Solvatochromic triazaborolopyridinium probes toward ultra-sensitive trace water detection in organic solvents

A series of triazaborolopyridiniums (TBP) has been synthesized by varying electron-tunning substituents. The TBP derivatives with electron-withdrawing groups, including cyano, and nitro moieties, exhibited solvatochromic properties due to the internal charge transfer (ICT) effect. The density functional theory (DFT) calculations suggested that the nitro-substituted TBP derivative (8) possessed the highest change in dipole moment upon excitation (Delta mu) yielding the most distinctly large stokes shift and low fluorescence quantum yields as the solvent polarity increased. In addition, compound 8 displayed an apparent fluorescence quenching in the presence of hydrogen bond-donating molecules (e.g. methanol, ethanol, water); therefore, it was chosen to be utilized as a trace water detector in four common solvents, including, tetrahydrofuran (THF), acetonitrile (MeCN), acetone, and dimethylformamide (DMF). The satisfying linear relationship between the ratios of the blank fluorescence intensity to the decreased fluorescence intensity (I-0/I) and the trace water content was used to determine the limits of detection (LOD) of water. The low LODs (0.028% for THF, 0.013% for MeCN, 0.021% for acetone, and 0.045% for DMF) confirmed the capability of compound 8 as a super-sensitive water sensor in organic solvents.