Tailoring surface-confined nanopores with photoresponsive groups

Article abstract of DOI:10.1002/anie.201303745

Opening light: Two-dimensional pores are formed by the self-assembly of azobenzene-functionalized triangular building blocks on graphite at the liquid-solid interface. These pores can selectively host a guest molecule. The pore size can be reversibly changed by irradiation at different wavelength which changes the number of guest molecules that are adsorbed (see scheme). Copyright

Full text of DOI:10.1002/anie.201303745

Angewandte
Chemie
DOI: 10.1002/anie.201303745
Tailored 2D Pores
Tailoring Surface-Confined Nanopores with Photoresponsive
Groups**
Kazukuni Tahara,* Koji Inukai, Jinne Adisoejoso, Hiroyuki Yamaga, Tatyana Balandina,
Matthew O. Blunt, Steven De Feyter,* and Yoshito Tobe*
Recently, the construction of two-dimensional (2D) porous
patterns on solid surfaces using molecular self-assembly has
become a subject of interest because of potential applications
in nanoscience and nanotechnology, for nanoreactors and
catalysts that may function cooperatively with substrate
surfaces and for molecular wires and 2D polymers generated
by surface-controlled reactions.[1] Surface-confined pores
within the 2D porous molecular networks can be used as
a host space to immobilize guest molecules at the surface.[2]
These molecular networks are typically observed by means of
scanning tunneling microscopy (STM) under ultrahigh
vacuum (UHV) conditions or at the liquid–solid interface.
One of the significant challenges in the design of 2D
porous system is physical (i.e. size and shape) or chemical (i.e.
electrostatic properties) modification of the interior space of
porous networks to construct tailored functional pores. There
are only a few studies which have investigated the influence
that chemical modification of the pore structure has on guest
co-adsorption.[3] However, none of them achieved specific
recognition of guest molecule(s) by shape and size comple-
mentarities between the guest and modified pore. Herein, we
report the construction of tailored 2D pores, which exhibit
a tight stoichiometric binding selectivity toward a guest
molecule. These pores are formed by self-assembly at the
liquid–solid interface of designer molecular building blocks
bearing photo-responsive groups. Moreover, the size of the
pores is reversibly altered by irradiation with light of different
wavelengths as demonstrated by a change in the number of
co-adsorbed guest molecules. This is, to our knowledge, the
first example of the construction of 2D pores which respond
to external stimuli in a specific manner.[4]
Among the various molecular building blocks, alkoxy-
lated dehydrobenzo[12]annulene (DBA) derivatives are
chosen because of their strong tendency to form porous
honeycomb patterns by the interdigitation of alkyl chains at
the liquid–solid interface,[5] tunability of pore size by varying
alkyl chain length, and their synthetic versatility in chemical
modification of the alkyl chains.[6] The design to tailor pore
environments is based on the introduction of functional
groups at the end of three of the DBAs six alkyl chains, in an
alternating fashion (Figure S1 in the Supporting Informa-
tion). Molecular modeling suggests that such DBAs form
a honeycomb structure in which the functional groups are
located inside the pores. By selection of functional groups we
can modify the physical and chemical environments of the
pores. In this case, photoresponsive azobenzene is chosen as
the functional group. In addition, two carboxy groups are
introduced to the azobenzene units to achieve high guest
selectivity by creating a confined space within a hydrogen-
bonded hexamer of dicarboxyazobenzene units: a cyclic
hexamer of isophthalic acid can immobilize one coronene
(COR) molecule on surfaces by size and shape recognition
(Figure 1a,b).[6] To locate the cyclic hexamer of the dicarbox-
yazobenzene units in the pore, the azobenzene units are
connected by meta-phenylene linkers at the end of shorter C12
chains. Moreover, taking into account the established photo-
isomerization of azobenzene derivatives at surfaces[7] and the
structural difference between a planar trans-configuration
and kinked 3D cis-configuration, the azobenzene units can
change the pore size and shape upon photoisomerization
(Figure 1c). This change in pore geometry also leads to
a change in the number of adsorbed COR guest molecules.
