ULTRAFAST PHOTOCHEMICAL PERICYCLIC-REACTIONS AND ISOMERIZATIONS OF SMALL POLYENES

Potential energy surfaces for pericyclic reactions can easily be predi cted by means of correlation diagrams. Whereas the initially excited s tate (called 1B in this paper) may vary from molecule to molecule, its population is collected by a lower, dark state (2A) of always the sam e nature, which corresponds to a two-electron excitation. From there, the system leaves to the ground states of educt and product(s) via a c onical intersection (CI). Using transient absorption and transient ion ization spectroscopy, we measured the lifetimes t(1B) and t(2A) of the se states after initiating electrocyclic ring opening of 1,3-cyclohexa diene and some of its derivatives (alpha-terpinene, alpha-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 ne eds some time to find the exit (the CI) from the 2A surface, whereas i n 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 nor bornene and norbornadiene (t(2A) = 220 and 210 fs) and the fast intern al conversions of cycloheptatriene (80 fs) and several UV stabilizers. The latter two processes are suggested to take the same pathway as th e pericyclic hydrogen shifts in these molecules. We also point out the similarity of the pericyclic potential surfaces to those of cia-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 bran chings and competing reactions play a role.