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(NH3)(4)(SO2)X]Cl-2 center dot H2O 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 SO2 coordination to the ruthenium ion. Solid-state metastable eta(1)-O-bound (MS1) and eta(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 SO2 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 d sigma-orbital rather than pi-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.