Aggregation-Induced-Emission-Active Macrocycle Exhibiting Analogous Triply and Singly Twisted M?bius Topologies

Article abstract of DOI:10.1002/chem.201502224

Molecules with M?bius topology have drawn increasing attention from scientists in a variety of fields, such as organic chemistry, inorganic chemistry, and material science. However, synthetic difficulties and the lack of functionality impede their fundame

Full text of DOI:10.1002/chem.201502224

DOI: 10.1002/chem.201502224
Communication
&
Macrocycle Topology
Aggregation-Induced-Emission-Active Macrocycle Exhibiting
Analogous Triply and Singly Twisted Mçbius Topologies
Erjing Wang,[a, b, c] Zikai He,[a, b] Engui Zhao,[a, b] Luming Meng,[b] Christian Schütt,[d]
Jacky W. Y. Lam,[a, b] Herman H. Y. Sung,[b] Ian D. Williams,[b] Xuhui Huang,[b] Rainer Herges,*[d]
and Ben Zhong Tang*[a, b, e]
properties as well as the challenges involved in their synthe-
sis.[1] Just like other molecular topologies, such as catenanes,
Abstract: Molecules with Mçbius topology have drawn in-
creasing attention from scientists in a variety of fields,
such as organic chemistry, inorganic chemistry, and mate-
rial science. However, synthetic difficulties and the lack of
functionality impede their fundamental understanding
and practical applications. Here, we report the facile syn-
thesis of an aggregation-induced-emission (AIE)-active
macrocycle (TPE-ET) and investigate its analogous triply
and singly twisted Mçbius topologies. Because of the
twisted and flexible nature of the tetraphenylethene units,
the macrocycle adjusts its conformations so as to accom-
modate different guest molecules in its crystals. Moreover,
theoretical studies including topological and electronic
calculations reveal the energetically favorable interconver-
sion process between triply and singly twisted topologies.
rotaxanes, and knots, non-orientable Mçbius bands have
aroused wide interest from synthetic chemists.[2] Continuous
efforts devoted to the synthesis of Mçbius-type molecules not-
withstanding,[3] it was not until 2003 that the first stable
Mçbius hydrocarbon was prepared by Herges et al.[4] The diffi-
culty in the synthesis of Mçbius-type molecules is mainly due
to the high strain caused by the twist.[5] Consequently, the syn-
thesis of multiply twisted Mçbius macrocycles should be even
more difficult owing to increased strain.[6]
In topological analysis, three parameters, linking number
(Lk), twist (Tw), and writhe (Wr), are applied to define the geom-
etry of a closed twisted ribbon. The three parameters are con-
[7]
˘
˘
nected by the Calugareanu theorem as: Lk =Wr +Tw. The link-
ing number Lk is an integer, with even numbers of Lk describ-
ing orientable and two-sided ribbons, whereas odd numbers
of Lk denote non-orientable and one-sided ribbons, such as
Mçbius systems. Generally, a closed ribbon with a higher order
of Lk tends to release its strain by projecting twist into writhe.
Rzepa[8] and Herges[9] proposed to apply this feature to con-
struct annulenes with higher Mçbius topologies. Guided by
this strategy, the first triply Mçbius dehydroannulene (Lk =3)
was successfully prepared in 2014.[10] As shown in Figure 1, the
Molecules with intriguing topologies stimulate the imagination
of scientists because of their (theoretically predicted) unusual
[a] E. Wang,+ Z. He,+ E. Zhao, J. W. Y. Lam, Prof. B. Z. Tang
HKUST Shenzhen Research Institute, No. 9 Yuexing 1st RD
South Area, Hi-tech Park Nanshan, Shenzhen 518057 (China)
E-mail: tangbenz@ust.hk
[b] E. Wang,+ Z. He,+ E. Zhao, L. Meng, J. W. Y. Lam, H. H. Y. Sung,
Prof. Dr. I. D. Williams, Prof. X. Huang, Prof. B. Z. Tang
Department of Chemistry, Division of Life Science
State Key Laboratory of Molecular Neuroscience
Institute for Advanced Study
Institute of Molecular Functional Materials
Division of Biomedical Engineering
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (China)
[c] E. Wang+
School of Materials Science and Engineering, Hubei University
No. 368, Youyi Avenue, Wuhan 430062 (China)
Figure 1. Structures of the first singly and triply Mçbius hydrocarbons.
[d] C. Schütt, Prof. R. Herges
Institute for Organic Chemistry, University of Kiel
Otto-Hahn-Platz 4, 24098 Kiel (Germany)iso-kiel.de
E-mail: rherges@oc.un
macrocycle is only slightly strained (Tw =1.42) because most of
the twist is transformed into writhe through the chiral 1,1’-bi-
naphthyl units. Another interesting feature of molecular
Mçbius bands is their ability to change the degree of twist
through topology flipping. However, to date, most reported
Mçbius systems have been made deliberately rigid to stabilize
the twisted conformation, with a few expanded porphyrins ex-
amples exhibiting reversible switching between different to-
pologies.[11]
[e] Prof. B. Z. Tang
Joint Research Laboratory
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology
Guangzhou 510640 (China)
[+] Both authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502224.
