SPECTROSCOPIC EVIDENCE FOR ASYMMETRIC POLARON STATES IN POLY[3-(4-OCTYLPHENYL)-THIOPHENE]

The strong electron-phonon coupling present in conjugated polymers has been amply demonstrated in the existence of solitons, polarons and bipolarons. For the non-degenerate ground state systems, the early theory of Campbell, Fesser and Bishop only considered Hamiltonians With explicit electron-hole symmetry. As a consequence, two of the allowed optical transitions of polarons are degenerate, and in total three absorption features are expected below the bandgap transition. We report spectroscopic observation of a polaron state in a polymer from the polythiophene family, poly[3-(4-octylphenyl)-thiophene]. This polymer shows four distinct optical transitions below the bandgap. This, we suggest, is due to the broken electron-hole symmetry due to coupling between the conjugated main chain and the aromatic side chain.