Self-assembly of chiral propeller-like supermolecules with unusual "sergeants-and-soldiers" and "majority-rules" effects

Article abstract of DOI:10.1002/asia.201301522

Chiral amplification is an interesting phenomenon in supramolecular chemistry mainly observed in complicated systems in which cooperative effect dominate. Herein, chiral, supramolecular, propeller-like architectures have been constructed through coassembl

Full text of DOI:10.1002/asia.201301522

COMMUNICATION
DOI: 10.1002/asia.201301522
Self-Assembly of Chiral Propeller-like Supermolecules with Unusual
“Sergeants-and-Soldiers” and “Majority-Rules” Effects
Bo Nie,[a, b] Tian-Guang Zhan,[b] Tian-You Zhou,[b] Ze-Yun Xiao,[b]
Guo-Fang Jiang,*[a] and Xin Zhao*[b]
Abstract: Chiral amplification is an interesting phenomenon
in supramolecular chemistry mainly observed in complicated
systems in which cooperative effect dominate. Herein,
chiral, supramolecular, propeller-like architectures have
been constructed through coassembly of an achiral disk-
shaped molecule and chiral amino acid derivatives driven by
intermolecular hydrogen bonding. Both the “sergeants-and-
soldiers” principle and “majority-rules” effect are applicable
in these discrete four-component supermolecules, which are
the simplest supramolecular system ever reported that ex-
hibit chiral amplification.
latter, the chirality of a system is determined by which
isomer is present in a slight excess in a pair of enantiomers.
These two principles were initially observed in the construc-
tion of chiral macromolecules[4] and have lately been ex-
tended to supramolecular polymers.[5] It was suggested that
such phenomena arose from the helix reversal penalty and
mismatch penalty,[6] which could be magnified in systems
consisting of a large number of building blocks, such as poly-
meric systems and extended macro/nanoscale aggregates.[7]
However, in simple discrete systems, the total energy penal-
ty might be too small to achieve chiral amplification due to
the lack of a cooperative effect. For this type of self-assem-
bled architecture, effective chiral amplification was only ob-
served in a few nine- and fifteen-component hydrogen-
bonded capsules developed by Reinhoudt et al.[8] We report
herein an unusual chiral amplification phenomenon in an
even simpler four-component supramolecular system con-
structed by the coassembly of achiral hexa-2-pyridyl-hexaa-
zatriphenylene (HPHAT), which is a hexaazatriphenylene
(HAT) derivative developed previously by us,[9] with amino
acid derivatives 1–3 (Scheme 1). Both sergeants-and-soldiers
and majority-rules effects are applicable to this simple
system. To the best of our knowledge, this is the simplest
The construction of chiral supramolecular architectures is
one of the central themes of self-assembly because it is not
only crucial to the understanding of chiral phenomena in
biological systems, but is also important for the rational
design of catalysts for asymmetric catalysis.[1] Direct incor-
poration of stereogenic centers into supramolecular tectons
is undoubtedly the most direct approach for the construction
of chiral supramolecular systems.[2] However, this method
sometimes suffers from tedious and time-consuming organic
synthesis. In this context, induction of supramolecular chiral-
ity, that is, the construction of chiral supramolecular archi-
tectures by coassembly of achiral tectons with chiral induc-
ers, has been widely exploited.[3] In the process of chiral
transfer from chiral inducers to achiral tectons, amplification
of chirality might occur. Amplification of chirality includes
two aspects: the sergeants-and-soldiers principle and the
majority-rules effect. In the former, the chirality of a system
is controlled by a few chiral units, the “sergeants”, whereas
the major analogues (the “soldiers”) are achiral. In the
[a] B. Nie, Prof. G.-F. Jiang
State Key Lab of Chemo/Biosensing and Chemometrics
College of Chemistry and Chemical Engineering
Hunan University, Changsha 410082 (P.R. China)
E-mail: gfjiang@hnu.edu.cn
[b] B. Nie, T.-G. Zhan, T.-Y. Zhou, Dr. Z.-Y. Xiao, Prof. X. Zhao
Laboratory of Materials Science
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (P.R. China)
E-mail: xzhao@mail.sioc.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301522.
Scheme 1. Chemical structures of HPHAT and amino acid derivatives 1–
3.
