Semiconducting nanowires from hairpin-shaped self-assembling sexithiophenes

Conjugated organic molecules can be designed to self-assemble from solution into nanostructures for functions such as charge transport, light emission, or light harvesting. We report here the design and synthesis of a novel hairpin-shaped self-assembling molecule containing electronically active sexithiophene moieties. In several nonpolar organic solvents, such as toluene or chlorocyclohexane, this compound was found to form organogels composed of nanofibers with uniform diameters of 3.0 (±0.3) nm. NMR analysis and spectroscopic measurements revealed that the self-assembly is driven by π-π interactions of the sexithiophene moieties and hydrogen bonding among the amide groups at the head of the hairpin. Field effect transistors built with this molecule revealed p-type semiconducting behavior and higher hole mobilities when films were cast from solvents that promote self-assembly. We propose that hydrogen bonding and π-π stacking act synergistically to create ordered stacking of sexithiophene moieties, thus providing an efficient pathway for charge carriers within the nanowires. The nanostructures formed exhibit unusually broad absorbance in their UV-vis spectrum, which we attribute to the coexistence of both H and J aggregates from face-to-face π-π stacking of sexithiophene moieties and hierarchical bundling of the nanowires. The large absorption range associated with self-assembly of the hairpin molecules makes them potentially useful in light harvesting for energy applications.