Effect of Silyl Ether Bulk on Excited-State Intramolecular Proton Transfer-Based Sensing of Extant Hydrogen Fluoride for G-Series Nerve Agent Detection

Luminescence-based sensing provides a method for the rapid detection of nerve agents. Previous approaches have generally focused on sensing materials containing a nucleophilic group that can react with the electrophilic phosphorus atom found in nerve agents. Herein we report an alternative approach for the detection of phosphonofluoridate-based G-series nerve agents that utilizes the fact they contain hydrogen fluoride. We have developed silylated sensing materials based on an excited-state intramolecular proton transfer (ESIPT) reporter compound, 2-[benzo[<i>d</i>]thiazol-2-yl]phenol. Thin films of differently silylated 2-[benzo[<i>d</i>]thiazol-2-yl]phenol were found to react with the hydrogen fluoride found in di-<i>iso</i>-propyl fluorophosphate (DFP), a simulant of sarin (G-series nerve agent), and turn on the ESIPT emission of the reporter compound. The use of the ESIPT emission reduced the impact of background fluorescence and improved the sensitivity of the detection. The effectiveness of the approach was dependent on the stability of the silyl protecting group used, with the least sterically hindered (trimethylsilyl) found to be too unstable to the ambient environment while the most sterically hindered, e.g., tri-<i>iso</i>-propylsilyl and <i>tert</i>-butyldiphenylsilyl were found to be insufficiently reactive to be useful in a real detection scenario. The sensing material composed of the <i>tert</i>-butyl dimethylsilyl protected 2-[benzo[<i>d</i>]thiazol-2-yl]phenol was found to have the best balance between stability under ambient conditions, and reactivity and selectivity to hydrogen fluoride. In a 3 s exposure, it could detect hydrogen fluoride down to a concentration of around 23 ppm in DFP with 99% purity.