Self-assembled two-dimensional hexagonal networks

Article abstract of DOI:10.1039/b206224k

A new compound, 1,3,5-tris(carboxymethoxy)benzene, which forms infinite two-dimensional hexagonal networks, has been synthesized. Its self-assembly behavior at gas-solid interfaces has been studied by scanning tunneling microscopy.

Full text of DOI:10.1039/b206224k

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J O U R N A L O F
Self-assembled two-dimensional hexagonal networks
Jun Lu,a,b Qing-dao Zeng,a Chen Wang,*a Qi-yu Zheng,a Lijun Wana and Chunli Bai*a
aCenter of Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences,
Beijing 100080, P. R. China. E-mail: wangch@infoc3.icas.ac.cn, clbai@infoc3.icas.ac.cn
bThe Graduate School of the Chinese Academy of Sciences, Beijing 100080, P. R. China
C H E M I S T R Y
Received 28th June 2002, Accepted 9th August 2002
First published as an Advance Article on the web 20th August 2002
A
new compound, 1,3,5-tris(carboxymethoxy)benzene,
for structuring metal nano-assemblies, or biomolecules in
sensor devices, only the surface is of interest. On the surface,
ultrathin organic layers, such as self-assembling and Lang-
muir–Blodgett films are constructed. These films represent a
novel class of materials with potential applications in advanced
optical and electronic systems.15,16 Recently, a lot of hydrogen-
bonded systems which form 3D crystal structures have been
assembled on solid supports to form highly stable 2D crystal
structures which can be visualized by scanning tunneling
microscopy (STM). STM is a powerful tool in the area of
surface and interface analysis due to its ultrahigh resolution
and adaptability to various environments. Organic species of
all types, from benzene to polymers, have been characterized by
STM.17 It is of interest to determine whether the 2D structure
of films resembles the intralayer alignment in the bulk 3D
crystal.18–20 A study of the adsorption of TMA to a single
crystal graphite surface under ultra high vacuum conditions
revealed that there are two coexisting phases that could be
imaged with sub-molecular resolution by STM.20 One of the
two structures is composed of sixfold rings of TMA molecules
with perfect arrangement of the hydrogen bonds, which is in
agreement with the intralayer alignment in the 3D crystal. To
date, we have not managed to obtain a single crystal of TCMB,
but high resolution STM images show that it also forms
hexagonal networks in the 2D crystal.
which forms infinite two-dimensional hexagonal net-
works, has been synthesized. Its self-assembly behavior
at gas–solid interfaces has been studied by scanning
tunneling microscopy.
Supramolecular structures formed by the self-assembly of
functional molecular building blocks are a promising class of
materials for future technologies.1–3 Particularly useful for
their fabrication is hydrogen bonding,4 which provides both
high selectivity and directionality.5 Among the various types of
supramolecular structures, the hydrogen-bonded 2D hexago-
nal network formed by trimesic acid (1,3,5-benzene tricar-
boxylic acid, H3TMA) is well known.6 Early works were
focused upon TMA itself and generated a large number of
inclusion compounds, both with and without catenation.7,8
That carboxylic acids can form homo- or heterodimers with
a variety of complementary functional groups (e.g. pyridines,
2-aminopyridines, and pyrimidines) has allowed functionalized
TMA,9 its deprotonated forms,10 and its metal complexes11,12
to be exploited as templates in crystal engineering studies.
Besides, TMA and many other molecules form host–guest
systems,13,14 which are of great significance in supramolecular
chemistry and crystal engineering.
As is the case for TMA itself, inclusion compounds and
host–guest systems containing TMA have been extensively
studied; it is now of interest to further explore other molecules
that have similar structures to TMA and behave like TMA.
These molecules could then provide new hydrogen-bonded
supramolecular synthons, which are the primary tools for
crystal engineering, and, thus, provide alternatives to TMA.
Here, we report the synthesis of 1,3,5-tris(carboxymethoxy)-
benzene (TCMB), which, like TMA, also possesses trigonal
exodentate functionality that facilitates self-assembly in two
dimensions.
The synthetic route to the designed molecule, TCMB, is
shown in Scheme 1. All chemicals used were purchased from
Acros and used without further purification.
1,3,5-Tris(ethoxycarbonylmethoxy)benzene (1) was synthe-
sized as follows: ethylbromoacetate (26.7 g, 0.16 mol) was
slowly added to a stirred mixture of anhydrous potassium
carbonate (22.1 g, 0.16 mol) in 500 ml acetone at room
temperature. Next, anhydrous phloroglucinol (5.04 g, 0.04 mol)
in 100 ml of acetone was added dropwise over a period of 1 h.
After addition, the mixture was refluxed overnight. The salts
formed were removed by filtration and the solvent volume was
reduced by rotary evaporation. After cooling to room tempera-
ture, the solution yielded 1 as a yellowish solid, which was
recrystallized from ethanol to provide colorless crystals. Yield:
While in supramolecular chemistry, host–guest complexes
have mainly been investigated in solution and as bulk crystals,
it is not always necessary to build three-dimensional struc-
tures. For potential technical applications, such as heterogenic
catalysis, molecular electronics, organic network templates
1
10.7 g (70%). Mp 65–66 uC. H NMR (300 MHz, CDCl3): d
Scheme 1
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This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b206224k

