The kinetics of photoinduced electron-transfer (ET) reactions have been measured in a series of synthetic donor-acceptor complexes. The electron donors are singlet or triplet excited iridium(I) dimers (Ir(2)), and the acceptors are N-alkylpyridinium groups covalently bound to phosphinite ligands on the Ir(2) core. Rate constants for excited-state ET range from 3.5 x 10(6) to 1.1 x 10(11) per second, and thermal back ET (pyridinium radical to Ir(2)(+)) rates vary from 2.0 x 10(10) to 6.7 x 10(7) per second. The variation of these rates with driving force is in remarkably good agreement with the Marcus theory prediction of a Gaussian free-energy dependence.

In the ideal photochemical energy storage system a photoinduced charge separation must be generated rapidly with minimum energy loss and be maintained by virtue of inefficient electron-hole recombination. These rigorous requirements necessitate a thorough understanding of the diverse parameters that determine the barriers to electron-transfer reactions:' free energy changes; donor-acceptor separation, orientation, and electronic coupling; inner-sphere reorganization energy; and solvent reorganization energy and dynamics. Clearly, in order to elucidate the contributions of these parameters to electron-transfer barriers, model electron-transfer systems must be prepared which can be selectively modified to vary one or more of these parameters. An important requirement for such systems is the covalent coupling of donor and acceptor into one molecule to surpass diffusional limits and permit study of extremely fast intramolecular electron-transfer reactions.' We report here results from a prototypal synthetic donor-acceptor system which exhibits photoinduced electrontransfer rate constants in the vicinity of 10l2 s-' at driving forces of ca. 1 eV and charge-recombination rates slower than 2 X 1O'O s-1.Our synthetic donor-acceptor complexes are based on [Ir(ppz')(CO)(PPh2(O(CHz)2RJ)]z (pz' = 3,5-dimethylpyrazolyl; Ph = C6H5),3 in which the Ir, core serves as the electron donor and the acceptors are incorporated into R on the terminal phosphines.When R is a poor electron acceptor such as -H or -+N(C2H5)3, these molecules luminesce from both singlet ('B) and triplet (3B) du*pu excited states4 The excited singlet state lifetimes of ca. 100 ps (Table I) for these two model complexes were measured by transient absorption spectroscopy following 30-ps excitation at 355 nm.' The triplet lifetimes, measured both by transient (1) (a) Marcus, R. 3707-3721. (2) Kinetics investigations of intramolecular electron transfer in closely coupled donor-acceptor complexes include the following: (a) Creutz, C.; Kroger, P.; Matsubara, T.; Netzel, T. L.; Sutin, N.(3) Compounds were recrystallized from acetonitrile solutions prior to quantum yield and kinetics measurements. A lone singlet for the aromatic protons on the bridging pyrazole ligands (6 = 5.6 ppm) indicates that the terminal phosphine ligands adopt a trans configuration with respect to the metal-metal axis. Preparative details will appear in a future publication.(a) Rice, S. F.; Gray, H. B. J. Am. Chem. SOC. 1981,103, 1593-1595. (b) Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H.Table I. Lifetimes and Quantum Yields of Singletand Triplet Excited States in[Ir(~-pz')(CO)(PPh2{O(CH,),R))I, R" %. -b 3@.,b ' T , PS 3 T , W S -*N(C2H5)3' 0.0015 0.034 90 1.25 -PY 2.5 x 10-5 4 0 -5 1 .5e OAll measurements performed in acetonitrile solution. bAci = 436 nm. Referenced to 9,, = 0.06 for *[Ru(bpy)J2+ (Caspar, J. V.; Meyer, T. J. J . A m . Chem. SOC. 1983, 105, 5583-5590). ' A, , = 355 nm for lifetime measurements. = 532 nm for lifetime measurements. e Estimated from -Hc.d 0.002...

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The intramolecular electron‐transfer reactions are investigated for the donor‐acceptor systems (I).