Intermolecular electron transfer rate in diffusion limited region: picosecond fluorescence studies

Abstract

The temporal profiles of the quenched fluorescence decay of the free base meso-tetraphenyl porphyrin (H₂TPP) and its Zn derivative (ZnTPP) with quenchers such as quinones and m-dinitrobenzene have been analysed by methods developed for short time regimes which are known to be diffusion influenced [N. Periasamy et al., J. Chem. Phys.88, 1638 (1988); 89, 4799 (1988); Chem. Phys. Lett.160, 457 (1989); N. Periasamy, Biophys. J.. 54, 961 (1988); R. Das and N. Periasamy, Chem. Phys. 136, 361 (1989); G.C. Joshi et al., J. Phys. Chem.94, 2908 (1990)]. These quenchers are known to participate in an electron transfer reaction leading to a charge separation. The intrinsic rate constant (k_a) derived from the analysis is examined as a function of the change in free energy in the electron transfer reaction. Such a comparison indicates that k_a can be related to the electron transfer rate, k_et. The electron transfer rates measured in acetonitrile (solvent reorganization energy, λ_s=1.35) and toluene (λ_s=0.1) do not indicate the existence of an inverted region as predicted by Marcus. The trend agrees with the findings of Rehm and Weller [Isr. J. Chem.8, 259 (1970)], except that the rate constants are at least one order of magnitude larger than the diffusion limited values.