Janusarene: A Homoditopic Molecular Host

Article abstract of DOI:10.1002/anie.201705451

A homoditopic molecular host, janusarene, is presented that has two back-to-back compactly arranged nanocavities for guest complexation. The unique two-face structural feature of janusarene allows it to bind and align various guest compounds concurrently, which include spherical pristine fullerene C₆₀ and planar polycyclic aromatic hydrocarbons (PAHs), such as pyrene, perylene, and 9,10-dimethylanthracene. The host–guest interactions were characterized by single-crystal X-ray diffraction. A pairwise encapsulation of the PAH guests by janusarene enables PAH dimers to be obtained that deliver spectroscopic properties distinct from those of PAHs dissolved in solution, or in the bulk state. A monotopic control host was also synthesized and used to characterize the host–guest complexing behavior in solution.

Full text of DOI:10.1002/anie.201705451

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Accepted Article
Title: Janusarene: A Homoditopic Molecular Host
Authors: Tao Li, Luoyi Fan, Hao Gong, Zeming Xia, Yanpeng Zhu,
Nianqiang Jiang, Gaofeng Liu, Long Jiang, yang li, and
Jiaobing Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705451
Angew. Chem. 10.1002/ange.201705451
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http://dx.doi.org/10.1002/ange.201705451

10.1002/anie.201705451
Angewandte Chemie International Edition
Janusarene: A Homoditopic Molecular Host
Tao Li, Luoyi Fan, Hao Gong, Zeming Xia, Yanpeng Zhu, Nianqiang Jiang, Long Jiang, Gaofeng Liu, Yang Li,
Jiaobing Wang*
Abstract: We report a kind of homoditopic molecular host,
janusarene, which has two back-to-back compactly arranged
nanocavities for guest complexation. The unique two-face structural
feature of janusarene allows it to bind and align various guest
compounds, concurrently, which include the spherical pristine
fullerene C60 and the planar polycyclic aromatic hydrocarbons
(PAHs) such as pyrene, perylene, and 9,10-dimethylanthracene. The
host-guest interactions were characterized by single crystal X-ray
diffraction. A pairwise encapsulation of the PAH guests by
janusarene enables us to obtain PAH dimers which deliver emerging
spectroscopic properties distinct from those of PAHs dissolved in
solution, or in the bulk state. Moreover, a monotopic control host
was synthesized and used to characterize the host-guest complexing
behaviour in solution.
in a pairwise manner, which leads to excimer emissions. Janusarene
combines the virtues of easy synthesis, flexible modification, and well-
defined architecture, these properties should benefit its further
elaborations for different applications as a modular supramolecular
building block in general.
The structure of janusarene mirrors that of a Janus head (Figure 1A),
with two concave aromatic faces fused together in a back-to-back style
and functioning as the binding cavities. We utilize the conformational
preference of hexaphenylbenzene14 (HPB, Figure 1B) platform to
control both the position and orientation of the structural components.
Specifically, free rotation of the peripheral phenyl rings (p-rings), along
the axis of their C-C bonds to the central phenyl ring (c-ring) of HPB, is
restricted due to steric hindrance15, providing a desired conformation to
construct janusarene. Six phenyl substituents are connected to each side
of the HPB scaffold at the meta-position of the p-rings, thereby fencing
two conical molecular spaces on the HPB “back”, which can
accommodate various guest compounds.
Advances in supramolecular chemistry depend heavily on the creations
of various molecular building blocks, among which the host compounds
such as crown ether, cyclodextrin, calixarene, cucurbituril, and the more
recently developed pillararene (CCCCP) are prominent examples1,2. The
popularity enjoyed by these CCCCP compounds and their derivatives
stems from the well-preorganized structures, highly predictable
noncovalent interactions, easy accessibility and modularity. These
properties enable the CCCCP compounds to contribute significantly to
different areas3-12 of supramolecular chemistry, including but not limited
to molecular recognition, self-assembly, molecular mechanics, and
biomimetics. It is self-evident that even after five decades of intensive
explorations, making of novel host compounds remains an important
target in the field of supramolecular chemistry.
