Emergent Self-Assembly of a Multicomponent Capsule via Iodine Capture

Article abstract of DOI:10.1021/jacs.0c11838

Described here is a three-component self-assembly system that displays emergent behavior that differs from that of its constituents. The system comprises an all-hydrocarbon octaaryl macrocycle cyclo[8](1,3-(4,6-dimethyl)benzene (D4d-CDMB-8), corannulene (Cora), and I2. No appreciable interaction is seen between any pair of these three-components, either in cyclohexane or under various crystallization conditions. On the other hand, when all three-components are mixed in cyclohexane and allowed to undergo crystallization, a supramolecular iodine-containing capsule, ((D4d-CDMB-8)3(Cora)2)I2, is obtained. This all-hydrocarbon capsule consists of three D4d-CDMB-8 and two Cora subunits and contains a centrally bound I2 molecule as inferred from single-crystal and powder X-ray diffraction studies as well as solid-state 13C NMR and Raman spectroscopy. These analyses were complemented by solution-phase 1H NMR and UV-vis spectroscopic studies. No evidence of I2 escape from the capsule is seen, even at high temperatures (e.g., up to 418 K). The bound I2 is likewise protected from reaction with alkali or standard reductants in aqueous solution (e.g., saturated NaOH(aq) or aqueous Na2S2O3). It was also found that a mixed powder containing D4d-CDMB-8 and Cora in a 3:2 molar ratio could capture saturated I2 vapor or iodine from aqueous sources (e.g., 1.0 mM I2 in NaCl (35 wt %) or I2 + NaI(aq) (1.0 mM each)). The present system displays structural and functional features that go beyond what would be expected on the basis of a simple sum-of-the-components analysis. As such, it illustrates a new approach to creating self-assembled ensembles with emergent features.

Full text of DOI:10.1021/jacs.0c11838

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Emergent Self-Assembly of a Multicomponent Capsule via Iodine
Capture
Yu-Dong Yang, Xu-Lang Chen, Jonathan L. Sessler,* and Han-Yuan Gong*
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ABSTRACT: Described here is a three-component self-assembly
system that displays emergent behavior that differs from that of its
constituents. The system comprises an all-hydrocarbon octaaryl
macrocycle cyclo[8](1,3-(4,6-dimethyl)benzene (D4d-CDMB-8),
corannulene (Cora), and I2. No appreciable interaction is seen
between any pair of these three-components, either in cyclohexane
or under various crystallization conditions. On the other hand,
when all three-components are mixed in cyclohexane and allowed
to undergo crystallization, a supramolecular iodine-containing
capsule, ((D4d-CDMB-8)3⊃(Cora)2)⊃I2, is obtained. This all-
hydrocarbon capsule consists of three D4d-CDMB-8 and two Cora
subunits and contains a centrally bound I2 molecule as inferred from single-crystal and powder X-ray diffraction studies as well as
1
solid-state 13C NMR and Raman spectroscopy. These analyses were complemented by solution-phase H NMR and UV−vis
spectroscopic studies. No evidence of I2 escape from the capsule is seen, even at high temperatures (e.g., up to 418 K). The bound I2
is likewise protected from reaction with alkali or standard reductants in aqueous solution (e.g., saturated NaOH(aq) or aqueous
Na2S2O3). It was also found that a mixed powder containing D4d-CDMB-8 and Cora in a 3:2 molar ratio could capture saturated I2
vapor or iodine from aqueous sources (e.g., 1.0 mM I2 in NaCl (35 wt %) or I2 + NaI(aq) (1.0 mM each)). The present system
displays structural and functional features that go beyond what would be expected on the basis of a simple sum-of-the-components
analysis. As such, it illustrates a new approach to creating self-assembled ensembles with emergent features.
works (SOFs),4 molecular cages,5 molecular capsules,6
INTRODUCTION
In 1875, George Henry Lewes expressed in philosophical terms
■
molecular knots,7 molecular machines,8 and so forth.9 In
the concept of emergence, basically codifying a phenomenon
that permeates the social and natural sciences wherein the
behavior of a system as a whole cannot be reduced to the sum
or difference of its individual components.1 Often the
emergent features are the result of complex, cooperative
interactions among the various constituents. For instance,
atoms may form small molecules via chemical bonds, which
then can undergo polymerization to yield macromolecules
(e.g., DNA and proteins) that permit cell construction. Further
organization can lead to tissues, organs, and ultimately living
entities. In this canonical sequence, multicomponent con-
structs at each level display emergent behavior in terms of both
structure and function. Synthetic self-assembled systems
displaying emergent behavior offer a means to understanding
the determinants underlying natural, chemical-based emergent
systems.
general, these systems rely on apparent pairwise interactions
(e.g., strong binding) between the individual components.
