Design and mechanistic study of a D-π-a fluorescent probe for sensitive water detection in organic media

For the sensitive detection of water content in organic solvents, we developed a donor-bridge-acceptor (D-π-A)-structured small-molecule fluorescent probe that operates through a photoinduced electron transfer mechanism, in which triphenylamine, thiophene and 3-nitropyridine function as the electron-donating unit, π-conjugated bridge and electron-accepting moiety, respectively. Increasing the water content from 0% to 100% in acetone and tetrahydrofuran solvents caused the fluorescence intensity of the probe to decrease with an excellent linear response, which indicates its suitability for the quantitative analysis of water content in organic solvents. In addition, the fluorescence intensity of the probe remained stable in phosphate-buffered saline solutions with pH values ranging from 6 to 9, thereby indicating its applicability in complex environments. The results of density functional theory, cyclic voltammetry and X-ray photoelectron spectroscopy analyses indicate that the luminescence mechanism involves an intramolecular photoinduced electron transfer process triggered by water molecules forming hydrogen bonds with the nitro groups in the probe, resulting in fluorescence quenching. Uncovering this reaction mechanism provides a novel approach for the rapid and visual detection of water content in organic solvents. Moreover, the probe can sensitively detect the water content in organic solvents over the full range, which is superior to other previously reported fluorescent probes for this task. Optimising the sensitivity and selectivity of the probe will expand its applicability in more complex environments.