiety and the rest of the complex on fluorescence quenching. Although a ligand with a one-carbon tether ({2-(diphenylphosphino)phenoxy}methylpyrene) was not successfully synthesized, a ligand with a two-carbon tether (2-{2-(diphenylphosphino)phenoxy}ethylpyrene) was obtained. However, attempts to coordinate this ligand with Ru were unsuccessful. A second strategy explored in this thesis is the effect of changing the halide ligands of complex 14 to either bromo (22) (tec-RuBr2(POC4Pyr-P,0)2) or iodo ligands (23) (fcc-Rul2(POC4Pyr-P,0)2). The different halogen ligands are expected to change the energies of the metal d-orbitals and possibly result in fluorescence quenching. This series of complexes with different halogen ligands has been fully characterized by N M R , i i MS, UV-vis and fluorescence spectroscopies and their reactivity toward CO studied. A similar geometric isomerization from the ttt isomer to the cct isomer was observed for 22 and 23 as for 14. Furthermore, fluorescence experiments were undertaken in order to explore the changes in fluorescence properties that accompany the reaction with CO. No excimer emission is observed even at high concentration before reaction with CO for 14, 22 or 23. After reaction with CO, excimer emission is observed for all complexes. From excitation, steady-state emission, and UV-vis spectroscopies, it has been determined that both inter- and intra-molecular excimers are observed that originate from loosely coupled or transient molecular pairs. ii i Table of Contents Abstract i i Table of Contents iv List of Tables viii List of Figures ix List of Schemes xiv List of Symbols and Abbreviations xvi Acknowledgements xviii Chapter One Introduction 1 1.1 Introduction to molecular sensors and the modular design approach 1 1.1.1 Development of luminescent chemosensors 2 1.2 A n introduction to photoluminescence theory 3 1.2.1 Relaxation processes from the singlet excited state 5 1.2.2 Relaxation processes from the triplet excited state 6 1.3 Deactivation of the excited state 7 1.3.1 Introduction to PET and EET 8 1.4 Introduction to the photophysical properties of pyrene 10 1.4.1 Pyrene excimers 11 1.4.2 Differentiating between static and dynamic excimers 12 1.4.3 Experiments to probe the nature of pyrene excimers 14 1.4.4 Further considerations for using pyrene in luminescent chemosensors 16 1.5 Sensing based on analyte coordination to a metal center 17 1.5.1 Metal ions as analytes in luminescent chemosensors 18 1.5.2 Metal ions as receptors in luminescent chemosensors 19 1.6 The hemilabile approach to analyte coordination and sensing 22 1.6.1 Phosphine ether hemilabile ligands 24 1.7 Pyrene as a pendant lumophore for hemilabile ligands 26 1.8 The current sensor, ta>RuCl2(POC4Pyr-P,0)2 27 1.8.1 Photophysical properties and luminescent response toward CO of tcc-RuCl 2(POC4Pyr-P,0) 2 28 iv 1.9 The scope of this thesis 29 1.10 References 31 Chapter Two Phosphine pyrenyl ether ligands with varying tether lengths 36 2.1 Motivation for the development of ligands with shorter tether lengths 36 2.2 Synthesis of short-tethered pyrenyl ethers 37 2.3 Characterization of POC2Pyr 40 2.4 POC2Pyr as a ligand on Ru(II) 41 2.5 Summary and outlook 41 2.5.1 Suggestions for future work 42 2.6 Experimental section 43 2.6.1 General details 43 2.6.2 Materials 44 2.6.3 Preparation and characterization of the phosphine pyrenyl ether ligands 44 2.6.3.1 Synthesis of (2-iodophenoxy)methylpyrene 44 2.6.3.2 Synthesis of (2-(diphenylphosphino)phenoxy)methyl-pyrene, POClPyr (Scheme 2.3) 45 2.6.3.3 Synthesis of (2-(diphenylphosphino)phenoxy)methyl-pyrene, POClPyr (Scheme 2.2) 45 2.6.3.4 Synthesis of (2-(diphenylphosphino)phenoxy)methyl-pyrene, POClPyr (Scheme 2.1) 46 2.6.3.5 Synthesis of 2-(l-pyrenyl)-1-ethanol 46 2.6.3.6 Synthesis of 2-(l-pyrenyl)-1-bromoethane 47 2.6.3.7 Synthesis of 2-(2-(diphenylphosphinophenoxy)ethyl-pyrene, POC2Pyr 48 2.6.4 Preparation of ruthenium complexes 48 2.6.4.1 Synthesis of /ran5,c/5,c/5-RuCl2(POC2Pyr-P,0)2 48 2.7 References 50 Chapter Three A series of luminescent Ru(II) complexes with different halo ligands 52 v 3.1 Promoting the quenching of pyrene luminescence 52 3.2 Synthesis and characterization of complexes with bromo and iodo ligands 52 3.2.1 Synthesis of complexes with bromo and iodo ligands 53 3.2.2 Characterization of fcc-RuX2(POC4Pyr-JP,0)2 (X = CI (14), Br (22), I (23)) 54 3.2.3 Absorption of 7cc-RuX2(POC4Pyr-JP,0)2 (X = CI (14), Br (22), I (23)) 54 3.3 ta>RuX2(POC4Pyr-P,0)2 (X = CI, Br, I) as luminescent molecular sensors 58 3.3.1 Reactivity of fcc-RuX2(POC4Pyr-P,0)2 (X = CI, Br, I) toward carbon monoxide 58 3.3.2 Reaction with carbon monoxide to form a fr-ans-dicarbonyl complex 58 3.3.3 Characterization of W-RuX 2(CO) 2(POC4Pyr-P) 2 (X - CI (15), Br (24), I (25)) 58 3.3.4 Absorption of W-RuX 2(CO) 2(POC4Pyr-P) 2 (X = CI (15), Br (24), I (25)) 60 3.3.5 Geometric isomerization to form a cis-dicarbonyl complex 61 3.3.6 Characterization of cc*-RuX 2(CO) 2(POC4Pyr-F) 2 (X = CI (16), Br (26), I (27)) ' 62 3.3.7 Further analysis of IR data for the geometric isomerization 63 3.3.8 Absorption of ccf-RuX 2(CO) 2(POC4Pyr-P) 2 (X = CI (15), Br (26), I (27)) 68 3.3.9 Reversibility of geometric isomerization 69 3.4 Summary and outlook 70 3.4.1 Suggestions of future work .'. 70 3.5 Experimental section 71 3.5.1 General details 71 3.5.2 Materials used 72 3.5.3 Preparation and characterization of Ru(II) complexes 72 3.5.3.1 Preparation and characterization of tcc-RuBr 2(POC4Pyr-F,0) 2 (22) 72 3.5.3.2 Preparation and characterization of tcc-RuI 2(POC4Pyr-P,0) 2 (23) 73 3.5.4 Reactions with CO 74 3.5.4.1 Preparation and characterization of ttt-RuBr 2(CO) 2(POC4Pyr-P) 2 (24) 74 3.5.4.2 Preparation and characterization of ttt-RuI 2(CO) 2(POC4Pyr-P) 2 (25) 75 3.5.4.3 Preparation and characterization of cct-RuBr 2(CO) 2(POC4Pyr-P) 2 (26) 75 3.5.4.4 Preparation and characterization of cct-RuI 2(CO) 2(POC4Pyr-P) 2 (27) 75 vi 3.7 References 77 Chapter Four Photoluminescent properties of ta>RuX2(POC4Pyr-.P,0)2, ttt-RuX2(CO)2(POC4Pyr-/>)2, and cc^-RuX 2(CO) 2(POC4Pyr-P) 2 (X = Cl , Br, I) 79 4.1 Luminescence response of fcc-RuX2(POC4Pyr-P,0)2 (X = C l (14), Br (22), I (23)) toward carbon monoxide 79 4.2 Photoluminescence of fcc-RuX2(POC4Pyr-P,0)2 (X = C l (14), Br (22), I (23)) 79 4.3 Photoluminescence of W-RuX 2(CO) 2(POC4Pyr-P) 2 (X = C l , Br, I) 82 4.3.1 Photoluminescence of «/-RuCl 2(CO) 2(POC4Pyr-P) 2 (15) 82 4.3.2 Photoluminescence of m-RuBr 2(CO) 2(POC4Pyr-P) 2 (24) 86 4.3.3 Photoluminescence of «/-RuI 2(CO) 2(POC4Pyr-P) 2 (25) 88 4.4 Photoluminescence of ccf-RuX 2(CO) 2(POC4Pyr-P) 2 (X = C l , Br, I) 91 4.4.1 Photoluminescence of cc^RuCl 2(CO) 2(POC4Pyr-F) 2 (16) 91 4.4.2 Photoluminescence of ccf-RuBr 2(CO) 2(POC4Pyr-P) 2 (26) 93 4.4.3 Photoluminescence of cc^-RuI2(CO)2(POC4Pyr-P)2 (27) 96 4.5 Summary and outlook 98 4.5.1 Suggestions for future work 99 4.6 Experimental section 102 4.6.1 General details 102 4.6.2 Materials used 103 4.7 References 104 vii List of Tables Table 1.1. Description of photophysical species for Figure 1.6 13 Table 3.1. Comparison of absorption properties of fcc-RuX2(POC4Pyr-.P,0)2 vs. tf/-RuX2(CO)2(POC4Pyr-/>)2 (X = Cl , Br, I) 61 Table 3.2. Comparison of absorption properties of fcc-RuX2(POC4Pyr-P,0)2 vs. «/-RuX 2(CO) 2(POC4Pyr-P) 2 and cct- RuX 2(CO) 2(POC4Pyr-P) 2 (X = Cl ,Br , I ) 68 viii List of Figures Figure 1.1. Chemosensor for free C a 2 + (X = C H 3 , H or Br). 8 2 Figure 1.2. Photophysical processes that occur during and after electronic excitation. Adapted from ref 15 .4 Figure 1.3. Schematic representation of (a) an OFF-ON chemosensor and (b) an ON-OFF chemosensor .8 Figure 1.4. Schematic representation of EET (F = organic fluorophore; F* = excited fluorophore; M = transition metal ion; M * = excited transition metal ion), (a) double electron exchange, (b) non-radiative decay and (c) the ground state. Adapted from ref 19 9 Figure 1.5. Schematic representation of relevant processes in PET (F = organic fluorophore; F* = excited fluorophore; M = transition metal ion), (a) electron transfer from the F L U M O to the M HOMO, (b) electron transfer from the M HOMO to the F HOMO and (c) no electron transfer after analyte binding.. Adapted from ref 18 10 Figure 1.6. Potential energy diagrams for pyrene excimer formation (a) in the absence of pyrene pre-association in the ground state and (b) with pyrene pre-association in the ground state. Adapted from ref 22 13 Figure 1.7. A 14-membered tetra-azamacrocycle linked to an anthracene fluorophore for N i 2 + detection.43 19 Figure 1.8. Cu coordinated with a dansyldiethylenetriamine-modified P-cyclodextrin ligand as a sensor for L- or D-alanine, L- or D-tryptophan, or L- or D-thyroxine (CD = cyclodextrin).50 20 ix Figure 1.9. Binuclear receptors as chemosensors for histidine, (a) a bisdien macrocycle (4) and (b) an octa-coordinate Z n 2 + ligand (5) (before exposure to the analyte; R = fluorophore).47,52 22 Figure 1.10. NO chemosensor incorporating a hemilabile ligand, before exposure to NO (6) and after reaction with NO (7). 5 5 ' 5 6 23 Figure 1.11. Fluorescent chemosensor incorporating a hemilabile ligand for the detection of pyrophosphate and citrate (M = C d 2 + ; A 2 " = anionic analyte).57 24 Figure 1.12. Rh + chemosensor that reacts reversibily with CO through the substitutionally labile ether ligand (Ar' = 2,4,6-trimethoxyphenyl).59....25 Figure 1.13. Reactivity of dichlorobis(o-(diphenylphosphino)anisole)ruthenium(II) toward C O . 6 2 26 Figure 1.14. Reactivity of/cc-RuCl 2(POC4Pyr-P,0) 2 toward CO (Pyr = pyrene).66.. .28 Figure 2.1. Target ligands with alkyl chains less than four carbon atoms in length.. .37 Figure 2.2. Possible compound for incorporating POC2Pyr into a R u 2 + complex 42 Figure 3.1. UV-vis absorption spectra for fcc-RuX2(POC4Pyr-P,0)2 (X = C l (14), Br (22), I (23)); in CH 2 C1 2 ; [/cc-RuX 2(POC4Pyr-P,0) 2] *10"5 M 56 Figure 3.2. Metal-based absorption band in the UV-vis spectrum of tcc-RuX 2(POC4Pyr-P,0) 2 (X = Cl (14), Br (22), I (23)); in CH 2 C1 2 ; [tcc-RuX 2(POC4Pyr- JP,0) 2] * 0.003 M 57 Figure 3.3. C-0 region of IR spectra for ^-RuX 2 (CO) 2 (POC4Pyr-F) 2 (X - C l (15), Br (24), I (25)); |>tf-RuX2(CO)2(POC4Pyr-P)2] * 0.0010 M ; all in CH 2 C1 2 60 x Figure 3.4. C-0 region of IR spectra for c^RiiX 2 (CO) 2 (POC4Pyr-P) 2 (X - CI (16), Br (26), I (27)); [W-RuX 2(CO) 2(POC4Pyr- JP) 2] * 0.0010 M ; all in CH 2 C1 2 63 Figure 3.5. IR spectra for the isomerization of #f-RuCl 2(CO) 2(POC4Pyr-P) 2 as a function of time 64 Figure 3.6. IR spectra for the isomerization of ftt-RuBr 2(CO) 2(POC4Pyr-P) 2 as a function of time 64 Figure 3.7. IR spectra for the geometric isomerization of ttt-RuI 2(CO) 2(POC4Pyr-P) 2 as a function of time 65 Figure 3.8. IR spectra for the geometric isomerization of ttt-RuCl 2(CO) 2(POC4Pyr-P) 2 corrected for t - 0 h data 66 Figure 3.9. IR spectra for the geometric isomerization of ttt-RuBr 2(CO) 2(POC4Pyr-P) 2 corrected for t = 0 h data 66 Figure 3.10. IR spectra for the geometric isomerization of ttt-RuI 2(CO) 2(POC4Pyr-P) 2 corrected for t = 0 h data 67 Figure 4.1. Excitation and emission spectra for fcc-RuX2(POC4Pyr-P,0)2 (X = CI (14), Br (22), I (23)); in CH 2 C1 2 ; [/cc-RuX2(POC4Pyr-P,0)2] « 10"6 M 81 Figure 4.2. Normalized emission spectra of W-RuCl 2(CO) 2(POC4Pyr-P) 2; ; U = 350nm; in CH 2 C1 2 : 83 Figure 4.3. Normalized excitation spectra for tf/-RuCl2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [15] * 10"5 M ; in CH 2 C1 2 ..85 xi Figure 4.4. I M / I E vs. wavelength for «/-RuCl 2(CO) 2(POC4Pyr-P) 2; [15] * 10"5 M ; in CH 2 C1 2 85 Figure 4.5. Normalized emission spectra of «/-RuBr 2(CO) 2(POC4Pyr-P) 2; Aex = 346nm; in CH 2 C1 2 . . . . . 87 Figure 4.6. Normalized excitation spectra for «^-RuBr 2(CO) 2(POC4Pyr-P) 2 at two different emission wavelengths; [24] « 10"5 M ; in CH 2 C1 2 87 Figure 4.7. I M / I E vs. wavelength for m-RuBr 2(CO) 2(POC4Pyr-P) 2; [24] « 10"5 M ; i n C H 2 C l 2 "...88 Figure 4.8. Normalized emission spectra of m-RuI 2(CO) 2(POC4Pyr-P) 2; ^ x = 348 nm; in CHC1 3 ...89 Figure 4.9. Normalized excitation spectra for f«-RuI 2(CO) 2(POC4Pyr-P) 2 at two different emission wavelengths; [25] * 10" 5M; in CHC1 3 90 Figure 4.10. IM/IE VS. wavelength for ^-RuI 2(CO) 2(POC4Pyr-P) 2; [25] * 10"5 M ; in CHC1 3 90 Figure 4.11. Normalized emission spectra of ccf-RuCl2(CO)2(POC4Pyr-P)2; = 344 nm; in CH 2 C1 2 92 Figure 4.12. Normalized excitation spectra for cc?-RuCl 2(CO) 2(POC4Pyr-P) 2 at two different emission wavelengths; [16] « 10" 5M; in CH 2 C1 2 92 Figure 4.13. IM/IE VS. wavelength for ccf-RuCl 2(CO) 2(POC4Pyr-P) 2; [16] ~ 10"5 M ; in CH 2 C1 2 93 Figure 4.14. Normalized emission spectra of cc^-RuBr 2(CO) 2(POC4Pyr-P) 2; ^ x = 346 nm; in CH 2 C1 2 94 xii Figure 4.15. Normalized excitation spectra for cc^-RuBr2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [26] * 10" 5M; in CH 2 C1 2 95 Figure 4.16. I M / I E vs. wavelength for cc/-RuBr 2(CO) 2(POC4Pyr-P) 2; [26] * IO"5 M ; in CH 2 C1 2 96 Figure 4.17. Normalized emission spectra of ccf-Rul2(CO)2(POC4Pyr-P)2; ^ x = 348 nm; in CH 2 C1 2 97 Figure 4.18. Normalized excitation spectra for cc/-Rul2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [27] « 10"5 M ; in CHCI3 97 Figure 4.19. I M / I E vs. wavelength for cc/-RuI 2(CO)2(POC4Pyr-P) 2; [27] « 10~5 M ; in CHCb 98 xiii List of Schemes Scheme 1.1. The structure of pyrene 11 Scheme 2.1. Synthesis of POClPyr via a Williamson ether synthesis using K O H as the base 39 Scheme 2.2. Synthesis of POClPyr via a Williamson ether synthesis using an amine base 39 Scheme 2.3. Synthesis of POClPyr via a 2-iodophenoxy intermediate 39 Scheme 2.4. Synthesis of POC2Pyr (18) 40 Scheme 2.5. Proposed synthesis of fcc-RuCl2(POC2Pyr-P,0)2 (Pyr = pyrene) 41 Scheme 2.6. Proposed synthesis for the ligand POC3Pyr 43 Scheme 3.1. Synthesis of fcc-RuBr2(PdC4Pyr-P,0)2 (22) 53 Scheme 3.2. Synthesis of fcc-RuI2(POC4Pyr-P,0)2 (23) 53 Scheme 3.3. Mechanism for the isomerization of complexes RuX 2 (CO) 2 L 2 (X = halogen ligand, L = ligand with phosphorus donor atom) 62 Scheme 3.4. Reactivity of fcc-RuX2(POC4Pyr-P,0)2 toward CO 71 Scheme 4.1. Proposed structure of an intermolecular excimer for ttt-RuCl 2(CO) 2(POC4Pyr-P) 2 (Pyr = pyrene) 82 Scheme 4.2. Proposed structure of an intramolecular excimer for ttt-RuCl 2(CO) 2(POC4Pyr-P) 2 83 Scheme 4.3. Proposed synthesis for water-soluble POC4Pyr using fuming sulfuric acid (Pyr = pyrene) 101 Scheme 4.4. Proposed synthesis for a phosphine solubilized with a carboxylate group 102 xiv Scheme 4.5. Proposed synthesis for a phosphine solubilized with a sulfonate group XV List of Symbols and Abbreviations Abbreviation Description A absorbance A Angstrom Ac acetate °C degrees Celsius Calcd. calculated cct cis, cis, trans 18-crown-6 (CH2CH2CH20)6, cyclic polyether for solubilizing 8 chemical shift (ppm) d doublet (NMR) dd doublet of doublets (NMR) DMSO dimethylsulfoxide e extinction coefficient (M^cm 1 ) EET electronic energy transfer EI electron impact equiv stoichiometric equivalent ES electrospray ESI electrospray ionization et al. et aliae etc. et cetera EtOH ethanol excimer excited state dimer F fluorescence or fluorophore FTIR Fourier transform infrared spectroscopy h hour hv photon HOMO highest occupied molecular orbital IC internal conversion IE intensity of pyrene excimer emission IM intensity of pyrene monomer emission IR infrared K degrees Kelvin KO'Bu potassium f-butoxide emission wavelength (nm) excitation wavelength (nm) wavelength at band maximum L U M O lowest unoccupied molecular orbital M molarity or transition metal ion m multiplet (NMR) MeOH methanol m/z mass-to-charge ratio MS mass spectroscopy or mass spectrometer xvi V IR absorption frequency (cm"1) nm nanometer N M R nuclear magnetic resonance ns nanosecond P phosphorescence P A ratio of the most intense absorption band to that of the adjacent minimum at shorter wavelength PET photoinduced electron transfer Ph phenyl P0C4Pyr 4-(2-(diphenylphosphino)phenoxy)butylpyrene P0C2Pyr 2-(2-(diphenylphosphino)phenoxy)ethylpyrene POClPyr (2-(diphenylphosphino)phenoxy)methylpyrene P0C3Pyr 3-(2-(diphenylphosphino)phenoxy)propylpyrene POL hemilabile phosphine ether ligand POMe 2-methoxyphenyldiphenylphosphine ppm parts per million Pyr/pyr pyrene R alkyl substituent R f retention factor (chromatography) RT room temperature s singlet (NMR) So singlet ground electronic state s, singlet first excited electronic state s2 singlet second excited electronic state ST singlet-triplet intersystem crossing t triplet (NMR) or time t tertiary T, triplet first excited electronic state THF tetrahydrofuran TMS tetramethylsilane TOF time-of-flight tec trans, cis, cis ttt trans, trans, trans U B C University of British Columbia U V ultraviolet vis visible V R vibrational relaxation vs. versus X halogen ligand xvii Acknowledgements I would like to thank my supervisor, Mike Wolf, for giving me the opportunity to work on this project. I've learned a lot in the past two years and it's