Thiophene/thieno[3,2-b]thiophene co-oligomers: fused-ring analogues of sexithiophene.

A series of six-ring oligothiophenes containing one to three degrees of ring fusion were assembled by a combination of metal-catalyzed Stille cross-coupling and oxidative homocoupling reactions. The effect of position and extent of ring fusion on the electronic properties was studied by UV-vis absorption and fluorescence spectroscopies, and these data were interpreted in the context of TD-DFT computational analysis. Within each set of regioisomers, a slight red shift is revealed in the onset of the UV-vis absorption spectra when the fused-ring unit is located nearer to the periphery of the oligomer, indicating a narrower HOMO-LUMO gap. Incorporation of the unit of ring fusion toward the interior of the oligomer results in a decrease in the longest wavelength emission maximum and a reduced Stokes shift, and is accompanied by an increase in fluorescence quantum yield.