The impact of vinylene bridges and side chain alkyl groups on the solid state structures of tricyanovinyl-substituted thiophenes

The goal of this work is to examine the solid state structures of compounds that have been designed for increased conjugation and solubility, as these factors are important if these compounds are to be used in the solid state. The impact of three commonly employed molecular design strategies on the solid state structures of three thiophene derivatives is reported herein. These strategies include: (i) introduction of a strong electron accepting group (2T-TCV, 1); (ii) increase in conjugation by introducing a vinylene bridge in the presence of a strong electron accepting group (2T-TCV with both a TCV group and a CC bridge, 2); and (iii) enhancing the solubility by introducing n-butyl side chain groups in the presence of both a strong electron accepting group and a CC bridge (2T-TCV containing a strong electron accepting group, a CC bridge and four n-butyl groups, 3). Compounds 1 and 2 crystallize with four molecules in the unit cell while the unit cell of compound 3 contains only two molecules. The torsion between the two thiophene rings increases from 4.39° to 5.50° to 5.75° for 1, 2, and 3, respectively. The short distances between adjacent molecules within the unit cell also increase from 2.84 Å in 2 to 3.47 Å in 3. We also note that while the sulfur atoms assume a syn conformation in both 1 and 2, they favor the anti-conformation in 3. DFT calculations show a small energy difference between the syn and anti-conformation for 1 and 2, i.e. 3.18 kJ mol^-1 and 3.19 kJ mol^-1, respectively; this energy difference is found to be greater for compound 3 with the anti-conformation being 17.47 kJ mol^-1 more stable than the syn conformation.