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  • Intricate Hydrogen-Bonded Networks: Binary and Ternary Combinations of Uracil, PTCDI,and Melamine

    Jules A. Gardener,*, Olga Y. Shvarova, G. Andrew D. Briggs, and Martin R. Castell*Department of Materials, UniVersity of Oxford, Oxford, OX1 3PH, United Kingdom

    ReceiVed: NoVember 29, 2009; ReVised Manuscript ReceiVed: January 31, 2010

    We report the formation of two- and three-component porous supramolecular networks from combinations ofuracil, PTCDI, and melamine. The structures, which are formed on Au(111) in ultra-high vacuum (UHV)and studied by scanning tunneling microscopy (STM), are stabilized by hydrogen bonds. We show that twobimolecular networks comprising uracil and PTCDI can be formed, one of which contains two pore geometriesand is composed of 28 molecules per unit cell. In addition, we observe two different ordered structures frommixtures of melamine and uracil. By combining all three of these species, we demonstrate the formation ofa single ternary structure that contains 33 molecules per unit cell. Our results demonstrate the capacity ofhydrogen bonding to produce highly complex structures, and open up the possibility of forming a wide rangeof new structures from combinations of nucleic bases and other small organic molecules.

    Introduction

    A range of porous two-dimensional supramolecular networkscan be constructed on surfaces through careful choice of themolecular components and the substrate.1 This is primarilyachieved through hydrogen bonding, metal-organic co-ordina-tion, van der Waals interactions, or covalent bonding. Here, weconcentrate on combinations of organic molecules with thecapacity to form hydrogen bonds. Such an approach is advanta-geous since hydrogen bonds are highly directional, leading toordered structures, but sufficiently weak to allow moleculardiffusion on surfaces at room temperature. These properties canresult in the formation of ordered molecular structures over largeareas. Further, some of these networks may contain pores andcan act as templates for the ordering of guest molecules suchas fullerenes.29

    Some nucleic bases and other small organic molecules canform monomer porous two-dimensional structures stabilized byhydrogen bonding.1014 However, a greater variety of networkscan be generated through the combination of two complementarymolecular species. For example, four different supramolecularnetworks of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)and1,3,4-triazine-2,4,6-triamine(melamine)havebeenreported,2,68,15,16

    of which three contain pores that act as absorption sites for guestmolecules. Bimolecular networks generally have more complexunit cells than their monomer counterparts, and offer a widerrange of pore sizes, geometries, and interpore separations.Recently, coronene-templated three-component networks havebeen reported, which are even larger and more intricate (wedefine intricacy in terms of the number of molecules per unitcell).17,18 However, these appear to show little capacity to hostmolecules other than coronene.

    In this work, we have combined two different systems thatare well-known to form hydrogen-bonded supramolecularnetworks: a nucleic base with PTCDI-melamine. The nucleicbase that we have chosen is uracil, a molecule that forms planar

    hydrogen-bonded structures in two and three dimensions.1921

    The chemical structures of the three molecules studied are shownin Figure 1. Uracil is prochiral; when constrained to lie parallelto a surface, it can adopt one of two possible orientations andthese have no mirror symmetry, giving rise to chirality.However, PTCDI and melamine are not chiral, even whenconstrained to two dimensions. Here, we deposit uracil, PTCDI,and melamine onto Au(111), by sublimation. Through thecareful selection and postdeposition processing of the componentmolecules, we demonstrate the formation of ordered binary andternary supramolecular networks, some of which have remark-ably intricate unit cells.

    Experimental Methods

    All experiments were performed in situ under UHV conditions(base pressure of 1 10-8 Pa). Au(111) films on mica(Agilent) were prepared by Ar+ sputtering and annealing to600 C, a procedure that produced large flat terraces displayingthe characteristic 22 3 reconstruction. Melamine (Aldrich),PTCDI (TCI Europe), and uracil (Aldrich) were sublimed attypical rates of 8-15 ML/h (ML)monolayers), 0.6-0.8 ML/h, and 0.1-0.3 ML/h (unless otherwise stated), respectively,onto the Au substrates, which were held at room temperature.The samples were subjected to postdeposition annealing for10-14 h; shorter anneals (typically a few hours) and lowertemperatures resulted in less ordered structures. A JEOLJSTM4500S STM was used to obtain constant current images(bias applied to the sample) at room temperature, usingelectrochemically etched W tips. All images were processedwith WSxM software.22

    * To whom correspondence should be addressed. gardener@fas.harvard.eduand martin.castell@materials.ox.ac.uk.

    Present address: Department of Physics, Harvard University, Cambridge,MA 02138.

