Molecular Face-Rotating Cube with Emergent Chiral and Fluorescence Properties

Article abstract of DOI:10.1021/jacs.7b07657

Chiral cage compounds are mainly constructed from chiral precursors or based on the symmetry breaking during coordination-driven self-assembly. Herein, we present a strategy to construct chiral organic cages by restricting the P or M rotational configuration of tetraphenylethylene (TPE) faces through dynamic covalent chemistry. The combination of graph theory, experimental characterizations and theoretical calculations suggests emergent chirality of cages is originated from complex arrangements of TPE faces with different orientational and rotational configurations. Accompanied by the generation of chirality, strong fluorescence also emerged during cage formation, even in dilute solutions with various solvents. In addition, the circularly polarized luminescence of the cages is realized as a synergy of their dual chiral and fluorescence properties. Chirality and fluorescence of cages are remarkably stable, because intramolecular flipping of phenyl rings in TPE faces is restricted, as indicated by calculations. This study provides insight into construct chiral cages by the rational design through graph theory, and might facilitate further design of cages and other supramolecular assemblies from aggregation-induced emission active building blocks.

Full text of DOI:10.1021/jacs.7b07657

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Communication
Molecular face-rotating cube with emergent chiral and fluorescence properties
Hang Qu, Yu Wang, Zhihao Li, XinChang Wang, Hongxun Fang, Zhongqun Tian, and Xiaoyu Cao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07657 • Publication Date (Web): 25 Aug 2017
Downloaded from http://pubs.acs.org on August 25, 2017
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Molecular face-∗rotating cube with emergent chiral and fluorescence properties∗
Hang Qu, Yu Wang, Zhihao Li, Xinchang Wang, Hongxun Fang, Zhongqun Tian, and Xiaoyu Cao
State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM and College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China
Received August 24, 2017; E-mail: roywangyu@gmail.com; xcao@xmu.edu.cn
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simple tetraphenylethylene faces through the combination of graph
theory and experimental investigations. Accompanied by the
generation of emergent chirality, strong fluorescence also emerged
during the cage formation, and remained strong in dilute solutions
with various solvents. In addition, we also characterized the
circularly polarized luminescence of the cages to investigate the
synergy of their dual chiral and fluorescence properties. The reason
leading to the emergent chirality and fluorescence was clarified by
theoretical calculations.
Tetraphenylethylene (TPE) is a well-known aggregation-induced
emission (AIE) chromophore,26–28 and due to the steric hindrance
between the phenyl rings,29 it exhibits either clockwise or
anticlockwise rotational patterns in its propeller-like P or M
configurations, respectively (Fig. 1A). To further develop the
strategy of face-rotating polyhedra, we decided to construct face-
rotating cubes (FRCs) from six tetragonal TPE faces and eight
triamine vertices (Fig. 1C), and investigate whether the chiral
and fluorescence properties can be emerged simultaneously in the
FRCs.
We further scrutinized the structure of TPE and found that its
rectangle motif presents two orientational configurations of the
vinyl bond (i.e., vertical and horizontal as shown in Fig. 1A), in
addition to the P and M rotational configurations of the phenyl
rings. The two orientational configurations of TPE might also
lead to various stereoisomers of the assembled structures, yet this
phenomenon has not been reported, despite that several metal-
organic frameworks,30–33 covalent-organic frameworks34,35 and
cages26,36–38 have been assembled from TPE.
When rectangle TPE motifs are patterned onto the cubic faces,
the independent orientational and rotational configurations would
generate numerous stereoisomers. Using the graph theory analysis
based on permutation groups (details in Supplementary Method),
we first determined that there are eight types of patterned cubes
if we only consider the vinyl orientations of TPE, including six
achiral cubes and a pair of chiral enantiomers. And when we
further consider the P and M rotational configurations, there are 224
types of patterned cubes, among which only four cubes are achiral
mesomers and the rest 220 cubes are chiral (Fig. 1B). Therefore,
there is a great chance to achieve emergent chirality, if the P and M
configurations of TPE can be fixed in the cubic structure.
