In chromophore-containing cyclobutane pyrimidine dimer (CPD) model systems, solvent effects on the splitting efficiency may depend on the length of the linker, the molecular conformation, and the oxidation potential of the donor. To further explore the relationship between chromophore structure and splitting efficiency, we prepared a series of substituted indole–T<>T model compounds 2 a–2 g and measured their splitting quantum yields in various solvents. Two reverse solvent effects were observed: an increase in splitting efficiency in solvents of lower polarity for models 2 a–2 d with an electron-donating group (EDG), and vice versa for models 2 e–2 g with an electron-withdrawing group (EWG). According to the Hammett equation, the negative value of the slope of the Hammett plot indicates that the indole moiety during the T<>T-splitting reaction loses negative charge, and the larger negative value implies that the repair reaction is more sensitive to substituent effects in low-polarity solvents. The EDGs of the models 2 a–2 d can delocalize the charge-separated state, and low-polarity solvents make it more stable, which leads to higher splitting efficiency in low-polarity solvents. Conversely, the EWGs of models 2 e–2 g favor destabilization of the charge-separated state, and high-polarity solvents decrease the destabilization and hence lead to more efficient splitting in high-polarity solvents.
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