Phenol-based styrylpyrylium dyes for trace water detection via chromogenic and fluorogenic responses

Abstract

Two new styrylpyrylium cations (1a,b) have been synthesized in only two steps with high yields. The presence of an ortho-nitrophenol moiety in their structures makes them very sensitive to deprotonation by traces of water in organic solvents. The presence of humidity in solutions of those dyes in acetone, acetonitrile and THF leads to the formation of a new band at a long wavelength (ca. 600–800 nm) resulting from the formation of a quinoidal structure. In parallel to the absorption changes, fast quenching of the fluorescence emission takes place. From the quantitative point of view, the best sensitivity is displayed by compound 1a, which can detect trace water with a limit of detection (LOD) of 0.003% vol. in acetone, and 0.004% vol. in acetonitrile, as determined by fluorescence spectroscopy. An interesting advantage of the sensors here reported is that the absorption changes taking place after the addition of water occur in the range 600–800 nm, which is uncommon in this research field. And mechanistically it has been concluded that the role of water is not to act as an acid, donating protons to a previously deprotonated sensor as in most reported sensors, but to act as a base, deprotonating directly the pyrylium dye, due to the presence of a phenolic functionality in its structure.