1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole complexes of mercury(II) and their photochromic properties

Abstract

The reaction of HgX2 (X=Cl, Br, I) with 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole (SRaaiNR′) in MeOH-ethyleneglycol mixture has synthesised [Hg(SRaaiNR′)X2]. The structure of the products has been established by microanalytical and spectral (IR, UV–Vis, 1H NMR) data, and a distorted Td-geometry has been confirmed by a single crystal X-ray diffraction study of [Hg(SMeaaiNEt)I2] (SMeaaiNEt=1-ethyl-2-{(o-thiomethyl)phenylazo}imidazole). The UV light irradiation in CH3CN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configurations) isomerisation of the coordinated SRaaiNR′ ligand about the –NN– bond. The rate of isomerisation follows: [Hg(SRaaiNR′)Cl2]<[Hg(SRaaiNR′)Br2]<[Hg(SRaaiNR′)I2]. The quantum yields (ϕE→Z) of the isomerisation of the complexes are lower than those of the free ligands. These may be due to increased mass and rotor volume of the complexes. The electronegativity sequence of X (I<Br<Cl) may regulate the molecular association and hence the effective mass of the rotor and the photoisomerisation rates. Thermal isomerisation determines the rate, activation energy (Ea), activation enthalpy (ΔH∗) and activation entropy (ΔS∗) of the Z→E transformation of the coordinated SRaaiNR′. The Eas of the thermal isomerisation of the complexes are lower than those of the free ligands.