Photoconversion bonding mechanism in ruthenium sulfur dioxide linkage photoisomers revealed by in situ diffraction

Three new ruthenium-sulfur dioxide linkage photoisomeric complexes in the [Ru(NH(3))(4)(SO(2))X]Cl(2)·H(2)O 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(2) coordination to the ruthenium ion. Solid-state metastable η(1)-O-bound (MS1) and η(2)-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(bound) electrons into the Ru dσ-orbital rather than π-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.