Structural, Electronic, and Spectroscopic Effects of Ligand Modification in RuCl 2 L 2 Complexes: A Comparative DFT and TD-DFT Study

The nature of the heterocyclic rings in azo ligands plays a crucial role in modulating the physicochemical properties of ruthenium complexes. However, the effects of replacing an azopyridine ligand with an azopyrrole analogue on the structural, thermodynamic, electronic, and photophysical properties of ruthenium complexes remain insufficiently understood. This study investigates the influence of ligand ring modification on the properties of RuCl 2 L 2 complexes, where L denotes either 2-phenylazopyridine (azpy) or 2-cyclopentadienylazopyrrole (azopyrrole). Ground-state geometries were optimized at the B3LYP/LANL2DZ level and characterized by vibrational frequency calculations. Thermodynamic and electronic properties were examined using Density Functional Theory (DFT), Natural Bond Orbital (NBO) analysis, and Time-Dependent DFT (TD-DFT) calculations. The results indicate that ligand substitution has a limited effect on the coordination geometry around the ruthenium center but significantly modifies the electronic distribution within the complexes. Among the three investigated isomers, the α-Cl isomer is predicted to be the most stable in both series. The formation of RuCl 2 (azpy) 2 is thermodynamically favorable, whereas the formation of RuCl 2 (azopyrrole) 2 is unfavorable at 298 K under the computational conditions considered. Frontier molecular orbital analysis reveals predominantly Metal-to-Ligand Charge-Transfer (MLCT) character for the azpy complexes, while the azopyrrole complexes exhibit a stronger Ligand-to-Metal Charge-Transfer (LMCT) contribution. The latter also display lower HOMO–LUMO gaps and chemical hardness values, together with higher electrophilicity indices. TD-DFT calculations predict absorption bands in the visible and near-infrared regions, highlighting ligand modification as a promising strategy for tuning the electronic and photophysical properties of ruthenium complexes.