Thermoresponsive Corannulene

Article abstract of DOI:10.1002/ejoc.201601405

We demonstrate herein that corannulene, a bowl-shaped polycyclic aromatic hydrocarbon, can assemble into larger supramolecular structures in water through a change in the solution temperature. This property is invoked through a simple five-fold symmetric

Full text of DOI:10.1002/ejoc.201601405

DOI: 10.1002/ejoc.201601405
Full Paper
Thermosensitive Bucky-Bowl
Thermoresponsive Corannulene
Surendra H. Mahadevegowda[a] and Mihaiela C. Stuparu*[a,b]
Abstract: We demonstrate herein that corannulene, a bowl- in solution, bright green luminescence can be observed in the
shaped polycyclic aromatic hydrocarbon, can assemble into fibrilar form. This unexpected and interesting behavior indicates
larger supramolecular structures in water through a change in that the corannulene nucleus presents a new motif for the de-
the solution temperature. This property is invoked through a sign of aggregation-induced emission (AIE) based luminogens.
simple five-fold symmetric substitution of the corannulene nu- To the best of our knowledge, this is the first report of a ther-
cleus with triethylene glycol units. Interestingly, an increase in moresponsive nonplanar polycyclic hydrocarbon derivative that
the solution temperature, which triggers the assembly process, can respond to a thermal trigger and assemble into larger emis-
also enhances the fluorescence emission properties of the as- sive structures.
sembled materials. Although the emission remains very weak
Introduction
temperature, they allow the molecule to dissolve in aqueous
media. However, an increase in temperature results in the en-
tropically driven removal of water molecules (an ordered state)
from the appended ethylene glycol chains and an overall reduc-
tion in water solubility of the material. Above a solution tem-
perature of 40 °C, the building block essentially becomes water-
insoluble and assembles to yield larger structures. Typically,
such a transition is a hallmark of thermosensitive polymers[22,23]
and very few small molecular motifs are known to emulate this
behavior.[24–26] Interestingly, during this work, we also observed
a weak enhancement of emission in solution upon thermally
triggered aggregation. In the fibrilar state, however, compara-
tively strong green luminescence could be seen. Therefore we
report herein on the thermal responsivity of a corannulene de-
rivative and the observation of aggregation-induced emis-
sion.[27,28] Both phenomena are of practical importance and
hitherto unknown in the realm of nonplanar aromatic hydro-
carbon chemistry.
Corannulene is a polycyclic aromatic hydrocarbon[1–3] with var-
ied structural[4–6] and electronic properties.[7–13] A glance at this
nonplanar molecule brings a stacking process to mind that
would ultimately result in the formation of larger structures,
such as a fiber, through supramolecular π–π interactions be-
tween aromatic bowls. In this context, however, only a few ex-
amples are known thus far. For example, Aida and co-workers
have established two different strategies for facilitating the
stacking process. One involves the substitution of the
corannulene nucleus with long alkyl chains and the other is
based upon hydrogen-bonding interactions between adjacent
assembly precursors.[14,15] Siegel and co-workers, on the other
hand, have shown that lipid modification or oligosaccharide
substitution of the corannulene scaffold can render the struc-
tures prone to aggregation into larger supramolecular assem-
blies.[16,17] Finally, Maeda and co-workers have also substituted
the corannulene unit with multiple alkyl chains to facilitate
aggregation.[18] None of these systems, however, respond to a
thermal stimulus. If achieved, such thermoresponsive behavior
could lead to the application of corannulene in the biomedical
arena.[19–21]
Results and Discussion
To synthesize the five-fold decorated corannulene, the synthetic
protocol developed by Siegel and co-workers was used.[29,30]
Initially, corannulene was treated with iodine chloride to access
1,3,5,7,9-pentachlorocorannulene. Subsequent modification of
this compound with thiolated triethylene glycol furnished the
targeted structure 1 in an isolated yield of 38 % (Scheme 1). 1H
and 13C NMR analyses confirmed the molecular structure (see
Figures S1–S3 in the Supporting Information).