been an enjoyable experience. I'd also like to thank the rest of the Wolf group for all of their support and advice. I am especially grateful to Carolyn Moorlag and Tracey Stott for all of their assistance. My thanks goes out to the support staff in the Department. I'd like to thank Ms. Marietta Austria and Ms. Liane Darge for their help with the N M R , the Withers group for letting me use their fluorometer and the Maclachlan group for the use of the UV-vis. Lastly, I'd like to thank Microanalytical Services for running the E A and MS samples. I am also very grateful to my family, Matt Chatterton, and Christina Mavinic. I changed my mind often in the past two years and they were always there for me, whether it be coming to the LSAT, listening to my endless discussion about getting into Law school and then choosing a Law school, or distracting me from studying with golf, skiing, or any other form of procrastination. I promise that one day I will actually leave school and get a job. xviii Chapter One Introduction 1.1 Introduction to molecular sensors and the modular design approach Molecule-based sensors, or chemosensors, are molecules that are able to bind selectively and reversibly an analyte of interest with a concomitant change in one or more measurable properties of the system.1 These properties can be optical (absorption or luminescence properties), electrochemical, magnetic, or mass spectrometric, amongst others. Special interest has been paid to the development of chemosensors due to their widespread applications in chemistry, biology, biochemistry, materials science, clinical and medical sciences, and cell biology.1 One of the simplest design approaches toward the development of chemosensors is the modular design approach. A modular chemosensor can be divided into three parts: a receptor, a reporter mechanism, and a spacer.3 The receptor is the site at which the analyte of interest binds, the reporter is the part of the molecule where measurable properties change in the presence and absence of the analyte (signal transduction), and the spacer joins the receptor and reporter together.3 A major advantage of this design approach is the ability to rationally adjust properties of the sensor by changing the molecular structure of any of the components.1 The versatility of this design strategy to synthesize chemosensors for anionic, cationic, and neutral analytes is limitless. It should also be noted that any of these three components can be combined into a single moiety. There are examples in the literature of chemosensors in which the receptor and reporter are combined into a singular molecular unit.4"7 This adaptation of the modular design approach eliminates the need for a spacer. 1 Amongst the first reported sensors using the modular design strategy was one for the detection of free C a 2 + (Figure l . l ) . 8 The ligand bis(o-aminophenoxy)ethane-N,N,N',N'-tetxaacctic acid (1) coordinates free C a 2 + (the analyte) via the carboxyl moieties of the ligand (the receptor).8 The receptor is connected to the phenoxy portion of the ligand via a methylene spacer. The UV-vis spectra of the free ligand and the complex are considerably different. Before C a 2 + coordination, the amine lone pair is conjugated with the phenoxy rings.8 After C a 2 + coordination, the conformation of the complex changes such that the conjugation between the phenoxy ring and amine is broken and a hypsochromic shift is observed in the UV-vis spectrum.8 < COOH HOOC N ' X ^ V " C O O H H O O C O ^ A. > Figure 1.1. Chemosensor for free C a 2 + (X = C H 3 , H or Br) As research into chemosensors has developed, one of the most attractive reporter mechanisms that has emerged is luminescence via fluorescent lumophores or fluorophores. 1.1.1 Development of luminescent chemosensors A lumophore is a molecule or group of molecules which gives rise to a particular emission band and a fluorophore is a molecule or group of molecules that fluoresces.9 2 These two terms will be used interchangeably throughout this thesis. There are several advantages to luminescence-based chemosensors. These include: high sensitivity (single molecule detection is possible), quick response time, low cost, direct and real-time detection with both spatial and temporal resolution, luminescence measurements can easily be performed, and the technique is versatile.3 Furthermore, the properties of the fluorescent lumophore can be tuned to achieve a specific response. Some of the possibilities are introducing proton-, energy-, or electron-transfer processes, the presence of heavy atom effects, changes of electronic density at different parts of the chemosensor, or destabilizing non-emissive excited states.1 A more detailed discussion of the specifics of luminescence theory will be included in the next section. Numerous examples of luminescent and non-luminescent molecular chemosensors based on the modular design approach exist in the literature. There are many excellent reviews on this topic.1'3"5'1 0"1 4 Several examples will be discussed throughout the current work but the reader is referred to these resources for further discussion. 1.2 An introduction to photoluminescence theory Photoluminescence can be defined as a phenomenon in which a molecule is excited by electromagnetic radiation to a state in which it itself emits light.9 Several textbooks provide an introduction to this topic. 9' 1 5" 1 7 Figure 1.2 provides a summary of the processes that will be discussed.15 3 When a molecule absorbs electromagnetic radiation (A), its energy increases by an amount equal to the energy of the absorbed photon.9 An electron is promoted from an originally occupied bonding or non-bonding orbital (a, n, or n), referred to as the ground state (So), to an originally antibonding or unoccupied molecular orbital (o* or 71*), referred to as the excited state (Si or S2).15 Each electronic state also has several vibrational levels associated with it. Figure 1.2. Photophysical processes that occur during and after electronic excitation. Adapted from ref 15. Electronic transitions occur very quickly and thus, are mostly likely to occur between vibrational levels with similar nuclear positions (Frank-Condon principle).1 6 An assumption that is made for the sake of simplicity is that the electron is in the zero vibrational level of the ground state and is promoted to one of several vibrational levels 4 of the excited state. Though this assumption is only achieved at absolute zero, it is a reasonable approximation from statistical mechanics.9 Figure 1.2 shows the possible electronic absorptions from the ground state to the various vibrational levels of the first and second excited states. An electron in a higher vibrational level of any given electronic state will descend to the lowest vibrational level via vibrational relaxation (VR). 1 7 A molecule vibrates with a frequency characteristic of that particular vibrational excited state.15 It gives up this excess vibrational energy in the form of kinetic energy that it imparts to other molecules with which it collides.1 5 Thus, the electron thermally descends to the lowest vibrational level of that particular electronic state.15 From the zero vibrational level of the first excited singlet state (Si), there are three processes that may occur: internal conversion (IC), fluorescence (F), or singlet-triplet intersystem crossing (ST). 1.2.1 Relaxation processes from the singlet excited states When the higher vibrational levels of the lower electronic state overlap with the lower vibrational levels of the higher electronic state, the upper and lower electronic states are in a transient thermal equilibrium.17 This equilibrium permits population of the lower electronic state and IC occurs to the higher vibrational levels of the lower electronic state. IC can also occur via tunneling if the gap between the two electronic states is small. 1 5 This type of relaxation process can occur between the first excited state and the ground state, the second excited state and the first excited state, etc. Another relaxation pathway from the first excited singlet state is fluorescence. Fluorescence is a radiative electronic transition resulting in the demotion of an electron 5 from the lowest vibrational level of the first excited state to any one of the vibrational levels of the ground state.9 The excess energy is released as visible or U V light whose frequency is directly related to the energy gap between the two electronic states.9 The occurrence of F depends on the difference in energy between the upper and lower states and on the number of vibrational states associated with each electronic state.15 Fluorescence competes with the nonradiative decay mechanisms (IC and ST). 1 7 For example, polyaromatic hydrocarbons, such as pyrene and anthracene, are rigid molecules with relatively few vibrational degrees of freedom. There is less vibrational level overlap between the first excited state and the ground state and IC is not as efficient.15 As a result, these molecules are highly fluorescent. 1.2.2 Relaxation processes from the triplet excited state States having zero spin angular momentum are defined as singlet states.16 If an electron from an occupied orbital is promoted to a higher, previously unoccupied orbital, it usually retains its original spin. 1 5 Therefore, an electron from a singlet ground state is promoted to a singlet excited state. Triplet states occur when there are two unpaired electrons and three possible orientations for the spin angular momentum vector in an external magnetic field.16 There is less electronic repulsion in any given triplet state than in the singlet state of the same electron configuration.15 As can be seen from Figure 1.2, the first excited triplet state (Ti) lies below the excited singlet state in energy. Often there is overlap between the vibrational levels of the singlet and triplet excited states and there is a probability that the triplet state can be populated from the excited singlet state by a 17 17 mechanism similar to IC. This process is termed ST and is a forbidden transition. Efficient ST is the result of spin-orbit coupling that reduces the "forbidddenness" of the 6 singlet-triplet transition. Spin-orbit coupling is more pronounced for heavy atoms and hence, this process is often referred to as the "heavy atom effect."9 From the excited triplet state, the electron may relax to the singlet ground state via the non-radiative process ST or via the radiative process phosphorescence.17 Phosphorescence is longer-lived than F as it is forbidden and can sometimes be recognized as a perceptible afterglow when the exciting source has been turned off.1 6 Phosphorescence will not be discussed in detail in the current work and the reader is referred to other sources for further information.9 ,15"17 1.3 D e a c t i v a t i o n o f the exc i t ed state There are numerous pathways by which the excited state of a lumophore can be manipulated. The fluorescence properties of a lumophore can be affected by substituents on the lumophore, solvent, pH, temperature, and lumophore concentration.9 A l l of these factors can perturb the energies of the excited states and change the luminescence properties observed.9 Some of these effects can completely extinguish or quench fluorescence, while others promote certain relaxation pathways. It should be noted that the effects observed depend on the nature of the excited state involved, which in turn depends on the orbitals involved. Common excited states are n-n*, n-n*, M L C T (metal-to-ligand charge transfer), L M C T (ligand-to-metal charge transfer), and ICT (internal charge-transfer). Further excited states are also possible. For the specific effects observed for each type of excited state, the reader is referred to a review on this topic.1 8 In terms of chemosensors, the imperative concept is that the sensor must show luminescence properties that change in the presence and absence of the analyte. In order 7 to maximize the signal-to-noise ratio, fluorescence should either be O N or OFF in the absence of the analyte, and then either OFF or ON in the presence of the analyte (Figure 1.3).18 Hence, these sensors are termed OFF-ON (Figure 1.3a) or ON-OFF (Figure 1.3b). Although there are several different quenching pathways through which ON-OFF sensing could be achieved, the two most common and prominent in terms of luminescent chemosensors are photoinduced electron transfer (PET), and electronic energy transfer (EET). a Figure 1.3. Schematic representation of (a) an OFF-ON chemosensor and (b) an ON-OFF chemosensor. 1.3.1 Introduction to PET and EET A metal can quench an excited fluorophore via EET i f the metal possesses empty or half-filled d-orbitals of appropriate energy.19 A schematic diagram outlining the EET process is shown in Figure 1.4.19 This is an example of an excited fluorophore (F*) and a d 9 metal ion (M) in an elongated-octahedral coordination environment.19 Deactivation 8 occurs through a double electron exchange (Figure 1.4a). The excited metal ion (M*) (Figure 1.4b) then undergoes rapid non-radiative decay to return to the ground state (Figure 1.4c).19 Chemosensors based on quenching via EET only are relatively rare in the literature and most examples involve Cu . 1 9 a b c T - h + - I ^ F* M F M * F M Figure 1.4. Schematic representation of EET (F = organic fluorophore; F* = excited fluorophore; M = transition metal ion; M * = excited transition metal ion), (a) double electron exchange, (b) non-radiative decay and (c) the ground state. Adapted from ref 19. The more common quenching mechanism for chemosensors is PET (Figure 1.5).18 Excitation of the fluorophore promotes an electron from the highest occupied molecular 1 R orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Electron transfer to effect quenching can occur in one of two ways: the electron transfers from the fluorophore L U M O to the metal HOMO (Figure 1.5a) or the electron transfers from the metal HOMO to the fluorophore HOMO (Figure 1.5b).18 A detectable radical intermediate is formed after either of these electron-transfer processes.18 To return to the ground state, an electron transfers from the metal HOMO to the fluorophore HOMO, or from the fluorophore L U M O to the metal H O M O . 1 8 Upon analyte binding, the potential of the metal HOMO is changed such that the relevant HOMO becomes lower in energy 9 than that of the fluorophore (Figure 1.5c).18 Now PET cannot occur and fluorescence is 1R not quenched. It should be noted that M in Figure 1.5 does not have to be a metal ion. There are numerous examples in the literature of organic molecules whose HOMO is of appropriate energy to act as a PET electron donor as shown in Figure 1.5b.3'20'21 a b e ^ - 4 -F* M F* M F* M Figure 1.5. Schematic representation of relevant processes in PET (F = organic fluorophore; F* = excited fluorophore; M = transition metal ion), (a) electron transfer from the F L U M O to the M HOMO, (b) electron transfer from the M H O M O to the F HOMO and (c) no electron transfer after analyte binding. Adapted from ref 18. The utility and versatility of these two deactivation pathways in the development of luminescent chemosensors based on OFF-ON or ON-OFF switching has been realized. Numerous examples of sensors based on these quenching mechanisms exist and detailed examples can be found in reviews. 1 ' 3 ' 1 0 ' 1 1 ' 1 9 1.4 Introduction to the photophysical properties of pyrene An ideal fluorophore has an intense and characteristically structured fluorescent 22 emission that is suitable for signaling the occurrence of the receptor-analyte interaction. Pyrene is amongst many ideal fluorophores for applications in chemosensors due to its well-studied photophysical properties ' ' and its established utility in chemosensors 10 with ON-OFF functionality via PET and/or EET. ' Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings and is planar in structure (Scheme 1.1). Scheme 1.1. The structure of pyrene. The absorption and emission spectra of pyrene are very well documented in numerous examples. The absorption spectrum of pyrene originates from a n-n* transition.23 Absorption spectra of 1-substituted pyrene molecules consist of three vibronic bands corresponding to 'La,  lBb, and 'Ba transitions, with the 'La band being of lowest energy.26 The emission spectrum of pyrene consists of five principle vibronic bands, although 16 vibronic bands can be observed in studies conducted at 4 K . 