Single-Crystal Optical Actuation Generated by 100% SO₂ Linkage Photoisomerization in a Ruthenium-Based Coordination Complex

Single-crystal optical actuators are emerging as a new field of materials chemistry because of their wide-ranging potential applications, from light-induced molecular motors to photosensing technologies. Ruthenium-based coordination complexes that contain sulfur dioxide linkage photoisomers have shown particular promise as optical actuators, given that they may exhibit either optical switching or nano-optomechanical transduction in their single-crystal form. The type of single-crystal optical actuation observed in a specific compound within this family of complexes depends upon the nature of the ligand that lies trans to this SO₂ linkage photoisomer, since this governs the type and extent of photoisomer (η²-(OS)O or η¹-OSO) that will form upon the application of light. We report the discovery of a new complex, trans-[Ru­(SO₂)­(NH₃)₄(3-iodo­pyridine)]­tosylate₂ (1), which forms an η¹-OSO photoisomer with 100% photoconversion upon the application of 505 nm light. The photoisomerization process in the ruthenium-based cation of 1 stimulates rotation and translation of the toluenic constituent of its neighboring anion, thereby affording nano-optomechnical transduction. We show that this η¹-OSO photoisomer transitions to its more thermally stable η²-(OS)­O photoisomer with an activation energy, E_a, of 11(2) kJ/mol using thermally activated single-crystal optical absorption spectroscopy. The application of external light with different wavelengths to 1 is also shown to cause a variation in its optical absorption spectral characteristics. This suggests that the photophysical properties of 1 may be tunable with light.