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Structural Studies and Photochromic Properties of Lead(II)-Halide Complexes of 1-CnH2n+1-2-(Arylazo) Imidazoles

Debashis Mallick, Bharati Chowdhury, Srikanta Jana, Chandana Sen, Sudipa Mondal and Chittaranjan Sinha

Pb(Raai-CnH2n+1)X2 has been synthesized by the reaction between PbX2 (X=Cl, Br, I) and 1-alkyl-2-(Arylazo) Imidazole (Raai- CnH2n+1, R=H, Me and n=4, 6, 8). The structures of the complexes have been established by spectral (UV-Vis, IR, 1H-NMR) data. UV light irradiation in DMF solution of the complexes exhibit E-to-Z (trans-to-cis) isomerisation of coordinated Raai-CnH2n+1 about- N=N-group. The reverse transformation, Z-to-E, is very slow with visible light irradiation while thermally accessible. The rate and quantum yields (φEZ) of the photoisomerisation of the complexes are lower than free ligand data. The activation energy (Ea) of reverse isomerisation (Z-to-E (cis-to-trans)) of [Pb(Raai-CnH2n+1)X2] is lower than that of free ligand data; the entropy of activation (ΔS*) is large negative in the complexes. Effect of halide (Cl-, Br- and I-) on the rate and quantum yields of photochromism is established in this work and has been supported by DFT computation. Slower rate of photoisomerisation of the coordinated ligands compared to the free ligand may be due to increase of mass of the complexes. However, the rate of isomerisation follows the sequence [Pb(Raai-CnH2n+1)Cl2] <[Pb(Raai-CnH2n+1)Br2] <[Pb(Raai-CnH2n+1)I2].