Influence of the anion nature on styryl dye crystal packing and feasibility of the direct and back [2 + 2] photocycloaddition reactions without single crystal degradation

Abstract

A family of styryl dyes of the 4-pyridine series, Et–Py⁺–CHCH–C₆H₃(OMe)₂ X⁻, with different anions X⁻, was synthesized. Their ability to undergo [2 + 2] photocycloaddition (PCA) in the polycrystalline state with the formation of 1,2,3,4-tetrasubstituted rctt cyclobutanes under irradiation with visible light was investigated by ¹H NMR spectroscopy. Crystal packing of the dyes and feasibility for the PCA reaction in single crystals were investigated by X-ray structural analysis. Small inorganic anions, such as ClO₄⁻, BF₄⁻, PF₆⁻ that do not form secondary interactions with the organic cation, aid the formation of cation stacking motifs of the “head-to-tail” type. In these stacks, any adjacent cations are arranged in an antiparallel fashion and approach one another at distances shorter than 4.2 Å, which corresponds to the Schmidt's rule for feasibility of PCA reaction in the solid state. As a result of PCA, rctt isomers of cyclobutane derivatives are formed. Success of the PCA reaction without single crystal degradation was only found for the dye containing BF₄⁻ anion. Upon irradiation with UV light of the cyclobutane product formed in the crystal, back phototransformation without single crystal degradation was observed. A number of weak interactions I⁻⋯H–C in the dye with I⁻ anion prevent significant atomic displacements that accompany the PCA transformation from being achieved. Occurrence of the bulky tosylate anion forming a set of hydrogen bonds with solvate acetonitrile and water molecules aids the formation of “head-to-head” type stacks, with a large separation between the adjacent ethylene bonds. In the dye containing bulky BPh₄⁻ anion, the main structure-forming role belongs to anions, which suppress the cation trend to form a stacking packing motif. The dye with picrate anion forms two types of crystals. Crystal packing of one of them excludes any PCA transformation, whereas, the other one permits it, but with single crystal degradation.