Hydrophobicity and CH/π-interaction-driven self-assembly of amphiphilic aromatic hydrocarbons into nanosheets

Article abstract of DOI:10.1039/c9cc08070h

The hydrophobicity and CH/π-interaction-driven self-assembly of an amphiphile that contains a biphenylanthracene group furnishes two-dimensional aggregates in dilute aqueous solution. A windmill-shaped molecular packing structure that arises from multiple intermolecular CH/π interactions of the aromatic hydrocarbons is the key motif for the self-assembly into micrometer-scale nanosheets.

Full text of DOI:10.1039/c9cc08070h

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Hydrophobicity and CH/p-interaction-driven
self-assembly of amphiphilic aromatic
hydrocarbons into nanosheets†
Cite this: Chem. Commun., 2019,
55, 14950
Received 14th October 2019,
Accepted 15th November 2019
a
Tsuyoshi Nishikawa,
Shigehiro Yamaguchi
Hiroki Narita,a Soichiro Ogi, *a Yoshikatsu Satob and
*
a
DOI: 10.1039/c9cc08070h
rsc.li/chemcomm
The hydrophobicity and CH/p-interaction-driven self-assembly of p-skeletons and hydrophilic chains self-assemble into self-
an amphiphile that contains a biphenylanthracene group furnishes standing sheet-like aggregates.6–9 However, the structural
two-dimensional aggregates in dilute aqueous solution. A windmill- requirements of amphiphiles for a self-assembly into self-
shaped molecular packing structure that arises from multiple inter- standing 2-D organic materials with well-defined packing struc-
molecular CH/p interactions of the aromatic hydrocarbons is the tures have not been fully established.
key motif for the self-assembly into micrometer-scale nanosheets.
In this regard, CH/p interactions, a type of hydrogen bond,
can be a useful means to control the packing mode in the self-
Two-dimensional (2-D) materials, represented by graphene and assembled structure.10 While an individual CH/p interaction
metal oxides, have recently received increasing attention due to is insufficient to overcome the entropic cost of arranging
their various functions derived from their dimensionality, p-skeletons, multiple CH/p interactions may cooperatively
which include e.g., a high charge-carrier mobility, effective facilitate such an alignment, as evident in the crystal structures
adsorption of target molecules, and a large surface area that of aromatic hydrocarbons.11 Anthracene is a representative
enables the accumulation of various functional groups.1 The example in which the CH/p interactions between the edge
use of such materials in aqueous media is highly attractive, C(sp2)–H bonds and the anthracene p-planes give rise to its
especially with regard to constructing bio-adaptive functional herringbone packing.12 Such CH/p hydrogen bonds would
materials.2 For example, the catalytic activity of enzymes can be presumably also work in water as the bond energy is derived
improved by fixation on graphene oxide.3
mostly from the dispersion force. Furthermore, orthogonal
In order to promote further development in this field, it is CH/p interaction can contribute to constructing higher dimen-
important to establish a methodology for the efficient prepara- sional structures such as 2-D sheets in comparison with the
tion of self-standing and well-defined 2-D structures. To this p–p interaction leading to 1-D arrangement of molecules.
end, the supramolecular approach represents an attractive Accordingly, we envisioned that an anthracene-based amphiphile
strategy due to the high modularity and tunability of organic could potentially self-assemble into well-defined structures
molecules and their self-assembling properties. While ionic (e.g., sheet-like aggregates) in aqueous media based on the synergy
interactions and hydrogen bonding have been widely used of hydrophobicity and CH/p interactions (Fig. 1).
to induce self-assembly in organic media,4,5 these interactions
Herein, we report that amphiphiles that contain a hydro-
are substantially perturbed by hydration in aqueous media. phobic biphenylanthracene group and a hydrophilic ethylene
In contrast, the use of hydrophobic effects may be an effective oxide chain afford sheet-like aggregates at concentrations as
way to induce self-assembly in aqueous solvents. Recent research low as 5 ꢀ 10ꢁ6 M in aqueous solvents. The key design feature
has demonstrated that amphiphiles that consist of hydrophobic is the orthogonal arrangement of the anthracene and the
biphenylene moiety, which have comparable skeletal lengths
to each other, thereby being suited to form a windmill-shaped
packing motif through multi-fold intermolecular CH/p interaction
between anthracene and biphenylene groups. The importance of
CH/p interaction for guiding the self-assembly into 2-D aggregates
is discussed by comparisons with self-assembling behavior of
several other relevant compounds.
