Role of conical intersection seam topography in the chemiexcitation of 1,2-dioxetanes

Chemiexcitation, the generation of electronic excited states by a thermal reaction initiated on the ground state, is an essential step in chemiluminescence, and it is mediated by the presence of a conical intersection that allows a nonadiabatic transition from ground state to excited state. Conical intersections classified as sloped favor chemiexcitation over ground state relaxation. The chemiexcitation yield of 1,2-dioxetanes is known to increase upon methylation. In this work we explore to which extent this trend can be attributed to changes in the conical intersection topography or accessibility. Since conical intersections are not isolated points, but continuous seams, we locate regions of the conical intersection seams that are close to the configuration space traversed by the molecules as they react on the ground state. We find that conical intersections are energetically and geometrically accessible from the reaction trajectory, and that topographies favorable to chemiexcitation are found in all three molecules studied. Nevertheless, the results suggest that dynamic effects are more important for explaining the different yields than the static features of the potential energy surfaces.

Excited-state distortions control the reactivities and regioselectivities of photochemical 4π-electrocyclizations of fluorobenzenes

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, C6F6-nHn, n=0–3 to study their S1→S0 decay in 4 ns. We trained neural networks with XMS-CASPT2(6,7)/aug-cc-pVDZ calculations, which reproduced the S1 absorption features with mean absolute errors of 0.04 eV (< 2 nm). The predicted S1 excited-state lifetimes for C6F4H2, C6F6, C6F5H, and C6F3H3 are 64, 40, 18, and 8 ps, respectively. The trend is in excellent agreement with the experimental lifetimes. Our calculations show that the pseudo Jahn-Teller distortions create the S1 minimum region that prolongs the excited-state lifetime of fluorobenzenes. The pseudo Jahn-Teller distortions reduce when fluorination decreases. Characterization of the surface hopping structures suggests that the S1 relaxation first involves a cis-trans isomerization of a 𝜋C-C-bond in the benzene ring, promoted by the pseudo-Jahn-Teller distortions. A branching plane analysis revealed that the conical intersections favoring 4π-electrocyclization are less energetically accessible through the S1 relaxation; lower-energy conical intersections resemble the reactant and favor reversion.