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 resona nce Raman scattering and calculated by a quantum chemical method. Rama n 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 st retching region. By changing the exciting laser line these lines exhib it 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 moderate ly toward lower frequencies with increasing chain length. The utilizat ion of thiophene rings as substituents results in a much more efficien t mode softening. In addition, the Raman response of FeCl3-doped sampl es is presented. Changes in relative intensities and weak line shifts were observed. Calculations on the ground state revealed that the quin onoid structure is by about 13 kcal/mol per repeat unit more stable th an the aromatic form. By using the scaled quantum mechanical oligomer force field (SQMOFF) method the Raman spectrum of the quinonoid form w as calculated. According to this the lines observed experimentally in the C=C stretching mode region originate from the inter-ring A(g) vibr ation and from an A(g) mode which mainly comprises intra-ring stretche s superposed with a B-2g mode. The vibrational results are consistent with the calculated quinonoid ground-state geometry.