Inter- and Intramolecular Interactions in Some Bromo- and Tricyanovinyl-Substituted Thiophenes and Ethylenedioxythiophenes

We
report herein on the competing inter- and intramolecular interactions
in seven structurally related thiophene and ethyelenedioxythiophene
(EDOT) molecules, substituted with bromine and/or tricyanovinyl (TCV)
groups in various combinations, using single crystal structural analyses.
Br···Br Interactions of less than 3.5 Å appear
to be dominant in the crystal structures of the dibromo EDOT molecules
and yet are almost nonexistent in 5,5″-dibromoterthiophene
(shortest Br···Br distances are >4.2 Å), indicating
a cooperative role involving the Br and the ethylenedioxy moiety.
Short Br···Br distances of 3.5 Å within stacks
and between adjacent stacks of molecules in crystalline dibromo EDOT
dimer (6) could be utilized for the preparation of highly
ordered polymers with the perfectly planar EDOT dimer as repeating
unit, similar to the work reported by Wudl. On the other hand, new dimeric motifs are formed in Br-EDOT-TCV
as strong S···N (3.03 Å) intermolecular interactions
in TCV-EDOT are replaced by competing N···Br (2.99
Å) interactions. Short intramolecular N···S distances
ranging from 3.2 to 3.3 Å are associated with small dihedral
angles between the TCV and thiophene planes ranging from 0.80 to 4.3
deg. A slight enhancement of molecular planarity apparently has a
profound impact on the extent of conjugation as evident from the CC
bond lengths (1.34–1.40 Å) and C–C (1.37–1.44
Å) within the thiophene rings. These findings suggest that N···S,
N···Br, and Br···Br inter- and intramolecular
interactions could be utilized as additional crystal engineering tools
to promote molecular planarity and arrangement of higher oligomers
in the solid state prior to polymerization of thiophene-based molecular
materials. On the basis of the current study, these interactions appear
to also enhance the stability of the structure and influence intramolecular
charge transfer and π-stack formation patterns.