In this report, we show that this can be achieved through
decoration of the corannulene nucleus with ethylene glycol
units. These units are polar and hydrophilic. Therefore, at room
[a] Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, 637371 Singapore, Singapore
E-mail: mstuparu@ntu.edu.sg
Having compound 1 in hand, first, its optical characteristics
were established. As can be seen in Figure 1, in solvents such
as methanol and acetonitrile, two major absorption bands lo-
cated at 250 and 335 nm can be observed. The absorption
coefficients (ε) of 1 at 335 nm were determined to be
http://www.mse.ntu.edu.sg/AboutUs/Organisation/Staff/Pages/
mstuparu.aspx
[b] School of Materials Science and Engineering, Nanyang Technological
University,
Nanyang Avenue, 639798 Singapore, Singapore
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201601405.
34000
M M
–1 cm–1 in methanol and 36000 –1 cm–1 in acetonitrile.
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Scheme 1. Synthesis of corannulene 1.
Figure 1. Absorption (top) and emission (bottom) spectra of 1 in water (blue
line), methanol (red line), and acetonitrile (black line) at room temperature.
In water, these absorption bands exhibit a significant decrease
in intensity (ε = 17000
M
–1 cm–1). The emission spectrum of 1
and above 40 °C the transmittance dropped suddenly as the
solution became opaque. Cooling the solution to room temper-
ature restored the optical clarity. Heating and cooling cycles
could be repeated many times, thereby indicating the reversibil-
ity of the thermally triggered assembly process (Figure 3).
upon excitation at 335 nm exhibits a similar trend; moderate
emission intensities are observed in methanol and acetonitrile,
whereas in water, a significant reduction in intensity is seen.
Next, dynamic light scattering (DLS) measurements were car-
ried out in water at room temperature (see Figure S4 in the
Supporting Information). The study showed that structures of
very small sizes (<5 nm) are present in the solution. This indi-
cates that intermolecular interactions at room temperature
must be limited to an oligomeric level (supramolecular dimers,
trimers, tetramers, etc. of 1). Moore and co-workers observed
similar behavior in the case of ethylene glycol appended phen-
ylacetylene macrocycles when dissolved in polar solvents.[31]
During the course of this study, we observed that heating,
originally intended to obtain a perfectly clear solution by assist-
ing the solubilization, as is common in synthetic laboratories,
had the opposite effect as the aqueous solution turned turbid
(Figure 2 and Figure S5 in the Supporting Information). This
observation revealed the sensitivity of the ethylene glycol ap-
pended corannulene 1 to changes in solution temperature. To
confirm this, temperature-dependent transmittance measure-
Figure 2. Digital pictures showing gradual changes in transmittance of an
aqueous solution (1 m
hotplate.
M) of 1 upon slowly increasing the temperature on a
The transmittance studies indicated conformational changes
ments were carried out (Figure 3). At room temperature, the of the molecule and the associated assembly process upon
aqueous solution remained clear. However, raising the tempera- thermal activation. This further motivated us to investigate the
ture led to a loss of the optical clarity of the aqueous solution, concomitant changes in the absorption and emission proper-
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of this, we believe that corannulene derivatives can be investi-
gated in the future as new aggregation-induced emission
probes.
Figure 4. Emission spectra of 1 in water in the temperature range 20–70 °C.
Figure 5. a) Bright-field, b) fluorescence excitation at 400 nm and collection
at 480–580 nm, and c) overlay of a and b confocal fluorescence microscopy
images (200 × 200 μm) obtained upon dropcasting an aqueous solution of 1
Figure 3. Temperature-dependent transmittance of an aqueous solution of 1
at 800 nm (top) and upon subjecting this solution to repeated heating and
cooling treatment (bottom).
(10 mM) maintained at 50 °C on a glass slide.
