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

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 <b>1</b>, photoexcitation populates an antibonding <i>d</i>_<i>yz</i>-<i>p</i>_<i>y</i> orbital, weakening the [Ru]-S bond and lowering the SO₂ photoisomerization barrier; in <b>2</b>−<b>4</b>, 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 <b>1</b>.

Response charge-density analyses visualize the corresponding photoinduced charge redistribution in real space, consistent with the finer ordering of the PCE: <b>3</b> < <b>2</b> < <b>4</b>. We additionally propose the Ru <i>d</i>-band center as a descriptor for PCE: within a common class of <i>trans</i>-ligand, the PCE tends to decrease as the <i>d</i>-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.