Description
Conical intersections (CIs) play a crucial role in determining the photochemical outcomes of excited-state molecular dynamics. The direct monitoring of CIs remains a big challenge for time-resolved spectroscopy [1] due to ultrafast timescales and strong coupling of nuclear and electronic motions. This challenge is even more emphasized when multiple CIs are in competition to each other.
Here, we computationally explore the potential of ultrafast electron diffraction (UED) to probe competing CIs in 2,5-Dichlorofuran (2,5-DCF). Like its parent molecule furan [2,3], it exhibits two main competing CI-mediated pathways - ring-opening and ring-puckering. We simulate the photo-induced dynamics of 2,5-DCF with semi-classical surface-hopping molecular dynamics based on XMS-CASPT2 theory. We then simulate the UED signal beyond the independent atom model by using ab-initio electron and nuclear densities. The incoming electron probe beam is scattered of these charge densities, providing structural information about the molecular photochemistry.
Our trajectory analysis shows that compared to furan, chlorine substitution pushes the relaxation time well above 100 fs, which is beneficial for current temporal resolution in UED [4]. In addition,rather than branching purely into puckering and opening, we find that most trajectories relax through a CI seam connecting these two between 100 fs and 300 fs.
The ensemble UED signal clearly reflects the mixed character of the dynamics. Nevertheless, we found distinct observable differences between the in-plane opening CI and the out-of-plane puckering CI especially in the static UED signal of the optimized CIs. This demonstrates the potential of UED in disentangling photochemical pathways with distinct structural differences.
