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462 Int. J. Nanotechnology, Vol. 3, No. 4, 2006

Copyright © 2006 Inderscience Enterprises Ltd.

Supramolecular chemistry at the liquid/solid interface probed by scanning tunnelling microscopy

S. De Feyter*, H. Uji-i, W. Mamdouh, A. Miura and J. Zhang Department of Chemistry, Laboratory of Photochemistry and Spectroscopy, Katholieke Universiteit Leuven, Celestijnenlaan 200-F, 3001 Leuven, Belgium Fax: +32 16 327990 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

P. Jonkheijm, A.P.H.J. Schenning and E.W. Meijer Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Fax: +31 40 245 1036 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

Z. Chen and F. Würthner Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany Fax: +49 931 888 5340 E-mail: [email protected] E-mail: [email protected]

N. Schuurmans, J. van Esch and B.L. Feringa Material Science Center, Stratingh Institute, Laboratory of Organic and Inorganic Molecular Chemistry, University of Groningen, Nijenborg 4, 9747 AG Groningen, The Netherlands Fax: +31 50 363 4296 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

Supramolecular chemistry at the liquid/solid interface 463

A.E. Dulcey and V. Percec Department of Chemistry, Vagelos Laboratories, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA Fax: +1 215 573 7888 E-mail: [email protected] E-mail: [email protected]

F.C. De Schryver* Department of Chemistry, Laboratory of Photochemistry and Spectroscopy, Katholieke Universiteit Leuven, Celestijnenlaan 200-F, 3001 Leuven, Belgium Fax: +32 16 327990 E-mail: [email protected] *Corresponding authors

Abstract: The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunnelling microscopy (STM) is one of the preferred methodologies to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It’s important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions to direct the ordering of both achiral and chiral molecules on the atomically flat surface. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.

Keywords: scanning tunnelling microscopy; supramolecular chemistry; physisorption; self-assembly; chirality.

Reference to this paper should be made as follows: De Feyter, S., Uji-i, H., Mamdouh, W., Miura, A., Zhang, J., Jonkheijm, P., Schenning, A.P.H.J., Meijer, E.W., Chen, Z., Würthner, F., Schuurmans, N., van Esch, J., Feringa, B.L., Dulcey, A.E., Percec, V. and De Schryver, F.C. (2006) ‘Supramolecular chemistry at the liquid/solid interface probed by scanning tunnelling microscopy’, Int. J. Nanotechnology, Vol. 3, No. 4, pp.462–479.

Biographical notes: Steven De Feyter (Kortrijk, Belgium, 1971) is a Senior Lecturer (Hoofddocent) at the Katholieke Universiteit Leuven in Belgium. After starting up scanning tunnelling microscopy during his PhD under the direction of Frans De Schryver in Leuven he moved for a postdoctoral position to the group of Professor Ahmed Zewail (California Institute of Technology, Pasadena) where he was involved in ultrafast organic femtochemistry. He rejoined the Leuven group in 1999. His current interests include the study of supramolecular chemistry and self-assembly phenomena at surfaces with scanning probe methods.

464 S. De Feyter et al.

Hiroshi Uji-i (Nagoya, Japan, 1973) developed scanning probe microscopy combined with optical microscopy during his PhD under the direction of Hiroshi Fukumura at Tohoku University in Japan. Then he moved to K.U.Leuven as a Postdoctoral fellow in the group of Frans De Schryver (2002). He currently studies organic and biological materials using single molecule fluorescence spectroscopy and scanning probe microscopy.

Wael Mamdouh received his PhD Degree from Katholieke Universiteit Leuven in 2005 and has recently joined the group of Prof. Flemming Besenbacher at the iNANO in Denmark as a postdoc. He is currently investigating DNA bases and their complexation with different guest molecules, the formation of DNA nanowires and creating supramolecular surfaces by means of scanning probe microscopes under variable conditions.

Atsushi Miura (Aomori, Japan, 1972) received his PhD Degree from Kwansei Gakuin University, Hyogo, Japan. After a Postdoctoral stay in the research group of Prof. F.C. De Schryver at K.U.Leuven, he joined the Nara Institute of Science and Technology in Japan. His current research interests deal with bionanoprocessing and its application in opto-electronic devices.

