摘要:
Zinc(II) complexes of 1-alkyl-2-(arylazo)imidazoles (RaaiR(/)) such as, [Zn(RaaiR(/))(DMF)X-2], [Zn(RaaiR(/))(2)X-2] (X = Cl, Br, I) and [Zn(RaaiR(/))(4)](ClO4)(2) are characterized by spectroscopic data. The structures have been confirmed by single crystal X-ray diffraction measurements of [Zn(MeaaiCH(2)Ph)(DMF)Cl-2] and [Zn(MeaaiCH(2)Ph)(2)I-2.] (MeaaiCH(2)Ph = 1-benzyl-2-(p-tolylazo)imidazole). These complexes are sensitive to irradiation of light and undergoes trans(t)-to-cis(c) isomerisation of coordinated azoimidazole upon UV light irradiation in DMF solution. Quantum yields (phi(t -> c)) are calculated and it is shown that free ligand shows higher phi(t -> c) than that of Zn(II) complexes. The reverse, cis-to-trans, transformation has been performed slowly with visible light irradiation and becomes fast with increasing temperature. The activation energy (E-a) is calculated by controlled temperature experiment. Effect of anions (Cl-, Br- and I-) and number of coordinated azoimidazoles (RaaiR(/)) [Zn(RaaiR(/))-; Zn(RaaiR(/))(2)-; Zn(RaaiR(/))(4)] on the rate and quantum yields of photochromism are examined. The rate of isomerisation follows the sequence [Zn(RaaiR(/))(n)Cl-2] < [Zn(RaaiR(/))(n)Br-2] < [Zn(RaaiR(/))(n)I-2] (n = 1 or 2) and [Zn(RaaiR(/))(DMF)X-2] > [Zn(RaaiR(/))(2)X-2] > [Zn(RaaiR(/))(4)](ClO4)(2). The effect of halide on the rate and quantum yields have been correlated with electronegativity in the order of I < Br < Cl. Gaussian 03 calculations of representative complexes are used to explain the difference in the rates and quantum yields of photoisomerisation. (C) 2014 Elsevier Ltd. All rights reserved.