Polymer-Bound Coumarin-Triphenyliminophosphorane Derivatives as Turn-On Fluorescence Sensors for Water Content Detection via Non-Hydrolytic Staudinger Reaction

Fluorometric optical sensors have recently been developed for analyzing environmental humidity and solvents' water content. Our group has demonstrated a series of coumarin-triphenyliminophosphorane (TPIPP) derivatives that exhibit a remarkable fluorescence efficiency under photoexcitation. We speculated that these derivatives could be developed into turn-on fluorescent probes suitable for the analysis of water content and humidity. First, the non-hydrolytic Staudinger reaction (NSR) was employed to modify the side chain of a styrene-4-diphenylphosphino styrene (DPPS) copolymer with coumarin-TPIPP fluorophores. When the DPPS content of the copolymer was higher, the fluorophores' fluorescence was weaker because of their dense packing, causing self-quenching. When the fluorophore-modified copolymer was placed in tetrahydrofuran containing water and a small amount of AcOH, hydrolysis occurred, and highly fluorescent 7-amino-3-cyanocoumarin (coumarin 1) was released in the solvent following the general Staudinger reduction route. The release of some fluorophores from the copolymer also resulted in less self-quenching, thus enhancing the fluorescence intensity. To take full advantage of this emission, we prepared polymer beads with crosslinked poly (styrene) and poly(DPPS) and coumarin fluorophores that were covalently linked to the beads through an efficient NSR. Compared with the nonbead form, the polymer beads required a much lower acid concentration for hydrolysis to occur and solvents' fluorescence turned on more quickly. The water content in three common solvents, tetrahydrofuran, acetonitrile, and methanol, was analyzed by immersing the modified beads for 5 min (AcOH concentration = 0.5%); the respective quantitation limits were 0.067, 0.013, and 0.057%, respectively. The proposed polymer material is a promising platform for moisture detection in the chemical refinery industry.