Jian Zhang (Heilongjian, China, 1975) received her PhD Degree with Prof. D.C. Yang from CIAC, Chinese Academy of Sciences in 2002. Afterwards, she obtained a Postdoctoral position in Prof. A. Janshoff´s group at University Mainz, Germany. Since 2004, she has worked at K.U.Leuven as a postdoctoral researcher to study supramolecular chemistry and self-assembly phenomena at surfaces with scanning probe methods.

Pascal Jonkheijm completed his PhD research in the group of Prof. Dr. E.W. Meijer at the University of Eindhoven in 2005. His research interests have been in macromolecular engineering of π-conjugated oligomers via supramolecular interactions into nanoscopic objects. Currently he is a Humboldt Post-doctoral Fellow in the chemical biology group of Professor Dr. H. Waldmann at the Max Planck Institute for Molecular Physiology in Dortmund with a focus on developing protein arrays in conjunction with new ligation chemistry.

A.P.H.J. Schenning received his PhD Degree at the University of Nijmegen in 1996 on supramolecular architectures based on porphyrin and receptor molecules with Dr. M.C. Feiters and Prof. Dr. R.J.M. Nolte. He is Associate Professor at the Eindhoven University of Technology. His research interests centre on plastic and supramolecular electronics.

E.W. “Bert” Meijer has been Professor of Organic Chemistry at the Eindhoven University of Technology since 1991 after 10 years at industry and obtaining a PhD in Groningen. His research interests are related to the design, synthesis, characterisation and possible applications of novel (macro)molecules and supramolecular systems with unconventional properties.

Zhijian Chen received his BSc from the Peking University and MSc from the Chinese Academy of Science. Since 2001 he is a PhD student in Professor Würthner’s group and working on supramolecular and material chemistry of perylene bisimide.


Frank Würthner (Villingen-Schwenningen, Germany, 1964) is Professor of organic chemistry at the University of Würzburg, Germany. His research interests are in the fields of functional dyes, supramolecular chemistry, and organic materials. Before moving to Würzburg in 2002 he was at the University of Stuttgart (PhD), MIT, BASF and at the University of Ulm. In 2002 he received the Arnold-Sommerfeld award of the Bavarian Academy of Science.

Norbert Schuurmans (Vlissingen, 1975) is a former PhD student in the group of Ben Feringa. He studied organic chemistry under the guidance of Ben Feringa, and stayed in his group for a PhD, under joint guidance of Ben Feringa and Jan van Esch. The focus of his PhD study was the synthesis of adsorbents in the context of surface patterning via self-assembly approaches.

Jan van Esch is Associate Professor of Supramolecular and Organic Chemistry at the University of Groningen. His research focuses on fundamental aspects of self-assembly phenomena by small molecules in solution and at interfaces, and the ability to exploit self-assembled objects in functional nanostructures. In 2004 he was a recipient of a VICI research grant from the Netherlands Research Foundation (NWO).

Ben L. Feringa received his PhD Degree from the University of Groningen in 1978 with Professor Hans Wynberg. He was a research scientist with Royal Dutch Shell and joined the University of Groningen in 1984 as a Lecturer, was appointed Professor at the same University in 1988 and in 2003 Jacobus van’t Hoff distinguished Professor in Molecular Sciences. He was the recipient of several international awards. In 2004 he was elected foreign honorary member of the American Academy of Arts and Sciences and in 2006 member of the Royal Netherlands Academy of Sciences. He has recently received the Spinoza award from the Netherlands Organisation of Scientific Research and the Prelog Medal of the ETH Zurich. He is the Scientific Editor of the RSC journal Organic & Biomolecular Chemistry.

Andrés E. Dulcey (Cali, Colombia, 1977) obtained his BS Degree in Chemistry from Ohio State University in 1999, where he did undergraduate research in the area of carbohydrate chemistry under the supervision of Professor Todd Lowary. Upon graduation he moved to the University of Pennsylvania to pursue his PhD in organic chemistry under the supervision of Professor Virgil Percec, where he is currently a graduate student performing research in the area of nature-inspired self-assembling dendritic molecules.

