Ground-State Geometry and Vibrations of Polyphenylenevinylene Oligomers

Conformational space of polyphenylenevinylene oligomers is systematically investigated computationally at energies relevant for room temperature dynamics in a solvent and in a solid state. Our calculations show that optimal oligomer structures are essentially planar. However, lack of a deep minimum at the planar geometry allows for large molecular deformations even at very low temperatures. At larger angles, rotational motion of dihedrals intermix with two orthogonal bending motions of the entire molecule. In a crystalline environment these degrees of freedom intermix with translational and rotational motions, whereas purely intramolecular modes are well separated. The reliability of our calculations is confirmed by an excellent match of the theoretical and experimental Raman spectra of crystalline stilbene in the entire spectral range including the low-frequency part. Obtained results provide important insights into nature of low-frequency vibrations, which play a key role in charge transport in organic semiconductors.