Ab initio investigation of conformational and excitation energies of phenylene vinylene oligomers

Abstract

Poly( para-phenylene vinylene) (PPV) and oligo(phenylene vinylene) (OPV) are well known conjugated polymers and oligomers that have been used as active electroluminescence/photoluminescence material in various devices such as, for example, light emitting diodes and lasers. Using the restricted Hartree–Fock (RHF/3-21G ∗) method we analyze the structure of the lowest energy state (the ground state) and various higher conformational energy states of OPVs in both gas phase and solvent. In particular, the planarity assumption is closely examined. This is followed by the use of restricted configuration interaction/singles (RCIS/3-21G ∗) approach to investigate the structure of the lowest (singlet) excitation energies. The RCIS/3-21G ∗ approach produces singlet excitation energies for OPVs that are in good agreement with the experimental values (in most cases differences are less than 0.2 eV). The results of RCIS/3-21G ∗ oligomer calculations show that delocalized singlet excited states are accompanied by a geometry relaxation in comparison to their ground state (HF) geometries. The geometry relaxation energy was estimated to be approximately 0.3 eV for PPV. The shape (i.e. the width and the depth) of the relaxation depends on the length of the oligomer for the shorter oligomers, eventually leveling off (between 5 and 10 phenylene vinylene units) for the longer chains.