However, in principle self-assembly can be achieved in the
absence of such direct interactions, thus mimicking the
emergent behavior seen in many natural systems.10 To the
best of our knowledge, there are no reports of multicomponent
artificial systems that self-assemble in the complete absence of
direct component-to-component pairwise interactions. Here
we report a self-assembled solid-state supramolecular capsule
that displays emergent behavior in that all three constituents
are necessary to drive the self-assembly, which occurs in the
absence of pairwise interactions (Scheme 1). A noteworthy
feature of this system is that all components are uncharged.
At present, emergent molecular systems based on covalent
bonds are reasonably well understood. However, it remains a
challenge to create emergent self-assembled systems organized
via weak intermolecular interactions.2 In recent years, several
artificial self-assembled systems involving three or more
components have been reported. Examples include metal−
organic frameworks (MOFs),3 supramolecular organic frame-
Received: November 11, 2020
https://dx.doi.org/10.1021/jacs.0c11838
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© XXXX American Chemical Society
A

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to remove it via sublimation at temperatures of up to 418 K
and a lack of reactivity when exposed to saturated NaOH(aq)
or aqueous Na2S2O3 solutions. It was also found that a solid-
state mixture of D4d-CDMB-8 and Cora was able to capture
iodine efficiently from I2 vapor or aqueous sources of I2. In
contrast, no evidence of interaction was seen for the
corresponding solid form of either D4d-CDMB-8 or Cora.
Effective entrapment of iodine thus required full participation
of all three constituents, namely, D4d-CDMB-8, Cora, and I2.
We believe that the approach reported here provides an
attractive strategy for storing small active molecules while
illustrating a possible new utility for synthetic emergent
systems.
Scheme 1. Schematic Representations of Three-Component
Self-Assembly
Studies of Possible Interactions in Solution. D4d-
CDMB-8, Cora, and I2 are all soluble in CYH. This solvent
was thus chosen to test the extent of pairwise interactions
between these species. In fact, no appreciable changes in the
RESULTS AND DISCUSSION
■
1
UV−vis or H NMR spectra were seen upon mixing any two
In previous work, we reported that the all-hydrocarbon
macrocycle, cyclo[8](1,3-(4,6-dimethyl)benzene (D4d-
CDMB-8), could act as a receptor for fullerenes, perylene,
and a series of solvent molecules (e.g., toluene, THF,
acetonitrile, nitrobenzene, etc.).11 These relatively large
substrates were found to be bound within the bowl-like cavity
of D4d-CDMB-8 via π−π and/or CH−π interactions.
However, smaller linear molecules (e.g., acetonitrile) were
found to thread through the center of the macrocycle, with the
resulting bound form being stabilized by possible CH−π
interactions. These findings led us to consider that D4d-
CDMB-8 might interact with an appropriately sized curved
aromatic compound to form a capsule-like construct capable of
capturing small linear molecules. As discussed further below,
this design expectation was realized in the case of corannulene
(Cora) and I2. Little evidence of binding was seen when D4d-
CDMB-8, Cora, or I2 was allowed to interact with one another
in a pairwise fashion in cyclohexane (CYH) in homogeneous
solution and under conditions favoring crystallization. In
contrast, when all three-components were mixed in CYH and
allowed to crystallize, a self-assembled capsule containing an
entrapped iodine molecule, (((D4d-CDMB-8)3⊃(Cora)2)⊃I2),
was obtained (Scheme 2). The normal chemical behavior
expected for I2 was suppressed as a result of being bound
within the self-assembled capsule, as evidenced by an inability
components in (deuterated) CYH or in fact all three
components at a final concentration of 0.50 or 3.0 mM in
each species (cf. Supporting Information Figures S1−S5). We
thus infer that any putative interspecies interactions are
insufficient to overcome solvation effects.
