Dihalo-bis[1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole]zinc(II): Structure, photochromism and DFT computation

Abstract

[Zn(SRaaiNR′)2X2] (SRaaiNR′, 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole; R=Me, Et; R′=Me, Et) has been characterized by different spectroscopic studies. The single crystal X-ray structure of [Zn(SMeaaiNEt)2I2] (where SMeaaiNEt=1-ethyl-2-{(o-thiomethyl)phenylazo}imidazole) shows distorted tetrahedral geometry of ZnN2I2 coordination sphere of two imidazolyl-N from two SMeaaiNEt and two iodides. The UV light irradiation in MeCN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configuration) of SRaaiNR′ about –NN–, respectively. Quantum yields (ϕE→Z) and the activation energy (Ea) of the isomerization of the complexes are lower than that of free ligand data. The rate of isomerisation follows [Zn(SRaaiNR′)2Cl2]<[Zn(SRaaiNR′)2Br2]<[Zn(SRaaiNR′)2I2]. The observation has been explained considering the molecular association that increases the effective mass and rotor volume of the complexes with electronegativity of X. The DFT computed results of optimized geometry of representative complexes are used to explain the difference in the rates and quantum yields of photoisomerisation.