ULTRAFAST CHARGE SEPARATION AND DRIVING-FORCE DEPENDENCE IN CYCLOPHANE-BRIDGED ZN-PORPHYRIN-QUINONE MOLECULES

The rate of charge separation in a series of cyclophane-bridged Zn-por phyrin-quinone systems has been investigated in nonpolar and polar sol vents by means of fluorescence upconversion. In all systems with drivi ng forces in the range 0.3-1.3 eV, ultrafast charge separation occurs with a rate constant of (2-5) x 10(12) s(-1). In combination with prev ious investigations on free-base porphyrin-quinone systems the driving force dependence can be probed from the (slightly endoergic) normal t o the moderately inverted region for the rate of charge separation alo ne. The (more limited) data for charge recombination in these systems are reasonably well reproduced by the same reorganization energies and electronic couplings that result from the analysis of the charge sepa ration. The data allow, for the first time, a satisfactory quantum mec hanical analysis of the driving force dependence in porphyrin-quinone systems of the charge separation alone and, consequently, the testing of the assumption on the comparability of charge separation and recomb ination by experimental means.