Jingbai Li, Steven A. Lopez
Apr 26, 2022
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Journal
Chemistry
Abstract
The photochemistry of benzene is complex and non-selective because numerous mechanistic pathways are accessible in the ground and excited states. Fluorination is a known strategy to increase the chemoselectivities for Dewar-benzenes via 4π-disrotatory electrocyclization. However, the origin of the chemo- and regioselectivities of fluorobenzenes remains unexplained because of experimental limitations in resolving the excited-state structures on ultrafast timescales. The computational cost of multiconfigurational nonadiabatic molecular dynamics simulations is also generally prohibitive. We now provide high-fidelity structural information and reaction outcome predictions with machine-learning-accelerated photodynamics simulations of a series of fluorobenzenes, C 6 F 6-n H n , n=0-3, to study their S 1 →S 0 decay in 4 ns. We trained neural networks with XMS-CASPT2(6,7)/aug-cc-pVDZ calculations, which reproduced the S 1 absorption features with mean absolute errors of 0.04 eV (< 2 nm). The predicted nonradiative decay constants for C 6 F 4 H 2 , C 6 F 6 , C 6 F 3 H 3 , and C 6 F 5 H are 116, 60, 28, and 12 ps, respectively, in line with the experiments. Our calculations show that the pseudo Jahn-Teller distortions create the S 1 minimum region that prolongs the excited-state lifetime of fluorobenzenes. The pseudo Jahn-Teller distortions reduce when fluorination decreases. Analysis of the S 1 dynamics suggests that the pseudo-Jahn-Teller distortions promote a cis-trans isomerization of a 𝜋 C-C -bond. We characterized the surface hopping points from our NAMD simulations and identified instantaneous nuclear momentum as a factor that promoted 𝜎-bond formation in the 4π-electrocyclization.