2D-Structures of Quadruple Hydrogen Bonded Oligo(p

NANO LETTERS

2D-Structures of Quadruple Hydrogen Bonded Oligo(p-phenylenevinylene)s on Graphite: Self-Assembly Behavior and Expression of Chirality

2004 Vol. 4, No. 7 1175-1179

A. Gesquie`re,†,‡,§ P. Jonkheijm,†,§ F. J. M. Hoeben,† A. P. H. J. Schenning,*,† S. De Feyter,*,‡ F. C. De Schryver,‡ and E. W. Meijer† Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, PO Box 513, 5600 MB EindhoVen, The Netherlands, and Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium Received January 27, 2004; Revised Manuscript Received May 13, 2004

ABSTRACT The two-dimensional pattern formation of chiral oligo(p-phenylenevinylene) derivatives of different lengths containing a self-complementary hydrogen bonding motif at the liquid/solid interface has been investigated and compared using scanning tunneling microscopy. Hydrogen bonding leads to dimer formation and the chirality of the molecules is expressed at the level of packing and orientation of the dimers with respect to the substrate symmetry. Differences in expression of molecular chirality are observed as a function of molecular length and the number of stereocenters they carry. Mixing of oligomers of different length does not lead to phase separation, but to the formation of heterodimers, stressing the important role of hydrogen bonding in the self-assembly process in both solution and at the liquid/solid interface.

The construction of supramolecular assemblies composed of functional molecular building blocks with a well-defined structure is an important topic in organic material chemistry.1 A vital aspect in this construction process is the self-assembly phenomenon at surfaces to obtain nanosized functional patterns.2 For two-dimensional (2D) supramolecular structures on surfaces, the self-assembly of the molecules is governed by both molecule-substrate and moleculemolecule interactions. The interaction between molecules and surface will guide molecular orientation in the overlayer, a phenomenon that has been extensively documented for highly oriented pyrolytic graphite (HOPG), especially when alkylated systems are investigated.3 Recently, the importance of substrate-molecule interactions was shown in supramolecular assembly processes.4,5 Intermolecular interactions will direct the mutual organization of the molecules. Hydrogenbond interactions have been assessed extensively in the selfassembly6 because hydrogen bonds are highly selective and directional, though moderately strong.7 This study aims at exploring the detailed intermolecular interactions and the effect of hydrogen bonding, length, and molecular chirality * Corresponding author. † Eindhoven University of Technology. ‡ Katholieke Universiteit Leuven. § Both Pascal Jonkheijm and Andre ´ Gesquie`re contributed equally to the work presented in this paper. 10.1021/nl049842c CCC: $27.50 Published on Web 06/19/2004

© 2004 American Chemical Society

on the 2D self-assembly process of a set of chiral oligo(pphenylenevinylene) derivatives of different lengths containing a self-complementary hydrogen bonding motif (Figure 1). A technique to access detailed information on selfassembly behavior of molecules at surfaces is scanning tunneling microscopy (STM). Noncovalent interactions such as hydrogen bond,8 host-guest,9 and dipole-dipole10 interactions have been exploited by STM, demonstrating spontaneous ordering of ensembles of molecules into larger structures. A vast amount of STM observations of single and double hydrogen-bond formation have been reported ranging from layers of simple fatty acids and isophthalic acid derivatives11 to the more complicated epitaxial grown layers of pyridylbenzoic acid, diimide derivatives, and complex multicomponent structures.12 Also, a number of π-conjugated rod-like molecules have been reported, for example, oligo13 and polythiophenes14 and oligo-(phenylene)ethynylenes.15 Recently, we presented an STM study of the ordering and bias-dependent contrast of a triad composed of two oligo(p-phenylenevinylene) (OPV) units and a central perylenediimide16 and the adsorption on graphite of a dimer, tetramer, and hexamer of oligo(p-phenylenevinylene)s, end-capped with tridodecyloxybenzene wedges.17 Furthermore, we investigated an OPV bearing a diaminotriazine hydrogen bonding unit (association constant approximately smaller than Ka ) 10 M-1 in chloroform) that forms hydrogen bonded

