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.