Design of styryl dye single crystals in the presence of low-molecular aromatic compounds and peculiarities of [2+2]-photocycloaddition in these single crystals

A variety of crystal packing motifs for styryl dye Et-Py+-CH=CH-C6H4OMe ClO4− crystallized from solvent mixtures containing different low-molecular aromatic compounds was investigated by X-ray diffraction method. The presence of some of these aromatic compounds was found to affect the mode of the aggregation of the dye structural units in the course of crystal nucleation, regardless of whether these compounds are involved in the growing crystal. The aromatic molecules containing two proton-donating groups can be involved in the crystal packing of the dye as a solvate that forms weak hydrogen bonds with perchlorate anions or other solvate molecules. The aromatic molecules without functional groups (except for benzene) are not involved in the growing crystal as additional building blocks. The syn-head-to-tail stacking with contiguous and antiparallel arrangement of the ethylene bonds is the main packing motif of dye cations in crystals of unsolvated and solvated forms. This is favorable for accomplishing the [2+2]-photocycloaddition (PCA) reaction resulting in the centrosymmetric rctt-isomer of a cyclobutane derivative. In the solvated forms of the crystals, the solvate molecules and perchlorate anions form loose layers between the cation stacks. These layers are capable of leveling internal stresses caused by the PCA reaction, thus promoting this reaction as a single-crystal-to-single-crystal transformation. Other conditions for the PCA reaction to proceed without single crystal degradation are (1) retention of the initial crystal symmetry in the course of the PCA and (2) stack separation into dimeric pairs with d1 < 4.2 Å < d2, where d1 and d2 are the distances between the ethylene bonds within the dimeric pairs and between them, respectively. The regularities found for the crystal packing and the PCA reaction in styryl dye single crystals may be used to design information recording and storage cells.