Virgil Percec (Siret, Romania 1946) is P. Roy Vagelos Professor of Chemistry at the University of Pennsylvania. His research pioneered the bridge between macromolecular and supramolecular chemistry. He is the author of 575 refereed publications and 800 invited and endowed lectures. He is Editor of the Journal of Polymer Science Polymer Chemistry, serves on the editorial board of 20 journals, and is the recipient of many awards.

Frans De Schryver (St-Niklaas, Belgium, 1939) is Emeritus Professor at the Katholieke Universiteit Leuven where he, after a postdoctoral stay with Professor C. Marvel, since 1969 has been involved in the study of excited states of organic synthetic systems. His research present activities combine fundamental photophysical methodologies, such as time resolved spectroscopy, with space resolution down to the single molecule level, e.g., scanning probe microscopy. He has received the Alexander von Humboldt (1993) and Max Planck (2001) Awards as well as the Porter Medal (1998).


1 Introduction

Control of the lateral assembly and spatial arrangement of micro- and nano-objects at interfaces is often a prerequisite when it comes to potential applications in the field of nanoscience and technology. To create two-dimensional (2D) patterns, one can take advantage of ‘active’ manipulation techniques such as photolithography or electron beam lithography [1], and ‘soft lithography’ techniques [2]. Scanning probe microscopy (SPM) techniques, such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM), are another class of techniques, which can be implemented for the controlled manipulation of matter [3–6]. Self-assembly methods provide an alternative approach to make defined structures with dimensions on the nanometer scale. Self-assembly is a natural phenomenon, which can be observed in many biological, chemical and physical processes. A textbook example of self-assembly on surfaces is the spontaneous formation of chemisorbed self-assembled monolayers (SAMs) [7], typically thiols on gold [8]. Chemisorption renders the modified substrate properties which differ significantly from the ‘naked’ substrate, which makes these chemisorbed self-assembled monolayers of prime interest for technological applications. Modification of the exposed groups at the monolayer/air interface leads to a wealth of possibilities to change the properties of the layer, which can be achieved on the nanometer scale by SPM [5,6,9,10]. In contrast to chemisorption, physisorption is not very suitable for making ‘permanent’ architectures. Nevertheless, these physisorbed adlayers are model systems to investigate the interplay between molecular structure and the formation of ordered assemblies in two dimensions. In addition, SPM techniques allow the investigation of molecular properties (conformation, reactivity, chirality, electronic properties) often at the single molecule level.

STM is one of the preferred techniques to investigate the ordering and properties of these self-assembled layers, in general monolayers, both under ultrahigh vacuum conditions as well as at the liquid/solid or air/solid interface. In STM, a metallic tip is brought very close to a conductive substrate and by applying a voltage between both conductive media, a tunnelling current through a classically impenetrable barrier results between the two electrodes. The direction of the tunnelling depends on the bias polarity. The exponential distance dependence of the tunnelling current leads to excellent control of the distance between the probe and the surface and very high resolution (atomic) on atomically flat conductive substrates can be achieved. For imaging purposes, the tip and substrate are scanned precisely relative to one another and the current is accurately monitored as a function of the lateral position. The contrast in STM images reflects both topography and electronic effects. In the constant height mode, the absolute vertical position of the probe remains constant during raster-scanning and the tunnelling current is plot as a function of the lateral position.

Successful monolayer formation and STM imaging require balanced molecule – substrate interactions: a too strong interaction immobilises the molecules and impedes self-assembly into ordered 2D layers. A too weak adsorbate – substrate interaction leads to a too high mobility and high-resolution STM imaging becomes impossible.

The organic liquid/solid interface provides a particularly interesting environment to carry out the self-assembly experiments and their investigation by scanning tunnelling microscopy [11]. Compared to sample preparation and measuring under UHV conditions, the liquid/solid interface has a number of advantages: 1) The experimental approach is straightforward and does not require a complicated or as expensive infrastructure.


