Figure 7. Energy levels of excited CT states and the local triplet excited state of 1,1'-binaphthyl.and CT-II states. The fact that racemization was not induced by photoexcitation of the CT-I band can be explained from this diagram. The triplet excited state of l.l'-binaphthyl, which is responsible for the racemization reaction, could not be formed energetically from the CT-I state.Photoexcitation of the CT-II band results in the following three processes: formation of a triplet excited state (racemization reaction), formation of ions, and fluorescence from the excited CT-I state as evidenced by laser photolysis, racemization, and fluorescence experiments. These facts indicate that intersystem crossing and ion formation processes compete with internal conversion from the CT-II to the CT-I state. The unusual intersystem crossing process from the upper excited singlet state is probably due to the large energy difference between the CT-II and CT-I states which consequently gives rise to a small Franck-Condon factor for the internal conversion process.18 The large energy difference slowed the internal CT-II 3BinaphthyI 'CT-I Ground State conversion rate and enabled other processes, such as intersystem crossing or ion formation, to compete with it. The enhancement of intersystem crossing from the CT-II singlet state may arise from a large spin-orbit coupling due to the CT interaction.19 This large spin-orbit coupling in the CT state and the small energy difference between the CT-II state and the local triplet excited state of , -binaphthyl may cause a considerable increase in the rate of intersystem crossing.
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