Water-soluble copper(i) complexes bearing 2,2′-bicinchoninic acid dipotassium salt with red-light absorption and repeatable colour change upon freezing operation

Abstract

We newly synthesized a copper(I) complex bearing a 2,2′-bicinchoninic acid dipotassium salt (biq(COOK)₂) ligand and characterized the complex by elemental analysis, NMR and UV-vis spectroscopy, and electrochemistry with the help of comparative compounds. The complex in water shows intense absorption in the red light region due to metal to ligand charge transfer. The wavelength value at the absorption maximum of the charge transfer, λ_{abs_max}, is 556 nm, and the value of the molar extinction coefficient, ε, at the shoulder of the absorption at 670 nm is 1 × 10³ M⁻¹ cm⁻¹. The water solution is purple, and the frozen one is green. The colour change is repeatable. The colour change is not observed when DMSO and ethanol are used. The UV-vis spectra related to absorption using an integrated sphere of the water solution and a frozen one show the repeatable colour change.