Solvent effects on photosensitized splitting of thymine cyclobutane dimer by an attached phenothiazine

The splitting quantum yields of the dimer by tethered chromophores exhibited different solvent effects. To further explore mechanism of the solvent effects, three covalently linked phenothiazine–dimer model compounds with a short linker, 1a–1c, were prepared. It was observed that solvent effect on dimer-splitting efficiency for phenothiazine–dimer systems is contrary to that of the other chromophore–dimer systems. Calculated results based on the Marcus theory showed that phenothiazine systems with a lower driving force induced by a lower value of Eox have a longer donor–acceptor distance between phenothiazine moiety and dimer unit, then gives a higher λs. Thus, back electron transfer would lie in the Marcus normal region for phenothiazine–dimer models, in which dimer-splitting is more efficient in higher polarity solvents. The value of redox potential between a donor and an acceptor should be a key leading to back electron transfer lying in the different Marcus regions and following two reverse solvent effects. Moreover, fluorescence spectra showed that the dual fluorescence gives a hint of charge-transfer complexes, and partial charge transfer would lead to lower splitting efficiency. However, some new insights into mechanisms of DNA photoreactivation mediated by photolyases were gained.