Hydrogen-Bonded Assembly and Solid-State [2 + 2] Photodimerization of a BPE–Citric Acid Salt

A previously reported crystalline assembly of the γ-form of trans-1,2-bis(4-pyridyl)ethylene (BPE) with citric acid (CA) was investigated under UV irradiation to examine its structural evolution and solid-state [2 + 2] photodimerization. Single-crystal X-ray diffraction was used to compare the BPE-CA (1) crystal before irradiation and after 30 min and a total of 2 h of irradiation. Structure 1 is a proton-transfer salt in which charge-assisted hydrogen bonding organizes the BPE components into parallel two-dimensional assemblies containing several offset olefin pairs. Selected neighboring C=C bonds satisfy the geometric requirements for topochemical photodimerization. Following irradiation, single-crystal diffraction data could still be collected with the same space group and closely similar unit-cell parameters. Complementary irradiation of ground crystals for 24 h, followed by dissolution and 1H NMR analysis in DMSO-d6, confirmed the quantitative formation of the two cyclobutane photodimers with rctt-tpcb as the major product. A separate photodimeric proton-transfer salt, Structure 3, was also identified crystallographically in crystals obtained under ambient laboratory illumination. Its photodimer topology is related to that of the previously reported α-form photoproduct, although its proton-transfer state distinguishes it from the reported neutral cocrystal. These findings show that proton transfer and hydrogen-bond-directed organization by CA govern the relative arrangement of the BPE olefins and promote solid-state [2 + 2] photodimerization. The crystallographic results further demonstrate that the lattice retains its single-crystalline character during UV-induced structural reorganization.