Induced chirality sensing through formation and aggregation of the chiral imines double winged with pyrenes or perylenes

Article abstract of DOI:10.1039/c8cc03660h

Reaction of chiral amines with benzaldehydes 3,5-disubstituted by two pyrenes or perylenes afforded corresponding double winged chiral imines, which aggregated to show significantly enhanced circular dichroism spectra at the transition bands of the chromo

Full text of DOI:10.1039/c8cc03660h

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Induced Chirality Sensing through Formation and Aggregation of
the Chiral Imines Double Winged with Pyrenes or Perylenes
Jianlin Wu,a Wenting Liang,b Tong Niu,a Wanhua Wu,a* Dayang Zhou,c Chunying Fan,a Jiecheng Ji,a
Guowei Gao,a* Jian Men,a Yonggang Yang,d and Cheng Yanga*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Reaction of chiral amines with benzaldehydes 3,5-disubstituted Further, chiral supramolecular assembly often leads to highly
by two pyrenes or perylenes afforded corresponding double responsive exciton coupling CD (ECCD)6 due to the electronic
winged chiral imines, which aggregated to show significantly coupling amongst proximal, asymmetrically oriented
enhanced circular dichroism spectra at the transition bands of the chromophores.7
chromophores in the mixture solutions of DMF and H2O.
Imine formation has recently been developed as an
attractive strategy for sensing of chiral amines.8 Sophisticated
designed aromatic aldehydes, such as dialdehyde
chromophores9 or aldehydes bearing coordination-sites,10
have been used for sensing chiral amines by inducing ECCD of
the chromophores. On the other hand, chiral aggregation of
chromophores often leads to strong ECCD signals.6,7 We
envision that proper combination of the imine formation and
chiral aggregation could lead to a convenient and highly
responsive ECCD-based chirality sensing. Herein, we report the
successful application of the strategy for chirality sensing of
amines.
The rapid emergence of chiral compounds produced by
asymmetric synthesis or from natural origin put forward the
requirement for high throughput detection of the absolute
configuration and optical purity of chiral products.1 Circular
dichroism (CD) is regarded as one of the most versatile and
widely-utilized chiroptical techniques,2 possessing the
advantage of directly differentiating molecular/supramolecular
chirality without the assistance of other chiral substances.
However, weak CD signals were often encountered with chiral
organic compounds.3 Moreover, most chiral molecules contain
no chromophore and have absorption only in the far or deep
UV region, which makes CD detection susceptible to
absorption interference from solvents or non-target
compounds. Supramolecular chirogenesis represents a viable
and promising approach for chiral differentiation,4 and we
have demonstrated that supramolecular self-assembly could
cause significant chiral recognition and photochirogenesis.5
Through supramolecular complexation, the chiral properties of
analytes could be efficiently transferred to a reporter to allow
for CD detection at the transition bands of the reporter.
CHO
CHO
1
2
OH
O
O
NH2
OH
OH
OH
OH
NH2
NH2
NH2
A1
A2
A3
A4
H
O
O
N
NH2
OH
S
OH
NH2
OH
NH2
O
A7
A6
A5
NH2
NH2
NH2
A10
and chiral amines A1-A10.
A9
Scheme 1. Double winged sensors
A8
1
and
2
We designed aromatic aldehydes
1 and 2 (Scheme 1), in
which two pyrenes or perylenes were substituted to the
central benzaldehyde for the purpose to facilitate the
chromophore aggregation. Ten chiral reagents, including chiral
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c
a
b
% H2O in DMF
% H O in DMF
2
5x106
4x106
3x106
2x106
1x106
0
6
4
2.0
1.5
1.0
0.5
0.0
% H2O in DMF
0%
0%
0%
20%
40%
50%
60%
70%
80%
20%
40%
50%
60%
70%
80%
20%
40%
50%
60%
70%
80%
2
ε
0
300
400
500
nm
600
Wavelength
/
400 450 500 550 600 650 700
Wavelength / nm
300
350
400
450
500
-2
Wavelength / nm
f
% H2O
d
e
0
20 40 50 55 60 70 80
12
8
4
0
0
500 1000 1500 2000 2500
Diameter / nm
Fig 1. Water content-dependent (a) UV−vis, (b) fluorescence and (c) CD spectra of the chiral imine (40
(upper) and emission (lower) of the chiral imine in DMF solution having various water contents. (e) SEM and (f) DLS measurements of the aggregated imine obtained
ꢁ
M) formed by reacting
1
with (S)-A6. (d) Light scattering
from the mixture of DMF/H2O (1:1 v/v)
.
