The dihydroazulene (DHA) / vinylheptafulvene (VHF) photochromism has been investigated, by studying the isomerization of 1,2,3,8a,9-pentahydrocyclopent[a]azulene-9,9-dicarbonitrile. The S1 reaction coordinate has a transition structure for adiabatic ring opening, connecting a DHA*-like intermediate to a much more stable VHF*-like structure. This VHF*-like structure (S1 minimum) is very close to an extended crossing seam (i.e., a conical intersection) between the excited and ground state potential energy surfaces. The reaction path intersects this extended crossing seam near the TS. 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. On the left hand side of the figure, we have plotted the three main co-ordinates involved in this reactivity problem. The X3 co-ordinate( x axis in the figure) connects the DHA and VHF equilibrium geometries and corresponds to the reaction co-ordinate. The co-ordinates X1 and X2 correspond to the branching space ( represented as the single co-ordinate).

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