Molecular design and crystal engineering of oligothiophenes using dicyanovinyl (DCV) and tricyanovinyl (TCV) groups

Abstract

Oligothiophenes constitute an important class of conjugated organic materials with unique optical and electrical properties. These properties not only depend on the molecular structure but also on solid-state packing. In this highlight, we summarize the results of our work on the synthesis and crystal structures of a series of oligothiophenes endowed with strong electron accepting groups, namely the dicyanovinyl (DCV) and tricyanovinyl (TCV) groups. We focus on the impact of the presence of these strong electron-accepting groups on the solid-state structures and the dominant inter- and intra-molecular interactions in these materials. The main features observed in these studies include the role of intramolecular CN⋯S interactions in enforcing molecular planarity while promoting π-stack formation, presumably by depleting electron density in the parent compounds. Additionally, some crystals were found to exhibit syn-conformations of sulfur atoms, a feature that is rare in oligothiophenes. We further examine the impact of the presence of molecular structural modifications commonly employed in thiophene chemistry, such as the introduction of vinylene bridges, ethylenedioxy groups or EDOT; bromo- and solubilizing alkyl groups, on solid-state packing motifs. The evolution and modulation of inter- and intra-molecular interactions upon such molecular modification can lead to a better understanding of their role in the observed properties of both oligomers and the corresponding polymers. Seventeen structurally related oligothiophenes, endowed with electron accepting groups, are included in this highlight. We will also briefly discuss how these groups influence the optical, vibrational, and electrochemical properties of these compounds as these are important properties for the development of optical, Raman and electrochemical sensors for biological and environmental applications.