Quantifying Crystallographically Independent Optical Switching Dynamics in Ru SO2 Photoisomers via Lock-and-Key Crystalline Environment

The photophysical properties associated with solid-state Ru-based SO2 linkage photoisomerism are shown to differ with single molecule recognition; this stands to afford superior optical resolution for data storage applications. Two compounds, [Ru(NH3)(4)SO2X]-tosylate(2) (X = isonicotinamide (1) and isonicotinic acid (2)), yield crystal structures, each with two chemically identical but crystallographically distinct Ru-based complexes (Ru01 and Ru02) and thus two different photoisomerizable SO2 environments. It was found that the SO2 photoconversion fraction in each crystallographically independent complex differed by over 20% (for 1: 51.0(12) % in Ru01 and 28.6(9) % in Ru02). These photophysical differences between neighboring molecules were attributed to the larger free volume around the ground-state SO2 in Ru01, allowing for higher photoconversion, i.e., a lock-and-key environment controls the photochemistry. Furthermore, the eta(2)-side-S,O-bound (MS2) metastable photoisomer in Ru01 was 20 K more thermally stable in both 1 and 2; photoinduced intermolecular interactions are shown to dictate this thermal stability.