Substituent and conformer effects on photochemistry investigated by ultrafast electron diffraction and excited state wavepacket simulations

Ultrafast photochemistry plays an important role in processes like human vision or the photosynthesis of vitamin D. A more thorough understanding of the underlying non-adiabatic dynamics through conical intersections between electronic states could help in tailoring the molecular structure of a reactant towards a desired photochemical outcome, e.g. by chemical substitution in specific positions. Chemical substitution, e.g. through hydrocarbon side chains, can also lead to the occurrence of conformers. The latter are often close enough in energy that several conformer geometries are populated in a room temperature gas phase sample.Not much is known so far about the influence of conformer geometries on photochemical reaction dynamics. 

We have investigated photochemical ring-opening dynamics in the molecule 1,3-cyclohexadiene (CHD) and its substituted derivatives  α-phellandrene (αPh) and  α-terpinene with specific sensitivity to the nuclear structure evolution using MeV ultrafast electron diffraction and ab initio multiple spawning (AIMS) simulations. In CHD, we find rapid ring-opening within < 200 fs upon internal conversion through a conical intersection with the electronic ground state. 

Substitution of the ring at the position of ring-opening with an isopropyl group as in α-phellandrene has far-reaching consequences for the photochemical dynamics. The substitution leads to the presence of several conformers of which we observe three in our static diffraction measurements. These three only differ by the rotation of the isopropyl group around the bond to the carbon ring. Our time-resolved experiments show a considerably slower appearance of the ring-opening signature in αPh than in CHD. Comparison with AIMS simulations only yields a high level of agreement if all three conformers are considered. The simulations reveal substantial differences in nature and time scale of the photochemical dynamics between the conformers. This marks to our knowledge the first real-time observation of conformer-specific photochemical dynamics.