2 3 Excited state pyrene has a relatively long lifetime on the order of 450 ns and a quantum efficiency of 0.60 in cyclohexane.23 The characteristic colour of emitted light from pyrene is 23 indigo-blue. The emission spectrum of pyrene is very sensitive to its environment. The intensity and the actual observation of the five principle bands changes dramatically with solvent and with chemical substitution on the pyrene molecule.23 1.4.1 Pyrene excimers Another interesting photophysical property of pyrene is efficient excimer formation. The classical definition of an excimer is a dimer that is associated in an electronic excited state but dissociated in the ground state.17 In other words, the 11 formation of an excimer requires the encounter of an electronically excited pyrene molecule with another pyrene molecule in the ground state to form an excited state 22 22 dimer. Light is then emitted from this excited state species. Two pyrene moieties must be sufficiently far apart such that when light is absorbed, excitation is limited to one of the pyrene molecules.22 If the excited pyrene molecule can diffuse towards another pyrene molecule during its fluorescence lifetime, then an excimer may form. 2 2 The propensity of pyrene to form excimers is dependent on its fluorescence lifetime, the probability of reaching a conformation suitable for excimer formation within the lifetime of the excited state, and the stabilization energy of the excimer.27 The existence of a pyrene excimer is characterized by blue-green emission and the observation of a broad, structureless band in the emission spectrum centered at approximately 480 nm. 2 2 There are also instances when an excimer-like emission is observed but there is no evidence that the pyrene molecules are separated before excitation. It was determined that these excimers originated from pyrene dimers that were associated with each other in the ground state, before excitation.22 Two terms are now accepted to differentiate the two types of excimers that are observed: dynamic excimers and static excimers. Dynamic excimer refers to the classical definition that the pyrene molecules are separated in the ground state prior to excitation and static excimer refers to an excimer that originates 22 from a pre-associated pyrene dimer. 1.4.2 Differentiating between static and dynamic excimers There are numerous ways to differentiate between static and dynamic pyrene excimers. However, before these experiments are discussed, an explanation of why these two types of excimers have different photophysical properties will be presented. 12 The differences between static and dynamic excimers can be understood via schematic potential energy diagrams of the pyrene excimers (Figure 1.6 and Table 1.1). a + Energy Py-Py b Energy Py* -s, •+Z Py + Py Py-Py P y - P y P y + P y Figure 1.6. Potential energy diagrams for pyrene excimer formation (a) in the absence of pyrene pre-association in the ground state and (b) with pyrene pre-association in the ground state. Adapted from ref 22. Table 1.1. Description of photophysical species for Figure 1.6. Symbol Description Py Pyrene in the ground state Py* Pyrene in the first excited singlet state Py-Py Pyrene dimer in the ground state Py-Py* Pyrene excimer in the first excited singlet state E * Dynamic excimer in the first excited singlet state D* Static excimer in the first excited singlet state 13 As can be seen from Figures 1.6, different processes occur for each type of excimer. The purely dynamic excimer case is shown in Figure 1.6a.22 Pyrene (Py) in the ground state is excited with energy equal to hv e x c . The excited pyrene molecule (often referred to as a monomer) (Py*) may fluoresce and return to the ground state.22 However, Py* may encounter a pyrene molecule in the ground state, form a dynamic excimer (E*), and return to the ground state emitting light with energy equal to hvfc.22 If some of the pyrene molecules are pre-associated in the ground state with another pyrene molecule in a dimer, then Figure 1.6b applies.22 The diagram illustrates how light is emitted from two different species with slightly different potential energies.22 This phenomenon has been determined from theoretical calculations and a full explanation into the exact energetics of static and dynamic excimers is provided elsewhere.22'27 The most important conclusion in the context of this thesis is that the presence of static excimers perturbs the absorption, excitation, and emission spectra of pyrene relative to 00 spectra obtained in the absence of static excimers. 1.4.3 Experiments to probe the nature of pyrene excimers The experiments that are used to distinguish between static and dynamic excimers include absorption, excitation, steady-state emission, and time-dependent fluorescence. In the context of this thesis, only the first three will be discussed. However, detailed examples on the utility of time-dependent fluorescence spectroscopy can be found in the literature.28"30 A clear indication of the presence of pre-associated pyrene molecules in the ground state is broadening of the absorption bands in the UV-vis spectrum accompanied by a red-shift in the maximum of each absorption band.22 A more precise manifestation 14 of these changes can be found by calculating the P A ratio. The P A ratio is the ratio of the most intense band to that of the adjacent minimum at shorter wavelength.22 This is an empirical way of calculating the extent of peak broadening observed for a specific 22 absorption band. Peak broadening is anticipated for pre-associated pyrene dimers due to increased vibrational fine structure. For 1-substituted pyrenyl compounds, the most intense band is usually the ! L a band. If the P A ratio is greater than 3, then the pyrene excimers originate entirely from 99 dynamic excimers. Conversely, i f the ratio is less than 3, then the pyrene molecules are pre-associated in the ground state.22 The value decreases in proportion to the extent of • 99 pre-association. For example, a P A value of 1.5 indicates that there is significant pre-association between the pyrene molecules in the ground state.31 An intermediate value between 1.5 and 2.5 usually indicates that the excimers originate from loosely coupled molecular pairs or aggregates of pyrene groups in the ground state from which the excimer forms through only a small displacement of an excited pyrene.31 The P A value is empirical and should be used with caution. Further experimentation should always accompany P A values to validate any conclusions that have been drawn. The excitation spectra obtained at the pyrene monomer emission wavelength and at the excimer emission wavelength are not superimposable in the presence of ground-state pyrene aggregates.22 The spectrum monitored at the excimer emission is red-shifted relative to that monitored at the monomer emission due to increased conjugation.22 Furthermore, the bands in the spectrum monitored at the excimer emission are broadened due to increased vibrational fine structure. If the red-shift of the most intense 15 absorption is between 1 and 4 nm, then there is significant pyrene pre-association in the ground state.22 A final experiment involves examining the intensity of monomer emission (IM) relative to the intensity of excimer emission (IE) in the steady-state emission spectrum. The ratio is termed the monomer: excimer emission intensity ratio ( W I E ) - This ratio does not depend on the excitation wavelength when there is no pyrene pre-association in the ground state.32'33 Excitation in the red-edge of the absorption band (for example, at -355 nm) results in lower W I E than excitation at ~330 nm if there is ground state association. Examination of the excitation spectrum confirms the validity of this result. At the red-edge of the excitation spectrum, excimer excitation is dominant.32 More excimers than monomers are excited, and the IM/IE ratio from the emission spectrum would decrease at those wavelengths. These changes in IM/IE would only be significant when the excimer excitation spectrum is shifted at least 1 nm from the monomer excitation spectrum, the exact situation when pre-association is significant.32 It must be noted that the results of these three experiments should be used in concert to determine the nature of the excimers forming in a particular system. Conclusions cannot be drawn from a single experiment. Most examples in the literature include multiple experiments to validate the results obtained and interpretations made. 1.4.4 Further considerations for using pyrene in luminescent chemosensors Pyrene has found numerous applications in luminescent chemosensors. Not only has pyrene been used in ON-OFF sensors, it is also used in ratiometric sensors. There are examples in the literature that demonstrate how, before exposure to the analyte, the pyrene emission is dominated either by monomer or excimer emission; after exposure to 16 the analyte, the exact opposite is observed. ' For example, a calix[4]crown bearing an ionophoric cavity on the lower rim and two pyrene groups on the upper rim was synthesized. In the absence of an alkali metal cation, the pyrenes can easily form excimers due to the flexibility of the system.37 After alkali metal cation coordination, the rigidity of the sensor is increased and the pyrenes can no longer form excimers.37 Hence, only monomer emission is observed. Pyrene molecules can interact with each other both intra- and intermolecularly. Intra- and intermolecular excimer formation has been used to probe polymer conformation38 as well as D N A and R N A secondary and tertiary structure.34 In general, intermolecular excimer formation is negligible at concentrations less than 1 x 10"5 M . 1 7 This observation can be used as a guide for differentiating between these two types of excimers. 1.5 Sensing based on analyte coordination to a metal center Transition metals, alkali and alkali earth metals, and lanthanides expand the versatility and potential utility of chemosensors. The choice of metal ion affects the number and type of donor atoms required for potential ligands, the structure of the complex, the formal charge and/or the presence or absence of dipoles (both permanent and induced), and the solvation requirements of the complex.3 9 Furthermore, metal ions play a significant role in a variety of ON-OFF chemosensors as either the analyte or the receptor due to their functionality in PET, EET, or the heavy atom effect, as well as other fluorescence quenching phenomena. If the metal ion is not participating in the quenching mechanism directly, then it can provide a coordination site for an analyte to act as a PET electron donor. 17 1.5.1 Metal ions as analytes in luminescent chemosensors Receptors for metal ion analytes are designed such that the receptor is tailored to the specific electronic and steric requirements for the coordination of a specific metal ion. For example, an aza-15-crown-5 receptor can detect extracellular potassium,40 and a bispyrenyl calix[4]arene-based receptor incorporating two hydroxamic acid functionalities41 and the ligand 7Y2-(4-dimethylaminobenzyl)-iVl^l-bis[2-(4-dimethylaminobenzylamino)ethyl]ethane-l,2-diamine are sensitive to both N i 2 + and C u 2 + . 4 2 These receptors are then linked to a fluorophore such as pyrene, naphthalene, anthracene, or fluorescein, amongst many others. These sensors are ON-OFF or OFF-ON depending on whether the presence of the metal ion revives fluorescence or quenches fluorescence. One of the more common designs for metal ion detection and OFF-ON sensing is shown in Figure 1.7.43 A macrocyclic ring is bonded to a fluorophore via an alkyl chain. The 14-membered tetra-aza macrocycle (2) can coordinate N i 2 + . In the absence of N i 2 + and at basic pH, the lone pair of electrons on the nitrogen at the 1-position of the macrocyclic ring can quench anthracene fluorescence via PET 4 3 However, after N i 2 + coordination, this lone pair is coordinated to the metal ion and can no longer be involved in PET 4 3 A revival in fluorescence is then observed.43 There are numerous examples of these types of sensors. However, the focus of the current work is on the metal ion acting as a receptor; for more information on sensors for metal ion analytes, the reader is referred to the literature.1'3'44"46 18 2 Figure 1.7. A 14-membered tetra-azamacrocycle linked to an anthracene fluorophore for N i 2 + detection.43 1.5.2 Metal ions as receptors in luminescent chemosensors Transition metal centers can be used as chemosensor receptors when they are coordinatively unsaturated with an open position for a further ligand, the analyte.47 An advantage of the analyte coordinating to the metal center is that metal-ligand interactions can be significantly stronger than hydrogen bonding and other van der Waals interactions 4 8 The type of metal center used depends on the analyte of interest, the binding constant between the metal center and analyte, and the steric demands of the analyte.39 For example, a Zn 2 +complex of an acridine-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) is a nucleobase receptor molecule for deoxythymidine and uridine 4 9 Selectivity is achieved via coordination of the imide functionality of these nucleobases to zinc. 4 9 Other nucleobases do not have an imide moiety and the binding constant between Z n 2 + and an imide is higher than between Z n 2 + and other nucleobase 19 functional groups. Analyte coordination is reported via fluorescence quenching after nucleobase coordination.49 A metal ion does not exist in solution with empty coordination sites. Therefore, it is very common for analyte coordination to a metal ion receptor to involve ligand substitution. The metal center must have at least one ligand that is substitutionally labile and the analyte must have a larger binding constant with the metal center than the labile ligand. These requirements have been achieved in several examples. A dansyldiethylenetriamine-modified P-cyclodextrin is a tridentate ligand for C u 2 + (3) (Figure 1.8).50 The metal ion quenches dansyl fluorescence via the heavy atom effect.50 In the presence of L- or D-alanine, L- or D-tryptophan, or L- or D-thyroxine, the fluorescence of the dansyl moiety is revived due to displacement of the dansyl group from Cu 2 + and coordination of a bidentate organic ligand.5 0 The driving force for substitution is the preference of C u 2 + to be tetracoordinate versus tricoordinate.50 + Figure 1.8. Cu coordinated with a dansyldiethylenetriamine-modified P-cyclodextrin ligand as a sensor for L- or D-alanine, L- or D-tryptophan, or L- or D-thyroxine (CD = cyclodextrin).50 20 A similar example that involves quenching via spin-orbit coupling (the heavy atom effect) is the dirhodium tetracarboxylate complex [Rh2(/i-02CR)4(L)2] (R = C H 3 ; L = dansyl-imidazole or dansyl-piperazine).51 These complexes are only weakly fluorescent but when the fluorophores are substituted by NO, there is a 15-fold increase in fluorescence since the fluorophores are no longer in the coordination sphere.51 Another interesting class of metal ion receptors is binuclear receptors. Two binuclear complexes have been designed as sensors for histidine (Figure 1.9). In the bisdien macrocycle in Figure 1.9a (4), the cavity between the C u 2 + centers is large enough to accommodate the imidizolate moiety of histidine.52 Prior to exposure to histidine, the cavity is occupied by the carboxylate group of a fluorophore such as fluorescein.52 The fluorescence of the fluorophore is quenched.52 The fluorophore can then be substituted by histidine and fluorescence is restored after the fluorophore leaves the coordination sphere. An alternate methodology to achieve the same goal is shown in Figure 1.9b.47 In this design, the fluorophore is incorporated into the backbone of the octa-coordinate Z n 2 + ligand (5).47 Before exposure to histidine, the anthracene moiety is emissive. Fluorescence is quenched via PET after histidine coordination via hydroxide ligand displacement since the imidizolate moiety of histidine can act as an electron donor in PET. 4 7 The two metal ions act as a scaffold to hold the analyte in position so PET can 47 occur. Although these examples demonstrate efficient analyte detection, none of them exhibit reversible analyte coordination. As a result, they are of limited practical utility. One way of promoting reversibility is through the use of hemilabile ligands. 21 a b 4 5 Figure 1.9. Binuclear receptors as chemosensors for histidine, (a) a bisdien macrocycle (4) and (b) an octa-coordinate Z n 2 + ligand (5) (before exposure to the analyte; R = fluorophore).47'52 1.6 The hemilabile approach to analyte coordination and sensing Hemilabile ligands are polydentate ligands that contain at least two different types of chemical functionalities capable of binding to one or more metal centers. The functionalities are chosen to be different in order to increase the differentiation between their resulting interaction with a metal center(s). One donor is substitutionally labile while the other remains firmly bound to the metal center(s).53 For example, a hard and soft donor functionality can be included in the same ligand. Interest in these types of ligands has been based in studying reversible coordination, stoichiometric and catalytic activation, and transport of small molecules. Hemilabile ligands have been used in these applications because they can provide open 22 coordination sites at a metal center during a reaction that are "masked" in the ground state, or they can stabilize reactive intermediates.53 Recent interest has also been in the utility of hemilabile ligands incorporated into the receptor moiety of molecular sensors. There are distinct advantages to using hemilabile ligands. The labile donor is often easily substituted by the analyte so that the response is efficient and dramatic.53 Also, since the labile donor does not leave the coordination sphere, there is a higher probability that reaction with the analyte will be reversible and the starting material will be regenerated.53 The labile donor is often regarded as an intramolecular solvent molecule.54 However, these complexes are much more stable than the corresponding solvent adducts due to the chelate effect. / 6 7 Figure 1.10. NO chemosensor incorporating a hemilabile ligand, before exposure to NO (6) and after reaction with NO (7). 