1180
J . Am. Chem. SOC.1988, 110, 1180-1 186
responsible for the difference in bond angles between it and the other more related derivatives.
Acknowledgment. The support of this research by the National Science Foundation (Grant CHE8504737) is gratefully acknowledged. We also thank the University of Massachusetts Computing Center for generous allocation of computer time.
Supplementary Material Available: Thermal parameters and additional bond lengths and angles (Tables S1 and S2, respectively, for 1, Tables S 3 and S 5 for 2, Tables S6 and S7 for 3) and fixed hydrogen atom parameters for 2 (Table S4) (8 pages); listings of calculated and observed structure factor amplitudes for 1-3 (32 pages). Ordering information is given on any current masthead page.
Photoinduced Intramolecular Electron Transfer in Peptide-Bridged Molecules Kirk S. Schanze*l and Kenneth Sauer Contribution from the Department of Chemistry and Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720. Received August 1 1 , 1986
Abstract: Two series of molecules have been prepared and characterized in which a polypyridyl Ru(I1) complex is linked to p-dimethoxybenzene (DMB) and p-benzoquinone (Q) moieties by peptide bridges containing the amino acid L-proline (Pro). The photophysical properties of the metal-to-ligand charge transfer (MLCT) excited state of the Ru(I1) chromophore have been examined for the complexes with 0, 1, 2, 3, and 4 intervening Pro residues. Steady state and time resolved luminescence experiments on the Pro-bridged DMB system show that the properties of the Ru MLCT excited state are only slightly modified from those of an unsubstituted model complex by the presence of covalently attached DMB peptides. Experiments on the Pro-bridged Ru-Q complexes show that the Q site quenches the yield and lifetime of the Ru MLCT emission. Furthermore, the quenching efficiency is diminished as the number of peptide spacers is increased. The quenching process is ascribed to long-range intramolecular Ru-to-Q electron transfer. This hypothesis is supported by time-resolved luminescence data which suggest that the average electron transfer rate falls sharply with an increase in the peptide bridge length.
There has been considerable interest recently in exploring the role of distance in the rate of electron transfer between donor and acceptor sites that are spatially separated.2 Studies involving various chemical and physical approaches have addressed this important problem. Rates have been measured for electron transfer between donor and acceptor sites that are randomly held at fixed distances by protein distributed in glassy frameworks"gb8 or between sites that are separated by peptide (1) Pysent address: Department of Chemistry, University of Florida, Gainesville, FL 3261 1. (2) For reviews, see: (a) Marcus, R. A,; Sutin, N. Eiochim. Eiophys. Acta 1985,811,265-322. (b) Newton, M. D.; Sutin, N. Annu. Rev. Phys. Chem. 1984, 35, 437-480. (c) De Vault, D. Quantum-Mechanical Tunnelling in Eiological Systems; 2nd ed.;Cambridge University Press: New York, 1984. (3) (a) Beitz, J. V.; Miller, J. R. J. Chem. Phys. 1979, 71,4579-4595. (b) Miller, J. R.; Beitz, J. V. J . Chem. Phys. 1981, 74, 6746-6756. (c) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J . Am. Chem. SOC. 1984, 106, 5057-5068. (d) Miller, J. R.; Hartman, K. R.; Abrash, S.J . Am. Chem. Soc. 