Vibrational spectra of neutral and doped oligothiophenes and polythiophene

We have measured the infrared, Raman and inelastic neutron scattering (INS) spectra of a series of oligothiophenes (bithiophene, terthiophene, quarterthiophene, sexithiophene and octithiophene) and polythiophene, both pristine and after doping with iodine. The spectra of the pristine (i.e. neutral) systems show a rapid convergence towards the spectrum of polythiophene, such that the spectra of sexithiophene and octithiophene are almost indistinguishable from that of polythiophene. The spectra, in combination with periodic density functional theory calculations, have also provided the first complete assignment of polythiophene. In contrast to the infrared and Raman spectra that show dramatic changes on doping, the INS spectra show only small changes. Isolated molecule DFT calculations show that the molecular structures are not greatly modified on doping and since the INS spectrum largely depends on the structure, this does not change much. In contrast, as shown by others, the electronic structure is greatly modified and this accounts for the major changes in the infrared and Raman spectra.