Abstract: Chiral cage compounds are mainly constructed from
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chiral precursors or based on the symmetry breaking during
coordination-driven self-assembly. Herein, we present an alternative
strategy to construct chiral organic cages by restricting the P
or M rotational configuration of tetraphenylethylene (TPE) faces
through dynamic covalent chemistry. The combination of graph
theory, experimental characterizations and theoretical calculations
suggests that the emergent chirality of cages is originated from
the complex arrangements of TPE faces with different orientational
and rotational configurations. Accompanied by the generation
of chirality, strong fluorescence also emerged during the cage
formation, even in dilute solutions with various solvents. In addition,
the circularly polarized luminescence of the cages is realized as
a synergy of their dual chiral and fluorescence properties. The
chirality and fluorescence of cages are remarkably stable, because
the intramolecular flipping of the phenyl rings in TPE faces is
extremely restricted, as indicated by theoretical calculations. This
study provides insight to construct chiral cages by the rational
design through graph theory, and might facilitate the further
design of the cages and other supramolecular assemblies from the
aggregation-induced emission active building blocks.
Cage compounds are important in a wide variety of applications
including molecular sensing,1,2 separation3–5 and catalysis.6–8
Using metal-organic coordination,9–13 hydrogen bonding14–16 and
dynamic covalent chemistry,17–20 chemists have synthesized many
cage compounds with various sizes and shapes. Among these
cages, the chiral ones are particularly interesting because chirality
increases the structural complexity as well as functionality of cage
themselves and their hierarchical assemblies.4,21,22 For instance,
many applications of cages are closely related to chirality, e.g.,
chiral separation3,5 and asymmetric catalysis.8
Generally, two strategies have been established to construct
chiral cages: by using chiral precursors,3,13,19,21 or based on the
chirality at metal centers in ∆ and Λ configurations.8,23 Inspired
by the mathematic study of Buckminster Fuller, we have recently
reported a novel strategy to generate emergent chirality of organic
cages by assembling achiral two-dimensional truxene building
blocks into chiral octahedra through dynamic imine chemistry.24
The truxene faces exhibited either clockwise or anticlockwise
rotational patterns on the octahedron faces, resulting in five types
of chiral face-rotating polyhedra (FRP) in the thermodynamic
and kinetic products. More recently, Fujita et al. reported a
more sophisticated emergent chirality in a tetravalent Goldberg
polyhedron,25 and the graph theory analysis suggested that the cage
chirality was generated from the asymmetric arrangement of the
triangular and square facial patterns. These studies illustrate the
complexity of the emergent chirality of cages, and indicate the
importance of introducing graph theory into the investigation of
sophisticated polyhedral cages.
The facial building block 4,4’,4”,4’”-(ethene-1,1,2,2-
tetrayl)tetrabenzaldehyde (ETTBA) consists of a TPE core and four
surrounding aldehyde groups for reversible imine formation (Fig.
1C). This facial building block was synthesized from benzopheone
within three steps with high yields (Fig. S1), whereas the vertex
building block (tris(2-aminoethyl)amine, TREN) is commercially
available. To form the cubes, the methanol solution of TREN (8
eq) was layered onto the chloroform solution of ETTBA (6 eq) at
ambient temperature for 10 days without stirring (Fig. 1C).
Mass spectrometry analysis confirmed that ETTBA6TREN8 was
formed exclusively, exhibiting a molecular weight of 3403.80 (Fig.
S2). Nevertheless, we measured more than five single crystals
isolated from the reaction solution, and found that all of them were
Here, we present how complex cage structures evolve out of
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Figure 2. Chiral-HPLC and CD analyses of the cubes 1 and 2.
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a, Chiral HPLC spectrum contains four peaks corresponding to (6M)-1,
(2P4M)-2, (6P)-1 and (4P2M)-2 in a 3:1:3:1 ratio. The absorbance for all
cubes was recorded at 300 nm. b, CD spectra of the separated cubes (6M)-1,
(2P4M)-2, (6P)-1 and (4P2M)-2 in the mixed solvent of dichloromethane
and methanol (v:v = 1:1).
Figure 1. Synthesis of the chiral cubes with different facial rotational
and orientational configurations. (A), A TPE motif exhibits two rotational
modes (P or M) of the phenyl groups and two orientational modes
(horizontal or vertical) of the vinyl group in two dimensions. (B), Rotational
and orientational modes of the TPE motif generate 224 types of cubes when
they are painted on the six faces of a cube. Nevertheless, only four types
of cubes were found in the experimental product as shown in entries D and
E. (C), Schematic of the synthesis of organic cubes from six ETTBA faces
and eight TREN vertices. (D,E), Cartoons to show the TPE configurations
in the homo-directional cubes (6P)-1 and (6M)-1, and the hetero-directional
cubes (4P2M)-2 and (2P4M)-2.
co-crystals that cannot be precisely solved, suggesting there are
more than one stereoisomers in the product. Further separation and
characterization revealed two pairs of enantiomers, i.e., the homo-
directional (6P)-1 and (6M)-1, and the hetero-directional (4P2M)-2
and (2P4M)-2, as shown in Figs. 1D and 1E.
