Synthesis, structure and photochromism of zinc(II) complexes of alkylthioarylazoimidazoles

The [Zn(SRaaiNR′)(H2O)X2] (SRaaiNR′ = 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole) complexes have been characterized by different spectral data and a single crystal structure in one case. UV light irradiation in acetonitrile solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configuration) isomerisation of the coordinated SRaaiNR′ ligand about the -NN- bond. The rate of isomerisation follow: [Zn(SRaaiNR′)(H2O)Cl2] < [Zn(SRaaiNR′)(H2O)Br2] < [Zn(SRaaiNR′)(H2O)I2]. The quantum yields (ϕE→Z) and activation energies (Ea) of the isomerisation of the complexes are lower than those of the free ligands. The decrease in electronegativity of X increases the rate of E-to-Z photoisomerisation.