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

While 1,2,3,8a,9-pentahydrocyclopent[a]azulene-9,9-dicarbonitrile (1-DHA) undergoes a photochemical ring-opening reaction with a quantum yield of nearly 1 to the corresponding vinylheptafulvene isomer (1-VHF), the backward (ring-closure) reaction does not occur photochemically but as thermally assisted ground-state reaction with an Arrhenius activation energy of about 79 kJ mol−1 in silicone. The dynamics of the photochemical ring-opening reaction of 1-DHA in acetone solution was investigated using femtosecond-resolved transient absorption spectroscopy with an apparatus time resolution of about 560 fs. Both decay of transient absorption of S1-excited 1-DHA and rise of the stationary S0–S1 absorption of the photoproduct occur monoexponentially with a lifetime τDHA of about 600 fs. Since the ring-opening reaction quantum yield is of about 1, τDHA corresponds to the ring-opening reaction time constant τRO. It is conceivable to conclude from these spectroscopic data that the ring-opening reaction of 1-DHA proceeds across a conical intersection to the S0 potential energy hypersurface.