Isomeric thermally activated delayed fluorescence emitters based on indolo[2,3-b]acridine electron-donor: a compromising optimization for efficient orange-red organic light-emitting diodesElectronic supplementary information (ESI) available. See DOI: 10.1039/c8tc06081a

• Main Authors: Chen, Jia-Xiong, Tao, Wen-Wen, Xiao, Ya-Fang, Tian, Shuang, Chen, Wen-Cheng, Wang, Kai, Yu, Jia, Geng, Feng-Xia, Zhang, Xiao-Hong, Lee, Chun-Sing
• Format: Journal Article
• Language: English
• Published: 07.03.2019
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/c8tc06081a

Energy differences (Δ E ST s) between the lowest singlet and triplet excited states as well as the oscillator strengths ( f s) are two contradictory factors controlling performance of thermally activated delayed fluorescence (TADF) emitters. To figure out their optimum balance, we developed two orange-red isomeric TADF emitters 12-(13,13-dimethyl-5-phenyl-5,13-dihydro-11 H -indolo[2,3- b ]acridin-11-yl)dibenzo[ a , c ]dipyrido[3,2- h :2′,3′- j ]phenazine ( IDAC-BPPZ ) and 12-(13,13-dimethyl-11-phenyl-11,13-dihydro-5 H -indolo[2,3- b ]acridin-5-yl)dibenzo[ a , c ]dipyrido[3,2- h :2′,3′- j ]phenazine ( ACID-BPPZ ), which are constructed with the same electron-donor (D) and the same electron-acceptor (A) cores via different connecting sites. ACID-BPPZ has a weaker D-A conjugation and realizes an extremely small Δ E ST of 0.02 eV, but its f is also very small (0.001). These factors are better compromised in IDAC-BPPZ which has a more conjugated D-A connections. This successfully realizes a much higher f of 0.068 and a reasonably small Δ E ST of 0.07 eV. As a result, the orange-red OLEDs using IDAC-BPPZ as emitter achieve a maximum external quantum efficiency 18.3% comparing to that of 14.7% in the ACID-BPPZ -based device. The results suggest that a balance between Δ E ST and f is important for developing high performance TADF emitters. A combination of small Δ E ST and higher f in an orange-red TADF emitter results in better performances.