RAMAN-SPECTRA OF POLY(2,3-R,R-THIENO[3,4-B]PYRAZINE) - A NEW LOW-BAND-GAP POLYMER

Vibrational modes of the new processable polymer poly(2,3-R,R-thieno[3,4-b]pyrazine) with a band gap of about 0.9 eV were measured by resonance Raman scattering and calculated by a quantum chemical method. Raman spectroscopy of the pristine polymer revealed a strong line at about 1520 cm(-1) and doublet structure at around 1560 cm(-1) in the C=C stretching region. By changing the exciting laser line these lines exhibit a dispersion which is about 16 cm(-1)/eV for the line at 1520 cm(-1). Substitution at the 2,3-positions with alkyl groups has only a weak influence on the vibrational properties. Several modes shift moderately toward lower frequencies with increasing chain length. The utilization of thiophene rings as substituents results in a much more efficient mode softening. In addition, the Raman response of FeCl3-doped samples is presented. Changes in relative intensities and weak line shifts were observed. Calculations on the ground state revealed that the quinonoid structure is by about 13 kcal/mol per repeat unit more stable than the aromatic form. By using the scaled quantum mechanical oligomer force field (SQMOFF) method the Raman spectrum of the quinonoid form was calculated. According to this the lines observed experimentally in the C=C stretching mode region originate from the inter-ring A(g) vibration and from an A(g) mode which mainly comprises intra-ring stretches superposed with a B-2g mode. The vibrational results are consistent with the calculated quinonoid ground-state geometry.