Molecular-strain engineering of double-walled tetrahedra

Article abstract of DOI:10.1016/j.chempr.2021.05.004

The synthesis of multi-walled nanostructures represents a formidable challenge owing to the limitations imposed by existing synthetic strategies. Herein, we report a strategy involving molecular-strain engineering (MSE) for constructing double-walled polyhedra from strained bow-shaped macrocycles called molecular bows (MBs). As a proof of concept, four double-walled tetrahedra have been assembled from L-shaped dual-panels subtending ～70.5° and producing MBs, which incorporate porphyrin and o-xylylene units, forming strained shape-persistent conformations. The head-to-tail closed-loop stacking of these units, driven by complementary non-covalent bonding interactions, leads to a large porphyrin-lined tetrahedron encapsulating a small o-xylylene-lined tetrahedron. The synergistic aromatic shielding effects emanating from the porphyrin tetrahedron result in the largest upfield chemical shifts of ?7.7 ppm for protons and ?8.8 ppm for carbons from those for the free dual panels. This demonstration, which features the launching of MSE for constructing sophisticated supramolecular nanostructures, could herald a fresh approach to the fabrication of multi-porphyrin photocatalytic systems.

Full text of DOI:10.1016/j.chempr.2021.05.004

ll
Article
Molecular-strain engineering of double-walled
tetrahedra
Min Tang, Yimin Liang, Xingyu
Lu, ..., Shangshang Wang, Lin
Wu, Zhichang Liu
liu@westlake.edu.cn
Highlights
Double-walled tetrahedra are
constructed as a hybrid model
from L-shaped dual panels
L-shaped dual panels feature
strained bow-like macrocycles
called molecular bows
The assembly of building blocks is
driven solely by non-covalent
bonding interactions
The porphyrin-incorporated
double-walled tetrahedra
enhance photocatalytic activity
Inspired by the tension-rigidified architecture of a bow, we present a molecular-
strain engineering strategy for constructing double-walled polyhedra from
strained bow-like macrocycles called molecular bows (MBs). Four double-walled
tetrahedra have been assembled from L-shaped dual panels subtending ꢀ70.5ꢁ
and producing MBs, which incorporate porphyrin and o-xylylene units, forming
strained shape-persistent conformations. The head-to-tail closed-loop stacking
between panels, driven by non-covalent bonding interactions, results in a small
benzene tetrahedron nesting supramolecularly inside a large porphyrin
tetrahedron.
Tang et al., Chem 7, 2160–2174
August 12, 2021 ª 2021 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2021.05.004

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Molecular-strain engineering
of double-walled tetrahedra
1
1
1
1
1
1
1
Min Tang, Yimin Liang, Xingyu Lu, Xiaohe Miao, Liang Jiang, Jiali Liu, Lifang Bian,
Shangshang Wang, Lin Wu, and Zhichang Liu1,2,3,*
1
1
SUMMARY
The bigger picture
The synthesis of multi-walled nanostructures represents a formi-
dable challenge owing to the limitations imposed by existing syn-
thetic strategies. Herein, we report a strategy involving molecular-
strain engineering (MSE) for constructing double-walled polyhedra
from strained bow-shaped macrocycles called molecular bows
Multi-walled nanostructures are
architectures that have
implications for materials science.
Their synthesis, however, remains
a formidable challenge on
(
MBs). As a proof of concept, four double-walled tetrahedra have
account of their complex (super)
structures known as the Russian
doll and Parchment models, along
with limitations that eliminate the
use of all existing synthetic
strategies. We report the
ꢁ
been assembled from L-shaped dual-panels subtending ꢀ70.5
and producing MBs, which incorporate porphyrin and o-xylylene
units, forming strained shape-persistent conformations. The head-
to-tail closed-loop stacking of these units, driven by complementary
non-covalent bonding interactions, leads to a large porphyrin-lined
tetrahedron encapsulating a small o-xylylene-lined tetrahedron. The
synergistic aromatic shielding effects emanating from the porphyrin
tetrahedron result in the largest upfield chemical shifts of ꢂ7.7 ppm
for protons and ꢂ8.8 ppm for carbons from those for the free dual
panels. This demonstration, which features the launching of MSE
for constructing sophisticated supramolecular nanostructures,
could herald a fresh approach to the fabrication of multi-porphyrin
photocatalytic systems.
construction of double-walled
tetrahedra, employing a blend of
the Russian doll and Parchment
models, from L-shaped dual
panels composed of strained
bow-like macrocycles called
molecular bows (MBs). The head-
to-tail closed-loop stacking of four
L-shaped dual panels, driven
solely by non-covalent
INTRODUCTION
interactions with conformational
self-templation, leads to the
formation of a large porphyrin-
lined tetrahedron, which
1
–3
Although the synthesis of multi-walled nanostructures represented
by one-
dimensional (1D) multi-walled carbon nanotubes (MWCNTs) is a remarkable feat
of nanotechnology and materials science, constructing 3D multi-walled nanostruc-
tures remains a formidable challenge, owing to the limitations associated with all
encapsulates a small o-xylylene-
lined tetrahedron. The
4
,5
of the present synthetic strategies. It follows that it is of interest to develop stra-
development of MBs with tunable
conformations from flexible units
provides a powerful molecular-
strain engineering approach to
assemble more complex
tegies for building superstructures of greater complexity and diversity than is pres-
6
–9
10–14
ently the norm.