We introduce here a kind of novel molecular host, janusarene (Figure
1), which has two back-to-back compactly arranged, nanoscale binding
cavities to complex with various guests, including the spherical fullerene
and the planar polycyclic aromatic hydrocarbons (PAHs) such as pyrene,
perylene, and 9,10-dimethylanthracene (DMA). In contrast to the
contemporary widely-employed monotopic CCCCP derivatives,
janusarene is homoditopic in nature13. This unique structure feature
allows janusarene to bind and align guest molecules concurrently, by
forming an alternate host-guest assembly. Moreover, to fully occupy the
nanoscale binding cavities, PAH guests are encapsulated by janusarene
Fig. 1. (A) Space filling molecular model of J1 optimized using molecular mechanics
(left, side view; right, top view). Part of the structure is shown as ball-and-stick model
to visualize the cavity. The depth (ca. 6Å, distance between the centroid of HPB and the
plane of the oxygen atoms at the edge) and width (ca. 16Å, distance between the
opposite oxygen atoms at the edge) of the cavity are indicated with arrows. (B)
Synthesis and molecular structure of janusarene J1-3. J1, R = OMe; J2, R = CH3; J3,
R = H. Central, peripheral, and fencing phenyl rings of janusarene are labelled as c, p,
and f, respectively.
[*] T. Li, L. Fan, H. Gong, Z. Xia, Y. Zhu, N. Jiang, Dr. L. Jiang, Prof. G. Liu,
Prof. J. Wang
School of School of Chemistry
Sun Yat-Sen University
Guangzhou 510275 (China)
E-mail: wangjb5@mail.sysu.edu.cn
The synthesis of janusarene is efficient and convenient (Figure 1B).
Nineteen phenyl rings (seven of them constitute the HPB “back”, the
other twelve fence around the two faces) are incorporated into
janusarene, in a single step, through a cobalt catalysed cyclo-
trimerisation in good to excellent yield. The alkyne precursors are
obtained readily using commercially available starting materials via a
Prof. Y. Li
Instrumental Analysis and Research Center
Sun Yat-Sen University
Guangzhou 510275 (China)
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10.1002/anie.201705451
Angewandte Chemie International Edition
Suzuki cross-coupling. In the present study, a methoxy-substituted
janusarene J1 is used in host-guest complexations. Janusarene
homologues J2-3 are prepared to demonstrate the modularity of the
synthesis. Detailed synthesis and characterization of all the related
compounds are provided in the supporting materials.
15.9 Å (Figure S4). Closer examination of the crystal structure reveals
intimate contact between all f-rings of J1 and C60. For each C60, there
are 38 neighbouring sp2 carbon atoms from janusarene with a C-C
distance < 4.0 Å (nearest C-C distance, 3.296 Å), indicating a strong
concave-convex interaction. Moreover, the twelvefold C−H···π
interactions between the inner protons of the HPB “back” and the π
system of fullerene (C−H···π distance, 2.913 Å) should also contribute
to the stabilization of the J1-C60 assembly.
Inspired by the well-established host-guest chemistry of the
calixarene derivatives16,17, we reasoned that janusarene might bind
pristine fullerene via a concave-convex interaction18-28. This envision
was further supported by a basic molecular modelling study (Figure S2),
which indicated a perfect shape complementarity between C60 and the
binding cavity of janusarene. Indeed, when a colourless solution of J1
was added to a violet-coloured and transparent solution of C60 in toluene
at room temperature, a dark yellow precipitate formed immediately upon
mixing (Figure S3), indicating an evident noncovalent interaction.
Due to exceptional concave-convex complementarity, janusarene can
bind and align fullerene C60 in a general and reliable manner.
Crystallographic analysis (Figure S5) of the J2-C60 and J3-C60 1:1
complexes demonstrates
a
one-dimensional divalent alternate
complexation similar with the J1-C60 complex.
We find that janusarene is adaptable to bind and align non-spherical
polycyclic aromatic hydrocarbons (PAHs), such as pyrene, perylene,
and DMA. Despite a significant variation in the size and shape compared
with fullerene, these PAH guests are encapsulated and arranged in a
packing mode similar to that of the J1-C60 complex, as proved by the
single crystal structures29 (Figure 2C-F, S6). We observe an alternate
one-dimensional columnar structure, in which the PAH guests locate in
the cavity formed by the two neighbouring J1 hosts. However, to fully
occupy the nanoscale binding cavity, a pair of parallelly aligned PAH
guests, instead of a single fullerene, are sandwiched between the
janusarene hosts (J1: PAH = 1:2, mole ratio). The PAH dimers are slight
displaced from the cofacial arrangement with short interplanar distances
(ca. 3.5 Å), revealing a π-π interaction. This structural feature in crystals
is maintained for all three J1-PAH complexes. In the crystal structure,
C−H···π interactions, van der Waals forces, and π-π stacking, cooperate
to stabilize the complex (Figure 2D, F).