Studies of Mixtures Containing Two Components
under Crystallization Conditions. CYH solutions contain-
ing D4d-CDMB-8 (3.0 mM) and 1 molar equiv of either Cora
or I2 (but not both concurrently) were subject to slow
evaporation at room temperature (298 K). Under these
conditions, only single crystals of [D4d-CDMB-8⊃CYH·
(CYH)4.5] and [Cora] were obtained. No evidence of iodine
in the crystals was seen, leading us to infer that this inherently
volatile molecule evaporated off during the crystallization
attempts.
In the single crystal [D4d-CDMB-8⊃CYH·(CYH)4.5], one
CYH molecule is located between two neighboring D4d-
CDMB-8 macrocycles. The CYH-to-benzene C−C distances
vary from 3.5 to 3.8 Å, leading us to propose that the CYH
guest is held in place via CH−π interactions (cf. Supporting
Information Figure S6). One D4d-CDMB-8 and one CYH
molecule form a repeat unit that presumably favors further 1D
packing (Figure 1).
Scheme 2. Schematic Representations of the Self-Assembly of Two- and Three-Component Mixtures Containing D4d-CDMB-8,
Corannulene (Cora), and/or I2 in Cyclohexane (CYH) or in the Solid State
B
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Cora edge and the benzene centroids of the D4d-CDMB-8
macrocycle is ca. 3.5 Å. These distances may reflect the
presence of stabilizing π−π donor−acceptor and attractive
CH−π interactions, respectively. The iodine molecule is
centered within the capsule and threads through the central
D4d-CDMB-8 macrocycle. No obvious interactions between
the iodine molecule and the surrounding macrocycle were
inferred on the basis of the large I−C separations (all >4.4 Å;
see Figure 2c). The iodine molecule also inserts into two
capping Cora subunits within the overall capsule structure.
The distance between the iodine atoms and the center of the
closest Cora centroid (i.e., bowl bottom) is ca. 3.6 Å (Figure
2d). While not a proof, such a separation may reflect a
stabilizing I···π halogen bonding interaction.
Gratifyingly, the crystallization procedure leading to
[(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·5CYH] could be
run easily on a gram scale. Of note is the fact that the powder
X-ray diffraction (PXRD) spectrum of the gram-scale sample
proved to be in good agreement with the PXRD pattern
simulated on the basis of the single-crystal data (cf. Supporting
Information Figure S12).
Spectral Studies of the Self-Assembly Process in CYH.
To obtain further insights into the three-component self-
assembly of D4d-CDMB-8 with Cora and I2, solution-phase
UV−vis spectroscopic analyses were carried out in CYH under
conditions mimicking those used to obtain single crystals.
Specifically, CYH solutions containing one or two components
of D4d-CDMB-8, Cora, and I2 (10.0 mM in each) were set in
small airtight vials on the bench and held at 298 K for 60 h.
Crystals, as colorless prisms, were observed in the case of the
various D4d-CDMB-8-containing vials. A decrease in the UV−
vis absorbance intensity was seen in the resulting supernatants.
Note that the supernatant was diluted by a factor of 100 prior
Figure 1. Extended 1D packing arrangement, (D4d-CDMB-
8⊃CYH)n, seen in the solid state for single crystals of [D4d-CDMB-
8⊃CYH·(CYH)4.5].
Single-Crystal Structure of the Iodine Complex
Obtained upon Self-Assembly of D4d-CDMB-8, Cora,
and I2. A mixture of D4d-CDMB-8, Cora, and I2 (each at a
concentration of 3.0 mM) in CYH was subject to slow
evaporation at room temperature (298 K) over the course of 2
days. X-ray diffraction analysis revealed a supramolecular
iodine capsule, ([(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·
5CYH]), comprising three macrocycles, two Cora compo-
nents, and one iodine molecule. Individual iodine-containing
capsule units, ((D4d-CDMB-8)3⊃(Cora)2)⊃I2, combine to
form as an infinite structure with CYH molecules playing a
possible stabilizing role (Figure 2e−g).