The synthesis of azobenzene-functionalized DBAs 1 and 2
is described in Supporting information (Scheme S1 and S2).
DBAs 1 and 2 reveal very similar spectroscopic properties in
solution. A 1-octanoic acid solution of all-trans 1 has an
absorption band at 315 nm arising from p–p* absorptions of
the DBA core and the azobenzene units and a weak band
(435 nm) corresponding to an n–p* transition of the azoben-
zene chromophore (Figure S2).[8] Upon irradiation with UV
light (313 nm) of a solution of all-trans 1 in [D8]THF,
a photostationary state containing 57% of the cis-azobenzene
unit was achieved within a few minutes as indicated by
1H NMR spectroscopy. If we assume all the azobenzene units
in 1 exhibit identical photoisomerization efficiency, the
distribution of 1 with one, two, and three cis-azobenzene
unit(s) becomes 31.6%, 41.9%, and 18.5%, respectively, and
[*] Dr. K. Tahara, K. Inukai, H. Yamaga, Prof. Dr. Y. Tobe
Division of Frontier Materials Science
Graduate School of Engineering Science, Osaka University
1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
E-mail: tahara@chem.es.osaka-u.ac.jp
tobe@chem.es.osaka-u.ac.jp
Dr. J. Adisoejoso, Dr. T. Balandina, Dr. M. O. Blunt, Prof. S. De Feyter
Department of Chemistry
Division of Molecular Imaging and Photonics, Laboratory of
Photochemistry and Spectroscopy, KU Leuven
Celestijnenlaan 200 F, 3001 Leuven (Belgium)
E-mail: Steven.DeFeyter@chem.kuleuven.be
[**] This work is supported by Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science, and Technology
(Japan), the Fund of Scientific Research—Flanders (FWO),
K.U.Leuven (GOA), the Belgian Federal Science Policy Office
through IAP, NMP4-SL-2008-214340 and JSPS and FWO under the
Japan–Belgium Research Cooperative Program. J.A. is grateful to the
Agency for Innovation by Science and Technology in Flanders (IWT).
We thank Prof. F. C. De Schryver for fruitful discussion.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303745.
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and COR (5.5 ꢀ 10À4 m) in 1:1 volumetric ratio. As a result,
we observed one COR molecule immobilized in most of
the pores formed by trans-1 (Figure 2b and S6). Statistical
analysis revealed that 96.5% of the pores were filled with
one COR molecule, whereas only 3.0 and 0.5% of the
pores accommodated two and three COR molecules,
respectively (Figure 3, trans-1 premixed with COR).
Moreover, by using a premixed sample of trans-1 (1.9 ꢀ
10À5 m), hexakis(phenylethynyl)benzene (Figure 1a, 3.7 ꢀ
10À6 m) and COR (5.0 ꢀ 10À6 m) in 1:1:1 volumetric ratio,
only COR was immobilized within the pores even in the
presence of hexakis(phenylethynyl)benzene which has
larger affinity to graphite (Figure S7). In addition, note
that the pore honeycomb network formed by DBA 3
bearing only C14 alkyl chains (without azobenzene groups)
host an undefined number of COR molecules in the
nanopore (Figure S8).[10] In the latter case, the pores are
imaged as fuzzy bright features. These results demonstrate
that the nanopores can be tailored by the functional
groups attached to the terminal of the alkyl chains of
DBA and can exhibit high selectivity to host a specific
guest molecule.
The effect of the photoisomerization of the azoben-
zene units was studied. A solution of PS-1 containing 57%
of cis-azobenzene moieties (2.0 ꢀ 10À6 m) formed mostly
disordered structures though with some small honeycomb
domains (Figure S9). In the honeycomb domains, six
azobenzene units were clearly observed in the pores,
indicating that the honeycomb structure must consist of
solely trans-azobenzene units. This implies that larger
adsorption affinity of the trans-azobenzene unit[11] and the
hydrogen bonding between the dicarboxy groups in the
trans-azobenzene units stabilizes the honeycomb struc-
ture of 1. However, because PS-1 containing cis-azoben-
zene units did not form large domains of the honeycomb
structure, 2 was also investigated to gather statistically
relevant data. Contrary to PS-1, the formation of honeycomb
domains of comparable size to those of trans-2 was also
confirmed for the sample in the PS state of 2 (Figure S10 and
S11). However, the number of observed azobenzene units in
the monolayer formed by PS-2 decreased from 91.9% for the
non-irradiated solution to 68.9%. We attribute these unob-
served azobenzene units mainly to the cis-isomer.
