Porous molecular networks formed by the self-assembly of positively-charged trigonal building blocks at the liquid/solid interfaces

Article abstract of DOI:10.1039/c4cc01576b

Tris-(2-hydroxybenzylidene)triaminoguanidinium salts having six alkyl chains with proper spacing served as new molecular building blocks for the formation of porous honeycomb networks by van der Waals interaction between interdigitated alkyl chains at the

Full text of DOI:10.1039/c4cc01576b

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Porous molecular networks formed by the
self-assembly of positively-charged trigonal
building blocks at the liquid/solid interfaces†
Cite this: Chem. Commun., 2014,
50, 7683
Received 2nd March 2014,
Accepted 21st May 2014
Kazukuni Tahara,a Maria L. Abraham,b Kosuke Igawa,c Keisuke Katayama,a
Iris M. Oppel*b and Yoshito Tobe*a
DOI: 10.1039/c4cc01576b
www.rsc.org/chemcomm
Tris-(2-hydroxybenzylidene)triaminoguanidinium salts having six alkyl to utilize the latter interaction is its superior versatility in terms of
chains with proper spacing served as new molecular building blocks for modification of the pore size and functionality for chirality control and
the formation of porous honeycomb networks by van der Waals inter- selective binding of guest molecules. We reported that triangle-shaped
action between interdigitated alkyl chains at the liquid/graphite interfaces. phenyleneethynylene macrocycles, dehydrobenzo[12]annulenes, substi-
tuted with six alkyl or alkoxy groups formed honeycomb networks at
Two-dimensional (2D) molecular networks formed by the self- the liquid/solid interfaces.6 The group of Attias and Charra reported the
assembly of organic molecules on solid surfaces have attracted formation of porous honeycomb networks by hexaalkoxy-substituted
increasing attention in connection with their promising uses in 1,3,5-tristyrylbenzene derivatives.7a,b The key structural feature common
nanoscale patterning, nanoreactors, molecular electronics and 2D in these building blocks is a trigonal-shaped core and three pairs of
polymer synthesis.1 Among various 2D molecular networks, porous alkyl chains attached to it, wherein each pair is located at an optimal
networks have become a subject of intense interest because surface- chain–chain spacing (ca. 1 nm) for van der Waals interaction with a
confined pores in the network can accommodate guest molecules to pair of alkyl chains of an adjacent molecule, forming a noncovalent
form multi-component 2D structures, which are important for con- linkage by the interdigitated alkyl chains.6–8 Another example reported
struction of complex patterns.2 Scanning tunneling microscopy (STM) thereafter also meets the above structural criteria of the building block
is a powerful tool to investigate these networks at sub-molecular level for honeycomb network formation.9 However, in the previous exam-
resolution under ultrahigh vacuum (UHV) conditions1a or at a liquid/ ples, the trigonal cores of the building blocks consist of nonpolar
solid interface.3
hydrocarbons.6 To endow additional functions to porous molecular
Among the porous patterns hexagonal honeycomb type molecular networks of this type, the exploration of new molecular building blocks
networks are the most common, because epitaxial effects of the is strongly desired. In this respect, porous molecular networks contain-
hexagonally symmetric surfaces can be utilized to control the network ing polar or charged trigonal cores are intriguing targets, because such
patterns. In general, for the construction of honeycomb type porous networks would possess binding sites that may serve to host ions or
molecular networks, C3 symmetric molecular building blocks are used molecules10 and platform freestanding functional groups on surfaces11
and they are connected by directional intermolecular interactions to in addition to pores with guest entrapping ability.2,6,7
leave an open space between them. Though there are many examples
In this context, we herein delineate the construction of the
for such networks formed by hydrogen bonding4 or coordination honeycomb type porous molecular network formed from building
bonding,5 only a few molecules are known that form honeycomb type blocks consisting of a positively charged trigonal core and six alkyl
networks via van der Waals interactions.6,7 The major advantage chains attached with proper spacing. For this purpose, we synthe-
sised C3 symmetric tris-(3,5-didecyl-2-hydroxy-benzylidene)triamino-
guanidinium salts as molecular building blocks (Fig. 1a) and
investigated their self-assembling behaviours at the liquid/graphite
interfaces by means of STM. These salts were chosen as their central
a Division of Frontier Materials Science, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan.
