Preparation of few-layer two-dimensional polymers by self-assembly of bola-amphiphilic small molecules

Article abstract of DOI:10.1002/pola.29444

Two bola-amphiphilic small molecules, based on the diphenylanthracene skeleton structure, namely, BASM-1 and its functionalized small molecule BASM-2, were designed and synthesized. The self-assembly behavior and mechanism of these two molecules in aqueou

Full text of DOI:10.1002/pola.29444

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Preparation of Few-Layer Two-Dimensional Polymers By Self-Assembly
of Bola-Amphiphilic Small Molecules
1
,2
1,2
1,2
1,2
1,2
Na Zhang , Taisheng Wang , Xiaohai Bu, Qiong Wu, Zewu Zhang
1
School of Materials Science and Engineering, Nanjing Institute of Technology, Nanjing 211167, People’s Republic of China
Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, People’s Republic of China
2
Correspondence to: N. Zhang (E-mail: zhangna@njit.edu.cn); T. Wang (E-mail: saw11020@njit.edu.cn)
Received 13 May 2019; Revised 8 July 2019; accepted 10 July 2019
DOI: 10.1002/pola.29444
ABSTRACT: Two bola-amphiphilic small molecules, based on the
diphenylanthracene skeleton structure, namely, BASM-1 and its
functionalized small molecule BASM-2, were designed and syn-
thesized. The self-assembly behavior and mechanism of these
two molecules in aqueous solution were studied. The supramo-
lecular two-dimensional (2D) layer and the covalent 2D poly-
mers were, respectively, prepared by these two molecules.
What is more, the transverse size of the covalent 2D polymer
laminates increased with the extension of the polymerization
time. Atomic force microscopy results showed that both free-
standing single-layer 2D polymers and few layer laminates with
two to three molecular layers were obtained. So our work pro-
vides a simple and efficient method for directly preparing inde-
pendent both supramolecular 2D polymers and covalent 2D
polymers in liquid phase which is of great significance. © 2019
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019
KEYWORDS: bola-amphiphilic; covalent two-dimensional poly-
mer; diphenylanthracene; self-assembly; supramolecular two-
dimensional layer
2
2–28
INTRODUCTION In recent years, two-dimensional (2D) materials
have attracted extensive attention and their emergence empha-
sizes the important influence of atomic-scale molecular structure
and can obtain various unique assembly structures.
Due to
the bipolar nature of BASM, planar assembly structures are
expected to be formed. However, of all the papers that reported
the self-assembly of these amphiphilic molecules, only a few
1–9
and dimension on material properties.
Among them, organic
2
9–31
2D materials (organic supramolecular 2D layer and organic cova-
BASM accidentally formed 2D structures.
In most cases, the
lent 2D polymer) have attracted people’s attention recently,
resulting assemblies were zero-dimensional or one-dimensional
3
2–41
because they have more special properties, such as light weight,
structures, such as spherical micelles and fibers.
In general,
1
0–17
good flexibility, structural tunability, and high adaptability.
when the packing parameters (which represent the ability to
aggregate between adjacent molecules) of amphiphilic molecules
were small, it was easy to obtain zero-dimensional or one-
dimensional structures. When the stacking parameters were
Distinct from graphene, a 2D polymer prepared by organic syn-
thesis offers more flexibility in structural tailoring and opportu-
nity for atomic level structure–property investigations. Due to its
structure peculiarities, 2D polymers might serve as optoelec-
tronic device, molecular electronics, sensor and drug release plat-
forms, and so on. Recently, more and more people are engaged in
the synthesis of organic 2D materials and some achievements
4
2
large, it was easy to form a 2D plane structure. Therefore, if the
packing parameters of BASM could be designed properly, these
molecules would greatly broaden the range of organic 2D
structures.
1
8,19
have been made.
There are two main methods for synthesis
In this article, we aim to develop a simple solution self-assembly
method for the direct preparation of independent organic 2D
polymers. Inspired by the above research, we first designed and
synthesized two new ionic BASMs, named BASM-1 and BASM-2,
respectively (Fig. 1), in which the hydrophobic part was a large
conjugated structure. We suggested that the molecular stacking
parameters could be increased by increasing the area and rigidity
of the hydrophobic part. In addition, due to the presence of ionic
groups in the hydrophilic parts at both ends of the molecules,
electrostatic repulsion could weaken the interlayer interaction.
of 2D polymers: “top-down” synthesis and “bottom-up” synthe-
sis. Compared with the top-down method, molecular self-
assembly is a bottom-up method which is simpler and more effi-
cient to prepare 2D planar structures, especially for organic 2D
1
3,20,21
structures.
