Enhance the fluorescence and singlet oxygen generation ability of BODIPY: Modification on the meso-phenyl unit with electron withdrawing groups

Herein, meso-R-carboxyphenyl substituted BODIPY compounds were synthesized and characterized (R = CH3, COOH, NO2, H and Br4). The fluorescence, excited triplet state and singlet oxygen formation properties for these compounds were measured in nonpolar and polar solvents by UV-vis absorption, steady-state and time-resolved fluorescence methods, as well as laser flash photolysis technique. These BODIPYs exhibit very different fluorescence quantum yield and lifetime value, as well as singlet oxygen formation capability in polar solvents. In contrast, these compounds show very similar spectral shape and position, emit bright green fluorescence in non polar solvents. The introduction of an ortho-COOH on the benzene unit leads to a 20-fold increase in the fluorescence quantum yield. The further introduction of an electron withdrawing NO2 or COOH on the phenyl makes the BODIPY dye show much higher capability to generate singlet oxygen (up to 5-fold increase) and causes a sharp decrease in both fluorescence quantum yield and lifetime value. The increase in solvent polarity enhances singlet oxygen generation and intramolecular fluorescence quenching. The results are explained by the presence of intramolecular photoinduced electron/charge transfer from BODIPY core to the carboxyphenyl moiety.