2D networks of rhombic-shaped fused dehydrobenzo[12]annulenes: Structural variations under concentration control

Article abstract of DOI:10.1021/ja904481j

A series of alkyl- and alkoxy-substituted rhombic-shaped bisDBA derivatives 1a-d, 2a, and 2b were synthesized for the purpose of the formation of porous networks at the 1,2,4-trichlorobenzene (TCB)/graphite interface. Depending on the alkyl-chain length and the solute concentration, bisDBAs exhibit five network structures, three porous structures (porous A, B, and C), and two nonporous structures (nonporous D and E), which are attributed to their rhombic core shape and the position of the substituents. BisDBAs 1a and 1b with the shorter alkyl chains favorably form a porous structure, whereas bisDBAs 1c and 1d with the longer alkyl chains are prone to form nonporous structures. However, upon dilution, nonporous structures are typically transformed into porous ones, a trend that can be understood by the effect of surface coverage, molecular density, and intermolecular interactions on the system's enthalpy. Furthermore, porous structures are stabilized by the coadsorption of solvent molecules. The most intriguing porous structure, the Kagome pattern, was formed for all compounds at least to some extent, and the size of its triangular and hexagonal pores could be tuned by the alkyl-chain length. The present study proves that the concentration control is a powerful and general tool for the construction of porous networks at the liquid-solid interface.

Full text of DOI:10.1021/ja904481j

Published on Web 11/06/2009
2D Networks of Rhombic-Shaped Fused
Dehydrobenzo[12]annulenes: Structural Variations under
Concentration Control
Kazukuni Tahara,† Satoshi Okuhata,† Jinne Adisoejoso,‡ Shengbin Lei,‡
Takumi Fujita,† Steven De Feyter,‡,* and Yoshito Tobe†,*
DiVision of Frontier Materials Science, Graduate School of Engineering Science, Osaka
UniVersity, Toyonaka, Osaka 560-8531 Japan and Department of Chemistry, DiVision of
Molecular and Nanomaterials, Laboratory of Photochemistry and Spectroscopy, and Institute of
Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen (KULeuVen),
Celestijnenlaan 200 F, B-3001 LeuVen, Belgium
Received June 3, 2009; E-mail: Steven.DeFeyter@chem.kuleuven.be (S.D.F.);
tobe@chem.es.osaka-u.ac.jp (Y.T.)
Abstract: A series of alkyl- and alkoxy-substituted rhombic-shaped bisDBA derivatives 1a-d, 2a, and 2b
were synthesized for the purpose of the formation of porous networks at the 1,2,4-trichlorobenzene (TCB)/
graphite interface. Depending on the alkyl-chain length and the solute concentration, bisDBAs exhibit five
network structures, three porous structures (porous A, B, and C), and two nonporous structures (nonporous
D and E), which are attributed to their rhombic core shape and the position of the substituents. BisDBAs
1a and 1b with the shorter alkyl chains favorably form a porous structure, whereas bisDBAs 1c and 1d
with the longer alkyl chains are prone to form nonporous structures. However, upon dilution, nonporous
structures are typically transformed into porous ones, a trend that can be understood by the effect of surface
coverage, molecular density, and intermolecular interactions on the system’s enthalpy. Furthermore, porous
structures are stabilized by the coadsorption of solvent molecules. The most intriguing porous structure,
the Kagome´ pattern, was formed for all compounds at least to some extent, and the size of its triangular
and hexagonal pores could be tuned by the alkyl-chain length. The present study proves that the
concentration control is a powerful and general tool for the construction of porous networks at the liquid-solid
interface.
Introduction
design and control of the structure and functionality of molecular
networks requires insight in the correlation between molecular
Construction of 2D molecular networks on solid surfaces
based on self-assembly1 is a subject of intense interest owing
to the perspective of various applications in the field of
nanoscience and nanotechnology.2 Supramolecular self-assembly
can become an alternative for current lithographic techniques
to create surface patterns in the low nanometer regime.3 The
structural features (shape, nature, and position of interacting
sites) as well as molecular electronic properties and the topology
of surface-confined molecular architectures. This strategy is
known as crystal engineering and has rapidly developed in 2D
systems.1,4,5 Among various types of molecular networks, porous
2D molecular networks have received a lot of attention because
such surface-confined molecular networks can accommodate
* To whom correspondence should be addressed.
† Osaka University.
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‡ K.U. Leuven.