Bola-amphiphilic small molecules (BASMs) are amphiphilic small
molecules with hydrophobic parts in the middle and hydrophilic
parts at both ends. It has been reported in literature that such
amphiphilic small molecules have rich self-assembly behaviors
Additional supporting information may be found in the online version of this article.
2019 Wiley Periodicals, Inc.
©
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precipitation collection, and washing with ether. The raw
product could be recrystallized with water/glacial acetic acid
(
3/2) solution to obtain colorless acicular crystals with a yield
of 70%. Alternatively, the crude product could be dissolved in
ethanol, filtered to remove insoluble impurities, collected in
filtrate, and rotated to evaporate to obtain brown solids, with
1
a yield of 85%. H NMR [400 MHz, d
6
- dimethyl sulfoxide
(
4
DMSO)]: δ 9.34 (s, 4H), δ 7.63 (t, 4H), δ 7.53 (t, 2H), 7.38 (d,
H), 6.65 (s, 4H).
Synthesis of 9,10-Diphenyl-2,3,6,7-Tetraphenyl(ethyl
oxybutyrate) Anthracene (3)
Compound 2 (0.28 g, 0.72 mmol) and 20 mL acetone were added
to a 100 mL three-necked flask and stirred with nitrogen gas,
followed by ethyl 4-bromobutyrate (0.702 g, 3.6 mmol) and
potassium carbonate (0.9 g, 6.48 mmol). The mixture was
FIGURE 1 The structures of BASM-1 and BASM-2. [Color figure
can be viewed at wileyonlinelibrary.com]
ꢀ
refluxed at 60 C for 48 h, and the reaction was monitored by
TLC (eluent: methylene chloride/ethyl acetate = 20/1). After the
reaction was finished, the insoluble potassium carbonate was fil-
tered out and filtrate was collected. Added 50 mL of dic-
hloromethane to the filtrate, washed it three times with water,
collected the organic phase, dried the anhydrous sodium sulfate,
removed the solvent by rotary evaporation, purified the residue
by column chromatography (eluent: dichloromethane/ethyl ace-
Therefore, we expected to obtain the thin layer 2D structures.
BASM-2 with the reactive functional groups in the middle part
was synthesized in order to further obtain covalent-linked 2D
polymers, in which the obtained 2D supramolecular structure
could be polymerized in layers through the Suzuki coupling
reaction.
tate = 20/1), and finally got the solid product, the yield was
EXPERIMENTAL
1
3
61.3%. H NMR (400 MHz, CDCl ): δ 7.51 (m, 10H), 6.79 (s, 4H),
Synthesis of 9,10-Diphenyl -2,3,6,7-Tetramethoxy
Anthracene (1)
4.12 (q, J = 5.4 Hz, 8H), 3.86 (t, J = 4.5 Hz, 8H), 2.47 (t, J = 5.4 Hz,
8H), 2.07 (m, 8H), 1.24 (t, J = 5.4 Hz, 12H).