1982, 104, 4296-4298. (4) (a) Strauch, S.;McLendon, G.; McGuire, M.; Guarr, T. J. Phys. Chem. 1983,87,3579-3581. (b) Guarr, T.; McGuire, M.; McLendon, G. J . Am. Chem. SOC.1985, 107, 5104-5111. (c) McLendon, G.; Guarr, T.; McGuire, M.; Simolo, K.; Strauch, S.;Taylor, K. Coord. Chem. Reo. 1985, 64, 113-124. (5) Domingue, R. P.; Fayer, M. D. J . Chem. Phys. 1985,83, 2242-2251. (6) (a) Winkler, J. R.; Nocera, D. G.; Yocum, K. M.; Bordignon, E.; Gray, H. B. J . Am. Chem. SOC.1982,104,5798-5800. (b) Nocera, D. G.; Winkler, J. R.; Yocum, K. M.; Bordignon, E.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 5145-5150. (c) Crutchley, R. J.; Ellis, W. R., Jr.; Gray, H. B. J . Am. Chem. SOC.1985, 107, 5002-5004. (d) Kostic, N. M.; Margalit, R.; Che, C.-M.; Gray, H. B. J. Am. Chem. SOC.1983,105,7765-7767. (e) Mayo, S. L.; Ellis, W. R., Jr.; Crutchley, R. J.; Gray, H. B. Science 1986, 233, 948-952. (7) (a) McGourty, J. L.; Blough, N. V.; Hoffman, B. M. J . Am. Chem. SOC.1983, 105,4470-4472. @) Peterson-Kennedy, S.E.; McGourty, J. L.; Hoffman, B. M. J. Am. Chem. SOC.1984, 106, 5010-5012. (c) Peterson-
Kennedy, S. E.; McGourty, J. L.; Ho, P. S.; Sutoris, C. J.; Liang, N.; Zemel, H.; Blough, N. V.; Margoliash, E.; Hoffman, B. M. Coord. Chem. Reo. 1985, 64, 125-133. (8) (a) McLendon, G. L.; Winkler, J. R.; Nocera, D. G.; Mauk, M. R.; Mauk, A. G.; Gray, H. B. J . Am. Chem. SOC.1985, 107, 739-740. (b) McLendon, G.; Miller, J. R. J . Am. Chem. SOC.1985, 107, 7811-7816. (c) Simolo, K. P.; McLendon, G. L.; Mauk, M. R.; Mauk, A. G. J . Am. Chem. SOC.1984, 106, 5012-5013.
Scheme I
oligomers,1° rigid carbon-carbon bond frameworks,Il-l6 or by various alipha&17*18and aromatic spacers." Several goals are (9) (a) hied, S.S.;Kuehn, C.; Worosila, G. J . Am. Chem. Soc. 1984, 106, 1722-1726. (10) (a) Isied, S. S.;Vassilian, A. J. Am. Chem. SOC. 1984, 106, 1726-1732. (b) Isied, S. S.;Vassilian, A. J . Am. Chem. SOC.1984, 106, 1732-1736. (c) Isied, S.S.;Vassilian, A.; Magnuson, R. H.; Schwartz, H. A. J. Am. Chem. SOC.1985, 107, 7432-7438. (11) (a) Calcaterra, L. T.; Closs, G. L.; Miller, J. R. J . Am. Chem. SOC. 1983,105,670-671. (b) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J . Am. Chem. Soc. 1984,106,3047-3049. (c) Closs, G. L.; Calcaterra,L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R.J . Phys. Chem. 1986, 90,3673-3683. (d)
Penfield, K. W.; Miller, J. R.; Paddon-Row, M. N.; Cotsaris, E.; Oliver, A. M; Hush, N. S.J . Am. Chem. SOC.1987, 109, 5061-5065. (12) (a) Pasman, P.; Koper, N. W.; Verhoeven, J. W. R e d . Trau. Chim. Pays-Bas 1982, 101, 363-364. (b) Pasman, P.; Mes, G. F.; Koper, N. W.; Verhoeven, J. W. J. Am. Chem. SOC.1985, 107, 5839-5843. (13) (a) Wasielewski, M. R.; Niemczyk, M. P.J . Am. Chem. SOC.1984, 106, 5043-5045. (b) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J. Am. Chem. SOC.1985, 107, 1080-1082. (c) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. SOC.1985, 107, 5562-5563. (14) (a) Hush, N. S.Coord. Chem. Rev. 1985.64, 135-157. (b) Oevering,
H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J. Am. Chem. SOC.1987, 109, 3258-3269. (15) (a) Joran, A. D.; Leland, B. A.; Geller, G. G.; Hopfield, J. J.; Dervan, P. B. J . Am. Chem. SOC.1984, 106, 6090-6092. (b) Leland, E.A,; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A. H.; Dervan, P. B. J. Phys. Chem. 1985, 89, 5571-5573.