The aforementioned optical microscopy observation then
motivated us to investigate the morphology of the assembled
ties as a function of temperature. Interestingly, the UV/Vis ab- architectures. Initially, atomic force microscopy in tapping mode
sorption spectrum remained unchanged upon increasing the was employed to avoid any contact between the probe tip and
temperature of the aqueous solution of 1 (see Figure S6 in the the material. This study indicated that the heights of the fibers
Supporting Information). However, changes were observed in are in the range of 10–100 nm, while their lengths are in the
the emission spectra. Namely, the emission intensity increased micrometer range (see Figure S7 in the Supporting Informa-
with increasing temperature (Figure 4). The quantum yields for tion). Encouraged by these results, we studied a sample of 1 by
the fluorescence (ꢀf) of 1 at 20 and 70 °C were calculated to scanning electron microscopy (SEM) in low-energy ionization
be 0.4 and 1.2 %, respectively. This result suggests that the ther- (LEI) mode. The aqueous solution used for the SEM experiments
mally triggered stacking of the corannulene nucleus restricts were deliberately made higher in concentration (20 mM) than
molecular motion and leads to enhanced emission, as is known used so far and a slow evaporation of water was ensured so as
for thermoresponsive polymers capable of aggregation-induced to facilitate the aggregation process and observation of longer
emission.[32–36] Although the emission remained very weak, we structures. This change in experimental conditions indeed re-
believe that the underlying concept can be harnessed to de- sulted in the formation of uninterrupted supramolecular fibers
velop better emitters in solution. Nonetheless, inspired by these of hundreds of micrometers in length. The high density of these
observations, we dropcast an aqueous solution of 1 onto a glass fibers can be seen in Figure 6. Such a large fibrilar structure,
slide and maintained the temperature at 50 °C (Figure 5). formed upon thermal assembly of a nonplanar polycyclic aro-
Evaporation of the solvent left a solid network of fibers that matic hydrocarbon derivative, is truly unprecedented. The
emitted strongly when excited at 400 nm. This suggests that chemical composition of the fibers was then confirmed by us-
although in solution the enhancement of emission is limited, a ing energy-dispersive X-ray spectroscopy, which revealed the
significant response can be achieved in the solid state. In light presence of carbon, hydrogen, and sulfur atoms in the assem-
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Figure 6. SEM image of the self-assembled fibres from a thermoreponsive assembly of 1 (scale bar = 100 μm).
bled structure in a ratio of 7.6:1.7:0.6, which is in agreement
with the theoretically calculated ratio of 7.3:2:0.6 (see Figure S8
in the Supporting Information).
To further investigate the assembly process, variable-temper-
ature 1H NMR spectroscopy was employed using deuteriated
water as the solvent. At room temperature, the signal arising
from the corannulene nucleus can be seen at 6.7 ppm and
those of the ethylene oxide units can be seen in the region of
3–3.8 ppm. Upon increasing the temperature, the signals arising
from the aromatic and aliphatic protons broaden and move
downfield (Figure 7). The most pronounced shift appears above
the solution temperature of 40 °C. This suggests the thermally
triggered formation of larger supramolecular structures in
which the building block loses its mobility and is frozen. This is
in perfect agreement with previous studies on ethylene glycol
based thermoresponsive polymeric systems[36] and reflects the
thermal behavior, discussed in the previous sections, of the
present system. In deuteriated acetonitrile, a solvent in which
the thermally triggered assembly process cannot occur, minor
changes were observed in the peak positions and the signals
remained sharp upon changing the solution temperature (see
Figure S9 in the Supporting Information).
Figure 7. 1H NMR spectra of 1 in deuteriated water at different temperatures.
The range 3–4 ppm shows signals from protons of the ethylene glycol based
side-chains, and the signal in the range 6.6–8 ppm arises from the cor-
annulene core.
