Photosalient and Liquefaction Behavior of Salt Crystals

Controlling the alignment of olefin-containing molecules within a crystalline lattice is essential not only to drive light-induced solid-state [2+2] cycloaddition reactions but also to achieve macroscopic photomechanical motion and photoinduced crystal-to-liquid transitions. Thereby advancing the development of actuators, light-melt adhesion, and photolithographic technologies. In this study, we designed a series of acid-assisted molecular salts that precisely direct the reactive olefin-pairs into the prerequisite geometry required for [2+2] photoreaction. The neutral 3-ThPy (3-ThPy = E-(3-Thienyl)vinyl-4-pyridine) (1) is photo-inert in the solid state as the olefin-pairs are separated by 6.28 Å exceeding Schmidt's criteria. However, protonation with different acids resulted in the head-to-tail (HT) alignment of the olefin moieties, primarily driven by cation···π interactions. The resulting salts are [(3-ThPy-H)(NO₃)(HNO₃)] (2), [(3-ThPy-H)(HSO₄)·H₂O] (3), and [(3-ThPy-H)(OTf)] (4), which undergoes solid-state [2+2] photoreaction when subjected to UV or sunlight, affording stereoselective rctt-products. Single crystals of 2-4 exhibit distinct photomechanical responses upon UV exposure, manifests as breaking, jumping, melting, and bending, triggered by solid-state [2+2] photoreaction. The dominant non-covalent interactions governing crystal packing and molecular orientation differ among the salts, as revealed by Hirshfeld surface analysis. Furthermore, DFT calculations provide insight into the electronic structures of neutral 3-ThPy, the protonated {3-ThPy-H}⁺ cation, and the dimeric species {3-ptcb-H₂}²⁺.