From molecular states to band structure: Theoretical investigation of momentum dependent excitations in phenylene based organic materials

The electronic states in conjugated organic materials are usually characterized either by quantum-chemical approaches relying on a molecular picture, or by solid-state band structure models requiring infinite periodicity of the chains. In this contribution we present calculations showing that a band structure like character is inherent also in molecular systems and discuss the gradual evolution of a full band structure upon increasing the number of repeat units in oligomer molecules. The main feature of a band structure approach is the description of the electronic eigenstates in momentum space. We have therefore developed a method to describe momentum dependent excitation processes in conjugated organic materials based on a post-Hartree-Fock approach including electron-electron correlation effects. The results of the simulations performed for oligo(phenylene)s of different chain lengths agree very well with experimental electron energy loss spectroscopy (EELS) data.