It is known that increasing the degree of polymerization can
modulate thermal behavior. To examine whether this strategy ures S10–S21 in the Supporting Information). Upon heating,
is valid for the corannulene-based ethylene glycol system, new however, no turbidity was observed in aqueous samples of 2
derivatives 2 and 3 were synthesized in isolated yields of 43 or 3 (Figure 9). In these systems, small oligomeric aggregates
and 52 %, respectively. The ethylene glycol segments in 2 and may exist, however, overall, the material remains soluble in wa-
3 each carry 7 and 16 units, respectively (Figure 8 and Fig- ter in the investigated temperature range of 10–100 °C.
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changes in a biologically pertinent temperature range along
with its emissive characteristics indicate the potential utility of
the presented nonplanar aromatic building block in biologically
relevant thermoresponsive materials.
Experimental Section
Synthesis of 1: Sodium pieces (231 mg, 10.03 mmol) in absolute
ethanol (5 mL) were stirred for 40 min and then 2-[2-(2-methoxy-
ethoxy)ethoxy]ethane-1-thiol (1.80 g, 10.03 mmol) was added drop-
wise and the reaction mixture stirred at room temperature. After
2 h, the volatiles were removed under reduced pressure and the
crude dried under vacuum for 2 h. The obtained salt was dissolved
in N,N′-dimethylethyleneurea (DMEU; 2 mL), and then 1,3,5,7,9-pen-
tachlorocorannulene (265 mg, 0.627 mmol) was added in one por-
tion and the reaction mixture stirred further at room temperature.
After 10 d, the reaction mixture was diluted with toluene (300 mL),
washed with water (3 × 200 mL), and the organic layer was washed
once with brine (100 mL), dried with Na2SO4, and concentrated.
SiO2 column chromatography of the obtained crude mixture using
50 % EtOAc in hexane and then 10 % MeOH in EtOAc as eluents
afforded the corannulene pentasulfide 1 (272 mg, 0.238 mmol,
38 %) as a brown solid. M.p. 48–49 °C. 1H NMR (400 MHz, CDCl3):
δ = 7.98 (s, 5 H), 3.79 (t, J = 6.7 Hz, 10 H), 3.60–3.65 (m), 3.49–3.51
(m), 3.33–3.36 (m) ppm. 13C NMR (100 MHz, CDCl3): δ = 136.1, 134.8,
131.4, 125.7, 71.9, 70.5, 70.5, 70.4, 69.7, 59.0, 34.5 ppm. IR (neat):
Figure 8. Chemical structures of corannulene derivatives 2 and 3 with longer
ethylene glycol segments attached to the corannulene core.
νmax = 3055, 2985, 2306, 1423, 1265 cm–1. HRMS (ESI): calcd. for
˜
C55H81O15S5 [M + H]+ 1141.4179; found 1141.4246.
Synthesis of PEG-Thiol (Total Number of Ethyleneoxy Units =
7): Et3N (15.9 mL, 114.28 mmol) and DMAP (140 mg, 1.14 mmol)
were added to a stirred solution of poly(ethylene glycol) mono-
methyl ether 350 (20 g, 57.14 mmol) in anhydrous CH2Cl2 (200 mL)
at room temp. The reaction mixture was then cooled to 0 °C and
p-toluenesulfonyl chloride (13.07 g, 68.57 mmol) was added por-
tionwise and then the reaction mixture was warmed to room temp.
