Crystal Design of Photodimerization and Proton Dynamics in Stilbene Dicarboxylate Salts

Solid-state [2 + 2] photodimerization reaction of stilbene derivatives has been utilized for the preparation of functional organic materials, which occurs at a confined structural condition of the nearest-neighboring parallel −C═C– bond arrangement less than 4.2 Å. Therefore, the crystal engineering approach to control the molecular arrangement is an essential point of view for the success of an ideal solid-state photoreaction. Simple stilbene dicarboxylic acid (H2SDC) and alkylamines (CnH2n+1NH2) were combined to form single crystals of (CnH2n+1NH3+)(HSDC–), CnHSDC (n = 1 and 2) and (CnH2n+1NH3+)2(SDC2–), and CnSDC (n = 1 and 3–6) to control the molecular arrangement of photoreactive stilbene −C═C– units. Among them, single crystals of C1HSDC, C1SDC, and C3SDC resulted in the suitable −C═C– arrangement for the [2 + 2] photodimerization. Among them, formed a quantitative [2 + 2] photodimerized product of the cyclization reaction after the 120 min photoirradiation with 16 mW cm–2. On the contrary, the single crystals of C2HSDC obtained by crystallization from DMF formed relatively strong one-dimensional (1D) O–H+···O hydrogen-bonding (HSDC–)∞ chain with an O···O distance around ∼2.4 Å, which showed a structural phase transition at 210 K based on the differential scanning calorimetry, temperature-variable X-ray structural analyses, and dielectric spectra. The conformational and proton dynamics of HSDC– in the hydrogen-bonding chains are a driving force to occur the structural phase transition.