From linear quaterthiophene to sulflower: A comparative theoretical study

In this paper, we report a theoretical study of four types of thiophene-based oligomers showing the same number of CC double bonds and very different molecular structures. The comparative study has been performed on the basis of B3LYP/6-31G∗∗ calculations. The way the thiophene rings are linked together has a remarkable influence on the molecular and electronic properties. Linear quaterthiophene and heptathienoacene show similar aromatic structures but a loss of π-conjugation is detected for the latter due to the condensation of thiophene rings. A blue shift of the most intense electronic transition is predicted for fused heptathienoacene compared with non-fused quaterthiophene. Cyclic quaterthiophene exhibits quinoid thiophene rings folded in an envelope shape and should be visualized as a sulphur-bridged, cis-transoid polyenic chain. Circularly-fused sulflower presents a loss of π-conjugation with respect to linear systems due to its highly-branched conjugated backbone. The loss of conjugation and the high symmetry of the molecule determine that sulflower shows no optical absorption in the visible or near-UV.