Effect of structure on excited-state intramolecular proton transfer-based sensors for phosphonofluoridate G-series nerve agent vapour detection†

Excited-state intramolecular proton transfer (ESIPT) emitters are unique in that the emission is significantly red shifted relative to the absorption spectra. Herein we explore the effect of substituents on the ability of thin films of 2-[1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl]phenol-based ESIPT reporter compounds to detect hydrogen fluoride found in G-series nerve agents containing a phosphonofluoridate moiety. When the hydroxyl group of the 2-[1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl]phenol-based reporter compounds was protected as a silyl ether the photoluminescence emission spectra had vibrational structure and emission maxima at around 370 nm. The silyl protecting groups could be cleaved upon exposure to hydrogen fluoride in the G-series nerve agent simulant, di-iso-propyl fluorophosphate, leading to ESIPT emission with a peak maximum at around 470 nm, thus allowing identification of the presence of hydrogen fluoride. Films of the sensing materials with the different silyl protecting groups were found to have different stabilities to ambient conditions and reactivity with hydrogen fluoride, with the larger silyl ethers such as triethylsilyl and t-butyldimethyl silyl performing better overall when compared to the smaller trimethylsilyl ether. Steric encumberance or addition of polar solubilising groups was found to reduce the sensing capability. The optimal sensing material was lipophilic and contained a t-butyldimethyl silyl protecting group, with films capable of detecting hydrogen fluoride at a concentration of 0.1 ppm which, based on a sarin purity of 99%, would enable sarin to be detected at 1.2 ppm, which is below the LC₅₀ five minute exposure limit for sarin of 1.6–3.2 ppm.