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

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.