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To design high-order Mçbius molecules with switchable to-
pologies, two factors need to be considered: a well-defined
chiral “writhe precursor” to release the strain in the Mçbius
structure and a relatively flexible conformation to allow topolo-
gy flipping. These properties are met by tetraphenylethene
(TPE), which exhibits novel aggregation-induced emission (AIE)
owing to the propeller-like structure and which has recently
been extensively applied in chemo/biosensors.[12] The intrinsic
turn generated by the adjacent phenyl rings and their free ro-
tational motion with respect to the central double bond make
TPEs excellent candidates for the construction of switchable
Mçbius rings. However, to the best of our knowledge, utilizing
TPE units to construct multiply Mçbius topologies remains un-
explored, despite the considerable research interest in con-
structing macrocycles with TPE motifs.[13,14] Herein, we report
the first example of a TPE-containing ring (TPE-ET), which ex-
hibits analogous singly and triply twisted Mçbius topologies,
as well as AIE characteristics.
is almost non-emissive in homogeneous solutions of THF and
highly emissive when it forms aggregates in THF/water mix-
tures that have water fractions higher than 90% (Figure S4 in
the Supporting Information). Similar properties are observed
for TPEyne (Figure S5 in the Supporting Information). The emis-
sion peak of TPE-ET in the aggregated state is located at
457 nm, which is 20 nm blueshifted compared with TPEyne,
verifying that the cyclization rigidifies and twists the conforma-
tion of the TPE units. The AIE properties should originate from
the well-known restriction of intramolecular rotation pro-
cess.[12b] The excited states in solution decay through nonradia-
tive pathways through rotations of the phenyl groups. Upon
aggregation, these intramolecular rotations are suppressed.
Thus, the radiative decay pathway is populated. The AIE char-
acteristics also reveal the flexible nature of the TPE-ET struc-
ture.
As a propeller-shaped molecule, when twisting in one direc-
tion, achiral TPE can take two chiral conformations (P or M as
shown in Figure 2).[16] In solution, they exist as a racemate due
The structure and synthetic route to TPE-ET are shown in
Scheme 1. A one-step modified Sonogashira coupling of an in
situ deprotected monomer led to the cyclization, yielding the
trimeric macrocycle TPE-ET. The isolated yield is 10%, which
Figure 2. Structures of the two chiral TPE conformations and their four pos-
sible combinations in the trimeric macrocycle.
to the fast interconversion. However, in the crystalline state,
the molecular conformation is fixed and can be easily deter-
mined by X-ray diffraction analysis. Two kinds of crystals of
TPE-ET suitable for X-ray crystallography were obtained from
CHCl3/cyclohexane (C-1) and CHCl3/n-hexane (C-2) solutions
through slow evaporation. As shown in Figure 3, most of the
crystals prepared from CHCl3/cyclohexane are hexagonal or
prism-like, whereas those obtained from CHCl3/n-hexane are
rod-like. We also tried to grow crystals of TPE-ET in CHCl3/n-
hexane/cyclohexane. It turned out that the same crystal struc-
ture as C-1 was obtained, indicating that the TPE-ET macrocy-
Scheme 1. Synthesis of the TPE-ET macrocycle.
corresponds to a yield of 47% for each coupling including in
situ deprotection. It is lower than reported examples,[15] but
the synthesis is much more straightforward and easier com-
pared with the first triply Mçbius dehydroannulene.[10] TPE-ET
has moderate solubility in common organic solvents, such as
dichloromethane, chloroform, THF, and acetone. The structure
1
was fully characterized by H NMR and 13C NMR spectroscopy,
as well as high-resolution mass spectrometry (Figures S1 and
S2 in the Supporting Information).
We then investigated the photophysical properties of TPE-ET
and its building unit, 3,3’-(2,2-diphenylethene-1,1-diyl)bis(ethy-
nylbenzene) (TPEyne, Scheme S1 in the Supporting Informa-
tion). As shown in Figure S3 (see the Supporting Information),
TPE-ET exhibits a blueshifted absorption maximum compared
with TPEyne, indicating the higher degree of twisting in TPE-
ET. Moreover, TPE-ET also exhibits typical AIE characteristics. It
Figure 3. Images of TPE-ET crystals grown from CHCl3/cyclohexane and
CHCl3/n-hexane (A, C under bright field; B, D under UV-light irradiation;
wavelength: 330–380 nm; scale bar: 250 mm).
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cle of C-1 should be the thermodynamically more stable of the
two. Both types of crystals emit blue light under UV irradiation
in agreement with the AIE properties.
