Fused-ring poly- and oligothiophenes: Designed electronic materials.

Abstract

Thiophene-based conjugated materials have received much attention as a novel class of organic semiconducting materials due to their excellent electronic and optical properties. To determine the influence of planarization on the properties of these conjugated molecules, a series of fused-ring poly- and oligothiophenes were designed and synthesized. Their properties, including electronic/electrochemical properties and solid-state packing, were studied and compared to the non-fused analogs. Two series of planarized oligomers were synthesized. Fused-ring oligothiophenes, in which one or more thiophene rings were replaced by thieno[3,2-<italic> b</italic>]thiophene and dithieno[3,2-<italic>b</italic>:2',3'-<italic>d</italic>]thiophene units, were prepared by a combination of cross coupling and oxidative coupling reactions. Fully fused oligothienoacenes were synthesized by Pd-catalyzed coupling of Bu₃SnSSnBu₃ with beta-Br-substituted oligomers to introduce sulfur linkages followed by oxidative ring closure. Removable solubilizing groups were introduced for oligomers with poor solubility to facilitate purification. The structural similarity of these oligomers offers the flexibility to systematically explore the ring fusion effect on the properties of oligothiophenes. It was found that oligomers with the same number of double bonds, but varying in the number and position of sulfur linkages, display a similar longest wavelength of absorption in solution. However, the introduction of sulfur linkages into these oligothiophenes leads to a blue shift of the longest wavelength emission and a correspondingly smaller Stokes shift. Although ring fusion has little effect on the solution absorption spectra, the solid-state spectra are dramatically different because the number and position of sulfur linkages alters the intermolecular interactions. X-ray diffraction data suggest that introduction of sulfur linkages favors pi-stacked packing motifs in contrast to the herringbone packing of non-fused oligothiophenes. Incorporation of fused-ring units into conjugated polymers was investigated by synthesizing and characterizing a series of alkyl-substituted polythieno[3,2-b]thiophenes. The mono-alkyl substituted polymer prepared via Stille coupling of a differentially functionalized monomer displayed a longer lambda_max of absorption and a corresponding lower optical band gap when compared to regiorandom polymers. Introducing a second alkyl chain into the monomer leads to decreased conjugation as observed in both electronic spectra and electrochemistry of a dialkyl-substituted polymer.