Figure 3. Single-crystal structures of (6M)-1 and (2P4M)-2. Front and
diagonal views of the cubic structures of (6M)-1 (A,B) and (2P4M)-2 (C,D).
Carbon atoms are shown in grey, green or pink, nitrogen atoms are shown
in blue, and hydrogen atoms are omitted for clarity.
The product mixture was separated by high-performance liquid
chromatography (HPLC) with a chiral column (Daicel Chiralcel
IC), resulting in only four fractions of cage isomers in a 3:1:3:1
ratio (Figs. 2A and S3). These four fractions were separated and
characterized individually. Nuclear magnetic resonance (NMR)
analysis was not sufficient to precisely elucidate the structures of
the separated stereoisomers (Fig. S4), because of the notable signal
overlap of various phenyl protons and imine protons, which is a
common issue for large empty cages.25 Fortunately, we obtained
and analyzed all single crystals of the four separated stereoisomers,
confirming that the HPLC fractions eluting at 16.8, 19.3, 24.5 and
28.9 min are (6M)-1, (2P4M)-2, (6P)-1 and (4P2M)-2, respectively
(Fig. 2A).
Diffusion of diethyl ether into the chloroform solutions of (6M)-
1 and (6P)-1 gave the corresponding cube-shaped single crystals,
respectively, whereas the hexagonal-shaped single crystals of
(2P4M)-2 and (4P2M)-2 were individually grown by a similar
method by changing chloroform into the mixed solvent of
dichloromethane and methanol (v:v = 1:1). All FRCs have an
eight-capped cubic structure (Fig. 3), in which six TPE units
occupy six faces linked by eight tripodal TREN-linked vertices.
The homo-directional (6M)-1 structure has a high (T) symmetry,
and a same M configuration for each TPE faces (Figs. 3A and 3B).
The vinyl orientations in each two opposite TPE faces are parallel
with a same distance of 11.5 Å, whereas the vinyl orientations
in each two adjacent TPE faces are non-coplanar perpendicular.
Along the diagonal line (C3 symmetric axial), the three phenyl
rings linked to a same TREN vertex (pink rings in Fig. 3B) are
evenly distributed, exhibiting the same 60◦ dihedral angle to each
other. In addition, all imine bonds in (6M)-1 are in E conformation
and coplanar with the adjacent phenyl rings (Fig. S5A), suggesting
a strong conjugation between the adjacent imine bond and phenyl
ring.
The hetero-directional (2P4M)-2 structure (Figs. 3C and 3D) has
a lower (D2) symmetry in comparison with the homo-directional
cubes. As shown in Fig. 3C, the distance between two P faces
(top and bottom) is approximately 14% smaller than the distance
between two opposite M faces (front and back, or left and right).
The vinyl orientations in two opposite P faces are non-coplanar
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perpendicular, whereas the vinyl orientations in each two opposite
M faces are parallel. As shown in (Fig. 3D), each TREN vertex
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is linked to one P face and two M faces. Two phenyl rings from
the M faces (pink) have a 73◦ dihedral angle (Fig. S5B), whereas
the phenyl ring from the P face (green) is nearly parallel to one
of the pink phenyl rings. Despite that the vertex configuration
is dramatically different from that of (6M)-1, all imine bonds in
(2P4M)-2 are still in E conformation and coplanar with the adjacent
phenyl rings (Fig. S5B).
The (6P)-1 and (4P2M)-2 structures were assigned to be the
enantiomers of (6M)-1 and (2P4M)-2, respectively, as shown in
Fig. S6.
The emergent chirality of the four FRCs in solid state has been
confirmed by single-crystal analysis. During the HPLC separation,
the online optical rotation detection indicated that the four FRCs
are also chiroptical active in solution (Fig. S7).
