New thiophene-based azo ligands containing azo methine group in the main chain for the determination of copper(II) ions

Abstract

New kind of azo chromophores containing thiophene moiety and salicyaldimine-based ligand as side chains have been synthesized by a sequential process for optical sensing of Cu 2+ metal ions in organic solution. Compounds have been characterized by IR and UV–vis spectrophotometer, 1H NMR, 13C NMR, MS, TGA and CV instruments. Spectral characteristics of the synthesized compounds have been investigated in five organic solvents of different polarity. Polarizibility effects of the solvents on the spectral characteristics and the dipole moments in the excited state are estimated. The fluorescence quantum yields of the synthesized compounds have been calculated to be in the range of 0.0035–0.0095 in solvents of different polarity. The fluorescence emission quenching experiments between the synthesized compounds and electron donor (pyrene, anthracene), electron acceptor (Co 2+) compounds give the bimolecular quenching rate constants of 10 11 and 10 14 M −1 s −1, respectively. The free energies of photo-electron transfer process (Δ G ET) between the azo dyes and the quenchers have been found to be about −19 kcal/mol. Cyclic voltammetry studies indicate that synthesized azo ligands undergo two- or three-reversible reduction potentials (versus ferrocene) and give LUMO energy value of −3.07 eV which is lower than that of TiO 2 conduction band and has a band gap value of ∼2.5 eV. These results may point that synthesized azo dyes could be used as hole conducting materials in solid DSSC (Dye Sensitized Solar Cell) devices. Thermal decomposition behavior of the azo dyes gives more information about the structure of the studied materials. The photoisomerization behavior of the synthesized compounds has been investigated in ethyl acetate under Xe lamp irradiation in the fluorescence spectrophotometer for 1 h. Photoisomerization rate constants of cis– trans orientation ( k c–t) have been found to be about 10 −5 s −1. The complexation process of synthesized thienylidene azo dyes gives subtle changes in their absorption spectra.