E-mail: tobe@chem.es.osaka-u.ac.jp; Tel: +81-(0)6-6850-6225
b Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1,
52074 Aachen, Germany. E-mail: iris.oppel@ac.rwth-aachen.de;
Tel: +49-(0)241 8094649
core unit consists not only a C3 symmetric shape (H6L) but also
potential coordination pockets, which are accessible after deproto-
nation. The construction of a variety of coordination compounds
due to a synthetic versatility in functionalization at the peripheral
aryl group is a major feature of this compounds.12 These molecules
formed porous honeycomb structures stabilized by van der Waals
interactions between interdigitated alkyl chains with adjacent
c Department of Chemistry, Graduate School of Science, Osaka University,
Toyonaka, Osaka 560-0043, Japan
† Electronic supplementary information (ESI) available: Experimental details,
STM images, details of theoretical simulations, and synthesis. See DOI:
10.1039/c4cc01576b
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Fig. 1 (a) Chemical structures of tris-(3,5-didecyl-2-hydroxybenzyl-
idene)triaminoguanidinium salts, 1a (X = Cl), 1b (X = NO3), and 1c (X = SCN)
and tris-(5-dodecyl-2-hydroxy-benzylidene)triaminoguanidinium chloride 2.
(b) Optimised geometry of cation 1+ at the B3LYP/6-31g(d) level of theory
under vacuum.
molecules. Moreover, versatility in the selection of a counter anion
was established by the formation of the identical networks irrespec-
tive of the counter anions employed.
The central core of H6L can adopt four isomeric forms (denoted
as sss-, ssa-, saa-, and aaa-isomers) in consequence of syn- or anti-
conformation of the three phenyl groups with respect to the CQN-
imine units. Intramolecular hydrogen bonding between the imine
and hydroxy groups stabilizes the syn-conformation, while inter-
molecular hydrogen bonds are formed to adjacent electronegative
Fig. 2 (a, b) An STM image and a network model of the porous structure of 1a
formed at the PO/graphite interface (concentration; 3.0 ꢀ 10ꢁ6 M, tunneling
atoms retaining the anti-conformation. Therefore, the H6L cation is parameters; Iset = 0.20 nA and Vset = ꢁ0.39 V). The yellow circle highlights the
space between the point end of the alkyl chain and the p-core. (c, d) An
expected to adopt the sss-conformation in a non-polar environ-
STM image and a network model of the non-porous structure of 1a formed
ment.13 To locate three pairs of alkyl chains at optimal positions
at the PO/graphite interface (1.0 ꢀ 10ꢁ4 M, Iset = 0.20 nA and Vset = ꢁ0.39 V).
for intermolecular van der Waals interaction, we designed tris-(3,5-
In model (d), one alkyl chain per molecule is omitted for clarity. White arrows in
didecyl-2-hydroxybenzylidene)triamino-guanidinium salts 1a–c in
which two decyl chains are attached to each aryl group at 3- and
5-positions (Fig. 1a). To confirm if 1a–c meet the structural criterion
(a) and (c) indicate the directions of main symmetry axes of underlying graphite.
(i.e. ca. 1 nm-spacing), the geometry optimisation of cation 1+ was four interdigitated alkyl chains,14,15 revealing the formation of a
performed with C3h symmetric constraint using the B3LYP/6-31g(d) honeycomb structure. The orientations of the four interdigitated alkyl
level of theory (Fig. 1b). As a result, the C1–C40 interatomic distance chains are parallel to the main symmetry axes of the underlying
was estimated to be 0.87 nm, in accordance with the optimal chain graphite surface. Unit cell parameters are a = b = 4.1 ꢂ 0.1 nm,
distances. On the basis of this consideration, cation 1+ is expected to g = 60 ꢂ 21. From these parameters and the appearance in the image,
form a porous honeycomb type molecular network. To compare the a tentative network model was constructed as shown in Fig. 2b. The
network forming properties, tris-(5-dodecyl-2-hydroxybenzylidene)- molecular core adopts the sss-conformation. Note that there is a small
triamino-guanidinium chloride 2 having only one long alkyl chain open space between the point end of alkyl chains and the central core
at each aryl group was also examined.