5 5 ' 5 6 23 An example of the utility of hemilabile ligands in chemosensors is provided by Lippard et al. (Figure 1.10).55'56 This C o 2 + sensor (6) is sensitive toward NO. ' Displacement of two nitrogen atoms of the ligand by NO (7) results in a dramatic luminescence response.55'56 Another example is a C d 2 + sensor for pyrophosphate and citrate (8) (Figure 1.11). When the Cd center is coordinated via the five nitrogen donors of the ligand, the fluorophore is quenched via spin orbit coupling.5 7 The anionic analyte displaces the axial ligand (9) and moves the fluorophore far enough away from the metal center such that fluorescence is restored.57 8 9 Figure 1.11. Fluorescent chemosensor incorporating a hemilabile ligand for the detection of pyrophosphate and citrate (M = C d 2 + ; A 2 " = anionic analyte).57 1.6.1 Phosphine ether hemilabile ligands Another type of hemilabile ligand that has received significant attention in the literature are phosphine ether ligands.5 4'5 8 There are numerous types of hemilabile 24 ligands that combine a phosphorus donor with an oxygen donor. 5 4 ' 5 8 The phosphorus donor is substitutional^ inert while the ether moiety is substitutional^ labile when the ligand is coordinated to a second or third row transition metal. Since the ether moiety is usually weakly coordinated, there are numerous analytes for which this ligand can be used in chemosensors. H 3co O C H , H 3 C / O Rh C O .-Ps ' A r ^ ^ ^At" 'Ar + 10 + CO CO H3CO, H3CO 11 Figure 1.12. Rh + chemosensor that reacts reversibly with CO through the substitutional^ labile ether ligand (Ar' = 2,4,6-trimethoxyphenyl) 59 Phosphine ether hemilabile ligands have been successfully incorporated into cationic Rh + and R h 2 + complexes (10) that react reversibly with CO (Figure 1.12).59-61 Due to changes in the IR and electronic absorption spectra, these complexes can act as chemosensors. Furthermore, Ru complexes that contain hemilabile phosphine ether ligands (12) have also been synthesized (Figure 1.13). ' These complexes can also show reactivity toward C O . 6 3 25 12 13 Figure 1.13. Reactivity of dichlorobis(o-(diphenylphosphino)anisole)ruthenium(II) toward C O . 6 2 1.7 Pyrene as a pendant lumophore for hemilabile ligands One disadvantage of the later examples in the last section was that signal transduction involved a mechanism other than luminescence. In order to further develop chemosensors based on phosphine ether hemilabile ligands, it would be advantageous to incorporate a fluorophore into the ligand. A suitable fluorophore for this goal is pyrene. It is easier to control side-reactions with pyrene since it has no reactive functional groups and pyrene-containing starting materials are readily available. Lastly, it has been shown in the literature that pyrene fluorescence can be quenched by open-shelled transition metal ions via PET and/or EET, or the heavy atom effect and thus, pyrene-containing chemosensors can display the desirable ON-OFF response to an analyte. 2 4 , 2 5 ' 6 4 , 6 5 The synthesis of a pyrene-containing phosphine ether hemilabile ligand has been 66 2"r" realized. The next section will discuss the utility of this ligand in a Ru complex and in the sensing of CO. 26 1.8 A CO sensor, fcc-RuCl2(POC4Pyr-P,0)2 Using the above mentioned design principles, the sensor molecule previously developed in our laboratory, fcc-RuCl2(POC4Pyr-P,0)2, (POC4Pyr = 4-{2-(diphenylphosphino)phenoxy}butylpyrene) (14) is shown in Figure 1.14.66 The metal center is attached to the receptor via the easily substituted ether-oxygen of the hemilabile ligand, the alkyl chain is the linker, and the pyrene moiety is the luminescent reporter mechanism. It was designed such that in the absence of the analyte, the pyrene moiety would be close enough to the R u 2 + center such that its fluorescence would be quenched via PET and/or EET, or the heavy atom effect.66 Upon coordination of CO, the pyrene moiety would move farther away from the metal center and its fluorescence would not be quenched.66 N M R , MS, IR, UV-vis absorption, and fluorescence spectroscopies were used to characterize the complex and analyze its reactivity in solution toward carbon monoxide.66 It was found that CO displaces both of the weakly-bonded ether ligands to form the complex W-RuCl 2(CO) 2(POC4Pyr-P) 2 (15).66 The reaction with CO is accompanied by a drastic colour change of the solution.66 The initial /rans-dicarbonyl product is a kinetic complex and undergoes isomerization after excess CO is removed from the system.66 The thermodynamic product, ccf-RuCl 2(CO) 2(POC4Pyr-P) 2 (16), is then formed.66 27 I C 1 \ Pyr(H2C)4 (CH2)4Pyr 14 Pyr(H 2C) 4 ' + 2 CO °%. | ' / P h 2 'Ru ' PhjP^I X O CI ,(CH2)4Pyr Isomerization Pyr(H2C)4-° \ I / P h 2 Ph 2 P^I CI CI (CH2)4Pyr 15 16 Figure 1.14. Reactivity of fcc-RuCl2(POC4Pyr-P,0)2 (14) toward CO (Pyr = pyrene) 66 1.8.1 Photophysical properties and luminescence response toward CO of tcc-RuCl2(POC4Pyr-P,0)2 The fluorescence spectra for the sensor, and each of its isomers after reaction with CO, are unique. Before reaction with CO, the fluorescence emission spectrum of 14 is composed entirely of pyrene monomer emission.66 The frans-dicarbonyl kinetic product 15 in dilute solution has an emission spectrum that is dominated by pyrene monomer emission but there is also evidence of some excimer emission. The amount of excimer emission increases as the concentration in solution is increased. This observation has led to the conclusion that the excimers are predominantly intermolecular in nature.66 The cis-28 dicarbonyl thermodynamic complex 16 has an emission spectrum dominated by excimer emission, even at very low concentration. This observation is associated with excimers that are forming intramolecularly.66 Some further areas of study for this system include probing the nature of the excimers. Studies have yet to be completed in order to determine whether the excimers of each isomer are dynamic or static. Also, the reversibility of the geometric isomerization has yet to be studied. There is evidence in the literature regarding the mechanism of isomerization and that the process should be reversible. 5 8 ' 6 7 ' 6 8 1.9 The scope of this thesis The fluorescence of the pyrene moieties of 14 is not completely quenched before exposure to CO such that the complex acts as an OFF-ON sensor. Two possible ways of promoting quenching are by changing the energies of the metal d-orbitals or by bringing the pyrene moieties closer to the metal center. One way of changing the energies of the metal d-orbitals is by exchanging the two chloro ligands for other halogens, such as bromo or iodo ligands. The difference in the ligand-field splitting parameter of the d-orbitals at Ru , generated by changing the halogen ligands, could be sufficient to promote fluorescence quenching. As mentioned, another way to promote quenching is to bring the pyrene moieties closer to the R u 2 + center. If the tether length could be shortened to a one-carbon, two-carbon, or three-carbon tether, then pyrene fluorescence may be quenched via PET and/or EET, or the heavy atom effect. The focus of this thesis is on two areas: the synthesis of new ligands with shorter tether lengths and the incorporation of these ligands into metal complexes (Chapter 2), and the synthesis and characterization of analogous complexes to 14 with bromo and iodo 29 ligands (Chapter 3). The photophysical and sensing properties of the complexes with three different halogen ligands will be discussed. Furthermore, more in depth studies into the nature of excimer formation are included (Chapter 4). 30 1.10 References (1) Prodi, L . ; Bolletta, F.; Montalti, M . ; Zaccheroni, N . Coord. Chem. Rev. 2000, 205, 59-83. (2) Haes, A . J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002,124, 10596-10604. (3) Valeur, B. ; Leray, I. Coord. Chem. Rev. 2000, 205, 3-40. (4) Keefe, M . FL; Benkstein, K . D.; Hupp, J. T. Coord. Chem. Rev. 2000, 205, 201-228. (5) de Silva, A . P.; Fox, D. B.; Huxley, A . J. M . ; McClenaghan, N . D.; Roiron, J. Coord. Chem. Rev. 1999,185-186, 297-306. (6) Fernandez, E. J.; L6pez-de-Luzuriaga, J. M . ; Monge, M . ; Olmos, M . E.; Perez, J.; Laguna, A . ; Mohamed, A . A. ; Fackler, Jr., J. P. J. Am. Chem. Soc. 2003,125, 2022-2023. (7) Buss, C. E.; Mann, K . R. J. Am. Chem. Soc. 2002,124, 1031-1039. (8) Tsien, R. Y . Biochem. 1980,19, 2396-2404. (9) Berlman, I. B. Handbook of Fluorescence ofAromatic Molecules; Academic Press, Inc.: New York, 1965. (10) de Silva, A . 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Soc, Dalton Trans. 1976,953-961. 35 Chapter Two Phosphine pyrenyl ether ligands with varying tether lengths 2.1 Motivation for the development of ligands with shorter tether lengths The quenching of aromatic hydrocarbon fluorescence by transition metal ions is a well-known phenomenon.1 Sensors based on photoinduced electron transfer (PET) processes have been developed in which pyrene luminescence has been quenched in the presence of open-shelled metal ions such as Fe , Cu and N i . Furthermore, a Zn sensor based on an alkyl pyrene group covalently bonded to an aza-18-crown-6 at the nitrogen position has been developed in which the quenching of pyrene luminescence was shown to depend on the length of the tether between the receptor moiety of the sensor and the pyrene.4 It is possible that a similar phenomenon may occur with a derivative of/cc-RuCl 2(POC4Pyr-P,0) 2 (POC4Pyr = 4-{2-(diphenylphosphino)phenoxy}butylpyrene) that incorporates a ligand with a shorter tether. This approach could result in complexes that show a different response to CO than the current sensor molecule in that the response could be OFF-ON instead of a change in the excimer: monomer fluorescence ratio. In order to probe the effects of a shorter tether length versus the four-carbon tether in terms of fluorescence quenching, there are three possible target ligands (Figure 2.1), each with an alkyl tether length of less than four carbon atoms. 36 Figure 2.1. Target ligands with alkyl chains less than four carbon atoms in length. 2.2 Synthesis of short-tethered pyrenyl ethers Previously, attempts have been made toward the synthesis of the one-carbon tethered ligand, {2-(diphenylphosphino)phenoxy}methylpyrene (POClPyr), but these were unsuccessful.5 Strategies considered include: treating 2-bromophenol with base followed by the addition of bromomethylpyrene to produce a bromophenoxymethylpyrene intermediate and using this intermediate in a Grignard or organolithium reaction, a crown-ether mediated Williamson ether synthesis using 2-diphenylphosphinophenol and bromomethylpyrene, and a Williamson ether synthesis using hydroxymethylpyrene, base, and CICH2CH2PPI12.5 It was found that the pyrene moiety was incompatible with the formation of Grignard and organolithium reagents due to reduction of pyrene by the metals, and that bromomethylpyrene alkylated the phosphine in the second proposed synthetic route.5 It is known that triaryl-substituted phosphines can be alkylated by benzyl halides.6 In the last attempt, it is believed that the major product was vinyldiphenylphosphine, the product of base-mediated HC1 elimination from C l C H 2 C H 2 P P h 2 . 5 37 Further strategies were considered by this author (Schemes 2.1-2.3), but a successful route was still not found. It is believed from 3 1 P{'H} and lH N M R spectroscopy that the major products of reaction for Schemes 2.1 and 2.2 were alkylated phosphine and a small amount of starting material. Bromomethylpyrene is a very good alkylating agent and thus, should not be used in any further attempts at this synthesis. The route shown in Scheme 2.3 was successful (as determined from 3 1 P{ 1 H} N M R spectroscopy), but the yield was so low that the route is impractical. The major product in this case was alkylated phosphine and starting material. A possible reason the ligand is difficult to synthesize is steric crowding. Since the tether length is so short, the pyrene moeity must be brought into very close proximity with the phosphinophenoxy portion of the ligand. The steric repulsion between the pyrene and diphenylphosphine moieties is very high and energetically unfavourable. As a result, the products obtained from the reactions attempted were either undesired products of side-reactions or starting materials. A ligand with a two-carbon tether 2-{2-(diphenylphosphino)phenoxy}ethylpyrene (POC2Pyr) proved to be a much easier target. A strategy was designed that utilizes reactions that are established in the literature or were used in the synthesis of the four-carbon tethered ligand (Scheme 2.4).7'8 38 Scheme 2.1. Synthesis of P O C l P y r via a Williamson ether synthesis using K O H as the base. Scheme 2.2. Synthesis of P O C l P y r via a Williamson ether synthesis using an amine base. Scheme 2.3. Synthesis of POC1 Pyr via a 2-iodophenoxy intermediate. 3 9 P P h 2 -I.KO'Bu, THF 18-crown-6 OH 18 Scheme 2.4. Synthesis of POC2Pyr (18). 2.3 Characterization of POC2Pyr Compound POC2Pyr was characterized by solution methods. N M R spectroscopy showed one singlet in the 3 1 P{'H} N M R (£-15.2 in CDC13) and the ! H N M R spectrum was fully assigned. Attempts were also made to characterize the ligand by UV-vis absorption spectroscopy and fluorescence spectroscopy. Both of these techniques showed impurities although the expected characteristic peaks of the pyrene 71-71* transition were observed in both. These impurities originate from the starting material and are extremely difficult to remove completely. The compound was pure by N M R spectroscopy but not by elemental analysis. Further studies should be done with this compound to determine a practical way to increase the purity of the ligand. Nonetheless, the POC2Pyr obtained was pure enough to conduct preliminary studies into the utility of the compound as a ligand on R u 2 + . 40 2.4 P0C2Pyr as a ligand on RiT Attempts to coordinate POC2Pyr to Ru in an analogous way to POC4pyr were unsuccessful (Scheme 2.5). Each time the reaction was attempted the only products obtained were a black precipitate and oxidized ligand. The presence of oxidized ligand 31 1 was confirmed by J , P{ 'H} N M R and since the ligand was oxidized, it is likely that the black precipitate is reduced ruthenium. A possible explanation for this observation is steric crowding. The pyrene moieties are much closer to the rest of the ligand in POC2Pyr compared to POC4Pyr and as a result, there is greater steric repulsion between the two ligands around the metal center. Scheme 2.5. Proposed synthesis of ta>RuCl2(POC2Pyr-.P,0)2 (Pyr = pyrene). 2.5 Summary and outlook Although the full potential for this area of research has yet to be fulfilled, significant strides have been made. There is still potential for using fcc-RuCl2(POC4Pyr-P,0)i as an OFF-ON switch by developing ligands with an alkyl tether lengths less than four carbon atoms. Although POClPyr now seems an unlikely potential ligand, the utility of POC2Pyr in a R u 2 + complex should be explored further. It is hoped that the research presented in this chapter will be a foundation for exploring synthetic routes for 41 developing the ligand POC3Pyr, and incorporating POC2Pyr and POC3Pyr into Ru complexes. 2.5.1 Suggestions for future work As it appears that the two-carbon tethered ligand is too bulky to coordinate two ligands around the ruthenium center, complexes of the type shown in Figure 2.2 could be synthesized that incorporate only one ligand. It is anticipated that these complexes would show a similar response to CO except that only one CO molecule would be coordinated instead of two. The fluorescence response may be slightly different since intramolecular excimer formation would not be possible. 