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Organic electronics have become an area of immense interest due to their promise of low-cost and large-area solution-processable applications. Within this field, polythiophenes have become the benchmark polymer semiconductor material in high-performance devices such as organic field-effect transistors and organic photovoltaics. There has been a great deal of effort over the years to study these materials in the solid state to understand how molecular structure is linked to device performance. However, polythiophenes exhibit quite complicated morphologies owing to the intrinsic complexities of polymeric solids. That is, crystallization in polymers is “frustrated” by the entropic barriers associated with chain entanglements and chain folding. It is further aggravated by the stiff backbones and long side chains characteristic of conjugated polymers such as polythiophenes. The result is that these polymers are extremely difficult to isolate as defect-free single crystals. As such, their detailed molecular packing remains elusive [1].

Oligothiophenes have historically garnered a great deal of attention in attempts to determine the effective conjugation length of polythiophenes. The electrical properties of oligothiophenes, of up to 96 thiophene units, have been studied in efforts to find the saturation point of conjugation [2]. However, these low-molecular-weight materials have been largely overlooked as model systems for studying the solid-state properties of their polymer analogs. Perhaps this is due to the tedious synthetic routes for producing these oligomers. However, crystallization of oligomers is much more straightforward than that of polymers because of their shorter chain lengths and monodispersity. Therefore, the formation of single crystals from oligothiophenes may pave the road toward the solid-state packing of polythiophenes, and help to bridge the structure-property gap in our understanding of these materials.

In a recent study, researchers grew large single-crystals from 12-unit oligomers of didodecylquaterthiophene. The crystals were the longest thiophenes ever characterized by x-ray structure analysis [3]. The 12-unit oligomers were found to have a markedly different structure than that of 4-unit oligomers of didodecylquaterthiophene previously characterized [4]. In a separate study, thin film transistors were fabricated from various molecular weight fractions of didodecylquaterthiophene polymer isolated by soxhlet extraction. As expected, mobility increased with chain length, except at the lowest molecular weight studied, which remarkably exhibited mobility nearing that of the high-molecular-weight polymer [5]. These results have inspired our group to work with low-molecular-weight thiophene oligomers, particularly as single crystals. The idea is to understand how specific structural changes in the crystal packing directly affect the electronic properties of the material.

One avenue that has been explored for studying the molecular arrangements of organic materials is directed self-assembly on highly ordered pyrolitic graphite (HOPG) [6]. HOPG consists of stacked sheets of carbon atoms arranged in a hexagonal lattice. These sheets can be cleaved to expose an atomically flat surface whose planar sp2 carbon atoms interact with, and thereby order, organic monolayers. The resulting films can then be imaged using scanning tunneling microscopy to reveal the details of the molecular order on the atomic scale. In their work on oligothiophene 12-unit oligomers, Azumi and co-workers noticed a difference between the molecular packing of their oligomer on the surface of graphene, and that of the bulk phase. This month's cover image features a colorful demonstration that there may be much to learn at the interface between graphene and polythiophenes.

The image is an optical micrograph of a thiophene oligomer self-assembled on HOPG. There is no false-color in this image; rather, the colors produced are the result of birefringence in the ordered crystal.

The authors gratefully acknowledge the Energy Frontier Research Center funded by the US Department of Energy (DE-SC000108) and the NSF Materials Research Science and Engineering Center on Polymers at UMass Amherst (DMR-0820506).

Further Reading
[1] J.A. Lim et al. Materials Today, 13 (5) (2010), p. 14
[2] T. Izumi et al. J A Chem Soc, 125 (2003), p. 18
[3] R. Azumi et al. J Mater Chem, 16 (2006), p. 8
[4] R. Azumi et al. Chem Eur J, 6 (2000), p. 4
[5] P. Pingel et al. Appl Phys A, 95 (2009), p. 1
[6] J.P. Rabe et al. Science, 253 (1991), p. 5018