Vibronic splitting of the electronic origin in two conformers of the 3-tolunitrile dimer.

3-tolunitrile (3-TN) can from three different dimers, which differ in the relative orientation of the methyl groups. We determined the geometry changes of two of these conformers of 3-TN upon electronic excitation via a Franck-Condon fit of the vibronic intensities in the fluorescence emission spectra. Both aromatic rings expand upon electronic excitation, showing a delocalized one-photon excitation of the cluster. The conformer with the smaller COM distance shows an unusual order of the split components of the electronic origin. We attribute this changed order to the stronger CT contributions to the splitting and a partial breakdown of the point dipole approximation, made in the Frenkel type interaction.