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(2)dcbpy = 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) + pi(ppy) -->pi*(bpy) with MLCT and LLCT characters. The calculated phosphorescence at 628 and 668 nm for and is mainly bpy-based intraligand pi* -->pi transition, while 743 and 827 nm emission for and are mixed characters of (3)MLCT and (3)LLCT {[pi*(bpy)] --> [d(Ir) + pi(ppy)]}. Moreover, the reasons for different transition characters and phosphorescence quantum yield between Rh and Ir species, and between and are discussed in this paper.