Ultrafast charge separation from the S₂ excited state of directly linked porphyrin - Imide dyads: First unequivocal observation of the whole bell-shaped energy-gap law and its solvent dependencies

Photoinduced charge separation (CS) due to electron transfer from the S₂ state of zinc porphyrin (ZP) to a directly linked imide (I) has been studied by means of femtosecond fluorescence up-conversion and femtosecond-picosecond transient absorption spectral measurements. S₂ fluorescence decay curves of ZP-I dyads in various solvents were all single-exponential, from which accurate rate constants for the CS reaction were obtained. The whole bell-shaped energy-gap law (EGL) has been confirmed unambiguously in polar solvents tetrahydrofurane, acetonitrile, and triacetin as the first example for CS, which can be reproduced on the basis of a nonadiabatic mechanism considering solvent reorganization and intrachromophore high-frequency vibrations (hfm) approximated by an averaged single mode. Mainly the top and inverted regions have been observed in nonpolar solvents cyclohexane and toluene. This result may be ascribed to the large decrease of the solvent reorganization energy in nonpolar solvents, which causes a large shift in the reaction coordinate for CS, making the S₂-state surface partially embedded in that of the CS state and transforming the EGL for the 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 inverted regions observed in triacetin indicate strongly that very fast solvent relaxation should take place despite the very long τ_L of 125 ps at 20°C (see the text). Thus, by using specifically designed ZP-I linked systems, we have directly demonstrated that both the intramolecular hfm and ultrafast solvent relaxation play very important roles in the ultrafast CS from the S₂ state and that we can regulate its EGL by controlling the nature of the solvent.