a Department of Chemistry, Graduate School of Science, Nagoya University, Furo,
Chikusa, Nagoya, 464-8602, Japan. E-mail: yamaguchi@chem.nagoya-u.ac.jp
b Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo,
Chikusa, Nagoya, 464-8602, Japan
† Electronic supplementary information (ESI) available: Experimental procedures,
spectral data, details of computational studies, and crystallographic data for 3,
9-([1,10-biphenyl]-4-yl)anthracene, and 6. CCDC 1824326, 1953600 and 1953601. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9cc08070h
Initially, we investigated the self-assembling behavior of
an anthracene bearing a hydrophilic ethylene oxide chain 1
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Consequently, we introduced p-phenylene or 4,40-biphenylene
units between the anthracene and the hydrophilic chain of 1
(2 and 3 in Fig. 1b) in order to induce the formation of multiple
CH/p interactions. Solutions of 2 and 3 in 10/90 (v/v) 1-PrOH/H2O
with a total concentration of 5 ꢀ 10ꢁ6 M were prepared in the
same manner as that of 1. Whereas phenylene-containing 2
formed particles with a size of less than 1 mm (Fig. 2b),
biphenylene-containing 3 self-assembled into sheet-like aggre-
gates with a size on the mm scale (Fig. 2c). Since 3 is molecularly
dispersed at 5 ꢀ 10ꢁ6 M in pure 1-PrOH, the hydrophobicity of
the biphenylanthracene moiety is assumed to induce the self-
assembly at such a low concentration in the presence of water.
A TEM image of a freshly prepared sample of 3 revealed the
presence of unstructured aggregates, which were transformed
into sheet-like aggregates after standing for several hours
(Fig. S4, ESI†). Monitoring the time-dependent transformation
by UV-vis absorption spectroscopy revealed that this process is
completed after 12 h (Fig. S5, ESI†).
The morphology of the sheet-like aggregates of 3 was further
studied by atomic force microscopy (AFM). A sample was
prepared by drop casting a 10/90 (v/v) 1-PrOH/H2O solution of
3 on a silicon wafer. An AFM image confirmed the sheet-like
morphology with a mm-scale lateral size similar to that observed
by TEM (Fig. 2d). Importantly, the thicknesses of the sheets
(27 ꢂ 6 nm) is by two to three orders of magnitude lower than
the lateral size, and this pronounced aspect ratio strongly
suggests the presence of 2-D aggregates.
The sheet morphology for the aggregates of 3 was also
confirmed by fluorescence microscopy. A sample was prepared
by drop-casting a 10/90 (v/v) 1-PrOH/H2O solution of 3 on a
glass substrate in a manner similar to that of the AFM sample.
The 2-D sheet exhibited blue emission, and several individual
spectra were directly recorded at different points on the aggre-
gates by the spectral imaging using a confocal laser microscope
(Fig. 2e). The thus obtained spectra were essentially identical,
indicating that the morphology of the 2-D sheets is uniform
(Fig. S6, ESI†). It should be noted that an identical 2-D-sheet
morphology was observed on the different substrates such
as carbon films, silicon wafers, and glass plates. These micro-
scopic results indicate that the self-assembly of 3 into sheets
occurs in solution prior to the drop-casting onto substrates and
not on the surface of the substrates.
Fig. 1 (a) Schematic representation of the synergy between hydrophobi-
city and CH/p-interactions that leads to the self-assembly of amphiphilic
aromatic hydrocarbons and (b) chemical structures of 1–3.
in aqueous media. For the preparation of an aggregated
sample, a 1-propanol (1-PrOH) solution of 1 (5 ꢀ 10ꢁ5 M) was
added to distilled water until the water content reached 90 vol%
with a total concentration of 5 ꢀ 10ꢁ6 M. The solution was
shaken and then allowed to stand at room temperature
(B20 1C) for 12 h. A small portion of the solution was placed
onto a carbon-coated copper grid and examined by transmis-
sion electron microscopy (TEM) (Fig. S1, ESI†). A TEM image
of this sample showed only the formation of unstructured
aggregates of 1 (Fig. 2a). Even at a concentration of 5 ꢀ 10ꢁ4 M,
the morphology of 1 was particulate, suggesting that its molecular
structure is not suitable for self-assembly into higher-order struc-
tures (Fig. S2 and S3, ESI†).
