Even parity states in small band gap p-conjugated polymers: polydithienothiophenes

Using the photon energy induced blue shift of the resonant Raman scattering frequencies in inhomogeneous films ('RRS dispersion'), we extract the energy of the lowest even parity state of pi-conjugated polymers. We show that in the small band gap (approximate to1.1 eV) poly(dithienothiophenes), the experimentally observed unusually large RRS dispersion yields for the lowest even parity state (2A(g)) an energy, which is in the vicinity of the optically allowed state (1B(u)). (C) 2004 Elsevier B.V. All rights reserved.