2) Though the UHV environment provides excellent control leading to unprecedented high resolution [12], not all species can be adapted to UHV, such as those with relatively low thermal stability or big size. The demands set on the properties (size and function) of the molecules under investigation at the liquid/solid interface are more relaxed. 3) The choice of solvent can be tuned in function of the particular solute and/or substrate. Typically, the solvent has a low vapour pressure, is non-conductive (electrochemically inert), and shows a lower affinity for the substrate than the solute. 4) The dynamic exchange of molecules adsorbed on the surface and in the liquid phase promotes repair of defects in the self-assembled layers. As a result, the liquid/solid interface approach in combination with STM imaging is becoming increasingly popular to induce and investigate self-assembly on surfaces [13,14]. Additional control of the monolayer formation can be achieved under potential control in aqueous solutions. Under electrochemical conditions, adsorbate-substrate interactions can be modulated by the surface charge density. Electrochemical environments offer therefore additional possibilities to control surface dynamics and monolayer structure via the surface charge and to image those structures by means of electrochemical scanning tunnelling microscopy (EC-STM) [15–17].

In this contribution, we give an overview of our latest efforts related to the study of monolayer formation and their properties at the organic liquid/solid interface with a focus on molecular conformation and its relation to monolayer formation and structure, the expression of molecular chirality and bias dependent imaging of molecules.

2 Results and discussion

Hydrogen bonding and control of monolayer structure

Hydrogen bonding has been amply explored for self-assembly purposes in solution and in three-dimensional (3D) crystals [18–20]. Hydrogen bonding has been exploited for self-assembly at surfaces too, including the liquid/solid interface [21]. Such studies clearly show that the hydrogen bonding pattern is not only governed by the anticipated intermolecular interactions but also by molecule-substrate interactions.

A first important issue is the control of molecular conformation, guided by hydrogen bonding, and the subsequent control of the ordering of molecules in two-dimensional crystals.

Foldamers

The observation that linear alkyl fragments featuring amide or urea groups form lamellar structures reminiscent of the β-sheets found in nature upon adsorption at the liquid/solid interface of HOPG prompted us to introduce a β-turn mimic in these systems as a next logic step. A 2D turn mimic should obey the requirements that (i) the entire structure is flat, (ii) the alkyl groups are spaced by approximately 5.0 Ǻ, that is the optimal distance for H-bonding, and (iii) the amide moieties are kept in registry, with respect to position as well as orientation.

We successfully designed a 2D turn element for oligo-amide sequences. The catechol moiety (Figure 1) is flat, bifunctional, and derivatives are synthetically accessible; the ortho-substitution pattern endows the structure with the necessary geometry for making turns. A systematic conformational search identified the conformation most likely


to fold in a plane involving the formation of an intramolecular hydrogen bond (Figure 1). The length of the spacers between the catechol and amide moieties plays an important role in the folding process. As forecasted by the calculations, derivatives obeying the rule n = m + 1 or n = m + 3 give (Figure 1) folded structures upon adsorption at the liquid/solid interface. However, in general no long range ordering is observed and only in case where the amide groups are positioned asymmetrically with respect to the length of the alkyl chains (foldamer 1), 2D crystals of perfectly folded molecules are formed. These results constitute a promising approach towards surface patterning and extension of the concept towards derivatives [22].

Figure 1 Top: molecular models of the foldamers. Bottom: Constant-height STM image of a monolayer of foldamer 1 (m = 3, n = 6) at the 1-octanol/graphite interface. Insets: line profiles and a tentative model of packing

Intermolecular hydrogen bonding

Tuning the intermolecular interactions by tailoring the non-covalent interactions indeed leads to control of the self-assembly process. Of special interest are those molecules with ‘functional’ moieties, such as conjugated oligomers. p-phenylene vinylenes are such an important class. Three types of chiral p-phenylene vinylenes have been investigated (Figure 2), which differ in their hydrogen bond forming abilities [23–26]. They all carry (S)-2-methyl butoxy groups along the backbone. In the first type, oligo-p-phenylene vinylenes which are at both termini functionalised with three dodecyl chains (2) (Figure 2A), self-assemble in highly organised 2D crystals on graphite by spontaneous self-assembly [23]. They form rows and the bright features correspond to the conjugated backbones, of which one is indicated by a white oval. Alkyl chains are interdigitated.