0002-7863/88/ 15 10-1 180%01.50/0 Q 1988 American Chemical Society
Electron Transfer in Peptide- Bridged Molecules
J . Am. Chem.Soc., Vol. 110, No. 4, 1988 1181
Chart I common to each of these studies: to establish quantitatively the S t r u c t u r e s and A b b r e v i a t i o n s dependence of the electron transfer rate on the distance between donor and acceptor sites,2a to determine the relation between distance dependence and reaction driving f ~ r ~ e , and ~ ~to, ~ ~ , ~ ~ * ~ ~ learn how the composition of the molecular framework, spacer, or solvent, which separates the donor and acceptor, affects the distance d e p e n d e n ~ e . ' ~ ~ Pioneering work on intramolecular energy transfer suggested '2 that oligopeptides which contain repeating L-proline units could be useful as spacers by allowing systematic variation in the distance between terminal pendant groups2' Taking this lead, Isied and co-workers recently demonstrated that oligo(L-proline) can be useful as a spacer in the study of intramolecular electron transfer R= COCH, : RuNAc reactions.lOb-cAlthough the donor-acceptor systems designed by Isied's group demonstrate the utility of peptide spacers, their results are difficult to interpret because in several instances intramolecular n = 0 RuDMB electron transfer rates are slow, allowing competition from peptide conformational isomerization and bimolecular electron transfer paths. Clearly, it would be desirable to develop a system that n = 1 - 4 RuP,DMB utilizes an oligopeptide spacer but which has an intrinsically higher rate for electron transfer.lobgC We have designed and characterized a system that utilizes a series of oligo(L-proline) spacers to separate an electron donor n = O RuOH2 and acceptor pair which under favorable conditions undergo an exceedingly rapid electron transfer reaction. The system is based n = I - 4 RuP,QH, on the use of an electronically excited state as electron donor. Recent experiments have proven that such systems can be extremely useful in the study of fast electron transfer reactions because the reactant, *D-A, is photochemically excited and the n = 0 RuQ kinetics of its decay by intramolecular electron transfer to the product, D+-A-, are readily monitored by time resolved emission n = 1 - 4 RuP,O or absorption techniq~es.'~-'~ The system described herein utilizes a polypyridyl Ru(I1) complex as a photoexcited electron donor and a p-benzoquinone moiety as an electron acceptor. The rate of photoinduced intramolecular Ru to quinone electron transfer X B L 867-2729 (k& process, Scheme I) can be deduced from emission experiments. The Ru(I1) complex was selected because its lowest excited state This report describes the characterization and photophysical is based on a metal-to-ligand charge transfer (MLCT) transition properties of two series of peptide-bridged complexes, RuP,DMB and therefore displays features amenable to the study of intraand RuP,Q (see structures and abbreviations for definition of the molecular electron transfer reactions: (1) MLCT excited states nomenclature used throughout, Chart I).26 The RuP,DMB are strongly redox active-indeed, *Ru(bpy)$+ is both a good system was examined to provide information regarding the effect oxidant and a good reductant.22 (2) Polypyridine Ru(I1) MLCT of the peptide upon the normal decay parameters of the Ru MLCT excited states are luminescent, allowing measurement of excited excited state. Results on the RuP,Q system show that the rate state kinetics with time resolved luminescence technique^.^^ (3) of R u to quinone electron transfer is strongly affected by the MLCT excited states are relatively long lived, allowing reactions number of intervening peptide spacers. with comparatively slow kinetics to compete effectively with normal excited state decay processes.23 The quinone moiety was Results and Discussion chosen as acceptor due to its facile reduction24and because of its Synthesis and Characterization of the Peptide-Linked Comimportance as an electron acceptor in many biological redox plexes. The general strategy for synthesis of the Ru-quinone systems.2s complexes followed techniques developed for synthesis of amide and ester linked porphyrinquinone molecule^.