Theoretical Elucidation of Luminescence Characteristics and Phosgene Recognition Mechanism of BODIPY-Based...

Phosgene (COCl₂) is a highly toxic industrial gas, and exploring the photophysical sensing mechanism of fluorescent probes is critical for rational probe optimization. Herein, time-dependent density functional theory (TD-DFT) and thermal vibration correlation function (TVCF) calculations were performed to clarify the excited-state decay mechanism of the 8-EDAB probe. The free 8-EDAB exhibits extremely weak fluorescence (Φ = 0.0045%), while its phosgene cyclized product displays significantly enhanced emission (Φ = 12.44%). The fluorescence turn-on behavior is attributed to cyclization-induced intramolecular charge transfer (ICT) suppression, which greatly reduces the internal conversion rate (k_ic from 10¹¹ to 10⁹ s^-1) and switches the dominant excited-state deactivation pathway from nonradiative decay to radiative fluorescence. Both molecules exhibit two-photon absorption capability, revealing potential bioimaging applicability. Furthermore, we rationally designed two hybrid local charge-transfer (HLCT) derivatives, 8-EDAB-S-3NH₂ and 8-EDAB-5CH₃, whose cyclized products achieve ultrahigh fluorescence quantum yields of 97.28 and 84.39%, respectively. The improved emission originates from dramatically suppressed internal conversion caused by molecular rigidification, rather than reverse intersystem crossing (RISC) triplet harvesting. Unlike the original experimental work, this study provides the first quantitative TVCF-based mode-resolved analysis of internal conversion suppression and demonstrates a predictive HLCT-based design strategy for high-performance phosgene probes.