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A R T I C L E S
Tahara et al.
guest molecules or atomic clusters via so-called 2D host-guest
chemistry, resulting in the formation of multicomponent
nanostructures.6,7 These molecular networks are typically
observed by means of scanning tunneling microscopy (STM)
under ultrahigh vacuum (UHV) conditions or at the liquid-solid
interface.1
length of linear alkanes15,16 or alkyl chains of alkylated
molecules.17 More specifically, epitaxial stabilization by match-
ing of 2D lattice registry between the molecular layer and the
substrate layer is also important for the molecular arrangement.18
Obviously, solvent plays an important role too at a liquid/solid
interface. For instance, it affects the adsorption-desorption
equilibrium,19 and solvent molecules can even become part of
the molecular network (so-called coadsorption).20 In addition,
solute concentration affects network formation especially for
nanoporous systems, an aspect that has been revealed only
recently. More specifically, in the case of alkoxylated triangular
dehydrobenzo[12]annulene (DBA) derivatives, two patterns
(nonporous linear and porous honeycomb structures) appear
depending on the concentration; nonporous linear structures are
formed at high solute concentrations, whereas porous honey-
comb structures are favored upon dilution.21 The relative
abundance of both structures also depends on the alkoxy chain
length. The tendency to form porous honeycomb structures
decreases with increasing alkyl-chain length, which is supported
by semiquantitative thermodynamics modeling. A second
beautiful example of the concentration effect is the binary
mixture of trimesic acid (TMA) and 1,3,5-tris(4-carboxylphe-
nyl)benzene (BTB).22 This mixture exhibits two polymorphs
of TMA, one 2D pattern from BTB, and three patterns of
cocrystals depending on the ratios of both components and their
concentrations. A phase diagram of the six patterns was
presented based on a thermodynamic equilibrium model.
Moreover, the effect of solute concentration was also demon-
strated for the formation of a porous molecular network from
The control of molecule-molecule interactions is crucial
especially for the formation of such porous molecular networks:
typically, discrete molecular building-blocks have to be con-
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coordination,10 or simply based upon van der Waals interactions
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molecule-molecule interactions, molecule-substrate interac-
tions are also playing a key role in determining the network
topologies. For example, it is well-documented that the molec-
ular network structure can be affected by tuning molecule-
graphite van der Waals interactions as a result of changing the
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17584 J. AM. CHEM. SOC. VOL. 131, NO. 48, 2009

Rhombic-Shaped Fused Dehydrobenzo[12]annulenes
A R T I C L E S
Figure 1. (a) Schematic view of dense packings of triangles and rhombi. (b) Three different types of alkyl-chain interdigitation: [2 + 2] (triangular DBAs)
and [2 + 1] as well as a combination of [2 + 2] and [1 + 1] (rhombic bisDBAs). The yellow lines connecting the center of tiles highlight the different
packing patterns.
melamine and linear bis-uracyl as well as diimide derivatives.23
A threshold value of the hydrogen bond energy for the formation
Chart 1. Molecular Structures of Rhombic-Shaped BisDBAs 1a-d,
2a, and 2b
of the porous network was estimated. Hence, for a successful
and rational construction of 2D molecular networks, tuning the
different interaction modes discussed above as well as the solute
concentration is essential.24
Among the various molecular networks, the porous Kagome´
structure is particularly intriguing, not only because of the
combination of both regularly arranged triangular and hexagonal
pores in the network into which different guest molecule(s) or
molecular cluster(s) can be entrapped forming multicomponents
self-assembly patterns but also because of its relevance in the
field of spin-frustrated magnetic materials.25 Decyl-substituted
rhombic-shaped bis(dehydrobenzo[12]annulene) (1a, bisDBA,
Chart 1),12 tetracarboxy-substituted p-bis(phenylethynyl)-
benzene26 and azobenzene,27 and tetracyanoquinquephenyl28
have been shown to form 2D molecular Kagome´ networks. The
apparently very different molecular building blocks share one
common feature; they possess four sites (i.e., four nodes)
dominating the directional intermolecular interactions via alkyl-
chain interdigitations (van der Waals interactions) in the case
of 1a and via hydrogen bonding of four carboxy or cyano groups
for the other three examples mentioned. More recently, the
construction of Kagome´-type metal-organic frameworks involv-
ing Au atoms and (hexapyridyl)porphirin has also been dem-
onstrated.29 Additionally, a template (coronene)-induced for-
mation of a Kagome´ network has been achieved for a
tetracarboxy-substituted p-quaterphenyl at a liquid-solid inter-
face. In the absence of the template molecule, which is hosted
in the hexagonal pores of the network, another polymorph is
formed.30 Despite this remarkable progress, size and functional-
ity tuning of Kagome´ networks is not achieved yet, and the
anticipated particular host-guest chemistry of Kagome´ networks
- a network containing two types of pores - has only recently
been explored.31
The following characteristics of the rhombic-shaped bisDBAs
should be mentioned. First, there are two dense periodic
rhombus tilings in two dimensions possible (part a of Figure 1,
middle and right), one corresponding a Kagome´ type pattern
(part a of Figure 1, middle), whereas there is only one for
triangles (part a of Figure 1, left) because of the lower symmetry
of the rhombus. Note that aperiodic rhombus tilings exist, as
recently demonstrated for a hydrogen bonded supramolecular
system.32 However, in case of the hexa-substituted bisDBAs,
the molecules do not preserve the D2h symmetry of the intrinsic
rhombus, which is a requisite for the rhombus tiling shown in
part a of Figure 1. Second, there are different motifs regarding
the alkyl-chain interactions. The six peripheral substituents of
the triangular DBAs lead to a [2 + 2]-type alkyl-chain
interdigitation at all triangular sides (two alkyl chains per
triangular side). In contrast, not all sides of the rhombus can be
bridged via [2 + 2]-type interactions as there are only six alkyl
chains for four nodes. The rhombus-shaped bisDBA system is
expected to form two different motifs of interdigitating alkyl
chains: a [2 + 1] type at all sides or a combination of [2 + 2]
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A R T I C L E S
Tahara et al.
