A photochromic AgBF4 complex with cis-1,2-dicyano-1,2-bis(2′,4′,5′-trimethyl-3′-thienyl)ethene: Synthesis, structure and photochromism

Abstract

A new coordination compound was isolated by reaction of AgBF4 with cis-1,2-dicyano-1,2-bis(2′,4′,5′-trimethyl-3′-thienyl) ethene (cis-dbe) in benzene with the aim of suppressing cis-trans photo-isomerization of cis-dbe by the steric hindrance from metal coordination. The complex was revealed as a dinuclear structure by single crystal X-ray diffraction, where two Ag(I) ions are bridged by two cis-dbe molecules via their four N atoms of cyano groups. The benzene, employed as crystallized solvent in synthesis, was revealed to coordinate with metal center. The ligand presents as cis configuration and the two thiophene rings adopt anti-parallel conformation with reactive C distance of 3.46Å, which is short enough for the photo-cyclization. These structural features are favorable for the ring-closure reaction. On the other hand, BF4− is existed as counter anion sandwiched between the neighboring dinuclear complexes via extensive intramolecular hydrogen bonds and Van de Waals contacts. Also, strong π-π interactions exist in the complex. These weak interactions are theoretically disadvantageous to the photo-cyclization.As expected, the cis-trans photo-isomerization was successfully suppressed by involving of cis-dbe into coordination bonds and rational selecting of 405nm excitation light as evidenced by UV–Vis spectra and 1H NMR studies. And the reversible ring-closing and ring-opening reactions with color inter-conversion between yellow and red were not prohibited in solution, solid state and PMMA films, suggesting an indistinctive effect of metal coordination on ring closure.