Highly water-soluble and pH-sensitive colorimetric sensors based on a D-π-A heterocyclic azo chromosphere

Two novel pH-sensitive colorimetric sensors based on a D pi A heterocyclic azo chromosphere (D1, D2) were designed and synthesized. They contained a nitro group as an electron-withdrawing group (A), a naphthalene of coupling component as an electron-donating unit (D), and SO3Na as water-soluble group. The electron-donating and electron-accepting groups were connected by an azo pi-conjugated linker. Their chemical structures were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (H-1 NMR, and C-13 NMR). Ultravioletvisible spectra (UVvis) were obtained and the pH sensitivity was investigated. The mechanism for the visible color change was determined by FT-IR and H-1 NMR. The two novel dyes had a pronounced bathochromic effect, good water solubility, and brighter color in aqueous solution. The two dyes exhibited excellent pH sensitivity and a large wavelength shift in weak acid solution because of the effect of pullpush electron in the intra-molecules. The sensitivity of the two dyes in different pH solutions was accompanied by a rapid and reversible change in color from blue to green for D1 in a pH range of 5.66.4 and from red to blue for D2 in a pH range of 4.25.0. The wavelength shifts were 75 nm for D1, and 140 nm for D2. The visible color of a mixed solution was very sensitive to pH in a narrow range. A fine and versatile color changes can be observed in a weak acid solution by naked-eyes. The remarkable visible color change is attributed to a change in the tautomerism from a hydrazone structure to an azo structure. The two dye molecules based on a D-pi-A structure and their mixture, as a new chemisonsors, have potential and important application in pH determination in cell biology, pharmacology and related fields. (C) 2014 Elsevier B.V. All rights reserved.