Photochemical Ring-Opening of Cyclohexadiene: Quantum Wavepacket Dynamics on a Global Ab Initio Potential Energy Surface

Abstract

We have assembled a global CASSCF potential energy surface for the excited 2A state of the cyclohexadiene -hexatriene system, in two degrees of freedom, with full relaxation in all other degrees of freedom. Quantum wavepacket dynamics on this surface yields simple interpretations of recent experimental data on the ultrafast photochemical ring-opening of cyclohexadiene as well as predictions on preferred product configurations. The feasibility of this system as a model for fulgide molecular switches is discussed.

Ultrafast Dynamics of 1,3-Cyclohexadiene in Highly Excited States

“…Previous work has associated the spectral width to dispersion of molecular structure [26]. This is consistent with the theoretically derived notion that the wave packet remains well focused during the transition to the ground state [8]. Challenges in the near future include the time and energy-deconvoluted analysis of the peak shifts in order to get the true molecular binding energy variations.…”

“…Challenges in the near future include the time and energy-deconvoluted analysis of the peak shifts in order to get the true molecular binding energy variations. These could then be compared to wavepacket dynamics calculations such as those conducted by Schönborn et al [8].…”

“…Of particular interest are the geometric changes occurring in the molecule after excitation into the 1B-valence state and as the molecule travels through the electronic states, as such structural motions have eluded direct experimental observation. As has been pointed out, [5,6,8,9] the wave packet remains quite narrow during the reaction, implying that "structure" is a well-defined quantity even though it rapidly evolves. Moreover, it is unknown if higher-lying electronic states follow ring-opening pathways similar to that following the 1B excitation, or if other unique mechanisms emerge.…”

“…Moreover, it is unknown if higher-lying electronic states follow ring-opening pathways similar to that following the 1B excitation, or if other unique mechanisms emerge. Because the dynamics is determined by the rapid motions of wave packets on intricately sculptured surfaces [8], a dependence on the excitation energy is to be expected.…”

From the (1B) Spectroscopic State to the Photochemical Product of the Ultrafast Ring-Opening of 1,3-Cyclohexadiene: A Spectral Observation of the Complete Reaction Path

“…It is that state that correlates with the ground state of the reaction product, 1,3,5-hexatriene (HT) and hence enables the ring-opening in accordance with the Woodward-Hoffmann rules. 5,6 A symmetry breaking motion away from the minimum energy path guides the wavepacket to a conical intersection between the 2A and 1A electronic surfaces, 7,8,9 which facilitates the decay to the electronic ground state of the reaction product.…”