Photoswitchable molecular pincers: synthesis, self-assembly into sandwich complexes and ion-selective intramolecular [2+2]-photocycloaddition of an unsaturated bis-15-crown-5 ether

Bis-crown-containing styryl dyes (E,E)-2a,b having the structure and properties of molecular pincers were synthesised. In acetonitrile solution, dye (E,E)-2b is able to form intramolecular sandwich complexes with Ca²⁺ and Ba²⁺ cations. These complexes undergo E–Z-photoisomerisation and intramolecular stereoselective [2+2]-photocycloaddition to afford cyclobutane derivatives 6a,b, whose structures were studied by ¹H NMR spectroscopy. Molecular-mechanics and semiempirical quantum-chemical calculations were performed to interpret experimental data.