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-CnH2n+1-2-(arylazo)imidazole (Raai-CnH2n+1 where n = 4, 6, 8) has isolated iodide bridging dimeric complexes, [Hg(RaaiCnH2n+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-CnH2n+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-CnH2n+1 in the complexes exhibit similar behaviour in DMF solution. Quantum yields (ϕE→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 (Ea) is calculated by controlled temperature experiment.

[Hg(RaaiCnH2n+1)(μ-I)(I)]2 (Raai-CnH2n+1, 1-alkyl-2-(arylazo)imidazole and n = 4, 6, 8) are synthesised and characterised by spectroscopic techniques. [Hg(1-C6H13-2-(p-tolylazo)imidazole)(μ-I)(I)]2 is structurally confirmed by single crystal X-ray diffraction study. The free and coordinated ligands show E-to-Z photoisomerisation upon UV light irradiation. The reverse transformation, Z-to-E, is very slow with visible light irradiation while isomerises rapidly upon heating. Quantum yields (ϕE→Z) of E-to-Z isomerisation are calculated and free ligand shows higher ϕ than their complex phase. The activation energy (Ea) of free ligands is three times than that of coordinated one.Figure optionsDownload as PowerPoint slideHighlights
► n-Butyl-, n-hexyl and n-octyl group containing 1-alkyl-2-(arylazo)imidazole (Raai-CnH2n+1, n = 4, 6, 8) are synthesised.
► [Hg(Raai-CnH2n+1)(μ-I)(I)]2 are synthesised and characterised by spectroscopic techniques.
► [Hg(Raai-C6H13)(μ-I)(I)]2 has been structurally confirmed by single crystal X-ray structure.
► Free and coordinated Raai-CnH2n+1 show E-to-Z photoisomerisation upon UV light irradiation.
► Z-to-E isomerisation is achieved by thermal route and activation energy is calculated.