Different electronic structures and spectroscopic properties of cationic [M(ppy)₂(N^∧N)]⁺ (M = Rh, Ir; N ^∧N = Hcmbpy, H₂dcbpy), a DFT study

We report a comparative quantum-chemical study of the electronic structures and optical properties of a series of cationic complexes [M(ppy) ₂(N^∧N)]⁺ (N^∧N = Hcmbpy = 4-carboxy-4′-methylbpy, M = Rh (Rh1), Ir(Ir2); N^∧N = H ₂dcbpy = 4,4′-dicarboxy-bpy, M = Rh (Rh3) and Ir (Ir4)). The theoretical calculation reveals that the increased number of -COOH groups on the bpy ligand can decrease the energy levels of LUMO more than HOMO and narrow down the HOMO-LUMO energy gaps, which results in the red-shifted of the lowest-lying absorption and phosphorescent spectra. The lowest-lying singlet absorptions were categorized as d(M,M = Rh or Ir) + π(ppy) →π*(bpy) with MLCT and LLCT characters. The calculated phosphorescence at 628 and 668 nm for Rh1 and Rh3 is mainly bpy-based intraligand π* →π transition, while 743 and 827 nm emission for Ir2 and Ir4 are mixed characters of ³MLCT and ³LLCT {[π*(bpy)] → [d(Ir) + π(ppy)]}. Moreover, the reasons for different transition characters and phosphorescence quantum yield between Rh and Ir species, and between Ir2 and Ir4 are discussed in this paper.