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The further circular dichroism (CD) analysis of the separated
FRCs (Fig. 2B) showed a strong positive Cotton effect in the
spectrum of (6M)-1, and a negative mirror-image spectrum for
(6P)-1. (2P4M)-2 and (4P2M)-2 also showed a pair of mirror-
image spectra with a relatively weaker intensity, and their CD
spectral signs were dominated by the major faces, i.e., M and
P, respectively. Comparing to the CD spectrum of (6M)-1,
an extra small peak at 360 nm was shown in the spectrum of
(2P4M)-2, which can be rationalized by the extra absorption at the
corresponding region as shown in the UV-Vis spectra (Fig. 4A). All
CD spectra of the four FRCs are consistent with the (ZINDO/S)-
predicted CD spectra as shown in Fig. S8, further confirming the
structures revealed by single-crystal X-ray analysis.
The correlation between TPE configurations and CD spectral
signs in FRCs is similar to that in small chiral molecules
synthesized from single TPE unit,29 in which TPE monomers
in M or P configurations also generate positive or negative Cotton
effects, respectively. Nevertheless, the CD intensities of FRCs,
e.g., 1.4 × 106 deg cm2 dmol−1 for FRC 1, are 100 times stronger
than that of chiral TPE monomers,29 indicating a strong nonlinear
effect of the chiroptical properties in FRCs.
In addition, the separated FRCs showed a remarkable stability
against racemization. For instance, the CD and HPLC spectra of
(6P)-1 remains the same even after being heated in the chloroform
solution at 60◦C for 24 hours (Fig. S9). This indicates that M
or P configuration of FRCs is restricted in solution, and there is
an extremely high energy barrier in converting (6P)-1 into (6M)-1
through changing the rotational configuration of all six faces. The
stability of the separated chiral cages provides important potential
for their further applications as chiral porous materials.39,40
Comparing with the absorption spectum of ETTBA monomer,
the spectra of FRCs are notably blue-shifted. This can be
rationalized by the decrease of the conjugation degree of TPE core,
as a result of the restriction of the phenyl flipping in FRCs.41 In
addition, the spectrum of 1 is more blue-shifted than that of 2,
which is in accord with the larger twist of the TPE core in 1, as
indicated by the analysis of the dihedral angles between the phenyl
rings and the vinyl bonds (Fig. S5).
The emergent fluorescence property of FRCs is also resulted
from the restriction of phenyl flipping.42–44 In contrast to the
low fluorescence activity of the ETTBA solution, strong blue
fluorescence with a peak at 427 nm was observed in the solutions
of 1 and 2 (Figs. 4B and 4C). In addition, the fluorescence quantum
yields in toluene for FRCs 1 and 2 were measured to be 26.0% and
30.9% respectively, by using 9,10-diphenylanthracene as standard.
The fluorescence of FRCs can be further tuned by solvent, generally
showing an increase in fluorescence intensity upon the decrease of
solvent polarity (Fig. S10).
Figure 4. Fluorescence and CPL analyses of the cubes 1 and 2. (A),
UV-vis spectra of ETTBA (50 µM, black) and the cubes 1 (8 µM, red)
and 2 (8 µM, blue) in the mixed solvent of dichloromethane and methanol
(v:v = 1:1). (B), Fluorescence spectra of ETTBA (50 µM, black) and the
cubes 1 (8 µM, red) and 2 (8 µM, blue) in chloroform, excited at 340
nm. (C), Fluorescence images of ETTBA, 1 and 2 solutions under the 365
nm UV light. (D), CPL spectra of ETTBA (0.6 mM, black), (6M)-1 (0.1
mM, magenta), (6P)-1 (0.1 mM, yellow), (2P4M)-2 (0.1 mM, green) and
(4P2M)-2 (0.1 mM, cyan) in chloroform, excited at 340 nm.
Based on the emergent chirality and fluorescence of FRCs,
we envisioned that the FRCs would also generate the circularly
polarized luminescence (CPL).45–47 Indeed, intensive mirror-
image CPL signals of (6M)-1 (positive) and (6P)-1 (negative) were
observed at 450 nm in their chloroform solutions (Fig. 4D), with a
large CPL dissymmetric factor (g1lum = ± 1.1 × 10−3). The CPL
spectra of (2P4M)-2 and (4P2M)-2 is similar to that of (6M)-1 and
(6P)-1, respectively, with a slightly smaller dissymmetric factor
(g2lum = ± 9.3 × 10−4).
