Pseudodimeric complexes of 4-styrylpyridine derivatives: Structure–property relationships and a stereospecific [2+2]-cross-photocycloaddition in solution

The study addresses the spectral and photochemical properties of N-(3-ammoniopropyl)-4-styrylpyridinium diperchlorate and its substituted analogs containing either electron-donating (OMe, SMe, NMe2) or electron-withdrawing (NO2, Cl) groups on the benzene ring. These styryl dyes in MeCN form pseudodimeric complexes with uncharged 18-crown-6-containing 4-styrylpyridine due to hydrogen bonding between the ammonium group and the crown ether oxygen atoms. The stability constants of the complexes were determined by spectrophotometric and 1H NMR titration methods. Owing to complexation, the dyes containing OMe or SMe groups and the dye with unsubstituted benzene ring undergo a stereospecific [2 + 2]-cross-photocycloaddition to the crown-containing styrylpyridine to give unsymmetrical cyclobutane derivatives as single rctt isomers. The structure of cyclobutanes was confirmed by X-ray diffraction analysis. The most probable conformations of the pseudodimeric complexes in MeCN were determined by density functional theory calculations. The cross-photocycloaddition quantum yields, measured upon selective excitation of the styryl dye, and other relevant data suggested that the barrier for this photoreaction increases with an increase in the reorganization energy of the singlet excited state of the dye.