Structure, photochromism and mesogenic property of mercury(II) complexes of 1-alkyl-2-(arylazo)imidazoles

Department of Chemistry, Jadavpur University, Kolkata-700 032, India

E-mail : c_r_sinha@yahoo.com

[Hg(Raai-C_nH_2n+1)(µ-I)I]₂ (1-alkyl-2-(arylazo)imidazole, Raai-C_nH_2n+1, R = H, Me and n = 10, 12, 14, 16, 18, 20, 22) are characterized by spectroscopic data. The structure has been confirmed in case of [Hg(Haai-C₁₆H₃₃)(µ- I)I]₂ by single crystal X-ray diffraction measurement. Ligands exist at ambient condition in E (trans) geometry about azo (-N=N-) group in the chelated motif of the complex. The complexes in DMF solution show E-to-Z (trans-to-cis) isomerisation of coordinated Raai-C_nH_2n+1 upon UV light irradiation. The reverse transformation, Z-to-E (cis-to-trans), is very slow with visible light irradiation and has been carried out by thermal treatment. Quantum yields (ɸE→Z ) of Eto-Z isomerisation are calculated and free ligand shows higher ɸ than their complex phase. The activation energy (Ea ) of Z-to-E isomerisation is calculated by controlled temperature experiment (298–308 K). With increasing chain length of -C_nH_2n+1 in the coordinated ligand the activation energy (E_a ) barrier and the activation entropy (ΔS*) decreases. [Hg(Meaai-C₁₈H₃₇)(µ -I)I]₂ shows metallomesogenic property and undergoes phase transitions : Cr(50.1ºC)–SmA(73.8ºC)- I phase sequence on heating and I(131°C)–SmB(71ºC)-Cr on cooling. The SmA phase appears as typical Banana shaped on heating cycle while it turns to SmB phase Mosaic texture on cooling.