While scaffolded multi-walled polyhedra
and Russian doll
10,12,14,19,23–26
1
5–22
complexes
have been constructed using metal coordination,
1
1,27,28
16–18,20,21,29,30
DNA origami,
and host-guest complexation,
closed-surface
4
multi-walled superstructures are extremely rare because of the need to design
outer and inner building blocks that complement each other geometrically and
stereoelectronically.
supramolecular multi-walled
Platonic solids.
MWCNTs are composed of multiple rolled layers of graphene assembled by means of
interlayer [p∙∙∙p] interactions. The special 1D multi-walled superstructure endows
MWCNTs with exceptional mechanical and electrical properties, enabling a broad range
3
1
of applications. Despite having two open ends, MWCNTs exhibit a nearly perfect
multi-walled superstructure, which relies on the complementarity between rolled
graphene layers expressed (Figure 1A) in the form of two models—namely, the Russian
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Figure 1. Design of the dual panel and its assembly into a double-walled tetrahedron
(
(
A) A hybrid version of the Russian doll and the Parchment models.
B) Unfolded exploded view of the building block showing a large triangle and a small triangle
connected by a trapezoid by two folding lines.
(
C and D) Top and side-on views of the building block adopting a ridge-tile-like conformation with
ꢁ
an angle of 70.5 .
(
E and F) Illustration of the assembly steps ①–④ involving four building blocks into a double-walled
tetrahedron (F) in a closed-loop complementary manner. For clarity, the four identical building
blocks in (E) are depicted in different colors, while the outer tetrahedron in (F) is shown as being
semitransparent.
See also Figure S42.
doll and the Parchment models. In the Russian doll model, multiple single-wall carbon
nanotubes of different diameters become nested in concentric cylinders, whereas in
the Parchment model, a single sheet of graphene rolls around itself, resembling a
rolled-up piece of paper. Upon blending the salient features of both models, we can
create (Figure 1A, right) a hybrid version. This model requires the design of a class of
shape-persistent dual panels, wherein the inner small and outer large panels, which
are complementary in geometrical and stereoelectronic features, are arranged with a
prescribed angle between them in order to enable the assembly of a desired double-
walled superstructure. By employing this design strategy, a range of double-walled su-
perstructures can be constructed by the cooperative head-to-tail assembly of multiple
building blocks to form closed loops, on account of the precisely defined co-conforma-
tions existing between the panels. Such a paradigm requires neither multiple preformed
single-walled components (Russian doll model) nor a single large continuous surface
1
Key Laboratory of Precise Synthesis of Functional
Molecules of Zhejiang Province, School of
Science, Westlake University, and Westlake
Institute for Advanced Study, 18 Shilongshan
Road, Hangzhou, Zhejiang 310024, China
2Twitter: @zhichang_liu
3Lead contact
(
Parchment model) but only carefully designed building blocks with well-defined confor-
*Correspondence: liu@westlake.edu.cn
https://doi.org/10.1016/j.chempr.2021.05.004
mational features so that the complementary multivalent recognition between the
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panels is optimized in a mutually cooperative manner, leading to the formation of the
3
2–34
desired multi-walled superstructures.
RESULTS AND DISCUSSION
Constructing double-walled tetrahedra (DWTs) from molecular bows
We have constructed the simplest double-walled tetrahedron (DWT) as a proof of
concept ( Figure S42A) of the viability of the hybrid model in a 3D setting. Consid-
ꢁ
35,36
ering the angle of ꢀ70.5 (Figure S42B) within a regular tetrahedron,
we first de-
signed (Figures 1B–1D) a shape-persistent dual panel, wherein a large triangular
panel and a small one are obliged to adopt conformations with an angle of
ꢁ
˚
ꢀ
70.5 and an offset of ꢀ3.6 A between the planes of the inner and outer panels
˚
in order to attain the ideal [p∙∙∙p] interaction distance of ꢀ3.4 A. Four building
blocks with their precisely defined conformations can now assemble to form (Fig-
ure 1E) a closed loop by head-to-tail stacking of the large outer panel of one building
block above the small inner panel of an adjacent building block in a highly synergistic
manner. Consequently, a closed-surface DWT is formed (Figure 1F) in a concentric
manner, wherein a tetrahedron composed of four inner panels nests inside a larger
tetrahedron assembled from four outer panels. When it comes to the molecule-level
design of such building blocks, however, integrating (1) two complementary planar
ꢁ
units—which (2) intersect at an angle of ꢀ70.5 —(3) within a shape-persistent molec-
ular skeleton is not straightforward to achieve on account that existing unstrained
molecular geometries are limited by the hybridization states of carbon. For instance,
although the angle between the two adjacent benzenoid substituents on aromatic
ꢁ
five-membered rings is ꢀ72 , this conformation is not shape-persistent because of
the rotation around s-bonds and occurs only when they are perpendicular to the
five-membered ring.
