PET-based bisBODIPY photosensitizers for highly efficient excited triplet state and singlet oxygen generation: tuning photosensitizing ability by dihedral angles

Herein, four covalent BODIPY heterodimers that differ by dihedral angles were shown to be highly efficient excited triplet state (T-1) photosensitizers (PSs) for singlet oxygen formation with a quantum yield (FD) of up to 0.94 as compared to their respective monomers, which had only negligible phi(Delta) of ca. 0.060. More interestingly, these PSs generate T-1 via charge recombination mechanism rather than traditional inter-system crossing. The photosensitizing ability of dimers is easily tuned by either the dihedral angle (between the two linked BODIPYs) or solvent polarity. Laser flash photolysis, time-resolved and steady state fluorescence, quantum chemical calculation, as well as thermodynamic analysis were employed to study the associated photophysical process to reveal the T-1 formation mechanism: photo-induced electron transfer (PET) followed by charge recombination. Due to its heavy-atom-free nature, polarity selectivity, high efficiency, and easy tunability, this PET-based PS and its mechanism are very useful in developing new PS for photodynamic therapy of tumors, photobiology, and organic photochemistry.