Cadmium(II)-imidazolylazo dyes: Synthesis, structure and photochromism

Abstract

1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole acts as imidazolyl-N donor ligand to Cd(II) although they have potential three donor centres namely N(imidazole), N(azo) and –SR. Two series of complexes [Cd(SRaaiNR/)2X2] (X=Cl, Br, I) and [Cd(SRaaiNR/)4](ClO4)2 are synthesized with these ligands. These complexes are spectroscopically (IR, UV–Vis, 1H NMR) characterised and, the single crystal X-ray structures of [Cd(SEtaaiNEt)2I2] and [Cd(SEtaaiNEt)4](ClO4)2 (where SEtaaiNEt=1-ethyl-2-{(o-thioalkyl)phenylazo}imidazole) have confirmed the distorted tetrahedral structures. The UV light irradiation in MeCN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configuration about –NN–, respectively) isomerisation of the coordinated azoimidazole. The rate of isomerisation follows the sequence : [Cd(SRaaiNR/)4](ClO4)2<[Cd(SRaaiNR/)2Cl2]<[Cd(SRaaiNR/)2Br2]<[Cd(SRaaiNR/)2I2]. Quantum yields (ϕE→Z) and the activation energy (Ea) of the isomerisation of the complexes are lower than that of free ligand data. The observation has been explained considering the molecular association that increases the mass and rotor volume of the complexes.