The mixture of veratrole (3.98 g, 28.8 mmol), benzaldehyde
(
(
1.02 g, 9.6 mmol), and phosphotungstic acid (PWA, H PW O )
Synthesis of 9,10-Diphenyl-2,3,6,7-Tetra(oxybutyric acid)
Anthracene (4)
3
12 40
7 mol %) (1.935 g, 67.2 mmol) was first added into a 100 mL
ꢀ
flask. The mixture was stirred at 75 C for 30 min. The reaction
process was monitored by thin-layer chromatography (TLC) (elu-
ent: petroleum ether/ethyl acetate = 1/4). Then benzaldehyde
Compound 3 (0.2 g, 0.235 mmol), sodium hydroxide (0.15 g,
3.76 mmol), 5 mL water, and 10 mL ethanol were added into a
ꢀ
25 mL flask. The mixture was reflux stirred at 60 C for 12 h until
(
2.04 g, 19.2 mmol), PWA (2 mol %) (0.553 g, 19.2 mmol), acetic
completely dissolved. After the reaction was complete, the mix-
ture solution was directly dried, and then 10 mL of water was
added. The mixture was acidified with dilute hydrochloric acid,
anhydride (3.92 g, 38.4 mmol), and 20 mL glacial acetic acid
were added to the reaction system, and then stirred for 90 min at
7
ture, 20 mL of dichloromethane was added to the system. Filter
away the phosphotungstate hydrate and collect the filtrate. The
organic phase was washed with saturated sodium bicarbonate
solution, and the solvent was removed by rotatory evaporation
after drying with anhydrous sodium sulfate. The residue was sep-
arated and purified by column chromatography (eluent: petro-
leum ether/ethyl acetate = 1/4) to obtain solid products with a
ꢀ
5 C. After the reaction mixture was cooled to room tempera-
and the solid products were separated out, filtered, and collected.
1
The yield was 99%. H NMR (400 MHz, d
6
- DMSO): δ 7.61 (m,
6H), 7.42 (d, 4H), 6.71 (s, 4H), 3.74 (t, 8H), 2.32 (t, 8H), 1.89
(m, 8H).
Synthesis of 9,10-Diphenyl-2,3,6,7-Tetra(sodium
oxybutyrate) Anthracene (BASM-1)
First, Compound 4 (0.113 g, 0.134 mmol), sodium hydroxide
(0.022 g, 0.536 mmol), and 10 mL water were added to the
round-bottom flask. Then, the mixture was stirred until
1
yield of 73%. Proton nuclear magnetic resonance ( H NMR;
4
anthracene), 3.73 (s, 12 H, OCH
00 MHz, CDCl
3
): δ = 7.69 (m, 10 H), 6.82 (s, 4 H, 1, 4, 5, 8 - H
).
3
completely dissolved under room temperature (or stirring 3 h at
ꢀ
6
0 C). Filter, collect filtrate, directly spin dry, and then obtain
1
Synthesis of 9,10-Diphenyl-2,3,6,7-Tetrahydroxy
Anthracene (2)
Compound 1 (0.45 g, 1 mmol), 47% hydrobromic acid aque-
ous solution (60 mL), and glacial acetic acid (60 mL) were
mixed and added into a 250 mL flask. The mixture was stirred
solid product BASM-1. The yield was 100%. H NMR (400 MHz,
D2O): δ 7.45 (m, 6H), 6.88 (d, 4H), 6.58 (s, 4H), 3.73 (t, 8H), 2.13
(t, 8H), 1.86 (m, 8H).
Synthesis of 9,10-Bis(4-bromophenyl)-
2,3,6,7-Tetramethoxy Anthracene (5)
ꢀ
with nitrogen for 10 min, and then slowly heated to 105 C
under nitrogen protection. Under the above conditions, the
mixture refluxed overnight for 24 h and then slowly cooled to
room temperature in the atmosphere of nitrogen. Filtration,
The mixture of veratrole (3.98 g, 28.8 mmol), 4-bromobenzal-
dehyde (1.78 g, 9.6 mmol), and PWA (7 mol %) (1.935 g,
67.2 mmol) was first added into a 100 mL flask. The mixture was
2
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ꢀ
stirred at 75 C for 30 min. The reaction process was monitored
by TLC (eluent: petroleum ether/ethyl acetate = 1/4). Then,
Synthesis of 9,10-Bis(4-bromophenyl)-2,3,6,7-Tetra
(oxybutyric acid) Anthracene (8)
4
-bromobenzaldehyde (3.55 g, 19.2 mmol), PWA (2 mol %)
Compound 7 (0.2 g, 0.198 mmol), sodium hydroxide (0.13 g,
3.16 mmol), 5 mL water, and 10 mL ethanol were added into a
25 mL flask. The mixture was reflux stirred at 60 C for 12 h until
completely dissolved. After the reaction was complete, the mix-
ture solution was directly dried, and then 10 mL of water was
added. The mixture was acidified with dilute hydrochloric acid,
(
0.553 g, 19.2 mmol), acetic anhydride (3.92 g, 38.4 mmol), and
ꢀ
20 mL glacial acetic acid were added to the reaction system, and
ꢀ
then stirred for 90 min at 75 C. After the reaction mixture was
cooled to room temperature, 20 mL of dichloromethane was
added to the system. Filter away the phosphotungstate hydrate
and collect the filtrate. The organic phase was washed with satu-
rated sodium bicarbonate solution, and the solvent was removed
by rotatory evaporation after drying with anhydrous sodium sul-
fate. The residue was separated and purified by column chroma-
and the solid products were separated out, filtered, and collected.