21 Figure 2.2. Possible compound for incorporating POC2Pyr into a Ru complex. Observing the effects of a shorter tether could also be achieved by using a three-carbon tethered ligand. A proposed synthesis is shown in Scheme 2.6. This synthetic route has previously been used to synthesis anthracene-containing alcohols.9 42 H 3 0 + , H 20 reflux B r 2 , P P h 3 Scheme 2.6. Proposed synthesis for the ligand POC3Pyr. 2.6 Experimental section 2.6.1 General details N M R spectra were acquired on Bruker AC-200, Avance 300 or Avance 400 instruments. Residual protonated solvent peaks were used as internal ] H references.(vs. TMS at SO). 31?{l~R} N M R spectra were referenced to 85 % H 3 P 0 4 (SO). The U B C Department of Chemistry Microanalytical Services Laboratory performed the elemental analyses. Mass spectra were also recorded by the U B C Department of Chemistry Microanalytical Services Laboratory. Electron Impact (EI) mass spectra were recorded using a Kratos MS-50 double focusing sector mass spectrometer equipped with an 43 electron impact ion source. A l l reactions were carried out under a nitrogen atmosphere unless otherwise stated. 2.6.2 Materials Chemicals were used as received from the supplier (Aldrich) unless otherwise specified. Deuterated solvents were used as received from Cambridge Isotope Labs. Bromomethylpyrene10 and 2-diphenylphosphinophenol11'12 were prepared as described elsewhere. Pyreneacetic acid was purified according to literature procedure prior to use.13 Ethanol was de-gassed and acetonitrile was dried over 4A molecular sieves overnight prior to use. Triethylamine was distilled from sodium ethoxide under nitrogen and THF was distilled from sodium/benzophenone under nitrogen. CH2CI2 was passed over an alumina column under nitrogen prior to use. 2.6.3 Preparation and characterization of the phosphine pyrenyl ether ligands 2.6.3.1 Synthesis of (2-iodophenoxy)methylpyrene K O H (0.058 g, 1.04 mmol) was dissolved in warm ethanol (50 mL). A constant pressure addition funnel was charged with 2-iodophenol (0.230 g, 1.05 mmol) dissolved in ethanol (15 mL). The solution was added as a steady stream to the base and the reaction mixture was left stirring at room temperature for 2.5 h. The addition funnel was then charged with bromomethylpyrene (0.350 g, 0.90 mmol) dissolved in THF (10 mL). The electrophile was added dropwise and the reaction mixture was left stirring for 18 h at 65 °C. The solvent was removed under reduced pressure to yield a brown oil. The brown oil was suspended in CH2CI2 (50 mL), distilled water (100 mL) was added, and the suspension was stirred for 20 minutes. The organic layer was isolated and the aqueous layer was extracted with CH2CI2 (2 x 50 mL). The combined organic extracts were 44 washed with distilled water (50 mL) and dilute NaHC03 (50 mL), and dried using Na2S04. Decolourizing carbon was added, and the solution was gravity filtered. The solvent was then removed under reduced pressure to yield the desired product. Yield: 78% as a tan solid. *H N M R (200 MHz, 25 °C, CDC13): £8.38 - 7.90 (m, 9H, pyrene), 7.48 (d, IH, Ph), 7.15 (t, IH, Ph), 6.95 (d, IH, Ph), 6.60 (t, IH, Ph), 5.21 (s, 2H, CH2). 2.6.3.2 Synthesis of {2-(diphenylphosphino)phenoxy}methylpyrene, POClPyr (Scheme 2.3) The following route follows a strategy found in the literature for other phosphinophenoxy compounds.14 (2-Iodophenoxy)methylpyrene (0.250 g, 0.58 mmol) was dissolved in acetonitrile (40 mL) and heated to 50 °C. Diphenylphosphine (0.1 mL, 0.58 mmol) and triethylamine (0.08 mL, 0.58 mmol) were added and after approximately 10 minutes, Pd(OAc)2 (0.0006 g, 0.003 mmol) was also added. The reaction mixture was heated to reflux for 44 h. The solvent was removed under reduced pressure to yield a light brown oil. The oil was dissolved in CH2CI2 and washed with distilled water (3 x 75 mL). The isolated organic layer was dried using N ^ S C U and the solvent was removed under reduced pressure to yield an oil. N M R (400 MHz, 25 °C, CDC1 3): £-15.90 (s). 2.6.3.3 Synthesis of {2-(diphenylphosphino)phenoxy}methylpyrene, POClPyr (Scheme 2.2) The following is based on a route from the literature for a similar phosphinophenoxy compound.15 A solution of 2-diphenylphosphinophenol (0.331 g, 1.19 mmol) and bromomethylpyrene (0.400 g, 1.19 mmol) in triethylamine (90 mL) and CH2CI2 (20 mL) was heated for 48 hours at 80 °C. After the reaction time, a fine white 45 powder had precipitated out of solution. Additional CH2CI2 (100 mL) was added and the amine was neutralized with dilute HC1. The solution was stirred for 20 minutes. The organic layer was isolated and then washed with distilled water (2 x 50mL), dried with Na2S04, and the solvent was removed under reduced pressure to yield a yellow oil. The oil was re-dissolved in CH 2 C1 2 and the product was precipitated out using hexanes. 2.6.3.4 Synthesis of {2-(diphenylphosphino)phenoxy}methylpyrene, POClPyr (Scheme . 2.1) K O H (0.100 g, 1.79 mmol) was dissolved in warm ethanol (5 mL). A n addition funnel was charged with 2-diphenylphosphinophenol dissolved in ethanol (10 mL) and acetone (5 mL). The 2-diphenylphosphinophenol was added as a continuous stream to the base and the reaction mixture was left stirring at room temperature for 2 h to yield a cloudy, pale yellow solution. The addition funnel was then charged with bromomethylpyrene (0.425 g, 1.10 mmol) dissolved in THF (5 mL). This solution was added dropwise and during the addition, the reaction mixture became a darker yellow and a fine precipitate formed. The reaction mixture was heated to 60 °C and was left stirring for 3 h. The solvent was removed under reduced pressure to yield an orange solid. The product was re-dissolved in CH2CI2 and distilled water was added. The aqueous layer was isolated and extracted with CH2CI2 (2 x 50 mL). The combined organic extracts were washed once with distilled water (50 mL) and twice with NaHCC>3, and dried using Na2S04. The solvent was then removed under reduced pressure to yield an orange solid. 2.6.3.5 Synthesis of 2-(l-pyrenyl)-1-ethanol THF (60 mL) was added to L i A l H 4 (0.730 g, 19.24 mmol). In a separate flask, pyreneacetic acid (0.500 g, 1.92 mmol) was dissolved in THF (40 mL). The acetic acid 46 was added dropwise to the LiAlFLj solution and the reaction mixture was heated to reflux for 4 h. The excess LiAltLj was destroyed according to the method of Steinhardt (a stirred reduction mixture from n grams of LiAlFL} is treated with successive dropwise additions of n mL H2O, n mL 15% NaOH, and 3n mL H2O, this produces a dry granular precipitate which is easy to filter and wash).16 The reaction mixture was then gravity filtered to remove the precipitate that had formed. The filtrate was diluted with diethyl ether (50 mL), dried using MgS04, and gravity filtered. The solvent was removed under reduced pressure to yield a yellow oil. The product was purified by column chromatography (silica gel, CH2CI2; collected 3 r d band, Rf = 0.29) to yield the desired compound as a white powder. Yield: 72%. Elemental analysis calcd. for CigH^O (%): C, 87.80; H , 5.69; found: C, 87.40; H, 5.67. EI-MS: m/z = 246. ! H N M R (200 MHz, 25 °C, C D C I 3 ) : £8.34 - 7.79 (m, 9H, pyrene), 4.10 (t, 2H, CH2), 3.62 (t, 2H, CH2). 2.6.3.6 Synthesis of 2-(l-pyrenyl)-1-bromoethane A solution of PPh 3 (0.426 g, 1.63 mmol) and imidazole (0.224 g, 3.26 mmol) in CH2CI2 (50 mL) was cooled to 0°C. Bromine (0.04 mL, 0.78 mmol) was added dropwise and the mixture was stirred for 20 minutes at 0 °C. During this time, a white precipitate formed. 2-(l-Pyrenyl)-1-ethanol (0.200 g, 0.81 mmol) was dissolved in CH 2 C1 2 (25 mL) and added to the stirring reaction mixture as a steady stream. The reaction mixture was left stirring at 0 °C for 25 minutes and at room temperature overnight. Saturated NaHC03 solution (50 mL) was added and the suspension was stirred for 20 minutes. The organic and aqueous layers were separated and the aqueous layer was extracted with CH2CI2 (3 x 40 mL). The combined organic extracts were dried with MgS04, gravity filtered, and the solvent was removed under reduced pressure to yield a pale yellow 47 powder. The product was purified by column chromatography (silica gel, CH2CI2; collected 1s t band, Rf = 0.94) to yield the desired compound as a white powder. Yield: 88%. Elemental analysis calcd. for C i 8 H 1 3 B r (%): C, 69.90; H , 4.21; found: C, 69.55; H, 4.14. EI-MS: m/z = 309. ' H N M R (300 MHz, 25 °C, C D C I 3 ) : £8.26 - 7.84 (m, 9H, pyrene), 3.89 (m, 2H, CH2), 3.76 (m, 2H, CH2). 2.6.3.7 Synthesis of 2-{2-(diphenylphosphino)phenoxy}ethylpyrene, POC2Pyr THF (50 mL) was added to 2-diphenylphosphinophenol (0.235 g, 0.84 mmol) and 18-crown-6 (0.222 g, 0.84 mmol). Potassium tert-butoxide in THF (1.0M, 0.84 mL, 0.84 mmol) was added dropwise and the reaction mixture was stirred for 20 minutes at room temperature. In a separate flask, 2-(l-pyrenyl)-l-bromoethane (0.258 g, 0.84 mmol) was dissolved in THF (20 mL) and added to the basic solution dropwise. The reaction mixture was stirred for 16 h at room temperature. The solvent was then removed under reduced pressure to yield a pale yellow powder. The product was purified by column chromatography (silica gel, CH2CI2; collected 1 s t band, Rf = 0.94) to yield the product as a pale yellow powder. Yield: 44%. EI-MS: m/z = 507. 3*P N M R (200 MHz, 25 °C, CDCI3): £-15.20 (s). ' H N M R (200 MHz, 25 °C, CDCI3): £8.35 - 7.65 (m, 25H, pyrene and Ph), 5.88 (t, 2H, CH2), 5.55 (t, 2H, CH2). 2.6.4 Preparation of ruthenium complexes 2.6.4.1 Synthesis oftcc-RuCl2(POC2Pyr-P,0)2 POC2Pyr (0.321 g, 0.63 mmol) was heated in ethanol (60 mL) to reflux temperature. Toluene (20 mL) was then added to ensure that the ligand was completely dissolved. Distilled water (15 mL) was added to RuCl 3 -xH 2 0 (0.0659 g, 0.32 mmol) and the solution was sonicated for 10 minutes followed by heating with a heat gun. This 48 treatment was repeated two more times. The solution was diluted with an equal volume of ethanol (15 mL) and was added to the ligand solution at reflux temperature as a steady stream. The reaction mixture was heated to reflux temperature for 24 h. Over the course of the reaction time, the solution remained black and opaque. The reaction mixture was gravity filtered to yield a dark yellow solution. The solvent was removed under reduced pressure to yield a brown solid. 49 2.7 References (1) de Silva, A . P.; Gunaratne, H. Q. N . ; Gunnlaugsson, T.; Huxley, A . J. M . ; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97,1515-1566. (2) Bodenant, B. ; Fages, F.; Delville, M. -H . J. Am. Chem. Soc. 1998,120, 7511-7519. (3) Bodenant, B. ; Weil, T.; Businelli-Pourcel, M . ; Fages, F.; Barbe, B.; Pianet, I.; Laguerre, M . J. Org. Chem. 1999, 64, 7034-7039. (4) Ji, H.-F.; Dabestani, R.; Brown, G. M . ; Hettich, R. L . Photochem. Photobiol. 1999, 69,513-516, (5) Rogers, C. W. Ph.D. Thesis, University of British Columbia, Dec. 2001. (6) Cairns, S. M . ; McEwen, W. E. Heteroatom Chem. 1990,1, 9-19. (7) Yang, N . C ; Minsek, D. W.; Johnson, D. G.; Larson, J. R.; Petrich, J. W.; Gerald III, R.; Wasielewski, M . R. Tetrahedron 1989, 45, 4669-4681. (8) Rogers, C. W.; Wolf, M . O. Angew. Chem. Int. Ed. 2002, 41, 1898-1900. (9) Stewart, F. H . C. Aust. J. Chem. 1960,13, 478-487. (10) Duhamel, J.; Yekta, A. ; Hu, Y . Z.; Winnik, M . A . Macromolecules 1992,25, 7024-7030. (11) Horner, L. ; Simons, G. Phosph. Sulf. 1983,14, 189-209. (12) Empsall, H . D.; Shaw, B . L. ; Turtle, B. L . J. Chem. Soc. Dalton Trans. 1976, 1500-1506. (13) Chan, W. K. ; Sui. Z.; Ortiz de Montellano, P. R. Chem. Res. Toxicol. 1993, 6, 38-45. (14) Herd, O.; HeBler, A. ; Hingst, M . ; Tepper, M . ; Stelzer, O. J. Organomet. Chem. 1996, 522, 69-76. 50 Baker, M . J.; Pringle, P. G. J. Chem. Soc., Chem. Commun. 1993, 314-316. Steinhardt, C. K. , as cited in: Fieser, L . F.; Fieser, M . Reagents for Organic Synthesis, Vol. 1; Wiley: New York, 1967; p. 584. 51 Chapter Three A series of luminescent Ru 2 + complexes with different halo ligands 3.1 P r o m o t i n g the q u e n c h i n g o f p y r e n e l uminescence One possible way of promoting pyrene quenching by the metal center for tcc-RuCl 2 (P0C4Pyr-P,0) 2 (14) (P0C4Pyr = 4-{2-(diphenylphosphino)phenoxy}butylpyrene) via PET is by changing the energies of the metal d-orbitals. Substituting the chloro ligands for bromo or iodo ligands may achieve this goal. The spectrochemical series decreases in the order CT > Br" > I". Thus, 10 Dq is larger i f the complex has chloro ligands than if it has iodo ligands. Pyrene quenching via PET will occur if, as a result of changes in 10 Dq, the pyrene L U M O is above the metal eg orbitals instead of below the eg orbitals. Alternatively, PET can occur from the metal t 2 g orbitals to the pyrene HOMO. The exact energies of the relevent orbitals can be calculated using electrochemistry and UV-vis absorption spectroscopy. However, due to decomposition of the complex during electrochemistry experiments, the energies of the orbitals for complex 14 could not be calculated.1 3.2 Synthes i s a n d c h a r a c t e r i z a t i o n o f complexes w i t h b r o m o a n d i o d o l i g a n d s In the literature, there are numerous examples of similar phosphine complexes for which a series of chloro, bromo, and iodo complexes were synthesized. " In general there are two synthetic strategies that are used: substitution of the chloro ligands with bromo or iodo ligands using the potassium or sodium salt of the desired halogen ligand, or the use of starting materials containing the desired halogen ligand.6 52 3.2.1 Synthesis of complexes with bromo and iodo ligands Initially, the bromo analogue of 14 was synthesized by trying the substitution strategy using NaBr; however, for unknown reasons, this route was unsuccessful. Substitution in this way to form the bromo analogue has not been successful for other similar complexes in the literature.2'5 However, using R u B r 3 x H 2 0 as the starting material, and following the same synthetic strategy as for 14, the desired complex was obtained (Scheme 3.1). RuBr 3-xH 20 Scheme 3.1. Synthesis of fcc-RuBr2(POC4Pyr-P,0)2 (22). The iodo analogue was easily synthesized by the substitution route using Nal (Scheme 3.2). Scheme 3.2. Synthesis of fcc-RuI2(POC4Pyr-P,0)2 (23). 53 3.2.2 Characterization oftcc-RuX2(POC4Pyr-P,0)2 (X = CI (14), Br (22), 1(23)) Complexes 14, 22, and 23 were characterized by solution methods. No solvent systems were found that yielded crystals suitable for X-ray crystallographic analysis. Previously, 14 was fully characterized and its spectral characteristics in solution compared to the complex fcc-RuCl2(POMe-P,0)2 (POMe = 2-methoxyphenyldiphenylphosphine) for which a solid-state structure is known.1 These assignments were used to aid in the assignment of the structures of 22 and 23. In the 3 1 P N M R spectrum of each complex, there is a singlet at 8 63.7 for 14, £64.7 for 22, and 8 66.0 for 23 (all in CDCI3). A downfield shift as the ligands are changed from chloro to iodo is expected. As the halogen becomes less electronegative, there is more electron density at the R u 2 + center and the phosphorus nucleus is more shielded, resulting in a lower field chemical shift.8 Since these chemical shift values are similar and singlets are observed for all three complexes, it has been assumed that the complexes all have the same geometry; the two halogen ligands are /raws-disposed to one another and the two PjO-coordinated phosphine ether ligands are coordinated such that the phosphines are ds-disposed to one another and are chemically equivalent. To confirm the relative 1 "X 1 stereochemistry of the phosphines, a series of C{ H} N M R experiments have been completed for 14.9 It has been assumed that the same assignments hold for 22 and 23. 3.2.3 UV-vis absorption spectrum oftcc-RuX2(POC4Pyr-P,0)2 (X = CI (14), Br (22), 1(23)) The UV-vis absorption spectra of 22 and 23 are essentially identical to that of 14 (Figure 3.1). For each complex, the structured absorptions in the U V region arise predominantly from pyrene-based n-n* transitions.10 Weaker absorptions due to 54 transitions within the triphenylphosphino portion of the molecule are buried beneath the strong pyrenyl n-n* bands.1 Furthermore, each complex has a weak, metal-based visible absorption band that gives rise to the observed colour (Figure 3.2). The transition is at 517 nm (e 5n = 5.8 ± 0.3 x 102 rvT'cm"1 in CH 2C1 2) for the red complex 14, at 542 nm (e542 = 5.3 ± 0.2 x 102 M^cm" 1 in CH 2C1 2) for the purple-red complex 22, and at 574 nm (E574 = 5.2 ± 0.4 x 102 IVrW 1 in CH 2C1 2) for the green complex 23. The P A values for each complex have been calculated based on the most intense absorption, the *L a band. The dichloro complex 14 has P A = 2.50, dibromo complex 22 has P A = 2.14, and di-iodo complex 23 has P A = 2.34. A l l three of these results can be interpreted as indicating that there is some ground-state interaction between the pyrene moieties. However, the values are still relatively close to 3 and as such, the pyrenes do not appear to be close enough to form a ground-state dimer.11 55 300 400 500 600 Wavelength (nm) Figure 3.1. UV-vis absorption spectra for fcc-RuX2(POC4Pyr-P,0)2 (X = CI (14) (22), I (23)); in CH 2 C1 2 ; [fcc-RuX 2(POC4Pyr-P,0) 2] * l ( r 5 M . 