Dihydroazulene/vinylheptafulvene photochromism: dynamics of the photochemical ring-opening reaction

While 1,2,3,8a,9-pentahydrocyclopent[a]azulene-9,9-dicarbonitrile (1-DHA) u ndergoes a photochemical ring-opening reaction with a quantum yield of near ly 1 to the corresponding vinylheptafulvene isomer (1-VHF), the backward (r ing-closure) reaction does not occur photochemically but as thermally assis ted ground-state reaction with an Arrhenius activation energy of about 79 k J mol(-1) in silicone. The dynamics of the photochemical ring-opening react ion of 1-DHA in acetone solution was investigated using femtosecond-resolve d transient absorption spectroscopy with an apparatus time resolution of ab out 560 fs. Both decay of transient absorption of S-1-excited 1-DHA and ris e of the stationary S-0-S-1 absorption of the photoproduct occur monoexpone ntially with a lifetime tau(DHA) of about 600 fs. Since the ring-opening re action quantum yield is of about 1, tau(DHA) corresponds to the ring-openin g reaction time constant tau(RO). It is conceivable to conclude from these spectroscopic data that the ring-opening reaction of 1-DHA proceeds across a conical intersection to the S-0 potential energy hypersurface. (C) 2000 E lsevier Science B.V. All rights reserved.