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

Abstract

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 = H₂dcbpy = 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.