Modifying the meso-phenyl with electron donating amino groups strongly enhances BODIPY's ability as good singlet oxygen photosensitizer

We synthesized three 1,3,5,7-tetramethyl-BODIPY (1a, 2a, 3a) and three 3,5-dimethyl-BODIPY (1b, 2b, 3b) photosensitizers, in which the meso position was modified by different para-aminophenyl groups. The singlet oxygen generation and photophysical properties of the BODIPYs were studied by UV-vis absorption spectra, fluorescence emission spectra methods in various solvents. It was found that these BODIPY derivatives were environment -sensitive and fluorescent photosensitizers, which show high efficiency for generating singlet oxygen in medium polar environments but little photoactivity in polar and non polar solvents. In the mean time, they are highly fluorescent in nonpolar solvents but much less emissive in polar solvents, ICT emission occurs in medium polar solvents and correlated with singlet oxygen formation efficiency. The results are explained by PCT (photoinduced charge transfer) mechanism. The presence of the methyl groups at 1 and 7 positions disables the rotation of the phenyl ring, and therefore, the corresponding BODIPY 1a, 2a, and 3a were characterized by significant higher singlet oxygen generation efficiency and fluorescence quantum yield than that of BODIPY 1b, 2b, and 3b.