1
The yield was 98%. H NMR (400 MHz, d - DMSO): δ 12.1 (s,
6
4H), δ 7.83 (d, J = 6.0 Hz, 4H), 7.38 (d, J = 6.0 Hz, 4H), 6.68 (s,
4H), 3.79 (t, J = 5.4 Hz, 8H), 2.35 (t, J = 5.4 Hz, 8H), 1.90 (m, 8H).
tography (eluent: petroleum ether/ethyl acetate = 1/4) to obtain
1
solid products with a yield of 65%. H NMR (400 MHz, CDCl
3
):
Synthesis of 9,10-Bis(4-bromophenyl)-2,3,6,7-Tetra
δ = 7.75 (d, J = 8.2 Hz, 4 H, 2, 6 - H phenyl), 7.37 (d, J = 8.25 Hz,
(
sodium oxybutyrate) Anthracene (BASM-2)
First, Compound 8 (0.161 g, 0.177 mmol), sodium hydroxide
0.028 g, 0.708 mmol), and 10 mL water were added to the
4
H, 3, 5 - H phenyl), 6.76 (s, 4 H, 1, 4, 5, 8 - H anthracene), 3.76 (s,
1
2 H, OCH ).
3
(
round-bottom flask. Then, the mixture was stirred until completely
ꢀ
dissolved under room temperature (or stirring 3 h at 60 C). Filter,
Synthesis of 9,10-Bis(4-bromophenyl)-
,3,6,7-Tetrahydroxy Anthracene (6)
Compound 5 (0.15 g, 0.025 mmol), 47% hydrobromic acid
aqueous solution (20 mL), and glacial acetic acid (20 mL)
were mixed and added into a 250 mL flask. The mixture was
collect filtrate, directly spin dry, and then obtain solid product
2
1
BASM-2. The yield was 100%. H NMR (400 MHz, D O): δ 7.56
2
(t, 4H), 6.66 (d, 4H), 6.53 (s, 4H), 3.76 (t, 8H), 2.23 (t, 8H), 1.96
(m, 8H).
stirred with nitrogen for 10 min, and then slowly heated to
Synthesis of 1,3,5-Triborate Ester Benzene (9)
First, 1,3,5-tribromobenzene (0.5 g, 1.6 mmol), bis(pinacolato)
diboron (1.46 g, 5.76 mmol), potassium acetate (0.95 g,
ꢀ
105 C under nitrogen protection. Under the above conditions,
the mixture refluxed overnight for 24 h and then slowly
cooled to room temperature in the atmosphere of nitrogen.
Filtration, precipitation collection, and washing with ether.
The raw product could be recrystallized with water/glacial
acetic acid (3/2) solution to obtain colorless acicular crystals
with a yield of 65%. Alternatively, the crude product could be
dissolved in ethanol, filtered to remove insoluble impurities,
9
0
.6 mmol), Tetrakis(triphenylphosphine)palladium(0) (39.2 mg,
.048 mmol) and 10 mL refined dimethylformamide (DMF) were
added to 20 mL stoker reactor. After three times of freezing–vac-
uum–nitrogen–thawing operation, the reaction mixture was
ꢀ
stirred under 90 C for 24 h. After the reaction, the mixture was
poured into the beaker, and 100 mL dichloromethane was added.
Then, the oil was washed with water (3 × 50 mL), the organic
layer was dried with Na SO and concentrated in vacuum. The
2 4
crude product was purified by column chromatography (eluent:
collected in filtrate, and rotated to evaporate to obtain brown
1
solids, with a yield of 83.3%. H NMR (400 MHz, d - DMSO):
6
δ 7.80 (d, J = 8.4 Hz, 4H), 7.34 (d, J = 8.4 Hz, 4H), 6.62 (s, 4H).
petroleum ether/ethyl acetate = 10/1), and the solid product was
1
obtained with a yield of 51.5%. H NMR (400 MHz, CDCl ): δ 8.36
3
Synthesis of 9,10-Bis(4-bromophenyl)-2,3,6,7-Tetraphenyl
(
s, 3H), 1.34 (s, 36H).
