O 1. him-"NW, . if: '0 x... awn... a». «o . 5%.“? Jun: 3...), .4... _ , at . inngll“ All! *0 7.10:1‘r'niol fl!!! 3.401 o y. . ‘ ‘ .t 3...“. g? . .. at; (15....va v5... ' Q. o x n. .sfldflt...l. “3 3 vigil»! . 1.. ‘ a l, 1‘ ‘ O to lvglnvliv, In Anzac}. 1 ‘4 '9 '1 nlt'; brudhflru . , . In!!!) a! fin...” v . Ilv! :. 33.1 13;}: .‘ In. t .0. .r IO . . . ...( . .1, :. Io. 1):: u lvnflu3..l:. .viv . v .v ‘v C’Oiu‘ l/llllmllilllilllllllm 3 1293 00794 9331 This is to certify that the dissertation entitled A Time-Resolved Study of Electron Transfer Mechanisms: Beyond Outer-Sphere Electron Transfer presented by Claudia Turro has been accepted towards fulfillment of the requirements for PhD Chemistry degree in ajor professor Date August 27, 1992 MSU is an Affirmative Action/Equal Opportunity Inxlirulmn 0 12771 I _. fl LIBRARY Michigan State University \A / PLACE IN RETURN BOX to remove thls checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE ____T___ a ,' MSU Is An Affirmative Action/Equal Opportunity Institution cmmwt A TIME-RESOLVED STUDY OF ELECTRON TRANSFER MECHANISMS: BEYOND OUTER-SPHERE ELECTRON TRANSFER By Claudia Turro A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1992 ABSTRACT A TIME-RESOLVED STUDY OF ELECTRON TRANSFER MECHANISMS: BEYOND OUTER-SPHERE ELECTRON TRANSFER By Claudia Turro One of the primary objectives of our research has been to understand some of the fundamental mechanistic issues underlying electron transfer reactions. We have designed molecular systems to provide information on several aspects of charge transfer reactions, including the role of protons in long-distance electron transfer, bimolecular donor/ acceptor pair reactivity at high driving forces, and excited state multiclectron transformations. In our systems a photon can initiate the reaction by placing the molecules in an excited electronic state, which permits the reactant and product concentrations to be monitored as a function of time. These dynamic measurements are conducted by exciting the molecules a with short light pulse, and following the progress of the reaction by optical methods, such as transient absorption spectroscopy and emission lifetime. Electron transfer reactions through a proton interface have been conducted by the design of hydrogen-bonded donor/ acceptor pairs, where the donor, a carboxylic acid derivative of a Zn-substituted porphyrin, transfers an electron to several aromatic acceptors from its excited state. The charge separation and subsequent charge recombination rates have been determined utilizing picosecond transient absorption spectroscopy, for protiated and deuterated donors and acceptors. A slight attenuation in the rates is observed, as compared to through-bond electron transfer. The transient absorption technique has also been utilized to characterize the excited states that lead to two-electron reactivity in quadruply-bonded inorganic complexes of the type M2C14(L)n, where M = molybdenum or tungsten and L = monodentate (n = 4) or bidentate (n = 2) phosphine ligands. It was observed that the reactions do not proceed directly from an excited electronic state, but from a conformationally- distorted intermediate formed following light excitation. The distorted intermediate is believed to possess favorable characteristics that permit the observed two-electron oxidative-addition reactions. The driving force dependence of the bimolecular electron transfer rates between RuII complexes and cytochrome c has been determined. A decrease in charge separation rate at high driving forces, the inverted region, wasobserved for this donor/ acceptor series, which is unusual in bimolecular reactions. The factors that govern bimolecular inverted region behavior have been considered. ACKNOWLEDGMENTS The work presented here would have never been possible without the people and atmosphere at Michigan State. In my several years here, I have learned much more than science. In the absence of the technical help from the machine shop, the glass shop, and electronic shop, I would have been here a lot longer. Without Marty’s wonderfully orchestrated pulsed devices none of the measurements presented here would have been possible. I’ve had many helpful discussions and collaborations with several MSU professors, including Shelagh Ferguson-Miller on proteins, Cukier on ET theory, Wagner on photochemistry, and Laser Lab faculty such as Jerry Babcock and Kris Berglund on qualitative and quantitative beer drinking skills. I want to especially thank Colleen Partigianoni for introducing me the to photochemical processes of quadruply-bonded compounds. Colleen was also known for doing unexpected things anytime, anyplace, which kept things interesting. Janice Kadis has told me (and the whole world) things I never thought physically possible, and Carolyn Hsu believed it all. Many interesting discussions flourished on Friday afternoons while at Lab Happy Hour, a tradition started by Randy King, which still lingers, now led by Doug Motry, JP Kirby, and Sara Helvoight - if she really joins the group. Which brings us to group parties: Zoe Pikraminou and Mark Torgerson were responsible for some of the finest. Mark Newsham helped me build my first instrument, and introduced me to the Nocera group (good?). I also got to know a few people while here and to learn from them. Jeong-A Yu attempted to teach me Korean, and Tony Oertling showed me, iv in a practical manner, to drink tequila with “a little salt and lime” while watching 4‘h of July fireworks from the roof of the Chemistry building (not everything I learned was good). The friendships Of José Centeno, Juan Lépez-Carriga, Hak-Hyun Nam, Elaine Harnon, Julie Jackson, and Renne Day were also invaluable. Friendships began and creativity peaked at establishments such as The Peanut Barrel, Harrison RoadHouse, The Green Door, Trippers, Dags, and the Old Babbock group hangout, Cabaret. The scientific insight and thoughtful guidance of George Leroi and Dan Nocera, along with their friendship have provided the underpinnings and shaped my graduate career. George always had words of wisdom and encouragement. Dan invariably had words, not necessarily wise or encouraging, but usually Sincere and forceful. Special friends, whose contributions range too far to be mentioned, are Sandy Nelson and Mariangel Gasalla. The support, love, and patience of my family has been invaluable. Their understanding of the short and far between visits is commendable. Seeing my family was always refreshing and provided new strength. They always encouraged me to continue in the pursuit of my scientific endeavors, although I know it was hard for them to understand why they took so long and occupied so much of my time. The most important collaboration here, in more than one sense, was that with Jeff. We tripped over the same picosecond barriers, but always (somehow), we were able to help each other go on - especially in the early days, when nothing worked and our spectra were upsidedown. Once we had data, our scientific discussions became routine, using each other as “The Guide of Fast Phenomena and Excited State Processes”. Without him, the ps TA would probably still have crosstalk, the saturable absorber still be misaligned, and the continuum uncollimated. However, the friendship that grew from this, with him and his family, is by far more important. Without him, things would have been very different. TABLE OF CONTENTS LIST OF TABLES .................................................... viii LIST OF FIGURES ..................................................... x INTRODUCTION ........................................................ 1 References ............................................ 6 CHAPTER I ELECTRON TRANSFER THROUGH HYDROGEN BONDED INTERFACES ............................... 8 A. BACKGROUND ................................. 8 B. EXPERIMENTAL METHODS ...................... 18 C. RESULTS AND DISCUSSION ...................... 28 1. Electron Transfer in l-H and 1—D ............. 28 2. Electron Transfer in Systems 2 and 3 .......... 49 3. Comparison of the Three Protiated Systems 57 D. REFERENCES .................................. 59 CHAPTER II TRANSIENT ABSORPTION SPECTROSCOPY OF MD AND w QUADRUPLY-BONDED DIMERS ........... 69 A. BACKGROUND ................................. 69 B. EXPERIMENTAL METHODS ...................... 79 C. RESULTS AND DISCUSSION ...................... 81 1. D2h Complexes: M2C14(FP)2 ................. 81 2. D2,, Complexes: M2Cl4(PR3)4 ................. 90 D. CONCLUDING REMARKS ......................... 105 E. REFERENCES .................................. 105 vi CHAPTER III THE DRIVING FORCE DEPENDENCE OF BI- MOLECULAR PROTEIN ELECTRON TRANSFER: *Ru(L)§*/CYTOCHROME c SYSTEM ................ 111 A. BACKGROUND ................................ 1 1 1 B. EXPERIMENTAL METHODS ................... 1 14 C. THEORY ...................................... 120 D. RESULTS AND DISCUSSION ................... 122 E. REFERENCES ................................. 136 APPENDIX .......................................................... 1 4 1 vii Table VIII IX X LIST OF TABLES Structures and Reduction Potentials (vs NHE) of the Acceptors 1 - 3 in their Protiated and Deuterated Forms ................. Observed 1H NMR Shifts and FWHM of Carboxy Proton Resonances of CD2C12 Solutions of DNBCOOH and ZnPCOOH at Selected Concentrations of the Acid ............ Observed 1H NMR Shifts and FWHM of Carboxy Proton Resonances of CD2C12 Solutions of DNBCOOH/ZnPCOOH at Selected Concentrations of the Acid Mixture ................. Monomer and Dimer Vibrational Frequencies of DNBCOOH and ZnPCOOH in the OH and CO Stretching Regions ........ Calculated Self-Association Binding Constants for DNBCOOH and ZnPCOOH in CH2C12 at Room Temperature ............. Concentration Dependence of the Charge Separation and Charge Recombination Rates of 1-H in CH2C12 .............. Comparison of Charge Separation and Recombination ET Rates for the Three Acceptors with ZnPCOOH and their Respective Driving Forces ............................................. Ground State Electronic Absorption Maxima of Several Edge- Sharing Bioctahedral Complexes in the Visible Region ........ Lifetimes and Emissive Quantum Yields of the 55* Excited State of MozCl4(PR3)4 Complexes and Non-Emissive Transient Lifetimes .................................................. Cone Angles ((b) of PR3 Ligands ............................ viii Page 17 3O 3O 30 33 57 87 91 102 XI Emission Lifetimes and Excited State Redox Potentials of the Ru" Complexes Utilized in this Study ....................... XII Driving Force and Observed Rates for the ET Reactions between the MLCT Excited State of Ru11 Complexes and Cytochrome c in the Oxidized (Fem) and Reduced (Fen) States XIII Driving Forces, Calculated Values of the Rates of Diffusion and Electronic Coupling, and Observed Bimolecular Electron Transfer Rates between Neutral and Negative Complexes and Femcytochrome c .......................................... XIV Comparison of Reorganization Energy and Electronic Coupling in ET Reactions involving Cytochrome c ..................... XV Comparison of Electronic Coupling in ET Reactions of Organic and Inorganic Donor/ Acceptor Systems ..................... AI IR Absorbances of DNBCOOH Utilized in the Self-Association Binding Constant Calculations Described in Chapter I ........ AII IR Absorbances of ZnPCOOH Utilized in the Self-Association Binding Constant Calculations Described in Chapter I ........ AIII IR Absorbances of ZnPCOOH and DNBCOOH Utilized in the Hetero-Association Binding Constant Calculation Described in Chapter I .................................................. ix 123 123 128 131 132 141 141 142 Figure LIST OF FIGURES Schematic representation of PSII showing the electron transfer pathway and rates ......................................... Pictorial representation of (a) cytochrome c oxidase in the inner mitochondrial membrane and (b) the reduction of oxygen at the bimetallic center ............................. Schematic diagram of the picosecond laser system, pulsed dye amplification, and transient absorption spectrometer ......... 1H NMR spectra of CD2C12 solutions containing DNBCOOH and ZnPCOOH where the concentration of each component is 4x10‘4 M (top), 1x 10‘3 M (middle), and 3 x 10'3 M (bottom). The carboxy proton is denoted by (*) in all spectra .......... FTIR spectra in the CO stretching region of CHZCIZ solutions of (A) DNBCOOH, (b) ZnPCOOH, and (c) DNBCOOH and ZnPCOOH. The concentrations of each component for (a), (b), and (c) are 1.6x10‘3 M (top). 1.0x 10-3 M (middle), and 4.0x 10'4 M (bottom) ..................................... Plot of the left side of eq 5 vs [DNBCOOH] ................ (a) Emission spectra and (b) luminescence decays of CH2C12 solutions of ZnPCOOD in the absence (tap trace) and presence (bottom trace) of 10'2 M DNBCOOD ............. Transient absorption spectrum of a CH2C12 solution of TCNE and ZnPCOOCH3 collected 1 us after the laser pulse (10 us, 532 nm) .................................................. Transient absorption spectra of ZnPCOOH collected 15 ps (—) and 1.5 ns (- - —) after the excitation pulse ....... '. . . . Page 11 13 20 31 32 35 36 38 41 10 ll 12 l3 14 15 l6 l7 18 19 Transient absorption spectra of ZnPCOOD collected 15 ps (—-) and 1.5 ns (— — -) after the excitation pulse ........... Transient (a) rise and (b) decay of the 11m“ excited state of a lo-3 M CHZCIZ solution of ZnPCOOH in the 625 - 760 nm region .................................................... Transient absorption spectra at various delay times after the 580 nm, 3 ps excitation pulse of ZnPCOOH (1.4x 10'3 M) and DNBCOOH (4x 10-2 M) in CH2C12 ........................ Transient (a) rise and (b) decay of a CHzClz solution containing [ZnPCOOH] = 1.6x 10‘3 M and [DNBCOOH] = 4.0x10‘2 M in the 625 - 760 nm region .................... Transient decay of the 11m" excited state of a 10'3 M CHzClz solution of ZnPCOOCH3 containing 5x10‘2 M DNB- COOCHzCH3 in the 625 - 760 nm region ................... Transient absorption spectra at selected delays following the pump pulse of CH2C12 solutions of [ZnPCOOH] = 10'3 M and [DNTCOOH] = 10'2 M, showing the decays (a) prior to and (b) after 300 ps ........................................... Plot of —ln(AOD) vs time Showing the triphasic decay of the ZnPCOOH/DNTCOOH pair .............................. Transient absorption spectra at selected delays following the pump pulse of CH2C12 solutions of [ZnPCOOH] = 10’3 M and [DNTCOOH] = 10-2 M, after subtraction of line signal ..... Transient absorption spectra collected at selected delay times after the pump pulse of CH2C12 solutions containing ZnPCOOH and AQCOOH, after subtraction of 11m“ signal . . Plot of —ln(AOD) vs time showing the biphasic decay of the ZnPCOOH/AQCOOH pair ................................ 42 43 47 49 50 52 54 55 20 21 22 23 24 25 26 27 28 29 Transient absorption Spectra of a CHzClz solution containing [ZnPCOOD] = 10-3 M and [DNTCOOD] = 10-2 M collected 20 ps (—) and 2 ns (- - -) after the excitation pulse ........ Molecular orbital diagram derived by uniting two d4 ML4 fragments to form the bimetallic Mng quadruply-bonded complex .................................................. Valence bond description of the electronic states formed by the d,‘y orbitals, as well as the corresponding MO formalism . . Qualitative MO diagram showing correlation between D2,, and D2,, geometries ........................................... Electronic absorption spectra of ( ) MozCl4(dppm)2 and (- — —) W2Cl4(dppm)2 in CH2C12 ........................... Electronic absorption spectra of ( )W2C14(PBU3)4 and (- - -) M02C14(PBU3)4 in CH2C12 .......................... Transient absorption spectra of MozCl4(dmpm)2 in CH2C12 collected 2, 20, and 50 ps following the 600 nm, 3 ps excitation pulse. The decay of the bleaching at 630 nm is shown in the inset ......................................... (a) Transient absorption spectrum of M02C14(dmpm)2 in CH2C12 recorded 1 us after 355 nm, 10 ns excitation and (b) electronic absorption spectrum of independently-prepared M02C16(dppm)2 ................................... . ..... (a) Transient absorption spectra of W2Cl4(dppm)2 in benzene recorded 100 ns and 4 us after 532 nm, 10 ns excitation. (b) Absorption spectra of W2C16(PEt3)4 (- - -) and W2C16(dppm)2 ( ) in toluene and CHZC 12, respectively .................. Proposed mechanism for the foi' nation fo the long-lived transient in D2,, complexes following high-energy excitation .. xii 56 72 74 75 76 77 82 84 85 89 30 31 32 33 34 35 36 37 38 39 Transient absorption spectrum of M02C14(PHPh2)4 in CH2C12 collected 2 ps after the 600 nm, 3 ps, excitation. The inset shows the decay of the absorbance at 420 nm ............... Transient absorption spectrum of W2Cl4(PEt3)4 in toluene recorded 70 us after the 683 nm excitation pulse; the inset shows the spectral profile in the near UV region ............. Transient absorption spectrum of W2CI4(PBU3)4 in CH2C12 recorded 70 us after the 683 nm excitation pulse; the inset shows the spectral profile in the near UV region ............. Transient absorption spectrum of W2Cl4(PPh2Me)4 in THF recorded 70 us after 683 nm, 10 ns excitation ............... Decays of (a) W2Cl4(PMe3)4 and (b) W2CI4(PBU3)4 following 683 nm excitation; in both cases the top curve is that of the transient absorption Signal at 500 nm and the bottom corresponds to the luminescence decay at 800 nm ........... Transient absorption spectra recorded 60 ns after 532 nm excitation of (a) MozCl4(PBu3)4, (b) M02C14(PMe2Ph)4, and (C) M02CI4(PPIT2MC)4 in CH2C12 .......................... Decays of W2Cl4(PPh2Me)4 in CH2C12 followed at (A) 400 nm and (O) 440 nm after 532 nm, 10 ns excitation ............... Log—log plot of the emission quantum yield vs the 185* lifetime of complexes in the MozCl4(PR3)4 series ............ Schematic representation of the ligands utilized in the driving force dependence study of the ET rate ...................... Driving force dependence of the ET rate in (a) fixed-distance systems and (b) diffusion controlled reactions ............... Schematic representation of the normal (-AG < A ) , activationless (-AG = A), and inverted (-AG > A) regimes of ' electron transfer (see text) ................................. xiii 92 93 94 95 97 98 99 101 113 115 116 41 42 43 45 Stem-Volmer plots of the quenching of 6 x 10‘5 M solutions of Ru(diMe-phen)32+ in pH = 7, p. = 0.1 M phosphate buffer by (a) ferrocytochrome c and (b) ferricytochrome c, showing the linear fit through the data points ............................ Transient absorption spectrum obtained from a u = 0.1 M, pH = 7.4 2phosphate buffer solution of 1x10'3 M Ru(diMe- phen)3 * and 1x10“3 M fem-cytochrome c following 532 nm, 10 ns excitation ........................................... Plots of the electron transfer rates of Ru11 complexes with (a) Fe"I cytochrome c and (b) FeII cyochrome c, with their respective calculated kobs (solid curve), kact (dashed curve), and diffusion rates ........................................ Stern-VOImer plots of Ru(bps)34‘ (0), Ru(phen)2(bps) (A), and Ru(phen)2(CN)2 (Cl) ....................................... Schematic diagram of cytochrome c viewed from the (a) top and (b) front. The dashed lines inidicate regions I, II, and III, whith some approximate residue numbers, where proteins, small anions, and cations react with the protein, respectively .. Plot of the data points from ref. 52, showing the calculated rates, kobs (solid curve) and k? (dased curve). The diffusion limit, km”, is shown at 7.1x 10 M‘ls‘l ...................... xiv 124 125 126 129 131 135 INTRODUCTION The mechanisms of electron transfer reactions have been in question since the earliest studies of ions in solution. The seminal work of Taube in the 1950s led to the distinction between inner— and outer-sphere electron transfer pathways."3 Inner sphere refers to those inorganic reactions where an atom is shared between the reagents in the precursor complex prior to electron transfer, and in most instances the bridging ligand is transferred from the electron acceptor to the donor. In outer-sphere electron transfer the reactants diffuse together and an electron is transferred without net bond changes. Parallel reactions are observed in organic chemistry, although the distinction between the two mechanisms is not as evident and has only recently been emphasized“5 The transfer of one electron is commonly observed throughout organic and inorganic chemistry. In addition to the discrete transfer of an electron from a donor to an acceptor and the formation of charge transfer complexes, many other reactions fall into this category. These include organic radical reactions, group transfers, and heterolytic bond cleavage. In inorganic chemistry, one-electron reactions are those in which the valence of the metal changes by one, as observed in the common addition or elimination of Charged ligands. These may be exemplified by halides, cyanide, Cflfboxylates, or alkyl oxides. Another class of electron transfer is the movement of more than one electron during the course of a reaction. Many inorganic reactions proceed with a net two-electron change, such as oxidative-addition and reductive- elirnination of X2 (X = halide), H2, and HX, among others.6 Similar behavior is observed in organic reactions, with the transfer of hydride, oxygen atom, or halonum ion (X+). Moreover, typical organic mechanisms propose the movement of many electrons, often in pairs. To distinguish between a concerted transfer of more than one electron and a path composed of several consecutive one-electron steps one must directly observe the intermediates, unless the one-electron reaction is sufficiently slow to afford the observation of products from radical traps. A benchmark example, where a once-thought concerted two-electron transfer was shown to proceed via two consecutive one-electron steps is the PtH/th reaction.7 The dynamic detection of the one-electron intermediates is difficult in fast reactions. The methods available in the past to determine the rates of formation of intermediates and their identity have included stopped flow, jump methods, electrochemical techniques, and low-temperature spectroscopy. However, with the advent of pulsed lasers and fast electronics it has been possible in the last twenty years to directly measure reaction rates faster than 1010 S4. Some of these methods along with their corresponding range of detection times are shown in Scheme 1.8 The techniques listed in Scheme I have been previously described in detail and will, therefore, not be discussed here. It is apparent from Scheme I that optical techniques have surpassed the others in accessing very fast reactions. These advances have rested primarily on the duration of laser pulses, which have become increasingly N .iv auhoi ~N-U.Ivc m \ 9:2. 0 FIG F N Flo F — FIOF O Flo F T0 F TOP NIOF Clo F ”IO F _ _ _ _ _ _ _ i\\|J1 58mm occideqi c0298? mnocadsaa Enw omega. moEomEga m_m>_o_umm omEa 9.22530 co_mm_Em afiz... 2298.5; E22 mmm £28328 EOE o_.=oo_w 9:2. Snowmen. 30E “mm“. 26.”. cooosw H 050:0” shorter, now reaching a few femtoseconds. Since only reaction times longer than the duration of the excitation pulse can be measured, the laser pulse width is a critical parameter. A laser pulse can be utilized to initiate a reaction, and detection techniques, including electronic absorption, Raman, time-resolved emission, or infrared absorption can be used to follow the decay and determine the identity of the intermediates. These techniques permit measurement of the fastest rates; thus the upper limit of the rates of photoinduced reactions is higher than those found in other methods. Donor/ acceptor studies which feature the transfer of one electron from an electronically excited state have taken advantage of these fast techniques. However, these systems usually involve simple one- electron transfer, and are often within the confines Of outer-sphere electron transfer.9 A detailed description of the work in this area is presented in the Background section of Chapter 1. Many atom-ransfer reactions in organic systems have also been probed in this manner, although they are generally one-electron in nature.10 Fast laser techniques, however, can be utilized to elucidate the fundamental mechanistic questions of more diverse reactivity, in addition to simple one-electron outer—sphere electron transfer, if such reactions are photochemically designed. This is the emphasis of the work described herein. Three different electron transfer reaction mechanisms have been elucidated in this thesis. These included 1) the role of hydrogen bonds in the electron transfer pathway on the picosecond time scale (Chapter I); 2) the nature of excited intermediates formed upon light absorption in bimetallic complexes, that are known to lead to two-electron oxidative addition reactions in the presence of substrates (Chapter II); and 3) the photoinitiated bimolecular electron transfer rates at very high driving forces (Chapter III). Photoinduced electron transfer is predicated on the production of an electronic excited state upon light absorption which is either strongly oxidizing or reducing in nature, and it can therefore undergo facile electron transfer with donors and acceptors. Hydrogen bonds are believed to play an important role in the mediation of long-range electron transfer in biological systems.”12 The effect of hydrogen bonds on the electron transfer pathway has been determined in model systems by monitoring the rates of charge separation and charge recombination in protiated and deuterated systems. In these particular systems an electron is transferred from a Zn-substituted porphyrin, placed in its lmr* excited state by excitation with a laser pulse, to several acceptors. In these systems the donor and acceptor are hydrogen- bonded by a carboxylic acid interface. The results of this study are presented in Chapter I. The photoexcitation of the reactants is particularly interesting in systems which are known to undergo two-electron oxidative addition when excited with light, such as the Mo and W quadruply-bonded dimers.l3 It is of interest to follow the mechanism of such transformations, since radical reactions are not observed outside the solvent cage. A concerted two- electron process in this system would implicate a three-atom transition state, whereas two sequential one-electron transformations would afford a two- atom transition state with the concomitant formation of radicals. The initial studies of these systems are presented in Chapter H, where the excited states and reactive intermediates observed in inert solvents are discussed. The photoinitiated electron transfer rates for the bimolecular reactions between Ru complexes and oxidized or reduced cytochrome c have been determined from emission lifetime quenching measurements. An investigation of the driving force dependence of the electron transfer rate was conducted, and a decrease of the rates in the highly exergonic systems was observed. These results, discussed in Chapter III, are particularly interesting, since inverted region electron transfer has only rarely been Observed for bimolecular donor/ acceptor pairs. References 10. Seaborg, G. T. Chem. Rev. 1940,27, 199. Symposium on Electron Transfer Processes J. Phys. Chem. 1952, 56, 801-910. - Taube, H. Electron Transfer Reactions of Complex Ions in Solution; Academic Press: New York; 1970. Pross, A. Acc. Chem. Res. 1985, 18, 212. Eberson, L. Electron Transfer Reactions in Organic Chemistry; Springer-Verlag: Berlin; 1987. Jordan, R. B. Reaction Mechanisms of Inorganic and Organometallic Systems; Oxford University Press: New York, 1991. Taube, H. In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D. B.; Endicott, J. F., Eds.; ACS Symposium Series, vol 198; American Chemical Society: Washington, DC, 1982; p 151. Jonah, C. D. In Chemical Reactivity in Liquids Fundamental Aspects; Plenum: New York, 1988; pp 1 - 14 and references therein. For recent reviews on photoinduced electron transfer see Chem. Rev. 1992, 92, 365-490. Baltrop, J. A.; Coyle, J. D. Excited States in Organic Chemistry; John Wiley: Bristrol, 1975. 11. Williams, R. J. P. In Electron Transfer in Biology and the Solid State; Johnson, M. K.; King, R. B.; Kutz, D. M., Jr.; Kutal, C.; Norton, M. L; Scott, R. A., Eds.; Advaces in Chemistry Series 226; American Chemical Society: Washington DC; 1990, pp 3-26. 12. Topics in Photosynthesis: The Photosystems; Barber, J., Ed.; Elsevier: Amsterdam, 1991. 13. Partigianoni, C. Ph.D. Dissertation, Michigan State University, 1991. CHAPTER I ELECTRON TRANSFER THROUGH HYDROGEN BONDED INTERFACES A - BACKGROUND Proton associated electron transfer reactions are crucial in vital biological processes, especially those involving energy storage and cornxrersion."2 Protons play both active and passive roles in electron transfer events. The most recognized form of coupling between electrons and Pro tons is an active one, where a substrate is protonated or deprotonated upon its reduction or oxidation. This behavior leads in some instances to the VeCtorial movement of protons across a membrane, which is typically Concomitant with the movement of electrons in the opposite direction.l'2 Another manner in which protons are moved across a membrane is by the acti on of proton pumps, which are often encountered in biological trarlSmembrane assemblies, such as ATP synthase, Na+lK+ ATPases, Ca2+ ATP ases, and bacteriorhodopsin.3 The pumping mode of action is believed to i rImolve changes in protonation of residues of the helical protein structures that comprise the enzyme, resulting in some backbone distortions. This r . . . . . - p Qteln motIon Is In turn governed by the redox state of the active centers or the conformation of a chromophore.“'10 The passive role of protons in modulating electron transfer rates has only recently been investigated in detail. This type of modulation is believed to take place by the transfer of an electron through one or several hydrogen bonds in long-range electron transfer in proteins. The experimental and theoretical investigations have been limited to small proteins such as cytochrome c, cytochrome b5, plastocyanin, and azurin, where it is believed that hydrogen bonds within the protein provide better coupling for electron transfer than longer covalently- bound pathways.”13 The movement of protons coupled to electron transfer can take the form of directed but Opposite motion of protons and electrons across a membrane. This establishes an electrochemical proton gradient, which is transformed into chemical energy by the production of high energy molecules such as ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and inorganic phosphate by the enzyme ATP synthase, and N ADPH from the reduction of NADP+ (nicotinamide adenine dinucleotide Pho Sphate) by protonation of the nicotinamide ring. The energy stored in libese molecules is then utilized to perform life-sustaining functions in the cell such as the reduction C02 to carbohydrates in the dark reactions of photosynthesis and the breakdown of carbohydrates, fats, and sugars following respiration. Active proton/ electron coupling is evident in many systems such as in oxidases and in the photosynthetic cycles. Protonation is concomitant with eleCtron transfer within xanthine oxidase, an enzyme which catalyzes the xanthine hydroxylation to uric acid and dioxygen reduction to peroxide or S‘1I3€.