Butterfly Molecules: How Cross-Stacking Determines Bulk Physical Properties

Butterfly shaped molecules are fascinating pi-conjugated compounds that exhibit high fluorescence efficiencies in solid state. In this work, the photophysical properties of a set of four butterfly molecules are studied in solution and solid phase using density functional theory calculations. Semiconducting properties have also been evaluated to shed light into the potential applications of these compounds in electroluminescent devices. Reorganization energy has been calculated for both electronic excitation processes and ionizations showing that, when the molecules relax from both the excited charged state and the excited neutral state, there is a group of normal modes which always assist the relaxation process, irrespective of the electronic nature of the excited state. To our knowledge, this is the first work where this interesting conclusion has been evidenced by theoretical calculations. In addition, DFT calculations have been performed to elucidate the main geometric changes that occur in both the ground and excited (neutral) state when going from solution to solid state and also after the ionization process. A significant planarization of the electronic excited (neutral) state is predicted in both solution and solid phase that could restrict nonradiative deactivation pathways through intramolecular motions favoring the fluorescence emission. Our results point out that the selected compounds could be good candidates as materials for OLEDs due their excellent predicted intrinsic photophysical and charge transport properties.