In the second and the third type, the molecules are functionalised by hydrogen bonding groups such as ureido-s-triazine (type 2) (3) (Figure 2B) or 2,5-diamino-triazine groups (4a and 4b) (type 3) (Figure 2C). The ureido-s-triazine derivatised oligo-p-phenylene vinylenes show linear dimerisation via self-complementary hydrogen bonding, as expected [24]. The molecules are indeed stacked in parallel, though not equidistant rows. The fact that the conjugated backbones forming a dimer are slightly shifted is in line with the hydrogen bonding pattern formed. In contrast, the 2,5-diamino-triazine derivatised oligo p-phenylene vinylenes show cyclic hexamer formation [25,26]. By functionalising the oligomers the ordering can be controlled.

Figure 2 STM images and chemical structures of A) type 1 (2) at the 1,2,4-trichlorobenzene/graphite interface. Image size: 10.7 × 10.7 nm2. B) type 2 (3) at the 1,2,4-trichlorobenzene/graphite interface. Image size: 12.1 × 12.1 nm2 and C) type 3 (4b) (n = 2) at the 1-phenyloctane/graphite interface. Image size: 18.4 × 18.4 nm2. Hydrogen bonding has a strong effect on the supramolecular architecture. The white ovals indicate a conjugated backbone. D,E) High-resolution STM image of a CW and CCW rosette of type 3 oligomers 4a and 4b, respectively. The arrow indicates the ‘rotation direction’ of the rosette

Intermolecular hydrogen bonding in multicomponent complexes

Given that the fabrication of highly ordered monocomponent supramolecular structures by design at surfaces is not always trivial, the controlled formation of multicomponent composites with a well-defined ordering forms an even bigger challenge. We have compared the complex formation of two distinctly different but related systems which are designed to form trimers based upon hydrogen bonding [27]. The central unit of the anticipated trimer is a merocyanine dye (6) or a substituted perylene bisimide derivative (7). Both dyes have two sets of hydrogen bonding sites, i.e., -CO-NH-CO- (imide), where NH is a hydrogen bond donor (D) and CO a hydrogen bond acceptor (A), giving rise to a A-D-A sequence. Most importantly, the relative orientation of the hydrogen bonding


units differs in both molecules: in 7 both A-D-A sets are parallel but facing opposite directions, while in 6 these sets are at an angle of ~120°. For the formation of well-defined heterocomplexes, an alkylated diamino triazine derivative (5) having a D-A-D hydrogen bonding set, i.e., NH-N-NH, is selected to form complementary hydrogen bonding with the A-D-A sequence. This type of interaction should favour heterocomplex formation as recently found in solution [28]. It is anticipated that the interaction between 5 and 6 will give rise to a termolecular complex having a triangle configuration. Also 5 and 7 are expected to form a termolecular complex, though now in a linear fashion (see Figure 3). This difference may also strongly affect their long range ordering. As anticipated upon mixing the two components, termolecular complexes are formed at the liquid/solid interface which is a clear indication that hydrogen bonding is indeed involved (Figures 4 and 5).

Figure 3 Scheme of the anticipated termolecular complexes 5:6:5 and 5:7:5

However, there are important differences between both systems. A 5/6 mixture leads to the formation of true 2D crystals: heterocomplexes are exclusively formed and they organise in large domains. In contrast, 5:7:5 complexes are formed only locally, they assemble in rows, they do not cover complete domains, and they coexist with non-complexed 5-dimers within the same domain.


Figure 4 STM images of monolayers of 5/6 at the liquid/solid interface self-assembled from a solution with the 5/6 components in a 2:1 ratio. A) Image size is 11.5 × 11.5 nm2. Unit cell is indicated. B) Tentative molecular model with unit cell

Figure 5 STM images of monolayers of 5/7 at the liquid/solid interface self-assembled from a solution with the 5/7 components in a 1:9 ratio. A) Image size is 23.5 × 23.5 nm2. Unit cell is indicated. B) Image size is 12.3 × 8.8 nm2. C) Tentative model. Unit cell is indicated. Pairs of grey dashed arrows indicate the formation of homodimers of 5. 5:7:5 complexes are indicated by a black solid arrow (7) between two grey solid arrows (5). The 5:7:5 complexes do not cover the complete substrate

