Department of Chemistry, Mrinalini Datta Mahavidyapith, Kolkata-700 051, India E-mail: dmchemmdm51@gmail.com Departmet of Chemistry, Sripat Singh Collge, Murshidabad-742 123, West Bengal, India Department of Chemistry, Raja N. L. Khan College, Gope Palace, Paschim Medinipur-721 102, West Bengal, India Department of Chemistry, Aliah University, New Town, Kolkata 700 156, India Department of Chemistry, Ramakrishna Mission Residential College, Kolkata-700 103, India Department of Chemistry,

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... r University, Kolkata-700 032, India Manuscript received online 16 June 2019, revised and accepted 10 July 2019 1-Alkyl-2-methyl-4-(p-nitro-phenylazo)imidazole complexes of Cd(II), Cd[{p-NO2 -aai(Me)R}2 ]X2 (2-4) [{p-NO2 -aai(Me)R = 1-alkyl-2-methyl-4-(para-nitro-phenylazo)imidazole}, {R = -CH3 (1a), -C2 H5 (1b)} (X = Cl, Br, I)] have been synthesized and characterized by spectral (UV-Vis, IR, 1H NMR) data and have been confirmed by single crystal X-ray diffraction study in the case p-NO2 -aai(2-Me)1-C2 H5 (1b) and Cd[{p-NO2 -aai(2-Me)1-C2 H5 }2I2 ] (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 (ɸE→Z ) of E-to-Z isomerisation are higher for free ligand, 1, than their complexes, 2-4. The activation energies (Ea ) and activation entropies (ΔS) 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 [...]