Raman spectra of the new low bandgap polymer poly(2,3-substituted thieno [3,4-b]-pyrazine)

Summary form only given. Vibrational modes of a new processable polymer with a bandgap as low as 0.95 eV have been measured by resonance Raman scattering and calculated by a quantum chemical method. Raman spectroscopy of the pristine polymer revealed two strong lines at about 1520 cm/sup -1/ and 1563 cm/sup -1/ in the C=C stretching mode region. By changing the exciting laser line these modes exhibit a dispersion of 14 and 25 cm/sup -1//eV, respectively. Substitution at the 2,3-positicns with alkyl groups has only a weak influence on the vibrational properties. The mode at 1520 cm/sup -1/ shifts moderately towards lower frequencies with increasing chain length. However, the utilization of thiophene rings as substituents results in a much higher mode softening. In addition, the Raman response of FeCl/sub 3/-doped samples is presented. A change in relative intensities and different line shifts were observed. In order to determine the structure of the ground state Raman spectra were calculated for the quinonoid and aromatic form and compared with the experimental data. It turned cut that the two strong lines experimentally observed originate from an interring vibration and from a mode which mainly comes from the thiopbene rings along the polymeric backbone.