High energy excited states in conjugated polymers and charge-transfer solids.

Abstract

Within the framework of the extended-Hubbard model, theoretical studies on the intensities of two-photon absorption (TPA) of the even-parity states in conjugated polymers show relatively large contributions from the exciton mA(g) state. The TPA intensities due to the 2A(g) as well as other sub-gap even-parity states are demonstrated to be extremely weak in the long chain limit, independent of their locations relative to the optically allowed exciton. We show our results have important implications for the interpretation of third harmonic generation (THG) and TPA spectra of several conjugated polymers. We have also probed the higher energy states that are reached by two-electron excitations from the ground state. Evidence for the biexciton, a bound state of two excitons that occurs below the two-electron continuum, is found. The lowest biexciton state is of even parity, call be reached by optical excitation from the 1Bᵤ exciton, and also by two-photon excitation from the ground state. Interpretation of a recent picosecond photoinduced absorption experiment on polyparavinylene (PPV) and TPA on polysilanes (PS) in terms of the biexciton will be given. Multiexciton states have experimentally found in a linear chain mixed-stack charge-transfer solid. Our theoretical studies based on the extended-Hubbard Hamiltonian demonstrate that such stable multiexciton states in this class of organic systems are due to the Coulomb interactions.