The vibronic band structure of the excitonically coupled S₁←S₀ and S₂←S₀ excitations of the 2-aminopyridine self-dimer (2AP)₂ is analyzed using a linear vibronic coupling model . A major vibronic coupling contribution arises from the intramolecular 6a’ vibration. However, the low energy intermolecular shear (χ’) and stretching (σ’) vibrational excitations also exhibit excitonic splittings, which is not explained by theory.
The S₁/S₂ electronic energy gap calculated by the RICC2 method is 340 cm^-1. The vibronic coupling to the Franck-Condon active intramolecular vibrations reduces this by a factor of 40 resulting in an experimental excitonic splitting of 11 cm^-1.

This work was carried out in the group of Prof. Samuel Leutwyler.

References:

• Philipp Ottiger, Samuel Leutwyler and Horst Köppel;
  "S₁/S₂ excitonic splittings and vibronic coupling in the excited state of the jet-cooled 2-aminopyridine dimer"
  J. Chem. Phys., 131, 204308/1-11, (2009); doi:10.1063/1.3266937.