Vibrational analysis as a function of the chain-length of well-barrier-well π-conjugated thiophene-based oligomers

We have analyzed the size dependence of the infrared and Raman active modes of two neutral thiophene-based oligomers in the solid state. The compounds have a well-barrier-well structure, where the well parts are two bithiophene or terthiophene end-moieties and the barrier part is a non-conjugating ethylene spacer. Relevant features are correlated with the vibrational data previously collected on a series of medium-size α,αʹ-dimethyl end-capped oligothienyls. Experiments indicate the existence of a rather effective conjugation in the case of 1,2-Di-[5-(2,2′:5′,2″-terthienyl)]-ethane, in spite of the introduction of sp3 defects into the oligomer backbone