Cyclohexadiene Revisited: A Time-Resolved Photoelectron Spectroscopy and ab Initio Study

Abstract

We have reinvestigated the excited state dynamics of cyclohexa-1,3-diene (CHD) with time-resolved photoelectron spectroscopy and fewest switches surface hopping molecular dynamics based on linear response time-dependent density functional theory after excitation to the lowest lying ππ* (1B) state. The combination of both theory and experiment revealed several new results: First, the dynamics progress on one single excited state surface. After an incubation time of 35 ± 10 fs on the excited state, the dynamics proceed to the ground state in an additional 60 ± 10 fs, either via a conrotatory ring-opening to hexatriene or back to the CHD ground state. Moreover, ring-opening predominantly occurs when the wavepacket crosses the region of strong nonadiabatic coupling with a positive velocity in the bond alternation coordinate. After 100 fs, trajectories remaining in the excited state must return to the CHD ground state. This extra time delay induces a revival of the photoelectron signal and is an experimental confirmation of the previously formulated model of two parallel reaction channels with distinct time constants. Finally, our simulations suggest that after the initially formed cis-Z-cis HT rotamer the trans-Z-trans isomer is formed, before the thermodynamical equilibrium of three possible rotamers is reached after 1 ps.

Ab initio calculation of inelastic scattering

“…67 However, TD-DFT is generally not appropriate for the simulation of complex photochemical reactions in isolated molecules, and is known to yield results that are quite divergent from established reaction paths. 69 The approach presented in this paper covers the ground between these two extremes. It calculates inelastic scattering matrix elements at a level of ab initio theory congruent with state-of-the-art quantum molecular dynamics simulations, and is therefore well placed to evaluate inelastic contributions to the signals observed in ultrafast X-ray scattering experiments.…”

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

“…This yields a pericyclic minimum in the first excited state, which acts as photochemical funnel that provides an efficient path for internal conversion. [14][15][16] The reaction has been studied extensively using a wide array of time-resolved techniques in solution and gas phase, including resonance Raman spectroscopy, [17][18][19] transient absorption, 12,20,21 photoelectron spectroscopy, [22][23][24] and dissociative intense-field ionisation. [25][26][27] Computational studies include electronic structure calculations 16,[28][29][30][31][32][33] and dynamics, [34][35][36][37][38][39][40][41][42] as well as joint theoretical and experimental work.…”

“…[25][26][27] Computational studies include electronic structure calculations 16,[28][29][30][31][32][33] and dynamics, [34][35][36][37][38][39][40][41][42] as well as joint theoretical and experimental work. 23,[43][44][45] It is notable that despite such extensive study, the understanding of the reaction is not complete. Apparent discrepancies include the branching ratio between ringopen and ring-closed products, possibly related to whether the experiment is performed in solution or gas phase.…”

Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction

“…1B (9,14). According to recent simulations (15), the strict separation into a bright 1B state and a dark 2A state may not be exact; however, this labeling convention conveniently differentiates the characteristically distinct regions of the excited-state potential energy surface. As the wave packet moves along the reaction coordinate with concurrent conversion of the photon energy into nuclear dynamics, a reduction in the p-p* splitting is expected near the pericyclic minimum, consistent with the narrowing gap between the groundand excited-state potential energy surfaces.…”