Photoresponsive Ru Complex—Gold Nanoparticle Hybrids for Theranostics: A Theoretical Study of Electronic Structure and Luminescence Based Detection

Photoactivatable nitric oxide donors (photoNORMs) are promising agents for con-trolled NO release and real-time optical tracking in biomedical theranostics. Here, we report a comprehensive density functional theory (DFT) and time-dependent DFT (TDDFT) study on a series of hybrid ruthenium–gold nanocluster systems of the gen-eral formula [(L)Ru(NO)(SH)@Au₂₀], where L = salen, bpb, porphyrin, or phthalocya-nine. Structural and bonding analyses reveal that the Ru–NO bond maintains a formal {RuNO}⁶ configuration with pronounced Ru→π*(NO) backbonding, leading to partial reduction of the NO ligand and an elongated N–O bond. NBO, NEDA, and ETS–NOCV analyses confirm that Ru–NO bonding is dominated by charge-transfer and polariza-tion components, while Ru–S and Au–S linkages exhibit delocalized, donor–acceptor character coupling the molecular chromophore with the metallic cluster. TDDFT re-sults reproduce visible–NIR absorption features arising from mixed metal-to-ligand and cluster-mediated charge-transfer transitions. The calculated zero–zero transition and reorganization energies predict NIR-II emission (1.8–3.8 μm), a region of high bi-omedical transparency, making these systems ideal candidates for luminescence-based NO sensing and therapy. This study establishes fundamental design principles for next-generation Ru-based photoNORMs integrated with plasmonic gold nanoclusters, highlighting their potential as multifunctional, optically trackable theranostic platforms.