Ultrafast exciton quenching upon geometry deformation in molecular aggregates

The measured absorption spectra could successfully be interpreted assuming a dipole-dipole coupling in the excited states and a single effective vibrational mode for each monomer. An additional intermolecular torsional mode was required to explain the fluorescence spectroscopic measurements indicating a long radiative lifetime of 33 ns and a low fluorescence yield. Up to this point, a consistent picture of the absorption and emission properties could be established. Whereas a more fundamental insight into the decay dynamics of the optically excited state and the reaction path which leads to the fluorescing state is missing up to date.

Femtosecond transient absorption measurements demonstrate that the excited state decays non-radiatively on an ultrafast time-scale of 215 fs. Due to the large moment of inertia, the torsional motion is too slow to reach the geometry where a curve crossing occurs. Thus, there must exist a different dimer geometry change which makes the non-adiabatic transition effective. We determine potential energy curves along a reaction coordinate relating the Franck-Condon geometry at which photon absorption occurs to a charge transfer configuration where the first monomer exhibits the anion geometry and the second the geometry of the cation.

Including phenomenologically an energy dispersion into modes different from the reaction coordinate (being inter-, intra-molecular or solvent modes), leads to results which are in excellent agreement with the experimental findings. Furthermore, it is shown that a coherent superposition of states in the reaction coordinate and other internal modes which are excited coherently may lead to quantum beats with the same oscillation period as seen in the measured transient absorption spectrum.