Letter pubs.acs.org/Langmuir
Self-Assembly of Isomeric Monofunctionalized Thiophenes Laura E. Heller, Julianne Whitleigh, Danielle F. Roth, Elisabeth M. Oherlein, Felicia R. Lucci, Kristopher J. Kolonko,† and Katherine E. Plass* Department of Chemistry, Franklin & Marshall College, Lancaster, Pennsylvania 17604, United States S Supporting Information *
ABSTRACT: Controlling the self-assembly of thiophene-containing molecules and polymers requires a strong fundamental understanding of the relationship between molecular features and structure-directing forces. Here, the effects of ring-substitution position on the twodimensional self-assembly of monosubstituted thiophenes at the phenyloctane/HOPG interface are studied using scanning tunneling microscopy (STM). The influence of π···π-stacking, hydrogenbonding, and alkyl-chain interactions are explored computationally. Alteration of the amide attachment point from the 2- to the 3-position induces transformation from head-to-tail packing to head-to-head packing. This may be attributed to canceling of lateral dipoles.
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INTRODUCTION The organization of thiophene-containing molecules and polymers at solid interfaces can have a profound influence on interfacial properties. For example, thiophene-containing thin film transistors transport charge only in the first few layers1 and facile charge separation in polythiophene/fullerene photovoltaics has been attributed to interface morphology.2 Despite the influence of the self-assembly of thiophene-containing molecules on the behavior of electronic devices, a comprehensive understanding of the most fundamental driving forces behind thiophene crystallization is lacking. Notably, no statistical analyses of packing motifs in crystal structures of thiophenes in the Cambridge Structural Database have been reported. Systematic studies of the crystal behavior of simple thiophene-containing molecules at interfaces and comparison with analogous hydrocarbons, for example, are also needed. Monolayers formed at the interface between a liquid and a weakly interacting substrate like highly oriented pyrolytic graphite (HOPG) represent a regime in which packing features can be closely correlated to changes in molecular structure.3,4 While monolayers of many structurally elaborate thiophenecontaining molecules have been imaged at the liquid/HOPG interface,5−9 few simple thiophenes have been examined.10,11 While the influence of π−π, van der Waals, and hydrogenbonding interactions on self-assembly has been studied in detail, dipole−dipole interactions have been less intensively investigated and are more controversial. At the graphite interface, dipole−dipole interactions influence the self-assembly of polar fluorenone complexes,12 anthracenes with ether side chains,13 hexabenzocoronene derivatives,14 and halogen-substituted oligothiophenes.15 The role of molecular dipoles in inducing centrosymmetric packing in three-dimensional crystals has been intensely debated.16−18 The influence of the dipole moment inherent to the thiophene ring on self-assembly has not been explored in either of these contexts. © XXXX American Chemical Society
Here we study the self-assembly of substitutional isomers of monofunctionalized thiophenes at the liquid/solid interface (Scheme 1) using scanning tunneling microscopy (STM). Scheme 1. Chemical Structures of Amide-Functionalized Thiophene Compounds, the Self-Assembly of Which Is Examined Here
Molecules consist of an octadecyl chain attached to a thiophene ring by an amide-linkage in either the 2- or the 3-position, called 2-amide and 3-amide, respectively. These were synthesized from octadecylamine and the appropriately substituted carboxylic acid (see the Supporting Information (SI) for synthetic details and characterization). The observed packing behavior is interpreted in light of the literature on analogous species at the liquid/HOPG interface10,19 and the dipole-induced centrosymmetricity of three-dimensional crystals.16−18 This represents one of the most fundamental studies of thiophene self-assembly, uncovering how heterocycle selfassembly differs from that of the more-studied hydrocarbons.20 Comparison of the patterns that these structurally related species form provides insight into the competition between various intermolecular interactions that govern the packing of thiophenes. Received: August 5, 2012 Revised: October 6, 2012
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Figure 1. STM images of the self-assembled monolayers formed at the phenyloctane-HOPG interface by the isomeric species 2-amide (a) and 3amide (c). The column propagation directions are indicated by blue arrows. The tail-to-head direction is indicated by white arrows. Molecular models of the two-dimensional packing of 2-amide (b) and 3-amide (d) are shown. Image (a) was collected with set current 200 pA and bias of 1000 mV (sample positive). Image (c) was collected with set current 300 pA and bias of −620 mV (sample negative).