Moreover, also the conditions to achieve heterocomplex formation and 2D ordering are different. For the 5/6 system, the solution contains the compounds in a 2:1 ratio, anticipating and leading to termolecular (2:1) complex formation. Adding the compounds in sequence did not give rise to formation of monolayers of the complex: only domains with the pure compounds were observed. In order to successfully form 5:7:5 complexes at the liquid/solid interface, it was necessary to add a large excess of 7, typically in a 1:9 ratio. Both 5 and 6 show a higher affinity for graphite than 7 as expressed by the fact that both 5 and 6 self-assemble on the surface into ordered monolayers with laterally immobilised molecules, while it was not possible to visualise 7 under analogous experimental conditions. The very high excess of 7 required to induce heterocomplex adsorption, in combination with the fact that 7 co-adsorbs with 5 only gradually in time, leads to the conclusion that the formation of a 5-layer is at least kinetically favoured compared to 7 and heterocomplex adsorption.


The data presented suggest some guidelines for the successful preparation of heterocomplexes based upon hydrogen bonding at the liquid/solid interface and the creation of two-dimensional networks. Heterocomplex formation on surfaces will be favoured if in addition to non-covalent interactions between the different partners which exceed the intermolecular interactions between molecules of the same kind, the tendency of adsorption of the heterocomplexes is higher or comparable to the pure components. If so, heterocomplexes formed in solution may successfully adsorb on the substrate and grow to 2D crystals. In the case that heterocomplexes have a lower affinity for adsorption on the substrate than (one of) the pure components, the expected higher affinity of (one of) the pure compounds for the substrate will favour the adsorption of the pure compound(s) and destabilise the heterocomplexes formed in solution. Therefore, heterocomplex formation on the substrate should be disfavoured. However, in case the heterocomplex formation does not disturb significantly the 2D ordering of the dominant species (i.e., 5 for the 5/7 system), heterocomplex formation might still be observed. Upon successful heterocomplex formation on the substrate, the formation of 2D crystalline domains will be favoured if the structure of the heterocomplexes is not compatible with the ordering of (one of) the pure components.

The role of solvent

The solvent plays an important but often underestimated role in the 2D ordering of the molecules at the liquid/solid interface. An example is the solvent dependent self-assembly of a monodendron 8 at the liquid/solid interface [29].

At the 1-octanol/graphite interface, tetramers are formed which appear distorted, and from row to row, the tetramers are tilted with respect to each other (Figure 6A–C). Tentatively, each bright spot can be assigned to the location of a phenyl ring. The grey-like striped features are the alkyl chains. A unit cell is indicated in Figure 6A, which contains eight molecules. The distance between two adjacent alkyl chains is approximately 0.46 nm. The formation of tetramers might be due to hydrogen bond formation via the carboxylic group of one monodendron and the hydroxyl group of another monodendron. The phenyl rings are surrounded by alkyl chains. The distance between the tetramers along the vector a measures 3.80 ± 0.06 nm. The alkyl chains between the tetramers along this direction are most likely directed to the solution.


Figure 6 A, B, C) STM-images and model of monodendron 8 physisorbed at the 1-octanol/graphite interface A) Image size: 23.5 nm × 23.5 nm2. Unit cell is indicated. B) Image size: 8.4 nm × 8.4 nm2. C) Tentative molecular model. The alkyl chains along the unit cell vector a, and the hydroxyl group containing alkyl parts within the tetramers have been omitted. D, E, F) STM-images and model of monodendron 8 physisorbed at the 1-phenyloctane/graphite interface D) Image size: 23 nm × 23 nm2. Unit cell is indicated. E) Image size: 12.8 nm × 12.8 nm. The location of the co-adsorbed 1-phenyloctane solvent molecules is indicated by arrows. F) Tentative molecular model. The 1-phenyloctane molecules are indicated by arrows

