NANO LETTERS
Losing the Expression of Molecular Chirality in Self-Assembled Physisorbed Monolayers
2005 Vol. 5, No. 7 1395-1398
Jian Zhang,† Andre´ Gesquie`re,† Michel Sieffert,‡ Markus Klapper,‡ Klaus Mu1 llen,‡ Frans C. De Schryver,† and Steven De Feyter*† Molecular and Nano Materials, Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, Katholieke UniVersiteit LeuVen (K. U. LeuVen), Celestijnenlaan 200 F, B-3001 LeuVen, Belgium, and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55021, Mainz, Germany Received April 19, 2005
ABSTRACT STM imaging on graphite of the S-enantiomer of a chiral diacetylene isophthalic acid derivative reveals that molecular chirality is not expressed in the monolayer due to a specific molecular conformation preventing the stereogenic center to transfer its chiral information.
The existence and induction of chirality is among the most intriguing and inspiring phenomena in nature.1 Adsorption of a chiral molecule on a surface leads to a chiral entity, and when two-dimensional (2D) crystals are formed, they are chiral too.2-9 Also, adsorption of achiral molecules often produces chiral 2D clusters or crystals.10-19 However, the 2D crystals of chiral molecules and achiral molecules are rather different. On one hand, achiral molecules form an equal amount of mirror-image type domains. On the other hand, adsorption of almost all pure enantiomers leads to the exclusive formation of one of the possible 2D diastereomeric arrangementssthe formation of enantiomorphous structuress while the corresponding opposite enantiomer gives rise to the mirror-image monolayer. In all these cases where chiral molecules are adsorbed on the surface the chiral groups feel or interact with the surface. However, what is the fate of those systems where the chiral groups do not touch the surface as a result of conformational or packing constraints? The spontaneous self-assembly of the S-enantiomer of a chiral diacetylene containing isophthalic acid derivative ((S)1) (Figure 1) into physisorbed monolayers has been investigated at the liquid/solid interface by scanning tunneling microscopy (STM). This compound combines a number of functionalities of related derivatives (2 and 3), which have been investigated before.4,20,21 These common functionalities are: 1) a hydrogen bonding isophthalic acid group, 2) a diacetylene moiety which under the appropriate conditions * Corresponding author. E-mail:
[email protected]. Fax: 32(0)16327990; Tel: 32(0)16327921. † Katholieke Universiteit Leuven. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. 10.1021/nl050717q CCC: $30.25 Published on Web 06/07/2005
© 2005 American Chemical Society
Figure 1. Chemical structures of compounds 1-4.
(orientation, intermolecular distance, irradiation with UVlight, STM tip stimulation) might give rise to polydiacetylene formation as observed for 220,21 and 3) a 2-methylbutoxy group carrying a stereogenic center, which if in contact with the surface is expected to express the molecule’s chirality. Such expression of chirality was observed for 3 by its formation of enantiomorphous monolayers,4 i.e., only one of the two mirror-image type patterns is formed. However, it has been shown before for compound 4 that the 2-methylbutoxy group can be bent away from the surface when located at the end of an alkyl chain. Though, even in that
case (compound 4),22 most of the molecules in a given domain still had their alkyl chains fully adsorbed on the substrate, and molecular chirality was expressed in the monolayers formed. Preliminary modeling studies indicated that for compound 1 the combination of a diacetylene and 2-methylbutoxy group can cause strain in the monolayer packing if a similar arrangement of the molecules as observed for 2 and 3 would be formed. Given this apparent strain in the 2D crystal, questions that can be asked are whether the monolayer structure previously observed for 2 (rows of molecules with interdigitated alkyl chains) will be observed for 1, and whether the molecular chirality of 1 will be expressed in its adlayers. A drop of a solution (0.6 mg/mL) of (S)-1 in 1-octanol was applied to a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG). Upon spontaneous monolayer formation, STM images were acquired in the variable current mode by scanning the STM tip immersed in solution (Figure 2). The measured tunneling currents are converted into a gray scale: black (white) refers to a low (high) measured tunneling current. In contrast, at the 1,2,4-trichlorobenzene/graphite interface a quasi-hexagonal arrangement of spherical type structures was observed, though with a weak contrast. In other solvents such as 1-phenyloctane, tetradecane, and 1-octanoic acid no monolayer formation at all could be observed. Figure 2 reveals a 2D arrangement of molecules with submolecular resolution. The aromatic isophthalic acid groups appear as bright spots and the distance between these aromatic groups along a lamella axis is 0.96 ( 0.03 nm. Two different spacings are found between adjacent rows of (S)-1 headgroups. The smaller spacing (∆L2 ) 1.09 ( 0.07 nm) corresponds to the dimension of the 1-octanol solvent molecules. The co-adsorption of 1-octanol molecules in a monolayer formed by alkylated isophthalic acid derivatives has been observed by us before.10 However, as mentioned above, for this particular molecule, co-adsorption of the 1-octanol molecules is essential for monolayer formation, which indicates that the typical lamella-type structure for alkylated isophthalic acid derivatives involving interdigitation of alkyl chains, is not a stable configuration for this system. Indeed, the image resolution and quality of the (S)-1 lamellae does not reach the quality observed for other diacetylene containing compounds such as 2 investigated by us: in contrast to 2, no diacetylene groups could be visualized. From the empty space in the tentative model (Figure 2B), it is clear that the packing is not optimal because a favorable enthalpy change upon physisorption requires a maximized adsorbate density. Moreover, in contrast to 2, light-induced polymerization could not be achieved.20,21 These observations are in line with the fact that the width of the larger lamellae ((S)-1 lamellae: ∆L1 ) 2.39 ( 0.07 nm) is much smaller than the dimension of interdigitating (S)-1 molecules with fully extended alkyl chains (3.05 nm). However, the experimentally obtained value is in good agreement with the distance between the isophthalic acid group and the oxygen atom in the alkoxy chain (2.43 nm). This is a strong indication that 1396
Figure 2. (A) STM image of an ordered monolayer of (S)-1 formed by physisorption at the 1-octanol/graphite interface. The image size is 14 × 14 nm2. Iset ) 0.56 nA, and Vbias ) -0.744 V. ∆L1 indicates a lamella formed by (S)-1. ∆L2 corresponds to the width of a lamella built up by 1-octanol molecules. (B) Tentative molecular model for the two-dimensional packing of (S)-1 molecules in the area shown in A.
