The anomalous fluorescence of azulene - emission from S₂ rather than S₁ - was first recognised by Beer and Longuet-Higgins forty years ago. Femtosecond laser studies and spectroscopic linewidth measurements have now established that radiationless decay from S₁ to the ground state takes place in less than a picosecond. Our results show how such ultrafast S₁ decay can be explained by relaxation through an unavoided S₁/S₀ crossing - i.e. a conical intersection.

Img:Azulene Scheme The relaxation coordinate linking the Franck-Condon geometry with the S₁ minimum in C_2v symmetry is illustrated in the figure (bond lengths in angstrom, energies in kcal/mol). This coordinate involves compression of the transannular bond. At the S₁ minimum, the S₀-S₁ energy gap is approximately half that at the Franck-Condon geometry . Following this coordinate through the S₁ minimum the gap continues to shrink, leading ultimately to a conical intersection. Img:Azulene Ray-traced PES

We have modelled the decay using MMVB dynamics. Packets of trajectories were computed, starting from the Franck-Condon geometry, with initial conditions determined by random sampling of each excited state normal mode within an energy threshold.

Img:Azulene Trajectory The figure shows a potential energy plot for a typical trajectory with low initial kinetic energy. The azulene molecule starts at the Franck-Condon geometry, and from this point it begins to fall toward the minimum on the S₁ surface. During the descent, the energy separation between the two states decreases, and at the lowest point on the S₁ trajectory it is ~15 kcal/mol. The kinetic energy developed is sufficient to take the molecule through the conical intersection where it returns to the ground state and begins to execute large amplitude vibrations. The geometry at which the surface hop takes place is shown top-right. The energy profiles along the trajectory reflect the S₁ relaxation coordinate discussed above: the azulene conical intersection is sloped.

The dynamics simulations suggest that fast internal conversion will approach 100% efficiency for vertically excited azulene molecules, decay taking place within a few femtoseconds of excitation. At low kinetic energies, nearly all of the surface-hops occur within the first half vibrational period.

Subsequent experiments supported the proposed model:

A. J. Wurzer, T. Wilhelm, J. Piel, E. Riedle, Chem. Phys. Lett. 299, 296-302 (1999).
E. W.-G. Diau, S. De Feyter, A. H. Zewail, J. Chem. Phys. 110, 9785-9788 (1999).

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A Study of Azulene S1/S0 Conical Intersection Decay with MMVB Dynamics

"The Azulene S1 State Decays via a Conical Intersection: A CASSCF Study with MMVB Dynamics"

A Study of Azulene S₁/S₀ Conical Intersection Decay with MMVB Dynamics

"The Azulene S₁ State Decays via a Conical Intersection: A CASSCF Study with MMVB Dynamics"