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-porphyrin-quinone systems has been investigated in nonpolar and polar solvents by means of fluorescence upconversion. In all systems with driving 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 previous investigations on free-base porphyrin-quinone systems the driving force dependence can be probed from the (slightly endoergic) normal to the moderately inverted region for the rate of charge separation alone. 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 separation. The data allow, for the first time, a satisfactory quantum mechanical 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 recombination by experimental means.