3
7–42
Inspired by the tension-rigidified architecture
of a bow (Figure 2A), we propose
a strategy we refer to as molecular-strain engineering (MSE) to facilitate the con-
struction of DWTs. MSE implies the use of strain to impose intramolecular tension
on molecules, which, on account of their precisely tunable strained conformations,
can exhibit enhanced performance in certain situations, such as the control-of-as-
4
3
sembly processes and reaction selectivities. We designed a class of bow-like
shape-persistent macrocycles—called molecular bows (MBs)—which consist of a
rigid conjugated aromatic unit (bow-limb) with a flexible chain (bowstring) incorpo-
rating an aromatic unit under tension joining both ends of the bow-limb. Owing to
the geometrical incompatibility between the bow-limb and bowstring, an aromatic
unit incorporated non-axisymmetrically into a bowstring folds up as a consequence
of tension to form a shape-persistent angle with the bow-limb aromatic unit. This
angle can be adjusted with precision by altering the strength of tension—that is,
the bowstring length.
Assembly of DWTs and their structural characterization
In order to realize this MSE strategy, we chose the readily modifiable meso-tetraphe-
4
4
nylporphyrin (TPP) as the bow-limb and some smaller aromatic rings—e.g., ben-
zene or furan—in the bowstring component to construct MBs based on cooperative
complementarities between matching steric and [p∙∙∙p] interactions involving the
aromatic residues in the bow-limb and bowstring. Molecular modeling was em-
ployed to screen the conformational properties of the MBs and the resulting super-
structures of the DWTs. We discovered that, upon linking (Figure 2B) the 5,10-meso
positions of the porphyrin unit with an o-xylylene unit, both energy-minimized struc-
tures of the resulting free-base and zinc porphyrins, MB-1 and MB-2, respectively,
adopt (Figure 2C) a ridge-tile-like (L-shaped) shape-persistent conformation,
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Figure 2. Molecular design of shape-persistent dual panels
(
(
A) Tension-rigidified structure of a bow.
B) Structural formulas of MB-1 and MB-2. Blue arrows indicate the directions of the stretching force
and gray arrows the folding up of the o-xylylene unit.
(
C) Ball-and-stick representation of the energy-minimized structure of MB-1 adopting a shape-
ꢁ
persistent ridge-tile-like conformation with an angle of 71 .
(
D) Schematic view of shape-persistent dual panel.
Porphyrin and o-xylylene units in (B) and (C) are depicted as red.
ꢁ
ꢁ
wherein the angle between the aromatic planes is fixed at 71 (close to 70.5 ), thus
satisfying (Figure 2D) the three critical design elements. Quantum chemistry calcu-
lations confirmed (Figure S52; Tables S7–S12) that both MB-1 and MB-2 are strain-
induced shape-persistent macrocycles, with strain energies of 10.6 and 11.2 kcal
ꢂ1
mol , respectively. With the designed MB-1 and MB-2 confirmed by both molecular
and quantum mechanical calculations, we turned our attention to their syntheses.
4
5
By taking advantage of the scrambling process in porphyrin synthesis, MB-1 was pre-
pared in one step (see supplemental information) as a purple solid from a reaction be-
0
tween a,a -bis(2-formylphenyl-1-yloxy)-o-xylene (1, Figures S1 and S2) and dipyrro-4-t-
butylphenylmethane (2, Figures S3, S4, and S17). MB-2 was synthesized by metalation
of MB-1 with Zn(OAc)
2
. The structures of both MB-1 and MB-2 were characterized by
1
13
H and C NMR spectroscopies (Figures S5–S10 and S18–S26) and mass spectrometry
1
(
Figure S36). The H NMR spectrum (Figure S5) of MB-1 reveals four sets of resonances
for the heterotopic b-protons (Hb1ꢂb4) on the porphyrin ring, an observation, which can
be rationalized by the AABB-constitution of the TPP unit and its bow-shaped structure,
1
resulting in C
s
symmetry. In comparison with the H NMR spectrum of 1 (Figure S1), the
o-xylylene resonances (Ha,b,c) in MB-1 are all shifted upfield by up to ꢂ0.5 ppm. These
appreciable changes in chemical shifts indicate that the o-xylylene bridge is shielded
by the porphyrin plane because of its location above this plane. This observation is
consistent with the computationally derived conformation of MB-1. In particular, Hc1
and Hc2 associated with two ꢂCH ꢂ linkers exhibit two doublets, indicating that,
2
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upon stretching, both these linkers are constrained as a result of the rigid conformation
that exists between the o-xylylene and porphyrin units.
Variable-temperature (VT) NMR spectroscopy was carried out in order to investigate
2 2
the assembly of MB-1 in solution. Upon decreasing the temperature of a CD Cl so-
ꢁ
ꢁ
lution of MB-1 from +30 C to ꢂ90 C, we observed (Figure S29) significant upfield
chemical shifts for the Ha,b,c and NH resonances, accompanied by significant broad-
ening of the Ha,b,c resonances, as well as changes in the line shapes, from sharp to
broad and back to sharp again, of the resonances for the b- and NH protons on
the porphyrin unit. These observations suggest that (1) the intermolecular aggrega-
tion between the o-xylylene and porphyrin planes through [p∙∙∙p] stacking results
in strong mutual shielding between the two planes and that (2) MB-1 undergoes a
step-by-step assembly process to form the DWT-1.
