A series of planar beta-linked oligothiophenes based on thieno[3,2-b]thiophene and dithieno[3,2-b:2',3'-d]thiophene fused units were synthesized. The optical data indicate a blue shift of the absorption maximum in comparison to the alpha-linked analogues due to cross-conjugation between fused rings. The crystal structures of 3,3'-bi(thieno[3,2-b]thiophene) and 3,3';6',3"-ter(thieno-[3,2-b]thiophene) reveal edge-to-face pi-stacked dimer motifs, whereas the crystal structure of 3,3'-bis(dithieno[3,2-b:2',3'-d]thiophene) consists of face-to-face pi-stacked molecules. [structure: see text]

A series of 3,6-dinonylthieno[3,2-b]thiophene-based polymers containing either 2,2‘-bithiophene, thieno[3,2-b]thiophene, or ethynylene spacer units were synthesized by Stille and oxidative coupling reactions. Their electronic properties were studied by UV−vis absorption and fluorescence spectroscopies in solution and in the solid state and compared to poly(3,6-dinonylthieno[3,2-b]thiophene). The introduction of these spacers led to a substantial red shift in the longest wavelength of absorption, indicating that these modifications restore, to a significant extent, the planarity of the system resulting in increased conjugation. Model compounds representing the basic structural unit of each polymer were synthesized, and computations were performed to understand the role of the spacer in dictating the conformation of the polymer backbone.

The effect of position and degree of sulfur oxidation on the fundamental properties of a series of planarized thieno[3,2-b]thiophene S,S-dioxides derivatives was studied. The optical data reveal a red shift in the longest wavelength of absorption relative to the nonoxidized analogues that is indicative of a reduced HOMO-LUMO gap. The position of oxidation rather than the extent of oxidation is the most critical factor in controlling electronic properties. Single-crystal analysis reveals that some of the oligothiophene S,S-dioxides studied present pi-pi interactions which are completely absent in the nonoxidized analogue.

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