Nature of photoexcitations in poly (paraphenylene vinylene) and its oligomers

Within a theoretical approach including both electron-phonon and electron-electron interaction terms, we investigate the relaxation processes of photoexcited states in oligomers and long chains of poly(paraphenylene vinylene), PPV. With regard to the vibronic structure, we find that the Huang-Rhys factor decreases monotonically as chain length increases. Our direct calculations indicate that in PPV the difference in binding energy between two polarons and one singlet (triplet) polaron-exciton decreases (increases) with the Hubbard U term; the long-range interaction V terms stabilize both the singlet and triplet polaron-excitons. The theoretically simulated photoinduced absorption spectra agree with experiment both for singlet and triplet states.