1
13
H and C NMR spectroscopic investigations of MB-2 confirm (Figure 3) the selective
assembly of MB-2 into the corresponding DWT-2 in the solution phase. At 1.2 mM con-
1
3
centration in CDCl , MB-2 displays (Figure 3B) an H NMR spectrum composed of 17 res-
onances for heterotopic protons—similar to those recorded for MB-1 (Figure 2B) in Fig-
1
ure S5—suggesting (Figure 3A) the presence of a plane (s) of symmetry in MB-2. The H
1
3
and C NMR spectra of MB-2 are, however, both highly concentration dependent.
1
Upon increasing the concentration of MB-2, its H NMR spectrum (Figure S32) exhibits
a new set of resonances, which arise and increase in intensity compared with those
observed for MB-2 at low concentrations. When the concentration of MB-2 reaches
7
5.9 mM, the new set of resonances become the dominant signals, whereas the original
resonances for MB-2 weaken in intensity so as to become unrecognizable. Diffusion or-
dered spectroscopy (DOSY) of MB-2 at 1.2 and 75.9 mM reveals (Figures 3F and 3G) two
ꢂ10
ꢂ10
2
distinct single bands with diffusion coefficients (D) of 6.87 3 10
and 3.50 3 10
m
ꢂ1
s
, which correspond to diameters of approximately 1.20 and 2.36 nm, respectively, for
MB-2 and DWT-2. The diffusion coefficients indicate that, although MB-2 exists as a
collection of free monomers at 1.2 mM, it assembles, to all intents and purposes, into
4
the [MB-2] tetramer—namely, DWT-2—at an MB-2 concentration of 75.9 mM. After as-
signing the new set of resonances to DWT-2 and observing the equilibrium 4 MB-2 $
1
DWT-2 ([MB-2]
4
) to be slower than the H NMR timescale, we were able to obtain an as-
9
ꢂ3
sociation constant, K
a
1
, of (1.24 G 0.32) 3 10 M in CDCl
3
at room temperature by car-
46
rying out (Figure S32) H NMR dilution experiments. Upon decreasing the temperature
ꢁ
1
of the 75.9 mM solution of MB-2 to ꢂ42 C, we obtained well-resolved H (Figure 3C) and
1
3
C (Figure 3E) NMR spectra for DWT-2 on account of its >99% purity, based on integra-
1
1
tion of its resonances compared with those for MB-2. Hꢂ H COSY NMR spectroscopic
analysis enabled (Figure S20) us to assign unambiguously all 34 resonances to two het-
erotopic t-butyl groups (each with their three isochronous methyl groups) and the other
3
2 heterotopic protons in the MB-2 building block. This number (34) is double the num-
1
ber (17) of resonances present in the H NMR spectrum of the completely dissociated
MB-2, an observation, which indicates the breaking of the C symmetry of the MB-2
building block within DWT-2, leading to the anisochronous nature of all the protons,
s
1
1
including the two t-butyl groups. The Hꢂ H ROESY spectrum of DWT-2 exhibits (Fig-
ure S26) cross peaks between H and Hb2/Hb3, Hc1 and Hb4, as well as between Hc1
and Hb2 —all of which are spatially close in the expected superstructure (Figure 3A) of
0
a
0
DWT-2 but far away from each other within the completely dissociated MB-2, an obser-
vation that confirms the formation of DWT-2 in the solution phase.
,b,b ,c1,c1 ,c2,c2
Remarkably, all eight o-xylylene resonances (Figure 3C) for Ha,a exhibit
0 0 0 0
substantial upfield shifts from 4.5 ꢀ 6.5 ppm in the completely dissociated MB-2 to
ꢂ1.5 ꢀ 3 ppm in DWT-2, commensurate, to the best of our knowledge, with an
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Figure 3. NMR spectroscopic characterization of MB-2 and DWT-2
(
(
(
A) Assembly of four MB-2 building blocks into DWT-2.
1
B and C) H NMR spectra (600.23 MHz, CDCl
3
) of MB-2 (B, 1.2 mM, 298 K, total 17 resonances) and DWT-2 (C, 75.9 mM, 231 K, total 34 resonances).
1
3
D and E) C NMR Spectra (CDCl
3
) of MB-2 (D, 125.78 MHz, 1.2 mM, 298 K, total 27 resonances) and DWT-2 (E, 150.94 MHz, 75.9 mM, 231 K, total 56
resonances). Magenta :, C, - annotate a-carbons, b-carbons, and meso-carbons on porphyrin, respectively.
F and G) DOSY spectra (600.23 MHz, CDCl , 298 K) of MB-2 (F, 1.2 mM) and DWT-2 (G, 75.9 mM).
Concentrations of all samples are initial concentrations of MB-2. See also Figures S7–S9, S29, S30, S32, S33, and S35.