After 48 h, the volatiles were evaporated under reduced pressure
and the obtained crude was purified by SiO2 column chromatogra-
phy to afford the viscous tosylate in a yield of 92 % (26.5 g,
52.55 mmol). 1H NMR (400 MHz, CDCl3): δ = 7.75 (d, J = 8.2 Hz, 2
H), 7.30 (d, J = 8.2 Hz, 2 H), 4.11 (br. t, J = 4.8 Hz, 2 H), 3.48–3.65
(m), 3.32 (s, 3 H, OMe), 2.40 (s, 3 H, Ar-Me) ppm. The obtained
tosylate (26.5 g, 52.45 mmol) was dissolved in absolute EtOH
(190 mL) and treated with thiourea (4.79 g, 62.94 mmol) for 10 h at
100 °C. Then the reaction mixture was brought to room temp. and
the volatiles evaporated under reduced pressure. Aqueous NaOH
solution (20 wt.-%, 200 mL) was added to the obtained liquid and
the reaction mixture heated at reflux for 5 h. Then the reaction
mixture was cooled with ice-cold water and acidified with 15 % HCl
to pH ≈ 2. The solution was extracted with EtOAc (300 mL), the
aqueous layer was saturated with NaCl and extracted further with
EtOAc (2 × 200 mL), the combined organic layers were washed once
with brine, dried with Na2SO4, and the solvent evaporated under
reduced pressure. The obtained crude was purified by SiO2 column
chromatography with 50 % EtOAc in hexane (v/v) to 10 % MeOH in
EtOAc as eluents to afford MeO-(ethyleneoxy)n-SH (10.8 g,
29.5 mmol, 52 %). 1H NMR (400 MHz, CDCl3): δ = 3.48–3.59 (m),
3.27–3.32 (m, 3 H), 2.62–2.66 (m, 2 H), 1.51–1.56 (m, 1 H, -SH) ppm.
13C NMR (100 MHz, CDCl3): δ = 72.6, 71.6, 70.3, 70.2, 69.9, 58.7,
Figure 9. Digital pictures showing aqueous solutions of 2 (left) and 3 (right)
at different temperatures.
Conclusions
Corannulene, when modified with chemically polar triethylene
glycol units, is soluble in water at room temperature. However,
increasing the temperature to above 40 °C causes turbidity and
assembly of the building blocks into larger structures. The as-
sembly process is completely reversible and can be carried out
a number of times with no negative effects on the properties
of the corannulene. Scanning electron and atomic force micros-
copy analyses indicate that the thermally triggered assembly
process produces large fibrilar structures of micrometer length.
Interestingly, the assembly process enhances the emission ca-
pability of the corannulene derivative. These results are remark-
able because corannulene and its derivatives typically show lit-
tle tendency to aggregate to form larger supramolecular struc-
24.0 ppm. IR (neat): νmax = 3529, 2870, 2553, 1967, 1639, 1458, 1350,
˜
1249, 1103 cm–1
.
Synthesis of PEG-Thiol (Total Number of Ethyleneoxy Units =
tures or to emit light. The triggering of structural and property 16): Poly(ethylene glycol) monomethyl ether 750 (20 g, 26.66 mmol)
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was treated with Et3N (5.39 g, 32.0 mmol), DMAP (65 mg,
0.533 mmol), and p-toluenesulfonyl chloride (6.1 g, 32 mmol) for
3 d to afford a crude tosylate. Then the crude mixture was purified
once with brine (100 mL), dried with Na2SO4, and concentrated.
SiO2 column chromatography of the obtained crude mixture with
1:1 (v/v) CHCl3/acetone and then with 2:2:1 (v/v/v) CHCl3/MeOH/
by SiO2 column chromatography with 1:1 hexane/toluene and then acetone as eluents afforded the corannulene pentasulfide 3 as a
1:3 (v/v) MeOH/toluene as eluents to afford the pure tosylate
brown viscous liquid (620 mg, 0.152 mmol, 52 %). 1H NMR
(21.69 g, 23.99 mmol, 90 %). 1H NMR (400 MHz, CDCl3): δ = 7.73 (d, (400 MHz, CDCl3): δ = 7.94 (s, 5 H), 3.75 (t, J = 6.8 Hz, 10 H), 3.49–
J = 8.3 Hz, 2 H), 7.28 (d, J = 8.1 Hz), 4.09 (br. t, J = 4.8 Hz, 2 H),
3.66 (m), 3.28–3.33 (m) ppm. 13C NMR (100 MHz, CDCl3): δ = 136.1,
3.47–3.63 (m, OMe), 3.31 (s, 3 H), 2.39 (s, 3 H, Ar-Me) ppm. 13C NMR 134.7, 131.3, 125.6, 71.8, 70.4, 70.3, 69.6, 58.8, 34.4 ppm.