formed by reacting 1 with (S)-A6, were only slightly changed in
(A1-A7) or without (A8-A10) an additional 20% water relative to that in pure DMF, but significantly
amines with
functional group, were selected as the analytes. The double deceased and broadened when the water content was
winged benzaldehydes were synthesized by the Pd-catalyzed increased to 40%. This suggested an aggregation of the chiral
Sonogashira cross-coupling reaction by reacting 3,5- imine which should be driven mainly by the hydrophobic
bisethynylbenzaldehyde
bromoperylene, respectively, and were characterized by NMR
and HRMS.11 The UV-vis spectra of
and were apparently
with 1-bromopyrene or 1-
interaction and the π−π stacking. The UV-vis baseline
apparently raised in 40% water, indicative of an aggregation
that caused scattering of light. Similar UV-vis spectrum was
seen in 50% water.
1
2
bathochromic shifted comparing with pyrene and perylene (Fig.
S8 and S10) 11 to show strong absorption in the visible range.
Both 1 and 2 readily formed imines by reacting with chiral
Slightly unexpected, further increasing the water contents
led to less broad absorption spectra with dropped baseline,
implying a decline of aggregation sizes. Such a water content-
dependent aggregation was confirmed by Tyndall scattering
experiments (Figure 1d), as strong scattering was observed in
solutions containing 40% and 50% water, which however
reduced in solutions containing more water. In pure DMF, a
broad fluorescence spectrum (Fig. 1b, black line) was observed,
which could be assigned as a mixture of the monomer and
excimer fluorescence. Adding water led to a fluorescence
quenching firstly, but then an increase of the fluorescence at
the longer wavelength when further increasing water content
(Fig. 1b, 1d). Interestingly, the chiral imine showed strong CD
signal in 40% and 50% water, but only weak or negligible CD
spectra in the solutions with other water contents (Fig. 1c).
All above results demonstrated that the aggregating sizes
and models of the chiral imine changed with the solvent
1
amines, and the H NMR tracing experiment indicated that the
aldehyde hydrogen proton completely disappeared within 30
min after the addition of an amine (Fig. S12).11 However, all
imines thus formed in DMF showed negligible CD signals in the
S0-S1 transition of both
1 and 2. The CD silence is presumably
due to two reasons: There is no driving force to fix the
chromophore groups in a chiral fashion. Also, the chiral groups
are far from the pyrene or perylene units and cannot strongly
disturb the transition of chromophores.
In order to verify the hypothesis of aggregation-induced
chirality sensing, we examined the solvent-induced
aggregation behaviour of the chiral imines by adding water
into the DMF solution of chiral imines. It turned out that the
aggregation was highly water content-dependent. As
illustrated in the Fig. 1a, the UV-vis spectra of chiral imine,
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Dotted and solid lines represent the CD spectra measured in DMF and in DMF /
H2O (1:1, v/v), respectively.