Scheme 1. Syntheses of BisDBA Derivatives 1a-d, 2a, and 2b
and [1 + 1] types (part b of Figure 1). Another characteristic
of the 2D networks of the rhombic-shaped bisDBAs is their
2D chirality (Supporting Information for detail).
Gaining insight into the parameters controlling the outcome
of supramolecular self-assembly on surfaces requires a system-
atic approach. The bisDBA system is in particular an intriguing
case to be explored because of the possible control over the
anticipated pattern variety. Therefore, the supramolecular
network formation of alkylated or alkoxylated bisDBAs 1a-d,
2a, and 2b at a liquid-solid interface has been investigated
systematically using STM, highlighting the effect and role of
the alkyl-chain length, and concentration dependency. In total,
five structures are observed, three porous (porous A, porous B,
and porous C) and two nonporous networks (nonporous D and
nonporous E). These phenomena are discussed in terms of
molecule-molecule, molecule-substrate, molecule-solvent,
and solvent-substrate interactions and lead to a practical guide
for the formation of porous (Kagome´) networks and their size
control, specifically, and tuning the architecture of supramo-
lecular networks composed from alkylated molecules in general.
Results and Discussion
1. Synthesis. The synthesis of bisDBA 1a having six decyl
chains (R ) C10H21) was reported previously.33 Following the
previous procedure, we synthesized alkylated or alkoxylated
bisDBAs 1b-d and 2a-b (R ) C12H25; 1b, C14H29; 1c, C16H33;
1d, OC10H21; 2a, OC9H19; 2b) from diethynylbenzene derivatives
and tetrabromotolanes by the in situ deprotection Pd(0) catalyzed
coupling protocol (Scheme 1). Details are described in the
Supporting Information.
2. Self-Assembly of BisDBAs at the 1,2,4-Trichlorobenzene
(TCB)/Graphite Interface. 2.1. Five Structures Formed by Bis-
DBA Derivatives. All STM observations were performed at a
1,2,4-trichlorobenzene (TCB)/graphite interface at 19-24 °C.
To allow stabilization of the system, all images were recorded
by starting 1 to 2 h after bringing a drop of the TCB solution
of bisDBAs 1a-d, 2a, and 2b on top of the graphite substrate.
The monolayer formation of bisDBAs was investigated at the
interface at various solute concentrations (from 10-4 to 10-6
M) to evaluate the effect of different types of substituents
(alkoxy or alkyl), the length of the alkyl chains, and the solute
concentration on the geometry of the 2D networks. Figure 2
summarizes the five observed structures, the tentative molecular
models constructed by molecular mechanics (MM) simulations
with the MM3 parameters, and unit cells, ranked by increasing
network density (dstructure, vide infra for the definition).
2.2. Porous Structures. First, we present the features of the
three porous structures (porous A, B, and C). The porous A
Figure 2. Overview of the five structures formed by the bisDBA
derivatives: porous A (Kagome´), porous B, porous C, nonporous D, and
nonporous E. The main symmetry directions of graphite 〈1, -2, 1, 0〉 are
indicated in pink in the STM images (central column). Tentative network
models (right column) are the result of molecular mechanics simulations
with MM3 parameters using the experimental unit cell parameters as a
periodic boundary condition (Supporting Information). In the model of the
nonporous E structure, two alkyl chains per molecule orienting to the solution
phase are omitted for clarity (Iscl ) 0.09 nA, Vbias ) -0.20 V for the porous
A, Iscl ) 0.40 nA, Vbias ) -1.30 V for the porous B, Iscl ) 0.45 nA, Vbias
)
-0.08 V for the porous C, Iscl ) 0.06 nA, Vbias ) -0.30 V for the nonporous
D, and Iscl ) 0.45 nA, Vbias ) -0.25 V for the nonporous E).
structure corresponds to a Kagome´ pattern (Figure 2, top row).