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RESULTS AND DISCUSSION An STM image of the two-dimensional packing pattern of the 2-amide at the phenyloctane/HOPG interface is shown in Figure 1a. This image is marked by lamellae of large bright spots indicative of a high tunneling current due to low ionization potential thiophene rings. This contrast is similar to that seen in STM images of 3-n-octadecylthiophene.10 These bright spots are 0.49 nm apart (see Table 1). This periodicity
minimum energy conformation (SI Figure S2). Notably, the amide groups form a hydrogen-bonded chain typical of secondary amides in two-19,23,24 and three-dimensional crystals25 though unlike the dimers25 and more elaborate morphologies generated by primary amides.26 The alkyl chains separating the columns of thiophene rings are offset by two methylene groups to give the 60° angle observed between the chains and the propagation direction. In this calculated pattern, the offset alkyl chains allow the amide functionalities to align and hydrogen bond (H···O distance = 2.19 Å), as observed for other alkyl-substituted amides19,23,24 and carbamates.27 S···H− N hydrogen bonds are not enabled in this geometry. This chain offset suggests that the hydrogen bonding is stronger than the intracolumn π···π interactions, as discussed below. In the simulation of the observed packing, the thiophene rings are angled with respect to the plane of the alkyl chains, allowing close-packing of the alkyl chains and necessary proximity for the hydrogen-bonds. The 60° angle between the alkyl chains and the column propagation direction observed for 2-amide provides insight into the relative influences of π-stacking, hydrogen bonding of the amide functionalities, and van der Waals interactions of the alkyl chains. Two possible packings were compared via molecular mechanics (SI Figure S3) and density functional theory (DFT; SI Figure S4) calculations: one with the observed 60° angle and one with a 90° angle. Either packing would be in registry with the HOPG substrate, so molecule−substrate interactions can be ruled out as the driving force for selection between these possible packings. When 2-amide molecules assemble with a 60° angle between chain and column direction, a N−H···O hydrogen-bond is possible. In comparison, molecular models of the 2-amide adopting a 90° angle show that in this configuration N−H···O hydrogen-bonds are not possible. Closer π···π interactions and greater contact between alkyl chains exist than in the 60° geometry (SI Figure S3). While both geometries have thiophene rings packed with a plane-to-plane distance typical of π-stacks (0.36−0.34 nm),28
Table 1. Comparison of the Measured and Calculated TwoDimensional Packing Patterns of 2-Amide and 3-Amide periodicity ∥ to column (nm) periodicity ⊥ to column (nm) alkyl chain angle with column (deg)
2-amide
3-amide
measured calculated measured calculated measured
0.49 ± 0.06 0.52 2.5 ± 0.3 2.81 60 ± 5
0.57 ± 0.09 0.52 5.5 ± 0.2 5.67 60 ± 5
calculated
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corresponds to the distance between alkyl chains along the column propagation direction and is similar to that observed for other ring-substituted21 or amide-containing chains.22 The “zigzag” pattern of the hydrogen atoms (smaller bright spots) indicates that the carbon backbone is parallel to the substrate. The mismatch between the intracolumn molecular spacing and the underlying graphite lattice gives rise to a moiré pattern, seen in Figure 2 and SI Figure S1. The columns of thiophene rings are separated by a distance of 2.5 nm due to alkyl chains set at a 60° angle. The packing pattern shown in Figure 1b simulates the features of the 2-amide two-dimensional crystal. Molecular mechanics calculations (see the SI) simulated the periodic spacings both parallel and perpendicular to the propagation direction within error of the STM measurements (Table 1). The 151.1° S−C−C−O torsion angle reflects the gas-phase B
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intracolumn interactions for the 2-amide and 3-amide. The spacing between bright columns of thiophene rings, however, is doubled (Table 1). This indicates a head-to-head arrangement of the 3-amide molecules in which thiophene rings in one column point toward the thiophene rings in the adjacent column. The space between adjacent rings is clearly seen in SI Figure S7. Such an arrangement is shown in the molecular model in Figure 1d. In the 3-amide packing pattern, as in that of the 2-amide, defects are observed. Rarely, head-to-tail packing for a column interrupts the dominant head-to-head packing (Figure 1c). More frequently, the alkyl chain direction alters between adjacent columns that meet at the methyl end of the molecule. Notably, the alkyl chains on either side of the thiophene rings are always at 180°. The thiophene rings seem to be interacting in a geometry-specific manner distinct from the weak, nonspecific interactions between alkyl-chains and thiophenes seen between columns in the 2-amide packing. The distinct directionality of the dipole moments for the 2amide and 3-amide could induce the observed difference in the orientation of adjacent columns. For both the 2-amide and 3amide, the dipole moment calculated by DFT lies primarily along the CO bond, parallel to the column propagation direction (SI Figure S2). Both 2-amide and 3-amide molecules self-assemble such that hydrogen-bonds are formed within the column along this dipole. The columns necessarily align the dipoles of adjacent molecules. A drive toward global centrosymmetricity