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

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posted on 20.02.2016, 08:30 by John T. Henssler, Adam J. Matzger
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.