Room-Temperature Columnar Liquid Crystalline Materials Based on Pyrazino[2,3-g]quinoxaline for Bright Green Organic Light-Emitting Diodes

Abstract

We report several molecules based on a central pyrazino[2,3-g]quinoxaline unit which is substituted with eight peripheral flexible tails of varying length. The out-of-plane conformation of the phenyl moieties made the self-assembly very sensitive to minute variation in the molecular structure. The compounds with very short and branched peripheral chains did not stabilize any liquid crystalline phase, while the medium- to long-chain homologues exhibited columnar phases. All the compounds exhibited high extinction coefficient and good greenish-yellow emission behavior in solution and solid state. Solvent-dependent aggregation behavior was noticed, and they could form long fibers of several micrometers in length. One of the columnar liquid crystalline materials was utilized to realize organic light-emitting diodes (OLEDs) either as the sole emissive material or as a dopant at low concentration in different host materials. Higher efficiency and bright green emission was achieved in the host–guest OLED, with CBP host, at 3 wt % concentration.