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Guo-Fang Jiang, Xin Zhao et al.
for the structures of the complexes). This might be attribut-
ed to steric hindrance generated by the secondary ammoni-
um of the amino acid derivatives. The first evidence was ob-
tained from a UV/Vis dilution experiment. It revealed that
no red- or blueshift of the absorption peaks of the HPHAT–
(d-1) complex occurred and the absorbance of the complex
obeyed the Beer–Lambert law if its concentration was dilut-
ed from 0.1 to 0.01 mm (Figure S6 in the Supporting Infor-
mation); this suggested that no aggregation occurred in solu-
tion over this range of concentration. Furthermore, almost
the same change in the UV/Vis spectrum of HPHAT was
observed when it bound to chiral salts 1 and 2, which con-
tained different side chains. If the resulting HPHAT–ammo-
nium salt complexes further aggregated, aggregates with dis-
similar structures should form as a result of different steric
hindrance of the side chains of the two chiral amino acid de-
rivatives, which should lead to different changes in the UV/
Vis absorption of HPHAT. This result further confirmed the
existence of discrete complexes in dilute solution. The for-
mation of discrete supermolecules without further aggrega-
tion in solution was also confirmed by dynamic light scatter-
ing (DLS) experiments, which gave a hydrodynamic diame-
ter (Dh) of 2.2 nm for a solution of the 1:3 mixture of
HPHAT–(d-1) at 0.1 mm (Figure S7 in the Supporting Infor-
mation); this corresponded to the size of the expected single
complex estimated from the crystal structure of HPHAT
(the diameter of HPHAT is 1.7 nm).[9a] ESI-MS results for
a solution of HPHAT and l-1 (1:3) in dichloromethane dis-
played strong peaks at m/z 1979.8 and 1829.7, which corre-
Figure 1. 1H NMR titration spectra of HPHAT with d-1 in CDCl3 at
258C. The concentration of HPHAT was 5ꢁ10À3 m.
supramolecular system ever reported in which these two ef-
fects are operational.
The coassembly behavior of HPHAT and amino acid
ester triflate salts 1–3 was first investigated through
1H NMR titration experiments (Figure 1 and Figures S1 and
S2 in the Supporting Information). Upon the incremental
addition of d-1, the signals of H-5 and H-6 of the pyridyl
unit of HPHAT exhibited considerable downfield shifts; this
suggested the formation of intermolecular hydrogen bonds
between the pyridine nitrogen atom of HPHAT and the
three hydrogen atoms of the ammonium salt. Only a very
small shift was exhibited after approximately three equiva-
lents of the salts were added; this suggested that the bond-
ing was saturated by the formation of a complex with 1:3
stoichiometry. The formation of intermolecular hydrogen
bonds was also evidenced by IR spectroscopy, which showed
a dramatic attenuation of the C=N stretching vibration of
pyridyls and sharpening of the vibration of an ammonium
group after HPHAT was mixed with d-1 (1:3; Figure S3 in
the Supporting Information). The coassembly behavior was
also studied by UV/Vis titration experiments. Upon increas-
ing the amount of amino acid ester salts, the UV/Vis absorp-
tion of HPHAT decreased and exhibited a redshift (Fig-
ure S4 in the Supporting Information). The UV/Vis titration
experiments also showed saturation of complexation after
the addition of three equivalents of the salts, which again
suggested a 1:3 binding model for HPHAT and the amino
acid ester salts. On the basis of the UV/Vis titration data,
the apparent association constants between HPHAT and
compounds 1–3 were estimated to be 1.3 (Æ0.31)ꢁ104, 3.3
(Æ0.54)ꢁ104, and 1.7 (Æ0.20)ꢁ104 mÀ1 (Figure S5 in the
Supporting Information),[10] respectively, and suggested re-
markably high stability of the complexes.
sponded to the exact masses of complexes [HPHAT+3
N
1)+H+]+ and [HPHAT+3
G
À
this again corroborated the existence of a discrete 1:3
HPHAT–(l-1) complex (Figure S8 in the Supporting Infor-
mation).
The crystal structure of HPHAT revealed that the six pyr-
idyl units were not coplanar, but twisted out of the plane of
the HAT core as a result of steric hindrance between the hy-
drogen atoms at the 3-position; this gave HPHAT a propel-
ler-like conformation.[9a] The whole HPHAT molecule is
À
achiral as a result of rotation of the single C C bond that
connects the pyridyl units and the HAT core. However,
once bound to a chiral ammonium salt, a spatially asymmet-
ric environment might be created by fixing the twisted ori-
entation of the pyridyl units in a certain direction, which
therefore induces the formation of chiral complexes. Circu-
lar dichroism (CD) spectroscopy was employed to investi-
gate this possibility. The CD spectrum of HPHAT was re-
corded in dichloromethane and was found to be CD silent,
which was consistent with its achiral character. However,
upon mixing HPHAT with l-1, the mixture displayed a posi-
tive Cotton effect in the range of l=280–400 nm (Figure 2).