56 Wavelength (nm) Figure 3.2. Metal-based absorption band in the UV-vis spectrum of tcc-RuX 2(POC4Pyr-P,0) 2 (X = Cl (14), Br (22), I (23)); in CH 2 C1 2 ; [fcc-RuX2(POC4Pyr-P,0)2] * 0.003 M . 57 3.3 fcc-RuX2(POC4Pyr-P,0)2 (X = Cl, Br, I) as luminescent molecular sensors 3.3.1 Reactivity of tcc-RuX2(POC4Pyr-P,0)2 (X = Cl, Br, I) toward carbon monoxide It has been established in the literature that complexes of the type RuX 2 (POL-P,0)2 (X = halogen, POL = hemilabile phosphine ether ligand) react with CO to form complexes of the type RuX 2 (CO) 2 (POL-P) 2 or RuX 2 (CO)(POL-P,0)(POL-P). 2 ' 3 , 1 2 ~ 1 6 The same observation was found with 14 and accompanying changes in photophysical properties have been described.1 In this section, the reaction with CO for 22 and 23 will be discussed and comparisons made between the three complexes. 3.3.2 Reaction with carbon monoxide to form a trans-dicarbonyl complex If a solution of any of the complexes 14, 22 and 23 in aromatic or chlorinated solvents is exposed to CO, the colour of the solution changes immediately. The dichloro complex changes from red to green-yellow, the dibromo complex changes from purple-red to yellow, and the di-iodo complex changes from green to orange. A discussion of the chemistry involved in the reaction with CO will follow in the subsequent sections as well as detailed discussion about the characterization and absorption properties of each dicarbonyl complex. 3.3.3 Characterization ofttt-RuX2(CO)2(POC4Pyr-P)2 (X = Cl (15), Br (24), I (25)) Complexes 14, 22 and 23 all underwent a dramatic colour change after reaction with CO. It was anticipated that the weakly-bonded ether ligands would be displaced by CO to form a dicarbonyl complex. To elucidate the structure of the complexes, solution methods were used. Immediately after exposure to CO, the 3 1 P{ 1 H} N M R spectrum of each complex showed a singlet at £27.1 for 15, £26.0 for 24, and £22.5 for 25 (all in CDCI3). A l l of these chemical shift values are shifted upfield relative to the chemical 58 shift values prior to reaction with CO. Carbon monoxide is a 7t-accepting ligand; electron density is drawn away from the metal center and as a result, the phosphorus nuclei are deshielded.17 Previously, a series of 1 3 C{'H} N M R experiments were conducted using 13C-labeled carbon monoxide to assign the relative stereochemistry of 15 as ttt? These experiments were not repeated for 24 and 25 but it has been assumed that the same relative stereochemistry applies for 24 and 25 since the 3 1 P N M R chemical shifts are similar for the three complexes. IR spectroscopy was used to further support these conclusions. If the complexes have two trans disposed CO ligands, one absorption is expected in the C-0 stretching region. As can be seen in Figure 3.3, each complex has one absorption in the C-0 stretching region (Vco = 2007 cm"1 for 15, V c o = 2001 cm"1 for 24, vco = 2007 cm"1 for 25, all in CH2CI2). Upon coordination, there is a shift of the C-0 absorption to lower energy compared to free CO (vto = 2143 cm"1), consistent with rc-backbonding from the Ru center to the CO ligands. The rc-backbonding results in an increase of electron density in the antibonding orbitals of CO and as such, the C-0 bond is weakened. Using this data it was concluded that immediately after reaction with CO, the resulting complexes are all /#-RuX2(CO)2(POC4Pyr-.P)2. These findings and stereochemical assignment are consistent with literature results for similar complexes. 1 2 ' 1 3 ' 1 6 ' 1 8 59 —I I 1 1 h-2020 2000 1980 Wavenumber ( c m 1 ) Figure 3.3. C-0 region of IR spectra for ftf-RuX 2(CO) 2(POC4Pyr-P) 2 (X = C l (15), Br (24), I (25)); [^-RuX 2(CO) 2(POC4Pyr-P) 2] « 0.0010 M ; all in CH 2 C1 2 . 3.3.4 UV-vis absorption ofttt-RuX2(CO)2(POC4Pyr-P)2 (X= Cl (15), Br (24), 1(25)) The UV-vis absorption spectra of all three complexes 15, 24 and 25 were essentially identical. The structured absorption bands in the UV-region are assigned to pyrene-based TC-TV* transitions and are of indentical structure to the spectra before exposure to CO. A summary of key observations is provided in Table 3.1. For each frans-dicarbonyl complex, the weak metal-based transition is blue-shifted relative to its position before exposure to CO. The ether is a weak-field ligand while CO is a strong-field ligand.1 7 Therefore, substitution of the ether by CO causes the d-d transition absorption band to shift to higher energy. The P A values were calculated for each frans-dicarbonyl complex and changed very little relative to those calculated before exposure to CO. There is some interaction 60 between the pyrenes but they are not close enough to form ^--stacked dimers. This conclusion is further supported by the observation that the maximum absorption of the ' L A band is not red-shifted." Table 3.1. Comparison of absorption properties of fcc-RuX2(POC4Pyr-P,0)2 vs. ttt-RuX 2(CO) 2(POC4Pyr-P) 2 (X - CI, Br, I). Complex X ( ' L A band) (nm) a X (d-d transition) (nm) P A /cc-RuCl 2(POC4Pyr-P,0) 2 (14) 345 517 2.50 W-RuCl 2(CO) 2(POC4Pyr-P) 2 (15) 345 440 2.65 fcc-RuBr2(POC4Pyr-P,0)2 (22) 345 542 2.14 «/-RuBr2(CO)2(POC4Pyr-P)2 (24) 345 455 2.45 fcc-RuI2(POC4Pyr-P,0)2 (23) 345b 574" 2.34 ftt-RuI2(CO)2(POC4Pyr-P)2 (25) 345" 500b 2.28 C H 2 C 1 2 ; b CHC1 3 . 3.3.5 Geometric isomerization to form a cis-dicarbonyl complex It is well known in the literature that complexes of the type #f-RuX2(CO)2(PR3)2 isomerize to form the thermodynamically more stable isomer ccf-RuX2(CO)2(PR3)2.13'16 The mechanism of this isomerization from the initial kinetic isomer to the thermodynamically preferred isomer typically occurs through a dissociative mechanism.3 Thus, the mechanism involves first the dissociation of CO to form a 5-coordinate intermediate (II), rearrangement of the coordination sphere around the metal center ( I I I) , and then re-coordination of CO (IV) (Scheme 3.3).4 This mechanism is consistent with the observation that isomerization does not occur until all of the excess CO is removed 61 from solutions of 15, 24, or 25. Excess CO is removed by sparging the solutions with N 2 or by degassing them via repeated freeze-pump-thaw cycles. L L CO CO 1^  -co I/* /> +co |# X Ru X - X R u ^ . X R i L * • X Ru CO o c ' l + c o | > - / I L L X X I II III IV Scheme 3.3. Mechanism for the isomerization of complexes RuX2(CO)2L2 (X = halogen ligand, L = ligand with phosphorus donor atom). 3.3.6 Characterization ofcct-RuX2(CO)2(POC4Pyr-P)2 (X= Cl (16), Br (26), 1(27)) Like the ttt isomers, the cct isomers contain two equivalent phosphines, as indicated by the singlet resonance in the 3 1 P{ 1 H} N M R spectrum for each complex (CDC1 3: 813.9 for 16, 810.2 for 26, and £5.1 for 27). The dichloro complex has previously been studied using 1 3 C{'H} N M R experiments with 1 3C-labeled carbon monoxide in order to verify the stereochemical assignment as cct? Again, it has been assumed that the same assignments apply for 26 and 27. IR spectroscopy was also used to characterize the complexes. Two absorptions in the C-0 stretching region are expected for c^RuX 2 (CO) 2 (POC4Pyr-P) 2 . As can be seen from Figure 3.4, this result was observed for each complex (vco = 2059, 1998 cm"1 for 16, vco = 2059, 2000 cm"1 for 26, and vCo = 2055, 1989 cm"1 for 27, all in CH 2C1 2). Thus, the IR spectra support the assignment of the fhermodynamically preferred product as cct. 62 J - H 1 1 1 1 1 1 1 1 1 1 2060 2040 2020 2000 1980 Wavenumber ( c m 1 ) Figure 3.4. C-0 region of IR spectra for cc/-RuX 2(CO) 2(POC4Pyr-P) 2 (X = CI (16), Br (26), I (27)); [tf/-RuX2(CO)2(POC4Pyr-P)2] * 0.0010 M ; all in CH 2 C1 2 . 3.3.7 Further analysis of IR data for the geometric isomerization Examples exist in the literature that show how the geometric isomerization can be 1 "\ monitored using IR spectroscopy. By obtaining spectra over the course of the isomerization, insight into the rate of isomerization, and confirmation of the mechanism of isomerization can be obtained: Figures 3.5, 3.6, and 3.7 show the IR spectra of the three complexes taken at different time intervals (all in CH 2C1 2). At t = 0 h, the only absorption in each spectrum is that of W-RuX 2(CO) 2(POC4Pyr-P) 2. The absorption at -2057 cm"1 increases in intensity as the time elapsed increases to 120 h. 63 2? l = Oh t = 2h t = 5h t-23h t = 28h t = 52h t=120h ^ \ 1 v 1 V\ \ \\ Vi 1 ' 1 1 1 1 1 2080 2040 2000 Wavenumber (cm1) Figure 3.5. IR spectra for the isomerization of «/-RuCl 2(CO) 2(POC4Pyr-P) 2 as a function of time. t = 0h t = 2h — t = 5h t = 23h t = 28h t = 52h t = 120h 2080 2040 2000 1960 Wavenumber (cm1) Figure 3.6. IR spectra for the isomerization of «/-RuBr 2(CO) 2(POC4Pyr-P) 2 as a function of time. 64 i 1 1 1 1 1 1 2080 2040 2000 1960 Wavenumber ( c m 1 ) Figure 3.7. IR spectra for the isomerization of «/-Rul2(CO)2(POC4Pyr-P)2 as a function of time. Alternative representations of this data are shown in Figures 3.8, 3.9, and 3.10. In these figures the t = 0 h data has been substracted from each subsequent data set. Thus, positive peaks are decreasing in amplitude while negative peaks are increasing in amplitude. For each analogue, there is a small absorption at lower wavenumbers (v= 1975 cm"1 for 16, v= 1972 cm"1 for 26, and v= 1980 cm"1 for 27). This absorption is tentatively assigned to the CO ligand of the 5-coordinate intermediates of isomerization (II and III). Other similar 5-coordinate complexes have CO absorptions in the same part of the C-0 stretching region. 1 2 ' 1 9 65 — I — 2080 + 2040 2000 1960 Wavenumber (cm 1 ) Figure 3.8. IR spectra for the geometric isomerization of tf^RuCi2(CO)2(POC4Pyr-P)2 corrected for t = 0 h data. t = 2 h — t = 5 h t = 23h t = 28h t = 52h H 1 1 ' 1 1 h-2080 2040 2000 1960 Wavenumber (cm 1 ) Figure 3.9. IR spectra for the geometric isomerization of /«-RuBr 2(CO) 2(POC4Pyr-P)2 corrected for t = 0 h data. 66 — I > 1 1 — 2080 2040 W a v e n u m b e r ( c m " 1 ) 2000 Figure 3.10. IR spectra for the geometric isomerization of W-RuI2(CO)2(POC4Pyr-.P)2 corrected for t = 0 h data. The same IR spectroscopy data has been utilized for an approximate kinetic analysis. For each complex, the intensity of the peak at ~2057 cm"1 was measured and plotted against time. Analysis of the slope of this plot yielded a relative value for the rate of isomerization. It was found that the rate decreased in the order CI" > Br" > T. Though the kinetic analysis warrants further study, this preliminary result supports a dissociative mechanism for geometric isomerization. The rate of isomerization should increase with any factor that promotes bond-breaking in a dissociative mechanism.20 Chloro is the most electronegative ligand in this halide series and as a result, there is less electron density on Ru 2 + . Less electron density on R u 2 + results in less ^-backbonding to the n-accepting CO ligands and weaker Ru-C bonds. As a result, the rate of bond-breaking, 67 and thus the rate of isomerization, will be faster with chloro ligands and slower with iodo ligands. 3.3.8 UV-vis absorption ofcct-RuX2(CO)2(POC4Pyr-P)2 (X= Cl (16), Br (26), 1(27)) Table 3.2. Comparison of absorption properties of /cc-RuX 2(POC4Pyr-P,0) 2 vs. ttt-RuX 2(CO) 2(POC4Pyr-P) 2 and cct- RuX 2(CO) 2(POC4Pyr-P) 2 (X - C l , Br, I). Complex X ( ! L a band) (nm)8 X (d-d transition) (nm)a PA fcc-RuCl2(POC4Pyr-P,0)2 (14) 345 517 2.50 /tf-RuCl 2(CO) 2(POC4Pyr-P) 2 (15) 345 440 2.65 crt-RuCl 2(CO) 2(POC4Pyr-P) 2 (16) 345 440 2.24 fcc-RuBr2(POC4Pyr-P,0)2 (22) 345 542 2.14 W-RuBr 2(CO) 2(POC4Pyr-P) 2 (24) 345 455 2.45 crt-RuBr 2(CO) 2(POC4Pyr-P) 2 (26) 345 455 1.84 fcc-RuI2(POC4Pyr-P,0)2 (23) 345b 574b 2.34 W-RuI 2(CO) 2(POC4Pyr-P) 2 (25) 345b 500" 2.28 cc/-RuI 2(CO) 2(POC4Pyr-P) 2 (27) 345b 500b 2.08 C H 2 C 1 2 ; b CHC1 3 . A summary of the UV-vis absorption data for 14-16 and 22-27 is provided in Table 3.2. The spectra are consistent with the expected results of absorption from pyrene-based 71-71* transitions in the UV-region. The position of the metal-based d-d transition for each cct complex was unchanged from the ttt isomer. The P A values for each cct isomer are smaller than the values obtained for the corresponding ttt isomer. This result indicates that there is more pyrene pre-association 68 in the ground state for the cct isomer than for the ttt isomer. However, it doesn't appear that a ground-state dimer forms since the most intense absorption, the lLa band, is not red-shifted relative to the position of this absorption for the trans-dicarbonyl complex.11 The conformation of the cct isomer apparently allows the pyrenes to associate more easily than the conformation of the ttt isomer. Crystal structures of the isomers might aid in determining why there is an increase in pyrene pre-association after geometric isomerization. 3.3.9 Reversibility of geometric isomerization There are examples of compounds with methyiphosphine ligands where geometric isomerization from the frans-dicarbonyl product to the c/s-dicarbonyl product is reversible with U V irradiation.3 , 4'1 6 A solution of cc^-RuCl 2(CO) 2(POC4Pyr-P) 2 in CH 2 C1 2 was irradiated with U V light for 7 h. The products of irradiation could not be determined from N M R or IR spectroscopy and it appears that U V irradiation leads to extensive photochemical reactions. A second experiment was conducted in which visible light was used to test the reversibility of the geometric isomerization. It was anticipated that exciting the d-d transition would weaken the Ru-C bond of the carbon monoxide ligand and result in the formation of the frans-dicarbonyl product. The progress of the photochemical experiment was monitored by IR spectroscopy. However, no trans-dicarbonyl product was observed during irradiation for 10 h. It is not unusual for the geometric isomerization to be irreversible. There are several examples in the literature where the same result was found. 1 3 ' 1 8 The phosphine ligands in these cases were benzylphosphines 69 which are more similar to POC4Pyr. It has been postulated that the cw-dicarbonyl product is thermally too stable relative to the fraws-dicarbonyl product.18 3.4 Summary and outlook Three complexes of the type fcc-RuX2(POC4Pyr-P,0)2 (X = C l , Br, I) have been successfully synthesized and characterized by solution methods. The reactivity of these complexes with CO has been studied using N M R , IR, and UV-vis spectroscopies (Scheme 3.4). Very similar results were observed for the dibromo and di-iodo complexes as for the previously characterized dichloro complex. A l l three complexes react with CO to initially form a kinetic fraws-dicarbonyl complex. The reaction with CO is accompanied by a drastic colour change of the complex in solution. When excess CO is removed from the system, the complexes isomerize to form the thermodynamic product, ccf-RuX 2(CO) 2(POC4Pyr-P) 2. It is likely that the mechanism of isomerization is dissociative; a preliminary kinetic analysis supports this claim. Somewhat surprisingly, it was found that the isomerization is not reversible. 3.4.1 Suggestions for future work Further attempts should be made toward growing crystals of the complexes that are suitable for X-ray crystallographic analysis. Crystal structures would further verify the stereochemical assignments that have been made as well as possibly provide insight into why pyrene pre-association in the ground-state increases after geometric isomerization. 70 Ph 2 x I x \ Pyr(H2C)4 (CH2)4Pyr X = C I 14 X = B r 22 X = I 2 3 + 2 CO Pyr(H 2C) 4" °°\ | / P h 2 Ru* Ph2P^I CO • 2 X X = C 1 1 5 X = B r 24 X = I 2 5 O' ,(CH 2) 4Pyr Isomerization Pyr(H2C)4 ° \ I / P h 2 'Ru' Ph2P^I X X X = C 1 1 6 X = B r 26 X = I 2 7 -(CH2)4Pyr O Scheme 3.4. Reactivity of /cc-RuX 2(POC4Pyr-P,0) 2 toward CO. 3.5 Experimental Section 3.5.1 General Details General synthetic and instrumental details (NMR spectrometers) are described in Chapter Two. IR spectroscopic measurements were made with a B O M E M MB155S FTIR spectrometer using solution samples. A l l IR spectra were corrected for solvent unless otherwise noted by subtracting the appropriate solvent spectrum from each data 71 set. UV-vis spectra were obtained using a Cary 5000 Varian Cary UV-vis-near-IR spectrophotometer. A 1-cm quartz cuvette was used. Electrospray (ES) mass spectra were obtained on a Micromass L C T time-of-flight (TOF) mass spectrometer equipped with an ES ion source. The samples were analyzed in MeOH: CH2CI2 (1:1) at 100 p.M. Reversibility of geometric isomerization using U V light was tested by shining U V light from a Model UVGL-58 Mineralight® Lamp (UVP) on two separate samples. One sample was irradiated with 254 nm light and the other with 366 nm light. 3.5.2 Materials used Chemicals were used as received from the supplier (Aldrich) unless otherwise noted. Deuterated solvents were used as received from Cambridge Isotope Labs. Carbon monoxide was obtained from Praxair and was used as received. Spectroscopic grade CH2CI2 and CHCI3 (Fisher) were used for UV-vis and IR spectroscopic measurements. 