The high propensity of 3 to form 2-D nanosheets should be
related to its packing structure. To gain insight into the inter-
molecular interactions between the biphenylanthracene moieties
in the aggregates, X-ray powder diffraction (XRPD) measurements
were carried out (Fig. 3). An aggregate sample was prepared at a
concentration of 5 ꢀ 10ꢁ5 M in 10/90 (v/v) 1-PrOH/H2O. TEM and
AFM images confirmed that the thus obtained aggregates exhibit
a morphology that is similar to those prepared at 5 ꢀ 10ꢁ6
M
(Fig. S7 and S8, ESI†). The XRPD measurements on the
nanosheets, collected by freeze-drying, showed sharp peaks,
which is indicative of a well-ordered molecular packing of 3 in
the aggregated state (Fig. 3a).
Fig. 2 (a–c) TEM images of (a) 1, (b) 2, and (c) 3 in the aggregated states;
(d) AFM height image of sheet-like aggregates obtained from a solution of
3 in 10/90 (v/v) 1-PrOH/H2O together with a cross-section analysis
corresponding to the red arrow; (e) result of the spectral imaging and
normalized fluorescent spectra at positions A, B, and C of 3.
To determine the molecular packing, we also tried to prepare a
single crystal for a single-crystal X-ray diffraction analysis by slowly
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molecule (Fig. S10, ESI†). In addition, the biphenyl moiety
exhibits a dihedral angle of 37.61. This comparison clearly
suggests that the amphiphilic structure is essential for indu-
cing the windmill-shaped alignment of the biphenylanthracene
group. A nature of rigidity and subsequent high crystallinity of
the aromatic moiety would contribute to segregation from the
flexible ethylene oxide chains and efficient formation of the
intermolecular CH/p interactions.14
A key feature to realize the crystalline growth into sheet-like
aggregates is the precise design of the oligomeric core packing
structure.9,14 In this regard, our design relies on the multiple
intermolecular CH/p interactions between the anthracene and
biphenyl moieties. It is reasonable to assume that the presence
of hydrophilic chains on the surface of the sheets decreases the
surface free energy of hydrophobic aggregates toward highly
polar solvents. Indeed, XRD analyses revealed that a line cut
along the out-of-plane direction exhibits the (010) plane, which
is parallel to the a axis (Fig. S11, ESI†). Namely, the windmill-
shaped oligomers are arranged along the in-plane direction of
nanosheets via the CH/p interactions between the biphenylene
moiety and the anthracene plane of adjacent molecules.
Consequently, the thickness of a monolayer is B2.6 nm, which
suggests that the sheets shown in Fig. 2d consists of 8–12 layers
of the unit cell.
Fig. 3 (a) A XRPD pattern of sheet-like aggregates (red) and a single-
crystal XRD pattern (blue) of 3; (b) windmill-shaped molecular packing in
sheet-like aggregates of 3; (c and d) two types of intermolecular CH/p
interactions between the anthracene and biphenyl units of 3.
Finally, the aggregate morphology of reference compounds
4–6 was compared to that of 3 in order to better understand the
cooling a hot solution of 3 in 1-PrOH to ambient temperature. The impact of the CH/p interactions on the hydrophobicity and the
XRD pattern of the single crystal matched well with that of the CH/p-interaction-driven self-assembly into nanosheets (Fig. 4a
nanosheets obtained from the solution in 10/90 (v/v) 1-PrOH/H2O. and Fig. S12, ESI†). TEM images revealed that amphiphile 4,
Specifically, the single crystal and the sheet-like aggregates exhibit which contains an anthracene inserted between a biphenylene
an essentially identical packing motif and the crystallographic moiety, did not form a sheet-like structure under the conditions
data can be used for a discussion of the molecular packing in the used for 3. In addition, amphiphile 5, in which bulky isopropyl
nanosheets.13
groups were introduced at the o,o-positions of the phenylene
The X-ray crystallographic analysis of 3 revealed a packing group to prevent CH/p interactions, only produced undefined
structure that is characterized by a symmetric windmill-shaped
alignment of four molecules (Fig. 3b), while the hydrophilic
ethylene oxide chains are oriented outward with respect to the
windmill core. Notably, the biphenylanthracene moieties of the
four molecules are close to each other and engage in two kinds
of intermolecular CH/p interactions. Firstly, the C–H bonds at
the 2,3-positions of the anthracene point toward the biphenyl
benzene plane of an adjacent molecule with a nonbonded
Hꢃ ꢃ ꢃC distance of B2.75 Å, which demonstrates that the
anthracene and biphenyl moieties act as hydrogen-donor and
-acceptor, respectively (Fig. 3c and Fig. S9a, ESI†). This inter-
action is responsible for the anti-parallel orientation of two
molecules. Secondly, CH/p interactions also occur in a vertical
direction between the hydrogen atoms of the biphenylene
moiety and the anthracene plane of an adjacent molecule with
nonbonding Hꢃ ꢃ ꢃC distances of B2.8 Å (Fig. 3d and Fig. S9b,
ESI†).10c Importantly, the biphenyl moiety adopts a coplanar
conformation with a dihedral angle of 27.01, which renders the
CH/p interaction effective. The packing structure is fundamen-
tally different from that of 9-([1,10-biphenyl]-4-yl)anthracene, in
Fig. 4 (a) Chemical structures of 4–6; (b) TEM image of 6 in the aggre-
which the C–H bonds at the 4,5,10-positions of the anthracene
are pointed toward the anthracene plane of an adjacent
gated state; (c) zeta potential distributions of aggregated 3 (red line) and 6
(blue line).
