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

Abstract

The [Zn(SRaaiNR′)(H2O)X2] (SRaaiNR′=1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole; X=Cl, Br, I) complexes have been characterized by spectroscopic studies. The X-ray structure of [Zn(SEtaaiNEt)(H2O)Cl2] (where SEtaaiNEt=1-ethyl-2-{(o-thioalkyl)phenylazo}imidazole) shows bidentate imidazolyl-N, azo-N chelation of SEtaaiNEt and the other donors are H2O and 2Cl−. UV light irradiation to a MeCN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configurations about –NN–, respectively) isomerisation of the coordinated azoimidazole. The rate of isomerisation follows the sequence: [Zn(SRaaiNR′)(H2O)Cl2]<[Zn(SRaaiNR′)(H2O)Br2]<[Zn(SRaaiNR′)(H2O)I2]. The quantum yields (ϕE→Z) and the activation energies (Ea) of the isomerisation of the complexes are lower than those of the free ligands. The observation has been explained considering the molecular association that increases the mass and rotor volume of the complexes.