The guest-binding capability was explored by in situ
irradiation of the already formed honeycomb structure of
trans-1 at the 1-octanoic acid/graphite interface. To avoid UV
light absorption by the COR molecules we chose a sequential
addition method: a saturated solution of COR in 1-octanoic
acid (5.5 ꢀ 10À4 m) was drop cast after irradiation. First, the
difference resulting from the preparation method was inves-
tigated. The proportion of pores with different numbers of
COR molecules before light irradiation was as follows: one
COR 86.2%, two CORs 4.0%, three CORs 0.5%, and fuzzy
feature 9.2% (Figure 3, sequential addition of COR). The
proportion of the pores displaying fuzzy features increases
compared with that formed from the premixed sample. At the
liquid–solid interface different preparation methods can
sometimes produce different outcomes because kinetic fac-
tors can influence the self-assembly process.[12] By irradiation
Figure 1. a) Structural formula of DBAs, trans-1, trans-2 and 3, coronene
(COR), and hexakis(phenylethynyl)benzene. b) Molecular models of a honey-
comb structure formed by self-assembly of trans-1 with all-trans-azobenzene
units (trans-1) in which a hydrogen-bonded hexamer of dicarboxyazoben-
zene units creates a size and shape confined space. c) The same pore
as (b) with one of the dicarboxyazobenzene units adopting a cis configura-
tion. Note that there is no space to accommodate more than one COR,
except if the COR molecules are adsorbed on top of the first layer or if the
not all azobenzene units are adsorbed.
the remaining 8.0% is all-trans 1 (Figure S3). We abbreviate
such a mixture at the photostationary state of 1 as PS-
1 hereafter. Upon irradiation of visible light (> 400 nm) to the
solution of PS-1 or heating it at 558C, the cis-azobenzene
units were transformed back to all-trans 1. PS-1 was stable
thermally and its half-life time at room temperature was two
days. Virtually identical results were obtained with all-trans 2
in solution leading to PS-2 (Figure S4).
To form a nanostructured 2D layer, a 1-octanoic acid
solution of trans-1 (1.0 ꢀ 10À5 m) was drop cast on graphite and
the resulting self-assembled monolayer was observed by
means of STM. Bright triangular features correspond to the
DBA p-cores as a result of their higher tunneling efficiency.
The large apparent height of these features relates to the
higher tunneling efficiency of the aromatic part of the
molecule.[9] The interdigitated alkyl chains were also imaged
clearly (Figure 2a and S5), revealing the formation of
a honeycomb network of all-trans 1. Moreover, nearly all
the azobenzene units directed towards the center of the
nanowell were visualized, suggesting the formation of a cyclic
hexamer of the dicarboxyazobenzene units in the honeycomb
pore by hydrogen-bonding interactions.
Prior to any irradiation, the guest-binding capability of the
hexagonal pores formed by trans-1 toward COR was inves-
tigated by using a premixed sample of trans-1 (3.0 ꢀ 10À6 m)
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desorption of one azobenzene unit generates enough space
to accommodate an additional COR molecule (Figure S13).
These results indicate that isomerization of one or two trans-
azobenzene unit(s) took place to leave free space in the pore,
affecting the distribution in the number of guest molecules
within the pores. Similarly, the number of COR molecules co-
adsorbed in the pores of PS-2 was larger than that for trans-2
(Figure S14 and S15), though the stoichiometric selectivity
was not as strict as that observed for 1 owing to the absence of
a hydrogen-bonding network in the pores formed by 2.