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aggregates. These results confirm that the CH/p interactions Conflicts of interest
between the biphenyl and anthracene moieties in 3 are essen-
The authors declare the absence of any potential conflicts of
interest.
tial to promote the observed 2-D self-assembling behavior.
On the other hand, replacement of the methoxy group at the
end of the hydrophilic chain with a hydroxy group in 6 afforded
belt-like aggregates, which hierarchically assemble into sheet-
like aggregates (Fig. 4b and Fig. S13, S14, ESI†). Notably, the
thickness of the aggregates obtained from a solution at
5 ꢀ 10ꢁ6 M in 10/90 (v/v) 1-PrOH/H2O was B3–6 nm
(Fig. S14b and c, ESI†), which is much thinner than those
obtained for 3. The XRPD pattern of the nanosheets of 6 was
consistent with that observed for its single crystals, which also
show a windmill-shaped packing structure that is essentially
similar to that of 3 (Fig. S15 and S16, ESI†). Nanosheets of 6
exhibit negative zeta potentials (Fig. 4c), whereas the zeta
potentials results imply that the surface properties of the
biphenylanthracene-based nanosheets are tunable by varying
the terminal group of the hydrophilic chain.
Notes and references
1 (a) C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj,
Angew. Chem., Int. Ed., 2009, 48, 7752–7777; (b) G. R. Bhimanapati,
Z. Lin, V. Meunier, Y. Jung, J. Cha, S. Das, D. Xiao, Y. Son, M. S.
Strano, V. R. Cooper, L. Liang, S. G. Louie, E. Ringe, W. Zhou,
S. S. Kim, R. R. Naik, B. G. Sumpter, H. Terrones, F. Xia, Y. Wang,
J. Zhu, D. Akinwande, N. Alem, J. A. Schuller, R. E. Schaak,
M. Terrones and J. A. Robinson, ACS Nano, 2015, 9, 11509–11539.
2 (a) X. Sun, Z. Liu, K. Welsher, J. T. Robinson, A. Goodwin, S. Zaric
and H. Dai, Nano Res., 2008, 1, 203–212; (b) D. Chimene, D. L. Alge
and A. K. Gaharwar, Adv. Mater., 2015, 27, 7261–7284.
3 (a) L. Jin, K. Yang, K. Yao, S. Zhang, H. Tao, S. T. Lee, Z. Liu and
R. Peng, ACS Nano, 2012, 6, 4864–4875; (b) Z. Liu, J. T. Robinson,
X. Sun and H. Dai, J. Am. Chem. Soc., 2008, 130, 10876–10877.
4 K. T. Nam, S. A. Shelby, P. H. Choi, A. B. Marciel, R. Chen, L. Tan,
T. K. Chu, R. A. Mesch, B. C. Lee, M. D. Connolly, C. Kisielowski and
R. N. Zuckermann, Nat. Mater., 2010, 9, 454–460.
In conclusion, we have introduced biphenylanthracene as a
useful motif to induce hydrophobicity and CH/p-interaction-
driven self-assembly into nanosheets. A combination of
biphenylanthracene moieties and long hydrophilic chains
affords amphiphiles that are able to engage in multiple
intermolecular CH/p interactions. The thus produced
windmill-shaped packing structures effectively form sheet-
like aggregates with a mm-scale lateral size at very low
concentrations. In this molecular design, changing the
terminal groups of the hydrophilic chains allows modifying
the surface character of the nanosheets. Further investiga-
tions into the development of functional materials based on
the two-dimensionality of the sheets are currently in progress
in our laboratory.
