Regiochemical Effects of Furan Substitution on the Electronic Properties and Solid-State Structure of Partial Fused-Ring Oligothiophenes

Oligomers containing the new fused-ring heterocyclic
conjugated
building block thieno­[3,2-b]­furan were synthesized,
and the effects associated with furan ring substitution into fused-ring
oligothiophenes on the electronic properties and solid-state structure
were assessed. A series of four-ring oligomers which vary in the degree
of furan ring substitution and the regiochemistry of placement were
synthesized via Stille cross-coupling and oxidative homocoupling strategies.
The electronic properties of these oligomers were studied by UV–vis
absorption and fluorescence spectroscopies. Substitution of furan
rings at the terminal positions yields oligomers with a narrower HOMO–LUMO
gap relative to the all-thiophene analogue 2,2′-bithieno­[3,2-b]­thiophene, and incorporation of furan rings at the interior
positions results in oligomers with an increase in rigidity and a
higher fluorescence quantum yield. Packing motifs of the oligomers
were determined using single crystal X-ray diffraction. In contrast
to the herringbone crystal packing observed for nonfused oligothiophenes,
oligofurans, thiophene–furan hybrid oligomers, and the all-thiophene
analogue 2,2′-bithieno­[3,2-b]­thiophene, all
three regioisomers derived from the dimerization of thieno­[3,2-b]­furan arrange in a π-stacked packing motif in the
solid state.