A mirror-symmetry CD signal was observed when d-1 was
used (Figure 2). Adding compounds l-2/d-2 to a solution of
HPHAT in dichloromethane led to a similar result (Fig-
ure S9 in the Supporting Information). Because compounds
1 and 2 did not exhibit UV/Vis absorption in this range, the
origin of the CD signals could be attributed to the induced
We previously found that the complex formed by the
coassembly of HPHAT and n-dodecyl ammonium triflate at
high concentration (10 mm) could further aggregate into fine
microbelts.[9a] However, in this system, spectroscopic results
suggested that no higher-order aggregates except discrete
1:3 complexes existed in dilute solutions (see Figure 3 below
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Guo-Fang Jiang, Xin Zhao et al.
Figure 2. CD spectra of the mixtures of HPHAT and l-1/d-1 (1:3) in
CH2Cl2 at 258C. The concentration of HPHAT was 4ꢁ10À5 m. The spectra
did not exhibit full mirror symmetry, which could be attributed to the ex-
cursion of the signals and instrumental errors. c HPHAT and l-1.
b HPHAT and d-1.
circular dichroism (ICD) of HPHAT, which should result
from the adoption of a certain asymmetric conformation of
the pyridyl units of HPHAT. As shown in Figure 3, achiral
HPHAT was induced to form one pair of enantiomeric com-
plexes after binding with l- and d-amino acid derivatives.
DFT calculations were performed to understand the origin
of the chiral structures. The results clearly revealed that the
six pyridyl units adopted M-helicity in the optimized geome-
try of the complex HPHAT–l-amino acid derivative (Fig-
ure S10 in the Supporting Information). Any other confor-
mations will result in steric hindrance between the substitu-
Figure 4. a) CD spectra of mixtures of HPHAT and (1+3) with variation
of the molar fraction of l-1 (positive region) and d-1 (negative region) in
CH2Cl2 at 258C. The molar ratio for HPHAT and the total amino acid
ester salts was 1:3 and the concentration of HPHAT was 4ꢁ10À5 m.
b) Plot of CD intensity at l=315 nm versus different molar fractions of
~
&
1 in the mixtures of 1 and 3. l-1. d-1.
1 as sergeants. As shown in
Figure 4, no Cotton effect was
detected for mixture of
a
HPHAT and 3. However, a solu-
tion of HPHAT and mixture of
l-1 (5%) and 3 (95%) exhibit
a CD signal that is similar in
shape to that of a mixture of
HPHAT and pure l-1 and its in-
tensity reached about one fifth
of that of the latter. Further-
more, upon increasing the con-
tent of l-1 in the mixture of l-
1 and 3, the intensity of the CD
signals first increased and then
became saturated when the
molar fraction of l-1 reached
40%, which indicated that the
sergeants-and-soldiers principle
Figure 3. Schematic illustration of the enantiomeric complexes of HPHAT–(l-1) and HPHAT–(d-1).
ent group on the chiral center of the amino acid derivative
and the pyridyls of HPHAT, which will destabilize the com-
plexes. As a result, asymmetric preference arises.
The sergeants-and-soldiers principle was then examined
for these propeller-like supermolecules. Glycine ester triflate
salt 3 was used as an achiral component (soldiers) and l-
was operational in this system. A sergeants-and-soldiers
principle experiment was also performed by using d-1 as the
sergeant and a similar phenomenon was observed (Fig-
ure 4a, negative region). The operation of the sergeants-
and-soldiers principle in this system was clearly demonstrat-
ed by plotting the CD intensity against the molar fraction of
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Guo-Fang Jiang, Xin Zhao et al.
chiral 1 (Figure 4b), which displayed a strong nonlinear de-
pendence.
ther relayed to the next pyridyl unit through the complexa-
tion of the ammonium salt. As a result, once one chiral spe-
cies bound to the two pyridyl units of one hydrogen-bonding
pocket, the chiral information was transferred from the two
pyridyl units to all other pyridyl units, although they were
hydrogen bonded to an achiral glycine derivative. In this
way, chiral transfer and amplification arose. This mecha-
nism, as illustrated in Figure 6, could explain the experimen-
The majority-rules effect was also examined to see if it
was applicable to this system. Mixtures of l-1 and d-1 with
different enantiomeric excess (ee) were used for this pur-
pose. As expected, no Cotton effect was observed at an ee
of 0%. However, a positive Cotton effect, which was similar
in shape to that of a mixture of HPHAT and pure l-1, was
observed when an ee of only 5% was reached for l-1, al-
though its intensity was just one fifth of the saturated one
(that of HPHAT and pure l-1; Figure 5a, positive region).
