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

•Zn(II)-1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole complexes are reported.•Structure is confirmed by single crystal X-ray diffraction study.•Photochromism, E-to-Z and Z-to-E, of coordinated ligand is examined.•DFT computation explains the light induced phenomena.

[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.

Graphical abstract[Zn(SRaaiNR′)2X2] (SRaaiNR′, 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole), are structurally characterized as a distorted tetrahedral geometry of ZnN2I2 coordination via two imidazolyl-N from two SEtaaiNEt and two iodides. The UV light irradiation in MeCN solution of [Zn(SRaaiNR′)2X2] 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 [Zn(SRaaiNR′)2Cl2] < [Zn(SRaaiNR′)2Br2] < [Zn(SRaaiNR′)2I2] which implies the molecular association that increases the mass and rotor volume of the complexes. DFT computation of representative complexes and free ligands are used to explain the difference in the rates and quantum yields of photoisomerisation.Figure optionsDownload full-size imageDownload as PowerPoint slide