(100 MHz, CDCl3): δ = 144.5, 132.8, 129.6, 127.7, 71.7, 70.5, 70.3,
70.2, 69.0, 68.4, 58.8, 21.4 ppm. IR (neat): νmax = 2873, 2746, 2349,
˜
1647, 1597, 1454, 1350, 1249, 1172, 1103 cm–1. The obtained tosyl-
ate (21.6 g, 23.88 mmol) was dissolved in absolute EtOH (120 mL)
Acknowledgments
and treated with thiourea (2.18 g, 28.65 mmol) for 8 h at 100 °C.
Financial support from NTU-Singapore (M4081566 and
Then the reaction mixture was cooled to room temp. and the sol-
M4011547 grants) is gratefully acknowledged.
vent removed under reduced pressure. An aqueous NaOH solution
(20 wt.-%, 120 mL) was added to the obtained liquid and the reac-
tion mixture heated at reflux for 5 h. Then the reaction mixture was
Keywords: Fused-ring systems · Corannulene · Aggregation ·
Supramolecular chemistry · Self-assembly · Thermosensitivity
cooled with ice-cold water and acidified with 15 % HCl to pH ≈ 2.
The solution was extracted with EtOAc (250 mL), the aqueous layer
was saturated with NaCl and extracted further with EtOAc (2 ×
250 mL). The combined organic layers were washed once with
brine, dried with Na2SO4, and the solvent removed under reduced
pressure. The obtained crude was purified by SiO2 column chroma-
tography with 100 % toluene to 50 % EtOH in toluene as eluents to
afford MeO-(ethyleneoxy)n-SH (12.5 g, 16.31 mmol, 61 %). 1H NMR
(500 MHz, CDCl3): δ = 3.59–3.64 (m), 3.52–3.54 (m, 2 H), 3.36 (s, 3
H, OMe), 2.66–2.70 (m, 2 H), 1.58 (t, J = 8.2 Hz, 1 H, SH) ppm. 13C
NMR (100 MHz, CDCl3): δ = 72.4, 71.4, 70.2, 70.1, 70.0, 69.7, 58.5,
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Synthesis of 2: Sodium pieces (169 mg, 7.34 mmol) in absolute
ethanol (2.5 mL) were stirred for 40 min and then poly(ethylene
glycol) methyl ether thiol (ethyleneoxy units = 7; 2.68 g, 7.34 mmol)
was added dropwise and the solution stirred at room temperature.
After 2 h, the volatiles were removed under reduced pressure and
the residue dried under vacuum for 2 h. The obtained sodium salt
was dissolved in DMEU (1.5 mL), and then 1,3,5,7,9-pentachloro-
corannulene (194 mg, 0.459 mmol) was added in one portion and
the reaction mixture stirred further at room temperature. After 10 d,
the reaction mixture was diluted with EtOAc (200 mL), water
(200 mL) was added, and the aqueous layer was saturated with
NaCl and further extracted with EtOAc (2 × 200 mL). The combined
organic layers were washed once with brine (100 mL), dried with
Na2SO4, and concentrated. SiO2 column chromatography of the ob-
tained crude mixture with 1:1 (v/v) toluene/acetone to 4:1 (v/v) tolu-
ene/MeOH as eluents afforded the corannulene pentasulfide 2 as a
brown liquid 400 mg, 0.197 mmol, 43 %). 1H NMR (400 MHz, CDCl3):
δ = 7.98 (s, 5 H), 3.79 (t, J = 6.8 Hz, 10 H), 3.59–3.65 (m), 3.52–3.55
(m, 16 H), 3.32–3.37 (m) ppm. 13C NMR (100 MHz, CDCl3): δ = 136.03,
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