Probing competitive photochemical pathways of 2,5-dichlorofuran via surface hopping dynamics and ultrafast electron diffraction

Conical intersections (CIs) play a crucial role in determining the photochemical outcomes of excited-state molecular dynamics. Such is the case for systems such as furan and many of its derivatives, which exhibit two main competing pathways-ring-opening and ring-puckering that are mediated by two distinct CIs. Since even single CIs are hard to detect spectroscopically, disentangling the contribution between multiple competing CIs is an exceptionally difficult challenge. In this study, we theoretically evaluate the possibility to disentangle competing CI-mediated pathways in 2,5-dichlorofuran with ultrafast electron diffraction (UED). We simulate the photochemistry of 2,5-dichlorofuran with semi-classical surface-hopping molecular dynamics based on XMS-CASPT2 theory. This approach captures non-adiabatic transitions between electronic states at CIs. Compared to furan, we find that chlorine substitution pushes the relaxation time well above 100 fs, which is beneficial for current temporal resolution in UED. We simulate UED patterns that involve scattering off electronic and nuclear charge densities, thereby being sensitive to structural changes in the molecule. In the static UED patterns of the optimized conical intersection structures, we find an observable difference between puckering and opening. The pair distribution functions of the UED signals we calculated exhibit signature peaks of the opening CI at 6.1 Å and of the puckering CI at 3.7 Å. Because the trajectories primarily exhibit mixed puckering/opening character instead of a clean branching, these differences are not clearly visible in the ensemble UED pattern.