Molecular vibrations of [n]oligoacenes (n=2−5 and 10) and phonon dispersion relations of polyacene

As model compounds for nanosize carbon clusters, the phonon dispersion curves of polyacene are constructed based on density functional theory calculations for [n]oligoacenes (n=2−5, 10, and 15). Complete vibrational assignments are given for the observed Fourier-transform infrared and Raman spectra of [n]oligoacenes (n=2−5). Raman intensity distributions by the 1064-nm excitation are well reproduced by the polarizability-approximation calculations for naphthalene and anthracene, whereas several bands of naphthacene and pentacene at 1700−1100 cm−1 are calculated to be enhanced by the resonance Raman effect. It is found from vibronic calculations that the coupled ag modes between the Kekulé deformation and joint CC stretching give rise to the Raman enhancements of the Franck–Condon type, and that the b3g mode corresponding to the graphite G mode is enhanced by vibronic coupling between the L1a(B11u) and B1b(B12u) states. The phonon dispersion curves of polyacene provide a uniform foundation for understanding molecular vibrations of the oligoacenes in terms of the phase difference. The mode correlated with the defect-sensitive D mode of the bulk carbon networks is also found for the present one-dimensional system.