TRANSIENT OPTICAL-ABSORPTION AND TIME-RESOLVED RESONANCE RAMAN EXPERIMENTS ON COVALENTLY-LINKED PORPHYRIN-QUINONE SYSTEMS

Light-induced triplet electron transfer (ET) and subsequent triplet ra dical-pair (RP) recombination in two covalently linked porphyrin-quino ne systems in highly viscous ethanol has been investigated by both tra nsient optical absorption and time-resolved resonance Raman spectrosco py with a time resolution of 10 ns. The temperature dependence of the rates is measured between 155 and 200 K and compared with predictions of solvent-controlled adiabatic electron-transfer theory. It is shown that the triplet ET in the normal region (exergonicity Delta G(0) < so lvent reorganization energy lambda(s)) is correctly described. The mec hanism of the RP recombination in the inverted region (Delta G(0) > la mbda(s)) depends on the dynamics of the exchange interaction J, on the triplet-singlet mixing of the radical pair states and on the singlet recombination rate. An intermolecular ET process leading to a dispropo rtionation reaction of the quinone moieties is also observed.