Structure and photochromism of Zn(II) thiocyanato complexes of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole

Zinc(II)-thiocyanato complexes of 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole), [Zn(SRaaiNR/)(SCN)(μ-SCN)2]n, are spectroscopically characterized. The single crystal X-ray structure of [Zn(SMeaaiNEt)(μ-SCN)(SCN)2]n shows μ-SCN 1D chain followed by hydrogen bonded 3D supramolecular structure. 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: [Zn(SEtaaiNEt)(SCN)(μ-SCN)2] < [Zn(SMeaaiNEt)(SCN)(μ-SCN)2] ∼ [Zn(SEtaaiNMe)(SCN)(μ-SCN)2] < [Zn(SMeaaiNMe)(SCN)(μ-SCN)2]. Quantum yields (ϕE→Z) and the activation energy (Ea) of the photoisomerisation of the complexes are less than that of free ligand data. This is explained on considering the increase in effective rotor volume of the complexes (complex/rotor volume (cm3)/E-to-Z isomerisation rate (s−1): [Zn(SMeaaiNMe)(SCN)2]/218.515/2.827; [Zn(SMeaaiNEt)(SCN)2]/267.206/2.227; [Zn(SEtaaiNMe)(SCN)]/263.399/2.309; [Zn(SEtaaiNEt)(SCN)]/275.447/1.801) and hence the lowering of rate of photoisomerisation.

[Zn(SRaaiNR/)(SCN)(μ-SCN)2]n, (SRaaiNR/ = 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole) exhibit light irradiated structural change referring to trans and cis-configuration about NN of chelated ligand. The structure shows μ-SCN 1D chain followed by hydrogen bonded 3D supramolecular structure. The optical isomerisation is dependent on the molar mass and molar volume; with increase in mass and volume the isomerisation rate decreases. DFT computation of the optimized structure has explained the photophysical phenomenon.Figure optionsDownload as PowerPoint slide