DYNAMICAL SOLVENT EFFECTS IN INVERTED REGION ELECTRON-TRANSFER

The solvent dependence of the charge recombination (CR) electron transfer rate constants of cofacially linked magnesium/free base diporphyrin (Mg-H2) and zinc porphyrin/modified free base chlorin (Zn-H2(-C(CN)2)) complexes has been investigated by picosecond transient absorption spectroscopy. The photogenerated Mg+-H2- and Zn+-H2(-C(CN)2)- charge separated pairs undergo facile reaction in the Marcus inverted region to produce ground state species, with the latter exhibiting faster reaction rates owing to less activated CR. We find that the CR rate constants k(obs)(CR) are strongly solvent dependent in a homologous series of nitriles and acetates. Although they are slow with respect to solvent motion (k(obs)(Mg+-H2-) = (0.6-4.0) x 10(9) s-1; k(obs)(Zn+-H2(-C(CN)2)- = (1.6-8.3) x 10(10) s-1), a linear correlation is observed between k(obs)(CR) and the inverse of the solvent relaxation time (1/tau(s) = (1.5-9.1 ) x 10(11) s-1). For the Mg+-H2- cofacial pair this correlation is observed only when solvents of the same series are considered, whereas for Zn+-H2(-C(CN)2)- this linear correlation is independent of the solvent series. These observations are analyzed within the context of recent theoretical predictions for solvent-controlled adiabatic electron transfer reactions.