may induce the observed frequent alterations in the alkyl chain angle for the 2-amide. While the dipole moment is dominated by the amide group, there is a component of the dipole directed along the thiophene that alters with ring attachment position (SI Figures S2 and S8). Rotating the thiophene ring in the 2-amide by 180° increases the dipole from 3.17 D for the minimum energy conformation to 4.01 D, a difference of 0.84 D. A similar change in conformation in the 3-amide alters the calculated dipole moment far less. It changes from 3.71 to 3.26 D, a difference of 0.45 D. The smaller change in net dipole upon switching ring orientation indicates that a greater percentage of the dipole component in 3-amide is nearly perpendicular to the net dipole. Thus, a possible explanation for the head-to-head arrangement of 3-amide molecules is a tendency to cancel the net lateral dipoles. In the case of 3-amide, this head-to-head packing has the additional benefit of canceling the amide dipoles, as well. A similar dipole-canceling argument underlies proposed dodecanal packing on graphite.29 Bond dipoles have been shown to influence molecular orientation over short distances in threedimensional crystals.17 The role of dipole−dipole interactions in producing the head-to-head packing for 3-amide is substantiated by examination of the monolayer defects and of the monolayers of analogous molecules,19,23 as well as calculation. In Figure 1c, it is apparent that the alkyl chain angle defects occur where the alkyl tails meet, but not on either side of the column of thiophene rings. In other words, the head-to-head interaction is directional, like that between dipoles. The alkyl tails associate through nondirectional van der Waals interactions. Furthermore, examination of STM images of 3-octadecylthiophene and other monoamide-substituted aromatics at the liquid−HOPG interface reveals a pattern that supports the role of lateral dipole canceling. Species without a dipole component lateral to the amide, including amide-substituted benzene and napthalene species, exhibit head-to-tail packing,19,23 like the 2-amide.
the offset of the rings is very different. DFT calculations show that the smaller π···π offset in the 90° geometry allows greater delocalization of π-electrons (SI Figure S4). The preference of π-interactions in the 90° geometry is supported by the observation that 3-n-octadecylthiophene preferentially adopts a pattern in which there is a 90° angle between the column propagation direction and alkyl chains (SI Figure S5).10 This allows the thiophene rings to adopt face-to-face π-stacking with expected distances. The 2-amide molecules thus sacrifice stronger π···π and van der Waals interactions in favor of hydrogen-bonds. This stands in contrast to longer chain carbamates, that adopt both a 90° and a 60° geometry.27 Frequent defects interrupt the periodicity of the selfassembled monolayer of the 2-amide, as seen in the STM image in Figure 2. Two defect types are observed within
Figure 2. Large scale (68 nm × 68 nm) STM image of the selfassembled monolayer formed at the phenyloctane−HOPG interface by 2-amide, showing the frequent defects in alkyl chain direction (arrows point from methyl group to thiophene ring) and in head-totail packing (circles indicate tail-to-tail defects). Image was collected with a set current 100 pA and bias of 1500 mV (sample positive).
domains. Infrequently, tail-to-tail packing is observed, rather than the expected head-to-tail packing. Examples of such defects can be seen three times in the 68 nm × 68 nm area shown in Figure 2 (circled). These defects are separated by regions that continue to pack in a head-to-tail fashion. More frequently, the alkyl chains switch direction within the head-totail packing pattern. The head-to-tail direction is noted in Figure 2 with arrows. Defects are noted by a change in arrow direction. Observation of these two types of defects suggests that the intercolumn interactions in 2-amide are weak and nonspecific. The lack of head-to-head defects, however, suggests an energetic penalty for thiophene-thiophene interactions in the 2-amide packing. The 3-amide exhibits a two-dimensional packing pattern (Figure 1c, SI Figures S6, and S7) distinct from that formed by the 2-amide. The STM image in Figure 1c shows unbroken columns of thiophene rings, with alkyl chains at 60° to these columns, as in the 2-amide. Thus, the same close-packing alkylchains, hydrogen-bonding, and π···π interactions dominate C
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Though the amide-dipoles would cancel for these species if they adopted head-to-head packing (SI Figure S9), calculations suggest that the dipole stabilization would be less for the 2amide than for the 3-amide (SI Tables S1 and S2) and that the packing may be less dense (SI Figure S9) or involve conformational strain that increases the total energy (SI Table S2). Head-to-head packing is observed for 3octadecylthiophene (SI Figure S5)10 and 4-substituted pyridine species.19 Despite the change in attachment chemistry from alkyl to amide, the sulfur atom forms the electron-rich end of the local ring dipole. N-Octadecylisonicotinamide19 is analogous to 3-amide in that the thiophene is replaced by a pyridine ring substituted at the 4-position. Both have aromatic rings with heteroatoms placed to align the aromatic dipole nearly perpendicular to the amide dipole, and both adopt head-tohead packing. The extension of this pattern to a substituted pyridine suggests that S···S interactions between columns are not structure-directing for the 3-amide.
