The reasons for ligand-dependent quantum yields and absorption spectrum of four Polypyridylruthenium(II) complexes with a tetrazolate-based ligand: TDDFT study

The quantum yield, lifetime, and absorption spectrum of four [Ru(bpy) ₂L]⁺ [where bpy is 2,2'-bipyridyl; L is represented by the deprotonated form of 2-(1H-tetrazol-5-yl)pyridine (L1) or 2-(1H-tetrazol-5-y1) pyrazine (L2)], as well, as their methylated complexes [Ru(bpy) ₂LMe]²⁺ (RuL1Me and RuL2Me) are closely ligand dependent. In this paper, density functional theory (DFT) and time-dependent DFT (TDDFT) were performed to compare the above properties among these complexes. The calculated results reveal that the replacement of pyridine by pyrazine or the attachment: of a CH₃ group to the tetrazolate ring greatly increases the π-accepting ability of the ancillary ligands. The stronger π-accepting ability of the ancillary ligands in RuL2 results in stronger interaction with polar solvents and a relaxed geometrical structure; these are the main reasons for the lowest quantum yields and the shortest lifetime, The electron-donating CH₃ group buffers the interaction between RuL2Me and the solvent, and together with the enhanced MLCT participation, these account for the highest quantum yields and the longest lifetime, This theoretical contribution allows the factors determining the efficiency of radiative and nonradiative decay pathways in this series of complexes to be rationalized and also allows design of new ruthenium(II) complexes with higher phosphorescence efficiency.