Two new Bodipy-carbazole derivatives as metal-free photosensitizers in photocatalytic oxidation of 1,5-dihydroxynaphthalene

Two new metal-free photosensitizers of Bodipy-carbazole derivatives, Phenyl-Bodipy-Carbazole (BC) and Carbazole-Bodipy-Carbazole (CBC), were synthesized. The transient absorption spectroscopy and the density functional theory calculations verify that the triplet Bodipy unit is produced in the dyads via an intramolecular charge transfer in the Franck-Condon excitation. Moreover, an apparent dependence on the solvent polarity was observed for these triplet states lifetimes, e.g. 9.6 and 118.2 μs in cyclohexane , while, 204.7 and 464.7 μs in dichloroethane for BC and CBC, respectively, as well as the triplet state quantum yields. With the increase of solvent polarity, the prolonged lifetime and the decreased quantum yield of the triplet BC and CBC convince us the charge-transfer mechanism for ISC process. In addition, the photo-oxidation rate constants of 1,5-dihydroxynaphthalene with these triplet photosensitizers are determined to be 1.11 × 10 −3  min -1 and 2.54 × 10 −3  min -1 for BC and CBC, respectively.