Unraveling the Sensing Mechanism of an ESIPT-Based Ratiometric Fluorescent Probe for the Detection of Phosgene: A Theoretical Study.

Excited state intramolecular proton transfer (ESIPT) process of a novel fluorescence probe 1 and its sensing mechanism for phosgene have been studied theoretically. The optimized geometric configurations and infrared spectroscopy analysis of probe 1 indicate that the intramolecular hydrogen bond (N₁-H₁···N₂) is strengthened upon excitation. Potential energy curves confirm that the energy barrier of probe 1 is smaller in the S₁ state (6.21 kcal/mol) than that in the S₀ state (15.47 kcal/mol), which promotes the occurrence of the ESIPT process. Theoretical calculations show that the absorption and fluorescence spectra of product are both red-shifted (91 and 77 nm, respectively) compared to the probe 1 due to the obvious charge transfer extent. The electron density difference indicates that the charge transfer distance of product (1.83 Å) is larger than that of probe 1 (1.49 Å), which results in the red-shift of emission of product compared to that of probe 1. Therefore, probe 1 can detect phosgene through the fluorescence variation induced by the large extent of charge transition. This work not only provides a theoretical foundation for designing ESIPT-based fluorescent sensors but also highlights their potential in real-time monitoring of toxic gases.