Theoretical investigation on the fluorescent sensing mechanism for recognizing formaldehyde: TDDFT calculation and excited-state nonadiabatic dynamics

Inspired by the activity-based sensing method, the hydrazine-modified naphthalene derivative (Naph1) was synthesized and used as a fluorescent probe to detect formaldehyde (FA) in living cells. Through the condensation reaction between the probe Naph1 and analyte FA, researchers observed a ∼14 folds enhancement of fluorescent signal around 510 nm in an experiment, realizing the high selectivity and sensitivity detection of FA. However, a theoretical understanding of the sensing mechanism was not provided in the experimental work. Given this, the light-up fluorescent detecting mechanism was in-depth unveiled by performing the time-dependent density functional theory (TDDFT) and the complete active space self-consistent field (CASSCF) theoretical calculations on excited-state intramolecular proton transfer (ESIPT) and non-adiabatic excited-state dynamics simulation. The deactivation channel of S ₁/T ₂ intersystem crossing (ISC) was turned off to successfully recognize FA. Insight into the ESIPT-based fluorescent detecting mechanism indicated that ESIPT was essential to light-up fluorescent probes. This work would provide a new viewpoint to develop ESIPT-based fluorescent probes for detecting reactive carbon species in vivo or vitio.