Synthesis, structure, photochromism, mesogenic property and DFT computations of silver(I) complexes of long chain alkyl group containing 1-alkyl-2-(arylazo)imidazoles

The complexes [Ag(Raai-CnH2n+1)2]ClO4 (where Raai-CnH2n+1 refer to 1-alkyl-2-(arylazo)imidazoles; R = H, Me and n = 4–22 (even numbers)) have been characterized by spectroscopic studies (UV–Vis, FT-IR, 1H NMR). The X-ray structure of [Ag(Haai-C22H45)2]ClO4 confirms a distorted tetrahedral geometry. UV light irradiation (365–370 nm) of an acetonitrile solution of [Ag(Raai-CnH2n+1)2]ClO4 shows E-to-Z (trans-to-cis) isomerization of the coordinated Raai-CnH2n+1 group. The reverse transformation, Z-to-E (cis-to-trans), is very slow upon visible light irradiation, but becomes significantly faster on increasing the temperature (298–313 K). The rates and quantum yields (ϕE→Z) of the E-to-Z isomerization of the free ligands are higher than those of the complexes, which have proved the effect of rotor volume, rotor mass and the 1-alkyl chain length (–CnH2n+1). [Ag(Haai-C18H37)2]ClO4 shows a metallomesogenic property and undergoes phase transitions: the Cr (46.7 °C) – SmC (106.4 °C) – I phase sequence on heating and I (106.4 °C) – SmA (46.4 °C) – Cr on cooling. The SmC phase appears as the typical fan-shaped texture. DFT computations on the optimized structure were carried out to explain the spectral properties.

Graphical abstract[Ag(Raai-CnH2n+1)2]ClO4 (Raai-CnH2n+1 = 1-alkyl-2-(arylazo)imidazole, R = H, Me and n = 4–22) have been characterized. The X-ray structure of [Ag(Haai-C22H45)2]ClO4 confirms the structure. UV light irradiation (365–370 nm) of a solution of the complexes shows E-to-Z isomerisation of the coordinated ligand. The reverse transformation, Z-to-E, is a thermal process (298–313 K). The rates and the quantum yields (ϕE→Z) of E-to-Z isomerisation of the free ligands are higher than those of the complexes. The activation energy is much lower and ΔS∗ is highly negative in the complexes. [Ag(Haai-C18H37)2]ClO4 shows the Cr (46.7 °C) – SmC (106.4 m °C) – I phase sequence on heating and I (106.4 °C) – SmA (46.4 °C) – Cr on cooling. The SmC phase appears as a typical fan-shaped texture.Figure optionsDownload full-size imageDownload as PowerPoint slide