The synthesis, structure and photochromism of mercury(II)-iodide complexes of 1-CnH2n+1-2-(arylazo)imidazoles (n = 4, 6, 8)

The reaction of HgI₂ with 1-C_nH_2n+1-2-(arylazo)imidazole (Raai-C_nH_2n+1 where n = 4, 6, 8) has isolated iodide bridging dimeric complexes, Hg(RaaiC_nH_2n+1)(μ-I)(I)]₂. The structures of the ligand and the complexes have been established by spectral (UV-Vis, IR, ¹H NMR) data. One of these complexes Hg(1-hexyl-2-(p-tolylazo)imidazole)(μ-I)(I)]₂ has been structurally confirmed by single crystal X-ray diffraction study. The ligand, Raai-C_nH_2n+1 exists at ambient condition in trans geometry about azo (-NN-) group; the UV light irradiation in MeOH solution shows E-to-Z isomerisation. The reverse transformation, Z-to-E, is very slow with visible light irradiation while isomerises rapidly on heating. The coordinated ligand, Raai-C_nH_2n+1 in the complexes exhibit similar behaviour in DMF solution. Quantum yields (?_E→Z) of E-to-Z isomerisation are higher for free ligands than that of their metal complexes. The Z-to-E isomerisation is a thermally induced process. The activation energy (E_a) is calculated by controlled temperature experiment.