Structure, electrochemistry and photochromism of [Cu(RaaiR′)(PPh3)X] (RaaiR′ = 1-alkyl-2-(arylazo)imidazole; X = Cl, Br, I) and correlation with theoretical calculations

The reaction between CuX (X = Cl, Br, I), PPh3 and 1-alkyl-2-(arylazo)imidazole (RaaiR′) has synthesized [Cu(RaaiR′)(PPh3)X]. The composition has been established by spectroscopic (UV–Vis, IR, 1H NMR) data and the single crystal X-ray diffraction study of [Cu(MeaaiH)(PPh3)Cl] and [Cu(MeaaiH)(PPh3)Br] (MeaaiH = 2-(p-tolylazo)imidazole) have confirmed the structures. These complexes show trans-to-cis (E-to-Z) photoisomerisation upon UV light irradiation. Quantum yields (ϕE→Z) of [Cu(RaaiR′)(PPh3)X] are lower than the free ligand values. The rate of isomerisation follows the sequence [Cu(RaaiR′)(PPh3)Cl] < [Cu(RaaiR′)(PPh3)Br] < [Cu(RaaiR′)(PPh3)I]. The cis-to-trans (Z-to-E) isomerisation is very slow upon light irradiation and has been achieved by a thermal route. The activation energy (Ea) of the Z-to-E isomerisation has been calculated by a controlled temperature reaction. DFT calculations of the optimized geometry of representative complexes have been used to determine the composition and energy of the molecular levels.

Graphical abstractCu((RaaiR′)(PPh3)X complexes are obtained by the reaction of CuX (X = Cl, Br, I), PPh3 and 1-alkyl-2-(arylylazo)imidazole (RaaiR′). An X-ray diffraction study establishes the structure of the complexes. They show trans-to-cis photoisomerisation upon UV light irradiation and the reverse transformation, cis-to-trans, is thermal isomerisation. The rate of isomerisation decreases with the increasing electronegativity of X in the complex. The activation energy (Ea) of the cis-to-trans isomerisation has been calculated and correlated with DFT computed resultsFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Copper(I)-1-alkyl-2-(arylazo)imidazole-PPh3 complexes are characterized. ► The single crystal X-ray structures of the complexes confirm the distorted Td-geometry. ► The azoimidazole undergoes a trans-to-cis photoisomerisation, and the reverse is achieved by a thermal route. ► Cyclic voltammetry shows a high potential Cu(II)/Cu(I) redox couple. ► DFT calculations explain the spectra, redox and photoisomerisation properties.