Synthesis and Properties of Meso-unsubstituted 3-Pyrrolyl Boron Dipyrromethene

We report the synthesis of meso-free 3-pyrrolyl boron dipyrrin (3-pyrrolyl BODIPY) and fully unsubstituted meso-free boron dipyrrin (parent BODIPY) in one pot under simple reaction conditions by treating meso-free dipyrromethane with pyrrole in CHCl3 followed by oxidation with DDQ, neutralization with triethylamine and complexation with BF3.OEt2. The compounds were separated by column chromatography on silica and isolated in 6–10 % yields. The compounds are characterized by HR-MS mass, NMR, absorption, electrochemical and fluorescence techniques. The meso-free 3-pyrrolyl BODIPY exhibit red shifted absorption and emission bands compared to meso-free BODIPY. The meso-free BODIPY exhibit green fluorescence and meso-free 3-pyrrolyl BODIPY exhibit orange fluorescence in solution. Furthermore, compared to meso-phenyl 3-pyrrolyl BODIPY, the meso-free 3-pyrrolyl BODIPY is more strongly fluorescent with nearly 41 % increase in quantum yield. Electrochemical studies showed that meso-free 3-pyrrolyl BODIPY exhibit one irreversible reduction and one ill-defined oxidation indicating that the compound is not stable under redox conditions. Computational studies revealed that meso-free pyrrolyl BODIPY has reduced HOMO-LUMO gap compared to parent meso-free BODIPY. Furthermore, the meso-free 3-pyrrolyl BODIPY exhibit much higher quantum yield compared to meso-aryl analogue of 3-pyrrolyl BODIPY.