Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion (Ru(bpy)32+). Solvent effects

Abstract

The excited-state lifetime of the metal-to-ligand charge-transfer (MLCT) excited state or states of Ru(bpy)3Z+ has been measured in a series of solvents at a series of temperatures. The data can be fit to the equation r(T)-' = k + k' exp (-(AE'/k,T)) where k is the sum of the radiative (k,) and nonradiative (k,,) rate constants for decay of the MLCT state(s) and the temperature-dependent term involves a thermally activated transition from the MLCT state to a low-lying state or states presumably d-d in character. From a combination of lifetime and emission quantum yield measurements, values for k, and k,, have been obtained in the series of solvents. From the variations of the various kinetic parameters with solvent the following conclusions can be reached: (1) k, is only slightly solvent dependent; (2) the variations in k,, and emission energy with solvent are in quantitative agreement with the predictions of the energy gap law for radiationless transitions; and (3) the solvent dependence of the kinetic parameters k'O and AE', which characterize the MLCT - dd transition, can be considered in the context of electron-transfer theory including the observation of a linear relationship between In k'O and AE'(Barc1ay-Butter plot). In a final section the implications of solvent effects on the use of Ru(bpy)32+* as a sensitizer are discussed. We have shown that the energy gap law can be applied to nonradiative decay in a series of polypyridyl complexes of OS(II).'~~ The studies were based on the metal-to-ligand charge-transfer (MLCT) excited states of the two series of complexes (phen)- OSI*L,~+ and (bpy)OsL42+ (bpy is 2,2'-bipyridine, phen is 1,lO- phenanthroline; L = '/2bpy, 1/2phen, pyridine, PR,, Me2S0, CH,CN, ...). The nature of the experiment was to show from excited-state lifetime and emission measurements that plots of In k,, vs. E,, are linear where k,, is the nonradiative decay rate constant and E,, the emission energy. In addition, it was possible to account for the origin of the solvent dependence of k,, on the basis of the energy gap law using the series of Os-phen based MLCT excited states., In these studies, radiative rate constants, k,, for excited-state decay, were shown to be relatively insensitive to variations either in complex or in solvent. The earlier studies based on the Os(I1) complexes are part of a larger effort to explore in detail the photochemical and pho- tophysical properties of MLCT excited states. In many ways the parent compound associated with MLCT excited states is Ru- (b~y),~+. The excited-state electronic structures of R~(bpy),~+ and related complexes have been investigated by spectroscopic studie~,~ low-temperature emission and lifetime measurement^,^