Charge-Transfer States in Pentacene: Dimer versus Crystal

According to recent ab initio calculations, the energy gap between the two oppositely polarized charge-transfer (CT) states of a model pentacene dimer is anomalously large, attaining 0.8 eV. Here we introduce the self consistent charge field approach to evaluate. the electrostatic stabilization energies of the pertinent states in the pentacene crystal represented by a dedicated multiscale model system containing the model dimer as its core. We demonstrate that, Contrary to common wisdom, the lower of the two CT states is barely affected by the crystalline environment whereas the upper one undergoes a large red shift. Effectively, embedding of the dimer in the crystal bulk reduces, the pertinent splitting by an order of-magnitude, because most of the intradimer charge-quadrupole interactions are compensated by similar interactions with surrounding molecules. This resolves the apparent contradiction between the ab initio result obtained for the dimer and the splitting of about 0.04 eV resulting from microelectrostatic calculations for the crystal.