Janus-type diiridium complexes with functional carbene cyclometalates for high-energy phosphorescence

We designed and synthesized four diiridium Janus carbene complexes, namely, Ir2(Js1)(LBM)4, Ir2(Js2)(LBM)4, Ir2(Js3)(LBM)4 and Ir2(Js3)(LIM)4, with Janus pro-chelates (Js1H₄)²⁺, (Js2H₄)²⁺ and (Js3H₄)²⁺, representing 1,7-dimethyl-3,5-diphenyl-3,5-dihydrobenzo[1,2-<i>d</i>:4,5-<i>d</i>']diimidazole-1,7-diium, 3,5-bis(3-(<i>tert</i>-butyl)phenyl)-1,7-dimethyl-3,5-dihydrobenzo[1,2-<i>d</i>:4,5-<i>d</i>']diimidazole-1,7-diium and 1,7-bis(4-(<i>tert</i>-butyl)phenyl)-3,5-diphenyl-3,5-dihydrodiimidazo[4,5-<i>b</i>:4',5'-<i>e</i>]pyridine-1,7-diium, together with bidentate pro-chelates (LBMH₂)⁺ and (LIMH₂)⁺, indicating 1,3-bis(4-(<i>tert</i>-butyl)phenyl)-5,6-dimethyl-1<i>H</i>-benzo[<i>d</i>]imidazol-3-ium and 1,3-bis(3-(<i>tert</i>-butyl)phenyl)-1<i>H</i>-imidazol-3-ium, respectively. These Janus complexes exhibited structured emission in degassed toluene at RT, while conversion from Ir2(Js1)(LBM)4 and Ir2(Js2)(LBM)4 to Ir2(Js3)(LBM)4 caused a notable red-shift in their first emission peak maximum, from 422 and 423 nm to 449 nm, which could be attributed to the stabilization of the ππ* transition gap governed by substituting the central benzo- with a pyrido-bridge in the Janus carbene chelates. They exhibited high photoluminescent quantum yields (PLQYs) of 44%-78% and slow radiative rate constants (<i>k</i>_r) of 4.1-5.3 × 10⁴ s^-1, which were greatly deviated from that of their mononuclear counterparts f-Ir(LBM)3 and f-Ir(LBM)2(LBI) with PLQY of 4.7% and 4.6% and <i>k</i>_r of 8.8 and 21.3 × 10⁵ s^-1, respectively, where pro-chelate (LBIH₂)⁺ stands for 5,6-dimethyl-1,3-di-(<i>tert</i>-butyl)phenyl-1H-benzo[<i>d</i>]imidazol-3-ium. These photophysical characteristics were verified using TD-DFT and spin-orbit coupling (SOC)-TDDFT calculations.