Raman spectroscopic studies of regioregular poly(3-alkylthiophenes)

Raman spectra of neutral and electrochemically doped regioregular poly(3-decylthiophene) were studied using different excitation lines, namely, 457.9, 514.5, 676, and 1064 nm. Contrary to the case of nonregioregular poly(3-decylthiophene) obtained by polymerization with FeCl3, in regioregular polymer the position of the principal band (due to the C-alpha=C-beta stretchings) is essentially independent of the excitation wavelength, proving its high structural homogeneity and extended conjugation. Raman spectroelectrochemical studies of oxidative doping of regioregular poly(3-decylthiophene) can be clearly correlated with cyclic voltammetry and UV-visible-near-IR spectroelectrochemistry provided that appropriate excitation wavelengths are selected. In particular, the use of the red (676.4 nm) and near-IR (1064 nm) excitation lines enables us to register the doping induced changes in the Raman spectra, whereas the green (514.5 nm) line always reveals the vibrations from the undoped segments of the polymer chain independently of the doping level. Finally, vibrational calculations, based on the symmetrized dynamical matrix, performed for undoped and doped poly(3-decylthiophene) led to a good agreement between the calculated and experimentally observed frequencies.