Halochromic properties of new nitrophenol-based azochromophores

A number of new azo dyes were synthesized using azo-coupling of nitrohydroxyphenols (2-nitroresorcinol and nitrophloroglicinol) with ortho-aminophenol and its nitro-and sulfonic-substituted derivatives. The dye mole-cules contain multiple hydroxy groups which can be easily deprotonated, and their ionization affects the con-jugated & pi;-electron system of the dye molecule leading to the significant spectral changes. Spectrophotometric titration of the obtained dyes with an acid and an alkali revealed their halochromic behaviour, i.e. an ability to alter color upon pH change. All the dyes absorb light with wavelength shorter 500 nm in acidic media, and with wavelength longer 500 nm in alkaline media. Variation of substituents in the dye molecules allows to tune the pH range of the color change. The obtained dye series covers the pH range from 0 to 10. DFT calculations of the dye molecules revealed their structures and preferential isomeric forms. TD-DFT calculations of the excited states allowed to clarify the acid-base equilibrium based on the spectrophotometric titration, and to determine the nature of the observed absorption bands. The dyes were adsorbed on polyamide fabrics, and the dyed fabrics reversibly changed their color upon treating with acids and alkalis. These fabrics with applied dyes showed good resistance to aggressive environments and mechanical influences. Results of tests of the dyed fabrics showed that the obtained materials can be used as flexible textile pH sensors.

Automated summary

A series of new azo dyes were synthesized by azo-coupling of nitrohydroxyphenols with ortho-aminophenol and its derivatives. The dyes exhibited halochromic behavior, changing color with pH, and had good absorption properties in acidic and alkaline media. The structures and isomeric forms of the dyes were determined through DFT calculations, and the acid-base equilibrium was clarified using TD-DFT calculations. The dyed fabrics showed reversible color changes in response to acids and alkalis, making them suitable as flexible textile pH sensors.