Heavy-atom-free charge transfer photosensitizers: Tuning the efficiency of BODIPY in singlet oxygen generation via intramolecular electron donor-acceptor interaction

To test the tunability of charge transfer (CT)-based BODIPY photosensitizers in generating singlet oxygen (1Δg), twelve meso-phenyl-BODIPY (donor-acceptor) type compounds have been synthesized and fully characterized, in which the phenyl moiety is modified with respective 0, 1, 2 and 3 methoxy groups to increase its electron-donating ability. The UV-Vis absorption spectra, fluorescence emission spectra, fluorescence quantum yield, fluorescence lifetime, excited triplet state formation, and singlet oxygen formation properties are measured. DFT quantum chemical computation is also carried out to explain the experiments. The occurrence of intra-molecular CT is confirmed by UV-Vis absorption, fluorescence properties and quantum chemical computation. The triplet excited state formation is evidenced by laser flash photolysis technique. The quantitative photosensitized singlet oxygen formation is demonstrated by DPBF (diphenylisobenzofuran) chemical trapping method. This type of BODIPY CT photosensitizers show good tunability in generating singlet oxygen (1Δg). When the number of methoxy group on the donor is increased (so that CT is enhanced), the efficiency of singlet oxygen generation becomes higher from 0.070 to 0.30. When solvent polarity is increased (CT is also enhanced), the efficiency of singlet oxygen generation is also increased significantly. The increase in singlet oxygen generation is accompanied by the decrease in fluorescence quantum yield and fluorescence lifetime values. These facts show that a higher CT efficiency in a simple phenyl-BODIPY donor-acceptor conjugate can lead to significant higher quantum yield of singlet oxygen generation. These results are useful in designing novel CT-based heavy-atom-free photosensitizers for photodynamic therapy of tumor.