As shown in the tentative model, all of the alkyl chains are
adsorbed on the surface (the number of adsorbed alkyl chains:
m ) 6), aligning parallel to one of graphite’s symmetry axes
(the 〈1, -2, 1, 0〉 directions). Each π-core is bridged via a [2 +
1]-type alkyl-chain interdigitation. The bisDBA cores adopt three
different orientations. The Kagome´ structure contains two
different types of pores, triangular and hexagonal ones.
In the porous B structure (Figure 2, second row), extended
alkyl chains bridge the gap between adjacent DBA cores; this
(33) Sonoda, M.; Sakai, Y.; Yoshimura, T.; Tobe, Y.; Kamada, K. Chem.
Lett. 2004, 972–973.
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Rhombic-Shaped Fused Dehydrobenzo[12]annulenes
A R T I C L E S
leads to all [2 + 1]-type alkyl-chain interdigitation interactions
between adjacent molecules. All alkyl chains align along two
of the main symmetry axes of graphite (m ) 6). The porous B
structure has a rectangular pore.34 In addition, whereas all
molecules of the porous A structure are adsorbed with the same
enantiotopic face within a given domain (conglomerate forma-
tion),35 this is not the case for the porous B pattern (racemate
formation, Supporting Information).
The porous C structure resembles the porous B structure as
far as the number of adsorbed alkyl chains (m ) 6) and the
formation of rectangular pores are concerned (Figure 2, third
row).34 However, both structures can be clearly distinguished
by comparison of the alignment of the alkyl chains in both
patterns. For both patterns, all adsorbed alkyl chains run parallel
to one of the main symmetry axes of graphite. However, the
porous C pattern is characterized by twice [2 + 2] and twice [1
+ 1] alkyl-chain interdigitation interactions,36 whereas for the
porous B-type alignment, alkyl-chain interdigitation interactions
between adjacent molecules are all [2 + 1] type.
2.3. Nonporous Structures. Next, we discuss the structural
aspects of the two densely packed nonporous structures,
nonporous D and E. Similar to the porous B and C structures,
the π-conjugated cores are oriented identically throughout a
domain. In the nonporous D structure (Figure 2, fourth row),
all alkyl chains are adsorbed (m ) 6) along one of the graphite
symmetric axes. Two of them most likely have gauche kinks
to achieve favorable intermolecular van der Waals interactions.
On the other hand, in the nonporous E structure four alkyl
chains per molecule are adsorbed (m ) 4) with their axis parallel
to a main symmetry axis of the graphite surface (Figure 2,
bottom row). The other two alkyl chains are most likely to be
exposed to the solution phase; each molecule is connected via
two [2 + 2]-type alkyl-chain interdigitation interactions.
2.4. Concentration and Substituent Effects. The network
coverage (θnetwork) of each structure was calculated by analyzing
more than 10 large-area images (ca. 100 nm × 100 nm) for
each compound at different concentrations. The concentration-
dependent evolution of the network coverage of bisDBAs 1b,
1c, and 1d is summarized in Figure 3 and in Tables S1-S3 of
the Supporting Information. Details of the unit cell parameters
for all observed structures of all compounds are provided in
Table 1. The network density (dstructure) was estimated by dividing
the unit cell area (area) by the number of molecules in the unit
cell (Z). For a molecule with a given alkyl-chain length, the
order of the network density (dstructure) of each structure is as
follows: porous A e porous B < porous C < nonporous D e
nonporous E (Figure 4).
Figure 3. Concentration-dependent surface coverage of the five structures,
porous A (θA, red), porous B (θB, orange), porous C (θC, green), nonporous
D (θD, blue), and nonporous E (θE, black), of bisDBAs 1b (a), 1c (b), and
1d (c) at the TCB-graphite interface.
solute concentration, the porous C structure is gradually replaced
by the porous A (Kagome´) structure (Figure S6 of the Sup-
porting Information). The concentration-dependent change of
the surface coverage of each pattern is summarized in part a of
Figure 3. The Kagome´ structure appears at a solute concentration
of 4.0 × 10-6 M, and its surface coverage (θA) reaches 30% at
the lowest concentration probed (1.5 × 10-6 M).
BisDBA 1a exclusively forms the porous A (Kagome´)
structure for a wide concentration range (from 10-4 to 10-6 M).12
In contrast, the bisDBA analogues with longer alkyl chains form
a variety of concentration-dependent structural variations.
