Ternary molecular switching in a single-crystal optical actuator with correlated crystal strain

A growing portfolio of single-crystal optical actuators is forging a new class of photonic materials that hold prospects for quantum technologies. Ruthenium-based complexes that exhibit this phenomenon via SO₂-linkage photoisomerisation are of particular interest since they display multiple metastable states, once induced by green light; yet, complete photoconversion into each SO₂-isomeric state is rarely achieved. We discover a new complex, trans-[Ru(SO₂)(NH₃)₄(4-bromopyridine)]tosylate₂, that produces 100% photoconverted η¹-OSO isomeric crystal structures at 90 K, which fully transition into η²-(OS)O photoisomers upon warming to 100 K, while the dark-state η¹-SO₂ structure is wholly recovered by heating the crystal to room temperature. Crystal structures and optical-absorption profiles of each state are captured via in-situ light-induced single-crystal X-ray diffraction and optical-absorption spectroscopy. Results show that both photoisomeric species behave as optical switches, but with distinct optical properties. The photoisomerisation process causes thermally-reversible micro- and nanoscopic crystal strain, as characterised by optical microscopy and in-situ light-induced atomic-force microscopy.