(yellow circle in Fig. 2b) probably due to steric repulsion between the
The condensation reaction of readily prepared 3,5-didecyl-2- methyl group and the hydroxy oxygen atom.
hydroxy-benzaldehyde and triaminoguanidinimum salts afforded
A high resolution image of the linear structure is shown in Fig. 2c.
1a–c in 41–50% yields (see ESI,† Scheme S1). The reference com- In the image, the bright bow-tie features correspond to two adjacent
pound 2 was synthesised in 50% yield. The 1H NMR spectra in non- molecular cores of 1a and the darker parts are composed of at least
polar solvents (CDCl3 or 1,1,2,2-tetrachloroethane-d2) indicate that all four interdigitated alkyl chains. Unit cell parameters are a = 2.5 ꢂ
compounds adopt C3 symmetric geometries (see ESI,† Fig. S12–S14). 0.1 nm, b = 3.9 ꢂ 0.1 nm, g = 84 ꢂ 11. A tentative network model is
On the other hand, in THF-d8 compounds 1 adopt C1 symmetric shown in Fig. 2d. The molecular core adopts the sss-conformation.
geometries with saa-conformation at the CQN-imine units (see ESI,† Though five alkyl chains including the fifth one adopting an ‘‘L’’
Fig. S15–S18). This observation supports that the C3 symmetric shape are shown in Fig. 2d to fill the open space between 1a, it is not
geometry is stabilised by intramolecular hydrogen bonds.
certain how many of them are indeed adsorbed on the surface,
Next, STM observations of monolayers formed by self-assembly of because they are not visualised. If only four alkyl chains are adsorbed
1 or 2 were performed at the liquid/solid interfaces at room tempera- on the surface, with the remaining two exposed in solution, the open
ture. By dropping a 1-phenyloctane (PO) solution of 1a at a concen- space must be filled with solvent molecules.
tration of 1.2 ꢀ 10ꢁ5 M onto a graphite surface, the formation of two
It is well known that solvent16 and solute concentration17 play
different networks, a porous honeycomb and non-porous linear significant roles in network patterns. Indeed, the molecular network
networks, was observed (see ESI,† Fig. S1). Fig. 2a displays an STM of 1a showed a distinct concentration dependency (see ESI,† Fig. S1).
image of the monolayer formed by 1a, where the bright trefoil features At a concentration of 3.0 ꢀ 10ꢁ6 M exclusive formation of the porous
correspond to the central p-core and the darker parts are composed of pattern was observed, whereas at 1.0 ꢀ 10ꢁ4 M the surface was
7684 | Chem. Commun., 2014, 50, 7683--7685
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Cadet Program’’. We thank Dr W. Bettray and C. Dittmer (IOC,
RWTH Aachen University) for MS-measurements.
Notes and references
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the counter anions were not visualised in the image, presumably 13 In crystalline state, H6L chloride and tris(5-bromo-2-hydroxybenzylidene)-
triaminoguanidinium chloride adopted saa- and aaa-conformation,
respectively, because of interactions between the chloride anion and
hydroxy groups. See ref. 12b and c.
because of their dynamic behaviour in the liquid phase or low
tunneling efficiency.19
´
In summary, we have shown that tris-(2-hydroxy-benzylidene)tri- 14 R. Lazzaroni, A. Calderone, G. Lambin, J. P. Rabe and J. L. Bredas,
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15 Orbital distributions of HOMO and HOMO ꢁ 1 of 1+ are located at
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serve as templates for complex 2D patterns formed by multiple
interactions. Additionally, since H6L salts are known to form transi-
tion metal complexes,12b,20 construction of 2D metal arrays which may
serve as tailor made catalysts would become possible.
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Externship, the Deutsche Forschungsgemeinschaft through the
International Research Training Group SeleCa (IGRK 1628), and
Programs Leading Graduate Schools: ‘‘Interactive Material Science
´
¨
20 I. M. Oppel (nee Mu¨ller) and K. Focker, Angew. Chem., Int. Ed., 2008,
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Chem. Commun., 2014, 50, 7683--7685 | 7685