Conjugated systems alternating benzene/thiophene condensed rings: effective conjugation and electrochemistry

Thiophene-based oligomers are a boundless class of organic semiconductors and starting materials for the preparation of electroactive films with attractive functional properties for energy, optoelectronic and sensor applications. Starting from the parent model series of linear -oligothiophenes, of high planarity and high conjugation efficiency but low solubility, many structure modifications have been developed so far, aiming to improve solubility and processability (e.g. by inserting alkyl chain substituents or adopting branched structures), to tune HOMO and LUMO energies and localization on the conjugated system (e. g. by inductive effects of appropriate substituents) and to enhance 3D properties, including chirality (e. g. purposedly inserting distorsions along the main conjugated backbone). The effective conjugation of the main conjugated backbone is a key parameter in determining the electronic properties of these molecular materials, such as their HOMO-LUMO energy gaps, which regularly decrease with increasing effective conjugation. Effective conjugation depends on the nature, the number, and the connectivity of the aromatic rings constituting the -conjugated system. We are currently investigating by electro¬chemical techniques the case of conjugated systems consisting of alternating condensed benzene and thiophene rings(1), resulting in higher conjugation with respect to both all-benzene and all-thiophene condensed systems, and having free  terminal thiophene positions. In this context, we will discuss the effect of ring connectivity contrasting the electrochemical behaviour of two benzodithiophene isomers (Figure 1), accounting for different planarity and different HOMO and LUMO energies and gaps; further tuning of HOMO and/or LUMO, in terms of both position and energy, can be achieved by adding convenient substituents on either the aromatic and/or the heteroaromatic rings. Furthermore, in the case of parent molecule 1b, increasing the number of condensed rings re-sults in helicoidal bending (for nring  4) affording the thiahelicene series, hinging on a helix-like conducting backbone with attractive 3D features and intramolecular -stacking effects.