To demonstrate the reversibility of this process the
enlarged pore size was shrunk to the initial size by cis-to-
trans isomerization of the azobenzene units. The network
formed from all-trans-1 was irradiated with 320 nm light and
a solution containing COR was added as above. This mixture
was then irradiated with long-wavelength light (l > 400 nm)
for 10 min. Note, in this case, COR would not interfere with
the absorption of the visible light by the cis-azobenzene unit
of 1. The distribution of the number of COR molecules
changed as follows: one COR, 92.3%; two CORs, 2.8%; fuzzy
features, 4.8% (Figure 3 second irradiation (> 400 nm)). No
pores containing three or four CORs were detected. The
recovery of the stoichiometric selectivity is attributed to the
reduction of the pore size by cis-to-trans isomerization of the
azobenzene units. This process causes excess COR molecules
to be desorbed from the pore. Co-adsorption of the guest was
thus controlled reversibly by light irradiation.
The question remains where the in situ photo-induced
reaction takes place, on the surface or in solution? It is
established that the presence of surfaces might quench the
photo-isomerization reaction.[7a–e,13] Therefore, we performed
an experiment in which half of the sample area was protected
from the light irradiation, by placing an aluminum foil a few
mm above the graphite substrate. Statistical analysis of three
different experiments showed the distribution in the number
of adsorbed COR molecules in the masked area was virtually
identical to that after sequential addition of COR, except for
the distribution of fuzzy pores. On the other hand, the
distribution in the unmasked area was comparable with that
obtained in the other illumination experiments (Figure S16
and Table S1). These results render the rapid diffusion and
adsorption–desorption dynamics of DBA 1 at the interface
unlikely, and support the view that photoisomerization of the
azobenzene units of 1 occurs while they are adsorbed on the
graphite surface.
Figure 2. a) A honeycomb structure formed by trans-1 at the 1-octanoic
acid/graphite interface (concentration; 1.0ꢀ10À5 m, tunneling condi-
tions; Iset =0.26 nA, Vset =À0.20 V). Statistical analysis shows 97% of
azobenzene units were visualized. b) A self-assembled monolayer
formed from a mixture of trans-1 and COR at the 1-octanoic acid/
graphite interface (concentrations of original solutions; 3.0ꢀ10À6 m for
1 and 5.5ꢀ10À4 m for COR, tunneling conditions; Iset =0.27 nA,
V
set =À0.13 V). c) A self-assembled monolayer formed by in situ UV
irradiation of trans-1 followed by addition of COR (concentrations of
original solutions; 1.0ꢀ10À5 m for 1 and 5.5ꢀ10À4 m for COR,
I
set =0.10 nA, Vset =À0.20 V). The white arrows correspond to the main
symmetric axes of graphite. The colored hexagons indicate the pores
containing four CORs (red), two CORs (yellow), and those with fuzzy
images (blue).
Figure 3. Percentage distributions of the number of adsorbed COR in
the pore formed by DBA 1. from left to right: premixed sample of
trans-1 and COR; sequential addition of COR for pre-formed honey-
comb structure of trans-1; after first irradiation of light (320 nm); after
second irradiation of light (>400 nm).
We have shown that tailored nanopores in 2D molecular
networks constructed from designer molecular building
blocks can host a specific guest molecule with high selectivity.
We have also demonstrated that the size of the pores can be
reversibly altered by irradiation with light of different wave-
lengths as shown by the change of the number of co-adsorbed
guest molecules. These insights may provide opportunities for
the creation of novel functional molecular self-assemblies on
surfaces.
of (320 Æ 10) nm light for 10 min followed by drop casting
a saturated solution of COR, the number of the pores
containing more than two COR molecules increased (Fig-
ure 2c and S12). Statistical analysis revealed the following
distribution: one COR, 73.0%; two CORs, 16.3%; three
CORs, 2.9%; four CORs, 0.2%; fuzzy feature, 7.5%
(Figure 3, first irradiation (320 nm)). Molecular modeling
indicates that the trans-to-cis isomerization followed by
Received: May 2, 2013
Published online: June 26, 2013
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Keywords: annulenes · photochemistry · porous materials ·
scanning tunneling microscopy · self-assembly
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