Figure 1. Molecular structures of oligo(p-phenylenevinylene)s equipped with a ureido-s-triazine array capable to form four hydrogen bonds. OPV3 (n ) 1), OPV4 (n ) 2), and OPV5 (n ) 3) have an increasing conjugation length from trimer, tetramer, to pentamer.

hexameric rosettes.18 We now report an STM study on the self-assembly of chiral oligo(p-phenylenevinylene)s (OPV3, OPV4, and OPV5, Figure 1)5 that differ in conjugation length at the solution-HOPG interface into 2D supramolecular crystals. The π-conjugated OPVs are equipped with a selfcomplementary ureido-s-triazine hydrogen bonding array (DADA array; D ) hydrogen-bonding donor, A ) hydrogenbonding acceptor) having a dimerization constant in the order of 20 000 M-1 in chloroform. All oligomers are substituted with two, four, or six enantiomerically pure (S)-2-methylbutoxy side chains on the OPV3, OPV4, and OPV5 backbone, respectively.5 Quasi-constant height STM images obtained at the liquidsolid 1,2,4-trichlorobenzene/graphite interface (Figures 2-4) of the different oligomers show the same contrast characteristics; the π-conjugated segments appear as the brightest areas while the alkyl tails lie in the dark parts.17,19 The long bright rods as observed in Figure 2A often appear as two individual bright rods separated by a dark trough, which can, for instance, clearly be observed for OPV4 in Figure 2C and OPV5 in Figure 3. This dark trough corresponds to the location of the hydrogen bonded moieties. The difference in chemical nature between the bright rods and dark troughs is also confirmed by their different contrast response to changes in the bias voltage (not shown). Occasionally, individual phenyl rings are resolved in the rods. By in-situ calibration with crystalline graphite (e.g., Figure 2D), the packing parameters of the 2D crystals were determined. The individual bright rods correspond in length to the conjugated parts of the respective dimeric structures studied (Table 1). The conjugated segments as well as the aliphatic side chains are lying with their long axes parallel to the basal plane of the graphite substrate. The apparent dimer formation indicates hydrogen bonding between two OPV molecules which, based upon the ratio between their length and width, clearly compose all-trans vinylene bonds. The 2D pattern of the crystallized dimers further assembles into lamellae over hundreds of nanometers. The space between two dimers in a lamella is sufficient to accommodate a face-on orientation of the conjugated parts and fully extended chiral (S)-2methylbutoxy side chains, indicating that they adopt a highly ordered and favored conformation. For OPV4 (Figure 2) and 1176

Figure 2. STM images of dimeric OPV4 monolayers on HOPG. (A) Image size is 12.1 × 12.1 nm2; Iset ) 0.8 nA; Vbias ) -0.68 V. (B) Molecular model representing the 2D ordering in A. The conformation of OPV4 is based on molecular dynamics calculations of one molecule on graphite, neglecting nearest-neighbor interactions and omitting one dodecyloxy side chain. Only two out of three alkyl chains are visible in the STM images. A unit cell is indicated in the image. (C) Often, a clear shift is observed between the OPV rods in a dimer. Image size is 13.4 × 13.4 nm2. Iset ) 1.0 nA, Vbias ) -0.98 V. (D) By lowering the bias voltage during scanning the monolayer and underlying graphite surface can be imaged simultaneously. The dashed line indicates that the lamella axis runs parallel with a main symmetry axis of the graphite lattice. Image size is 13.4 × 13.4 nm2. Iset ) 1.0 nA, Vbias ) -0.004 V (for the lower part).

OPV5 (Figure 3), this leads to exclusively counterclockwise (CCW) oriented dimers with respect to the normal on the long lamella axis in lamellae with an exclusive symmetry-equivalent propagation direction with respect to the crystalline substrate (yellow line in Figure 2D). In the spacing between consecutive lamellae, two out of three dodecyloxy side chains per molecule could be detected in a side-by-side fashion following the 6-fold symmetry of the support. Note that the chains of two end-to-end molecules do not interdigitate like the fingers of a left and the right hand when clasped, but the two visible chains of one molecule always stay together. The location of the third chain is not revealed.17 On the contrary, for OPV3 both domains with counterclockwise and clockwise (CW) oriented dimers with respect to the normal on their respective long lamella axes are observed. These are, however, no mirror domains. The minority of the domains (packing II, top area in Figure 4A and Figure 4B) obtain a phase (CCW) similar to OPV4 and OPV5, whereas the majority (packing I, bottom area in Figure 4A and Figure 4C) of the domains contain lamellae with a CW orientation of the dimers. Each phase, however, is homochiral and shows an exclusive propagation direction with respect to the main symmetry axes of graphite (about Nano Lett., Vol. 4, No. 7, 2004