(ethyl oxybutyrate) Anthracene (7)
Compound 6 (0.4 g, 0.72 mmol) and 20 mL acetone were
added to a 100 mL three-necked flask and stirred with nitro-
gen gas, followed by ethyl 4-bromobutyrate (0.702 g,
Self-Assembly of BASM-1 and BASM-2 in Pure Water
First, 1 mg BASM-1 and 1 mg BASM-2 were weighed and placed
in bottle, and 1 mL distilled water was added, respectively. The
solution was fully dissolved by ultrasound for 5 min, and then
allowed to stand for 24 h to obtain the assembly solution with a
concentration of 1 mg mL− . The above assembly solutions were
numbered nos. 1 and 2, respectively.
3
.6 mmol) and potassium carbonate (0.9 g, 6.48 mmol). The
ꢀ
mixture was refluxed at 60 C for 48 h, and the reaction was
monitored by TLC (eluent: methylene chloride/ethyl ace-
tate = 20/1). After the reaction was finished, the insoluble
potassium carbonate was filtered out and filtrate was col-
lected. Added 50 mL of dichloromethane to the filtrate,
washed it three times with water, collected the organic phase,
dried the anhydrous sodium sulfate, removed the solvent by
rotary evaporation, purified the residue by column chroma-
tography (eluent: dichloromethane/ethyl acetate = 20/1), and
1
Self-Assembly of BASM-2 in a Mixture of Organic
Solvent/Water
First, six samples of BASM-2 with a mass of 1 mg were weighed
and placed in six bottles, and then 3 mL distilled water was
added, respectively. After being completely dissolved, six aque-
ous solutions were added to 3 mL dioxane, 3 mL tetrahydrofuran
(THF), 3 mL acetone, 3 mL DMF, 6 mL DMF, and 1.5 mL DMF,
drop by drop, respectively. Then, the above solutions were dia-
lysed 2 h with corresponding mixed solvent for assembly, and
1
finally got the solid product, the yield was 51%. H NMR
(
4
8
400 MHz, CDCl
3
): δ 7.72 (d, J = 6.0 Hz, 4H), 7.32 (d, J = 6.0 Hz,
H), 6.73 (s, 4H), 4.13 (q, J = 5.4 Hz, 8H), 3.88 (t, J = 4.5 Hz,
H), 2.49 (t, = 5.4 Hz, 8H), 2.09 (m, 8H), 1.24
J
(
t, J = 5.4 Hz, 12H).
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finally obtained six different assembly solutions, which were
(a) The intermediate hydrophobic part is a rigid large conju-
numbered nos. 3, 4, 5, 6, 7, and 8, respectively.
gate structure, with a strong π–π accumulation interaction;
(
b) Molecules have bipolar properties, so the hydrophobic
Synthesis of Few-Layer Covalent 2D Polymers
interactions will be observed in aqueous solutions, which will
further increase the degree of molecular aggregation; and
(c) The hydrophilic parts at both ends of the molecules also
have ionic groups and the highly polar aqueous solution has a
strong solvation effect on the ions, so the negative ions can be
dissociated, which eventually leads to the electrostatic repul-
sion between the negative ions, weakens the layer–layer accu-
mulation, and obtains the oligolayer 2D plane structure.
Therefore, we chose aqueous solution as the self-assembled
solvent system.
1
,3,5-Triborate ester benzene (1 mol), Tetrakis(triphenylpho-
sphine)palladium(0) (0.05 mol) and potassium carbonate
6 mol) were added to no. 6 assembly solution containing 1.5
(
mole BASM-2, in turn. After three times of freezing–vacuum–
nitrogen–thawing operation, the reaction mixture was stand-
ꢀ
ing under 90 C for 72 h. When the reaction was finished, the
reaction mixture was directly filtered to remove the insoluble
solids. The filtrate was dialyzed for 3 days with DMF/H2O
=
1/1 mixed solvent (molecular weight cutoff = 3500), to
remove the unreacted raw materials. The polymer solution
was precipitated with acetone to obtain a solid polymer.
