Compact N,N-diphenylaminophenyl–quinoxaline donor–acceptor-type dyes were synthesized, and the effects of substituents on the fluorescence properties were investigated. Introduction of electron-withdrawing groups to the quinoxaline acceptor caused a red shift of the emission, and furthermore, thermally activated delayed fluorescence was facilitated by the reduction of the energy gap between the excited singlet and triplet states.

Abstract

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) are expected as next-generation electroluminescent devices because they provide high device performance due to utilization of both singlet and triplet excitons. Here we report the development of intramolecular charge transfer dyes with a compact N,N-diphenylaminophenyl–quinoxaline donor–acceptor system for use as TADF emitters; namely, 4-(3-methylquinoxalin-2-yl)-N,N-diphenylaniline (1a), N,N-diphenyl-4-(3-(trifluoromethyl)quinoxalin-2-yl)aniline (1b), and 4-(6,7-difluoro-3-(trifluoromethyl)quinoxalin-2-yl)-N,N-diphenylaniline (1c). Dye 1a exhibited fluorescence at 464 nm in poly(methyl methacrylate) film. In contrast, introducing trifluoromethyl and/or fluoro groups to the quinoxaline skeleton resulted in red-shifted emission at 516 and 539 nm for 1b and 1c, respectively. These red shifts were predominantly induced by stabilization of the LUMO energy levels, and interestingly, 1b and 1c had higher photoluminescence (PL) quantum yields under nitrogen atmosphere than those under aerobic conditions. From the temperature dependence of the PL intensity and PL lifetimes, it was found that 1b and 1c exhibited TADF properties, although 1a showed only prompt fluorescence. We successfully fabricated a solution-processed OLED with 1c as an emitter. The device exhibited yellowish green electroluminescence, and the maximum external quantum yield of 3.02% indicated a relatively small contribution of TADF to overall electroluminescence.