Ultrafast photochemistry of cyclopentadiene: Competing hydrogen migration and electrocyclic ring closure

Probing by dissociative intense-laser field ionization, we found that cyclopentadiene relaxes from the initial 1B2 state, excited at 240 nm, within 37 fs to the dark 2A1 state; a doubly exponential decay (71 and 333 fs) leads from there to S0. To explain the short times, we invoke conical intersections which we associate with photochemical pericyclic reactions: electrocyclization and a hydrogen shift. The latter reaction is dominant, as concluded from the deuterium isotope effect. A reaction (19 ps) found after departure from 2A1 is ascribed to thermal back reaction of the initially formed electrocyclization product bicyclo[2,1,0]pentene in the hot ground state, recovering cyclopentadiene. Comparing this molecule with two others, benzene and cycloheptatriene, we point out that ultrafast internal conversion is not governed by densities of states and matrix elements, but by the pathway on potential surfaces; the matrix-element approach tacitly assumes vertical transitions. But probably ultrafast radiationless transitions are always accompanied by large displacements of atoms or groups similar to that in chemical reactions, so that photophysics is a consequence of photochemistry in such cases. Both have the initial path in common, typically till the last conical intersection.