THEORETICAL VIBRATIONAL-SPECTRA OF A BIPOLARONIC DEFECT IN OLIGOTHIOPHENES AND IN POLYTHIOPHENE - A COMBINED SEMIEMPIRICAL AND AB-INITIO STUDY

The vibrational spectra of isdlated, alpha,alpha'-linked oligothiophene dications, [H(C4H2S)(n)H](2+), with n = 4 and 5, have been computed at the ab initio SCF level. The results obtained are compared with earlier data for the cases n = 2 and 3, with semiempirical predictions as obtained for n = 2 to 15, and with analogous results on neutral benzenoid and quinonoid oligothiophenes. The trends in the computed harmonic force fields, vibrational frequencies, IR intensities, and Raman activities are monitored as a function of the chain length. The data are analyzed in conjunction with the trends in computed equilibrium geometries and charge distributions. Instead of discussing the complete spectra in detail, we emphasize the spectral region of the carbon-carbon stretching frequencies characteristic for conjugated systems. We also point out the relation between our data and the effective conjugation coordinate model by an analysis of the dipole moment derivatives with respect to the C-C stretching coordinates and appropriate linear combinations thereof. Many features of the IR spectra of the dications, in particular the appearance of several bands with exceedingly large intensities, may be rationalized in this way. We predict a fast convergence of the Raman spectra of the dications with respect to chain length elongation, since the most intense Raman bands Stem from normal modes largely localized at the defect center. On the other hand, the IR spectra of the dications converge very slowly, since the most intense bands originate from rather delocalized normal modes at the edges of the defect regions, where the C-C stretching force field itself is quite delocalized.