| Sumario: | Three new ruthenium-sulfur dioxide linkage photoisomeric complexes in the [Ru(NH)(SO)X]Cl∙HO family (X = pyridine (1); 3-chloropyridine (2); 4-chloropyridine (3)) have been developed in order to examine the effects of the trans-ligand on the nature of the photo-induced SO coordination to the ruthenium ion. Solid-state metastable η-O-bound (MS1) and η-side S,O-bound (MS2) photoisomers are crystallographically resolved by probing a light-induced crystal with in situ diffraction. This so-called photocrystallography reveals the highest known photoconversion fraction of 58(3)% (in 1) for any solid-state SO(2) linkage photoisomer. The decay of this MS1 into the MS2 state was modeled via first-order kinetics with a non-zero asymptote. Furthermore, the MS2 decay kinetics of the three compounds were examined according to their systematically varying trans-ligand X; this offers the first experimental evidence that the MS2 state is primarily stabilized by donation from the S∔O electrons into the Ru dσ-orbital rather than σ-backbonding as previously envisaged. This has important consequences for the optoelectronic application of these materials since this establishes, for the first time, a design protocol that will enable one to control their photoconversion levels.
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