Electron-withdrawing bulky group substituted carbazoles for blue TADF emitters: Simultaneous improvement of blue color purity and RISC rate constants.

Carbazole is a weak electron donor suitable to construct blue thermally activated delayed fluorescence (TADF) emitters, but the less repulsion effect of its five-member-ring structure can't guarantee the frontier molecular orbital separation and TADF feature. A group of carbazole-π-triazine based blue fluorescence emitters is designed by introducing the steric hindrance groups with different electronic properties at the donor or π-bridge. When electron-withdrawing pyridyl with moderate steric hindrance is introduced to the 1-site of 3,6-Di-tert-butyl-9H-carbazole, PyBuCz-TRZ and PyBuCz-MeTRZ gain the blue-shifted TADF. The observed charge transfer triplet excited state within the donor (3CT D) between carbazole and pyridyl units has different spatial orbital angular momentum and allowed spin-orbit coupling (SOC) with the charge transfer singlet excited state (1CT) of the whole molecule from carbazole donor to triazine acceptor, thus, acting as the media state to facilitate efficient multi-channel reverse intersystem crossing (RISC) process. This is the first time to introduce an electron-withdrawing group into the 1-position of carbazole donors to construct TADF molecules, not only achieving the blue-shifted TADF at 455 nm but also improving the external quantum efficiency to 15.3% in organic light-emitting diodes. This donor strategy should be valid to design more blue TADF materials. A donor design strategy for blue TADF is developed by incorporating electron-withdrawing and bulky pyridyl at 1-site of carbazole, not only leading to blue-shift of TADF but also facilitating multi-channel reverse intersystem crossing via the participation of the charge transfer triplet state of the donor (3CT D). An EQE of 15.3% is achieved in pure blue OLED of these TADF emitters. [Display omitted] • Electron-withdrawing pyridyl was introduced at 1-site of carbazole to construct thermally activated delayed fluorescence (TADF) emitters. • Blue shifted emission and improved EQE were simultaneously achieved in the pure blue OLEDs. • A new triplet state 3CT D was detected within the donor and facilitated multi-channel RISC process. • Too strong electron-withdrawing substituent on carbazole like cyano was proved to make TADF feature disappear. • This is the first report to incorporate electron-withdrawing substituent at 1-site of carbazole for blue TADF materials. [ABSTRACT FROM AUTHOR]