Ultrafast photochemical pericyclic reactions and isomerizations of small polyenes

Potential energy surfaces for pericyclic reactions can easily be predicted by means of correlation diagrams. Whereas the initially excited state (called 1 B in this paper) may vary from molecule to molecule, its population is collected by a lower, dark state (2A) of always the same nature, which corresponds to a two-electron excitation. From there, the system leaves to the ground states of educt and product(s) via a conical intersection (CI). Using transient absorption and transient ionization spectroscopy, we measured the lifetimes t_1B and t_2A of these states after initiating electrocyclic ring opening of 1,3-cyclohexadiene and some of its derivatives (α-terpinene, α-phellandrene and 7-dehydrocholesterol). t_2A can be considered the reaction time constant. Whereas t_2A = 5.2 ps for dehydrocholesterol. it is 80–100 fs for the other three molecules. We suggest that the former system needs some time to find the exit (the CI) from the 2A surface, whereas in the latter cases the CI is located right in the line of the steepest descent. We apply the same scheme to, and compile the time constants for, some other fast processes: the [1.3] sigmatropic reactions of norbornene and norbornadiene (t_2A = 220 and 210 fs) and the fast internal conversions of cycloheptatriene (80 fs) and several UV stabilizers. The latter two processes are suggested to take the same pathway as the pericyclic hydrogen shifts in these molecules. We also point out the similarity of the pericyclic potential surfaces to those of cis-trans isomerizations and give evidence that cis-stilbene (t_2A < 300 fs), too, is accelerated directly in the direction of the CI. For cis-trans isomerization of longer polyenes there are however several 2A minima and conical intersections. We also give examples where additional branchings and competing reactions play a role.