Weak Temperature Dependence of Electron Transfer Rates in Fixed-Distance Porphyrin-Quinone Model Systems

Abstract

Electron transfer rate constants of several derivatives of [5-(4'-(4-(2',5'-benzoquinonyl)bicyclo[2.2.2]octyl) phenyl)-2,3,7,8,12,13,17,18-octamethylporphyrinato]zinc(II) have been measured as a function of temperature in 2-methyltetrahydrofuran, toluene, and toluene-d_g. 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 described in terms of an initial (k_(ET)^0) and an average ET rate constant (k_(av)). The k_(ET)^0 values for the molecules with different driving forces, spanning a range of 0.2 eV, show parallel trends over the range of temperatures studied. The trends in k_(ET)^0 are described in terms of the effects of temperature-dependent changes in solvent dielectric constants on the barrier height. Good agreement is observed for the case of toluene solvent, using a semiclassical model, but poorer quantitative agreement is found for the 2-methyltetrahydrofuran data. The temperature dependence of k_(av) is described using a model incorporating an angle-dependent electronic coupling and interconversion of rotational 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 temperatures studied.