compositions. The strong scattering observed in 40-50% water
suggested relatively large aggregation sizes. Indeed, scanning
electron microscopy (SEM) study revealed that the aggregation
formed in 50% water led to nanoparticles with an average
diameter about 400 nm (Fig. 1e), which is consistent with the
dynamic light scattering (DLS) analyses (Fig. 1f). It is not yet
clear the mechanism for the solvent composition-dependent
aggregation changing but seems plausible due to a switching
of the solvent structure with the water contents. This is
partially supported by the fact that very similar solvent
composition-dependent aggregations were seen with the
We therefore selected DMF/H2O (1:1, v/v) as the solvent
for the CD sensing of chiral amines. CD measurements
demonstrated strong signal responses for all chiral amines
examined, which is in opposite to the CD-silence in pure DMF
(Fig. 2 and Fig. S20-S35). As shown in Fig. 2a, positive CD
signals peaked at 421 nm, 393 nm and 302 nm were seen for
the (R)-A1, while (S)-A1 showed completely opposite CD
spectrum, suggesting an efficient enantiodifferentiating
sensing. Very strong CD spectra were also observed with the
imine formed by 2 and (S)-A6, which showed unique spectral
response in the water content range of 40% to 60% (Fig. S13-
S19).11
perylene-based sensor 2. As exemplified in Fig. 2c and 2d,
almost no CD signal could be seen from the chiral imines in the
homogenous phase of DMF. However, significantly enhanced
CD signals were observed at the perlyene’s La band, with CD
1
spectra of the (R)- and (S)-amines exhibiting totally mirror
image, and the deviation from the perfect reflection is mainly
due to the deviation in controlling the imine formation and
20
10
0
aggregation
condition.
Such
a
strong
and
enantiodifferentiating CD response clearly demonstrated the
feasibility of the present system for sensing of chiral amines.
Moreover, the fine CD spectral properties were chiral guest-
300
350
400
450
λ / nm
-10
-20
dependent. For the sensing with
1 as an example, A1 gave a
CD maximum at the wavelength of 393 nm with a shoulder at
421 nm (Fig 2a), while A5 offered a maximum at 423 nm (Fig
2b). Very different relative peak intensities were also observed
with other chiral amines and with the sensor 2 (Fig. 2c, 2d). We
15
10
5
attribute the strong CD induced by aggregation to the exciton
coupling effect, which usually occur with chirally arranged
chromophores. The sensor 1 did not show typical bisignate
ECCD signals (Fig 2a,2b), which is presumably due to the
multiple coupling from different transitions among the closely
oriented chromophores that may lead to mutual cancellation
of opposite CD signals. The ECCD spectra are highly sensitive to
the stacking geometry of chromophores, the difference on the
shape and intensity implies a non-identical stacking structure
caused by different chiral amines. Adding achiral amine lead to
the decrease of the CD intensity (Fig.S36), suggesting the
chirality sensing is sensitive to amino impurity.
0
300
350
400
450
500
λ / nm
-5
-10
-15
30
20
10
0
300
350
400
450
500
λ / nm
-10
-20
-30
20
10
0
Scheme 2. The mechanism for the aggregation-induced chirality sensing.
We propose that the chiral stacking of the pyrene and
perylene groups should be responsible for the significant CD
response upon forming aggregations. As shown in scheme 2,
for non-aggregated chiral imines, the chiral moieties are non-
fixed and far from the chromophores, and therefore cannot
essentially disturb the optical properties of the chromophores.
However, when aggregation is formed by adding water, the
rotation of the chiral imine moiety is prohibited. Since simple
chiral amines A8-A10 exhibited strong CD response, the steric
interaction should play a main role in chirality transfer.
300
350
400
450
500
-10
λ / nm
-20
Fig. 2 CD spectra of 50
ꢁ
M chiral imines formed by (a)
1
with A1, (b) 1 with A5,
(c)
the CD spectra of chiral imines formed with (R)- and (S)- amines, respectively.
2
with A6 and (d)
2
with A7, measured at 25 °C. Red and black lines represent
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However, we do not rule out the possible effect from the and opens a new and promising avenue for sensing the
hydroxyl and carboxyl groups of A1-A7 in assisting the absolute configuration and enantiomeric composition of chiral
aggregation formation and chirality transfer. The chiral steric amines.
interaction will lead to staggered stacking of the
We acknowledge the support of this work by the National
chromophores to induce helical structure. In this way, the Natural Science Foundation of China (No. 21572142, 21372165,
chirality of the imine substituents was efficiently transferred to 21402129 and 21402110), National Key Research and
the helical stacking of chromophores to cause significantly Development Program of China (No. 2017YFA0505900),
improved ECCD response. The detailed analyses as for the Science
& Technology Department of Sichuan Province
correlation between the aggregated structure of the chiral (2017SZ0021), State Key Laboratory of Fine Chemicals (KF
imines and CD responses through theoretical calculation12 are 1508), Comprehensive Training Platform of Specialized
in progress.
