Synthesis, structure, spectral and redox properties of 3-pyrrolyl BODIPYs containing one to eight bromides at the pyrrole carbons

We report the regioselective bromination of <i>meso</i>-aryl 3-pyrrolyl BODIPY by introducing one to eight bromides at the pyrrole carbons of three pyrrole rings by treating <i>meso</i>-aryl 3-pyrrolyl BODIPY with <i>N</i>-bromosuccinimide/Br₂ in CH₂Cl₂ under mild reaction conditions. The crystal structures of five of these pyrrole-brominated <i>meso</i>-aryl 3-pyrrolyl BODIPYs indicated that the appended pyrrolyl group underwent distortion from the 12-atom mean plane of the BODIPY, and a maximum distortion of 40° was observed for octabrominated <i>meso</i>-aryl 3-pyrrolyl BODIPY. Upon increasing the number of bromides from one to eight at the pyrrole carbons of <i>meso</i>-aryl 3-pyrrolyl BODIPY, the following key observations were made: (i) the absorption bands experienced a bathochromic shift up to the introduction of five bromides, followed by a hypsochromic shift from six to eight bromides, indicating that the magnitude of the absorption band shifts was non-additive; (ii) the fluorescence band experienced bathochromic shifts along with a decrease in quantum yield and singlet state lifetime; (iii) the electrochemical studies indicated that brominated 3-pyrrolyl BODIPYs were electron deficient and underwent easier reductions with an increase in the number of bromides at the pyrrole carbons of 3-pyrrolyl BODIPY. A linear additive relationship was noted between <i>E</i>_1/2red and the number of bromide groups at the pyrrole carbons of 3-pyrrolyl BODIPY and (iv) the halogenated 3-pyrrolyl BODIPYs were highly efficient in generating singlet oxygen. A linear relationship was observed between the singlet oxygen quantum yields and the number of bromides at the pyrrole carbons of 3-pyrrolyl BODIPY. Furthermore, DFT and TD-DFT studies supported these experimental observations.