Synthesis, characterisation, photochromic study of cadmium(II) halide complexes of 1-alkyl-2-methyl-4-(p-nitro-phenylazo)imidazole and DFT computation

1-Alkyl-2-methyl-4-(p-nitro-phenylazo)imidazole complexes of Cd(ii), Cd[{p-NO2-aai(Me)R}(2)]X-2 (2-4) [{p-NO2-aai(Me)R = 1-alkyl-2-methy1-4-(para-nitro-phenylazo)imidazole}, {R = -CH3 (1a), -C2H5 (1b)} (X = CI, Br, I)]. have been synthesized and characterized by spectral (UV-Vis, IR, H-1 NMR) data and have been confirmed by single crystal X-ray diffraction study in the case p-NO2-aai(2-Me)1-C2H5 (1b) and Cd[{p-NO2-aai(2-Me)1-C2H5}(2)I-2] (4b). Both ligands and complexes undergo E(trans)-to-Z(cis) isomerization when irradiated with UV light. The reverse Z(cis)-to-E(trans) isomerization can be driven thermally in the dark. The rates and quantum yields (phi(E -> Z)) of E-to-Z isomerisation are higher for free ligand, 1, than their complexes, 2-4. The activation energies (E-a) and activation entropies (Delta S) of the Z-to-E isomerization are calculated by controlled temperature experiment (298-313 K). Effect of halides on the rate and quantum yields of photochromism is established and has been supported by DFT computation of optimized structures. Slow rate of photoisomerisation of coordinated ligands compare to the free ligands may be due to increased mass and rotor volume of the complexes. The rate of isomerization follows the sequence Cd[{p-NO2-aai(2-Me)1-C2H5}(2)CI2] < Cd[{p-NO2-aai(2-Me)1-C2H5)(2)Br-2] < Cd[{(p-NO2-aai(2-Me)1-C2H5}(2)I-2].