Periodic decay in the photoisomerisation of p-aminoazobenzene

Semiempirical OM2/MRCI surface-hopping simulations have been performed to study the E -> Z and Z -> E isomerisations of p-aminoazobenzene upon photoexcitation to the S-1 state (n pi*). The overall mechanism is similar to the one found previously for the unsubstituted parent system, although there is a moderate speedup of the decay to the ground state because of the steeper excited-state potential between the Franck-Condon region and the conical intersection seam. The decay dynamics to the ground state shows an oscillatory pattern that can be attributed to an out-of-plane rotation of the N-2 moiety. The reaction is thus initially driven by N-2 rotation, which triggers phenyl rotations around the C-N bonds. The Z isomer is produced most effectively when the phenyl rings rotate in phase.