Femtosecond dynamics of hydrogen migration and internal conversion in cycloheptatriene and derivatives studied by intense-field dissociative ionization

Abstract

In the photochemical [1,7]-sigmatropic H shift in 1,3,5-cycloheptatriene (CHT) and its 7-methyl derivative, we can distinguish three consecutive phases in the excited states, all of them having time constants <100 fs. The ultrafast rates prove a continuous pathway from the initially excited 1 A″ state via the dark state 2 A′ to the ground-state surface via two easily accessible conical intersections. We assign the first two times (10 and 60 fs in CHT) to travelling through two regions of 1 A″ and the third (70 fs in CHT) to the departure from 2 A′ and repopulation of the ground state. Perdeuteration of 7-methyl-CHT only lengthens the times for region 2 and 3 (by a factor of 1.5). So, the direction of motion changes after region 1 (Franck–Condon region), only then involving the hydrogen. We suggest that the geometries of both conical intersections are displaced from that of the ground state along two coordinates: one which is Franck–Condon active (a′ CC stretch+torsion) and one (a″) which shifts an H of the CH2 group.