Understanding ground- and excited-state properties of perylene tetracarboxylic acid bisimide crystals by means of quantum chemical computations.

Quantum chemical protocols explaining the crystal structures and the visible light absorption properties of 3,4:9,10-perylene tetracarboxylic acid bisimide (PBI) derivates are proposed. Dispersion-corrected density functional theory has provided an intermolecular potential energy of PBI dimers showing several energetically low-lying minima, which corresponds well with the packing of different PBI dyes in the solid state. While the dispersion interaction is found to be crucial for the binding strength, the minimum structures of the PESs are best explained by electrostatic interactions. Furthermore, a method is introduced, which reproduces the photon energies at the absorption maxima of PBI pigments within 0.1 eV. It is based on time-dependent Hartree-Fock (TD-HF) excitation energies calculated for PBI dimers with the next-neighbor arrangement in the pigment and incorporates crystal packing effects. This success provides clear evidence that the electronically excited states, which determine the color of these pigments, have no significant charge-transfer character. The developed protocols can be applied in a routine manner to understand and to predict the properties of such pigments, which are important materials for organic solar cells and (opto-)electronic devices.