Congestion effect of an annulated tetracyclic thiophene-containing fragment on 18-crown-6 ether: manifestation in complex formation in solution and membrane transfer properties

Abstract

Two novel macrocyclic compounds with a crown ether moiety annulated to a tetracyclic thiophene-containing aromatic fragment were synthesized by the Mallory–Katz photocyclization reaction from a single stilbene precursor. These compounds were produced by cyclization of its two conformers and differed in the degree of steric hindrance exerted by the tetracyclic fragment on the crown ether. Their complexation with ammonium and barium perchlorates was studied both in solution by NMR, UV-vis, and fluorescence spectroscopy, and in PVC membrane by crown ether-facilitated ion transfer between the aqueous and membrane phases. In solution, the congestion of the crown ether is manifested by different responses of the spectra of the two isomers to the addition of salts. Also, the sterically hindered isomer exhibits weaker binding to the studied ions than the unhindered one. Besides, congested isomer exhibit larger decrease of the diffusion coefficient on complexation. In PVC membrane, the observed binding constants are higher, although the difference between the congested and uncongested isomers is less pronounced. Analysis of possible conformations by the MMFF94 force field and DFT calculations was made to account for features of NMR spectral changes upon complexation.