Monitoring vibronic coherences and molecular aromaticity in photoexcited cyclooctatetraene with an X-ray probe: a simulation study

Understanding conical intersection (CI) dynamics and subsequent conformational changes is key for exploring and controlling photo-reactions in aromatic molecules. Monitoring of their time-resolved dynamics remains a formidable experimental challenge. In this study, we simulate the photoinduced S-3 to S-1 non-adiabatic dynamics of cyclooctatetraene (COT), involving multiple CIs with relaxation times in good agreement with experiment. We further investigate the possibility to directly probe the CI passages in COT by off-resonant X-ray Raman spectroscopy (TRUECARS) and time-resolved X-ray diffraction (TRXD). We find that these signals sensitively monitor key chemical features during the ultrafast dynamics. First, we distinguish two CIs by using TRUECARS signals with their appearances at different Raman shifts. Second, we demonstrate that TRXD, where X-ray photons scatter off electron densities, can resolve ultrafast changes in the aromaticity of COT. It can further distinguish between planar and non-planar geometries explored during the dynamics, as e.g. two different tetraradical-type CIs. The knowledge gained from these measurements can give unique insight into fundamental chemical properties that dynamically change during non-adiabatic passages.

Automated summary

Understanding conical intersection (CI) dynamics and subsequent conformational changes is crucial for exploring and controlling photo-reactions in aromatic molecules. In this study, we simulate the photoinduced S-3 to S-1 non-adiabatic dynamics of cyclooctatetraene (COT) and find good agreement with experimental relaxation times. We also investigate the potential of TRUECARS and TRXD to directly probe CI passages in COT, showing that these signals can sensitively monitor key chemical features and resolve ultrafast changes in aromaticity and geometries during the dynamics.