~.roxide.1“ Another example of proton movemtne coupled to electron tr . . . . . . ansfer events ls found In sarcosme oxrdase, where the oxrdatlve it: 31: ‘15 1‘ if H. 31'] it. 10 demethylation of sarcosine is accompanied by the reduction of 02 to hydrogen peroxide.15 Yet, the most intensively investigated example of proton movement coupled to electron transfer is that of the photosynthetic reaction center and cytochrome c oxidase. Photosystems I and 11 (PSI and PSII, respectively), lie across the thylakoid membrane and effect a transverse flux of electrons and protons. 16’” Their action is predicated on the absorption of photons by light harvesters, which through energy transfer populate the 11m" excited state of a chlorophyll dimer, the special pair (SP). Electron transfer from the excited chromophore to a nearby acceptor is followed by subsequent transfers to acceptors located at increasingly longer distances from the oxidized SP, thus 16'” Figure 1 depicts the avoiding electron/hole charge recombination. Spatial arrangement of the molecules in PSII, as determined from the crystal Structure of the reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides (the RC’s in these photosynthetic bacteria are believed to be very similar to PSH in green plants).13'19 The photophysics of PSII have been extensively investigated, and the rates for the primary electron transfer events are summarized in Figure l. I:OIIOwing these primary electron transfer events from the SP to pheophytin (PA), the two quinones, Q A and Q9, are reduced in series.”25 The SP cation I“aciical returns to its neutral state by the oxidation of water to 02 via the aetion of the oxygen-evolving center (OEC), after four sequential electron transfer events. It has been proposed that the oxidation of water at the CBC proceeds by a series of proton-coupled electron transfer reactions of an oxo- bridged cluster of manganese.”28 On the reduction side of the scheme, absorption of two consecutive photons leads to the two-electron reduction of QB . concomitant with the proton uptake from nearby ionizable residues to 11 02+2H+ QH2 .1. PhB ll -------M-1.3I?P.°P. I Q?” + 2H” ---------------------‘ GI 6. £00 ps II 4 ps 3 E.P.r.a£.€*1-----..- ..................................... ................................... Figure 1. Schematic representation of PSII showing the electron transfer pathway and rates. 12 produce QBH2.29‘32 The proton sources are believed to be glutamic and aspartic acids of the L subunit which are located near QB, although a serine residue may also play an important role in the protonation.”32 It has also been shown that hydrogen bonding to the carbonyl functionalities of both Q A and Q3 facilitates the electron transfer events to the quinones.33 Once protonated, QBH2 exchanges with a quinone from the quinone pool; this process ultimately leads to the translocation of protons across the membrane thus creating a proton gradient. The reduction of 02 to water during respiration can be thought of as the reverse of the photosynthetic oxidation of water by the DEC in photosynthesis. As is the case in the DEC, proton movement is coupled to electron transfer in the enzyme cytochrome c oxidase. Reduction of the enzyme by four electrons is accomplished by the sequential electrostatic binding of the reduced protein, ferrocytochrome c, which is the last carrier in title electron transport chain.“38 A schematic representation of cytochrome C oxidase is shown in Figure 2a, depicting its four redox-active metal centers: heme a, heme a 3, CuA, and (3113.34-38 The primary electron acceptor Within the enzyme is either heme a or Cu,,(,39'40 which are located in the Cytoplasmic side of the inner mitochondrial membrane, closest to the Cytochrome c binding site (Figure 2a). It is believed that the sequence of electron transfer events is initially cytochrome c —) CuA followed by CuA —-) heme a and heme a -) heme a 3/Cu3.41'45 Whereas no pH dependence of the electron transfer rates is observed in the absence of Oz, in its presence some of the rate constants are affected by the medium’s proton concentration, indicating the role of oxygen reduction on the proton pump activity of c:y‘iochrome c oxidase.42~46 The reduction of 02 takes place at the heme ‘13 /CuB site, where it binds and becomes successively protonated as its l3 3" ‘ ti ' ' II 1 02 + 4H+ + 4e' 2H20 . Fe2+ Cu ® i=2 G93 :95 9% -O--..------m.---—--- . “U + O I: : Fe3+ Cu2+ © Figure 2. Pictorial representation of (a) cytochrome c oxidase in the inner mitochondrial membrane and (b) the reduction of oxygen at the bimetallic center. l4 stepwise multielectron reduction is effected.”49 The mechanism of oxygen reduction proposed by WikstrOm is shown in Figure 2b, where the interplay between electron transfer and proton uptake can readily be observed.“47 As shown in Figure 2b, a 2H”/e‘ stoichiometry is necessary to effect the transitions from peroxy (P) to ferryl (F) intermediates and from F to the fully oxidized (O) heme a 3/ CUB center, where the latter is free of bound 0;. The protons involved in 02 reduction are taken up from the matrix Side of the inner mitochondrial membrane, thus producing a proton gradient. The number of protons translocated per electron transfer, however, is twice that generated from oxygen reduction alone.”38 The remaining protons are believed to be translocated through the action of a transmembrane proton pump, as described above, although the exact mechanism of action is not yet fu 11y understood.“2“S Unlike the long standing recognition of proton transfer in protein electron transfer events, the passive role of protons has only recently been ”'13 is now recognized. A limited amount of experimental evidence available to support the semi-empirical calculations performed on small 13rcteins,ll which predict that the magnitude of the electronic coupling is dictated by the type of pathway between the donor and acceptor in addition to their separation.13 The most striking evidence was obtained for Ru- modified cytochrome c, where the excited state electron donor Ru(bpy)2(im)(msx)2+ (bpy = 2,2’-bipyridine, inn = imidazole, Hisx = histidine at position X) is coordinated to several different histidines of the PI‘Qtein.12 The electron transfer rates from the excited state covalently- b011nd Ru complex to the protein’s heme can be determined, and they can then be correlated to the distance between the modified histidine and the heme and to possible electron tunneling pathways. The different pathways in; CO CO 15 include those which are completely covalently bound and those which contain hydrogen bonds or through-space jumps in conjunction with covalent bonds. Although the electron can take any pathway to reach the heme, only the most favorable ones are expected to contribute significantly to the rate. It was found that pathways containing hydrogen bonds are favored over much longer ones containing only covalent bonds.14 Calculations were performed which predict that one hydrogen bond corresponds to three covalent linkages, whereas one through-space jump can be correlated to ten covalently bonded atoms.13 The passive role of protons on the modulation of electron transfer rates has not been subjected to the rigorous experimental and theoretical treatment that has advanced the knowledge of fixed distance electron transfer in inorganic and organic compounds,5°'62 proteins,”70 and ¢E=nzymes.26 Of the active and passive proton involvement in electron transfer rates, we have begun model studies on the latter because it is synthetically more tractable. One approach to assessing the passive role of Protons in electron transfer rates is to combine the strategy of photoinduced fixed-distance electron transfer”61 with that of photoinduced proton transfer.”73 The strategy chosen here is to channel the electron as it travels from the photoexcited donor (*D) to an acceptor (A) through a proton interface, as is schematically depicted below kcs H] V 5y: 16 where kcs and kCR are the rate constants for charge separation and charge recombination, respectively. Careful design of the donor/acceptor system should preclude the electron from travelling through pathways other than the hydrogen-bonded interface. The propensity of carboxylic acids to dimerize in non-hydrogen bonding solvents of low polarity”76 allows us to prepare donor/acceptor systems such as / \ /O-"-‘-2H—O W 044+ ----- o/ where the photoexcitable donor is a Zn-substituted porphyrin in the protiated (ZnPCOOH) or deuterated form (ZnPCOOD). The ZnPCOOH 11m" excited State (E09 = 2.1 eV) is a powerful reducing agent, with E(ZnP+/*) = —1.3 V Vs NHE (since Em(ZnP+/°) = 0.80 V vs NHE),77 such that it can reduce a V ariety of acceptors including several substituted nitrobenzenes and dinitrobenzenes. The structures of the acceptors utilized in this study are Shown in Table I, in addition to their respective redcution potentials. The reduction potentials of the acceptors presented in Table I are those of their decarboxylated forms. The addition of the —COOH group to . the 2-position of 9,10-anthraquinone is known to make the reduction potential more positive by 0.06 V,78 whereas the reduction potential of 3,4- dinitrobenzoic acid only differs by 0.02 V from that of 3,4-dinitrobenzene.79 T‘hese potential changes, however, were determined by different methods, and therefore may not be comparable. The addition of a methyl group at the 4‘Position in methyl-3,5-dinitrobenzoate, to form the methyl ester of 3,5- 17 dinitro-p-toluic acid, increases the value of the reduction potential by 0.1 v.80 Therefore, we chose the reduction potential of 3,5-dinitrotoluic acid to be 0.1 V larger than that of 3,5-dinitrobenzoic acid. Although the reduction potentials of the acids have been measured, the values obtained by different methods are not in good agreement. We have therefore chosen to utilize the values of the decarboxylated compounds, since the reported values are in better agreement.81 Table I. Structures and Reduction Potentials (vs NHE) of the Acceptors l - 3 in their Protiated and Deuterated Forms. g D/A Systems Structure Em(A‘® / Va 141 and 1-D 2 0‘-2H -06 OZN 2—H and 2-D '06 3—H and 3—D ‘08 ‘ “Values from Ref. 81. 18 The rates of charge separation (kcs) and charge recombination (kCR) have been measured in these hydrogen-bonded donor/acceptor pairs using picosecond transient absorption spectrosc0py. The effect on the electron transfer rate of deuterium substitution of the carboxy groups of each reactant has been determined by comparison to the protiated analogs. B. EXPERIMENTAL METHODS All the acid acceptors, 3,4-dinitrobenzoic acid, 3,5-dinitrobenzoic acid, p-nitrobenzoic acid, 2,6-dinitro-p-toluic acid, and 9,10-anthraquinone- 2-carboxylic acid, as well as their methyl or ethyl esters were purchased from Aldrich. The porphyrin carboxylic acid was obtained from Professor C- K. Chang of the MSU Chemistry Department in the methyl ester form,82 and was subsequently treated with 18 M hydrochloric acid (~1 ml) in 88% f‘<)rmic acid (~10 ml) to obtain the protonated product. Following the removal of solvent with a rotovap, the product was refluxed in ectrometer. The samples were prepared with 0.5 ml ampules of d2- c1ichlorometl'lane, which were opened just prior to placement of the solvent i ‘31 a closed NMR tube and subsequent data collection. The transient absorption signals in the picosecond time regime were thained using the pump-probe technique. A schematic diagram of the laser and detection systems utilized to date is shown in Figure 3, where transients With lifetimes ranging from ~ 20 ps to ~ 3 us can be monitored. The master laser in the setup is a Coherent Antares 76-s mode-locked NszAG, whose 20 .cooothoomm 5:989. .565: 28 63853.5. o3 e815 .823... comm. 9.8883 on. co EEwE—O oemEonom .m charm 383 95 II a 1 III. Remy it ... _ ... . ...-v I I m E: crime 0 03 03,—. a m im1|l_ " n , ‘ malt... .n __...._..._ IlulL m . \c. I I 7 ~.0 no 2 an. I . . . . . 0 m..— E: «3 .. \. ttttttt m. ttttttttttt .S . . .. . . m a... 3x: .. . E: a... as u u . . . u .. u - . . NI I. - m h. 2 8N .. w in u u 3 ...0 .8m _ 539.80 Cam coon]M = 1 + KDZ[DNBCOOH]M (5) where [ZnPCOOH]0 represents the initial concentration of ZnPCOOH, [DNBCOOH]M and [ZnPCOOH1M are the monomer concentrations of each acid, KDD is the ZnPCOOH self-association binding constant, and Koz is the hetero-association binding constant. The monomer concentrations are Calculated from the absorbance of the monomer peaks and the 8M values 34 Table V. Calculated Self-Association Binding Constants for DNBCOOH and ZnPCOOH in CHZCIZ at Room Temperature. “ v(CO) region v(OH) region Acid KIM" eM/M-lem-l KIM" ” eM/M‘lcm'l KIM-1 DNB 220:90 1080i90 320i60 170:20 210:110 an 1000:1500 360:110 2600:900 120:60 c “Data listed in Tables AI — AIII in the Appendix. ”Determined from the ratio of AM/AD. ”Could not be determined due to large measurement error. obtained from the intercept of eq 4 at each frequency (Table III). In our case at the maximum of the each monomer v(CO) band there is considerable absorption from the other acid, and therefore the monomer concentrations were calculated from the two-equation two-unknown problem: A1751 = e137,”; b [DNBCOOH]M + efi's’l b [ZnPCOOH]M (6) A1744 = 2137?; b [DNBCOOH]M + 175, b [ZnPCOOH]M A plot of the left side of eq 5 vs [ZnPCOOI-flM should yield an intercept of unity and a slope = Koz- From the absorbance vs concentration data, analyzed according to eq 5, the linear plot shown in Figure 6 was Constructed; it has an intercept of 1.09 and a slope (= heteroassociation binding constant) of 600 i 250 M“. Evidence of electron transfer after heterodimer formation is obtained from static quenching experiments in which the intensity of the ZnPOOCH 35 1.4- 0 o 1.2 [ZnPCOOHL/[ZnPCOOHh — axodzttpcooii]M h 1.0 1 l 2 3 4 5 [DNBCOOth x104 M mt- V Figure 6. Plot of the left side of eq 5 vs [DNBCOOH]. fluorescence and the its emissive lifetime are monitored as a function of DNBCOOH concentration. Because the electron transfer rate for a donor/acceptor bound pair is expected to be much faster than the lifetime of the 11m“ excited state, emission from ZnPCOOH which is bound to an acceptor will not be observed. Since the donor/ acceptor pair does not emit, its lifetime cannot be monitored, and only the decay of unbound species will be observed.96 Therefore as the quencher concentration is increased the total emission of the sample decreases, while the fluorescence lifetime remains constant. Experiments involving static quenching of the ZnPCOOD emission by DNBCOOD are in agreement with this interpretation, inasmuch as the fluorescence decay remains constant (Figure 7). If the self-association Emission Intensity l I I 550 600 650 700 750 A I nm Emission Intensity I I L I I I 0 1 2 3 4 5 Time/X10418 Figure 7. (a) Emission spectra and (b) luminescence decays of CHZCIZ solutions of ZnPCOOD in the absence (top trace) and presence (bottom trace) of 10'2 M DNBCOOD. 37 binding constants obtained from IR spectroscopy are employed, analysis of the emission quenching yields a hetero-association binding constant of 316 M“1 for the deuterated acids in dichloromethane and 698 M‘1 for the ZnPCOOH/DNBCOOH pair in o-dichlorobenzene.97 The absorption spectrum of the ZnPCOOCHg' was obtained by collecting a transient absorption spectrum of a CHZCIZ solution of ZnPCOOCH3 (2.