Thiophenes Endowed with Electron-Accepting Groups: A Structural Study

Single crystal structures of six thiophene-containing molecules, endowed with strong electron-accepting groups, are reported. Each of the reported compounds is endowed with either one or two strong electron-accepting groups. These groups are dicyanovinyl (DCV) or –CH═C(CN)2; tricyanovinyl (TCV) or –C(CN)═(CN)2; cyanophenyl −Ph-CN and nitro −NO2. The molecules in this study are 2-([2,2′-bithiophen]-5-ylmethylene)malononitrile or 2T-DCV (1); 2-(thieno[3,2-b]thiophen-2-ylmethylene)malononitrile or thienothiophene-DCV (2); 2,2′-([2,2′-bithiophene]-5,5′-diylbis(methanylylidene))dimalononitrile or DCV-2T-DCV (3); 4,4′-(thiophene-2,5-diyl)dibenzonitrile (4); 2-(5″-(p-tolyl)[2,2′:5′,2″-terthiophen]-5-yl)ethene-1,1,2-tricarbonitrile (5); and 5,5′-dinitro-2,2′-bithiophene (6). A common and most noticeable feature found in these structures is the impact of the electron-accepting groups on promoting planarity and directing molecular packing in the solid state favoring the formation of π-stacks with varying distances ranging from 3.267 Å in 2 to 3.447 Å in 1. The presence of phenyl groups seems to result in less planar structures and favors herringbone motifs as seen in compounds 4 and 5. Planarity, close π-stacks, and the presence of additional strong interplanar and inter-stack interactions such as C–H···NC distances of 2.610 Å are also observed in 1–3 and 6. Hydrogen bonding between adjacent stacks was also observed. Intramolecular CN···S interactions with distances ranging from 3.267 to 3.364 Å were exhibited in all compounds with DCV or TCV groups. Such structural features are thought to be conducive to better charge transport in organic semiconductors. Our study found that fused thiophenes do not necessarily lead to more planar molecules in the presence of DCV groups presumably due to intramolecular N···S interactions. Case in point is compound 1, and a linear bithiophene derivative is found to be more planar than the fused bithiophene 2 even though both are endowed with the same electron-accepting group (DCV), as evident from experimental torsion angles. CCDC numbers are 2208323, 2208322, 2289032, 2270158, 2289048, and 2270317, for compounds 1–6, respectively.