Tert-Butyltriazine-Diphenylaminocarbazole based TADF materials: π-Bridge modification for enhanced kRISC and efficiency stability

Abstract

A series of novel D-π-A type thermally activated delayed fluorescence (TADF) emitters, namely DTPTCzDP, DTPTCzDP-CN and DTPTCzDP-Py, were developed with 2,4-di-tert-butyl-1,3,5-triazine (DTPT) as electron acceptors and N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-diamine (DAC-II) as donors. The π-bridge was varied from phenylene to cyanophenylene or pyridyl to tune the electronic and luminescence properties. The strong electron-donating DAC-II groups disperse the highest occupied molecular orbital (HOMO) and guarantee the effective separation of frontier molecular orbitals and the TADF feature despite the small dihedral angles. Incorporation of electron-withdrawing cyano or pyridyl on or as the π-bridge enhanced the triazine acceptor strength, not only leading to bathchromatic shift of the TADF, but also reducing the singlet and triplet energy splitting (ΔEST) and increasing the reverse intersystem crossing rate constant (kRISC). Particularly DTPTCzDP-CN exhibited high radiation rate constant kr > 107 s−1 and kRISC>106 s−1 simultaneously. The greenish-blue organic light-emitting diode of DTPTCzDP realized a maximum EQE of 20.13% (53.57 cd A−1) while the pale yellow device of DTPTCzDP-CN exhibited an EQE of 17.81%. All these devices showed slow efficiency roll-off, benefitting from the steric hindrance effect and intrinsic chemical stabilities of the bulky DTPT and DAC-II units in these emitters. Keywords: thermally activated delayed fluorescence (TADF), organic light-emitting diodes (OLEDs), reverse intersystem crossing (RISC), steric hindrance, tert-butyltriazine.