SOLVENT REORGANIZATION ENERGY IN EXCITED-STATE ELECTRON-TRANSFER REACTIONS - QUENCHING AND GEMINATE-PAIR BACK ELECTRON-TRANSFER

The temperature dependencies of the quenching rate constants (k(q)) an d cage escape yields of the redox products (eta(ce)) from the electron -transfer reaction of Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) with nin e aromatic amines in deaerated 1:1 (v/v) CH3CN/H2O solutions have been determined. Values of lambda, the solvent reorganization energy for e lectron-transfer quenching and back electron transfer within the solve nt cage, have been extracted from plots of log(k(q)T(1/2)) vs 1/T and log((eta(ce)(-1) - 1)T-1/2) vs 1/T, respectively. For the quenching pr ocess, lambda is not a constant value for the series of quenchers; in general, higher values of lambda are exhibited by primary amines and l ower values by tertiary amines. The structure and size of the quencher s and the nature of the ring substituents contribute to the value of l ambda. For the back-electron-transfer reaction within the geminate red ox pair formed in the quenching process, the more sterically hindered two-ring amines exhibit a higher value of lambda (1.1 +/- 0.08 eV) tha n do the majority of the one-ring amines (0.82 +/- 0.04 eV). A Marcus plot of log(eta(ce)(-1) - 1) vs Delta G degrees(bt) shows a correlatio n within only the inverted region for systems with the same lambda; th e earlier identification of the results for the same photosensitizer a nd quenchers as a bell-shaped curve is due to the coincidental overlap of two independent segments within the inverted region.