OUTLOOK Examination of the self-assembly of 2-amide and 3-amide at the phenyloctane−HOPG interface revealed that the dipole moment direction change that results from alteration of substituent position alters the packing pattern from head-totail to head-to-head. This reveals a fundamental difference in the self-assembly of heterocycles in comparison with benzene rings. This study reveals that even structurally simple thiophenes exhibit complex packing behavior. In light of findings that the self-assembly of poly(thiophenes) with large dipoles at fullerene interfaces improves photovoltaic performance,30 further systematic study of the influence of molecular dipole on the self-assembly of thiophene-containing molecules is needed. ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures and characterization, additional STM images, and calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Department of Chemistry, Siena College, Loudonville, New York 12110. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by Franklin & Marshall College start-up funds, COG funds, Hackman Endowment Student Stipends, as well as Donors of the American Chemical Society Petroleum Research Fund.
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ABBREVIATIONS STM, scanning tunneling microscope; HOPG, highly oriented pyrolytic graphite; 2-amide, N-octadecyl-2-thiophenecarboxamide; 3-amide, N-octadecyl-3-thiophenecarboxamide D
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(20) For example, among the 573 unique molecules studied at the liquid/HOPG interface compiled in the Two-Dimensional Structural Database (2DSD) (Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40, 287−293), there are 50 thiophene-containing entries versus 334 benzene-containing entries. (21) Plass, K. E.; Engle, K. M.; Matzger, A. J. Contrasting two- and three-dimensional crystal properties of isomeric dialkyl phthalates. J. Am. Chem. Soc. 2007, 129, 15211−15217. (22) Mali, K. S.; Van Averbeke, B.; Bhinde, T.; Brewer, A. Y.; Arnold, T.; Lazzaroni, R.; Clarke, S. M.; De Feyter, S. To mix or not to mix: 2D crystallization and mixing behavior of saturated and unsaturated aliphatic primary amides. ACS Nano 2011, 5, 9122−9137. (23) Qian, P.; Nanjo, H.; Yokoyama, T.; Miyashita, T.; Suzuki, T. M. STM observation of N-octadecylacrylamide and N-octadecylcinnamoylamide monolayers self-assembled on a graphite surface. Chem. Commun. 1998, 943−944. (24) Miyashita, N.; Mohwald, H.; Kurth, D. G. 2D structure of unsaturated fatty acid amide mono- and multilayer on graphite: Selfassembly and thermal behavior. Chem. Mater. 2007, 19, 4259−4262. (25) Dey, A.; Pidcock, E. The relevance of chirality in space group analysis: A database study of common hydrogen-bonding motifs and their symmetry preferences. CrystEngComm 2008, 10, 1258−1264. (26) Ahn, S.; Morrison, C. N.; Matzger, A. J. Highly symmetric 2D rhombic nanoporous networks arising from low symmetry amphiphiles. J. Am. Chem. Soc. 2009, 131, 7946−7947. (27) Kim, K.; Plass, K. E.; Matzger, A. J. Kinetic and thermodynamic forms of a two-dimensional crystal. Langmuir 2003, 19, 7149−7152. (28) Curtis, M. D.; Cao, J.; Kampf, J. W. Solid-state packing of conjugated oligomers: From π-stacks to the herringbone structure. J. Am. Chem. Soc. 2004, 126, 4318−4328. (29) Phillips, T. K.; Bhinde, T.; Clarke, S. M.; Lee, S. Y.; Mali, K. S.; De Feyter, S. Adsorption of aldehydes on a graphite substrate: Combined thermodynamic study of C6-C13 homologues with a structural and dynamical study of dodecanal. J. Phys. Chem. C 2010, 114, 6027−6034. (30) Lobez, J. M.; Andrew, T. L.; Bulovic, V.; Swager, T. M. Improving the performance of P3HT-fullerene solar cells with sidechain-functionalized poly(thiophene) additives: A new paradigm for polymer design. ACS Nano 2012, 6, 3044−3056.
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