title: Dihydroazulene/vinylheptafulvene photochromism: a model for one-way photochemistry via a conical intersection.
abstract: Dihydroazulene (DHA)/vinylheptafulvene (VHF) photochromism has been investigated by studying the isomerization of 1,2,3,8a,9-pentahydrocyclopent[a]azulene-9,9-dicarbonitrile through complete active space-self consistent field calculations on the ground (S(0)) and first excited (S(1)) states of smaller model compounds. In each case, the S(1) reaction coordinate is characterized by a transition structure for adiabatic ring opening, connecting a DHA-like intermediate to a much more stable VHF-like structure. This VHF-like structure is not a real S(1) minimum but a crossing (i.e., a conical intersection) between the excited- and ground-state potential energy surfaces. The existence of such a crossing is consistent with the lifetime of approximately 600 fs recently measured for the DHA-like intermediate on S(1) (Ern, J.; Petermann, M.; Mrozek, T.; Daub, J.; Kuldova, K.; Kryschi, C. Chem. Phys. 2000, 259, 331-337). The shape of the crossing is also crucial; it not only explains the fact that the quantum yield approaches 1.0 for the forward DHA --> VHF reaction, but also the lack of any fluorescence or photochemical back-reaction from VHF. These findings are supported by ab initio direct dynamics calculations. This work suggests that calculating and understanding the topology of excited-state potential energy surfaces will be useful in designing photochromic molecules.
authors.affiliation: Department of Chemistry, King's College London, Strand, London WC2R 2LS, UK.
authors.forename: Martial
authors.initials: M
authors.lastname: Boggio-Pasqua
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bearpark
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Hunt
authors.forename: Michael A
authors.initials: MA
authors.lastname: Robb