Arylazoimidazole complexes of lead(II)-halide and their photochromism

Abstract

Lead(II) complexes of 1-alkyl-2-(arylazo)imidazole (Raai-C_nH_2n+1), [Pb(Raai-C_nH_2n+1)X₂] (X = Cl, Br, I; and Raai-C_nH_2n+1, R = H, Me and n = 4, 6, 8) have been characterized by UV-vis, IR and ¹H-NMR spectroscopy. The coordinated Raai-C_nH_2n+1 in the complexes undergoes E-to-Z (trans-to-cis) isomerisation about the –N=N– group upon being irradiated with UV light in DMF solution. The rate and quantum yields of E-to-Z photoisomerisation (f_E®Z) of the complexes are poorer than the respective free ligand response and are also affected by the nature of halide present (Cl^-, Br^- and I^-). Variation in physicochemical parameters may be correlated with the effective mass of the photochrome. The rate of isomerisation follows the sequence: [Pb(Raai-C_nH_2n+1)Cl₂] < [Pb(Raai-C_nH_2n+1)Br₂] < [Pb(Raai-C_nH_2n+1)I₂]. The Z-to-E isomerisation has been carried out at varying temperatures (298–308 K) to determine the activation energy of Z-to-E (cis-to-trans) isomerisation
(E_a: 47.09–63.42 kJ mol^-1) and the entropy of activation (DS : 166.52 to –109.0 J mol^-1 K^-1) which is a large negative in the complexes. Theoretical calculation supports cleavage of Pb(II)-N(azo) bond followed by the –N=N– rotation in a three-coordinated symmetry rather than the four-coordinated symmetry.