Laboratory, College of Chemistry and Analytical & Testing
The excellent enantiomeric differentiation observed with Center, Sichuan University.
the present system encouraged us to explore its possible
application for measuring enantiomeric composition of chiral
amines. The CD spectra of solutions spanning the enantiomeric
composition between 100% S configuration and 100% R
Notes and references
1
(a) Z. Chen, Q. Wang, X. Wu, Z. Li and Y.-B. Jiang, Chem. Soc.
Rev., 2015, 44, 4249-4263. (b) L. You, D. Zha and E. V. Anslyn,
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configuration were measured at 25 °C. As shown in Fig. 3, the
CD intensity of imine formed by A5 was strongly dependent on
enantiomeric compositions. A calibration curve was built from
the CD amplitudes at 420 nm vs the enantiomeric excess (Fig. 3,
inset), which revealed a perfect linear relationship (R2 = 0.997),
2
(a) N. Berova, K. Nakanishi and R. Woody, Circular dichroism:
principles and applications, John Wiley & Sons, 2000. (b) V. V.
Borovkov, T. Harada, G. A. Hembury, Y. Inoue, R. Kuroda,
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Borovkov, T. Harada, Y. Inoue, and R. Kuroda, Angew. Chem.
Int. Ed. 2002, 41, 1378 –1381
Good linear relationships were also observed for A1, A3 and
A4 (Fig. S37-39). The CD signals of 5 unknown samples of
varying ee for each chiral amine were recorded to investigate
the sensing accuracy towards unknown samples, and the
corresponding calibration curves were used to calculate the
ee’s (Table S1). The average absolute errors of sensors towards
the analytes were determined to be below 3%, demonstrating
a potential for using the aggregation system for sensing the
enantiomeric composition. For practical chirality sensing with
the present system, precisely controlling the solvent
composition and reducing the interference from impurity
should be important.
3
4
S. F. Mason, Molecular optical activity and the chiral
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20
20
15
10
5
15
10
5
100% (R)-A5
0
-100
-50
0
50
100
ee%
-5
-10
-15
-20
0
300
350
400
450
500
550
-5
λ / nm
6
7
-10
-15
-20
100% (S)-A5
Fig. 3 CD spectra of the imine (30
ꢁ
different ee’s, measured in DMF/H2O (1:1, v/v) 25
M) formed by reaction of
1
and A5 of
(a) H. Miyaji, S. J. Hong, S. D. Jeong, D. W. Yoon, H. K. Na, J.
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°C. Inset: Plot of the
ellipticity at 420 nm as a function of %ee.
In summary, we established a convenient and highly
efficient supramolecular chirality sensing strategy through a
two-step chirality transition process: The double-winged
benzaldehydes were reacted with chiral amines to give
corresponding chiral imines, which formed chiral
8
9
(a) K. Wen, S. Yu, Z. Huang, L. Chen, M. Xiao, X. Yu and L. Pu,
J. Am. Chem. Soc., 2015, 137, 4517-4524. (b) L. A. Joyce, E. C.
Sherer and C. J. Welch, Chem. Sci., 2014, 5, 2855-2861.
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,
supramolecular assemblies through
a
solvent-induced
10 J. M. Dragna, G. Pescitelli, L. Tran, V. M. Lynch, E. V. Anslyn
and L. Di Bari, J. Am. Chem. Soc., 2012, 134, 4398-4407.
11 For details, see the electronic supplementary information.
12 G. Pescitelli, T. Bruhn, Chirality, 2016, 28, 466–474.
aggregation. The non-aggregated chiral imines were CD-silent,
while aggregated chiral imines showed highly strong CD
responses due to the chiral steric interaction amongst chiral
substituents in the aggregation. This work represents the first
implementation of induced chiroptical sensing by combining
the chiral imine formation and solvent-induced aggregation
4 | J. Name., 2012, 00, 1-3
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