Outer Sphere Metal-to-Ligand Charge Transfer in Organometallic Ion Pairs Lucian A. Lucia, Khalil Abboud, and Kirk S. Schanze* Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200 ReceiVed March 27, 1997 X A series of organometallic salts which comprise a fac-(b)Re I (CO) 3 (py) + cation (b ) 4,4′,5,5′-tetramethyl-2,2′- bipyridine (tmb), 2,2′-bipyridine (bpy), or 4,4′-dicarbomethoxy-2,2′-bipyridine (dmeb); py ) pyridine) paired with the Co(CO) 4 - anion have been prepared and subjected to photophysical study. In nonpolar solvents the salts feature a broad, low-intensity ion-pair charge transfer (IPCT) absorption band. The energy of the IPCT band decreases with the LUMO energy of the diimine ligand, suggesting that the orbital basis of the transition is d (Co) f π* (b). An X-ray crystal structure of [(bpy)Re I (CO) 3 (py) + ][Co(CO) 4 - ](2a) reveals that the anion occupies a lattice position which is directly below (or above) the plane defined by the bpy ligand, which supports the d (Co) f π* (b) IPCT assignment. Luminescence studies of the salts indicate that the dπ (Re) f π* (b) metal-to-ligand charge transfer (MLCT) excited state is quenched by reductive electron transfer from Co(CO) 4 - . Nominally IPCT and MLCT excitation of the organometallic ion pairs afford the same geminate radical pair, [(b •- )Re I (CO) 3 (py),Co(CO) 4 • ]. However, laser flash photolysis studies reveal that the rate of charge recombination within the geminate radical pair is significantly slower when MLCT excitation is applied. The slower rate of charge recombination is attributed to the fact that triplet state geminate pairs are produced via the triplet MLCT excited state manifold. Introduction Bimolecular photoinduced electron transfer between donors and acceptors in fluid solution leads to production of ion radicals (or neutral radicals) with varying degrees of efficiency. Detailed studies of the efficiency for charge separation in organic donor- acceptor pairs has led to a thorough understanding of the structure of geminate radical (ion) pairs and the dynamics of highly exothermic electron transfer within them. 1-36 Organic donor-acceptor systems feature a number of properties that facilitate studies of bimolecular photoinduced electron transfer, namely, high radiative rates for fluorescence from the locally excited (LE) states of the photoexcited acceptor (or donor), 37 as well as exciplex fluorescence 2,14,15 and/or Mulliken electron donor-acceptor (EDA) complex formation. 38 By taking ad- vantage of these unique properties, several research groups developed concise models which elucidate the factors that control competition between charge recombination, exciplex formation, and free radical (ion) formation in organic systems. 3-36 Bimolecular photoinduced electron transfer reactions involv- ing a transition metal ion as the donor and/or acceptor have also been examined in great detail. 39-47 A particular focus in * Corresponding author. 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