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

Abstract

We designed and synthesized four diiridium Janus carbene complexes, namely, Ir₂(Js1)(L_BM)₄, Ir₂(Js2)(L_BM)₄, Ir₂(Js3)(L_BM)₄ and Ir₂(Js3)(L_IM)₄, with Janus pro-chelates (Js1H₄)²⁺, (Js2H₄)²⁺ and (Js3H₄)²⁺, representing 1,7-dimethyl-3,5-diphenyl-3,5-dihydrobenzo[1,2-d:4,5-d‘]diimidazole-1,7-diium, 3,5-bis(3-(tert-butyl)phenyl)-1,7-dimethyl-3,5-dihydrobenzo[1,2-d:4,5-d′]diimidazole-1,7-diium and 1,7-bis(4-(tert-butyl)phenyl)-3,5-diphenyl-3,5-dihydrodiimidazo[4,5-b:4′,5′-e]pyridine-1,7-diium, together with bidentate pro-chelates (L_BMH₂)⁺ and (L_IMH₂)⁺, indicating 1,3-bis(4-(tert-butyl)phenyl)-5,6-dimethyl-1H-benzo[d]imidazol-3-ium and 1,3-bis(3-(tert-butyl)phenyl)-1H-imidazol-3-ium, respectively. These Janus complexes exhibited structured emission in degassed toluene at RT, while conversion from Ir₂(Js1)(L_BM)₄ and Ir₂(Js2)(L_BM)₄ to Ir₂(Js3)(L_BM)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 (k_r) of 4.1–5.3 × 10⁴ s⁻¹, which were greatly deviated from that of their mononuclear counterparts f-Ir(L_BM)₃ and f-Ir(L_BM)₂(L_BI) with PLQY of 4.7% and 4.6% and k_r of 8.8 and 21.3 × 10⁵ s⁻¹, respectively, where pro-chelate (L_BIH₂)⁺ stands for 5,6-dimethyl-1,3-di-(tert-butyl)phenyl-1H-benzo[d]imidazol-3-ium. These photophysical characteristics were verified using TD-DFT and spin–orbit coupling (SOC)-TDDFT calculations.