EFFICIENT PI-ELECTRONS DELOCALIZATION IN ALPHA,ALPHA'-DIMETHYL END-CAPPED OLIGOTHIOPHENES - A VIBRATIONAL SPECTROSCOPIC STUDY

alpha,alpha'-dimethyl substituted oligothiophenes with increasing chai n length (from the dimer up to the hexamer) were recently synthesized by chemical methods. In this paper we have investigated the vibrationa l Fourier transform-IR and Fourier transform-Raman spectra of solid al pha,alpha'-dimethyl substituted oligothiophenes in the neutral state. The data are consistent with the existence of a chain-length dependent pi electron delocalization: a large frequency dispersion with conjuga tion length is observed for some Raman and infrared active vibrational modes, particularly at the high-energy side of the aromatic C=C stret ching region. Vibrational assignments are proposed for the main vibrat ional features in the whole spectral range. This vibrational spectrosc opic analysis of the solid samples thus becomes a tool for deriving in formation on the structure of these neutral materials in solution and in the doped state. (C) 1996 American Institute of Physics.