Solvation and stabilization of ionic products of fluorescent water-content chemosensor in organic solvents

We investigated the mechanism and sensing efficiency of an anthracene-based fluorescent water-content chemosensor (AMG) containing a guanidine moiety in water-miscible organic solvents. Based on spectroscopic and theoretical results, we established that there are two functions of water for AMG in organic solvents, one of which is as a reagent for protonation and the other is in the solvation and stabilization of ionic products, such as protonated AMG (AMGH(+)) and hydroxide ion. Moreover, the effective solvation and stabilization of ionic products in organic solvents are critical for the mitigation of the deprotonation between them and the relatively high ratio of the fluorescence turn-on AMGH(+) species. In 1,4-dioxane with low polarity, AMGH(+) and hydroxide ion cannot be effectively solvated, and there is a strong positive correlation between the fluorescence intensity and volumetric fraction of water due to the improved solvation condition caused by the coexistent water. Conversely, the amount of water in polar solvents is less significant, as evidenced by the relatively small increase in the fluorescence intensity upon increasing the water content. This is because ionic products can be stabilized by polar organic solvent molecules without cooperation with water. Conclusively, the solvation and stabilization of ionic products produced by a chemical reaction at a chemical sensing site in a fluorescent chemosensor are important for determining the relative ratio between the fluorescence turn-on and -off species and the overall sensing efficiency of the fluorescent chemosensors.