Comparison between the J1-C60 and J1-PAH crystal structures
reveals that the shape of the binding cavity is adjustable to maximize the
host-guest complexation through a subtle change in the dihedral angles
(DA) of the p-c rings and that of the p-f rings (table S3). But, in general,
the two-face, biconcave structural feature of janusarene is maintained
through the unique conformation of the HPB platform, which displays
an average DAp-c of ca. 67o for all the crystal structures of the janusarene
complexes. From this point of view, janusarene is a well-preorganized
molecular host with appropriate tunability to complex with various
guests such as fullerene and PAHs, as demonstrated above.
Fig. 2. Single crystal X-ray structures of the J1-C60 1:1 (A, B, orthorhombic, Pnn2), J1-
pyrene 1:2 (C, D, monoclinic, C2/c), and J1-perylene 1:2 (E, F, monoclinic, C2/c)
complexes. An alternate linear assembly is observed in all cases, despite a significant
change in the geometry and electronic property of the guests.
A high-quality single crystal29 suitable for X-ray diffraction was
grown in a three-layer solution (o-xylene solution of C60, 0.5 mM, 4 mL,
top; dioxane, 1.5 mL, middle; carbon disulfide solution of J1, 0.5 mM,
4 mL, bottom). Crystallographic analysis reveals a one-dimensional
columnar structure (Figure 2A, B) of the J1-C60 1:1 complex. In this
structure, fullerene nestles inside the cavity formed by two adjacent
Fig. 3. Molecular structure of the control host C1. Replacing two f-rings of J1, on one
side of the HPB “back”, with a short aliphatic linker (circled area in the model) removes
one of its binding cavity, and results in the monotopic host. Space filling molecular
model of C1 was optimized with Spartan software, MM force field.
janusarene molecules, manifesting
a
divalent supramolecular
assembly30-33. In each column, a stacking period of 14.4 Å is observed
for both C60 and J1. The columns are organized into a hexagonal packing,
which is slightly disordered with an average intercolumnar distance of
To examine the host-guest interactions in solution, we designed and
synthesized a control host C1 with only one binding cavity (Figure 3).
The monotopic nature of C1 suggests that it may form discrete host-
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10.1002/anie.201705451
Angewandte Chemie International Edition
guest complex, instead of the extended assembly (for C60) as observed
for the homoditopic janusarene, thereby enabling us to perform the
titration experiments to quantify the binding event. As shown in Figure
4A, a broad absorption band round 450 nm appears and develops when
J1 is added to a solution of C60 in toluene. This spectral change can be
attributed to the change-transfer (CT)34 transition from the C1-C60
complex. The solution of C60 changes from violet to brown in the
presence of one equiv. of C1 (Figure S2). Cyclic voltammetry
measurement shows a negative shift (ca. 30 mV) of the reduction
potential of C60 when being complexed (Figure S13), a result in accord
with the expectation that C1 has an electron-donating property, which
increases the electron density of C60 and therefore makes it more difficult
to further incorporate electrons.
force applied by the conventional macrocycle hosts to encapsulate
different guests in an aqueous environment. Primary result indicates that
water soluble janusarene can associate hydrophobic guests in water with
strong binding interactions, which we will report soon in a separate
paper.
In the solid state, the homoditopic two-face nature of janusarene is
essential to produce the J1-PAH complexes with a 1:2 stoichiometry
(Figure 2). When the monotopic control host C1 was used to crystalize
with perylene by slow evaporation in a mixed DCM/methanol solution,
a single crystal of the perylene-C1 1:1 complex was obtained with only
one perylene residing in the binding cavity (Figure S7).
At last, we show that pairwise encapsulation of the PAHs guests by
janusarene brings about emergent fluorescent properties which are
absent for either free PAHs in solution, or their bulk aggregates. It is
well-known that the relative orientation of chromophores plays a crucial
role in determining the optical and electronic behaviour. In this context,
dimers of a variety of chromophores become attractive model research
targets, which may provide fundamental understanding of the basic
photophysical processes and attractive spectroscopic properties possibly
useful for molecular sensors, switches, light-emitting materials38-48
.
However, generation of such dimers of chromophore is neither
straightforward nor general using either covalent or noncovalent
approaches, a challenge well-addressed by janusarene as shown above.
