Toward the rational design of conjugated oligomers and polymers: Systematic study of the substituent effect in oligothiophenes

A systematic study of substituent electronic effects on a number of oligothiophene properties was performed using well-defined oligothiophenes of various chain lengths that were monosubstituted with classical electron-withdrawing or -donating groups. The main focus was to understand how the effect of a substituent on one thiophene ring was transmitted through the other thiophene rings in the oligomer. The physical and electrochemical properties were studied by optical absorption and emission spectroscopy and by cyclic voltammetry. The experimental data were complemented with theoretical calculations of HOMO-LUMO gaps, charge distribution, bond length alternations, and ¹³C NMR spectra. We show that substitution has a significant electronic effect on the substituted ring; however, the effect decreases strongly over conjugation. A semiquantitative evaluation of the substituent effect in a well-defined long conjugated system was obtained for the first time using the Hammett equation.