VIBRATIONAL STUDIES OF A SERIES OF ALPHA-OLIGOTHIOPHENES AS MODEL SYSTEMS OF POLYTHIOPHENE

Vibrational properties of a series of individual pi-conjugated alpha-c oupled oligothiophenes alpha-nT have been investigated by resonance ra man scattering (RRS) and infrared absorption spectroscopies. The exper imental frequencies obtained on crystalline evaporated alpha-nT (n = 4 , 6, and 8 thiophene rings) thin films are compared with those calcula ted using a symmetrized dynamical matrix model. Special attention is g iven to the 1300-1600 cm(-1) region where the C-alpha=C-beta and C-bet a-C-beta stretching vibrations are active. Also, vibrational spectra o f doped alpha-sexithiophene in the solid state are reported where we o bserve remarkable changes when using different excitation wavelengths. It appears that upon doping, the alpha-sexithiophene polymerizes and the consequent doped poly(sexithiophene) presents a distorted conforma tion around the inter-ring single bonds. In addition to a complete ass ignment of the various vibrational modes of each alpha-nT molecule, ou r results on well-defined model oligomers shed some light on the contr oversial parent polythiophene and poly(3-alkylthiophenes).