At the 1-phenyloctane/graphite interface, also tetramers are formed (Figure 6D–F). The alkyl chains are interdigitated and separated by approximately 0.46 nm and can be divided into two sub-sets, depending on their contrast. Those two subsets are oriented perpendicular to each other. From every monodendron, two alkyl chains are oriented almost perpendicular to the unit cell vector a, and those alkyl chains are oriented along one of the main symmetry axes of graphite. The third alkyl chain which appears with a higher contrast in the images is oriented along the unit cell vector a. The difference in contrast between the alkyl chains can be attributed to their different orientation with respect to the graphite lattice. Between two tetramers are four of the latter type of alkyl chains. As a result, the distance between two tetramers along unit cell vector a


(4.69 ± 0.07 nm) is longer than for the pattern at the 1-octanol/graphite interface. In the direction perpendicular to the unit cell vector a, there is an excess of two alkyl chains for every tetramer. Two solvent molecules of 1-phenyloctane are co-adsorbed per tetramer unit in the monolayer as illustrated in Figure 6E. The bright spots indicated by arrows in Figure 6E–F correspond to the phenyl groups of the co-adsorbed solvent molecules. These phenyl rings often appear rather fuzzy which suggests some motional freedom. This co-adsorption leads to a densely packed 2D monolayer.

It is remarkable that 1-phenyloctane is co-adsorbed and immobilised. 1-Phenyloctane molecules do not show any tendency to form immobilised monolayers on graphite. After all, this is one of the reasons why 1-phenyloctane is used as solvent. Although 1-phenyloctane and 1-octanol molecules are comparable in size, only the former one is co-adsorbed. This presents a clear example of the effect of solvent-solute interactions on the 2D ordering at the liquid/solid interface [30].

The expression of molecular chirality

The manifestation of molecular chirality in essentially 2D systems can be used to control the orientation of the building blocks on a substrate [31]. When 2D crystals are formed, they often appear to be chiral. This is often true, even for achiral molecules, but the 2D crystals of achiral molecules appear in an equal amount of mirror-image type domains in the absence of any discriminating influence. In almost all cases reported so far, enantiopure molecules lead to the exclusive formation of one of the possible 2D mirror-type arrangements [32–38]. For instance, in the case of 2 and 3, molecular chirality is expressed by the orientation of the molecules within the domains: the π-conjugated backbone is oriented counterclockwise with respect to the normal on a lamella axis. Stereospecific molecule-substrate interactions result in a preferred adsorption of the enantiomer eliminating one of both mirror-image type (enantiomorphous) packings (i.e., unit cells) [39]. Within these series of oligo-p-phenylene vinylene derivatives, compound 4a and 4b take a special place. For both molecules, the arrangement of the molecules in individual rosettes is unidirectional but different for the two oligomers discussed (Figure 2D–E). The 4a rosettes appear exclusively to rotate clockwise (CW) (Figure 2D) while in 4b rosettes, the molecules are exclusively arranged in a counterclockwise (CCW) fashion (Figure 2E). No rosettes of opposite chirality were found. Molecular chirality is transferred to the rosette structures which in turn form chiral 2D crystalline patterns. Intuitively, one would expect that the rosettes formed would show the same ‘rotation’ direction, independent of the number of stereogenic centres or conjugated oligomer length. The difference in the virtual rotation direction of the 4a and 4b rosettes can be explained by balanced molecule-molecule and molecule-substrate interactions. Molecules tend on one hand to minimise the free surface area (favoured by a CCW rotation) but in order to minimise steric interactions a less dense packing might be more favourable (CW rotation). The final 2D pattern is the result of a delicate balance between hydrogen bonding, van der Waals interactions between the alkyl chains and between the stereogenic centres and the substrate, and the free surface area, which is to a large extent affected by the ratio between the length of the conjugated backbone and the alkyl chains.

Electronic properties

The way the molecules appear in STM images often depends on experimental parameters of which the bias voltage is a critical one. Indeed, STM is sensitive to the electronic


properties of the adsorbates under investigation [40]. It was soon noticed that STM shows chemical sensitivity; certain functional groups (e.g., aromatic moieties, amines, sulphides) are more conductive than others (e.g., alkyl chains). Several theoretical approaches have been developed to explain the contrast of organic adsorbates in STM [41–43]. Bias dependent imaging has been used to probe the electronic levels involved in the tunnelling process [40], but 9 is unique for the following reasons: (1) it combines an electron donor (D = oligo-p-phenylene vinylene (OPV)) and electron acceptor part (A = perylene bisimide (PDI)), covalently linked to each other and (2) the donor and acceptor parts are relatively large and well-separated in space, allowing addressing them individually [44].

