Theory of photoexcitations in phenylene-based polymers

Abstract

We show that optical absorption spectra of polyphenylenes can be explained only within theoretical models that explicitly include the Coulomb interaction among the (pi) -electrons. We also show that the dominant effect of substitution on the electronic structure of polyphenylenes within Coulomb correlated models is broken spatial symmetry, while broken charge conjugation symmetry plays a rather weak role. The broken spatial symmetry has a subtle, and weak, effect on the optical absorption spectrum. Consequently, optical absorption spectra of unsubstituted polyphenylenes and the substituted derivatives are nearly identical. Comparison of theoretical and experimental absorption spectra leads to the conclusion that the exciton binding energy in a long chain of poly(para-phenylenevinylene) is about 0.9 eV. Such a large binding energy would be in agreement with nonlinear spectroscopic measurements and pump-probe experiments. However, the present work also indicates that the experimental polymers actually consist of short chains with the chain length distribution peaking at about 10 phenylene units. The gaps between the energy levels above the calculated continuum threshold are much too large for transport to be an intrachain process. Photoconductivity may be predominantly an interchain process, and probably measures the dissociation energy of the exciton which is different from the exciton binding energy.