Studies on 1-alkyl-2-(arylazo)imidazole complexes of lead(II) and their photoisomerization

Halide bridging lead(II) coordination polymer of 1-alkyl-2-(arylazo)imidazole (RaaiR′), [Pb(RaaiR′)(μ-X)2]n is spectroscopically characterized and, the single crystal X-ray structure of [Pb(MeaaiMe)(μ-Cl)2] (where MeaaiMe = 1-methyl-2-(p-tolylazo)imidazole) has confirmed the structure. The UV light irradiation in DMF solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configuration about –NN–, respectively) isomerization of the coordinated azoimidazole. The rate of isomerization follows: [Pb(RaaiR′)(μ-Cl)2]n < [Pb(RaaiR′)(μ-Br)2]n < [Pb(RaaiR′)(μ-I)2]n. Quantum yields (ϕE→Z) and the activation energy (Ea) of the isomerization of the complexes are lower than that of free ligand data. These may be due to increased mass and rotor volume of the complexes. The results are supported by DFT computation of optimized structures.