9 forms well-ordered 2D patterns when physisorbed from 1-phenyloctane on HOPG at the liquid/solid interface, as measured by STM in the constant-height mode (Figure 7). The tunnel path through the aromatic core is tip-D-graphite or tip-A-graphite and not tip-D-A-D-graphite. Of course, the tunnelling direction (tip-graphite or graphite-tip) depends on the bias polarity. Alkyl chains occupy the dark regions between the rows of bright rods. From the high-resolution images it appears that the bright rods consist of three parts: the central part is attributed to the location of the PDI part, the outermost parts correspond to the OPV moieties.

At high negative voltages (–1.19 V to –1.06 V), the OPV parts appear brighter compared to the central PDI moiety. By changing the bias voltage from negative to positive, the PDI part gradually becomes brighter (from –0.77 V to +0.80 V). Upon further increasing the positive bias voltage, the PDI parts become brighter than the OPV units. The bias-dependent contrast changes can be explained by the schematic energy diagrams proposed in Figure 7 which are based upon a number of assumptions: (1) The tunnelling junction is asymmetric with the molecules closer to the substrate than to the tip. (2) The interaction between the molecules and the substrate is weak; physisorption does not significantly affect the energy levels of HOMO and LUMO. (3) The molecular orbital that is closer in energy to the Fermi level of the negatively biased electrode (the electron source), affects the tunnelling process stronger. (4) OPV and PDI moieties are electronically decoupled or only weakly coupled as indicated by the absorption spectrum.


Figure 7 Top: Bias dependent imaging of 9 at the 1-phenyloctane/graphite interface. The applied sample bias is indicated below each image. Image size is 10.1 × 10.1 nm2. Iset = 0.40 nA. Arrows outside the images refer to OPV (heavy solid arrow) and PDI moieties (thin solid arrow). Bottom: Tentative Energy level diagrams illustrating the relative position of the HOMO (solid lines) and LUMO levels (dashed lines) of OPV and PDI with respect to the graphite and Pt/Ir tip Fermi levels upon applying a bias on the sample. Left: negative sample bias. Right: positive sample bias. Fermi levels of graphite (Ef,S) and STM tip (Ef,T) are indicated with dotted lines

As depicted in the left scheme, at negative sample bias, the HOMOOPV level is located close to the Fermi level of graphite. Therefore, electrons tunnelling from graphite to the tip couple stronger to the HOMOOPV level than to the other energy levels. At high positive bias voltages (right scheme), the LUMOPDI level is closer to the Fermi level of the tip than other states and accordingly is expected to affect the tunnelling process, from tip to substrate, to a greater extent. This simplified model is in line with experimental observations: OPV moieties appear brighter (higher tunnelling current) at negative sample bias while the PDI moiety appears brighter at high positive sample bias.

This behaviour has also been confirmed by scanning tunnelling spectroscopy (STS) measurements [45] where the voltage is ramped while the feedback loop is turned off and a current-voltage graph is obtained at a certain position on the substrate.

3 Conclusions

Scanning tunnelling microscopy is a powerful methodology to investigate the self-assembly at the liquid/solid and air/solid interface where it provides insight in the factors controlling the molecular ordering and electronic properties of molecules forming adlayers. In analogy with the situation in solution and 3D crystals, directional non-covalent interactions such as hydrogen bonding are ideal to direct the ordering of molecules at the liquid/solid and air/solid interface. However, in contrast to the behaviour


of these compounds in solution, the substrate plays an important role. The design of the molecules and the selection of a suitable liquid/solid environment are mandatory to tailor the molecule-molecule and molecule-substrate interactions.

Acknowledgements

The authors thank the Federal Science Policy through IUAP-V-03, the Institute for the promotion of innovation by Science and Technology in Flanders (IWT), and the Fund for Scientific Research-Flanders (FWO).

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