Unraveling the positive effect of twisted helicene structure on narrowband electroluminescence

Multi-resonance (MR) materials based on 1,4-BN-heteroarenes have attracted extensive attention in recent years for their narrowband electroluminescence. Extending the π-conjugation of MR skeletons is a widely adopted strategy to regulate their emission colors, but it inevitably induces structural distortion and undesirable vibronic couplings, thus broadening the emission bandwidth. Herein, we design and synthesize new MR emitters via π-extension of a classic MR backbone (CzBN) and disclose how the twisted structure plays a positive role in reducing the emission bandwidth. Specifically, π-extension of CzBN to form a [5]helicene substructure (BN-5H) induces serious vibrations, while further extending the helicene moiety to build a [7]helicene substructure (BN-7H) suppresses undesirable vibrations by locking the conformation. As a consequence, BN-7H achieves a smaller full-width at half-maximum (FWHM) of 28 nm compared with BN-5H (33 nm) in organic light-emitting diodes and longer device lifetime. These results overturn the traditional cognition of the detrimental effect of highly twisted structures on narrowband emission and offer a new design concept for the future development of narrowband electroluminescence materials.