Elucidating the sensing mechanism of a fluorescent probe for formaldehyde detection: A computational study under acidic and neutral conditions.

The probe HBT-BA1 based on excited-state intramolecular proton transfer (ESIPT) can achieve rapid optical responses and show promising potential for formaldehyde (FA) detection. However, the ESIPT-mediated luminescence regulation mechanism remains completely unexplored, particularly regarding how environmental conditions determine the subsequent reaction pathways of key intermediates. In this work, the systematic theoretical study on the environment-dependent, dual-pathway FA sensing mechanisms was performed by using quantum chemical methods. Under neutral condition, the probe captures FA and undergoes spontaneous photocyclization to emit strong deep-red light at 711 nm, with a large Stokes shift of up to 392 nm. Frontier molecular orbitals (FMOs) analysis confirms that cyclization reduces the energy gap of electronic transition and enhances electronic delocalization, thereby facilitating long-wavelength emission. In acidic environments, the intermediate undergoes acid-catalyzed hydrolysis to release the classic ESIPT fluorophore HBT, which spontaneously protonates to yield HBT+ with bright blue-violet emission at 413 nm. Overall, this work fully elucidates the environment-dependent recognition mechanism at molecular level, which provides important theoretical guidance for developing environment-adaptive probes for precise FA detection.