3.5.3 Preparation and characterization of Ru(II) complexes 3.5.3.1 Preparation and characterization of tcc-RuBr2(POC4Pyr-P\0)2 (22) POC4Pyr (0.3816 g, 0.71 mmol) was heated in ethanol (60 mL) to reflux temperature. Toluene (17 mL) was then added to ensure that the ligand was completely dissolved. Distilled water (15 mL) was added to RuBrs-xH^O (0.1215 g, 0.36 mmol) and the solution was sonicated for 10 minutes followed by heating with a heat gun. This treatment was repeated two more times. The ruthenium bromide solution was diluted with an equal volume of ethanol or acetone (15 mL) and was added to the ligand solution as a steady stream. The reaction was heated at reflux for 72 h. Over the course of the reaction time, the solution went from black and opaque to red-orange. The reaction mixture was hot-filtered to remove any excess undissolved ruthenium bromide trihydrate 72 and the filtrate was cooled for 12 h. The purple-red solid that had precipitated out of solution was isolated via filtration using a Buchner funnel. • In order to purify the complex, the precipitate was dissolved in CH 2 C1 2 (7.5 mL) and hexanes was added (100 mL). The CH 2 C1 2 was boiled off and the red-purple precipitate was collected by hot filtration and dried under vacuum for several days to yield the pure product. Yield: 29%. Elemental analysis calcd. for C 7 6 H 6 2 B r 2 0 2 P 2 R u (%): C, 66.49; H , 6.55; found: C, 66.71; H , 6.78; sample contains 11 equivalents of acetone that could not be removed in vacuo, confirmed by *H N M R . ESI-MS: m/z= 1249 ( M - B r ) + . 3 1P{'FJ} N M R (300 MHz, 25 °C, CDC1 3): £64.7 (s). ' H N M R (200 MHz, 25 °C, CDC1 3): £8.22 - 7.85 (m, 16H, pyrene), 7.68 (d, 3 J H H = 8.0 Hz, 2H, pyrene), 7.38 - 7.05 (m, 26H, Ph), 6.96 (m, 2H, Ph), 4.78 (m, 4H, pyrene-CH 2(CH 2) 2Ci/ 2-0), 3.11 (m, 4H, pyrene-CrY 2(CH 2) 2CH 2-0), 1.90 (m, 4H, pyrene-CH 2 CH 2 C// 2 CH 2 -0), 1.67 (m, 4H, pyrene-CH 2C/7 2CH 2CH 2-0); assignments based on previous experiments done for fcc-RuCl2(POC4Pyr-P,0)2. 3.5.3.2 Preparation and characterization of tcc-Rul2(POC4Pyr-PtO)2 (23) Acetone (15 mL) was added to fcc-RuCl2(POC4Pyr-P,0)2 (0.096 g, 0.08 mmol) and Nal (0.054 g, 0.36 mmol). The reaction was heated at reflux for 1.5 h. During the reaction time, the solution changed from red to green. The reaction mixture was cooled and a layer of hexanes was added to precipitate the product. The solution was cooled for 12 h and the precipitate was collected on a Buchner funnel. The product was washed with both hexanes and acetone. Drying the product in vacuo yielded the product as an emerald green solid. Yield: 90%. Elemental analysis calcd. for C?6H62I202P2Ru (%): C, 63.94; H, 4.81; found: C, 63.59; H , 4.41; sample contains 2 equivalents of acetone that could not be removed in vacuo, confirmed by ' H NMR. ESI-MS: m/z = 1297 (M -1) + . 73 3 1 P{'H} N M R (400 MHz, 25 °C, CDC1 3): £66.6 (s). ' H N M R (400 MHz, 25 °C, CDC1 3): 8 8.11 - 7.87 (m, 16H, pyrene), 7.69 (d, 2 J H H = 7.3 Hz, 2H, pyrene), 7.34 - 7.07 (m, 26H, Ph), 6.96 (m, 2H, Ph), 4.82 (m, 4H, pyrene-CH 2(CH 2) 2C// 2-0), 3.16 (m, 4H, pyrene-C# 2 (CH 2 ) 2 CH 2 -0) , 2.04 (m, 4H, pyrene-CH 2CH 2C# 2CH 2-0), 1.71 (m, 4H, pyrene-CH 2 Cr7 2 CH 2 CH 2 -0 ) ; assignments based on previous experiments done for tcc-RuCl 2(POC4Pyr-P,0) 2 . 3.5.4 Reactions with CO Solutions of 14, 22, or 23 in either CH 2 C1 2 or C H C I 3 (all were of varying concentration depending on the experiments being performed with the solutions), were sparged with CO until a colour change was observed. The initial fraws-dicarbonyl product was characterized immediately after exposure to CO. Each solution was degassed with nitrogen in order for the geometric isomerization to occur. No colour change was observed for any of the complexes after isomerization. The cw-dicarbonyl product of the di-iodo complex had better solubility in C H C I 3 than CH 2 C1 2 . The dichloro complex has previously been characterized and the characterization data can be found elsewhere.1 *H N M R assignments for the dibromo and di-iodo complexes were assumed to be the same as for the dichloro complex. 3.5.4.1 Preparation and characterization of ttt-RuBr2(CO)2(POC4Pyr-P)2 (24) Treatment of a solution of 22 with CO yielded 24 as a yellow product. IR (CH 2C1 2): vto = 2001 cm - 1 . 3 1 P{ ! H} N M R (300 MHz, 25 °C, CDC1 3): £26.0 (s). ! H N M R (200 MHz, 25 °C, CDCI3): £8.24 - 7.93 (m, 14H, pyrene), 7.80 - 7.58 (m, 8H, pyrene), 7.36 - 7.12 (overlapping m, 14H), 6.91 - 6.78 (overlapping m, 6H), 3.94 (m, 74 4H, pyrene-C# 2CH 2CH 2CH 2-0), 3.18 (m, 4H, pyrene-CH 2 CH 2 CH 2 C# 2 -0), 1.67 (m, 4H, pyrene-CH 2 CH 2 Ci/ 2 CH 2 -0) , 1.40 (m, 4H, pyrene-CH 2 C# 2 CH 2 CH 2 -0). 3.5.4.2 Preparation and characterization of ttt-RuI2(CO)2(POC4Pyr-P)2 (25) Treatment of a solution of 23 with CO yielded 25 as an orange product. IR (CH 2C1 2): vco = 2007 cm"1. ilP{lH} N M R (300 MHz, 25 °C, CDC1 3): £22.5 (s). ' H N M R (200 MHz, 25 °C, CDC1 3): £8.22 - 7.94 (m, 14H, pyrene), 7.86 - 7.64 (m, 8H, pyrene), 7.46 - 7.03 (overlapping m, 14H), 6.97 - 6.74 (overlapping m, 6H), 3.87 (m, 4H, pyrene-Ci/ 2 CH 2 CH 2 CH 2 -0) , 3.10 (m, 4H, pyrene-CH 2 CH 2 CH 2 C# 2 -0), 1.58 (m, 4H, pyrene-CH 2CH 2C# 2CH 2-0), 1.41 (m, 4H, pyrene-CH 2 C# 2 CH 2 CH 2 -0). 3.5.4.3 Preparation and characterization of cct-RuBr2(CO)2(POC4Pyr-P)2 (26) Solutions of 22 treated with CO in CH 2 C1 2 or CHC1 3 and degassed with nitrogen after the colour change undergo gradual conversion to the cw-dicarbonyl product 26. IR (CH 2C1 2): vco = 2059, 2000 cm - 1 . ^ P l ' H } N M R (300 MHz, 25 °C, CDC1 3): £ 10.2 (s). *H N M R (200 MHz, 25 °C, CDC1 3): £8.22 - 7.98 (overlapping m, 24H), 7.74 - 7.58 (overlapping m, 4H), 7.32 - 7.20 (overlapping m, 14H), 7.12 - 7.00 (overlapping m, 2H), 6.92 - 6.80 (overlapping m, 2H), 3.82 (m, 4H, pyrene-Ci/ 2 CH 2 CH 2 CH 2 -0) , 3.16 (m, 4H, pyrene-CH 2 CH 2 CH 2 Ci/ 2 -0), 1.43 (m, 8H, pyrene-CH 2C# 2C// 2CH 2-0). 3.5.4.4 Preparation and characterization of cct-RuI2(CO)2(POC4Pyr-P)2 (27) Solutions of 23 treated with CO in CH 2 C1 2 or CHC1 3 and degassed with nitrogen after the colour change undergo gradual conversion to the cz's-dicarbonyl product 27. IR (CH 2C1 2): vco = 2055, 1989 cm" 1 . 3 1P{'H} N M R (300 MHz, 25 °C, CDC1 3): £5.1 (s). [ H N M R (200 MHz, 25 °C, CDC1 3): £8.22 - 7.97 (overlapping m, 24H), 7.76 - 7.54 (overlapping m, 4H), 7.35 - 7.15 (overlapping m, 14H), 7.10 - 6.98 (overlapping m, 2H), 75' 6.92 - 6.83 (overlapping m, 2H), 3.81 (m, 4H, pyrene-C# 2 CH 2 CH 2 CH 2 -0), 3.18 (m, 4H, pyrene-CH 2CH 2CH 2C# 2-0), 1.50 (m, 8H, pyrene-CH 2Gr7 2Gf7 2CH 2-0). 76 3.6 References (1) Rogers, C. W. Ph.D. Thesis, University of British Columbia, Dec. 2001. (2) Gi l l , D. F.; Mann, B. E.; Shaw, B. L . J. Chem. Soc, Dalton Trans. 1973, 311-317. (3) Rauchfuss, T. B.; Patino, F. T.; Roundhill, D. M . Inorg. Chem. 1975,14, 652-656. (4) Barnard, C. F. J.; Daniels, J. A. ; Jeffery, J.; Mawby, R. J. J. Chem. Soc. Dalton Trans. 1976, 953-961. (5) Martin, M . ; Gevert, O.; Werner, H. J. Chem. Soc, Dalton Trans. 1996,11, 2275-2283. (6) Levison, J. J.; Robinson, S. D. J. Chem. Soc. (A) 1970, 4, 639-643. (7) Braunstein, P.; Chauvin, Y . ; Nahring, J.; Dusausoy, Y . ; Bayeul, D.; Tiripicchio, A. ; Ugozzoli, F. J. Chem. Soc, Dalton Trans. 1995, 5, 851-862. (8) Loudon, G. M . Organic Chemistry; 3 r d ed.; The Benjamin/Cummings Publishing Company, Inc.: USA, 1995. (9) Rogers, C. W.; Wolf, M . O. Angew. Chem. Int. Ed. 2002, 41,1898-1900. (10) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (11) Winnik, F . M . Chem. Rev. 1993, 93, 587-614. (12) Jeffrey, J. C ; Rauchfuss, T. B. Inorg. Chem. 1979,18, 2658-2666. (13) Krassowski, D. W.; Nelson, J. H.; Brower, K . R.; Hauenstein, D.; Jacobson, R. A . Inorg. Chem. 1988,27,4294-4307. (14) Lindner, E.; Geprags, M . ; Gierling, K. ; Fawzi, R.; Steimann, M . Inorg. Chem. 1995, 34,6106-6117. 77 (15) Lindner, E.; Pautz, S.; Haustein, M . Coord. Chem. Rev. 1996,155,145-162. (16) Bader, A. ; Lindner, E. Coord. Chem. Rev. 1991,108, 27-110. (17) Shriver, D. F.; Atkins, P. W. Inorganic Chemistry; 3 r d ed.; W. H . Freeman and Company: New York, 1999. (18) Wilkes, L . M . ; Nelson, J. H. ; Mitchener, J. P.; Babich, M . W.; Riley, W. C ; Helland, B. J.; Jacobson, R. A. ; Cheng, M . Y . ; Seff, K. ; McCusker, L . B. Inorg. Chem. 1982, 21, 1376-1382. (19) Haefner, S. C ; Dunbar, K . R.; Bender, C. J. Am. Chem. Soc. 1991,113, 9540-9553. (20) Henderson, R. A . The Mechanisms of Reactions at Transition Metal Sites; Oxford University Press: New York, 1995. 78 Chapter Four Photoluminescence properties of *cc-RuX2(POC4Pyr-P,0)2, ttt-RuX2(CO)2(POC4Pyr-P,0)2, and ccf-RuX2(CO)2(POC4Pyr-P)2 (X = CI, Br, I) 4.1 Luminescence response of fcc-RuX2(POC4Pyr-i>,0)2 (X = CI (14), Br (22), I (23)) toward carbon monoxide In previous work, transition metal complexes were designed and synthesized such that before exposure to an analyte, the fluorescence of the fluorophore is quenched by an open-shelled transition metal via PET and/or EET or the heavy atom effect.1"5 Addition of an analyte displaces the fluorescent ligand, or the fluorescent moiety of a ligand, from the quenching metal center. As a result, the ligand fluorescence is not quenched and the presence of the analyte can be detected. As the reactivity of fcc-RuX2(POC4Pyr-P,0)2 (X = CI, Br, I; POC4Pyr = 4-{2-(diphenylphosphino)phenoxy}butylpyrene) toward CO has been established, its utility as a luminescent molecule-based chemosensor will be addressed. It is desirable that the same response will be achieved as for sensors reported in the literature that have shown an OFF-ON fluorescent response toward N O 6 , 7 and pH. 8 Furthermore, comparisons between the three complexes with different halogen ligands will be made. 4.2 Photoluminescence of fcc-RuX2(POC4Pyr-P,0)2 (X = CI (14), Br (22), I (23)) The emission and excitation spectra for all three complexes are shown in Figure 4.1. Excitation of each complex in dilute solution (10"6M) with U V light leads to indigo-blue emission, as is typically observed for the pyrene monomer.9 Spectra from more concentrated solutions (> 10"4 M) are not shown because their excitation, and to a lesser 79 extent, their emission spectra, are distorted due to absorption effects. The emission and excitation spectra are near mirror images of each other and a small Stokes shift typical of fluorescent aromatic compounds is observed.9 The excitation spectra have the same general features as the absorption spectra. The emission and excitation spectra are almost identical for the three complexes (14, 22, and 23) bearing different halogen ligands. However, there are some differences in the intensities of the vibronic bands in each excitation spectrum. Pyrene emission is not completely quenched in any of the complexes. This result indicates that either the changes in metal d-orbital energies with different halogen ligands are not significant enough to lead to quenching of pyrene emission, or that the pyrene moieties are not close enough to the metal center for emission to be quenched. It has been reported that a tether length of four methylene groups is too long to observe significant quenching of pyrene emission via PET. 1 0 None of the emission spectra for complexes 14, 22 or 23 at any concentration studied show excimer emission. The lack of excimer emission indicates that the pyrene moieties do not ;r-stack inter- nor intramolecularly in the excited state. This observation contrasts with what was observed for the free ligand, in that at higher concentrations (> 1(TM), excimer emission was observed. The conformational constraints imposed by the ligand coordinating to R u 2 + appear to prevent ^--stacking. 80 c 0 •4—» c 80 60 40 20 3 c 0 240 160 + 80 + 3 CD 200 + 3 100 11 ' i i t / 14 Exci ta t ion (k = 3 9 8 n m ) v em ' 14 Emiss ion (A, = 3 4 5 n m ) 4- + 22 Exci ta t ion (k = 3 9 8 n m ) x em ' • 22 Emiss ion (k = 3 4 6 n m ) 1 23 Exci ta t ion (k = 3 9 8 n m ) x em ' 23 Emiss ion (k = 3 4 6 n m ) 300 400 500 Wavelength (nm) 600 Figure 4.1. Excitation and emission spectra for fcc-RuX2(POC4Pyr-P,0)2 (X = CI (14), Br (22), I (23)); in CH 2 C1 2 ; [/cc-RuX 2(POC4Pyr-P,0) 2] * 10" 6M. 81 4.3 Photoluminescence of W-RuX2(CO)2(POC4Pyr-P)2 (X = Cl, Br, I) 4.3.1 Photoluminescence ofttt-RuCl2(CO)2(POC4Pyr-P)2 (15) The fluorescence properties of this frans-dicarbonyl complex were previously studied by examining solutions of varying concentrations.12 It was found that upon CO coordination, the conformational freedom of the alkylpyrene moiety, and the distance between the metal center and pyrene, were increased.12 Both of these factors allowed the pyrenes to interact more strongly with one another and ultimately gave rise to the formation of excimers.12 In Figure 4.2, the emission spectra of 15 for solutions of varying concentration are 12 shown. The broad, structureless excimer emission band centered at ~470 nm increases with increasing complex concentration. This observation is consistent with the presence of intermolecular excimers (Scheme 4.1).1 3 It was also found that in dilute solution (10" M), a trace of excimer emission was observed.12 Since intermolecular excimer formation is insignificant at concentrations as low as 10" 6 M, 9 the excimers observed were intramolecular (Scheme 4.2). Scheme 4.1. Proposed structure of an intermolecular excimer for ttt-RuCl 2(CO) 2(POC4Pyr-P) 2 (Pyr = pyrene). 82 400 450 500 550 600 W a v e l e n g t h ( n m ) Figure 4.2. Normalized emission spectra of ^-RuCl 2 (CO) 2 (POC4Pyr-P) 2 ; = 350 nm; in CH 2 C1 2 . Scheme 4.2. Proposed structure of an intramolecular excimer for ttt-RuCl 2(CO) 2(POC4Pyr-P) 2. 83 Further experiments have now been conducted to determine whether the excimers observed for complex 15 are dynamic or static. The experiments described in Chapter One were carried out in order to verify the results obtained from each method. The normalized excitation data for the /ra«s-dicarbonyl complex are shown in Figure 4 .3 . In comparing each data set, Amax for excitation at the excimer emission wavelength is slightly red-shifted compared to excitation at the monomer emission wavelength (Amax = 344 nm at /Ljm - 3 9 7 nm; /U ax = 345 nm at / U m = 471 nm). Furthermore, the peak: valley ratio of the most intense band to that of the adjacent minimum at shorter wavelength decreases slightly. Both of these observations are consistent with the presence of static excimers.14 However, for an excimer to originate from entirely pre-formed aggregates, then the red-shift must be at least 3 nm. 1 5 If the red-shift is approximately 1 nm, then the excimer emission likely originates from transient pyrene aggregates or loosely coupled molecular pairs in the ground state.15'16 To confirm this result, the monomer: excimer intensity ratio (IM/IE) in the steady-state emission spectrum at various excitation wavelengths was monitored. The plot of IM/IE versus wavelength is shown in Figure 4.4. The plot is not a straight line within experimental error and there is a significant decrease in IM/IE at wavelengths longer than 345 nm. These observations are consistent with the occurrence of ground state interaction between the pyrene moieties.14 84 1 2 - - Exci tat ion (X e m = 397 n m ) Exci tat ion (X = 471 n m ) v am ' 240 280 320 360 W a v e l e n g t h (nm) Figure 4.3. Normalized excitation spectra for ^-RuCl2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [15] « 10" 5M; in CH2CI2. 15 340 360 Wavelength (nm) Figure 4.4. I M / I E vs. wavelength for W-RuCl 2(CO) 2(POC4Pyr-P) 2; [15] « 10"5 M ; in CH 2 C1 2 . 85 Using the data from these two fluorescence experiments and the P A values from the absorption data (see Chapter Three), it can be concluded that the excimers observed for complex 15 at low concentration (< 10"5 M) originate from loosely coupled or transient molecular pairs. At low concentration, these excimers are intramolecular in nature and at higher concentration, the excimers are a combination of intermolecular and intramolecular. 4.3.2 Photoluminescence ofttt-RuBr2(CO)2(POC4Pyr-P)2 (24) A similar series of experiments were carried out for 24 as for 15. The emission from a series of solutions of varying concentration is shown in Figure 4.5. The major difference between the two complexes is that the intensity of excimer emission at low concentration is much lower for 24 than for 15; essentially no intramolecular excimer forms for 24. A n explanation for this observation is that due to the difference in size between the chloro and bromo ligands, the alkylpyrene moiety cannot "fold over" the complex as readily to form an intramolecular excimer as shown in Scheme 4.2. Nonetheless, the very small amount of excimer that does exist at low concentration was probed to see if the excimers were dynamic or static. Figure 4.6 shows the excitation spectra at both monomer and excimer emission wavelengths. The red-shift of the most intense peak is ~0.5 nm, and the peak: valley ratio of the 'La band is slightly decreased for excitation at the excimer emission wavelength. A plot of IM/IE versus wavelength indicates the presence of static excimers (Figure 4.7). 