Fig.4. (A) UV-vis absorption spectral changes of fullerene C60 (2.0 × 10-4 M) upon
addition of C1 from 0 to 4.0 eq. Inset, titration curve with absorption at 450 nm as a
function of the equiv. of C1. (B) Partial 1H-NMR (8D-toluene) spectral changes of C1
(0.4 mM) upon addition of C60 from 0 to 2.4 eq. Resonance signals of the methoxy
protons at the rim of the binding cavity (blue arrow, uOMe) shift to the low field when
forming complex with C60, in contrast to the methoxy protons away from the binding
site, which show only modest spectral change (gray arrow, dOMe), inset, molecular
model of the C1-C60 1:1 complex.
We observe the excimer emissions of the PAH dimers for all three J1-
PAH complexes at room temperature. Compared with the solution state,
emission of the 0-0 band red-shifts ca. 93, 85, and 84 nm for pyrene,
perylene, and DMA, respectively. These emissions are also distinct from
that of the bulk PAHs, due to different orientation of the chromophore
49-52. For instance, J1-perylene complex emits light at 527 nm, which can
be attributed to the Y-type53-56 excimer, and displays a blue-shift of 52
nm from that of the bulk-state at 579 nm.
1H-NMR titration supports the formation of a C1-C60 complex.
Protons of the methoxy groups attached at the edge of the binding cavity
(uOMe) experience a significant down-field shift (> 0.05 ppm) in the
presence of fullerene (Figure 4B, blue arrow). Decreased electron
density of the anisole-f-ring upon complex formation is expected to be
responsible for the spectral changes. Job plot analysis indicates a 1:1
binding stoichiometry (Figure S9-10). An association constant of 1020
M-1 is derived from the titration curve (Figure S11-12). In solution, C1
does not form the sandwich complex as observed for J1 in the solid state
(Figure 2A). This discrepancy in the binding mode, as observed for other
reported fullerene complexes22,28, may be caused by the crystal packing
force. Currently, it is yet to be known whether the complexations in the
two binding cavities of one janusarene are coupled to any extent.
Fig.5. Normalized fluorescence spectra of pyrene (A, λex. = 310 nm), perylene (B, λex.
=
365 nm), and DMA (C, λex. = 310 nm) in solution (1.0 × 10-5M, DCM, black solid line),
bulk (black dash line), and encapsulated dimer (colored line, complex with J1) state at
room temperature. Inset, vitalized emission from the crystal of the complex.
In contrast to the spherical fullerene, titrations of J1 or C1 with the
PAH guests in various solvent conditions such as toluene,
dichloromethane (DCM), tetrahydrofuran, and mixed DCM/methanol
(1:1, v/v) indicate the absence of evident host-guest interactions. This
result suggests that formation of the J1-PAH complex occurs in and is
driven by the crystalizing process. It is noteworthy that the lack of strong
noncovalent interactions between janusarene and PAHs in common
organic solvents does not imply that janusarene has a limited potential
as a host in general, because when the solvophobic interactions are
brought to bear, significantly enhanced noncovalent binding would be
expected35-37. For instance, hydrophobic interaction is a major driving
In conclusion, we present a kind of readily accessible, homoditopic
molecular host, janusarene, which has back-to-back compactly arranged,
nanoscale binding cavities. Taking advantage of this unique structural
feature, janusarene has the ability to bind and align a variety of guest
compounds, different in shape and size, including the spherical fullerene
and planar PAHs. We anticipate that janusarene may find broad
applications in fields such as host-guest chemistry, molecular assembly,
biomimetics, and crystal engineering, which currently rely heavily on
the conventional CCCCP hosts.
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Angewandte Chemie International Edition
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This research was supported by the 1000 Young Talent Program (to
J.W.), the start-up fund from the Sun Yat-Sen University, the National
Natural Science Foundation of China (Grant Nos. 21372264), and the
Foundation of Guangzhou Science and Technology (201504010021).
We thank prof. Zhenguo Chi and Dr. Zhu Mao for assistance on the
measurement of solid state fluorecence.
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Keywords: Janusarene • Host • Assembly • Fullerene • Excimer
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10.1002/anie.201705451
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Two-face host: A kind of novel back-to-back
arranged homoditopic host compound,
janusarene, is presented. The unique two-face
structural feature of janusarene allows it to bind
and align guest compounds, varying in size and
shape, such as the spherical pristine fullerene
and the planar polycyclic aromatic hydrocarbons.
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