(
3
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unprecedented change (Dd = ꢂ7.7 ppm) in chemical shift for H
0
. In an attempt to
b
rationalize these large changes in chemical shifts, we carried out nucleus indepen-
dent chemical shift (NICS) calculations (Table S13) based on the structure of DWT-
2
on the right in Figure 3A. The differences in the calculated NICS values (Table
S4) for the chemical shifts of the pairs of heterotopic resonances for the o-xylylene
1
protons in DWT-2 match very well with the Dd values (Table S5) in the H NMR spec-
trum (Figure 3C) of DWT-2. These observations indicate that the assembled super-
structure of MB-2 at high concentration corresponds with DWT-2, wherein the asym-
metric [p∙∙∙p] stacking between o-xylylene and porphyrin units results in
4
7,48
substantial upfield shifts of the resonances for their protons,
reflecting the ani-
sochronous nature of all protons in their local anisotropic environments.
1
3
The C NMR spectrum of DWT-2 exhibits (Figure 3E) a total of 56 resonances—i.e.,
1
3
more than twice the 27 resonances observed in the C NMR spectrum of the
completely dissociated MB-2. 2D HSQC and HMBC spectroscopies allowed (Figures
S22 and S24) us to assign the 56 resonances to two heterotopic t-butyl groups (four
resonances) and all the other 52 heterotopic carbon atoms, including all 20 carbon
atoms on the porphyrin ring, confirming the anisochronous nature of all carbon
4
9
1
atoms in the MB-2 building block within DWT-2. In line with the H NMR spectrum
of DWT-2, all eight o-xylylene carbon resonances (Ca,a ) exhibit (Figures 3D
and 3E), to the best of our knowledge, unprecedented upfield shifts (Dd = ꢂ2.9 ꢀ
0
0 0 0
,b,b ,c,c ,t,t
1
3
ꢂ8.8 ppm) compared with those for the o-xylylene carbon resonances in the
C
NMR spectrum of the completely dissociated MB-2. The aromatic-walled nanospace
associated with DWT-2, resulting in strong shielding and anisotropic effects, is remi-
5
0
niscent of a diametrically opposite example reported by Nitschke, wherein an anti-
aromatic-walled nanospace is surrounded by four norcorroles, leading to strong de-
shielding and anisotropic effects.
In order to confirm the formation of DWT-1 and DWT-2 in the solution phase, we also
carried out electrospray ionization high-resolution mass spectrometry (ESI-HRMS)
experiments. ESI-HRMS analyses revealed (Figures S37A and S37B) major peaks at
+
mass-to-charge (m/z) ratios of 3,444.6333 for DWT-1 ([M + H] ) and 3,698.2754
+
2+
DWT-2 ([M + H] and [2M + 2H] overlapped), as well as isotopic distribution pat-
terns that match well with the calculated ones for the corresponding tetrahedra,
respectively, observations that indicate unambiguously the formation of DWT-1
and DWT-2 in the solution phase.
Single crystals of DWT-1 and DWT-2 suitable for X-ray diffraction were grown at
3
room temperature by slow liquid-liquid diffusion of MeOH into CHCl , respectively.
X-ray diffraction analyses (Table S1) provide unambiguous evidence for the forma-
tion of the double-walled tetrahedral superstructures, DWT-1 and DWT-2, in the
solid state. DWT-1 (Figures S43 and S48) and DWT-2 (Figures 4 and S44) are isostruc-
tural and composed of four MB-1 and four MB-2 building blocks, respectively. Both
the MB-1 and MB-2 building blocks adopt local C
s
symmetric ridge-tile-shaped con-
formations in which the mean planes of o-xylylene and porphyrin units subtend an-
ꢁ
ꢁ
gles of 71.4 and 68.3 in MB-1 and MB-2, respectively. Their computationally
derived conformations, which satisfy the requirements for the formation of dou-
ble-walled tetrahedra, are observed in the solid-state superstructures of DWT-1
and DWT-2. Here, we discuss the solid-state superstructure of DWT-2. The two
ꢁ
ꢁ
bond angles (Figure 4B) of 124 and 125 , which are associated with the two
ꢂCH
2
ꢂ linkers to the phenylene ring constituting the o-xylylene unit, are greater
ꢁ
than the normal angle of 120 . In addition, the porphyrin plane is slightly bent. These
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Figure 4. The solid-state superstructure of DWT-2
ꢁ
(
A and B) Side-on and front views of the solid-state structure (stick models) of MB-2 showing a tensioned macrocycle with a 68.3 angle between
porphyrin and o-xylylene planes.
C) The head-to-tail stacking of two building blocks illustrating the complementary [p∙∙∙p] and [C–H∙∙∙p] interactions between the porphyrin and o-
xylylene units. C, tan; H, white; O, red; N, blue; Zn, silver gray.
(
(
(
D) Step-by-step illustration of the assembly of four MB-2 building blocks into a DWT-2. 4-t-butylphenyl groups are omitted for the sake of clarity.
E and F) A small tetrahedron composed of four o-xylylene rings and a large tetrahedron enclosed by four porphyrin planes. Both porphyrin and o-
xylylene units in (C–F) are depicted as stick models superimposed with semitransparent space-filling models.
G) Front view of the S -symmetric DWT-2 showing that a large tetrahedron (stick model) composed of four porphyrin units encapsulates a small
tetrahedron (space-filling model) assembled from four o-xylylene units.
(
4
(
(
(
H) Space-filling model of DWT-2 showing a compact structure with a closed surface.
I–K) Front, top, and schematic views of the packing superstructure of five DWTs in different colors for clarity.
L) SEM image of single crystals of DWT-2.
