| Abstract
| - Photoinduced charge separation (CS) due to electron transfer from the S2 state of zinc porphyrin (ZP) to adirectly linked imide (I) has been studied by means of femtosecond fluorescence up-conversion andfemtosecond−picosecond transient absorption spectral measurements. S2 fluorescence decay curves of ZP−Idyads in various solvents were all single-exponential, from which accurate rate constants for the CS reactionwere obtained. The whole bell-shaped energy-gap law (EGL) has been confirmed unambiguously in polarsolvents tetrahydrofurane, acetonitrile, and triacetin as the first example for CS, which can be reproduced onthe basis of a nonadiabatic mechanism considering solvent reorganization and intrachromophore high-frequencyvibrations (hfm) approximated by an averaged single mode. Mainly the top and inverted regions have beenobserved in nonpolar solvents cyclohexane and toluene. This result may be ascribed to the large decrease ofthe solvent reorganization energy in nonpolar solvents, which causes a large shift in the reaction coordinatefor CS, making the S2-state surface partially embedded in that of the CS state and transforming the EGL forthe CS reaction into one that is somewhat analogous to that of the radiationless transition in the weak-coupling limit. The single-exponential fluorescence decay in 100 fs and the EGL with both normal and invertedregions observed in triacetin indicate strongly that very fast solvent relaxation should take place despite thevery long τL of 125 ps at 20 °C (see the text). Thus, by using specifically designed ZP−I linked systems, wehave directly demonstrated that both the intramolecular hfm and ultrafast solvent relaxation play very importantroles in the ultrafast CS from the S2 state and that we can regulate its EGL by controlling the nature of thesolvent.
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