Ligand Engineering Controls Near-Infrared Emission, Quantum Efficiency, and Photoluminescence Lifetimes in Dimetallic Ru(II) Complexes

Abstract The exceptional photophysical and electrochemical properties of Ru(II) complexes support broad applications in optoelectronic functional materials and bioimaging, particularly those Ru(II) complexes which show near-infrared (NIR) luminescence. This study reports ligands (L1–L6) and Ru(II) complexes (C1–C6) with methyl substituents, numbers of nitrogen atoms in the heterocyclic rings, and phenylene-bridge topology (meta versus para) that regulated their photophysical and electrochemical properties. Pairwise structure–property analysis revealed that these structural variables do not act in a simply additive manner. Pyridyl-to-pyrimidinyl replacement primarily lowered the MLCT-state energy and red-shifted the emission, whereas methyl substitution improved photoluminescence efficiency. The para-phenylene bridge in pyrimidinyl-containing frameworks, where it enhanced the quantum yield and emission lifetime. C6 exhibited the most red-shifted emission at 714 nm, whereas C4 showed the best overall photophysical performance, with NIR emission at 702 nm, a photoluminescence quantum yield of 59.4%, and an emission lifetime of 155.38 ns. The structure and photophysical behavior of C1–C6 were further explored computationally by density functional theory (DFT) calculations. Overall, these results demonstrated that ligand engineering effectively regulates NIR emission, quantum efficiency, excited-state lifetime, and electrochemical behavior, providing guidance for the design of high-performance Ru(II)-based functional materials.