    Figure 1. The molecular structures of (a) PTCDI, (b) melamine, and(c) uracil. Balls represent carbon (gray), oxygen (red), nitrogen (blue),and hydrogen (white) atoms.

    J. Phys. Chem. C 2010, 114, 58595866 5859

    10.1021/jp9113249 2010 American Chemical SocietyPublished on Web 03/10/2010

  • Results and Discussion

    First, we characterize the self-assembly of uracil afterdeposition onto a clean 22 3 reconstructed Au(111) filmon mica. We observe two different structures, as shown in Figure2. At moderately low coverages, chain-like features are formed,as shown in Figure 2a. These structures often form closed rings,but show no long-range ordering. High-resolution images, suchas Figure 2b, show that the chains comprise double rows ofuracil molecules, while pentagonal clusters are frequentlyobserved where multiple chains join (for example, as indicatedby an arrow in the figure). The uracil chains are similar to thoseformed by flat-lying cytosine or thymine on Au(111), stabilizedby in-plane hydrogen bonds;2325 any differences in molecularpositioning within the chains are attributed to the nonequivalenthydrogen bonding sites of different nucleic bases. At highermolecular coverages we find close-packed regions (Figure 2c).In this arrangement, each uracil is generally surrounded by sixother uracil molecules, again forming a hydrogen-bondednetwork.19,20

    New porous structures are observed after sequential depositionof 0.1-0.3 ML of uracil then 0.3-0.4 ML of PTCDI at room

    temperature, followed by annealing to 85 C for 10 h,examples of which are shown in Figures 3 and 4. On average,these structures cover a combined total of 34% of the Au(111)surface, while some residual close-packed PTCDI patchesremain (any excess uracil is likely to have desorbed from thesurface during annealing). The differences in size, shape, andelectronic structure of PTCDI and uracil allows them to bedistinguished in the STM images; PTCDI appear as brightrectangles, while uracil molecules are seen as smaller, roundfeatures. However, we cannot rule out the additional presenceof small adsorbate molecules (such as water) within thesestructures.

    The network shown in panels a and b of Figure 3 comprisesdouble rings of uracil, linked by parallel pairs of PTCDImolecules arranged in a cross-like manner. We will refer to thisstructure as the double-cross. The uracil double-rings areslightly elliptical in shape and show distinct similarities to thechain structure of Figure 2 since, in both of these structures,neighboring uracil molecules from each ring are aligned radially.However, the outer ring of the double-cross structure appearsto be missing four uracil molecules (one in each quadrant), the

    Figure 2. (a) Disordered chain-like structures of uracil (Vs ) +1.35 V, It ) 0.07 nA, 85 nm 85 nm). (b) A higher resolution image of the chains,which are double rows of uracil molecules (Vs ) +1.44 V, It ) 0.07 nA, 16.0 nm 12.0 nm). A black arrow highlights a pentagonal cluster ofuracil. (c) A region of close-packed uracil (Vs ) -1.40 V, It ) 0.10 nA, 16.0 nm 12.0 nm). Uracil was deposited at a rate of 0.9-1.0 ML/h forall images shown in this figure.

    5860 J. Phys. Chem. C, Vol. 114, No. 13, 2010 Gardener et al.

  • positions of which are instead occupied by the ends of fourPTCDI molecules. The other ends of these PTCDI link to theouter edge of neighboring uracil double-rings. The differentarrangements of uracil at either end of the PTCDI moleculeslead to one of the PTCDI protruding further out of the ringthan its parallel near-neighbor, and give rise to chirality. Thedouble-cross motif has two rotational axes and its unit cell is aparallelogram. This structure is therefore of the p2 chiral planegroup (as described in the Two-Dimensional Structural Data-base).26

    The double-cross network has two different porous sites. Thepore marked A in Figure 3a is outlined by uracil moleculesand is circular. In addition, a rectangular pore, framed by PTCDImolecules in the corners, is also formed (labeled B in Figure3a). A network possessing two different pore shapes isparticularly interesting since it opens up the possibility ofgenerating different packing arrangements of guest moleculesin close proximity to each other. The double-cross structure is

    rotated by 9((1) from the [112j] direction of the Au(111)surface. A preferred crystallographic orientational relationshipbetween the network and the Au(111) surface demonstrates theanisotropic nature of the substrate. This effect is not unexpectedas it is frequently observed for similar hydrogen-bondednetworks.68,15,19,27,28 This structures unit cell has dimensionsof a ) 5.1 ( 0.2 nm, b ) 4.7 ( 0.2 nm, and ) 81((1) andcontains 24 uracil and 4 PTCDI molecules.

    Our high-resolution images allow us to propose a po