All H atoms except two ꢂCH
2
ꢂ linkers in (A) and (B), one in (C), and all in (H), are omitted for the sake of clarity. Four building blocks in (DꢂH) are
depicted as red, blue, orange, and green for the sake of clarity. See also Figures S41 and S44 and Video S1.
Chem 7, 2160–2174, August 12, 2021 2167

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observations confirm that this macrocycle is strained and, as a result, dictates the for-
mation of the shape-persistent, ridge-tile-like conformation of MB-2.
Precise molecular recognition between the four MB-2 building blocks leads to the
formation of the double-walled tetrahedron DWT-2. The o-xylylene unit in each
MB-2 building block is embraced (Figure 4C) from one of its sides by another MB-
˚
2
building block, through (1) [p∙∙∙p] interactions (3.31 A) between the o-xylylene
˚
0∙∙∙p) between inter-
and porphyrin planes, (2) [C–H∙∙∙p] interactions (2.85 A, H
b
secting o-xylylene units, as well as (3) between two ꢂCH
2
ꢂ linkers and the porphyrin
˚
plane (2.50, 2.89 A, H∙∙∙p). The stereoelectronic interactions support strong aniso-
tropic and shielding effects resulting from the intermolecular asymmetric [p∙∙∙p]
1
3
1
and [C–H∙∙∙p] stacking evident in the C and H NMR spectra of DWT-2 at high
concentrations. Consequently, four MB-2 building blocks, directed by angles of
ꢁ
6
8.3 subtended by the o-xylylene and porphyrin planes, are held (Figure 4D; Video
S1) together in a highly cooperative and precise head-to-tail manner forming a
closed loop, i.e., DWT-2. In this discrete S -symmetric supramolecular DWT, four
4
benzenoid rings form (Figure 4E) a small tetrahedron that is nested inside a large
tetrahedral chamber enclosed (Figure 4F) by four porphyrin rings. This closed-sur-
face DWT (Figure 4H) constitutes a compact supermolecule with a diameter of
ꢀ
2.4 nm, matching well with the hydrodynamic diameter of ꢀ2.36 nm obtained
3
˚
from the DOSY NMR experiments in CDCl
3
. The calculated volume of 9,591 A
from the unit cell indicates that there are no voids inside DWT-2 and confirms that
the cavity of the large tetrahedron is fully occupied by the small tetrahedron in
both the solution-phase and solid-state superstructures.
ꢁ
The mean values of the angles in the small and large tetrahedra—namely, 72.4 and
ꢁ
ꢁ
7
3.4 , respectively—are within striking distance of that (70.5 ) of a regular tetrahe-
dron, making it possible for the rigid conformation of MB-2 to come together in units
of four to form DWT-2. The precision with which these four building blocks assemble,
using mutual [p∙∙∙p] and [C–H∙∙∙p] interactions between their o-xylylene and
porphyrin planes, into two nested tetrahedra at high concentrations is remarkable.
The DWT exhibits a blend of the characteristics of both the Russian doll and Parch-
ment models—i.e., all four MB-2 building blocks contain both outer and inner panels
from which an outer one in each building block is stacked on the outside of an inner
panel in an adjacent building block. In the overall superstructure of DWT, each dou-
ble-walled tetrahedron is surrounded (Figures 4I and 4J) by four congeners stabi-
˚
lized by weak [p∙∙∙p] interactions (3.8 A) between neighboring porphyrin rings in
5
1
a J-aggregated manner. This supramolecular packing is reflected (Figure 4K) in
an emergent manner on macroscopic scale (Figures 4L and S41) in the shape of
5
2
the single crystals of DWT-2, where the morphology corresponds to a z- axis-com-
pressed D4h-symmetrical tetragonal octahedron. Finally, the experimental powder
5
3
X-ray diffraction pattern of the rapidly precipitated sample of MB-2 matches (Fig-
ure S39) that derived from the crystal data, indicating that this sample corresponds
to the crystalline DWT-2.
Coordination of NO to the Co(II)porphyrin modulates the formation of DWT-3
5
4
Coordination assembly is a convenient and versatile strategy for the supramolec-
5
5
ular synthesis of discrete and elaborate multi-porphyrin architectures that mimic
naturally occurring systems. This strategy exploits axial coordination at the metal
5
6
centers of metalloporphyrins, leading to significant changes in properties between
the monomers and their assemblies. We have also demonstrated the ability of the
MSE strategy to produce a multi-Co(II)porphyrin tetrahedron by means of [p∙∙∙p]
and [C–H∙∙∙p] interactions.
2168 Chem 7, 2160–2174, August 12, 2021

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Figure 5. NO-modulated assembly of MB-3 into DWT-3
1
(
A and B) H NMR spectra (600.23 MHz, CDCl
3
, 298 K) of MB-3 (A, magenta) and its nitrosyl complex
MB-4 (B, green).
C) Structural formulas and single-crystal X-ray (super)structures of MB-3, MB-4 and DWT-3 as well
(
as the interconversion between them. Colors of elements in X-ray structures of MB-3 and MB-4 are
depicted as C, tan; H, white; O, red; N, blue; Co, pink. All H atoms are omitted for the sake of clarity.
Four building blocks in DWT-3 are depicted as red, blue, orange, and green for the sake of clarity.