The dual emergent chiral and fluorescence properties of FRCs
is unprecedented for cage compounds. Despite that a few TPE
cages have been reported,26,36–38 the chiral property of them has
not been investigated due to the difficulties in the separation and
characterization. Besides, the emergent fluorescence was mainly
presented in the small cages that formed from two close-packed
TPE faces,26,36 in which the steric hindrance between the opposite
TPE faces is strong enough to restrict the intramolecular flipping of
the TPE phenyl rings. By contrast, the larger TPE cages, in which
the steric hindrance between the opposite TPE faces is absent,
generate much weaker fluorescence in dilute solutions.37,38
The dual emergent properties of FRCs are clearly resulted from
the restriction of the intramolecular flipping of the TPE faces.32,42
Nevertheless, it remains an open question why the restriction of
flipping can be achieved in FRCs, since FRCs also have large
structures and weak steric hindrance between the opposite TPE
faces. To this end, we performed the density functional based tight
binding plus (DFTB+) calculations to compare the potential energy
surfaces (PES) of the phenyl flipping in free TPE monomer and in
(6M)-1.31,36
The PES for TPE monomer was constructed by synchronously
flipping the four phenyl rings with increasing the CPh-CPh-C=C
dihedral angle from 22.5 to 157.5◦, and the flipping energy barrier
was calculated to be 53.9 kJ mol−1 (Figs. 5A and 5B). Therefore,
the interconversion between (M)-TPE and (P)-TPE can happen
at room temperature, and hence the TPE monomer is neither
fluorescent nor chiroptical active in dilute solution. By contrast, if
one of the six TPE faces in (6M)-1 was flipped into P configuration
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(pathway in Fig. 5C), the energy of the resulted 5M1P structure
assemblies from AIE-active building blocks. In addition, this
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would be 58.8 kJ mol−1 higher than that of (6M)-1, and the energy
barrier from (6M)-1 to 5M1P is much larger than that in the reverse
direction. Therefore, when one TPE face in (6M)-1 was flipped, it
will easily flip back to the original (6M)-1 structure. Furthermore,
the complete interconversion from (6M)-1 to (6P)-1 involves the
flipping of all 24 phenyl rings (Fig. 5D). The energy barrier of this
pathway was calculated to be 224.9 kJ mol−1, thus explaining the
remarkable stability of (6M)-1 against racemization.
study provides a strategy to construct chiral cages by the rational
design through graph theory. The mathematic method based on
permutation groups is suitable for the analysis of all kinds of
polyhedra incorporating different kinds of faces or vertices, and
it will become more and more important as the complexity of the
research objects increases.
Acknowledgement The authors thank Stephen Z.D. Cheng,
He Tian, Xin Xu, Yiqin Gao, Peter J. Stang, E.W. (Bert) Meijer,
Wei Zhang, Minghua Liu, Benzhong Tang, Rongrong Hu, Hui
Zhang and Cheng Wang for discussions. We also thank Xiao Tang,
Kai Wu, Jijun Jiang, Lin Shi and Zhiyong Tang for assistance
in experiments. This work is supported by the 973 Program
(No.015CB856500), the NSFC (Nos. 21722304, 91427304,
21573181, 91227111 and 21102120) and the Fundamental
Research Funds for the Central Universities (No.0720160050)
of China.
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Supporting Information Available: The Supporting Information
includes experimental procedures and data, graph theory analysis,
Figures S1–15 and Table S1. This material is available free of charge
via the Internet at http://pubs.acs.org/.
References
Figure 5. DFTB+ calculations of the phenyl flipping in TPE monomer
and (6M)-1. (A), Potential energy surfaces of the different pathways to
flip the phenyl rings in TPE and (6M)-1. (B), Pathway to synchronously
flip the four phenyl rings of lowest energy structure of (M)-TPE to its
mirror-image (P)-TPE by increasing the CPh-CPh-C=C dihedral angle from
42.5◦ to 137.5◦. (C), Pathway to synchronously flip the four phenyl rings
on the top face of (6M)-1 with the TPE units on other faces fixed. (D),
Pathway to synchronously flip the 24 phenyl rings in (6M)-1 to its mirror-
image (6P)-1.
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crystal structures and is remarkably stable against racemization,
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cubes. Theoretical calculations reveal that the conjugation between
imine bonds and phenyl rings is important to the restriction of TPE
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great potential for chiral sensors and luminescent materials. The
method we used to restrict the intramolecular flipping of TPE units
by the conjugation between imine bonds and phenyl rings might
facilitate the further design of cages and other supramolecular
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