1 4 ' 1 7 ' 1 8 These results lead to the conclusion that the excimers of 24 are of a similar nature to those observed for 15: the excimers originate from loosely coupled or transient molecular pairs. 86 400 450 500 550 600 Wavelength (nm) Figure 4.5. Normalized emission spectra of ^RuBr 2 (CO) 2 (POC4Pyr-P) 2 ; /lex = 346 nm; in CH 2 C1 2 . Figure 4.6. Normalized excitation spectra for W-RuBr 2(CO) 2(POC4Pyr-P) 2 at two different emission wavelengths; [24] « 10" 5M; in CH 2 C1 2 . 87 1.2 . 1 , 1 340 360 Wavelength (nm) Figure 4.7. I M / I E VS. wavelength for ^RuBr 2(CO) 2(POC4Pyr-.P) 2; [24] w 10"5 M ; in CH 2 C1 2 . 4.3.3 Photoluminescence ofttt-RuI2(CO)2(POC4Pyr-P)2 (25) The fluorescence results for 25 are similar to those obtained for the previous two complexes. Figure 4.8 shows the emission spectra of 25 in solutions of varying concentration. There is essentially no excimer emission at very low concentration and the amount of excimer increases with increasing concentration. However, even in the most concentrated solution, there is still very little excimer relative to the other two complexes. It is likely that the steric bulk of the iodo ligands prevent the pyrene moieties from interacting with each other inter- nor intramolecularly. However, without a crystal structure to confirm the specific conformation of this complex, steric arguments are somewhat speculative. 88 400 500 600 Wavelength (nm) Figure 4.8. Normalized emission spectra of /#-RuI2(CO)2(POC4Pyr-P)2; Ae X = 348 nm; in C H C I 3 . To probe the nature of the excimers observed, the same two experiments were completed (Figures 4.9 and 4.10). The red-shift observed in the normalized excitation spectra is 0.5 nm and IM/IE decreases at wavelengths greater than 340 nm. Through the same analysis that was carried out for 15 and 24, the same conclusion can be drawn; the excimers originate from loosely coupled molecular pairs. 89 1.2 Excitation (X = 398 nm) 240 280 320 360 Wavelength (nm) Figure 4.9. Normalized excitation spectra for m-Rul2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [25] « 10"5 M ; in CHCI3. 0.8 + 0.4-i 1 1 1 1 340 360 Wavelength (nm) Figure 4.10. I M / I E vs. wavelength for /«-RuI2(CO)2(POC4Pyr-P)2; [25] * 10"5 M ; in C H C I 3 . 90 4.4 Photoluminescence of cct-RuX2(CO)2(POC4Pyr-P)2 (X = CI, Br, I) After excess CO has been removed from a solution of 15, 24, or 25, the trans-dicarbonyl complex undergoes a geometric isomerization to form the thermodynamically favoured c/s-dicarbonyl complex. Due to the different conformation of this isomer, these complexes may have different luminescence properties. 4.4.1 Photoluminescence ofcct-RuCl2(CO)2(POC4Pyr-P)2(16) Previously it was found that after isomerization, strong excimer emission was observed even in dilute solution (Figure 4.11).12 It was concluded that the pyrene moieties in 16 must be able to more easily interact with one another to form an intramolecular excimer.12 The cis configuration of the CO ligands and of the chloro ligands must produce a steric environment that more easily facilitates ^--stacking between 12 the pyrenes. However, pyrene monomer emission is still observed and the excimer emission increases dramatically with increasing solution concentration. Both of these results indicate a combination of excited-state species is in solution after excitation: intermolecular excimers, intramolecular excimers, and pyrene monomers. Further experiments have now been completed to determine whether the excimers are dynamic, static or transient molecular pairs as was observed for the kinetic trans-dicarbonyl complex. The results of the two experiments are shown in Figures 4.12 and 4.13. The same -0.5 nm red-shift in the excitation spectra and decrease in W I E is observed. Taken with the UV-vis spectroscopy results, it can be concluded that the excimers are transient or loosely coupled molecular pairs as was observed for 15. 91 in c £ c g w u> E UJ "O a> N 400 500 Wavelength (nm) 600 Figure 4.11. Normalized emission spectra of cc^-RuCl2(CO)2(POC4Pyr-P)2; 4* = 344 nm; in CH2CI2. 1.2-8 0-8 N 0.4 + 0.0-• Excitation X - 398 nm em Excitation X = 741 nm 240 280 320 Wavelength (nm) 360 Figure 4.12. Normalized excitation spectra for cc^-RuCl2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [16] « 10"5 M ; in CH2CI2. 92 1.0 0.8 + 0.4-I 1 1 1 1 340 360 Wavelength (nm) Figure 4.13. IM/IE vs. wavelength for cc/-RuCl 2(CO) 2(POC4Pyr-F) 2; [16] « 10"5 M ; in CH 2 C1 2 . 4.4.2 Photoluminescence ofcct-RuBr2(CO)2(POC4Pyr-P)2 (26) The results obtained for 26 are different than those obtained for 16. At low concentration the amount of excimer observed is very low (Figure 4.14). At high concentration, the amount of excimer observed for 26 is higher than that observed for 24. As the only difference between 16 and 26 is the halogen ligands, it can be concluded that the larger size of the bromo ligands impedes the formation of intramolecular excimers. However, the conformation of 26 still seems to facilitate the formation of intermolecular excimers as the amount of excimer is higher after geometric isomerization relative to the ttt isomer. 93 1.6 '</> c o 1.2 + 10"3M i r /M 10"5M 10"6M 450 500 Wave leng th (nm) 600 Figure 4.14. Normalized emission spectra of ccV-RuBr2(CO)2(POC4Pyr-.P)2; Ae X = 346 nm; in CH 2 C1 2 . Based on results from IR spectroscopy (see Chapter Three), an argument based on electronic differences has not been put forth to explain the difference in fluorescence between 16 and 26. If there was a significant difference in electron density at the metal center between the two complexes, then the energy of the C-0 absorption in the IR spectrum would be significantly different. The C-0 stretching bands of 16 and 26 are in the same region, with only minor differences between the two complexes ( v c o = 2059, 1998 cm"1 for 16, V c o = 2059, 2000 cm"1 for 26). These observations substantiate the conclusion that steric differences between the two complexes give rise to the observed differences in fluorescence. Nonetheless, electronic factors cannot be completely excluded due to spin-orbit coupling. Spin-orbit coupling would cause non-radiative relaxation to occur over fluorescence and is larger for heavier halogens. Quantum yield 94 or lifetime measurements would conclusively determine i f quenching was occuring for the complexes with heavier halogen ligands due to spin-orbit coupling. The excimers that were observed were studied to determine whether they were loosely coupled molecular pairs as was determined for 24. Indeed, the same observations were made to conclude that the excimers are neither entirely static nor entirely dynamic but transient molecular pairs (Figures 4.15 and 4.16). 240 280 320 360 Wavelength (nm) Figure 4.15. Normalized excitation spectra for cc^-RuBr2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [26] « 10"5 M ; in CH2CI2. 95 1.2 + 0.0-I I 1 1 1 340 360 Wavelength (nm) Figure 4.16. I M / I E vs. wavelength for c^RuBr 2 (CO) 2 (POC4Pyr-P) 2 ; [26] « 10"5 M ; in CH 2 C1 2 . 4.4.3 Photoluminescence ofcct-RuI2(CO)2(POC4Pyr-P)2(27) The fluorescence properties of 27 are most similar to those of 26. There is little excimer observed at low concentration but the amount of excimer observed at high concentration is larger for the cct isomer than for the ttt isomer (Figure 4.17). The same steric argument has been proposed. The large size of the iodo ligands prevents the pyrene moieties from interacting intramolecularly. The cct conformation seems to more easily facilitate the formation of intermolecular excimers. From the excitation and IM/IE experiments, it has been determined that the excimers observed are loosely coupled or transient molecular pairs (Figures 4.18 and 4.19). 96 400 500 600 Wavelength (nm) Figure 4.17. Normalized emission spectra of ccr-RuI 2(CO) 2(POC4Pyr-P) 2; = 348 nm; in CH 2 C1 2 . Figure 4.18. Normalized excitation spectra for cc?-RuI2(CO)2(POC4Pyr-P)2 at two different emission wavelengths; [27] « 10" 5M; in CHC1 3 . 97 — I 1 . 1 1 1 1 330 340 350 Wavelength (nm) Figure 4.19. I M / I E VS. wavelength for cc/-RuI 2(CO) 2(POC4Pyr-P) 2; [27] « 10"5 M ; in CHC1 3. 4.5 Summary and Outlook The fluorescence response toward CO of fcc-RuX2(POC4Pyr-P,0)2 (X = C l , Br, I) and the fluorescence properties of the isomers ^-RuX 2 (CO) 2 (POC4Pyr-P) 2 and cct-RuX 2(CO) 2(POC4Pyr-P) 2 have been studied. Before exposure to CO, each complex displays very similar fluorescence properties. The fluorescence properties change after exposure to CO and formation of the frans-dicarbonyl product. Instead of originating entirely from pyrene monomer, the emission observed is a combination of monomer and excimer emission. The excimers are mainly intermolecular in nature, although some intramolecular excimers are observed, and they arise primarily from loosely coupled or transient molecular pairs. 98 A large change in fluorescence intensity was not observed after exposure to CO: the emission intensity of each /rarcs-dicarbonyl complex was less than double that of the corresponding complex before exposure to CO. Though the results render the chemosensor inefficient, ligands with shorter tether lengths may increase pyrene fluorescence quenching via PET and/or EET or the heavy atom effect. This observation emphasizes the necessity of developing the ligands with shorter tether lengths that were described in Chapter Two. After geometric isomerization to form the ds-dicarbonyl complex, the fluorescence response changes in that the excimer emission intensity increases. The excimer emission at low concentration for complex 16 is significantly higher than for either of complexes 26 or 27 and likely originates from intramolecular excimers. The steric constraints imposed by the bromo and iodo ligands of complexes 26 and 27, respectively, impede the formation of intramolecular excimers. Excimer emission increases with increasing solution concentration for all complexes and thus, intermolecular excimers are also present. Evidence has been provided to indicate that the excimers for all c/s-dicarbonyl complexes originate from loosely coupled or transient molecular pairs. 4.5.1 Suggestions for future work There are some key areas toward which further research on this series of chemosensors should be directed. These areas include determination of the detection limits and other analytical parameters, water solubility, and incorporation of the sensor into the solid state. During a preliminary experiment using complex 14, it was found that exposure of a 1 x 10" 6M solution to an atmosphere containing 0.05% (ca. 850 ppm) CO resulted in a detectable change in emission.12 Further studies should be conducted to determine the 99 exact detection limits for each complex. A collaboration has been established with Prof. Purnendu K. Dasgupta of the Department of Chemistry and Biochemistry at Texas Tech University to further study the analytical properties and utility of the sensor in device fabrication. Their expertise could assist in determining the relevant detection limits. Also, the fluorescence lifetimes and quantum yields of each complex should be measured in order to determine with certainty the extent of fluorescence quenching that occurs before exposure to CO for complexes 14, 22, and 23. It was suggested by our collaborators that water solubility would be an asset for device fabrication using this sensor. Potential schemes for solubilizing the ligand are presented in Schemes 4.3-4.5. Scheme 4.3 is an overall synthesis for the ligand while Schemes 4.4 and 4.5 show other water-soluble phosphines that could be synthesized and used in place of 28. These proposed routes should be explored and may help to generate further methodologies. A l l of the synthetic routes shown involve solubilizing the phosphino moiety of the ligand and are based on examples from the literature.19"23 Furthermore, the complex shown in Chapter Two (Figure 2.4) may also be water soluble and is another synthetic target for achieving this goal. Lastly, in order to probe the utility of the complexes as chemosensors in devices, they could be incorporated into the solid state. For example, the complexes could be incorporated into polystyrene beads. There are examples in the literature that demonstrate general methodologies for incorporating chemosensors into the solid 100 1. 48% H B r , N 2 ) reflux 2. H 2 0 3. NaOAc 29 Scheme 4.3. Proposed synthesis for water-soluble POC4Pyr using fuming sulfuric acid (Pyr = pyrene). 101 PhPH, + PHPh 2 Pd(PPh 3) 4,NEt 3, C H 3 O H , 12h, 70°C C0 2 H PhPH 2 + PHPh, Pd(OAc)2, NEt 3 , C H 3 C N , 70h, 85°C C0 2 H 30 C0 2 H C0 2 H 30 Scheme 4.4. Proposed synthesis for a phosphine solubilized with a carboxylate group. 31 Scheme 4.5. Proposed synthesis for a phosphine solubilized with a sulfonate group. 4.6 Experimental section 4.6.1 General details Fluorescence measurements were made using a Varian Cary Eclipse spectrofluorometer in the laboratory of Prof. Stephen G. Withers (UBC Department of Chemistry). Solutions were in Suprasil quartz cells. A l l measurements were made in air-saturated solutions at room temperature. Excitation and emission slitwidths were both 5 nm except for 10"5 M solutions of 15,16 and 22-27 which had an excitation slitwidth of 5 nm and an emission slitwidth of 2.5 nm. Excitation spectra were normalized to the maximum value. Monomer: excimer emission intensity ratio (IM/IE) was calculated by taking the ratio of the emission intensity at 377 nm to the maximum emission intensity between 470 and 475 nm at excitation wavelengths between 275 and 355 nm (in 5 nm increments). The experiment was repeated three times. Each data set was normalized to 102 the maximum value prior to calculation of the mean W I E value for each excitation wavelength. The standard deviation was also calculated. 4.6.2 Materials used Spectroscopic grade CH2CI2 and C H C I 3 (Fisher) were used for fluorescence measurements. Both solvents produced negligible background luminescence at the excitation wavelengths that were used. 103 4.7 References (1) Bodenant, B.; Weil, T.; Businelli-Pourcel, M . ; Fages, F.; Barbe, B.; Pianet, I.; Laguerre, M . J. Org. Chem. 1999, 64, 7034-7039. (2) Bodenant, B.; Fages, F.; Delville, M. -H . J. Am. Chem. Soc. 1998,120, 7511-7519. (3) Wolf, C ; Mei , X . J. Am. Chem. Soc. 2003,125, 10651-10658. (4) Cabell, L . A. ; Best, M . D.; Lavigne, J. J.; Schneider, S. E.; Perreault, D. M . ; Monahan, M. -K. ; Anslyn, E. V . J. Chem. Soc, Perkin Trans. 2 2001, 315-323. (5) de Silva, A . P.; Fox, D. B.; Huxley, A. J. M . ; McClenaghan, N . D.; Roiron, J. Coord. Chem. Rev. 1999,185-186, 297-306. (6) Franz, K . J.; Singh, N . ; Lippard, S. J. Angew. Chem. Int. Ed. 2000, 39, 2120-2122. (7) Franz, K . J.; Singh, N . ; Spingler, B.; Lippard, S. J. Inorg. Chem. 2000, 39, 4081-4092. (8) Fabbrizzi, L. ; Licchelli, M . ; Pallavicini, P.; Parodi, L . Angew. Chem. Int. Ed. 1998, 37, 800-802. (9) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (10) Ji, H.-F.; Dabestani, R.; Brown, G. M . ; Hettich, R. L . Photochem. Photobiol. 1999,65,513-516. (11) Rogers, C. W. Ph.D. Thesis, University of British Columbia, Dec. 2001. (12) Rogers, C. W.; Wolf, M . O. Angew. Chem. Int. Ed. 2002, 41, 1898-1900. (13) Berlman, I. B. Handbook of Fluorescence ofAromatic Molecules; Academic Press Inc.: New York, 1965. (14) Winnik, F. M . Chem. Rev. 1993, 93, 587-614. 104 (15) Anghel, D. F.; Toca-Herrera, J. L. ; Winnik, F. M . ; Rettig, W.; v. Klitzing, R. Langmuir 2002,18, 5600-5606. (16) Yamazaki, I.; Winnik, F. M . ; Winnik, M . A. ; Tazuke, S. J. Phys. Chem. 1987, 91, 4213-4216. (17) Duhamel, J.; Kanagalingam, S.; O'Brien, T. J.; Ingratta, M . W. J. Am. Chem. Soc. 2003,125,12810-12822. (18) Winnik, F. M . ; Winnik, M . A. ; Tazuke, S.; Ober, C. K . Macromolecules 1987, 20, 38-44. (19) Eckl, R. W.; Priermeier, T.; Herrmann, W. A. J. Organomet. Chem. 1997, 532, 243-249. (20) Bitterer, F.; Herd, O.; Hessler, A . ; Kuhnel, M . ; Rettig, K. ; Stelzer, O.; Sheldrick, W. S.; Nagel. S.; R6sch, N . Inorg. Chem. 1996, 35, 4103-4113. (21) Herd, O.; Hoff, D.; Kottsieper, K . W.; Liek, C ; Wenz, K. ; Stelzer, O.; Sheldrick, W. S. Inorg. Chem. 2002, 41, 5034-5042. (22) Herd, O.; Hefiler, A. ; Hingst, M . ; Tepper, M . ; Stelzer, O. J. Organomet. Chem. 1996, 522, 69-76. (23) Herd, O.; Hefiler, A. ; Hingst, M . ; Machnitki, P.; Tepper, M . ; Stelzer, O. Catalysis Today 1998,42,413-420. (24) Smith, J.; Liras, J. L . ; Schneider, S. E.; Anslyn, E. J. Org. Chem. 1996, 61, 8811-8818. (25) Schneider, S. E.; Bishop, P. A. ; Salazar, M . A. ; Bishop, O. A . ; Anslyn, E. V . Tetrahedron 1998, 54, 15063-15086. 105 

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