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]­Cl2·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 η1-O-bound (MS1) and
η2-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–Obound 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.