With two extra methylene groups in the alkyl chain, bisDBA
1b forms the porous C structure (the third row in Figure 2) at
a solute concentration of 7.0 × 10-4 M. Upon decreasing the
The concentration-dependent surface coverage of the poly-
morphs of 1c is summarized in part b of Figure 3. A remarkable
change appears for this molecule. The relative surface coverage
of the two nonporous linear structures (nonporous D and E, the
fourth and bottom rows in Figure 2, respectively) does not
change at concentrations higher than 3.3 × 10-5 M. Upon
decreasing the solute concentration, the porous B structure
(Figure S7 of the Supporting Information) rapidly becomes the
dominant one (θB ) 81% at 1.0 × 10-5 M) with decreasing
coverage of the nonporous patterns. The porous C structure only
appears at domain boundaries (θC < 1%, Figure S8 of the
Supporting Information). The porous A structure appears upon
decreasing the concentration: its coverage reaches a maximum
(θA ) 7%) at 6.1 × 10-6 M (the top row in Figure 2). However,
upon further decreasing the concentration down to 2.0 × 10-6
M the Kagome´ structure disappears.
(34) Recently, Matzger et al. reported the formation of 2D rhomnic pores
by alkoxyformamide in which alkyl groups are interdigitated in a [2
+ 1] form. Ahn, S.; Morrison, C. N.; Matzger, A. J. J. Am. Chem.
Soc. 2009, 131, 7946–7947.
(35) Eliel, E. L.; Wilen, S. H. Stereochmistry of Organic Compounds; Jon
Wiley & Sons: New York, 1998; p 159.
(36) The porous B structure was observed for bisDBA 1a as the kinetic
phase within 10 min after sample preparation. See also ref 12b in
which it is referred to as the linear A structure. For bisDBA 1b, it
appears as the stable phase under our experimental conditions.
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A R T I C L E S
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Table 1. Structural Parameters of Monolayer Structure of BisDBAs 1a-d, 2a, and 2b
Unit Cell Parametersa
d,e
D (nm-2
)
compound
structure
plane group
a (nm)
b (nm)
γ (°)
Zb area (nm2)c
pore area (nm2)f azimuthal angles (°)g mh
1a
1b
porous A (Kagome´)
porous A (Kagome´)
porous C
porous A (Kagome´)
porous B
nonporous D
nonporous E
porous A (Kagome´)
porous B
nonporous D
nonporous E
porous C
P6
P6
P1
P6
P2gg
P1
P1
4.9 ( 0.1 4.9 ( 0.1 60 ( 1
5.4 ( 0.1 5.4 ( 0.1 61 ( 2
2.7 ( 0.1 2.8 ( 0.2 78 ( 2
5.9 ( 0.1 5.9 ( 0.1 59 ( 1
4.4 ( 0.1 4.4 ( 0.1 89 ( 1
3.0 ( 0.1 3.1 ( 0.1 48 ( 1
1.8 ( 0.1 3.0 ( 0.1 70 ( 2
6.4 ( 0.3 6.4 ( 0.3 60 ( 1
4.7 ( 0.1 4.8 ( 0.1 88 ( 1
3.0 ( 0.1 3.2 ( 0.1 51 ( 1
3
3
1
3
2
1
1
1
2
1
-
21.2
25.5
7.4
29.8
19.4
6.9
0.141 (11.9)
0.118 (11.3)
0.135 (13.0)
0.101 (10.9)
0.103 (11.2)
0.145 (15.6)
0.197 (15.8)
0.085 (10.2)
0.089 (10.6)
0.134 (16.1)
--
8.3 (0.39)
10.5 (0.41)
2.4 (0.32)
12.8 (0.43)
8.0 (0.41)
--
15 ( 1
6
6
6
6
6
6
4
6
6
6
--
6
6
14 ( 1
7 ( 1, 21 ( 3
12 ( 1
0 ( 1, 29 ( 1
7 ( 3, 19 ( 3
24 ( 1, 19 ( 2
13 ( 2
0 ( 1, 30 ( 1
8 ( 1, 21 ( 2
--
1c
5.1
--
1d
P6
35.5
22.6
7.5
--
6.6
16.7 (0.47)
10.0 (0.44)
--
P2gg
P1
--i
--
--
--
--
2a
2b
P1
P1
2.5 ( 0.1 2.7 ( 0.1 77 ( 2
2.4 ( 0.1 2.5 ( 0.1 78 ( 2
1
1
0.152 (13.7)
0.170 (14.3)
2.0 (0.30)
1.6 (0.28)
3 ( 2, 15 ( 4
6 ( 1, 26 ( 1
porous C
5.8
d
a Unit cells are shown in the respective STM images. b Number of molecules per unit cell. c Unit cell area. Network density (molecule/nm2).
e (number of adsorbed carbon atoms)/(area per unit cell) is given in parentheses. f The fraction of pore area (F) is given in parentheses. g Experimentally
determined azimutal angles between the underling graphite unit cell vector and unit cell vector a (left) or b (right) of the molecular network. h Number
of the adsorbed alkyl chains per bisDBA molecule. i Because of the lack of long-range order (Figure S10 of the Supporting Information), the unit cell
parameters were not determined.
