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, Cu^II(SRaaiNR′)(SCN)₂] (3) compound while identical reaction with Cu(MeCN)₄](ClO₄) yields -SCN- bridged coordination polymer, Cu^I(SRaaiNR′)(SCN)]_n (4). These two redox states Cu^II and Cu^I] are interconvertible; reduction of Cu^II(SRaaiNR′)(SCN)₂] by ascorbic acid yields Cu^I(SRaaiNR′)(SCN)]_n while the oxidation of Cu^I(SRaaiNR′)(SCN)]_n by H₂O₂ in presence of excess NH₄SCN affords Cu^II(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 Cu^I(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 (?_t→c) of trans-to-cis isomerisation are calculated and free ligands show higher ? than their Cu(I) complexes. The activation energy (E_a) 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.