BisBODIPY as PCT-based halogen free photosensitizers for highly efficient excited triplet state and singlet oxygen formation: Tuning the efficiency by different linking positions

Four covalent BODIPY hetero-dimers, formed by different linking positions at (2,2′), (2,8′), (3,8′) and (8,8′) respectively, were designed and synthesized to compare their ability for excited triplet state (T1) and singlet oxygen generation. In contrast to BODIPY monomers that show negligible photosensitizing ability, the four dimers are very efficient for T1 and singlet oxygen formation depending on the individual linking style and solvent polarity. Laser flash photolysis, time-resolved/steady state fluorescence, quantum chemical calculation and thermodynamic analysis revealed that the mechanism of triplet state T1 formation is different from the traditional intersystem crossing mechanism. The T1 formation is due to charge recombination of the twisted charge separation state (TCSS), while the TCSS is generated by photoinduced intramolecular charge transfer (PCT) from one BODIPY monomer moiety to another within the dimer. The PCT based photosensitizers are also medium polarity sensitive which may be very useful in designing and synthesizing of photosensitizers for photodynamic therapy of tumor, photobiology and organic photochemistry.