Table 1. Length of the Hydrogen Bonded Dimers Obtained by Molecular Dynamics (MD) and the Lattice Parameters of the Two-Dimensional Crystals of the Oligo(p-phenylenevinylene)s Derivatives MD (Å)

OPV3-I OPV3-II OPV4 OPV5

Figure 3. STM image of dimeric OPV5 monolayer on HOPG. Multiple domains covering a large area, image size 180 × 180 nm2, Iset ) 1.0 nA, Vbias ) -0.80 V. All molecules adopt the same counterclockwise orientation. Inset: Zoom of a domain, image size is 10.7 × 10.7 nm2, Iset ) 1.0 nA, Vbias ) -0.75 V.

Figure 4. STM images of dimeric OPV3 monolayers on HOPG. (A) Two lamellar domains with opposite handedness of the OPV backbone. Image size is 32.0 × 16.0 nm2, Iset ) 0.9 nA, Vbias ) -1.06 V. The dashed line indicates the orientation of a main graphite symmetry axis. (B) Zoom of domain with packing II (top part in A) where the π-system obtains a counterclockwise orientation with respect to an axis perpendicular to the lamellar axis. Image size is 8.9 × 8.9 nm2, Iset ) 0.9 nA, Vbias ) -1.06 V. (C) Zoom of domain with packing I (bottom part in A) where π-system obtains a clockwise orientation with respect to an axis perpendicular to the lamellar axis. Image size is 11.2 × 11.2 nm2, Iset ) 0.9 nA, Vbias ) -0.27 V. This image has been recorded at a different rotation angle.

-14° for packing I and 0° for packing II) as indicated in Figure 4A.20,21 The formation of homochiral phases is determined by the molecular conformation, which is mainly governed by the balance between the preferred adsorption of chiral methyl groups in the side chains and the central core of the dimers. Nano Lett., Vol. 4, No. 7, 2004

STM

length dimer

a (Å)

b (Å)

a (°)

47.0 47.0 57.8 73.5

19.8 ( 0.1 17.0 ( 0.1 19.9 ( 0.5 20.2 ( 0.3

48 ( 1 46 ( 1 54 ( 2 65 ( 1

75 ( 3 81 ( 3 85 ( 3 85 ( 3

The OPV hydrogen bonded dimers can be detected as one bright rod and also as two bands with a clear offset between the OPV units (Figure 2D and Figure 3 inset). These two manifestations might be caused by the conformation of the vinylene bonds as shown in Figure 5. For conformation I (Figure 5) an offset exists, while a linear dimer is observed for conformation II (Figure 5). Interestingly, the chirality of the phase is, in addition to the oVerall rotation direction of a dimer within a lamella, also reflected in the offset between the two opposite hydrogen bonding OPVs. The direction of the shift remains consistent for a particular homochiral phase throughout the collected data. In the lamellae with clockwise oriented dimers (only observed for OPV3: bottom part Figure 4A and Figure 4C), the groups in the right half of a lamella are always shifted downward with respect to the groups in the left half of a lamella (hydrogen bonding tape). In lamellae with counterclockwise oriented dimers (OPV3, OPV4, OPV5), the groups in the right half of a lamella are always shifted upward with respect to the groups in the left half of a lamella. Defects are often observed at domain boundaries that surround each crystal (Figure 6). At these frontiers the molecules are more loosely packed, and within the time scale of imaging dynamic boundaries are observed.22 In case of OPV3, also some other polymorphous structures are observed in addition to the ones (packing I and II) discussed above. From the results presented above, it is clear that a subtle balance between molecules and surface interactions determines the packing morphology at the liquid-solid interface. Mixing experiments of the OPVs with different chain length (OPV3 and OPV4) showed in solution the formation of heterodimers.23 It is a challenge to observe these heterodimers at the surface. The formation of mixed monolayers was indeed observed by mixing equimolar amounts of OPV3 and OPV4 (Figure 7). It indicates the existence of hydrogenbonded heterodimers in addition to homodimers, which are all stacked in lamellae on the graphite surface. The different combinations of homodimers and heterodimers are indicated in the image. The existence of this type of 2D pattern shows the dominance of hydrogen bonding. Without these strong intermolecular interactions, it is expected that the molecules would phase-separate into different domains. The dimerization constant in solution for OPV3 and OPV4 is the same5 and is most probably in 1,2,4-trichlorobenzene in the same order as determined in chloroform. For the STM experiments, 10-3 M solutions are used implying that most of the molecules are present as dimers in a statistical mixture of 1177