The self-assembly of BASM was in two directions in aqueous
solution. The inducing force of lamellar growth along the
x axis includes π–π accumulation and hydrophobic interaction.
Among them, π–π accumulation played a dominant role
because of the large conjugate area of the molecule itself. And
the induced force along the y axis was only hydrophobic inter-
action. So the main driving force along the x axis was π–π stac-
king interaction, and the main driving force along the y axis
was the hydrophobic interaction. The relative magnitude of
dominant forces in the two directions was mainly regulated
by the variation of hydrophobic effect, while the strength of
hydrophobic effect was affected by the water content in the
solvent. The higher the water content was, the stronger the
RESULTS AND DISCUSSION
The synthetic procedures of BASM-1 and BASM-2 are shown
1
13
in Scheme 1. The structures were characterized by H,
C
NMR spectroscopy, and mass spectroscopy. The results were
shown in Figures S1–S20.
Self-Assembly Mechanism of BASM-1 and BASM-2 in
Aqueous Solution
The molecular structure of BASM-1 and BASM-2 that we
designed and synthesized both have three characteristics:
SCHEME 1 Synthetic procedures of bola-amphiphilic BASM-1 and BASM-2.
4
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hydrophobic effect and the greater the inducing force of
growth along the y axis.
However, instead of forming a 2D structure, the small molecule
BASM-2 assembled into a nanoribbon-like structure (a few hun-
dreds of nanometers wide and a few tens of micrometers long).
Therefore, in order to prepare covalently bonded 2D polymers,
we studied the solution assembly conditions of BASM-2 in detail.
To claim the π–π interaction, X-ray diffraction (XRD)
(
Figs. S23 and S24) and ultraviolet–visible spectroscopy (UV–
vis) (Fig. S25) of BASM-1 and BASM-2 were measured. The
XRD results turned out that both BASM-1 and BASM-2
showed strong intermolecular accumulation in solid state, and
the accumulation of BASM-1 was more compact and orderly.
UV–vis results also proved that both the two compounds had
wide absorption peaks near 398–399 nm in aqueous solution
due to intermolecular π–π accumulation.
The previous research results showed that the reason why
BASM-2 could not obtain a 2D plane structure in pure water was
that its hydrophilic–hydrophobic effect was very strong, resulting
in a large steric effect between bromine atoms, and the induced
force of molecules growing along the x axis was less than that
growing along the y axis. In order to balance the growth rate of
molecules in all directions, we reduced the inducing force of mol-
ecules growing along the y axis by adjusting the solvent. When
using the mixed solution of organic solvent and water, the hydro-
phobic effects were weakened, the steric hindrance between bro-
mine atoms was reduced, and BASM-2 grew uniformly along all
directions to form regular nanosheet.
Self-Assembly of BASM-1 and BASM-2 in Pure Water
We first studied the self-assembly behavior of two BASM in pure
water, as shown in Figure 2. The results showed that BASM-1
could directly self-assemble in pure water to obtain 2D
nanosheet structure, and the surface of the sheet was smooth,
with the transverse size ranging from hundreds of nanometers to
dozens of micrometers. However, the self-assembly of BASM-2 in
pure water resulted in nanoribbon structure, and the width of
the nanoribbon was about hundreds of nanometers, the length
was about ten of micrometers. This was due to, in pure water, the
hydrophobic effect was very strong, which was much larger than
the intermolecular π–π stacking interaction, so the induced
growth force of BASM-1 along the x axis (the hydrophobic effect)
and y axis (π–π stacking interaction and the hydrophobic effect)
was close, and easy to form a uniform-sized slice structure. How-
ever, due to the steric hindrance of larger bromine atoms, which
limited the horizontal growth of the lamellar along the x axis,
BASM-2 could only form nanoribbon structures.