This work was supported by Grants-in-Aid for Young
Scientists (B) (KAKENHI grant JP17K14469 to S. O.) from the
Japan Society for the Promotion of Science (JSPS). This work
was partly supported by JSPS KAKENHI grant JP16H06280. The
authors thank Prof. Takahiro Sasamori (Nagoya City Univ.), and
Prof. Mao Minoura (Rikkyo Univ.), as well as Dr Masato Hirai,
Dr Naoki Ando, Mr Junichi Usuba, and Mr Masahiro Hayakawa
(Nagoya Univ.) for X-ray crystallographic analyses. The synchro-
tron X-ray crystallography experiment was performed at the
5 H. S. Jang, J. H. Lee, Y. S. Park, Y. O. Kim, J. Park, T. Y. Yang, K. Jin,
J. Lee, S. Park, J. M. You, K. W. Jeong, A. Shin, I. S. Oh, M. K. Kwon,
Y. I. Kim, H. H. Cho, H. N. Han, Y. Kim, Y. H. Chang, S. R. Paik,
K. T. Nam and Y. S. Lee, Nat. Commun., 2014, 5, 3665.
6 S. Ghosh, D. S. Philips, A. Saeki and A. Ajayaghosh, Adv. Mater.,
2017, 29, 1605408.
7 (a) A. S. Weingarten, R. V. Kazantsev, L. C. Palmer, M. McClendon,
A. R. Koltonow, A. P. S. Samuel, D. J. Kiebala, M. R. Wasielewski and
S. I. Stupp, Nat. Chem., 2014, 6, 964–970; (b) R. V. Kazantsev, A. J.
Dannenhoffer, A. S. Weingarten, B. T. Phelan, B. Harutyunyan,
T. Aytun, A. Narayanan, D. J. Fairfield, J. Boekhoven, H. Sai,
A. Senesi, P. I. O’Dogherty, L. C. Palmer, M. J. Bedzyk, M. R.
Wasielewski and S. I. Stupp, J. Am. Chem. Soc., 2017, 139, 6120–6127.
8 (a) E. Lee, J. K. Kim and M. Lee, Angew. Chem., Int. Ed., 2009, 48,
3657–3660; (b) H. J. Kim, T. Kim and M. Lee, Acc. Chem. Res., 2011,
44, 72–82.
9 (a) M. Vybornyi, A. V. Rudnev, S. M. Langenegger, T. Wandlowski,
¨
G. Calzaferri and R. Haner, Angew. Chem., Int. Ed., 2013, 52,
¨
11488–11493; (b) M. Vybornyi, A. Rudnev and R. Haner, Chem.
Mater., 2015, 27, 1426–1431.
10 (a) M. Nishio, M. Hirota and Y. Umezawa, The CH/p interaction. Evidence,
Nature, and Consequences, Wiley-VCH, New York, 1998; (b) M. Nishio,
Y. Umezawa, K. Honda, S. Tsuboyama and H. Suezawa, CrystEngComm,
2009, 11, 1757–1788; (c) H. Suezawa, S. Ishihara, Y. Umezawa,
S. Tsuboyama and M. Nishio, Eur. J. Org. Chem., 2004, 4816–4822.
11 (a) J. E. Anthony, Chem. Rev., 2006, 106, 5028–5048; (b) M. Nishio,
Y. Umezawa, J. Fantini, M. S. Weiss and P. Chakrabarti, Phys. Chem.
Chem. Phys., 2014, 16, 12648–12683.
12 C. P. Brock and J. D. Dunitz, Acta Crystallogr., Sect. B: Struct. Sci.,
1990, 46, 795–806.
BL40XU beamline of SPring-8 with the approval of the Japan 13 S. Saito, K. Nakakura and S. Yamaguchi, Angew. Chem., Int. Ed.,
2012, 51, 714–717.
14 (a) Y. Zheng, H. Zhou, D. Liu, G. Floudas, M. Wagner, K. Koynov,
Synchrotron Radiation Research Institute (JASRI; proposal
2018B1084, 2018B1275). The authors are grateful to Mr Tatsuo
M. Mezger, H. J. Butt and T. Ikeda, Angew. Chem., Int. Ed., 2013, 52,
Hikage for XRPD measurements.
4845–4848; (b) T. Ikeda, Langmuir, 2015, 31, 667–673.
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