“ = 532 nm, FWHM = 10 us) in the presence of the electron acceptor tetracyanoethylene (TCNE) in the nanosecond timescale.98 The bimolecular electron transfer takes place from the porphyrin’s 31m" excited state which has a strong absorption feature at 460 nm (E00 = 1.7 eV)‘58 to TCNE, whose radical anion spectrum has a characteristic vibronic progression in the 400 - 500 nm region with a maximum at 435 nm (e = 7100 M’lcm"‘).99 Addition of TCNE results in quenching of the porphyrin’s 31m“ excited state lifetime, with concomitant growth of the TCNE“ and ZnPCOOCHS’ features (Figure 8). The ZnPCOOCH§ absorption spectrum has a maximum at 675 nm with a shoulder at 640 nm, as shown in Figure 8, in agreement with spectral profiles previously reported for other Zn- substituted porphyrins.“’°*101 Attempts to obtain ZnPCOOH” spectra in the presence of TCNE or methylviologen were unsuccessful; presumably there was irreversible protonation of the acceptor following the electron transfer, since charge recombination is slow (us - ms) in these bimolecular systems. Bulk electrolysis of ZnP lead to demetallation of the porphyrin within minutes following cation formation. Oxidation with agents such as silver nitrate yield an absorption spectrum with a maximum at 640 nm,m which is shifted from that observed in the transient absorption experiments. The absorption spectra of a number of reduced substituted nitrobenzenes and 38 ml 103 M"t:m‘1 .h o 1 I l 400 500 600 700 — M nm Figure 8. Transient absorption spectrum of a CH2C12 solution of TCNE and ZnPCOOCI-I3 collected 1 us after the laser pulse (10 us, 532 nm) 39 dinitrobenzenes have been reported, and their spectra show absorption maxima at approximately 300 and 830 nm.79 The transient absorption spectra of the of CH2C12 solutions of ZnPCOOH (Figure 9) and ZnPCOOD (Figure 10), collected 15 ps and 1.5 ns after excitation with 580 nm amplified pulses, respectively, correspond to the lmr’“ and 31m* excited states. These spectra are in excellent agreement with those obtained for other Zn-substituted porphyrins.51°'1°3 The 31:1?“ absorption exhibits a blue shift of the large positive feature at 455 nm compared to the 11m“ absorption, possibly due to decreased bleaching in the Soret region. The singlet spectra, both in the red and blue regions, have an apparent 10 ps risetime and subsequently decay slowly over hundreds of picoseconds. The emissive lifetimes of the 11m* excited states are 1.4 and 1.5 us for ZnPCOOH and ZnPCOOD, respectively, measured by time- correlated single photon counting (see insets in Figures 9 and 10).104 Figure 11 shows the excited state spectrum of ZnPCOOD in the red, where the 11m” exhibits a peak at 660 run; while absorption due to the 31m" state is significant, yet does not exhibit marked features in the 630 - 750 nm region. Addition of DNBCOOH to CHZCIZ solutions of ZnPCOOH results in the growth of a new absorption feature with a maximum at 685 nm, as shown in Figure 12, which is attributable to the ZnPCOOH”. The decay rates at 600 run, where the 1m?“ excited state absorbs, and at 685 nm are biphasic and concentration independent in the range [ZnPCOOH] = 1.0 to 1.6x10'3 M and [DNBCOOH] = 5.0 to 40x10‘3 M.1°5 Table VI lists the decays of 1-H obtained at several donor and acceptor concentrations followed at 660 and 685 nm. The 20 ps decay is attributed to the disappearance of the 11t1t* excited state, quenched by fast electron transfer to DNBCOOH. Since kCS is fast compared to the rate constant for excited state Table VI. Concentration Dependence of the Charge Separation and Charge Recombination Rates of 1-H in CH2C12. [ZnPCOOH]/M [DNBCOOH]/M ltCS/lo10 s-l keg/1010s-1 1.0x 10-3 5.0x 10-3 3.7 a 1.0x 10-3 ,. 1.4x 10-2 4.2 a 1.6x 10-3 1.4x 10-2 “ 4.8 a 1.6x 10-3 1.9x 102 7.7 1.0 1.6x10‘3 4.0x10'2 50” 10" “Signal too weak to measure rates. b Signal at these concentrations is the most reliable and was measured at least ten times. A00 41 a 515 ‘ i 0.8— ’3 > s a , ‘1: c i a it .c. it. 0.6- d ."I'\ f‘ . I \ ‘1" \\\ —I‘L I I I I L I 04— : \ 01234567 3; \ Time / xto'“ s 1 \\ ill! "\\‘ 0.2"- .1 x \\ I \ \ \ 0.0 V I l I I 400 450 500 A/nm 550 Figure 9. Transient absorption spectra of ZnPCOOH collected 15 ps (———) and 1.5 ns (— — —) after the excitation pulse. AOD A00 42 0.3— .é‘ (I) 0.61- g .5 — I ".~"~\ __I I L L L 0'4 " ‘.\ 0 1 2 3 4 \ Time I no“ s 02* 'l. \"‘-‘ \\ \ t \ 0.0 . V I I I \4’ I I I I I 1 I I 400 450 500 Alnm 550 Figure 10. Transient absorption spectra of ZnPCOOD collected 15 ps (—) and 1.5 ns (---) after the excitation pulse. 43 0.12 r- (a) 0.10 " door 1 . PO . / ‘ I 5 A 0.0. o.“ 1 I I I I I 0.12 — 0.10 i- 008 P A CD. 0.06 P 0.04 I I I I g I 600 650 700 750 Wavelength I run Figure 11. Transient (a) rise and (b) decay of the 11m“ excited state of a 10‘3 M CH2C12 solution of ZnPCOOH in the 625 -- 760 nm region. 0.06 0 ps 0.04 r- 0.02 - 5 ps 0.04 - "0.02 - D g 0.04 r- 0.02 b 0.02 '- 70 ps 0.02 b I l I I 600 700 800 A/nm Figure 12. Transient absorption spectra at various delay times after the 580 nm, 3 ps excitation pulse of ZnPCOOH (1.4><10’3 M) and DNBCOOH (4x10’2 M) in CH2C12. 45 decay, it represents the forward electron transfer rate, kcs = 5.0(5)><1010 s“. The rate of charge recombination, kCR, is attributed to the longer decay, which corresponds to a rate constant of 1.0(2)x1010 s". Figure 13 shows the transient absorption spectra obtained with the deuterated pair, l-D, where the electron transfer rates were found to be kCS = 3.0(3) x 1010 s"1 and kg. = 6.2(3) x 109 s“. Esterification of the donor to the methyl ester, ZnPCOOCH3, and acceptor to the ethyl ester, DNBCOOEt,106 results only in bimolecular quenching of the porphyrin’s 11m" excited state. The transient absorption spectra of ZnPCOOCH3 at various delay times in the presence of 5 x10‘2 M DNBCOOEt are shown in Figure 14. The singlet peak at 660 nm decays monotonically with a lifetime of 500 ps. Owing to the short lifetime of the porphyrin 11m“ excited state, quenching by a diffusion controlled electron transfer (kq = 1.8x 1010 M‘ls'.'1) is inefficient; thus the singlet is only slightly quenched even with a large concentration of DNBCOOEt. Therefore the quantum yield for the production of charge separated product is low, and features due to the porphyrin cation features are not observed. The ET process in the hydrogen-bonded pre-associated pair (1) shows a pronounced deuterium isotope effect, kH/kD. The ratio for the rates of charge separation and recombination are 1.7(3) and 1.6(4), respectively. The magnitudes are consistent with the 1.7 — 2.0 deuterium isotope effects measured for the oxidation of a soluble analog of Vitamin E by organochloro peroxides.107 In this latter system, the rate determining step has been proposed to involve the transfer of an electron from substrate to the peroxy radical via a hydrogen bonding network formed by the incipient hydroperoxide and solvent. 0.12 0.10 0.08 0.06 A 0.0. 0.04 0.02 0.00 -0.02 0.10 i- )- 0.oe —- p3 10 , 0.06 a q 50 O - 160 q 0.04 r 0.02 - 0.00 1- -o.02 . A . 1 1 1 600 650 700 750 Wavelength I nm Figure 13. Transient (a) rise and (b) decay of a CHZCIZ solution containing [ZnPCOOH] = 1.6 x 10'3 M and [DNBCOOH] = 4.0 e 10‘3 M in the 625 -— 760 nm region. 47 0.00 l- 0.06 i- A 0.0. 0.04 P 0.02 - 600 650 700 750 Wavelength / nm Figure 14. Transient decay of the 11m“ excited state of a 10“3 M CHZCIZ solution of ZnPCOOCH3 containing 5 x 10'2 M DNBCOOCHZCH3 in the 625 — 760 nm region. 48 2. Electron Transfer in Systems 2 and 3 Preliminary studies have been conducted with two other acceptors, 3,5-c‘ .nitro-p-toluic acid (DNTCOOH) and 9,10-anthraquinone-2-carboxylic acid (AQCOOH). The driving forces for forward and back ET are 0.8 V and 1.4 V, respectively, for 3, which are nearly identical to those for system 1 of Section 1. DNTCOOH is more difficult to reduce than DNBCOOH, thus mal ng tie driving force for the ZnPCOOH excited state ET and subsequent charge recombinatic Tl 0.6 V 5nd 1.6 V, respectively. Therefore the electron transfer rates in the .atter system are expected to be slower than those for the two other acceptors. When DNTCOOH is added to solutions of ZnPCOOH, a new absorption feature with maximum at 460 nm is observed upon excitation. which can be attributed to ZnPCOOH“. The Spectral profiles at selected delay times are shown in Figure 15, where a slight decrease in intensity is observed from 0 to 300 ps (Figure 15a). This initial decay is followed by the disappearance of the transient 1n the 1.0 to 2.5 ns timescale (Figure lfb). As is evident from the plot 0. -. .(AOD460) vs time shown in Figure 16, the full decay is triphasic, with a short, 81 ps component, and two longer ones of 1.46 and 2.81 ns. The fast decay is assigned to the disappearance of the quenched 11:1?“ excited state, which is due only to ZnPCOOH hydrogen bonded to a DNTCOOH molecule. The 1.46 ns decay is consistent with that of the unquenched 11m“ due to unbound ZnPCOOH, whose 1m?“ excited state in the absence of quencher is 1.45 us (see Figure 9). At times longer than 1 ns the decay of the ZnPCOOH“ occurs over 2.81 ns and is attributed to charge recombination. 49 0.4 AOD 0.2 0.0 0.4 0.2 AOD 0.0 l 400 450 500 550 Alnm Figure 15. Transient absorption spectra at selected delay following the pump pulse of Clip, solutions of [ZnPCOOH] = 10‘3 M and [DNTCOOH] = 10-2 M showing the decays (a) prior to and (b) after 300 ps. 50 —In(AOD) l l l l l l 2.0 0.0 0.5 1.0 1.5 2.0 2.5 " Time / ns Figure 16. Plot of -ln(AOD) vs time showing the triphasic decay of the ZnPCOOH/DNTCOOH pair. 51 The contribution of ZnPCOOH“ to the transient spectrum can be enhanced by subtraction of the signal from the 11t7r* excited state of unbound ZnPCOOH. This signal should in principle decay with the same lifetime as that in the absence of acceptor, since in conjunction with the short lifetime of the porphyrin’s lmr’“ excited state (1.45 ns), bimolecular quenching is not expected to play a prominent role in our concentration range (ZnPCOOH ~10‘3 M; DNTCOOH ~10‘2 M). This allows subtraction of a certain fraction of the ZnPCOOI—I spectrum at each corresponding delay time. However, determination of the correct fraction to be subtracted is difficult; therefore these subtracted spectra have been utilized only to aid in the assignment of the transients in a qualitative manner. Once all the spectral contributions of the 11t1t"‘ of bound ZnPCOOH molecules, which was quenched by electron transfer, have decayed, the only 11m“ excited state signal should be from unbound ZnPCOOH. Thus at 800 ps, for example, one can begin subtracting fractional components of the ZnPCOOH 11m" spectrum until the region where only the singlet absorbs (420 - 440 nm) becomes zero. In our case this occurs when half of the ZnPCOOH spectra shown in Figure 9 is subtracted from those presented in Figure 15; the resulting spectra are shown in Figure 17. If one now subtracts this amount of the ZnPCOOH 11m" excited state spectrum from those obtained in the presence of acceptor at short times (0 to 100 ps), the decay of the singlet in the 420 - 440 nm region is observed. Owing to the slower time scale for charge recombination, the signal from ZnPCOOH+ at 460 nm remains of constant intensity over short times, after which it decays dramatically over 2.5 ns.. This observation is in agreement with the assignment of the short decay to the disappearance of the quenched ZnPCOOH thlt* excited state. 52 0.2 " AOD 0.0 l 400 450 500 550 Figure 17. Transient absorption spectra at selected delay following the pump pulse of CHZCIZ solutions of [ZnPCOOH] = 10'3 M and [DNTCOOH] = 10'2 M after subtraction of 11m“ signal. 53 The spectral profiles obtained by the subtraction of a fraction of the ZnPCOOH spectrum (0.7) from those obtained in the presence of AQCOOH are shown in Figure 18 (system 3—H). The multiplication constant is larger in this case because the acceptor concentration was smaller due to solubility problems; thus more unbound ZnPCOOH is expected to be present in solution. For this reason a concentration dependence was not followed and the exact concentration of quencher is unknown, although is likely to be in the 10‘3 to 10'2 M range. However, qualitatively two distinct absorption features are observed, one at 460 nm due to the ZnPCOOH” and the other with maximum at 490 nm, which is typical of reduced quinones.108 The decay profile (—ln(AOD460) vs time) of the unsubtracted spectra at 485 nm is shown in Figure 19, where two decays of 22 and 465 ps are observed. No charge transfer was observed in 2-D, even at high concentrations. Spectra of solutions containing [DNTCOOD] = 5x10“2 M, obtained 20 ps and 2 ns after the pump pulse, show no absorption due to the ZnPCOOD“, and only the 11m" and 31m* spectral profiles are evident from these data (Figure 20). This observation is puzzling, since it implies that either the donor and acceptor are not binding or that the binding constants are faster or comparable to the forward ET rate. If one assumes the previously determined deuterium isotope effect of 1.7 and utilizes kcs = 1.2)(1010 s‘1 for the protiated pair, then in the deuterated system kcs would be expected to be 7x109 3". If the rate constant for the dissociation of the donor/acceptor pair is of the order of 1010 s‘lM‘l, then charge separation would be impeded. Alternatively, the system may have a small enough binding constant such that the number of donor/ acceptor pairs in solution is not sufficiently high to be detected by transient absorption. To this end, the binding constant must be determined by IR spectroscopy and the static quenching experiment AOD 54 l 400 450 500 550 A / nm Figure 18. Transient absorption spectra at collected at selected delay times after the pump pulse of CHZClz solutions containing ZnPCOOH and AQCOOH, after subtraction of 11m* signal. 55 -ln(AOD) 1 l I 0 200 400 600 800 Time / ps Figure 19. Plot of —ln(AOD) vs time showing the biphasic decay of the ZnPCOOH/AQCOOH pair. 56 0.6 '- 0.4 F AOD 0.2 r 0.0 400 550 Alnm Figure 20. Transient absorption spectra of a CHZCIZ solution containing [ancoool = 10‘3 M and [DNTCOOH] = 10‘2 M collected 20 ps (——) and 2 ns (- - -) after the excitation pulse. 1 57 should be performed on this system to begin addressing the questions posed by these preliminary results. 3. Comparison of the Three Protiated Systems Three different protiated acceptors were utilized to form donor/ acceptor pairs of type n—H (n = l - 3), The rates of charge separation, kcs, and charge recombination, kCR, measured for the three systems are listed in Table VII, along with their respective driving forces. Systems 1 and 2 have similar forward and back driving forces; accordingly their forward ET rates are equal. However, the charge recombination rate of 2 is a factor of 4.6 slower than that of 1. It is not yet clear why these rates are so different. Although the driving forces for 1 - 3 are only approximate (Table VII), speculations stemming from their approximate values can be made. From classical transition state theory the electron transfer rate can be Table VII. Comparison of charge separation and recombination ET rates for the three acceptors with ZnPCOOH with their respective driving forces. Acceptor —AGCS (eV)a kcs(s'l) -Ach (eV)a ltCR(s-1) DNBCOOH 0.7 5.0x 1010 1.4 1.0x 1010 AQCOOH 0.7 5.0x 1010 1.4 2.1 x 109 DNTCOOH 0.5 1.2x 1010 1.6 3.6x 108 “The values of AG were calculated from the reduction potentials listed in Table I. 58 expressed as109 2 — AG + A 2 ket = A lVoexp(-l5(d-do)| exp{ (4}.ka ) } (7) where A is a pre-exponential factor, d and cl0 the edge-to-edge distance between donor and acceptor and the closest distance, respectively, B is the damping factor which determines the decay of the electronic coupling with distance, AG is the driving force, A the reorganization energy (assumed to be 1 V), k3 is Boltzmann's constant, and T is the temperature (298 K). From to eq 7, the charge separation in 3—H should be approximately four times slower than those in l-H and 2-H, whereas the charge recombination is expected to be attenuated by a factor of ~6. The value of lies is in agreement with this prediction. This relation for the charge recombination rate of 3—H is in agreement with that for 2- H, where a six-fold attenuation on the 2.15x109 5‘1 value for kCR is 3.58x108 s“, in excellent agreement with the value obtained experimentally. These values, however are not in agreement with the charge recombination rate measured for l-H. One possibility is proton abstraction in the reduced DNTCOOH. It has been shown that nitrobenzenes reduced by one electron readily abstract protons from neighboring methyl or methylene groups.110 In cases where this abstraction takes place following laser excitation, the system returns to the ground state with a lifetime of ~1 113.111.112 From our observed charge recombination rate, kcp = 3.6x 108 s“, it is not unreasonable to postulate that proton abstraction governs the back electron transfer rate. However, this explanation does not address the slow charge recombination rate observed in 2-H, where AQCOOH is the acceptor. 