We selected the mixed solution of different organic solvents
(
dioxane, THF, acetone, N,N-DMF) and water as the self-assembly
solvent, with the proportion of 1/1 (v/v). The transmission elec-
tron microscope (TEM) results are shown in Figure 3. The results
showed that BASM-2 self-assembled into one-dimensional
nanobelts and nanorods structure in dioxane/H
in THF/H O mixture, respectively. However, in acetone/H
DMF/H O mixture, 2D planar structure with low contrast and
2
O mixture and
2
2
O and
2
uniform smooth surface were obtained, and their lateral sizes
were about tens of more micrometers. Because of the different
polarities of the organic solvents we used (The order of polarity
of solvent is THF < dioxane < acetone < DMF.), different mor-
phologies were obtained in various mixed solvents. The main
driving force along the x axis was π–π stacking interaction, and
the main driving force along the y axis was the hydrophobic inter-
action. The relative magnitude of dominant forces in the two
directions was mainly regulated by the variation of hydrophobic
effect. The greater the polarity of solvent, the stronger the
Self-Assembly of BASM-2 in a Mixture of Organic
Solvent/Water
Although BASM-1 was self-assembled in pure water to obtain 2D
lamellar structure, it further proved that our method based on
solution self-assembly to prepare 2D layer was reasonable.
FIGURE 2 TEM images of BASM-1 (a,b) and BASM-2 (c,d) in
water.
FIGURE 3 TEM images of BASM-2 in organic solvent/water.
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hydrophobic effect, and the closer the growth force of molecules
along the x and y axes, which was conducive to the formation of
laminates. Considering that higher temperature was required for
the preparation of covalent 2D polymers in the later stage, we
chose DMF/water mixed solution as the solvent system for self-
assembly polymerization next.
Synthesis and Characteristic of Few-Layer Covalent 2D
Polymers
In order to prepare covalent 2D polymers, trifunctional monomer
1,3,5-triborate ester benzene (9) was added into the self-
assembly solution, and the Suzuki coupling of borate and
bromobenzene was used for the polymerization reaction. Since
1,3,5-triborate ester benzene was an oil-soluble monomer, it dif-
fused to the hydrophobic part in the middle of the 2D layer,
resulting that the polymerization reaction could not occur among
the layers, but only in the 2D layers. Therefore, in the XY plane,
bromobenzene coupled with Compound 9 to form covalent
bonds in the initial stage, and covalent 2D polymer was obtained.
In the later stage, bromobenzene at the edge of 2D polymer layer
coupled with Compound 9 to make the polymer layer grow con-
tinuously with the extension of time. In the z axis (i.e., perpendic-
ular to the XY plane), the electrostatic repulsion of ions
prevented the accumulation among the layers, resulting in thin
layers or even single-layer 2D polymers.
2
Self-Assembly of BASM-2 in a Mixture of DMF/H O with
Different Water Contents
In order to obtain the 2D laminates with the best morphology and
the largest size, we further studied the influence of water content
in the DMF/H O mixed solvent on the self-assembly behavior of
2
BASM-2. DMF/H
DMF/H O = 0/1 were selected as solvents (water content was
3, 50, 67, and 100%, respectively) for assembly and the results
2 2 2
O = 2/1, DMF/H O = 1/1, DMF/H O = 1/2, and
2
3
are shown in Figure 4. The results showed that low water content
or high water content was not conducive to the formation of 2D
lamellar. When the water content was 50%, the obtained lamellar
morphology was the best. This was because, when the water con-
tent was too low (33%), the hydrophobic effect was weak and
intermolecular π–π stacking interaction dominated, which led to
the induced force of BASM-2 along the x axis (the hydrophobic
effect) less than the induced force along y axis (π–π stacking
interaction and the hydrophobic effect), eventually forming
nanoribbon structures; when the water content was high (67 and
2
The polymerization conditions were as follows: DMF/H O = 1/1
as the solvent, Tetrakis(triphenylphosphine)palladium(0) as the
catalyst, potassium carbonate as the base, the concentration of
BASM-2 was 0.1 mg mL− , the temperature was 90 C, the poly-
merization was standing in the atmosphere of nitrogen, and the
polymerization time was 12, 24, 36, and 72 h, respectively. The
TEMs of the polymers are shown in Figure 5. The results showed
that with the extension of the polymerization time, the polymer
sheet grew gradually. When the polymerization time was 72 h,
the sheet size could reach 10 or even dozens of micrometers, and
the sheet surface was smooth and flat. The structure of the poly-
mer was characterized by infrared spectroscopy, as shown in
Figure S21. After polymerization, the stretching vibration peak
(C Br bond 595 cm− ) of BASM-2 and the characteristic peaks
1
ꢀ
100%), the hydrophobic interaction was dominant, but the steric
effect of bromine atoms along the x axis was so large that the
induced force of BASM-2 along the x axis (the hydrophobic inter-
action) was still less than the induced force along the y axis (π–π
stacking interaction and the hydrophobic interaction). Only when
the water content was moderate, the π–π stacking interaction and
the hydrophobic effect were close to each other, leading to the uni-
form growth of BASM-2 along all directions, and finally 2D plane
1
(B O bond 1337 cm− and C O bond 1273 cm
1
−1
) of
structures were obtained. So we chose DMF/H
mixed solution as the solvent system of self-assembly
polymerization.
