THEORY OF MOLECULAR EXCITONS IN THE PHENYL-BASED ORGANIC SEMICONDUCTORS

Starting from the Pariser-Parr-Pople model of pi conjugated systems, a model of the low-lying excitations of poly(paraphenylene) (PPP) and p oly(para-phenylenevinylene) (PPV) is introduced, based on the bonding HOMO and LUMO states of the repeat units. The inter-phenyl hybridisati on is treated as a phenomenological parameter. The model is solved usi ng exact diagonalisation and the density matrix renormalisation group method. The predicted exciton energies for PPV agree well with absorpt ion experiments on short oligomers and thin films. For a 15 phenylene- unit oligomer, an exciton gap of 2.7 eV is predicted, with a binding e nergy of ca. 1.4 eV. For PPP the exciton gap is 3.0 eV and the binding energy is ca. 1.7 eV. (C) 1997 Elsevier Science B.V.