Double-Bond Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene-1,2- and -4,5-dicarboxylates

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 S₃ to S₁ 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.

Nonadiabatic Molecular Dynamics Study of the Relaxation Pathways of Photoexcited Cyclooctatetraene

In the current study, we present nonadiabatic (NAMD) and adiabatic molecular dynamics simulations of the transition-state dynamics of photoexcited cyclooctatetraene (COT). The equilibrium-state structure and absorption spectra are analyzed using the semiempirical Austin Model 1 potential. The NAMD simulations are obtained by a surface-hopping algorithm. We analyzed in detail an active excited to ground state relaxation pathway accompanied by an S₂/S₃(D_2d) → S₁(D_8h) → S₀(D_4h) → S₀(D_2d) double-bond shifting mechanism. The simulated excitation lifetime is in good agreement with experiment. The first excited singlet state S₁ plays a crucial role in the photochemistry. The obtained critical molecular conformations, energy barrier, and transition-state lifetime results will provide a basis for further investigations of the bond-order inversion and photoswitching process of COT.

Cyclooctatetraene computational photo- and thermal chemistry: a reactivity model for conjugated hydrocarbons

We use ab initio CASSCF and CASPT2 computations to construct the composite multistate relaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that an efficient population of the dark excited state (S(1)) takes place after ultrafast decay from the spectroscopic excited state (S(2)). A planar D(8)(h)-symmetric minimum represents the collecting point on S(1). Nonadiabatic transitions to S(0) appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S(1) minimum following specific excited-state reaction paths. The higher-energy conical intersection belongs to the same type of intersections previously documented in linear and cyclic conjugated hydrocarbons and features a triangular -(CH)(3)- kink. This point mediates both cis --> trans photoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-bond shifting. The mapping of both photochemical and thermal reaction paths (including also Cope rearrangements, valence isomerizations, ring inversions, and double-bond shifting) has allowed us to draw a comprehensive reactivity scheme for cyclooctatetraene, which rationalizes the experimental observations and documents the complex network of photochemical and thermal reaction path interconnections. The factors controlling the selection and accessibility of a number of conjugated hydrocarbon prototype conical intersections and ground-state relaxation channels are discussed.