Ring fusion effects on the solid-state properties of α-oligothiophenes

The solid-state properties of a series of fused-ring oligothiophenes were studied by UV-visible absorption spectroscopy and single crystal X-ray diffraction. Although the degree of ring fusion has little effect on the solution absorption spectra, the solid-state spectra are dramatically different. Three thieno[3,2-b]thiophene-containing oligomers display spectral blue shifts when comparing the solid state to solution because they adopt a herringbone packing motif that leads to H aggregation. The solid-state structure of a dithieno[3,2-b:2',3'-d]thiophene-based oligomer contains both herringbone and slipped pi-pi interactions, and the overlap of transitions from these different interaction modes results in a broad absorption spectrum in the solid state. The fully fused pentathienoacene adopts a pi-stacked packing motif and displays a small blue shift in the solid-state spectrum compared to solution. Time-dependent density functional theory calculations of the electronic transitions of isolated molecules and interacting dimers provide support that the different behavior in the solid-state spectra is due to the unique intermolecular interactions arising in each packing motif.