Synthesis, Structure, and Spectroscopy of Green to Yellow Fluorescent Divinylbipyrroles

Green and yellow luminescent 4,4′-dimethyl-5,5′-divinyl-2,2′-bipyrrole derivatives bearing ester and amide were synthesized from blue luminescent 5,5′-diformyl-4,4′-dimethyl-2,2′-bipyrrole and diethyl malonate, 2,2-dimethyl-1,3-dioxane-4,6-dione, 1,3-dimethylbarbituric acid, and 1,3-dihexylbarbituric acid under Knoevenagel condensation conditions. All the synthesized products clearly showed a red-shifted π–π* transition absorption band in the visible region compared with the starting bipyrrole. Single-crystal X-ray diffraction analysis and 1H nuclear magnetic resonance (NMR) spectroscopy revealed that the carbonyl moiety interacts with NH to form an intramolecular hydrogen-bonding. Out of the synthesized compounds, a Knoevenagel adduct of 1,3-dimethylbarbituric acid showed fluorescence at 578 nm with an 88 % quantum efficiency in dichloromethane, which is the highest value for the bipyrrole system. The highly efficient yellow photoluminescence can be attributed to the rigid molecular structure of the compound. Density Functional Theory (DFT) calculation revealed that the rigid and planar structure is essential for the bathochromically shifted absorption and high photoluminescence quantum yield.