Competing Photoprocess-Based Sensor: Excitation-Wavelength-Dependent Turn-On and Turn-Off Signaling Response toward Picric Acid

Abstract Based on competing proton transfer and charge transfer, a sensor for picric acid (PA), 2-(4′-dimethylamino-2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (DMAHPIP-c), has been designed and synthesized. It exhibits a limit of detection (LOD) of 0.37 nM toward PA. Apart from absorption and steady state fluorescence, time-resolved fluorescence studies and NMR experiments were performed. Solvatochromic studies revealed that the fluorophore exhibits proton transfer or charge transfer depending on the nature of the environment. PA turns off the normal emission and proton transfer and turns on the intramolecular charge transfer emission. Thereby, the fluorophore can be tuned by excitation wavelength to act as not only a turn-off but also a turn-on sensor for PA. The sensor was tested in tap water and on a paper strip. The mechanism of sensing was established by experimental, density functional theory (DFT), and time-dependent DFT studies. The effect of pH on the absorption and fluorescence spectra was also studied.