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The organization of molecules at solid interfaces can have a profound influence on interfacial properties and influence lubrication, catalysis, and electron transfer processes. Thiophene units are common in organic semiconductors, which are themselves incorporated into devices where performance is interface dependent. Understanding the self-assembly of these species at solid interfaces is thus of great interest. Few studies, however, have focused on uncovering the effect that the distinct molecular properties of thiophenes have on self-assembly. This grant has supported the efforts of one undergraduate student to observe the self-assembly behavior of a series simple alkyl-decorated thiophene-containing (Figure 1) using scanning tunneling microscopy (STM). Comparison of the patterns that these structurally related species form at the liquid-graphite interface is expected to provide insight into the role of various intermolecular and substrate-molecule interactions on self-assembly specific to the thiophene chemical moiety.

Figure 1. The structurally related alkyl-decorated thiophene species whose self-assembly is being investigated at the liquid-solid interface. These species differ in the substitution position on the ring (1 versus 3 and 2 versus 4) and in the attachment chemistry (1 versus 2 and 3 versus 4).

A Franklin & Marshall College undergraduate, Felicia Lucci, has undertaken the STM observation of the monolayers formed by a series of thiophenes, shown in Figure 1, at the phenyloctane-graphite interface. Each molecule contains an octadecyl chain to slow molecular dynamics sufficiently to allow imaging on the time scale of the STM experiment. Several modes of alkyl chain attachment chemistry were explored to identify feasible synthetic routes that could be utilized with a variety of commercially available precursors. In this series, the octadecyl chain is attached to the thiophene either by a simple C-C bond (1 and 3) or via an ester-linkage (2 and 4). The alkyl chain is bound either in the 2 (1 and 2) or the 3 (3 and 4) position of the thiophene ring. Species 3 is commercially available. As the synthesis and purification of species 1 and 4 has progressed, STM imaging has been carried out on species 3 and 2 (Figure 2).

Figure 2. STM images of the self-assembled monolayers formed at the phenyloctane-graphite interface by species 3 (left) and 2 (right). These are current images collected in quasi-constant height mode. The bright yellow areas denote increased tunneling current.

Comparison between the monolayers of 2-octadecyl thiophene (3) and 2-octadecanoate thiophene (2) shows a distinct difference in the self-assembly of these two structurally similar species. Species 3 self-assembles into lamellae in which the alkyl chains are at 90° angles to the propagation direction. The alignment of the small bright spots that represent hydrogen atoms indicates that the carbon backbone is perpendicular to the substrate. The spacing between columns suggests that the thiophene rings of adjacent columns face one another. This packing would optimize neither the interactions between neighboring alkyl chains nor interactions with the graphite substrate. As this simple 3-alkyl thiophene is most similar to the units making up 3-hexyl polythiophene, a widely used electrically conductive polymer, fully understanding the cause of this unexpected packing will be a priority. The packing behavior of 2 is distinct from 3 in that there is a greater angle between the alkyl chains and the propagation direction. A difference in the contrast between adjacent columns may suggest an alternation in the arrangement of the ester-containing species visible either due to the ester functionality or the asymmetry of the ring.

These initial results suggest that the self-assembly of alkyl-decorated thiophene-containing species is sensitive to either the chain attachment chemistry or position, or both. Impending imaging of species 4 will help to determine the specific impact of each factor. Detailed computational modeling will aid in interpretation of the affect these molecular changes have on the interactions between molecules and with the substrate. Future work will seek to explore other modes of chain attachment to explore the effect this has on self-assembly. The behavior will be compared with that of structurally similar non-thiophene compounds, furans and benzenes. This work will clarify the effects of chemical substitution on the self-assembly of thiophenes, and provide an understanding of the similarities and differences between thiophene self-assembly and that of other aromatic rings.