Raman spectra and ground state of the new low bandgap polymer poly(thienopyrazine)

Raman modes of the new processable polymer poly(2,3-alkyl-thieno[3,4-b]-pyrazine) with a bandgap of about 0.9 eV were measured and calculated. Raman spectroscopy revealed two strong lines at about 1520 cm^-1 and 1560 cm^-1 in the CC stretching mode region. By changing the exciting laser line these modes exhibit a dispersion of 16 and 25 cm^-1/eV, respectively. 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 method we calculated the Raman spectrum of the quinonoid form and identified the mode at 1520 cm^-1 as the inter-ring A_g vibration.