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

•Two novel water-soluble pH chemosensors based on D–π–A were designed and synthesized.•Novel dyes had pronounced bathochromic effect, good water solubility, and brighter color.•The mixture of two dyes was very sensitive with pH change at the narrow range.•The mechanism of color change was due to the tautomerism between hydrazone and azo structure.

Two novel pH-sensitive colorimetric sensors based on a D–π–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 π-conjugated linker. Their chemical structures were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (1H NMR, and 13C NMR). Ultraviolet–visible spectra (UV–vis) were obtained and the pH sensitivity was investigated. The mechanism for the visible color change was determined by FT-IR and 1H 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 pull–push 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 5.6–6.4 and from red to blue for D2 in a pH range of 4.2–5.0. 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–π–A structure and their mixture, as a new chemisonsors, have potential and important application in pH determination in cell biology, pharmacology and related fields.