The polarized infrared and Raman spectra of alpha-T6 single crystal: An experimental and theoretical study

The polarized infrared absorption and Raman spectra on the bc plane of a si ngle crystal of alpha-sexithienyl (alpha-T6) have been recorded in the rang e from 50 up to 3500 cm(-1). The intermolecular interactions give rise to t he Davydov splitting of the ungerade phonon energy levels originated by the molecular pyramidalization along with the CCH bending modes. A mu-Raman in vestigation on the crystal surface allows identification of the perturbed l ocal packing of the molecules, which are spectroscopically crystal defects. The selective resonant enhancement of the scattering intensity of some Ram an modes, obtained by exciting at lambda(ex) = 632.8 nm, indicates that the se defects are associated with the lowering of the electronic excited state s due to the increased pi-orbital overlapping. Moreover, the resonant enhan cement allows detection of some of the gerade intramolecular phonons not ob served in the bulk. The comparison between the far-infrared absorption spec tra recorded at room temperature and at 80 K evidences three lattice phonon s. The spectral analysis, based on a simulation, which uses a scaled ab ini tio molecular force field, well interprets the experimental data assigning all the internal phonons and elucidates some of the effects of the intermol ecular interactions. The calculations are performed at the HF/6-31 G* level of the theory to obtain the harmonic frequencies and the dipole moment and polarizability tensor derivatives of the alpha-T6 molecule. Both polarized infrared and Raman spectra simulations are carried out calculating the opt ical response of a unit cell of the alpha-T6 single crystal belonging to th e C-2h factor group in the monoclinic P2(1)/n space group. (C) 2000 America n Institute of Physics. [S0021-9606(00)70513-7].