Effect of cyano substituent on the functional properties of blue emitting Imidazo[1,2-a]pyridine derivatives

Abstract

High fluorescence efficiency and balanced charge transportation in organic emitters are desirable characteristics for application in OLEDs. Generally, donor-acceptor compounds showing pronounced intramolecular charge transfer exhibit low fluorescence quantum yield. Here, we report D-A fluorophores containing CN-substituted 2-phenylimidazo[1,2-a]pyridine acceptor unit which exhibit good fluorescence characteristics. The positions of donors (triphenylamine/carbazole) and cyano substituents are respectively interchanged to get a series of isomers. All the fluorophores exhibit high fluorescence quantum yield (Фf≥ 0.70) and excellent thermal stability (Td10≥ 390 °C). A dye comprising triphenylamine unit at para-position of C2 phenyl ring exhibited red-shifted emission maxima with larger Stokes shift in the series. Moreover, all the isomers displayed excited-state solvatochromism attributable to the photo-induced intramolecular charge transfer between donors and cyano-imidazo[1,2-a]pyridine acceptor. Addition of the cyano group on imidazopyridine restrained the formation of aggregates in the solid-state and stabilized the lowest unoccupied molecular orbital. Among the all, doped devices constructed with 3 wt% of triphenylamine containing dye revealed excellent performance in the series with a maximum EQE of 6.9%, 2759 cd/m2 luminance, and CIE coordinates (0.16, 0.08) which is very close to the NTSC blue standard.