part of the chain, namely, the 2-methylbutoxy group is bent away from the surface, while the other part of the molecule is lying flat on the graphite surface, adopting an extended conformation (Figure 3). The chiral center in the 2-methylbutoxy groups may still influence the 2D packing of the molecules, even though this part of the chain is bent away from the substrate surface. To figure out whether the 2D expression of (S)-1 is affected by this, the orientation of the lamella axis, along unit cell vector a, is evaluated with respect to the symmetry axes of the graphite substrate underneath the monolayer. Therefore, the angle (θ) (see Figure 4A) between the lamella axis and the graphite’s main reference axis was determined for 74 Nano Lett., Vol. 5, No. 7, 2005
Figure 3. Top view and side view of a (S)-1 molecule with the 2-methylbutoxy group rotated away from the surface.
recorded under identical experimental conditions, except for lowering the bias voltage which allows imaging the graphite surface underneath the monolayer. The histogram in Figure 4B shows the distribution of the angle θ for (S)-1. In those cases where enantiopure molecules form enantiomorphous monolayer structures, it is generally observed that, due to molecule-substrate interactions, the molecules orient with a preferred orientation with respect to the substrate symmetry, which often is reflected in terms of the histogram of the angle θ, by an asymmetric distribution around θ ) 0, and more specifically, by pure positive or negative values.23 However, in Figure 4B, the distribution of the angle θ is rather symmetric around 0, resembling behavior similar to achiral molecules. The observation that only a small range of θ values is observed stresses the fact that the molecules indeed interact with the substrate. The symmetric distribution around θ ) 0, however, indicates that the chiral nature of the molecules is not expressed by the ordering of the molecules on the surface. This is most probably due to the fact that all the 2-methylbutoxy groups are bent away from the surface, directing into the liquid phase. In case of 4, within a given domain only for a fraction of the molecules the 2-methylbutoxy groups were bent away and 2D chirality was retained. The packing of the 2-methylbutoxy groups protruding in the liquid can be compared with the packing of chiral amphiphilic molecules in Langmuir or Langmuir-Blodgett films where it has been shown that often molecular chirality is expressed in the monolayer structure.24,25 The density of chiral centers in those monolayers is, however, much higher than the density of the chiral centers protruding to the liquid from the monolayer of (S)-1. We cannot exclude that in our case adjacent 2-methylbutoxy groups still interact. However, if present, these interactions are not strong enough to induce symmetry breaking. Acknowledgment. The authors thank the Federal Science Policy through IUAP-V-03 and the Institute for the promotion of innovation by Science and Technology in Flanders (IWT). Support from the Fund for Scientific ResearchFlanders (FWO), a Max-Planck Research Award, and the Marie-Curie RTN “Chextan” is acknowledged. S.D.F. is a postdoctoral fellow of FWO. Supporting Information Available: STM images of the self-assembly of (S)-1 at the 1,2,4-trichlorobenzene/graphite interface. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. (A) The main reference axis of graphite and symmetryequivalent directions are indicated by a black solid line and black dashed lines, respectively. The solid red lines indicate different orientations for lamellae of (S)-1. The angle between the graphite’s main reference axis and the lamella axis is defined as θ. (B) Histogram of the angle θ observed for 74 physisorbed monolayers of (S)-1 at the 1-octanol/graphite interface. The absolute error for the determination of θ is about 1°.
different domains. This correlation can be made because after imaging a monolayer each time the graphite lattice is Nano Lett., Vol. 5, No. 7, 2005
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Nano Lett., Vol. 5, No. 7, 2005