59 Comparison of the ET rates through hydrogen bonds with those reported for porphyrin/quinone systems at similar donor/ acceptor separation reveals that the proton interface attenuates the rate by approximately one 113 order of magnitude. These results are in agreement with recent calculations, which predict that the electronic coupling through one hydrogen bond corresponds to three covalently-bonded atoms.114 Our data show that although electron transfer is not as efficient as through covalent bonds, hydrogen bonded pathways for ET may play prominent roles where covalently-bound alternatives are very long or absent. Utilizing eq 7 for the ET rates in l—H and in Wasielewski's systems,113 where through-bond B = 0.84 and A is constant in both cases,"‘ one obtains a value of B = 1.0 - 1.2 for the ET through our hydrogen-bonded interface. This value of the damping factor is not unreasonable, since in proteins, where hydrogen bonds and are prominent, B ranges from 1 to 2.12°“5 D. REFERENCES 1. Stryer, L. Biochemistry 3rd Ed.; W. H. Freeman and Co: New York, 1988. 2. Zubay, G. Biochemistry 2nd Ed.; MacMillan Publishing Company: New York, 1988. Williams, R. J. P. Nature 1989, 338, 709. 4. Wikstrom, M.; Krab, K.; Saraste, M. In Cytochrome Oxidase: A Synthesis; Academic Press: New York, 1981. 5. Williams, R. J. P. In Electron Transfer in Biology and the Solid State; Johnson, M. K.; King, R. B.; Kutz, D. M., Jr.; Kutal, C.; Norton, M. L; 10. ll. 12. l3. 14. 15. 16. 17. 18. 60 Scott, R. A., Eds.; Advaces in Chemistry Series 226; American Chemical Society: Washington DC; 1990, pp 3-26. Scott, R. A. Annu. Rev. Biophys. Chem. 1989, 18, 137. Naqui, A.; Chance. B.; Cardenas, E. Annu. Rev. Biochem. 1986, 55, 137. Chan, 8. 1.; Li, P. M. Biochemistry 1990, 29, 1. Malmstrom, B. G. Chim. Scripta 1987, 27B, 67. Capaldi, R. A. Chemica Scripta 1987, 27B, 39. (a) Beratan, D. N.; Betts, J. N.; Onuchic, J. N. Science 1991, 252, 1285. (b) Betts, J. N.; Beratan, D. N.; Onuchic, J. N. J. Am. Chem. Soc. 1992, 114, 4043. ' Wuttke, D. S.; Bjerrum, M. J .; Winkler, J. R.; Gray, H. B. Science 1992, 256, 1007. Therien, M. J.; Selman, M.; Gray, H. B.; Chang, I-J.; Winkler, J. R. J. Am. Chem. Soc. 1990, 112, 2420. Hille, R. Biochemistry 1991, 30, 8522. Ali, S. N.; Zeller, H.-D.; Calisto, M. K.; Jorns, M. 8. Biochemistry 1991, 30, 10980. Hansson, O.; Wydrzynski, T. Photsynth. Res. 1990, 23, 131. Topics in Photosynthesis: The Photosystems; Barber, J ., Ed.; Elsevier: Amsterdam, 1991. (a) Deisenhofer, J .; Epp, 0.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180, 385. (b) Deisenhofer, J.; Epp, 0.; Miki, K.; Huber, R.; Michel, H. Nature (London) 1985, 318, 618. (c) Allen, J. P.; Feher, G.; Yeates, T. O.; Rees, D.; Deisenhofer, H.; Huber, R. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 8593. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 61 (a) Chang, C.-H.; Tiede, D.; Tang, J .; Smith, U.; Norris, J .; Schiffer, M. J. Mol. Biol. 1985, 186, 201. (b) El-Kabbani, 0.; Chang, C.-H.; Tiede, D.; Norris, J .; Schiffer, M. Biochemistry 1991, 30, 5361. (a) Fleming, G. R.; Martin, J .-L.; Breton, J. Nature (London) 1988, 333, 190. (b) Breton, J.; Martin, J.-L.; Fleming, G. R.; Lambry, J. C. Biochemistry 1988, 27, 8276. (a) Kirrnaier, C.; Holten, D. FEBS Lett. 1988, 239, 211. (b) Kirmaier, c.; Holten, D. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 3552. ' (a) Holzapfel, W.; Finkele, U.; Kaiser, W.; Oesterhelt, D.; Scheer, H.; Stilz, H. U.; Zinth, W. Chem. Phys. Lett. 1989, 160, 1. (b) Dressler, K.; Finkele, U. Lauterwasser, C.; Hamm, P.; Holzapfel, W.; Buchanan, 8.; Kaiser, W.; Michel, H.; Oesterhelt, D.; Scheer, H.; Stilz, H. U.; Zinth, W. In Structure and Function of Bacterial Reaction Centers; Michel- Beyerle, M. E., Ed.; Springer-Verlag: Berlin, 1990; pp 135-140. Woodbury, N. W.; Becker, M. Middendorf, D.; Parson, W. W. Biochemistry 1985, 24, 7516. Carithers, R. P.; Parson, W. W. Biochim. Biophys. Acta 1975, 387, 194. Vermeglio, A.; Clayton, R. K. Biochim. Biophys. Acta 1977, 461, 159. Vincent, J. B.; Christou, G. Adv. Inorg. Chem. 1989, 33, 197. Brudvig, G. W.; Crabtree, R. H. Prog. Inorg. Chem. 1989, 37, 99. Chan, M. K.; Armstrong, W. H. J. Am. Chem. Soc. 1990, 112, 4985. Paddock , M. L.; Rongey, S. H.; Feher, G.; Okamura, M. Y. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 6602. Takahashi. E.; Wraight, C. A. Biochim. Biophys. Acta 1990, 1020, 107. Takahashi, E.; Wraight, C. A. Biochemistry 1992, 31, 855. Paddock, M. L.; McPherson, P. H.; Feher, G.; Okamura, M. Y. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 6803. 33. 34. 35. 36. 37. 38. 39 41. 42. 43. 45. 46. 47. 48. 62 Buchanan, S.; Michel, H.; Gerwert, K. Biochemistry 1992, 31, 1314. Wikstrom, M.; Krab, K.; Saraste, M. In Cytochrome Oxidase: A Synthesis; Academic Press: New York, 1981. Williams, R. J. P. In Electron Transfer in Biology and the Solid State; Johnson, M. K.; King, R. B.; Kutz, D. M., Jr.; Kutal, C.; Norton, M. L; Scott, R. A., Eds.; Advaces in Chemistry Series 226; American Chemical Society: Washington DC, 1990; pp 3-26. Scott, R. A. Annu. Rev. Biophys. Chem. 1989, 18, 137. Naqui, A.; Chance. 8.; Cardenas, E. Annu. Rev. Biochem. 1986, 55, 137. Chan, S. 1.; Li, P. M. Biochemistry 1990, 29, 1. Pan, L.-P.; Hazzard, J. T.; Lin, J .; Tollin, G.; Chan, S. I. J. Am. Chem. Soc. 1991, 113, 5908. Hill, B. C. J. Biol. Chem. 1991, 266, 2219. Morgan, J. E.; Wikstrom, M. Biochemistry 1991, 30, 948. Oliveberg, M.; Malmstrom, B. G. Biochemistry 1991, 30 , 7053. Morgan, J. E.; Li, P. J.; Jang, D.-J.; El-Sayed, M. A.; Chan, S. 1. Biochemistry 1989, 28, 6975. Brzezinski, P.; Malmstrdm, B. G. Biochim. Biophys. Acta 1987, 894, 29. Boelens, R.; Wever, R.; Van Gelder, B. F. Biochim. Biophys. Acta 1982, 682, 264. (a) Wikstrom, M. Chemica Scripta 1987, 27B, 53. (b) Wikstrom, M. Nature 1989, 338, 776. Malmstr'o'm, B. G. Chim. Scripta 1987, 27B, 67. Williams, R. J. P. Nature 1989, 338, 709. 49 50. 51. 52. 53. 54. 63 . (a) Malmstrtim, B. G. Chem. Rev. 1990, 90, 1247. (b) Malmstr'o'm, B. G. Arch. Biochem. Biophys. 1990, 280, 233. (a) Wasielewski, M. R.; Johnson, D. G.; Svec, W. A.; Kersey, K. M.; Minsek, D. W. J. Am. Chem. Soc. 1988, 110, 7219. (b) Wasielewski, M. R.; Johnson, D. G.; Niemczyk, M. P.; Gaines, III, G. L.; O’Neil, M. P.; Svec, W. A. J. Am. Chem. Soc. 1990, 112, 6482. (c) Gaines, III, G. L.; O’Neil, M. P.; Svec, W. A.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem. Soc. 1991, 113, 719. (a) Bilsel, O.; Rodriguez, J .; Holten, D.; Girolami, G. S.; Milam, S. N.; Suslick, K. S. J. Am. Chem. Soc. 1990, 112, 4075. (b) Knapp, S.; Dhar, T. G. M.; Albaneze, J.; Gentemann, S.; Potenza, J. A.; Holten, D.; Schugar, H. J. J. Am. Chem. Soc. 1991, 113, 4010. (c) Rodriguez, J.; Kirmaier, C.; Johnson, M. R.; Friesner, R. A.; Holten, D.; Sessler, J. L. J. Am. Chem. Soc. 1991, 113, 1652. (a) Heller, D.; McLendon, G.; Rogalskyj, P. J. Am. Chem. Soc. 1987, 109, 604. (b) Helms, A.; Heller, D.; McLendon, G. J. Am. Chem. Soc. 1991, 113, 4325. (a) Closs, G. L.; Calcaterra, L. T. :1 Green, N. J .; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673. (b) Closs, G. L.; Miller, J. R. ‘ Science (Washington, DC) 1988, 240, 440. (c) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Piotrowiak, P. J. Am. Chem. Soc. 1989, 111, 3551. (a) Siemiarczuk, A.; McIntosh, A. R.; Ho, T.-F.; Stillman, M. J.; Roach, K. J .; Weedon, A. C; Bolton, J. R.; Connolly, J. S. J. Am. Chem. Soc. 1983, 105, 7224. (b) Schmidt, J. A.; McIntosh, A. R.; Weedon, A. C.; Bolton, J. R; Connolly, J. S.; Hurley, J. K.; Wasielewski, M. R. J. Am. Chem. Soc. 1988, 110, 1733. 55. 56. 57. 58. 59. 61. 62. 63. 64 Leland, B. A.; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A. H.; Dervan, P. B. J. Phys. Chem. 1985, 89, 5571. (a) Harrison, R. J .; Pearce, B.; Beddard, G. S.; Cowan, J. A.; Sanders, J. K. M. Chem. Phys. 1987, 116, 429. (b) Hunter, C. A.; Meah, M. N.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5773. (a) Batteas, J. D.; Harriman, A.; Kanda, Y.; Mataga, N.; Nowak, A. K. J. Am. Chem. Soc. 1990, 112, 126. (b) Osuka, A.; Maruyama, K.; Mataga, N.; Asahi, T.; Yamazaki, I.; Tamai, N. J. Am. Chem. Soc. 1990, 112, 4958. Hermant, R. M.; Bakker, N. A. C.; Scherer, T.; Krijnen, B.; Verhoeven, J. W. J. Am. Chem. Soc. 1990, 112, 1214. (b) Antolovich, M.; Keyte, P. J.; Oliver, A. M.; Paddon—Row, M. N.; Kroon, J.; Verhoever, J. W.; Jonker, S. S.; Warman, J. M. J. Phys. Chem. 1991, 95, 1933. Gust, D.; Moore, T. A. Science (Washington, DC) 1989, 244, 35. (b) Gust, D.; Moore, T. A.; Moore, A. L.; Gao, F.; Luttrull, D.; DeGraziano, J. J.; Ma, X. C.; Makings, L. R.; Lee, S.-J.; Trier, T. T.; Bittersman, E.; Seely, G. R.; Woodward, S.; Bensasson, R. V.; Rougee, M.; De Schryver, F. C.; Van der Auweraer, M. J. Am. Chem. Soc. 1991, 113, 3638. Meyer, T. J. Acc. Chem. Res. 1989, 22, 164. Perkins, T. A.; Humer, W.; Netzel, T. L.; Schanze, K. S. J. Phys. Chem. 1990, 94, 2229. Vassilian, A.; Wishart, J. F.; van Hemelryck, B.; Schwarz, H.; Idied, S. S. J. Am. Chem. Soc. 1990, 112, 7278. Chang, M.; Gray, H. B.; Winkler, J. R. J. Am. Chem. Soc. 1991, 113, 7056. 65. 66. 67. 68. 69. 70. 71. 72. 73. 65 (a) McLendon, G. Acc. Chem. Res. 1988, 21, 160. (b) Conklin, K. T.; McLendon, G. J. Am. Chem. Soc. 1988, 110, 3345. (a) Wallin, S. A.; Stemp, E. D. A.; Everest, A. M.; Nocek, J. M.; Netzel, T. L.; Hoffman, B. M. J. Am. Chem. Soc. 1991, 113, 1842. (b) Kuila, D.; Baxter, W. W.; Natan, M. J .; Hoffman, B. M. J. Phys. Chem. 1991, 95, 1. (c) Nocek, J. M.; Stemp, E. D. A.; Finnegan, M. G.; Koshy, T. 1.; Johnson, M. K.; Margoliash, E.; Mauk, A. G.; Smith, M.; Hoffman, B. M. J. Am. Chem. Soc. 1991, 113, 6822. Dixon, D. W.; Hong, X.; Woehler, S. E.; Mauk, A. G.; Sishta, B. P. J. Am. Chem. Soc. 1990, 112, 1082. Concar, D; W.; Whitford, D.; Pielak, G. J .; Williams, R. J. P. J. Am. Chem. Soc. 1991, 113, 2401. Sigel, H.; Sigel, A., Eds. Metals in Biological Systems; Marcel Dekker: New York, 1991; Vol. 27. Johnson, M. K.; King, R. B.; Kurtz, D. M.; Kutal, C.; Norton, M. L.; Scott, R. A., Eds.; Electron Transfer in Biology and the Solid State; American Chemical Society: Washington, DC, 1990; Vol. 226, Section 2. De Felippis, M. R.; Faraggi, M.; Kapper, M. H. J. Am. Chem. Soc. 1990, 112, 5640. (a) Barbara, P. F.; Jarzeba, W. Acc. Chem. Res. 1988, 21 , 195. (b) Smith, T. P.; Zaklika, K. A.; Takur, K.; Barbara, P. F. J. Am. Chem. Soc. 1991, 113, 4035. Swinney, T. C.; Kelley, D. F. J. Phys. Chem. 1991, 95, 2430. Held, A.; Plusquellic, D. F; Tomer, J. L.; Pratt, D. W. J. Phys. Chem. 1991, 95, 2877. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 66 (a) Millikan, R. C.; Pitzer, K. S. J. Am. Chem. Soc. 1958, 80, 3515. (b) Davis, Jr., J. C.; Pitzer, K. S. J. Phys. Chem. 1960, 64, 886. Chang, T.-T.; Yamaguchi, Y.; Miller, W. H.; Schaefer, 111, H. F. J. Am. Chem. Soc. 1987, 109, 7245. Wenograd, J .; Spur, R. A. J. Am. Chem. Soc. 1957, 79, 5844. Kalyasundaram, K.; Newman-Spailart, M. J. Phys. Chem. 1982, 86, 5163. . Clark, W. M. Oxidation-Reduction Potentials of Organic Systems; Krieger: Huntington, N.Y.; 1972. Neta, P.; Simic, M. G.; Hoffman, M. Z. J. Phys. Chem. 1976, 80, 2018. Kazakova, V. M.; Minina, N. E.; Piskov, V. B. Zh. Obs. Khim. 1982, 52, 836. Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Non- Aqueous Systems; Marcel Dekker: New York, 1970 This porphyrin was synthesized from condensation of 5,5’-dibromo— 3,3’-diethyl-4,4’-dimethyl-2,2’-dipyrrylmethene hydrobromide and 4- carboxymethyl-3,3’,4’,5,5’-pentamethyl-2,2’-dipyrrylmethene hydro- bromide in formic acid. Marty Raab, the MSU Chemistry Department electronic designer, modified the electronics to allow the device to operate at cavity dumper rates slower than 8 MHz. Ultrafast Light Pulses; Shapiro, S. L., Ed; Springer-Verlag: Berlin; 1977. Skoog, D. A. Principles of Instrumental Analysis 3rd Ed.; Saunders College: Philadelphia, 1985. Declemy, A.; Rulliere, C. Rev. Sci. Instrum. 1986, 57, 2733. Mussel, R. D.; Nocera, D. G. J. Am. Chem. Soc. 1986, 110, 2764. 88. 89. 91. 92. 93. 94. 95. 96. 97. 98. 99. 67 Joensten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974. Tucker, E. E.; Lippert, E. In The Hydrogen Bond; Schuster, P; Zundel, G.; Sandorfy, C., Eds.; North-Holland Pub. Co.: New York, 1976. Huggins, C. M.; Pimentel, G. C.; Shoolery, J. N. J. Phys. Chem. 1956, 60, 1311. Davis, Jr., J. C.; Pitzer, K. S. J. Phys. Chem. 1960, 64, 886. Muller, N .; Hughes, O. R. J. Phys. Chem. 1966, 70, 3975. Murthy, A. S. N.; Rao, C. N. R. Appl. Spectros. Rev. 1968, 2, 69. Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New York, 1975. Affsprung, H. E.; Christian, S. D.; Melnick, A. M. Spectrochim. Acta 1963, 20, 285 Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F. Coord. Chem. Rev. 1975, 15, 321. The ratio of 10/1, where 10 and I represent the integrated emission intensities in the absence and presence of a known quencher concentration, respectively, was utilized to calculate the number of ZnPCOOH molecules bound to DNBCOOH. The nanosecond transient absorption instrument has been previously described (Jackson, J. A.; Turro, C.; Newsham, M. D.; Nocera, D. G. J. Phys. Chem. 1990, 94, 4500) and is explained in more detail in Chapter II.B. Webster, 0. W.; Mahler, W.; Benson, R. E. J. Am. Chem. Soc. 1962, 84, 3678. 100. Nosaka, Y.; Kuwabara, A.; Miyama, H. J. Phys. Chem. 1986, 90, 1465. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 68 Fajer, J .; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem. Soc. 1970, 92, 3451. Oertling, W. A.; Salehi, A.; Chung, Y. C.; Leroi, G. E.; Chang, C. K.; Babcock, G. T. J. Phys. Chem. 1987, 91, 5887. Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111 , 6500. km = 575 nm, FW HM = 5 ps, item = 630 nm; instrument described in detail by Lawerence E. Bowman, Ph.D. Dissertation, Michigan State University, 1991. A larger range of concentrations could not be probed owing to the low signal intensities and insufficient solubility at low and higher concentrations, respectively. Because of synthetic inavailability, ethyl-3,5-dinitrobenzoate was utilized instead of the 3,4-substitued analog. tha, P.; Huie, R. E.; Maruthamuthu, P.; Steenken, S. J. Phys. Chem. 1989, 93, 7654. Umemoto, K. Chem. Lett. 1985, 1415. (a) Marcus, R. A. J. Chem. Phys. 1956, 24, 966. (b) Marcus, R. A. Ann. Rev. Phys. Chem. 1964, 15, 155. Fine, D. A.; Miles, M. H. Anal. Chim. Acta 1983, 153, 141. McClelland, R. A.; Steenken, S. Can. J. Chem. 1987, 65, 353. Gravel, D.; Giasson, R.; Blanchet, D.; Yip, R. W.; Sharma, D. K. Can. J. Chem. 1991, 69, 1193. Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J. Am. Chem. Soc. 1985, 107, 1080. Beratan, D. N.; Betts, J. N.; Onuchic, J. N. Science 1991, 252, 1285. Winkler, J. R.; Gray, H. B. Chem. Rev. 1992, 92, 369. CHAPTER II TRANSIENT ABSORPTION SPECTROSCOPY OF Mo AND W QUADRUPLY-BONDED DIMERS A. BACKGROUND Multielectron transformations are at the essence of energy conversion and storage schemes. Their importance in biology is manifested in the catalytic reduction of oxygen in respiration and the oxidation of water in photosynthesis."13 A crucial issue in the mechanism of multielectron reactions has been the determination of whether they proceed via two consecutive one-electron transfers or one concerted two-electrOn step.“’17 Although many reactions were believed to proceed in a concerted manner, as the detection timescales became faster they were shown to be composed of two one-electron steps.”17 Early kinetic methods included techniques such as stopped-flow and temperature-jump, which permit measurement of events in the 10'4 s and 10“5 s regimes, respectively. However, in systems where photons initiate the two-electron reactions, dynamic measurements can be conducted following excitation of the sample by a light pulse of short duration (10‘8 to 10‘12 s). The excited states, or intermediates, prepared in this manner should provide an insight into the requirements necessary to 69 70 effect photoinduced two-electron transformations. Following the excitation pulse their physical properties, including their electronic absorption spectrum, can be probed in the absence of substrate. Quadruply-bonded of molybdenum complexes, containing a MonMoII core and four bidentate bridging ligands, are known to undergo two-electron photochemsitry to yield an oxidized MomMoIII product.”20 However, these reactions were shown to proceed via radical mechanisms, rather than a concerted two-electron reaction. It was postulated that the rigid bridging ligands about the metal core precluded addition of the substrate to one of the metals, thus leading to the formation of radicals. To remove this constraint, complexes of the type M2c1,,(1'>“l>)2 and M2C14(PR3)4 (M = M0, w; P‘ P = bidentate phosphines; PR3 = trialkyl phosphines) were utilized, which contain flexible halides. The two types of complexes have different symmetries, D2,, and Du, as illustrated below, T T T T“ £_ 2° £3. 2" (“7“ 9.47“,” P 0 m3 I)2h I)2d The M2Cl4(PR3)4 complexes have an eclipsed conformation about metal core to preserve the metal-metal 5 bond; however, due to steric effects, each phosphine is eclipsed with a chloride on the adjacent metal, rather than another PR3 group. These complexes have been shown to undergo photoinduced two- electron oxidative addition in the presence of methyl iodide (MeI), and 71 unlike the reactivity observed with the rigid ligands, one electron intermediates outside the solvent cage were not produced.”24 The observed reaction of MozCl4(dppm)2 (dppm = bis(diphenylphosphino)methane) with Mel is shown in eq 8, Fh O-l Ph - Fh O-l Fh Flt-Al / 2‘ Iz-Ph PhAP/ “Pleat TLC; I}: hvot>435 nm) A‘_C"| |\G— I" p Ph Ph Ph Fh m7P\O-Iz/PVP11 H17? \CHz/ VP“ which yields an edge-sharing bioctahedral structure with the addition of both Me and I to the same metal center.23 This feature and the absence of ethane are indicative of the lack of radicals outside the solvent cage. Similar products have been observed in the thermal two-electron addition reactions of quadruply-bonded complexes, which yield products containing bridging ligands, oftentimes halides. A rearrangement of the ligation sphere about the MiM core in the transition from reactants to products is therefore expected.25 The different reactivity between the flexible and rigid complexes is intriguing since their excited states are similar in nature. A general MO diagram for MiM complexes is shown in Figure 21, where two ML4 fragments are brought together to create the metal-metal bonds. The dzz orbitals form the metal-metal 0' bond, while d,‘z and dyz orbitals give rise to two metal-metal 1: bonds. The fourth bond is 5 in nature, and is formed by the weak interaction of the dxy orbitals on each metal. For two (14 metals, as is the case with Mo11 and W", the highest occupied molecular orbital (HOMO) is the 8 orbital, while the lowest unoccupied molecular orbital 72 \ \‘ ‘ s ‘ \ I \ ’ I I’ ’ , I I \ H \ (en) ,’ ‘ I I I 1 ._ 1 .. ‘L—Ms“ __ \“ L’T *2 LIT L I z * (ITI L L y/ L L y/ Figure 21. Molecular orbital diagram derived by uniting two (14 ML, fragments to form the bimetallic Mng quadruply-bonded complex. 73 (LUMO) is formed from the corresponding antibonding interaction, 8*. Many spectroscopic studies have been conducted with these molecules and it is well-established that the lowest electronic transition is the dipole allowed 82 -—> 188’“, as evidenced from inspection of the general MO diagram.”29 Owing to the small overlap of the (1,.y orbitals valence bond theory provides a better description of the transitions involving the 8 and 8* molecular orbitals, as shown in Figure 22. In the ground lAlg state each electron is localized on the dxy orbital of each metal, whereas in the first allowed excited state, 1A2“, both electrons reside on the same metal center. In this model the triplet excited state lies just above the ground state rather than “ear the excited singlet, since electron pairing is necessary in the transition to the 1A2u state. The spectroscopic properties of the D2,, and Du complexes must be understood before transient absorption studies are undertaken. The correlation diagram for the D211 and Du geometries is shown in Figure 23, where the effect of a C4 rotation of one ML2X2 fragment about the M-M axis on the relative position of the molecular orbitals is followed through the staggered conformation, D2.29 The most significant difference between the two MO diagrams is the degeneracy of the 1: and 1t* orbitals in the D2d complex, whereas in the D211 case the degenerac is lifted. Representative ground state electronic absorption spectra for Mo and W homonuclear dimers are shown in Figure 24 (D2,, geometry) and Figure 25 (Du geometry). The visible transitions in the tungsten complex are red-shifted from those in the molybdenum analog, consistent with their metal-localized origins. The 88* transitions in MozCl4(dppm)2 and W2C12(dppm)2 appear at 634 and 710 nm, respectively, and the high energy absorption in the 350 — 400 nm region have been assigned as 11; —-> 8* in nature.29 The complete 74 25V 201- Energy / kK a I 8 I Figure 22. Valence bond description of the electronic states formed by the dxy orbitals, as well as the corresponding MO formalism 75 a: Baa—h O on" a .8 Eu «6 2032:: w confluence 3:35. am . O 5Q 502:3 . an v 85088 as O x a a a/ . o as .. u o s a e a. N co N no ~ o s do s a N an lameJII on s IIIIII II a .. an e Tanl s IIIIHIIIIIIINI I I s s llllnllll D I II It s up s I lllllliuls n s \Idl o 1 11 w a: LHVJNI awn .. . . so ..|e.| ~ \‘ Fa QIII III s / \ \~ I'll! II 3 II \ \ .o. l w 1.....uunn1111111111h . 33.41 INNIIA' shutsslt till 6 All \ssss . .11 111 as 11:11:11.1: .I\\ It I . o \.I\ I III s eeldluunnll... an a ..-- o IIII :— emeJxlv. ... 414.211..-.{unwilllb o III DQDINAJIIIllIav Illl n IIIII a o It III «unluu1111THa 1‘ an INx a.» opt a I v 76 afiszu 5 2883.6"; T .. i one £88328: Tl do good... 8:988 38:85 .8 came. . E: \ £98652, coo och com com 8v can u q - ‘ A ‘ .- \“ -"" eoueqlosqv 77 .550 e essence: T .. 1v a com con as «mama—30"? ATV mo «58% 5:888“ Segue—m .mn 95w:— aauemosqv 78 assignment of the “D2,, complexes in the visible and ultraviolet regions has been conducted for M2X4(PMe3)4 (M = Mo, W; X = Cl, Br, I; PMe3 = trimethylphosphine). In M02C14(PMe3)4 and W2Cl4(PMe3)4 the 8 8* transitions reach their respective maxima at 584 and 655 nm. The small band at 440 nm in the Mo complex has been assigned to be 1: -) 8*, and the strong absorption at 330 nm has been correlated with a 0(MP) —> 8* transition.28 A similar assignment has been provided for the tungsten analog for the absorptions at 490 (11: -—) 8*) and 296 nm (O(MP) —-> 8*), where the metal-localized transitions red-shift with respect to those in the molybdenum complex with the concomitant blue-shift of the ligand-to-metal charge transfer (LMCI') band.28 The metal-localized transitions lead to the movement of some electron density from one metal to the other, and can therefore be represented formally by hv ——> IIM_4_MII IMLMIII (9) The 188* excited state posseses a dipole moment of 4.0 D with respect to the non-polar ground state, suggesting a transfer of 0.4 electrons from one metal to the other.30 The two-electron photochemical reactions are believed to stem from the MMCT mixed valence states, where the oxidative addition takes place at the low-valent center to yield the oxidized IIIM—Mm product. Transient absorption studies have been undertaken to distinguish the excited state properties which permit the two-electron reactivity of the flexible complexes, while precluding that of the rigid analogs. Comparative transient absorption studies of M02C14(PMe3)4 and M02C14(PBu3)4 (PBu3 = tributylphosphine) have shown that the former decays monoexponentially to the ground state with the lifetime corresponding to that of the 188* emission, 79 whereas the 188’" state of the latter decays to a long-lived, non-emissive transient.31 Several possible explanations have been provided for the nature of this long-lived transient, such as population of the low-lying 388* or some other state that does not couple effectively with the ground state.31 A more plausible alternative is a conformational distortion, which can be accomplished by a ligation core rearrangement. Similar differences have been observed for complexes of the type M02C14(PP)2; when the bridging ligands, P-‘P, are dimethylphosphines the excited state decay takes place directly from the 188* excited state, whereas when RP = diphenylphosphines the long-lived non-emissive intermediate is observed.31 The difference in excited state properties between these complexes has been correlated to the differences in their cone angles.32 Recently, it has also been observed that two-electron oxidative-addition proceeds only with those complexes which exhibit the long-lived, non-emissive transient.22 In this Chapter the results of transient absorption spectroscopy of the M2Cl4(BP)2 and M2Cl4(PR3)4 bimetallic complexes (D2,, and Dzd symmetries, respectively) will be presented, and correlations between structure and reactivity will be made. These studies have elucidated some of the properties required to effect two- electron photochemistry. B. EXPERIMENTAL METHODS The complexes were synthesized by Dr. Partigianoni; the synthesis and sample preparation procedures have been previously described.21 The transient absorption spectroscopy was either conducted on the picosecond 80 timescale with the instrument described in Chapter LB or with nanosecond resolution. The nanosecond transient absorption measurements were made with the pulse-probe technique. The excitation source was a Quanta Ray DCRl Nd:YAG laser whose fundamental frequency was doubled or tripled with a Quanta Ray HG-l harmonic generator and the emanating beams separated with a Quanta Ray PHS-l prism harmonic separator. For experiments where the excitation was 683 nm, the second harmonic of a Quanta Ray DCR2-A, prepared by the Quanta Ray HG-2 and separated from the fundamental by an ESCO dichroic mirror, was sent through a 1 m long Raman shifter filled to 60 psi with H2 to obtain the first Stokes line. The laser excitation beam intercepted a white light probe beam generated from a ISO-W pulsed OSRAM Xe arc lamp (XBOlSO/ S) mounted in a Photon Technology International (PTI) A1000 lamp housing and driven with a PTI LPSlOOO power supply. The probe beam was collimated and refocused onto the sample (0.2 cm path length) by two 2 in, f/7.0 fused silica lenses. The excitation and probe beams were nearly collinear, with an incidence angle of 11°. The lamp was pulsed (5 Hz repetition rate) to ~12 A for a duration of 5 ms, and the white light was passed through a Uniblitz 23X mechanical shutter. The triggering of the Nd:YAG laser, pulsing of the lamp, and opening and closing of the shutter were orchestrated by synchronization electronics designed and built by Martin Rabb, Electronics Design Engineer, Chemistry Department, Michigan State University. The light transmitted by the sample was collimated by an f/7.0 lens and focused by a second lens (f/4.0) through an appropriate Schott cutoff filter (KV- or WG-series) onto the entrance slit of a SPEX 1680A monochromator. The signal obtained from a Hamamatsu R928 photomultiplier tube was amplified using a LeCroy 6103 dual amplifier/trigger. The amplifier output was 81 passed into a LeCroy TR8828D transient recorder, and the digitized signal was stored in two MM8104 memory modules arranged in a series configuration. The amplifier, digitizer, memory modules, and a LeCroy 6010 GPIB interface were housed in a LeCroy 8013A minicrate. Data, acquired and processed by a Compaq 386 computer equipped with a 40 megabyte hard disk, were typically averaged over 1000 pulses. A Tektronix DSA602A digitizing oscilloscope with a Tektronix 11A72 amplifier was also utilized to average the Signal from the photomultiplier tube. C. RESULTS AND DISCUSSION 1. D2,, Complexes: M2C|4(FP)2 Excitation of M02C14(dppm)2, M02C14(dmpm)2 M02C14(dppe)2, and Mo2C14(dmpe)2 (dmpm = bis(dimethylphosphino)methane; dppe = bis- (diphenylphosphino)ethane; dmpe = bis(dimethylphosphino)ethane) with wavelengths coincident with the 88* transition leads to the production of short-lived transients (40 ps - 2.4 ns). As reported by Dr. L]. Chang, all complexes exhibit transients corresponding to the 88* excited state with lifetimes of 40 - 100 ps, although the complexes containing dppm and dppe ligands were observed to decay biexponentially, with lifetimes of the longer component of ~ 2ns.32 The origin of the second transient was unknown. The transient absorption profiles of M02C14(dmpm)2 at several times following 3 ps excitation (600 nm) are shown in Figure 26. The prominent feature at 460 nm is typical of 88* excited states; its intensity decays monoexponentially to the ground state with a lifetime of 40 ps, The lifetime 82 3.0 0.04 A _ 8 3.5 3 e C T 2 p5 4.0 T e 0.02 0 .2. 2'. [do to 20 ps Tm ”5 50 ps 0 O 0.00 < l- -0.02 - 5° 98 20 ps 2 ps -0.04 - 1 I 1 I 1 I M nm Figure 26. Transient absorption spectra of M02C14(dmpm)2 in CH2C12 collected 2, 20, and 50 ps following the 600 nm, 3 ps excitation pulse. The decay of the bleaching at 630 nm is shown in the inset. 