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

Abstract

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.

Synthesis, Crystal Structure, and Photoluminescent Properties of 3,3′,4,4′-Tetraethyl-5,5′-divinyl-2,2′-bipyrrole Derivatives

“…NMR spectroscopy supported that synthesized divinylbipyrroles derivative ( 2 – 4 ) have good solubility in chloroform. Although 13 C NMR spectrum of the previously reported 1,3-dimethylbarbituric acid adduct, 4Me could not be obtained because of the poor solubility [ 22 ], its analog, 4 , afforded clear 13 C NMR spectrum at similar experimental condition. The 1 H NMR spectral studies revealed that compounds 3 and 4 have hydrogen bonding between NH and CO.…”

“…Herein we report synthesis of novel π-extended bipyrrole derivatives 2 – 4 ( Figure 1 B) using activated methylene compounds by means of aldol condensation reaction ( Scheme 1 ), study their photophysical properties and utilize time-dependent density functional theory (TD-DFT) calculations to rationalize the desirable bathochromic shifts observed experimentally. It was found that the introduction of four ethyl groups on the bipyrrole 3 and 3′-positions ( Figure 1 A) resulted in good solubility compared with the previously reported 3,3′-free derivatives [ 22 ]. The improvement of the solubility enabled us to obtain single crystals of 3, and 4, suitable for X-ray diffraction analysis.…”

“…Compound 1 was prepared according to literature method [ 12 ]. Novel divinylbipyrroles 2 – 4 bearing four ethyl groups were synthesized from compound 1 and corresponding activated methylene compounds, malononitrile, 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum’s acid), and 1,3-dimethylbarbituric acid, by means of an aldol condensation catalyzed by ammonium acetate ( Scheme 1 ) [ 22 ]. Crude products were precipitated out from the reaction mixture by the addition of water and collected by filtration.…”

“…3,3′,4,4′-tetraethyl-5,5′-diformyl-2,2′-bipyrrole ( 1 ) has been prepared according to the literature [ 12 ]. Compounds 2 – 4 were synthesized using the similar reaction condition reported in the previous paper [ 22 ].…”

“…A planar 2,2′-bipyrrole is known to show blue fluorescence upon UV excitation in both solid and solution state. We recently reported the methodology of tuning the fluorescence wavelength of bipyrrole-based fluorophores by utilizing their crystal structure and incorporating suitably placed π-extension [ 21 , 22 ]. In our previous study, a highly efficient photoluminescence was achieved ( λ FL = 578 nm, Φ FL = 88%) by extending π-conjugation, containing a rigid and flat molecular structure, but unfortunately this resulted in poor solubility and crystallinity of the compounds.…”