Dark- and Photoexcited-State Electronic Structures of [Ru(SO2)(NH3)4X]Y Complexes and Their Relation to SO2 Photoconversion Efficiency

Abstract Single crystals of the [Ru(SO2)(NH3)4X]Y family of complexes are promising photoswitches or photoactuators; they respond structurally to visible light, undergoing SO2 linkage photoisomerization with high spatiotemporal precision. However, the SO2 photoconversion efficiency (PCE) varies markedly with the trans-ligand X and no unifying molecular design principle is yet available. This study addresses this knowledge gap by performing electronic-structure calculations on 11 respective crystal structures of these complexes using reference crystal structures characterized by single-crystal X-ray diffraction under light irradiation. We focus on 4 structural archetypes of [Ru(SO2)(NH3)4X][tosylate]2, where X = H2O (1), 3-F-pyridine (2), 3-Cl-pyridine (3), and 3-Br-pyridine (4), to elucidate how the trans-ligands control the Ru-centered electronic rearrangement upon photoexcitation. The dark-state projected densities of states of these crystal structures are qualitatively similar across the series, exhibiting [Ru]-S π-bonding and [Ru]-X π/σ interactions. By contrast, electronic-structure calculations using time-dependent density functional theory reveal distinct photoexcited-state behavior: in 1, photoexcitation populates an antibonding dyz-py orbital, weakening the [Ru]-S bond and lowering the SO2 photoisomerization barrier; in 2−4, photoexcitation simultaneously populates this antibonding orbital and depopulates its bonding counterpart, weakening the [Ru]-S bond substantially more to afford a correspondingly higher PCE than in 1. Response charge-density analyses visualize the corresponding photoinduced charge redistribution in real space, consistent with the finer ordering of the PCE: 3 < 2 < 4. We additionally propose the Ru d-band center as a descriptor for PCE: within a common class of trans-ligand, the PCE tends to decrease as the d-band center shifts toward the Fermi level, as shown here for ten pyridyl-ligated structures, although other factors also contribute to the photoisomerization process. Together, these results provide a mechanistic account of photoexcited-state changes in the electronic structure of [Ru(SO2)(NH3)4X]Y complexes and suggest a practical screening metric for the rational molecular design of this family of high-PCE photoresponsive single crystals for quantum photosensing applications.