MODELING LONG-RANGE PHOTOSYNTHETIC ELECTRON-TRANSFER IN RIGIDLY BRIDGED PORPHYRIN QUINONE SYSTEMS

Photoinduced charge separation as well as subsequent charge recombination is studied in bichromophoric molecules containing a quinone unit (Q) and either a porphyrin (P) or a zinc porphyrin (ZnP) unit that are interconnected by a rigid saturated hydrocarbon bridge that unequivocally determines both the separation and the relative orientation of the chromophores. Across a bridge comprising a separation equivalent to two saturated carbon-carbon bonds (i.e., in P[2]Q), extensive direct overlap between the pi-systems of the chromophores is still possible and accordingly very fast photoinduced charge separation (typically on a 30-40-ps time scale) is observed. However, even across a bridge comprising an extended array of six saturated bonds, charge separation times in the order of 100 ps can still be realized if the driving force for this process is optimized by modification of the porphyrin and of the solvent (a minimum charge-separation time of 65 ps was observed for ZnP[6]Q in chloroform). This implies a rate of charge separation comparable to or somewhat higher than that of the charge transfer from pheophytin to quinone in natural photosynthesis in spite of the fact that the interchromophore distance in the latter process is slightly smaller. The time-resolved microwave conductivity method was employed to confirm the occurrence of photoinduced charge separation as well as to measure the rate of charge recombination in ZnP[6]Q. The latter was found to decrease dramatically at lower solvent polarity, thus indicating the importance of a relatively apolar environment for achieving long-lived charge separation and for storing as large a fraction as possible of the initial light energy used to initiate it.