Dithiolate auxiliary ligands as electronic modulators in ruthenium-based sensitizers: a DFT and TD-DFT study

Rational ligand engineering provides an efficient route for tailoring the electronic structure and photoinduced properties of ruthenium-based sensitizers. In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are employed to systematically investigate a series of Ru(ii) dyes with the general formula [Ru(dcbpy)(NCS)₂(L)], where L represents different dithiolate auxiliary ligands (dcdmp, dmit, pdt, and tdt). The impact of ligand electronic characteristics on frontier molecular orbitals, excited-state composition, optical absorption, and interfacial charge-transfer behavior is comprehensively analyzed. The results show that dithiolate ligands induce significant stabilization of the HOMO and narrowing of the HOMO-LUMO gap compared to the reference dye, thereby enhancing low-energy metal-to-ligand and ligand-to-ligand charge-transfer contributions. Among the investigated complexes, Ru-dcdmp exhibits the smallest energy gap (1.83 eV in the gas phase) and the most favorable energetic alignment for spontaneous electron injection into the TiO₂ conduction band. TD-DFT simulations reveal broadened and intensified absorption in the visible and near-infrared regions, with solvent-induced enhancement of oscillator strengths in acetonitrile. Explicit adsorption modeling on a (TiO₂)₈ cluster confirms strong dye-TiO₂ interactions, efficient electronic coupling, and effective charge delocalization from the Ru center toward the TiO₂ surface upon excitation. Most complexes exhibit a bidentate adsorption mode, whereas the Ru-dmit complex adopts a more asymmetric interfacial binding geometry. Overall, this work establishes clear structure-property relationships and highlights dithiolate ligands, particularly dcdmp, as effective electronic modulators for designing high-performance ruthenium sensitizers for dye-sensitized solar cell applications.