WEAK TEMPERATURE-DEPENDENCE OF ELECTRON-TRANSFER RATES IN FIXED-DISTANCE PORPHYRIN-QUINONE MODEL SYSTEMS

Electron transfer rate constants of several derivatives of ,3,7,8,12,1 3,17,18-octamethylporphyrinato]zinc(II) have been measured as a functi on of temperature in 2-methyltetrahydrofuran, toluene, and toluene-d(8 ). The observed temperature dependencies of the electron transfer rate constants are relatively weak in both the polar and nonpolar solvents . Nonexponential ET dynamics are observed at low temperatures and desc ribed in terms of an initial (k(ET)(0)) and an average ET rate constan t (k(av)). The k(ET)(0) values for the molecules with different drivin g forces, spanning a range of 0.2 eV, show parallel trends over the ra nge of temperatures studied. The trends in k(ET)(0) are described in t erms of the effects of temperature-dependent changes in solvent dielec tric constants on the barrier height. Good agreement is observed for t he case of toluene solvent, using a semiclassical model, but poorer qu antitative agreement is found for the 2-methyltetrahydrofuran data. Th e temperature dependence of k(av) is described using a model incorpora ting an angle-dependent electronic coupling and interconversion of rot ational conformers. A temperature-dependent solvent isotope effect is observed on going from toluene to toluene-d(8), with k(ET)(0)(toluene) /k(ET)(0)(toluene-d(8)) being as large at 1.5 over the range of temper atures studied.