Engineering of Cyano Functionalized Benzo[ d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes

Jie Yan; Sheng Fu Wang; Chao-Hsien Hsu; Emily Hsue-Chi Shi; Chi-Chi Wu; Pi-Tai Chou; Shek-Man Yiu; Yun Chi; Caifa You; I-Che Peng; Wen-Yi Hung

doi:10.1021/acsami.3c02671

Engineering of Cyano Functionalized Benzo[ d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes

ACS Appl Mater Interfaces. 2023 May 3;15(17):21333-21343. doi: 10.1021/acsami.3c02671. Epub 2023 Apr 19.

Authors

Affiliations

¹ Department of Materials Sciences and Engineering, Department of Chemistry, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon Tong 999077 Hong Kong SAR, China.
² Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.
³ Department of Optoelectronics and Materials Technology, National Taiwan Ocean University, Keelung 20224, Taiwan.

PMID: 37074734
DOI: 10.1021/acsami.3c02671

Abstract

In this study, we designed and synthesized three series of blue emitting homoleptic iridium(III) phosphors bearing 4-cyano-3-methyl-1-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (mfcp), 5-cyano-1-methyl-3-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (ofcp), and 1-(3-(tert-butyl)phenyl)-6-cyano-3-methyl-4-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (5-mfcp) cyclometalates, respectively. These iridium complexes exhibit intense phosphorescence in the high energy region of 435-513 nm in the solution state at RT, to which the relatively large T₁ → S₀ transition dipole moment is beneficial for serving as a pure emitter and an energy donor to the multiresonance thermally activated delayed fluorescence (MR-TADF) terminal emitters via Förster resonance energy transfer (FRET). The resulting OLEDs achieved true blue, narrow bandwidth EL with a max EQE of 16-19% and great suppression of efficiency roll-off with ν-DABNA and t-DABNA. We obtained the FRET efficiency up to 85% using titled Ir(III) phosphors f-Ir(mfcp)₃ and f-Ir(5-mfcp)₃ to achieve true blue narrow bandwidth emission. Importantly, we also provide analysis on the kinetic parameters involved in the energy transfer processes and, accordingly, propose feasible ways to improve the efficiency roll-off caused by the shortened radiative lifetime of hyperphosphorescence.

Keywords: benzonitrile; carbene; hyperphosphorescence; iridium; triplet-to-singlet FRET.