Figure 6. Schematic illustration of the origin of the sergeants-and-soldiers
principle on the basis of the transfer of chirality from the d-phenylala-
nine derivative to the other part of the complex.
tal results that the intensity of the CD signal becomes satu-
rated when the molar fraction of the sergeant reached 40%:
statistically 33.3% of sergeant is required to ensure that at
least one complex has one sergeant. For the origin of the
majority-rules effect, if l-1 is in excess in the mixture of l-
1 and d-1, the complex that has at least two molecules of l-
1 in the three hydrogen-bonding pockets of HPHAT will
dominate in solution. The chirality of the whole complex
should be dictated by the two molecules of l-1 that are hy-
drogen bonded to four pyridyl units, not by d-1 bound to
the other two pyridyl units. The chiral information will also
be transferred from the four pyridyl units to the other two
through the steric repulsion of the adjacent pyridyl units.
In summary, discrete chiral propeller-like supermolecules
have been constructed through the coassembly of achiral
disc-like molecules and chiral amino acid derivatives.
Thanks to intramolecular steric hindrance between the hy-
drogen-bonding segments, the operation of both the ser-
geants-and-soldiers principle and majority-rules effect could
be realized in these simple four-component supermolecules
through chiral transfer, which was mediated by steric repul-
sion. Although these two effects have already been observed
in supramolecular polymerization of small molecules, the
phenomena reported herein are unique because in supra-
molecular polymers the cooperativities of a large amount of
building blocks contribute greatly to the magnification of
the original chirality, but in our case the cooperative effect
is very low or there may not be any cooperativity in these
simple four-component supermolecules at all. This work
demonstrated that chiral amplification could also be applica-
ble in very simple supramolecular systems if a suitable chan-
Figure 5. a) CD spectra of mixtures of HPHAT and (l-1+d-1) at differ-
ent ee values for l-1 (positive region) and d-1 (negative region) in
CH2Cl2 at 258C. The molar ratio of HPHAT and the total amino acid
ester salts was 1:3 and the concentration of HPHAT was 4ꢁ10À5 m.
~
&
b) Plot of CD intensity at l=315 nm versus ee. ee of l-1. ee of d-1.
The intensity of the CD signal increased with increasing ee
of l-1 and remained almost constant after an ee of 40% was
reached. The use of excess d-1 resulted in mirror-symmetric
changes to the CD signals (Figure 5a, negative region). Plot-
ting the intensity of the CD signals at l=315 nm against the
ee revealed a significant nonlinear dependence (Figure 5b),
which strongly indicated the presence of the majority-rules
effect in this simple supramolecular system.
The origin of the sergeants-and-soldiers principle in these
supermolecules could be explained as follows: communica-
tion among all pyridyl units is accessible through the steric
repulsion of the adjacent pyridyl units. The bonding of one
pyridyl unit will certainly affect the bonding environment of
its neighboring pyridyl group. This information could be fur-
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Guo-Fang Jiang, Xin Zhao et al.
A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2012, 134,
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helpful to better understand the principles in asymmetric
catalytic processes in which asymmetric induction plays
a key role.[11] Moreover, extending the amplification of chir-
ality from complicated architectures to discrete, simple, self-
assembled objects might open up a way to the construction
of novel chiral supramolecular systems with various struc-
tures.
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Acknowledgements
This work was supported by the National Natural Science Foundation
(nos. 20972180, 21172249, and J1210040) and the Basic Research Devel-
opment Program of China (grant no. 2010CB833300). We thank Prof.
Zhan-Ting Li (Fudan University) for his helpful advice.
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Keywords: chirality
bonding · steric hindrance · supramolecular chemistry
·
cooperative effects
·
hydrogen
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Received: November 12, 2013
ˇ
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Bruijn, z. Tomovic, B. V. Averbeke, D. Beljonne, R. Lazzaroni,
Published online: January 23, 2014
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