Supramolecular assemblies of photochromic benzodithia-18-crown-6 ethers in crystals, solutions, and monolayers

Abstract

We studied the assembly of dithiacrown ether styryl dye (CSD) molecules in crystals, solutions, and films in the presence of metal cations. X-Ray diffraction data allowed us to conclude that the anion affects the supramolecular architecture of CSDs in the crystal, specifically, the type of stacking of the dye molecules. In solution, in the presence of Pb²⁺, CSD molecules with the betaine structure spontaneously form dimeric complexes consisting of two dye molecules and two metal cations, with a fixed mutual arrangement of the double bonds. The dimer complex is stable due to coordination between the anion substituent of one molecule and the metal cation located in the crown ether cavity of the other molecule. Irradiation of the dimer complexes leads to regio- and stereoselective [2 + 2]-cycloaddition, giving only one cyclobutane derivative of the eleven theoretically possible products. The other photoreaction studied for CSDs is reversible Z–E isomerization. Due to its specific structure, the betaine-type CSD is able to form the ‘anion-capped’ Z-isomer. Intramolecular coordination in the ‘anion-capped’ isomer enhances its stability and causes a sharp deceleration of its dark Z–E isomerization. The amphiphilic CSD forms relatively stable monolayers on distilled water and various aqueous salt subphases. The results obtained indicate that it is possible to distinguish between two types of the dye monolayer structures based on the presence of alkali or heavy metal cations in the aqueous subphase.