Contrary to the alkyl-substituted bisDBAs 1a-d, bisDBAs
2a and 2b with alkoxy substituents only form the porous C
structure at the wide concentration range probed (from 10-4 to
10-6 M). Details are described in Figures S12 and S13 of the
Supporting Information. This is probably because of the different
orientation of the alkoxy chains originating from the ether
functionality.
Figure 4. Summary of the observed molecular networks of bisDBAs and
their relative molecular density.
Two general trends are elucidated from the above results: 1)
at the level of the solute concentration: the lower the concentra-
tion, the larger the chance to find a low density pattern, and 2)
at the level of the alkyl-chain length: the shorter the alkyl chains
of the bisDBA derivatives, the stronger the tendency to form
porous networks.40
These results are in line with the experimental trends observed
for a related alkylated, triangular-shaped DBA system, which
gives rise to only two patterns, a porous (honeycomb) and a
nonporous (linear) one.21,41 A similar concentration-monolayer
density dependency has also been experimentally verified and
theoretically confirmed for an alkyl-chain free bicomponent
In case of bisDBA 1d, again, the relative surface coverage
of the two nonporous structures (nonporous D and E, Figures
S9 and S10 of the Supporting Information) does not change
anymore at concentrations higher than 1.0 × 10-5 M (part c of
Figure 3). Upon decreasing the solute concentration (< 4.0 ×
10-6 M), the porous B structure (Figure 2, the second row)
becomes the predominant structure (θB ) 86% at 1.5 × 10-6
M), a process that goes along with a drastic decrease of the
surface coverage of the nonporous D structure. At concentrations
below 2 × 10-6 M, the coverage of the Kagome´ structure
gradually increases reaching a maximum at the lowest concen-
tration probed (θA ) 19%, 1.3 × 10-6 M, Figure S11 of the
Supporting Information). Here we observe a remarkable transi-
tion of the chirality characteristics of the physisorbed patterns
as a function of surface coverage. For the high-density
nonporous structures, all molecules within a given 2D domain
have the same enantiotopic face directed to the surface and a
homochiral lattice is formed. Obviously, other domains show
the mirror-image-type pattern leading to a conglomerate. In
contrast, in the lower-density porous B phase, a domain exists
of a heterochiral lattice of molecules leading to a racemic
compound.37 The lowest-density pattern, the porous A or
Kagome´ pattern, forms again a homochiral phase at the level
of a domain though. These observations38 are intriguing in
relation to the Wallach’s rule39 established in the 3D system,
which states that a racemic lattice packs more densely than a
homochiral lattice.
(38) There are a number of reports on the formation of homochiral lattice
on the surface from chiral as well as prochiral molecules. See, for
example: (a) Stevens, F.; Dyer, D. J.; Walba, D. M. Angew. Chem.,
Int. Ed. 1996, 35, 900–901. (b) Corte´s, R.; Mascaraque, A.; Schmidt-
Weber, P.; Dil, H.; Kampen, T. U.; Horn, K. Nano Lett. 2008, 8, 4162–
4167. (c) Vidal, F.; Delvigne, E.; Stepanow, S.; Lin, N.; Barth, J. V.;
Kern, K. J. Am. Chem. Soc. 2005, 127, 10101–10106. (d) Rankin,
R. B.; Sholl, D. S. J. Phys. Chem. B 2005, 109, 16764–16773. (e)
Bo¨hringer, M.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed.
2000, 39, 792–795. (f) Huang, T.; Hu, Z.; Zhao, A.; Wang, H.; Wang,
B.; Yang, J.; Hou, J. G. J. Am. Chem. Soc. 2007, 129, 3857–3862. (g)
Fasel, R.; Parschau, M.; Ernst, K.-H. Nature 2006, 439, 449–452.
(39) Wallach, O. Liebigs Ann. Chem. 1895, 286, 90–143.
(40) Similar alkyl-chain length dependency was observed in the formation
of porous versus nonporous 2D molecular networks of alkoxyforma-
mides: See, ref 34.
(41) In our attempt to apply a thermodynamic model to evaluate the
concentration-dependent polymorph appearance of the bisDBAs;
however, we were confronted with difficulties: for example, in the
case of 1b, the high sensitivity of the network coverage to the solute
concentration (i.e. rapid morphology change within a narrow concen-
tration range) together with the large errors in the surface coverage
hamper application of the quantitative treatment (Supporting Informa-
tion). Furthermore, in the cases of 1c and 1d, the situation is more
complicated because of the coexistence of more than two polymorphs.
(42) The details of the molecular mechanics simulations (MM3) are
described in the Supporting Information.
(37) There are few reports on the network consisting of racemates from
both chiral (racemic) and prochiral molecules. (a) De Feyter, S.;
Gesquie`re, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver,
F. C. Angew. Chem., Int. Ed. 2001, 40, 3217–3220. (b) Hibino, M.;
Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544–
4547. (c) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am.