83 of this decay parallels that for the ground state bleaching at 630 nm, corresponding to the 88* transition (Figure 26, inset). In these complexes long-lived transients are not observed upon 88* excitation. The lack of photochemical activity of these complexes in the presence of substrates upon 88* excitation is in agreement with this observation.22 A long-lived transient is observed when benzene solutions of M02C14(dmpm)2 are excited at a higher energy (Am = 355 nm) than the 88* transition. This long-lived transient decays back to the ground state with a lifetime of 5 us. The spectrum obtained 1 us after the 10 ns pump pulse (Figure 27a) exhibits an absorption feature with a maximum at 520 nm. Because of an emissive impurity in the ligand, the spectrum could not be collected in the wavelength region below 450 nm. Owing to the strong ligand emission from dppm (71.max = 460 rim, 1 ~ 2 ns) similar transient spectroscopy experiments of MozCl4(dppm)2 with high energy excitation (hm = 355 nm) could not be conducted. However, in the tungsten analog, W2Cl4(dppm)2, the electronic absorption spectrum is red- shifted, thus allowing excitation at energies higher than 88* without populating the emissive state of the ligand. The transient absorption spectra of W2Cl4(dppm)2 in benzene, collected 100 ns and 4 us after the laser pulse, are shown in Figure 28a. The initial spectrum shows an intense absorption into the ultraviolet (it < 430 nm) and a peak with a maximum at 480 nm. However, the decay of the intensity with time is not monotonic. The decay of the transient at 440 nm is shown in the inset of Figure 28a, where there is a depopulation of *he initially prepared state followed by a rise in intensity with subsequent decay to the ground state over 100 us. The long-lived transients are not observed for 88* excitation of W2C14(dppm)2(}twe = 683 nm). 84 0.015 (a) 0.010- E i i 8 ii i i 0.005- 1 i 0.000 ' l 1 1 1 l ' 450 500 550 600 700 750 A/nm (b) 8 C i In 2 .5. ‘ .5. ‘ .5. . 7.. k/nm Figure 27. (3) Transient absorption spectrum of MozCl4(dmpm)2 in CHzClz recorded 1 us after 355 nm, 10 ns excitation and (b) electronic absorption spectrum of independently—prepared M02C16(dppm)2. 85 0.151 (a) I I - O.10l- o 0.. d T)" e I I I I 0 ° .u o lo 20 < o Time/us c 100ns - Go. I ..I 0.05.- . ~ Absorba nce l 1 1 l l 1 350 400 450 500 550 600 650 lt/nm Figure 28. (3) Transient absorption spectra of W2Cl4(dppm)2 in benzene recorded 100 ns and 4 us after 532 nm, 10 ns excitation. (b) Absorption spectra of W2C16(PEt3)4 (~- - -) and W2C16(dppm)2 ( ) in toluene and CHzClz, respectively. 86 The transient absorption spectra are similar to the electronic absorption spectra of MomMoIn edge-sharing biochtahedra, 4. The structure of these complexes is dictated by the two bridging chlorides, which are shared by the two octahedral metal centers to form an edge-sharing bioctahedron as shown below, m c. I cul Cl T€“—I7=dl‘ 8*) leads to the formation of the two-electron mixed-valence excited state. The foldover to edge-sharing bioctahedral geometry has a two-fold purpose: it stabilizes the charge on both metals by shifting it from the reduced to the oxidized center, and it induces the desired octahedral geometry about the high-valent metal. As shown in Figure 29 this foldover mechanism opens two coordination sites at the low-valent center, which makes it susceptible to two-electron oxidative addition. Indeed the photochemical reactivity of these complexes is in agreement with this mechanism, with reactions proceeding only upon it —> 8* excitation and the localization of the two-electron addition on one metal. 89 n8*,8n* 88* X' l /\ P P Clm:..'.'.', ,,,.11\\\\CIIIII:..,_ I I Cl’T‘CI/ T P P V Figure 29. Proposed mechanism for the formation of the long-lived transient in D2,, complexes following high-energy excitation. 90 2. D2,. Complexes: M2C14(PR3)4 The 188* excited state of complexes in the M2Cl4(PR3)4 (M = M0, W) series is for the most part highly emissive, and the luminescence lifetimes are typically in the 10 — 100 ns range. The emissive lifetimes and quantum yields are listed in Table IX for the M02C14(PR3)4 series, where PR3 = PMe3, PEt3, PBu3, PthMe, PMezPh, and PHPh2 (PEt3 = triethylphosphine, PthMe = diphenylmethyphosphine, PMezPh = dimethylphenylphosphine, and PHPh2 = diphenylphosphine). The transient absorption spectra for several of the Mo complexes following 88* excitation have been previously reported. For example, M02C14(PBu3)4 exhibits an absorption peak at 440 nm, which is characteristic of 88* spectra.”32 The transient absorption spectrum of M02Cl4(PHPh2)4 decays with the unusually short lifetime of 180 ps, has a maximum at 420 nm, as shown in Figure 30, and is consistent with other 188* spectra.“32 The transition giving rise to the absorption at 440 nm in the 188* excited state transient spectrum of M02X4(PBu3)4 (X = Cl, Br, 1) has been assigned to be either metal-metal based or metal-to-phosphine charge transfer, since its energy remains constant along the halide series.32 The transient absorption spectra collected following 188* excitation of the analogous tungsten series, W2Cl4(PR3)4, with PR3 = PEt3, PBu3, PthMe, are shown in Figures 31-33. In all cases there is weak absorption throughout the visible region (500 — 400 nm), with intensity increasing markedly toward the UV, in the 400 - 300 nm region. The transient decays of the tungsten complexes are comparable to the emissive lifetimes of the 88* excited state, which are in the 50 to 80 ns range. Unfortunately, with our present instrumentation, the decay of transients in this time regime cannot be accurately measured. The transient absorption and luminescence 91 Table IX. Lifetimes and Emissive Quantum Yields of the 88* Excited State of M02C14(PR3)4 Complexes and Non- Emissive Transient Lifetimes. PR3 rem / ns“ them“ 1: / nsb " PMe3 135 0.259 - PEt3 14 0.013 120 PBu3 21 0.013 120 PthMe 11.4 0.011 80 PMezPh c c 120 PHPh2 0.18 d — “Ref. 22. b Lifetime of long-lived transient; approximate values due to instrumental constraint (see text). ‘Not measured. “Emission not observed. 92 0.09 - 3 o s. C 0.06 l- T 0.03 r- 0. O < 0.00 0.03 )- 0.06 l I l l I L I l 400 ‘00 600 700 800 71./hm Figure 30. Transient absorption spectrum of MozCl4(PHPh2)4 in CH2C12 collected 2 ps after the 600 nm, 3 ps excitation. The inset shows the decay of the absorbance at 420 nm. 93 0.10- . I 0.03 '- 0 9 . -' . d " o 0.05 - 4 I .002- . ' .- . I O - 0.0 _ . ' ILLI. < . 300 350 400 llnm 0.01 - C .- C e..oe .e.e... . 0.00 ' 02...— C , l I I I 1 I 300 400 500 600 Figure 31. Transient absorption spectrum of W2C14(PEt3)4 in toluene recorded 70 ns after the 683 nm excitation pulse; the inset shows the spectral profile in the near UV region. 94 0.06 C ‘ 0.0:».- '. ojo.02p ° I- O . < h 0.01 . . . . . , o.oo ' ' L— 0. '- O 0053? p.03“) ‘ 450 A 550 A 650 < A/nm - O . . o . 0 . o . 0 . . . . . . 0.00L l l l 1 350 400 450 500 550 600 ll nm Figure 32. Transient absorption spectrum of W2Cl4(PBu3)4 in CH2C12 recorded 70 ns after the 683 nm excitation pulse; the inset shows the spectral profile in the near UV region. 95 0.20 0.15”} o’ 0.10~ g Q 0.05 - _ i i i i i i i {i iié g E 0.00 l l I l ‘ 350 400 450 500 550 l/nm Figure 33. Transient absorption spectrum of W2Cl4(PPh2Me)4 in THF recorded 70 us after 683 nm, 10 ns excitation. 96 decays of W2Cl4(PMe3)4 and W2Cl4(PBu3)4 are shown in Figure 34. .It was therefore concluded from inspection of the decays that the transient absorption signal is due to the 158* excited state. In addition to the signal from the 158* excited state, a second long- lived transient was observed for the molybdenum complexes when PR3 = PEt3, PBu3, PthMe, and PMezPh (the PMe3 and PHPh2 complexes only exhibit a transient due to 158*). In contrast to the bridged D2,, complexes discussed above, this long-lived transient is observed upon 88* excitation. The lifetimes of the long-lived non-emissive transients are listed in Table IX. The spectral profiles of the long-lived transients formed upon 155* excitation of M02C14(PR3)4 complexes, where PR3 = PBu3, PthMe, and PMeQPh, are shown in Figure 35, where there the signal exhibits a maximum at ~ 400 nm. These spectra are in good agreement with those reported for M02Cl4(PBu3)4 in dichloromethane and acetonitrile, where a peak at 390 nm is observed.31 Figure 36 compares the decay of MozCl4(PPh2Me)4 at 400 nm and 440 am. At the former wavelength the long-lived transient is at a maximum, whereas at 440 run only the 185* absorbs significantly. This difference is apparent from inspection of decays. At 400 run both the signal from the 185* excited state and that from the long-lived transient are present, whereas only the short-lived 185* signal appears at 440 nm. The nature of the long-lived transient remains unknown. However, phosphine dissociation has been ruled out as the origin of the non- luminescent transient, since the lifetime of the transient does not change upon addition of excess free phosphine to the solutions probed. No correlation was found in the comparison of phosphine basicity with the absence of the long-lived transient. 97 .8: com an .303 8:88:35— 05 2 3532.80 Boson 05 98 E: can a 336 5:983.“ .5653 05 Ho 35 mm 263 no. 05 830 58 5 38930 a: $0 $32.8 Amamagoaa g as A3530”? 3 do £88 .3 2:5 m: \ 95% m: \ 9:; com com cop o cowl com cow 0 00 _.I . _ _ _ _ . . _ _ fluent-nun |905!S 3v 5 98 0.02- . (a) O D b ‘2 0.01— . o O . . 0.00 l l L . 4 r . 0.02. (b) O O 8 . <1 0.01- o O O O 0.00 . . . . 0.02_ (C) O P 0 <1 . 0.01:— . o P O O 0.00 . . '. .__'.__ 380 400 420 440 460 - l/nm Figure 35. Transient absorption spectra recorded 60 us after 532 nm excitation of (a) M03C14(PBu3)4, (b) MozCl4(PMe2Ph)4, and (c) M02C14(Pph2MC)4 in CH2C12, 99 A ‘5‘ ‘5 ‘ f.» A a“ a e “‘5‘ .n a A A l .‘9 *QHS’“.‘I‘ ~ g c 9. E E a 5"), o o o o o o o ’0 Mmfi “WQ 1 1 l ' 1 -100 0 100 200 Time / ns Figure 36. Decays of W2Cl4(PPh2Me)4 in CH2C12 followed at (A) 400 nm and (o) 440 nm after 532 nm, 10 ns excitation. 100 There are marked differences in emissive quantum yields and 188* lifetimes among the molybdenum complexes, M02Cl4(PR3)4. The magnitude of these differences is clearly observed in Figure 37, where the two parameters are plotted. The square point corresponds to M02Cl4(PHPh2)4, whose emissive lifetime is two orders of magnitude shorter than those observed for most complexes. This excited state behavior is consistent with a faster deactivation pathway, different from that in action in the other complexes of the series. It may be suggested that the P—H bond, which has a higher vibrational frequency than the P—C bonds found in all the other complexes, may be involved in such deactivation. If this deactivation model is correct, the complex M02C14(PDPh2)4, with the deuterated ligand, should ex..1bit a longer lifetime than that of its protonated analog. In Figure 37, the open circle corresponds to M02C14(PMe3)4, whose lifetime and emissive quantum yield are one order of magnitude greater than those of M02Cl4(PEt3)4, M02Cl4(PBu3)4, and M02C14(PPh2Me)4. Since the long-lived transient is not observed in the PMe3 complex, the differences in 188* lifetime and emissive quantum yield suggests that deactivation of the 158* excited state in the PEt3, PBu3, and PthMe complexes takes place through the state giving rise to the long-lived transient.31 The lifetimes of the molybdenum PR3 = PMe3, PEt3, PBu3 complexes converge at 77K, which indicates that the formation of the long-lived transient from the 188* excited state is an activated process, consistent with a conformational rearrangement.31'41 Since the transition to the low-lying 388* is not expected to be activated, the long-lived transient is not believed stem from the 355*. It may be postulated that the size of the phosphine ligand, measured by its cone angle, plays a role in the formation of the non-emissive transient. In studies utilizing a series of MozCl4(PR3)4 complexes, including only PR3 101 0 0PM93 1 PEta A 2 .. .PBUa g PthMe 3 o: 2 ' 3 4 I PHPh2 - 1 1 lllllll l l llLlJll l 1 1111111 1 l l+jlll 2'1 0 1 2 3 IOQ(Tem) / x 10'9 s Figure 37. Log-log plot of the emission quantum yield vs the 188* lifetime of complexes in the M02C14(PR3)4 series. 102 = PMe3, PEt3, PPr3, and P8113, a correlation between the observation of the long-lived transient and the phosphine cone angle had been proposed.31 The long-lived transient was not observed for PMe3, which has the smallest cone angle (118°) compared to the other members of the series (~132°).42 The cone angles of the phosphines utilized in our extended series, which includes PMezPh. PthMe, and PHth, are listed in Table X. The correlation of cone angle with observation of the long-lived transient is obscured by PHth, since its value lies between that of PMth and those for PEt3 and PBu3. However, as discussed above, the formation of the long-lived transient may not be an issue in the photophysics of M02Cl4(PHPh2)4, since the 155* excited state may decay too quickly, by other non-radiative pathways, to Table X. Cone Angles ((1)) of PR3 Ligands. PR3 0 / degrees“ PMe3 1 18 PMezPh 122 PHth 128 PEt3 132 PBu3 132 PthMe 136 “Ref. 42; average value of the three angles, therefore PMezPh has a large angle (145°) on one side associated with the phenyl group. 103 permit the formation of the long-lived transient. If this is the case, then its cone angle should not be compared with the others, and the only complex that does not have a long-lived transient is M02Cl4(PMe3)4, with the smallest cone angle. The a and 1: bonds are symmetric about the metal-metal bond in D2d symmetry (Figure 23); therefore once the 8 bond is broken, by placing the molecule in the 188* excited state, the MC12(PR3)2 fragments may rotate about the central bond. Thus, with small phosphines, such as PMe3, the fragment can rotate freely, whereas when a larger phosphine is present there may be steric hindrance to free rotation. However, in the molecules containing the bulky phosphines, there is significant repulsion between the PR3 ligand and the adjacent Cl, as displayed by the large M-M-P and M— M—Cl bond angles, typically 105° and 108°, respectively.“48 To minimize these steric repulsions in the eclipsed conformation, the molecules may rotate to attain a staggered (D2) geometry, with subsequent relaxation of the bond angle to smaller values. Such rearrangement is shown in Equation 10, where after the initial rotation the phosphines become locked in place. Such behavior is not unexpected, since it is well known that MozCl4(PPh3)4 cannot be synthesized owing to the large phosphine.22 This observation is consistent with the postulated lack of steric strain in the M02C14(PMe3)4 104 complex, and the necessity of the sterically-hindered complexes to rearrange. The electronic absorption spectrum of M02Cl4(dppe)2, which is known to be somewhat staggered across the Mo—Mo bond (torsional angle of bidentate phosphine ~27°), is similar to that of long-lived transient.49 The transient absorption spectrum of M02Cl4(PBu3)4, reported by Winkler over a larger spectral range, has marked similarities with that of the staggered complex.31 The transient exhibits absorption maxima at 750, 470, and 390 nm, which are similar to those of M02C14(dppe)2 at 762, 548, 469, and 345 nm.49 The high energy absorption in M02Cl4(dppe)2 has been assigned to a ’ 1t—)8* transition, whereas in the M02C14(PR3)4 complexes the same transition appears at ~ 440 nm. Therefore it is not unreasonable to correlate the peak at 345 nm in the staggered complex with that at 390 nm in the transient. Points in the 500 .- 550 nm region were not collected in the transient spectrum, since the excitation was 532 nm; thus the small peak at 548 nm in M02C14(dppe)2 cannot be compared. The long-lived transient was not observed for any of the complexes probed from the W2Cl4(PR3)4 series or in MoWCl4(PMePh2)4. However, the metal-metal distances in the W2 and MoW complexes are 0.13 A and 0.08 A longer than those in the corresponding M02 series.“3“8 The longer distance between metals may indeed provide sufficient space about the metal-metal core to reduce the steric constraints of the large phosphines. If the observation of the long lived transient is indeed dictated by the steric repulsions of the phosphines, then it would be expected that in the complexes with the longer metal-metal bonds the non-emissive transient may not be observed. 