See also Figures S11–S14, S31, S45, S46, S49, and S50.
2
The cobalt complex MB-3 was synthesized by metalation of MB-1 with Co(OAc) .
1
The H NMR spectrum of MB-3 exhibits (Figure 5A) a set of broad resonances
ranging from 0 to 16 ppm, suggesting that it exists in a high-spin state. Concentra-
1
tion-dependent H NMR experiments revealed (Figure S34) the rapid assembly of
MB-3 building blocks, confirming the retention of the high-spin state of MB-3
upon aggregation in solution at concentrations of up to 94.6 mM. ESI-HRMS ana-
lyses revealed (Figure S37C) a major peak at a m/z ratio of 3,671.2974 for DWT-3
$+
2($+)
(
M
and [2M]
overlapped), and an isotopic distribution pattern that matches
well with the calculated one, an observation that confirms the formation of DWT-3
in the solution phase. Single crystals of MB-3 suitable for X-ray diffraction were
grown at room temperature by slow liquid-liquid diffusion of MeOH into CHCl
X-ray diffraction analysis confirmed (Figures 5C, S45, and S49; Table S2) the forma-
tion of a C -symmetrical double-walled tetrahedron DWT-3 from MB-3 in which the
3
.
2
2
+
Co center is tetracoordinated. In order to investigate the influence of axial coordi-
nation on the assembly and the properties of MB-3, we introduced nitric oxide (NO)
2
+
as an axial ligand on Co and prepared the nitrosyl complex MB-4 (Figures 5C and
1
S46). The H NMR spectrum of MB-4 exhibits (Figures 5B and S27) a set of well-
resolved resonances ranging from 0 to 9 ppm, confirming the low-spin state of
MB-4. Single crystals of MB-4 suitable for X-ray diffraction were grown by slow evap-
3
oration of its CHCl solution. X-ray diffraction analysis of MB-4 reveals (Figure 5C) the
2
+
coordination of a bent NO ligand to Co selectively positioned on the inner face of
the L-shaped conformation of MB-4 as a consequence of strain-induced negative
Chem 7, 2160–2174, August 12, 2021 2169

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Article
Figure 6. Photodegradation of dyes using DWTs as photocatalysts
(
A) Reaction scheme for the photodegradations of Rhodamine B (RhB) and crystal violet (CV) using
DWTs as photocatalysts.
B) Time-dependent UV-vis absorption spectra (lmax = 554 nm) following the photodegradation of
RhB carried out in aqueous solution in a 20-mL vial in the presence of H (0.5 mL) using DWT-2
(
2
O
2
(
(
3 mg) as a photocatalyst.
C) Comparison of photocatalytic performances of DWT-2 (red), TtPPZn (blue), and blank (black) for
the degradation of RhB. C
T is the reaction time.
0 T
and C indicate the initial and intermediate concentrations of RhB, while
See also Figures S53–S58.
curvature on the porphyrin plane. Since NO occupies the cavity of MB-4 in its L-
shaped conformation, it cannot form a corresponding DWT, but rather crystallizes
(
see Figure S50) as a head-to-tail dimer. These observations indicate that the forma-
tion of DWTs can be modulated by selective axial coordination of NO to the Co(II)
porphyrin.
Influence of substituents on the formation of DWTs
In order to explore the generality of this MSE strategy, we investigated the influ-
ence of substituents on the formation of DWTs. We synthesized (Figures S15,
S16, and S28) a monomer MB-5 in which the two t-butyl groups in MB-1 were re-
placed with two bromine atoms. Both HRMS and single-crystal X-ray crystallog-
raphy confirmed the formation of the tetrahedron DWT-5 in both the solution
phase and the solid state. ESI-HRMS analyses revealed (Figure S37D) a major
+
peak at a m/z ratio of 3,627.4260 for DWT-5 ([M + H] ) with an isotopic distribution
pattern that matches well with the calculated one, an observation that confirms the
formation of DWT-5 in the solution phase. The X-ray solid-state superstructure re-
vealed (Figures S47 and S51; Table S3) the fact that the tetrahedral superstructure
of DWT-5 is isostructural with DWT-1–3. In contrast to the 3D packing superstruc-
ture between the tetrahedra in DWT-1–3, only two porphyrin faces of DWT-5 form
[
p∙∙∙p] interactions (Figures S51E–S51G) with two neighboring tetrahedra assem-
bling into a 1D zigzag chain of tetrahedra and leaving two remaining porphyrin
2170 Chem 7, 2160–2174, August 12, 2021

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Article
faces of each tetrahedron in contact with solvent molecules. This observation indi-
cates that the formation of DWT-5 does not result from the packing of the
porphyrin dimers. The fact that changes in the peripheral substituents do not inter-
fere in the formation of the tetrahedral core proves that this class of double-walled
tetrahedra can be functionalized with a wide range of functional groups, allowing
us the opportunity to explore their potential applications.
Photocatalytic degradation of dyes by DWTs
Porphyrin nanostructures have been demonstrated to have photocatalytic activity in
5
7–59
biological photochemical processes.
