The bell-shaped energy gap dependence of the charge recombination reaction of geminate radical ion pairs produced by fluorescence quenching reaction in acetonitrile solution

In view of the theoretical prediction that the observation of the inverted region in the photoinduced charge separation will be difficult, as it is actually the case in the fluorescence quenching reaction, but the charge recombination of the produced geminate ion pair will show a clear-cut bell-shaped energy gap dependence, we have made systematic studies on the charge recombination processes of geminate ion pairs by directly observing their dynamics with ultrafast laser spectroscopy. We have obtained not only the results for the inverted region, but also the results for the top region as well as normal region, confirming the bell-shaped energy gap dependence of the charge recombination. On the basis of this result, some discussions on the nature of the inter- and intra-molecular ion pairs with respect to the charge recombination processes have been given in the case of some typical exciplexes and porphyrin-quinone systems.