Copper–thioarylazoimidazole complexes: Structures, photochromism and redox interconversion between Cu(II) ↔ Cu(I) and correlation with DFT calculation

Reaction of Cu(ClO4)2·6H2O, SRaaiNR′ (1-alkyl-2-[(o-thioalkyl)phenylazo]imidazole) and NH4SCN (1:1:2 mol ratio) affords distorted square pyramidal, [CuII(SRaaiNR′)(SCN)2] (3) compound while identical reaction with [Cu(MeCN)4](ClO4) yields –SCN– bridged coordination polymer, [CuI(SRaaiNR′)(SCN)]n (4). These two redox states [CuII and CuI] are interconvertible; reduction of [CuII(SRaaiNR′)(SCN)2] by ascorbic acid yields [CuI(SRaaiNR′)(SCN)]n while the oxidation of [CuI(SRaaiNR′)(SCN)]n by H2O2 in presence of excess NH4SCN affords [CuII(SRaaiNR′)(SCN)2]. They are structurally confirmed by single crystal X-ray diffraction study. Cyclic voltammogram of the complexes show Cu(II)/Cu(I) redox couple at ∼0.4 V and azo reductions at negative to SCE. UV light irradiation in MeCN solution of [CuI(SRaaiNR′)(SCN)]n (4) show trans-to-cis isomerisation of coordinated azoimidazole. The reverse transformation, cis-to-trans, is very slow with visible light irradiation while the process is thermally accessible. Quantum yields (ϕt→c) of trans-to-cis isomerisation are calculated and free ligands show higher ϕ than their Cu(I) complexes. The activation energy (Ea) of cis-to-trans isomerisation is calculated by controlled temperature experiment. Copper(II) complexes, 3, do not show photochromism. DFT and TDDFT calculation of representative complexes have been used to determine the composition and energy of molecular levels and results have been used to explain the solution spectra, photochromism and redox properties of the complexes.

Graphical abstractIn [CuII(SRaaiNR′)(SCN)2] the ligand, SRaaiNR′ (1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole) acts as tridentate N(imidazole), N(azo), S(thioether) donor while in Cu(I) compound, [CuI(SRaaiNR′)(SCN)]n, SRaaiNR′ acts as bidentate N(imidazole), N(azo) chelator. These two redox states [Cu(II) and Cu(I)] are interconvertible. [CuI(SRaaiNR′)(SCN)]n (4) shows trans-to-cis photochromism upon UV light irradiation. Quantum yields (ϕt→c) are lower than free ligand data. The activation energy (Ea) of cis-to-trans isomerisation is calculated by controlled temperature experiment. Copper(II) complexes, 3, do not show photochromism. The properties are supported by DFT computation.Figure optionsDownload full-size imageDownload as PowerPoint slide