PHOTOINDUCED ELECTRON-TRANSFER IN CYCLOPHANE-BRIDGED PORPHYRIN-QUINONE MOLECULES - A SUBPICOSECOND TRANSIENT ABSORPTION STUDY

Photoinduced charge separation and recombination in a series of cyclop hane-bridged porphyrin-quinone systems (PQ) in toluene solution have b een investigated by means of sub-ps time-resolved absorption spectrosc opy. Temporal profiles and transient spectra of the porphyrin cation ( P+) show the photoinduced formation and decay of the charge-separated state P(+)Q(-). The rate of charge recombination in these systems (P()Q(-) --> PQ) is investigated as a function of the reaction free energ y Delta G in the range (-Delta G) = 1.54-1.98 eV, which is in the so-c alled inverted region of electron transfer. Combining values for charg e separation and recombination the dependence of the electron transfer rate on Delta G fully covers the normal and inverted regions ((-Delta G) = (-)0.02-1.98 eV) with the rate varying over more than three orde rs of magnitude (approximate to 2 . 10(9) - 5 . 10(12) s(-1)).