^^^^* A detailed description of the synthetic methods and analytical results is given (16) Bolton, J. R.; Ho, T.-F.; Liauw, S.; Siemiarczuk, A.; Wan, C. S. K.; in the Experimental Section; aspects of the sythesis which are Weedon, A. C. J. Chem. SOC.Chem. Commun. 1985, 559-560. relevant to the photophysical experiments follow. The preparative (17) (a) McIntosh, A. R.; Siemiarczuk, A.; Bolton, J. R.; Stillman, M.J.; scheme involved the sequence RuP,,DMB RuP,QH2 RuP,Q. Weedon, A. C. J. Am. Chem. SOC.1983, 105, 7215-7223. (b) Ho, T.-F.; Siemiarczuk, A.; McIntosh, A. R.; Ho T.-F.; Stillman, M. J.; Roach, K. J.; The RuP,QH2 complexes were carefully purified by repeated Weedon, A. C.; Bolton, J. R.; Connolly, J. C. J. Am. Chem. SOC.1983, 105, chromatography and fully characterized by 13C and 'H N M R , 7224-7230. (c) Schmidt, J. A.; Siemiarczuk, A.; Weedon, A. C.; Bolton, J. FTIR, UV-vis, and elemental analysis. Following oxidation, the R. J. Am. Chem.Soc. 1985, 107, 6112-6114. RuP,Q complexes were characterized by 'H N M R and UV-vis (18) Moore, T.A.; Gust, D.; Mathis, P.; M i a l q , J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A.; Lehman, W. R.; spectroscopy. These techniques established that (1) oxidation Nemeth, G. A.; Moore, A. L. Nature (London) 1984, 307, 63C-632. could be effected in 90-95% yield and (2) the quinone complexes (19) Heitele, H.; Michel-Beyerle, M. E. J. Am. Chem. SOC.1985, 107, were free of all impurities except slight contamination by residual 8286-8288. RuP,QH2. Attempts to purify the RuP,Q complexes by a variety (20) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J. Am. Chem. Soc. 1984, 106,6858-6859. of techniques invariably resulted in complete reduction of the
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(21) (a) Stryer, L.; Haugland, R. P. Proc. N a f l .Acad. Sci. U.S.A.1967, 58, 719-726. (b) Gabor, G. Biopolymers 1968, 6, 809-816. (22) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K.J . Am. Chem. Soc. 1979,101,4815-4824. (23) Caspar, J. V.;Meyer, T.J. J. Am. Chem. Soc. 1983,105,5583-5590. (24) Wilford, J. H.; Archer, M. D.; Bolton, J. R.;Ho, T.-F.; Schmidt, J. A.; Weedon, A. C. J. Phys. Chem. 1985,89, 5395-5398. (25) Trumpower, B. L.Funcrion of Quinones in Energy Conserving Systems; Academic: New York, 1982.
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(26) Abbreviations used throughout this paper: bpy = 2,2'-bipyridine; 5-AP = 5-amino- 1,IO-phenanthroline; DMB = (2,5-dimethoxyphenyl)acetyl; QH2 = (2,5-dihydroxyphenyl)acetyl;Q = (2,5-benzoquinyl)acetyl; OBz = benzyloxy. (27) Kong, J. L. Y.; Loach, P. A. J. Heterocycl. Chem. 1980,17,737-744. (28) Ho, T.-F.; McIntosh, A. R.; Weedon, A. C. Can. J. Chem. 1984,62, 967-974.
1182 J . Am. Chem. SOC.,Vol. 110, No. 4, 1988 Table I. Steady State Emission Maxima and Intensities, CH2C12 S o h tion"Vb complex n RuNAc RuP-DMB RuP.Q 1.11 (599) 0.045 (598) 0 1.00 (599) 0.056 (601) 1 1.10 (602) 0.10 (602) 2 1.19 (603) 0.33 (602) 3 1.01 (601) 0.53 (600) 4 Intensities for emission at 600 nm relative to RuNAc. Estimated error *lo%. Emission wavelength maxima in nm given in parentheses. bAex = 450 nm, A, = 600 nm.
quinone. As a result, the luminescence experiments were carried out on RuP,Q samples that contained 5-10% residual RuP,QH2. The effects of this impurity on the photophysics are discussed below. Electrochemistry. Energetics of Excited State Electron Transfer. The excited state redox potentials for the Ru chromophore common to each of the complexes can be calculated from electrochemical data and the M L C T excited state energy. Cyclic voltammetry of RuNAc, RuDMB, and R u Q reveals that the first oxidation and reduction potentials for the R u center common to each of the complexes are +1.25 and -1.35 V, respectively (potentials vs SSCE). From these redox potentials and the MLCT excited state energy (2.1 eV) the following excited state potentials are calculated: El12(*Ru2+/Ru3+)= -0.85 V; E,,2(*Ru2+/Ru+) = +0.75 V.29 These potentials indicate that an electron acceptor with a reduction potential >-0.85 V or an electron donor with an oxidation potential
