INTRAMOLECULAR PHOTOINDUCED CHARGE SEPARATION AND CHARGE RECOMBINATION OF THE PRODUCT ION-PAIR STATES OF A SERIES OF FIXED-DISTANCE DYADS OF PORPHYRINS AND QUINONES - ENERGY-GAP AND TEMPERATURE DEPENDENCES OF THE RATE CONSTANTS

Intramolecular photoinduced charge separation (CS) and charge recombin ation (CR) of the product ion pair (IP) state of a series of fixed-dis tance dyads consisting of free-base porphyrin or zinc porphyrin and qu inones have been investigated by means of picosecond-femtosecond laser spectroscopies in order to examine the energy gap and temperature dep endences of CS and CR reactions in nonpolar media. Obtained CS rates w ere in the normal region, up to the neighborhood of the top region, an d CR rates were in the inverted region; their energy gap dependences a t room temperature were approximately reproduced by a semiclassical fo rmula taking into consideration the high-frequency quantum mode of nuc lear vibrations, although the CS rates near the top region did not sho w indication of the shift to the inverted region, contrary to the calc ulation. We have confirmed that the activation barrier for the CS reac tion increases with a decrease of the energy gap, while the CR process is activationless, indicating the dominant effect of the high-frequen cy quantum mode in the inverted region. However, we could hardly find optimum parameter values for reorganization energies, etc., in the the oretical equation which could reproduce quantitatively both the energy gap dependence and the temperature dependence of the CS and CR rates at the same time. We have examined also the solvent polarity effect up on the energy gap (-DELTAG(CS)) dependence of the CS rate constant (k( CS)) which showed a rather large systematic change corresponding to th e increase of the solvent reorganization energy with the increase of t he solvent polarity, while the energy gap (-DELTAG(CR)) dependence of the CR rate constant (k(CR)) showed little solvent polarity dependence , leading to the crossing between the k(CS) vs -DELTAG(CS) curve in th e normal to near the top region and the k(CR) vs -DELTAG(CR) in the in verted to near the top region. Implications of these results, which se em difficult to interpret on the basis of the conventional electron-tr ansfer theories, are discussed on the basis of the dominant effect of the quantum mechanical tunneling in the inverted region and/or the exi stence of nonlinear or some specific interactions between the IP state and the surrounding polar solvent.