Palladium(II)-iodo-{1-alkyl-2-(arylazo)imidazole} complexes: Synthesis, structure, dynamics of photochromism and DFT computation

Abstract

[Pd(Raai-CnH2n+1)2I)]2[Pd2I6] (Raai-CnH2n+1=1-alkyl-2-(arylazo)imidazole) has been characterized by the spectral data (UV–Vis, FTIR, Mass, 1H NMR). The single crystal X-ray structure of [Pd(Meaai-C2H5)2I]2[Pd2I6] shows that one Meaai-C2H5 acts as monodentate N(imidazolyl) donor, while the other one is bidentate N(imidazolyl), N(azo) chelator; the charge is neutralized with [Pd2I6]2− anion. UV light irradiation in DMF solution of the complexes show trans-to-cis isomerisation of Raai-CnH2n+1 about –NN– bond. Quantum yields (ϕt→c) of trans-to-cis isomerisation of the complexes are lower than that of the free ligand data. This observation is consistent with femtosecond transient absorption results of [Pd(Haai-C10H21)2][Pd2I6], which suggest that the trans→cis isomerization occurs in the monodentate azo-imidazole group and the bidentate azo-imidazole, because of chelation with Pd(II), does not exhibit photochromism. The isomerization proceeds with a time constant of ∼1.0ps in acetonitrile, 0.6ps in methanol, and 1.7ps in ethylene glycol, which are comparable to those the free azo-imidazole ligand. In contrast, the reverse transformation, i.e. cis-to-trans, is carried out by thermal process and the activation energy (Ea) of cis-to-trans isomerisation of the complexes is lower than that of free ligand. The spectral property and photochromic efficiency have been explained by DFT computation of optimized geometry of the complex.