Effect of Local Molecular Structure on the Chain-Length Dependence of the Electronic Properties of Thiophene-Based π-Conjugated Systems

Three series of thiophene-based π-conjugated oligomers built with different combinations of
thiophene cycles and double bonds have been synthesized and characterized. The analysis of the
chain length dependence of the electronic, electrochemical, and vibrational properties of the three
series of oligomers has been carried out using cyclic voltammetry, UV−vis, IR, and Raman
spectroscopies. These various investigations provide consistent results showing that incorporation
of ethylenic linkages in an oligothiophene structure leads to a faster decrease of the HOMO−LUMO
gap with chain extension due to the combined effects of enhanced planarity and lower overall
aromatic character of the system. Although the incorporation of two consecutive double bonds in
the system leads to a stabilization of the dicationic state, this structural modification does not
produce the expected further decrease of the HOMO−LUMO gap at large chain extension. This
phenomenon is discussed on the basis of an interplay between aromaticity and bond length
alternation.