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1.30 (t, 9H, J ~ 7.2 Hz), 4.28 (m, 6H, J ~ 7.2 Hz), 4.55 (s, 6H),
6.13 (s, 3H). 13C NMR (300 MHz, CDCl3): d 168.5, 159.6, 95.1,
65.3, 61.4, 14.1. FT-IR (KBr) n/cm21: 1749, 1607, 1219, 1159,
1091, 1035. EI-MS m/z: 384 (M1). Elemental analysis for
C18H24O9 (Mw 384.3) found (calc.): C 56.53 (56.22), H 6.57
(6.30)%.
1,3,5-Tris(carboxymethoxy)benzene (2) was prepared as fol-
lows: compound 1 (5 g, 0.013 mol) was refluxed in water with
0.8 g. of Dowex 50 W-x8 (H1 form). The ethanol formed upon
hydrolysis was distilled as the azeotrope in order to drive the
reaction to completion, thus forming the triacid 2. After 8 h of
reflux, the mixture was filtered to remove the resin. Upon
cooling, the triacid solidified and was recrystallized from hot
water. Yield: 3.6 g. (92%). Mp 285-287 uC. 1H NMR (300 MHz,
DMSO-d6): d 4.67 (s, 6H), 6.13 (s, 3H). 13C NMR (300 MHz,
Fig. 2 Simulated molecular model representing the assembly structure
of TCMB.
Due to the thermally activated diffusional motion of the
molecules on the inert substrate at room temperature, most
STM studies of organic molecules on graphite under ambient
conditions are focused upon long chain alkyl-substituted
molecules.23 To the best of our knowledge, TCMB is one of
the smallest organic molecules that has been imaged by STM
on graphite under ambient conditions without using the matrix
molecule coadsorption method. This shows that the strength
of the hydrogen-bonding between the TCMB molecules on
graphite is strong enough to support the two-dimensional
networks.
Attempts were also made to obtain STM images of
TMA. Maybe due to the lack of flexibility in its structure,
we failed to obtain any images of well-ordered arrays of TMA
on graphite.
In summary, it is illustrated that TCMB can form 2D
hexagonal networks. Furthermore, because TCMB has longer
chains than TMA, it seems likely that its structure is more
flexible than that of TMA, which may mean that TCMB will
form host–guest systems more easily.
DMSO-d6): d 170.4, 159.8, 94.7, 65.0. FT-IR (KBr) n/cm21
:
3200–2500, 1748, 1714, 1602, 1233, 1175. FAB-MS(2ve) m/z:
299 ([M 2 H]2). Elemental analysis for C12H12O9 (Mw 300.2)
found (calc.): C 52.67 (52.61), H 5.60 (5.30)%.
The solvent used in the STM experiments is a mixture of
toluene (HPLC grade, Aldrich) and ethanol (A.C., Aldrich).
The concentrations of all the solutions used are less than 1 mM.
Samples were prepared by depositing a drop of the above
solution on freshly cleaved highly oriented pyrolytic graphite
(HOPG). Experiments were performed with a Nanoscope
IIIA system (Digital Instruments, Santa Barbara, CA, USA)
operating under ambient conditions. STM tips were mechani-
cally firmed 90% Pt–10% Ir wires. All the STM images were
recorded using the constant current mode of operation. The
specific tunneling conditions are given in the figure captions.
Presented in Fig. 1(a) is a large scale STM image of TCMB
molecules adsorbed on a HOPG surface over a scan area of 66
6 66 nm2. The image illustrates that the TCMB molecules,
appearing as bright spots with 6-fold symmetry, form a well-
ordered two-dimensional array. Fig. 1(b) is a high resolution
image of TCMB. The diameters of the hexagonal cavities are
1.9 ¡ 0.1 nm, compared to 1.5 ¡ 0.1 nm of TMA.20 In the top
right corner of Fig. 1(b), a model (constructed using Hyper-
chem software21) of four self-assembled rings composed of
16 molecules of TCMB is superposed on the STM image. The
model and the image fit well. The blue hexagon at the bottom
of Fig. 1(b) represents a unit cell, with a lattice constant of
1.1 ¡ 0.1 nm. Fig. 2 shows a schematic picture of TCMB
hexagonal networks formed by hydrogen bonding. On the
left is a molecular model proposed on the basis of STM
observation using Hyperchem software; the model includes
seven cavities. On the right is a molecular model constructed
with ChemDraw Ultra22 which shows how a hexagonal cavity
is formed by the linking of six molecules in a circlar pattern via
hydrogen bonding. Each molecule is cooperatively connected
by three cavities.
Acknowledgements
The authors are grateful for financial support from the National
Natural Science Foundation (grant No. G20073053, 20103008)
and the National Key Project on Basic Research (grant No.
G2000077501).
Notes and references
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