2
O = 1/1 of the
1,3,5-triborate ester benzene disappeared, indicating the occur-
rence of Suzuki coupling reaction and the formation of covalent
2D polymers.
FIGURE 4 TEM images of BASM-2 in a mixture of DMF/H
2
O with
FIGURE 5 TEM images of 2D polymers formed at different times
different water contents.
2
in DMF/H O = 1:1 (a) 12 h, (b) 24 h, (c) 36 h, and (d) 72 h.
6
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preparation process, so we also got the 2D polymer with few
layers eventually (Fig. S22).
The synthesis mechanism of 2D polymers is shown in Figure 7.
First of all, the BASM-2 self-assembled in mixed solvent
(
2
DMF/H O = 1/1) to form a supramolecular 2D layer. Then, the
oil-soluble trifunctional monomer (9) was added to the above
assembly solution. Due to the hydrophobic interaction, the oil-
soluble monomer diffused to the middle hydrophobic part and
polymerized with BASM-2 in the 2D layer to obtain the covalent
2D polymers with small size. Finally, with the extension of the
polymerization time, the small polymer laminates were further
crosslinked to make the laminates grew up gradually and
obtained the 2D polymers with the transverse size of tens of
micrometers.
CONCLUSIONS
Two ionic BASMs, BASM-1 and BASM-2, were successfully
designed and synthesized, and supramolecular 2D planar
structures were obtained in pure water and organic
solvent/water, respectively, based on solution self-assembly.
Then, the self-assembly behavior and mechanism of two small
molecules in different solvents were studied in detail. The
results showed that BASM-1 directly self-assembled into
lamellar structure in pure water, while BASM-2 could only
self-assembled into regular supramolecular nanolayer in
FIGURE 6 AFM image and height profile of 2D polymers
(simulated monomer conformation giving
hcalc ≈ 2.26 nm).
[Color figure can be viewed at wileyonlinelibrary.com]
Next, we used atomic force microscopy (AFM) to characterize the
thickness of the polymers, as shown in Figure 6. The results
showed that theoretically simulated monolayer thickness was
2 2
DMF/H O = 1/1 and acetone/H O = 1/1 mixed solution.
2
.26 nm, and the measured experimentally thickness of polymer
Finally, the 2D polymers were obtained by the within-layer
crosslinking polymerization of BASM-2 nanosheets via Suzuki
coupling reaction. The formation of the polymerization reac-
tion was demonstrated by infrared spectroscopy, and the mor-
phology and thickness of the polymer sheets were
characterized by TEM and AFM. So our work provides a sim-
ple and efficient method for directly preparing independent
both supramolecular 2D polymers and covalent 2D polymers
in liquid phase which is of great significance.
sheet was about 2.65 nm. Considering the water layer on the sur-
face of the sample, the roughness of the substrate surface, the
repulsion between the substrate and the probe, and so forth, we
judged that the synthesized 2D polymer was single layer struc-
ture. Theoretically, electrostatic repulsion existed among poly-
mer layers, and 2D polymers with a single molecular thickness
4
3,44
could be obtained in aqueous solutions,
which was consis-
tent with the results. What is more, due to the large transverse
size (large specific surface area) of the polymer sheet layer, par-
tial accumulation was likely to occur during the sample
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of
China (grant no. 51803090), Natural Science Foundation of
Jiangsu (grant no. BK20181025), Introducing Talents Fund of
Nanjing Institute of Technology (YKJ201807 and YKJ201808)
for financial support.
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