In order to evaluate the photocatalytic
performance of these porphyrin DWTs, we carried (Figures 6 and S53–S58) out pho-
todegradation experiments on two typical industrial pollutants, namely, Rhodamine
2 2
B (RhB) and crystal violet (CV) in aqueous solutions in the presence of H O using
crystalline DWT-1 and DWT-2 as heterogeneous photocatalysts. For comparison,
meso-tetra(4-t-butylphenyl)porphyrin (TtPP), and its zinc complex TtPPZn, were
chosen as reference catalysts for DWT-1 and DWT-2, respectively. Blank experi-
ments without catalyst in the presence of H
assess the effect of photocatalysts. Upon irradiating aqueous solutions of RhB
and CV with visible light in the presence of H employing the photocatalysts
2 2
O were also conducted in order to
2
O
2
DWT-1, DWT-2, TtPP, and TtPPZn, a considerable decrease (Figures S55–S57) in
the intensities of the peaks decreased for RhB at 554 nm and for CV at 591 nm
were observed in each case in line with the photodegradation of RhB and CV.
The photodegradation of RhB and CV with DWT-1 and DWT-2 was (Figure S58)
much faster than those with TtPP and TtPPZn, whereas blank experiments without
a photocatalyst were the slowest reactions of all. In particular, the photodegrada-
tion rate constant of RhB with DWT-2 is (Figure 6C; Table S6) three times as much
as that for the one with TtPPZn and 4.7 times as much as that of the corresponding
blank experiment. These observations indicate that porphyrin photocatalysts accel-
erate the degradation of dyes and that DWT-2 has a better photocatalytic perfor-
mance than TtPPZn. In contrast to the planar TtPPZn, the formation of tetrahedra
enhances the photocatalytic activity of the porphyrin units, presumably because
the tetrahedron superstructure hinders the aggregation of porphyrin units. Conse-
quently, the recombination of electron-hole pairs, generated from visible-light irra-
6
0
diation, is suppressed, thus facilitating the production of photogenerated active
species such as $OH (Figure 6A) to transform dye molecules into oxidized products
on the surface of the DWTs.
Outlook
We have developed a MSE strategy, which operates in unison with molecular recog-
nition to realize the precise construction of closed-surface double-walled poly-
hedra. Employing a blend of the Russian doll and Parchment models, double-walled
tetrahedra can be assembled from judiciously designed dual-panel building blocks
with well-defined conformational features, rather than from multiple preformed sin-
gle-walled components (Russian doll model) or a single large continuous surface
(
Parchment model). The development of bow-like tensioned macrocycles provides
a powerful MSE approach to the construction of unusual shape-persistent dual
panels with precisely adjustable conformations from flexible units. Dual panels
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
with angles of 70.5 , 90 , 109.5 , 116.6 , and 138.2 subtended by two panels,
both of which are complementary to each other geometrically and stereoelectroni-
cally, can, in principle, act cooperatively in a supramolecular context to assemble,
not only double-walled tetrahedra but also cubes, octahedra, dodecahedra,
and icosahedra—namely, the five Platonic solids. This strategy lays the foundations
for the synthesis of supramolecular multi-walled architectures, the photoactive
Chem 7, 2160–2174, August 12, 2021 2171

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Article
nature of which might well result in promising supramolecular photocatalytic
systems.
EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources and reagents should be directed to
and will be fulfilled by the lead contact, Zhichang Liu (liu@westlake.edu.cn).
Materials availability
All unique/stable reagents generated in this study are available from the lead con-
tact with a completed materials transfer agreement.
Data and code availability
Crystallographic data for MB-1ꢂMB-5 have been deposited at the Cambridge Crystallo-
graphic Data Centre under CCDC: 1997220, 1997221, 2035701, 2035702, and 2062813,
respectively, which can be obtained via www.ccdc.cam.ac.uk/data_request/cif.
Full experimental procedures are provided in the supplemental information.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.chempr.
2
021.05.004.
ACKNOWLEDGMENTS
We thank Sir Fraser Stoddart for his assistance in editing the manuscript. We are
grateful for financial support from the National Natural Science Foundation of China
(
21971211), the Qiantang River Talent Foundation (QJD1902029), and Westlake
University. This work is supported by both the Instrumentation and Service Centers
for Molecular Science (ISCMS) and the Instrumentation and Service Centers for Phys-
ical Science (ISCPS) as well as Westlake University HPC Center. We also thank Profes-
sor Biaobiao Zhang at CAP for Solar Fuels @Westlake and Dr. Zhong Chen for their
help on photocatalysis, Drs. Xiaohuo Shi and Yinjuan Chen for their help on NMR and
mass spectroscopic analyses, and Shilong Zhang for his help on crystal structure
refinement, respectively.
AUTHOR CONTRIBUTIONS
Z.L. and M.T. conceived and designed the research and composed the manuscript.
M.T. and Z.L. carried out experiments and analyzed the data. M.T. and X.L. contrib-
uted to NMR spectroscopic analysis. M.T., X.M., and L.W. contributed to X-ray crys-
tallographic analysis. Z.L. is the principal investigator of the Laboratory for Supramo-
lecular Organic Functional Assemblies (SOFA LAB) and supervised the research. All
authors discussed and commented on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: November 2, 2020
Revised: December 22, 2020
Accepted: May 7, 2021
Published: June 10, 2021
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