Effect of Fluorination on Excited-State Dynamics in Methyl Salicylate: A Surface-Hopping Dynamics Study

Fluorination is known to modulate the photophysical properties of conjugated aromatic systems by changing the properties in excited states. Methyl salicylate (MS), which undergoes keto-enol tautomerization, plays a key role as a functional building block in the development of photoprotective materials, including UV filters and photostabilizers. While MS has been widely studied, the effects of fluorination on its excited-state dynamics remain unclear until now. In the present study, we show the effects of mono- and tetrafluorination(s) on the excited-state intramolecular proton transfer (ESIPT) and nonradiative deactivation pathways in MS. Electronic structure calculations are carried out at the RI-SCS-MP2 and RI-SCS-ADC(2) levels for locating the minima, and at the SA-CASSCF level for the conical intersection optimizations. In addition, the dynamics is studied using "on-the-fly" trajectory surface-hopping (TSH) simulations. In the S₁ state, the proton transfer (PT) is found to be ultrafast, completing within 33 fs for MS. Upon monofluorination, it shows a small decrease to ∼25 fs, and tetrafluorination does not change the PT time further. For each system, two S₁/S₀ MECI structures are identified: the first one occurs before and the other after the PT. In both, the ester group is twisted out of the benzene ring plane. TSH dynamical simulations were carried out for all three systems up to 1.8 ps. Results show marginal changes in excited-state relaxation dynamics upon fluorination(s). However, asymmetry created by a single fluorine substitution elevates the importance of one of the modes in monofluorinated MS.