Chem. Soc. 2004, 126, 5318–5322.
(43) EpiCalc was downloaded on the Ward’s group web site (http://
www.nyu.edu/fas/dept/chemistry/wardgroup/Software.html).
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Rhombic-Shaped Fused Dehydrobenzo[12]annulenes
A R T I C L E S
Table 2. Representative Parameters for Geometric Epitaxy
Calculated by the EpiCalc Programa
coincidence exists in each molecular network, indicating that
epitaxial stabilization plays a role in the monolayer formation.
However, without thermodynamic information for the epitaxial
effect, it is difficult to correlate it with the observed structural
variations. At bisDBA concentrations exceeding those required
for obtaining monolayer coverage, the close-packed nonporous
networks become more favorable because the total enthalpy is
decreased owing to the larger number of adsorbed molecules.
The concentration dependency of the self-assembly of bisDBAs
1a-d can therefore be understood as to arise from a balance
between the different stabilities of the structures normalized to
the single molecule level, including the stabilizing effect of
solvent coadsorption, and the molecular density of the patterns.
Calculated Unit Cell Parameters
compound
structure
b1 (nm) b2 (nm) ꢀ (°)
θcalcd
V/V0b
1a
1b
porous A (Kagome´) 4.860 4.865 59.2 15.25 0.50
porous A (Kagome´) 5.400 5.460 61.1 14.75 0.51
porous C
2.645 2.820 77.1 23.75 0.50
1c
porous A (Kagome´) 5.905 5.855 58.6 12.50 0.51
porous B
nonporous D
nonporous E
4.500 4.475 88.7
2.970 3.050 48.7
0.00 0.50
8.25 0.50
1.825 2.985 71.8 20.50 0.50
1d
porous A (Kagome´) 6.435 6.365 60.3 13.25 0.50
porous B
4.710 4.795 88.5
2.970 3.190 52.0
0.75 0.54
8.25 0.50
nonporous D
a EpiCalc simulations analyze the lattice registry by rotating an
overlayer lattice (b1, b2, and ꢀ) on a substrate lattice (a1, a2, and R)
through a series of azimuthal angles (θ). In present study, following
values are employed as the unit cell parameters of graphite substrate (a1
) a2 ) 0.246 nm and R ) 60°). See the Supporting Information for
details. b The V/V0 value is a dimensionless potential for each azimuthal
angle, which indicates the degree of commensurism between an
2.5. Selection of Porous Networks. Another issue of interest
is the selection of the porous networks. The differences in on
the one hand the energies of intermolecular and molecule-
substrate interactions and on the other hand the densities of the
three porous networks A-C are small. On the basis of these
grounds, it is difficult to understand the concentration-dependent
appearance of the different porous patterns. As stated above,
one can assume, however, that coadsorption of solvent molecules
plays a crucial role. Indeed, sometimes bright features were
observed in the hexagonal pore of the porous A structures and
the rectangular pore of the porous B and C structures (e.g., the
porous A structure in Figure S11 of the Supporting Information
and the porous B structure in Figure 2).44 Therefore, we modeled
bisDBA network structures coadsorbed with TCB clusters via
molecular mechanics simulations. In the simulation, TCB
molecules are assumed to be connected via weak CH-Cl
hydrogen bonds.45,46 As can be seen in Figure 5, the hexagonal
pores of the Kagome´ structure of bisDBAs 1a and 1d as well
as the rectangular pore of the porous B structure of bisDBA 1d
are occupied by TCB clusters. The same number of bright
features as those experimentally observed fits nicely to the
corresponding pores in the simulation, indicating that the
observed bright spots can be ascribed to the specific number of
coadsorbed TCB molecules. We believe that those images are
actually snapshots revealing temporary ordered solvent clusters,
which in general are dynamic in nature. Consequently, the
selection of the porous networks is attributed, at least partially,
to the coadsorption of solvent molecules that occupy the pores
as clusters.47 Interestingly, in other solvents than TCB (e.g.,
1-phenyloctane or 1-octanoic acid), no Kagome´ patterns are
overlayer and a substrate layer: V/V0 ) 1 for incommensurism, V/V0
)
0.5 for point-online coincidence, and V/V0 ) 0 for commensurism on a
nonhexagonal substrate.
