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 pi-stacked packing motif in the solid state.