Figure 5. Molecular models (HyperChem) and chemical structures illustrating two possible dimer conformations. The upper one shows a pronounced shift between the OPV rods (conformation I). The lower one shows the dimer as a linear rod (conformation II).

Figure 7. STM image of mixed lamellae of OPV3 and OPV4 by physisorption on a graphite surface from a concentrated 1,2,4trichlorobenzene solution. Image size is 21.0 × 21.0 nm2, Iset ) 1.5 nA, Vbias ∼ -0.8 V. Figure 6. STM images of rare monolayers showing peculiarities (for OPV3) or disorder (for all compounds, here shown for OPV4) at domain boundaries. (A) OPV3 dimers arranged as dimers. Image size is 33.5 × 33.5 nm2, Iset ) 0.9 nA, Vbias ) -1.13 V. (B) OPV3 dimers arrange as trimers. Image size is 15.6 × 15.6 nm2, Iset ) 0.9 nA, Vbias ) -0.95 V. (C) OPV4: image size is 16.0 × 16.0 nm2, Iset ) 0.8 nA, Vbias ) -0.75 V. (D) OPV4: image size is 11.9 × 11.9 nm2, Iset ) 0.8 nA, Vbias ) -0.75 V.

25% homodimers of OPV3, 50% heterodimers, and 25% monodimers of OPV4. On the substrate, however, the OPV3 are a minority, which is probably the result of a difference in physisorption energy of OPV3 and OPV4 with the substrate. Oligo(p-phenylenevinylene)s have been adsorbed at the liquid-solid interface. Quadruple hydrogen bonding plays the dominant role in the intermolecular interactions as in all cases dimers are formed on the surface. The phenylenevinylene derivatives’ stiff and linear backbone gains geometric advantage, contributing to extremely ordered aggregates. 2D assemblies show lamellae in which molecules are face-on oriented, in this way maximizing the overlap of their orbitals with those of graphite, resulting in a maximum enthalpic gain.24 The internal arrangement of the lamellae can provide the gain in adsorption energy. The lateral aliphatic dodecyl1178

oxy chains will stabilize the lamellae in 2D assemblies. The orientation of the monomers within the dimers, the orientation of the dimers within the lamellae, and the propagation direction of the lamellae with respect to the symmetry elements of the substrate, are determined by and are an expression of the chirality of the molecules. The longer oligomers, OPV4 and OPV5, form a similar phase, which is homochiral. In case of the short oligomer, OPV3, different polymorphs were observed. Nevertheless, each phase is still homochiral: for a given phase, one type of ordering is observed, not its mirror image. For this set of compounds, the formation of monophase (OPV4 and OPV5) or multiphase (OPV3) structures relates to the number of stereocenters and length of the conjugated backbone. Apparently, a certain number of chiral groups is needed to steer the internal arrangement to one unique preferred crystallite or phase. The number of stereocenters has, however, no influence on the “quality” of the expression of molecular chirality: each phase is homochiral. In mixtures of OPV3 and OPV4, heterodimers are adsorbed on graphite in addition to homodimers as a result of the strong self-complementary hydrogen bonding motif. Nano Lett., Vol. 4, No. 7, 2004

Acknowledgment. The authors thank the Federal Science Policy, trough IUAP-V-03, the Institute for the Promotion of Innovation by Sciences and Technology in Flanders (IWT), and the Fund for Scientific Research-Flanders (FWO). S.D.F. is a postdoctoral fellow of FWO.

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