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 recombination (CR) of the product ion pair (IP) state of a series of fixed-distance dyads consisting of free-base porphyrin or zinc porphyrin and quinones have been investigated by means of picosecond-femtosecond laser spectroscopies in order to examine the energy gap and temperature dependences of CS and CR reactions in nonpolar media. Obtained CS rates were in the normal region, up to the neighborhood of the top region, and CR rates were in the inverted region; their energy gap dependences at room temperature were approximately reproduced by a semiclassical formula taking into consideration the high-frequency quantum mode of nuclear vibrations, although the CS rates near the top region did not show indication of the shift to the inverted region, contrary to the calculation. We have confirmed that the activation barrier for the CS reaction increases with a decrease of the energy gap, while the CR process is activationless, indicating the dominant effect of the high-frequency quantum mode in the inverted region. However, we could hardly find optimum parameter values for reorganization energies, etc., in the theoretical 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 upon the energy gap (-DELTAG(CS)) dependence of the CS rate constant (k(CS)) which showed a rather large systematic change corresponding to the increase of the solvent reorganization energy with the increase of the 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 the normal to near the top region and the k(CR) vs -DELTAG(CR) in the inverted to near the top region. Implications of these results, which seem difficult to interpret on the basis of the conventional electron-transfer theories, are discussed on the basis of the dominant effect of the quantum mechanical tunneling in the inverted region and/or the existence of nonlinear or some specific interactions between the IP state and the surrounding polar solvent.