Tert-Butyltriazine-Diphenylaminocarbazole based TADF materials: π-Bridge modification for enhanced kRISC and efficiency stability. (August 2022)

• Record Type: Journal Article
• Authors: Shi, Chunlong; Liu, Di; Li, Jiuyan; He, Zhaolong; Song, Kai; Liu, Botao; Wu, Qi; Xu, Min
• Is Part Of: Dyes and pigments. Volume 204(2022)
• Publication Date: 2022-08
• Subjects: (TADF) thermally activated delayed fluorescence -- (OLEDs) organic light-emitting diodes -- (RISC) reverse intersystem crossing; Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2022.110430
• Languages: English
• ISSNs: 0143-7208

Abstract

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 N 3, N 3, N 6, N 6 -tetraphenyl-9 H -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 (Δ E ST ) and increasing the reverse intersystem crossing rate constant ( k RISC ). Particularly DTPTCzDP-CN exhibited high radiation rate constant k r > 10 7 s −1 and k RISC >10 6 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. Graphical abstract: Incorporating cyano group on the phenylene bridge of the thermally activated delayed fluorescence emitter containing tert-butyltriazine acceptor and diphenylaminocarbazole donor reduces the Δ E ST and realizes high k RISC > 10 6 S −1 and k R > 10 7 S −1 simultaneously, finally improves the efficiency stability of OLEDs. Image 1 Highlights: π-Bridge modification by incorporating cyano or pyridyl enhanced TADF feature through reducing Δ E ST and increasing k RISC to 10 6 s −1 . Their OLEDs exhibited EQEs of over 20% and slow efficiency roll-off based on the steric hindrance and stabilities of the bulky tert -butyl and diphenylamino groups. Utilization of strong electron-denoting diphenylaminocarbazole donors is proved as effective method to design TADF emitters even with small dihedral angles.