system.22 Note that for bisDBAs at the lowest concentration
probed, 2.0 × 10-6 M, the number of molecules within a droplet
(8 µL, 0.160 pmol) is not enough to cover a 1 cm2 piece of
graphite by (a combination of) porous motifs and obviously also
not by nonporous motifs. At these submonolayer conditions,
the porous motif is preferred. We have addressed the total
molecular interaction energy (sum of intermolecular and
molecule-substrate interactions at the level of a single molecule)
via MM3 simulations (Table S4 of the Supporting Informa-
tion).42 In general, only a slight stabilization for molecules in a
porous network is predicted at the individual molecule level,
and it is questionable if the calculated differences between the
different patterns have any predictive value also because
solvation and solvent coadsorption effects are not taken into
account. Especially, solvent coadsorption should play a role in
the stabilization of the porous structures (vide infra). Further-
more, to analyze possible epitaxy between the overlayer and
substrate, a lattice registry analysis was performed using
EpiSearch subroutine in the EpiCalc program.18,43 As a result,
for all observed structures a number of fits classified as the point-
on-line (POL) coincidence criteria were found within experi-
mental error under certain search conditions (Table 2, also
Supporting Information).18 This implies that at least one POL
Figure 5. Models obtained via molecular mechanics simulations of the porous networks with coadsorbed TCB molecules for the Kagome´ structure of 1a
(a), the Kagome´ structure of 1d (b), and the porous B structure of 1d (c).
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A R T I C L E S
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formed, regardless of the solute concentration, stressing the
active participation of TCB in the monolayer formation.
The main message of the present study is that concentration
control is a powerful and general approach for structural
selection, also in those cases where multiple pattterns coexist.
Even more, variation in the alkyl-chain length is an extra option
for structural selection. In addition, these prochiral molecules
form both racemic compound and conglomerate phases, though
no clear polymorph density dependency was observed, an
intriguing issue in relation to the Wallach’s rule in 3D crystals.
The knowledge of how to select and favor the formation of
specific patterns and in particular the porous templates will lead
to the development of novel 2D host-guest chemistry, such as
site-selective adsorption of two different components in size-
controlled Kagome´ lattices31 or guest-induced switching between
various porous structures.
Conclusions
We have accomplished STM measurements of a series of
alkyl- and alkoxy-substituted rhombic-shaped bisDBA deriva-
tives at the TCB/graphite interface. By changing the alkyl-chain
length (i.e., tuning molecule-substrate and molecule-molecule
interactions) and controlling the solute concentrations, bisDBAs
were observed to form five structures, three porous structures
(porous A, B, and C), and two nonporous structures (nonporous
D and E). BisDBAs 1a and 1b with the shorter alkyl chains
favorably form porous patterns, whereas bisDBAs 1c and 1d
with the longer alkyl chains are prone to forming nonporous
structures. However, upon diluting, the nonporous structures
were observed to transform into porous structures. Under dilute
conditions, partial formation of the Kagome´ motif was observed
for all compounds.
Acknowledgment. This work is supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, a Bilateral Program of
Japan Society for the Promotion of Science, the Fund of Scientific
Research - Flanders (FWO), K.U.Leuven (GOA), and the Belgian
Federal Science Policy Office through IAP-6/27. J.A. thanks the
Institute for the Promotion of Innovation by Science and Technol-
ogy in Flanders (IWT).
(44) A similar observation has been reported: See, ref 19f.
(45) (a) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063–5070.
(b) Aakero¨y, C. B.; Evans, T. A.; Seddon, K. R.; Pa´linko´, I. New.
J. Chem. 1999, 145–152. (c) Abel, M.; Oison, V.; Koudia, M.; Maurel,
C.; Katan, C.; Porte, L. ChemPhysChem 2006, 7, 82–85. (d) Oison,
V.; Koudia, M.; Abel, M.; Porte, L. Phys. ReV. B 2007, 75, 03428.
(46) It has been well-known that chorine exhibits weak Cl-H hydrogen
bonding (the distance ranges from 0.26 to 0.29 nm). Thus, we
performed MP2/6-31g* optimization of a hexamer of TCB as the
simplest case under vacuum. By comparison with six free TCB
molecules, the sum of intermolecular interactions was estimated to
be -2.48 kcal/mol. This indicates that by weak hydrogen bonding
interactions TCB clusters are stabilized.
(47) Control experiments for bisDBA 1d at the 1-phenyloctane/graphite
interface at dilute conditions (1.5 × 10-6 M) or at dry conditions by
the evaporation of TCB (1.3 × 10-6 M) resulted in the formation of
the porous B and nonporous E structures or nonporous D structure,
indicating the role of TCB coadsorption (also Figure S17 of the
Supporting Information).
Note Added after ASAP Publication. Two values in Table 2
and the citation in the last sentence of the Conclusions were
incorrect in the version published ASAP November 6, 2009. The
corrected version was published November 12, 2009.
Supporting Information Available: Synthetic procedures of
1a-d, 2a, and 2b, experimental details (both STM measure-
ments and molecular modeling), additional STM images,
optimized structure of 1a-d, 2a, and 2b. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA904481J
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17590 J. AM. CHEM. SOC. VOL. 131, NO. 48, 2009