Interestingly, the TPE-ET macrocycle adopts suitable confor-
mations so as to accommodate solvent molecules with differ-
ent shapes and sizes. In the C-1 crystal, it takes on a PPP or
MMM configuration with D3 symmetry, whereas in the C-2 crys-
tal a PMM or MPP conformation with quasi-C2 symmetry is pre-
ferred. In the crystal of C-1, the TPE-ET macrocycles in PPP and
MMM configuration stack layer by layer in alternating se-
quence, in a coaxial manner with a perpendicular rotation
angle of 608 (Figure 4C). This type of packing generates hexag-
onal channels, which are filled perfectly by cyclohexane mole-
cules in chair conformation. The packing is stabilized by three
CH to p interactions (2.884 Š) between cyclohexane and the
carbon–carbon triple bonds. In the crystal of C-2, TPE-ET adopt-
ing PMM conformation mainly forms two kinds of channels
(Figure 4D). The channels formed by the macrocycle cavities
are filled by n-hexane molecules in staggered conformation,
whereas the channels constructed by four adjacent TPE units
with PMPM conformation are filled by n-hexane molecules
with some disorder.
X-ray crystallography analysis revealed that both crystals
trap solvent molecules because of the intrinsic cavity of the
macrocycle and the nano-sized porous crystal structure. As
shown in Figure 4A, two cyclohexane molecules with chair
conformation are encapsulated in the center of the TPE-ET
macrocycle with coinciding C3 axes of cyclohexane and C-1. In
C-2, however, one n-hexane molecule with staggered confor-
mation stands in the macrocyclic cavity, while another n-
hexane molecule is surrounded by two TPE units with some
disorder (Figure 4B). In addition, one chloroform molecule is
trapped in the nano-sized pore of C-2 through strong CH to p
interactions (2.545 Š).
The X-ray crystallography analysis revealed the analogous
Mçbius topology of TPE-ET macrocycle. To check the Mçbius
topological nature of the macrocycle, we can either examine
the structure-based definition or check its Mçbius topology.
The structure-based definition relies on counting the transoid
units (the sum of all trans C=C bonds and s-trans CÀC
bonds),[17] which is not suitable for TPE-ET as it is neither annu-
lene nor dehydroannulene. Therefore, taking the p system as
a band, we construct a band orthogonal to the p system and
cut it down in the middle (Figure S8 in the Supporting Infor-
mation).[10] A trefoil knot is obtained from PPP Mçbius topolo-
gy, with a linking number of 3 (Figure 5B[18]). In contrast, one
ring (double the diameter) is obtained from PMM Mçbius to-
pology, with a linking number of 1 (Figure 5D[18]). The Mçbius
feature was further verified by density functional theory (DFT)
calculations (on B3LYP/6-31G(d,p) basis set). To directly visual-
ize the Mçbius topology, molecular orbitals lower than the
HOMO were examined, owing to their fewer nodes, which may
Figure 4. Single-crystal X-ray structures of C-1 and C-2. (A)–(B) Top view
showing that solvent molecules are included in macrocycle cavities and out-
side channels. (C) Molecular packing of C-1 viewed along the c axis. (D) Mo-
lecular packing of C-2 viewed along the a axis. Cyclohexane and n-hexane
molecules are drawn as space filling models, other atoms are shown as 50%
probability ellipsoids. The P enantiomer of the TPE unit is shown in red, the
M enantiomer is shown in blue. Other carbon, chlorine and hydrogen atoms
are shown in gray, green and white, respectively.
Figure 5. Topological diagrams to investigate the linking number Lk of the p
system.
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present Mçbius topology in a much clearer way. The calculated
frontier orbitals clearly show a triply twisted topology for PPP
conformation and a singly twisted topology for PPM conforma-
tion in the HOMOÀ3 orbital (Figure S9 in the Supporting Infor-
mation), corroborating the Mçbius topological nature of these
conformations.
Grants Council of Hong Kong (604913, 16301614, and N_
HKUST604/14), and the University Grants Committee of Hong
Kong (AoE/P-03/08). B.Z.T. thanks the support from Guangdong
Innovative Research Team Program (201101C0105067115).
Keywords: aggregation-induced emission
mçbius topology · topology flipping
· macrocycles ·
The Mçbius-analogous property of TPE-ET was further inves-
tigated by using the program ANEWWRITHM (F. Kçhler, Univer-
sity of Kiel).[19] Linking numbers of 3 and 1 were determined
for the PPP conformation in C-1 and the PMM conformation in
C-2, respectively, (enantiomers have linking numbers of oppo-
site sign). The results are consistent with their Mçbius proper-
ties and the DFT calculation results discussed above. The high
twist and low writhe values (Tw =2.257, Wr =0.743 in C-1 and
Tw =0.975, Wr =0.025 in C-2) reveal that only a relative small
fraction of twist is projected into writhe. The ten times larger
Wr proportion in the PPP conformation indicates its smaller tor-
sion and lower strain. The more extended structure of C-1 is
also supported by UV/Vis and photoluminescent (PL) spectra
(Figures S6 and S7 in the Supporting Information). The UV/Vis
absorption maxima of TPE-ET in C-1 form, C-2 form, and in so-
lution are 311, 309, and 307 nm, respectively, whereas the cor-
responding emission maxima are 466, 462, and 463 nm. These
results indicate that TPE-ET in C-1 form, with its triply twisted
system, is less strained, exhibiting redshifted absorption and
emission peaks.
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Acknowledgements
This work was partially supported by National Basic Research
Program of China (973 Program; 2013CB834701), the Research
Chem. Eur. J. 2015, 21, 11707 – 11711
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Published online on July 14, 2015
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