105 D. CONCLUDING REMARKS Upon 58* excitation, the D2,, complexes exhibit transient features due only to the 158* excited state. In contrast, the D2d complexes exhibit a longer-lived transient in addition to the 185* when excited with low-energy wavelengths, coincident with the 58* absorption. This long-lived transient in the Du complexes is believed to stem from a conformational rearrangement involving rotation about the metal-metal bond to minimize steric repulsions of bulky phosphines. Such rearrangement cannot occur in the bridged D2,, complexes, in agreement with the absence of long-lived transients in these complexes upon 88* excitation. The D2,, complexes, however, exhibit a long-lived transient when excited with higher-energy photons, coincident with their «8* transition. These non-emissive transients have been assigned to a conforrnationally distorted species, where the foldover of two chlorides to bridging positions form an edge-sharing bioctahedron. E. REFERENCES l. (a) Wikstréim, M. Nature 1989, 338, 776. (b) Wikstriim, M. Chemica Scripta 1987, 27B, 53. 2. (a) Malmstrbm, B. G. Chim. Scripta 1987, 27B, 67. (b) Oliveberg, M.; Brzezinski, P.; Malmstn'im, B. G. Biochim. Biophys. Acta 1989, 977, 322. (c) Malmstriim, B. G. Chem. Rev. 1990, 90, 1247. (d) Malmstrbm, B. G. Arch. Biochem. Biophys. 1990, 280, 233. 10. ll. 12. 13. 106 Williams, R. J. P. Nature 1989, 338, 709. Han, S.; Ching, Y.-C.; Rousseau, D. L. Proc. Natl. Acad. Sci. USA 1990, 87, 8408. Hansson, 0.; Wydrzynski, T. Photosynth. Res. 1990, 23, 131. Ghanotakis, D. F.; Yocum, C. F. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1990, 41, 255. (a) Brudvig, G. W.; Crabtree, R. H. Prog. Inorg. Chem. 1989, 37, 99. (b) Brudvig, G. W.; Beck, W. F. Annu. Rev. Biophys. Chem. 1989, 18, 25. (c) Thorp, H. H.; Brudvig, G. W. New J. Chem. 1991, 15, 479. Babcock, G. T. In New Comprehensive Biochemistry: Photosysnthesis; Amesz, J .; Ed.; Elsevier: New York, 1987; pp 125- 158. (a) Krishtakik, L. 1. Biochim. Biophys. Acta 1986, 849, 162. (b) Krishtakik, L. I. Bioelectrochem. Bioenerg. 1990, 23, 249. (a) Vincent, J. 13.; Christou, G. Adv. Inorg. Chem. 1989,33, 197. (b) Libby, E.; McCuster, J. K.; Schmitt, E. A.; Folting, K.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1991, 30, 3486. (a) Micklitz, W.; Bott, S. G.; Bentsen, J. G.; Lippard., S. J. J. Am. Chem. Soc. 1989, 111, 372. (b) Bentsen, J. G.; Micklitz, W.; Bott, S. G.; Lippard, S. J. J. Inorg. Biochem. 1989, 36, 226. (a) Chan, M. K.; Armstrong, W. H. J. Am. Chem. Soc. 1991, 113, 5055. (b) Chan, M. K.; Armstrong, W. H. J. Am. Chem. Soc. 1990, 112, 4985. Proserpio, D. M.; Hoffman, R.; Dismukes, G. C. J. Am. Chem Soc. 1992, 114, 4374. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 107 Taube, H. In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D. B.; Endicott, J. F., Eds.; ACS Symposium Series, vol 198; American Chemical Society: Washington, DC, 1982; p 151. Taube, H.; Gould, E. S. Acc. Chem. Res. 1969, 2, 321. Cox, L. T.; Collins, 8. B.; Martin, D. S. J. Inorg. Nucl. Chem. 1961, 17, 383. (a) Basolo, F.; Willis, P. H.; Pearson, R. G.; Wilkins, R. G. J. Inorg. Nucl. Chem. 1958, 6, 161. (b) Basolo, F.; Morris, M. L; Pearson, R. G. Discuss. Faraday Soc. 1960, 29, 80. (a) Erwin, D. K.; Geoffrey, D. K.; Gray, H. B. J. Am. Chem. Soc. 1977, 99, 3620. (b) Trogler, W. C.; Erwin, D. K.; Geoffrey, G. L.; Gray, H. B. J. Am. Chem. Soc. 1978, 100, 1160. Chang, H.; Nocera, D. G. J. Am. Chem. Soc. 1987, 109, 4901. Chang, LL; Nocera, D. G. Inorg. Chem. 1989, 28, 4309. Partigianoni, C. M. Ph.D. Dissertation, Michigan State University, 1991. Partigianoni, C. M.; Nocera, D. G. Inorg. Chem. 1990, 29, 2033. Partigianoni, C. M.; Turro, C.; Shin, Y. K.; Motry, D. H.; Kadis, J.; Dulebohn, J. 1.; Nocera, D. G. In Mixed Valency Systems: Applications in Chemistry, Physics and Biology; Prassides, K., Ed.; Kluwer: Netherlands, 1991; pp 91-106. Partigianoni, C. M.; Turro, C.; Hsu, C.; Chang, I—J.; Nocera, D. G. In Photosensitive Metal-Organic Systems: Mechanistic Principles and Recent Applications; ACS Symposium Series, Reidel: Amsterdam, 1992; pp 0000. (a) Cotton, F. A.; Mott, G. N. J. Am. Chem. Soc. 1982, 104, 5978. (b) Cotton, F. A.; Powell, G. L. J. Am. Chem. Soc. 1984, 106, 3371. (c) ' 26. 27. 28. 29. 30. 31. 32. 33. 34. 108 Cotton, F. A.; Diebold, M. P.; O’Connor, C. J .; Powell, G. L. J. Am. Chem. Soc. 1985, 107, 7438. (d) Chakravarty A. R.; Cotton, F. A.; Diebold, M. R; Lewis, D. B.; Roth, W. J. J. Am. Chem. Soc. 1986, 108, 971. (e) Agaskar, P. A.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R.; O’Connor, C. J. Inorg. Chem. 1987, 26, 4051. (f) Canich, J. M.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R. Inorg. Chem. 1988, 27, 804. (g) Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Falvello, L. R.; O’Connor, C. J .; Price, A. C. Inorg. Chem. 1991, 30, 2509. Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, 705. Manning, M. C.; Trogler, W. C. J. Am. Chem. Soc. 1983, 105, 5311. Miskowski, V. M.; Gray, H. B.; Hopkins, M. D. J. Am. Chem. Soc. 1992, 31, 2085. Agaskar, P. A.; Cotton, F. A.; Fraser, 1. F.; Manojlovic-Muir, L.; Muir, K. W.; Peacock, R. D. Inorg. Chem. 1986, 25, 2511. Zhang, X.; Kozik, M.; Sutin, N.; Winkler, J. R. In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, J. R.; Mataga, N.; McLendon, G., Eds.; Advances in Chemistry Series, American Chemical Society: Washington, DC, 1991; pp 247-264. Winkler, J. R.; Nocera, D. G.; Netzel, T. L. J. Am. Chem. Soc. 1986, 108, 4451. Chang, I-J., Ph.D. Dissertation, Michigan State University, 1988. Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Falvello, L. R.; O'Connor, C. J .; Price, A. C. Inorg. Chem. 1991, 30, 2509. Agaskar, P. A.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R.; O'Connor, C. J. O. Inorg. Chem. 1987, 26, 4051. 35. 36. 37. 38. 39. 41. 42. 43. 45. 47. 48. 109 (a) Cotton, F. A.; Powell, G. L. J. Am. Chem. Soc. 1984, 106, 3372. (b) Cotton, F. A.; Diebold, M. P.; O'Connor, C. 1.; Powell, G. L. J. Am. Chem. Soc. 1985, 107, 7438. . Canich, J. A. M.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R. Inorg. Chem. 1988, 27, 804. Fanwick, P. E.; Harwood, W. S.; Walton, R. A. Inorg. Chem. 1987, 26, 242. Saillant, R.; Hayden, J. L.; Wentworth, R. A. D. Inorg Chem. 1967, 6, 1497. Shaik, S.; Hoffman, R.; Fisel, R.; Summerville, R. H. J. Am. Chem. Soc. 1980, 102, 4555. Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metals; Wiley- Interscience: New York, 1982, p 221. Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid State Chem. 1985, 57, 112. Tolman, C. A. Chem. Rev. 1977, 77, 313. Cotton, F. A.; Extine, M. W.; Felthouse, T. R.; Kolthammer, B. W. 8.; Lay, D. G. J. Am. Chem. Soc. 1981, 103, 4040. Cotton, F. A.; Daniels, L. A.; Powell, G. L.; Kahaian, A. 1.; Smith, T. J.; Vogel, E. F. Inorg. Chim. Acta 1988, 144, 109. Cotton, F. A.; Czuchajowska, J.; Luck, R. L. J. Chem. Soc. Dalton Trans. 1991, 579. Cotton, F. A.; Jennings, J. G.; Price, A. C.; Vidyasagan, K. Inorg. Chem. 1990, 29, 4138. Luck, R. L.; Morris, R. H.; Sawyer, J. F. Inorg. Chem. 1987, 26, 2422. Cotton, F. A.; Falvello, L. R.; James, C. A.; Luck, R. L. Inorg. Chem. 1990, 29, 4759. 110 49. Bakir, M.; Cotton, F. A.; Falvello, L. R.; Simpson, C. Q.; Walton, R. A. Inorg. Chem. 1988, 27, 4197. CHAPTER III THE DRIVING FORCE DEPENDENCE or BIMOLECULAR PROTEIN ELECTRON TRANSFER: *Ru(L)§*/CYT0CHR0ME c SYSTEM A. BACKGROUND Measurements of electron transfer rates in the Marcus inverted region1 have been limited to systems where the donor/acceptor distance is fixed, such as those which are covalently-bound,2 in frozen media,3 in electrostatic complexes of proteins,“ or in covalently-modified proteins.° Only recently has inverted region electron transfer been observed for bimolecular reactions between an Irz complex and a series of pyridinium acceptors.8 It is now possible to design series of donors and acceptors of varying driving forces for which electron transfer kinetics in the bimolecular inverted region can be readily observed. The necessary conditions include an electron transfer rate sufficiently small compared to the rate of diffusion and a moderate reorganization energy to allow easy access of the inverted region. From the known electron transfer properties of cytochrome c, we have designed a series excited state ET reactions with Ru diimine complexes in which the inverted region is observed. The bimolecular cytochrome c ET with small molecules is characterized by a reorganization energy of 0.8 — 1.0 V which 111 112 allows access of the inverted region, and the quenching of Ru(phenfi+ by the protein is of the order of 108 M'ls’l, wich is similar to the estimated rate of diffusion. Described in this Chapter are the photoinduced bimolecular ET rates between Ru diimine complexes in their metal-to-ligand charge transfer (MLCT) excited state and cytochrome c in its reduced and oxidized states. For the exemplary complex Ru(phenfi+ (phen = LID-phenanthroline) the MLCT lies 2.1 eV above the ground state and is both a powerful oxidant (EV+ = - 0.87 V vs NHE) and reductant (E"' = 0.79 V vs NHE),9 whereas the protein is easily reduced or oxidized depending on its initial redox state (Emmn = 0.26 V vs NI-iE).10 Systematic variation of the substituents of the parent ligand or utilization of mixed-ligand complexes provides a wide variation of the excited state oxidation and reduction potentials,9 which in turn control the driving force of the *Runlcyt c ET reaction. A pictorial representation of the ligands utilized in this study is shown in Figure 38. Since in all complexes the excited state is MLCI’ in character, they should interact in an analogous manner with the protein thus providing a means to follow the driving force dependence of the ET rates without disturbing other rate-controlling parameters. The highly charged residues in cytochrome c induce an overall charge of +7.5 and +6.5 in the oxidized and reduced forms, respectively.10 The electron transfer rates between the Ru“ complexes chosen for our driving force dependence and cytochrome c are purely bimolecular, since binding is severely impeded by electrostatic repulsion. Nonetheless, the redox-active center of the protein, the heme, has an exposed edge, which is solvent accessible and is known to accelerate the ET rates of inorganic complexes containing hydrophobic ligands such as those in the phenanthroline family. 113 py bpy phen diMe-phen diOMe-phen Figure 38. Schematic representation of the ligands utilized in the driving force dependence study of the ET rate. 114 B. EXPERIMENTAL METHODS Horse heart cytochrome c Type VI was purchased from Sigma and was purified by standard methods.22 The oxidized protein, dissolved in tt = 10 mM, pH = 7.4 phosphate buffer, was loaded onto a CM52 (Whatman) cation exchange column, and was subsequently eluted with the same buffer at a higher ionic strength (1.1 = 0.1 M). Ferricytochrome c was reduced by addition of freshly prepared sodium ascorbate, or was fully oxidized with potassium ferrocyanide.23 The protein was then eluted from a Sephadex LH- 50 column with tt = 0.1 M, pH = 5 ammonium bicarbonate buffer, to allow separation of the reducing or oxidizing agents. After the eluent was frozen with a dry ice/acetone bath, the water and buffer were removed under vacuum (10‘3 torr). All manipulations of ferrocytochrome c were performed under N2, and the oxidation state was carefully monitored utilizing the characteristic absorption features. The reduced heme possesses a sharp band at 550 nm (e = 27.7 mM‘lcm‘l),2" whereas in the oxidized state a broad feature at 530 nm (e = 10.1 mM’10m‘l) is observed.” The ligands, pyridine (py), bipyridine (bpy), LID-phenanthroline (phen), 4,7—dimethyl-1,lO-phenanthroline (diMe-phen), and 4,7-dihydroxy- 1,10-phenanthroline (diOH-phen), and 4,7-di(p-phenylsulfonate)-1,10- phenanthroline (BPS) were purchased from Aldrich. The ligand 4,7- dimethoxy-l,10-phenanthroline (diOMe-phen) was synthesized by reported methods, where 4,7-dichloro-LID-phenanthroline (prepared from diOH- 27 Characterization was phen)2° was treated with NaOMe in benzene. conducted by 1H NMR [phen arom.: 8.90 ppm (Area = A = 5.66), 8.22 ppm (A = 6.11), 7.25 ppm (A = 5.80); OCH3: 4.15 ppm (A = 19.6)], melting point (200 °C), and elemental analysis (found: C = 64.6%, H = 4.2 %, N = 115 10.7%, 0 = 8.7%; calc.: C = 70.0%, H = 5.0 %, N = 11.7%, 0 = 13.3%). The RuII complexes were prepared by reported techniques.9'28 Typically RuCl3 was refluxed in ethanol/water (80/20) with a six equivalents of ligand, L, to form the red/ orange Ru"(L)§+ complexes. Ru(bpy)2(py) 3+ was prepared in a similar manner, where Ru(bpy)2C12 (purchased from Aldrich) was refluxed with a four-fold molar excess of py ligand in ethanol/water. After evaporation of the solvent, the product was readily precipitated from acetone with ether. The Ru complexes were characterized by comparison with their known electronic absorption and emission spectra, emissive lifetime, and redox potentials.9 The lifetime quenching measurements were performed with an instrument that has previously been described in detail.29 The luminescence decay was monitored at the emission maximum of the Ru complex, typically in the 600 - 630 nm range, following 532 nm, 8 ns excitation. Stock solutions (25 or 50 ml) containing 6x10‘5 M of each Ru complex were prepared in u = 0.1 M, pH = 7.4 phosphate buffer. The protein was dissolved in 200 pl of stock solution and placed in a 1 mm pathlength cuvette equipped with a stopcock; deoxygenation was attained by bubbling N2 for ~ 5 min and closing the stopcock prior to the lifetime measurement. The protein concentration was varied by addition of known volumes of stock solution with a 250 pl syringe to the initial 200 1.1], followed by deoxygenation and lifetime measurement. The concentration of cytochrome c was determined prior to each lifetime measurement from the absorbance at either 530 nm or 550 nm, depending on the protein’s oxidation state, and was corrected for the absorbance of the Ru complex at each wavelength (typically 0.05 — 0.07). In this manner the concentration of the Ru 116 complexes remained constant and at least two orders of magnitude smaller than the protein concentration, as required to perform our kinetic analysis. C. THEORY Marcus predicted that as the driving force of the electron transfer (ET) reaction increases, the rate will initially increase, reach a maximum, and then decrease.1 From the transition state formulation with parabolic potential energy surfaces the ET rate is given by1 (AG + W} kc! = Vet°XP{- 41kg (11) where vct represents the frequency of crossing the transition state, AG is the driving force, and 7. is the reorganization energy. It can be readily observed from eq 11 that the ET rate will be fastest when -AG = 1», as shown in Figure 39. In bimolecular reactions if the ET rate is much faster than the rate of diffusion, kdm, then the observation of the inverted behavior will be obscured by the limiting kinetics (Figure 3%).”12 The interaction between the reactant and product potential energy surfaces in the three regimes predicted by Marcus, -AG > 1., -AG = 7., and -AG < h, are depicted in Figure 40, where the reaction passes through the activationless state between the normal and inverted regions. The AG range where the rate decreases with increasing driving forces is referred to as the inverted region, and it is evident from Figure 39 that as either the ET rates or 1. become larger, moving the curve up or to the right, respectively, the inverted region will move to higher driving forces. 117 (a) 1 B" (b) l is” -AG —" Figure 39. Driving force dependence of the ET rate in (a) fixed-distance systems and (b) diffusion controlled reacr. ns. 118 2:. .2 u O