Two kinds of limiting geometric arrangements involving the
macrocycle and Cora are seen within each capsule unit. These
are referred to as bowl bottom binding (Figure 2a) and bowl
edge binding (Figure 2b), respectively. In the bowl bottom
binding mode, the mean distance between the carbon atoms
on the Cora edge and the benzene centroids of the D4d-
CDMB-8 macrocycle is ca. 3.4 Å. In the bowl edge binding
mode, the mean distance between the carbon atoms on the
Figure 2. Single-crystal X-ray structures showing various interactions among D4d-CDMB-8, Cora, I2, and CYH seen within the extended solid-state
structure, [(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·5CYH]n. (a and b) Bowl bottom and bowl edge spatial arrangements of the D4d-CDMB-8 and
Cora portions of the overall capsule. (c) View showing the threading of I2 through the central D4d-CDMB-8 macrocycle. (d) View showing the
insertion of I2 into the capping Cora. (e) Basic iodine-containing capsule unit (D4d-CDMB-8)3⊃(Cora)2)⊃I2. (f and g) Side and top views of the
extended 1D linear structure produced via self-assembly of individual capsules ((D4d-CDMB-8)3⊃(Cora)2)⊃I2 and CYH. Some CYH molecules
and hydrogen atoms have been omitted for clarity.
C
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to spectral analysis (cf. Supporting Information Figures S16
and S17).
Single crystals analogous to those used for the X-ray
1
diffraction analyses were also subject to solution-phase H
NMR spectroscopic analysis after dissolution in benzene-d6 (cf.
Supporting Information Figure S13). It was concluded that in
all cases the crystals consisted of [D4d-CDMB-8⊃CYH·
(CYH)4.5], the composition of which was ca. 50% D4d-
CDMB-8 on a per molar basis. No evidence of either D4d-
CDMB-8·Cora or D4d-CDMB-8·I2 complex formation was
observed. Moreover, no solid material or discernible change in
the UV−vis spectral features was seen in the solutions
containing only Cora, only I2, or a 1:1 mixture of the two
species (cf. Supporting Information Figure S17). In contrast,
brown crystals were produced upon stirring a CYH solution of
D4d-CDMB-8, Cora, and I2 (each at 10.0 mM) at 298 K for 60
h. UV−vis spectral studies of the CYH solution revealed an
absorbance decrease over the 250 to 620 nm spectral region.
On the basis of the decrease in the I2 absorbance at 520 nm,
the Cora absorbance at 320 nm, and the superimposed
absorbance of D4d-CDMB-8, Cora, and I2 at 268 nm, the
molar percentages of D4d-CDMB-8, Cora, and I2 in the solid
material were calculated to be 68, 42, and 18%, respectively (cf.
Supporting Information). These values agree with the 3:2:1
ratio seen in single crystals of [(((D4d-CDMB-8)3⊃(Cora)2·
1
CYH)⊃I2)·5CYH]. Moreover, UV−vis and H NMR spectro-
scopic studies were carried out by dissolving the single crystals
in CYH or benzene-d6. In aggregate, these studies allowed us
to confirm the identities and ratios of the components within
the crystals (cf. Supporting Information Figures S14 and S15).
Considered in conjunction with both the single-crystal and
powder X-ray diffraction analyses, these findings provide
support for the conclusion that only the supramolecular iodine
capsule ([(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·5CYH]) is
produced under these crystallization conditions.
Figure 3. Time-dependent crystallization studies of D4d-CDMB-8 and
((D4d-CDMB-8)3⊃(Cora)2)⊃I2 in CYH monitored by UV−vis
spectroscopy. (a) Plot of the percentage of D4d-CDMB-8 present in
crystalline form starting from CYH solutions of D4d-CDMB-8 (10.0
mM) either (i) pure, (ii) containing 1 molar equiv of Cora, or (iii)
containing 1 molar equiv of Cora and I2. (b) Plot of the ratio of each
component in the crystalline material obtained from a CYH solution
containing D4d-CDMB-8, Cora, and I2 (10.0 mM in each) as a
function of time. The ratios observed correspond to those seen in
((D4d-CDMB-8)3⊃(Cora)2)⊃I2. See the text for details.
Thermodynamic and Kinetic Analyses of Self-
Assembly under Conditions of Crystallization. Time-
dependent UV−vis spectroscopic studies were carried out to
compare the crystallization of D4d-CDMB-8 and the iodine
capsule ((D4d-CDMB-8)3⊃(Cora)2)⊃I2, respectively, in CYH.
In solutions containing either D4d-CDMB-8 or a mixture of
D4d-CDMB-8 and I2 (1:1 by concentration), essentially
complete crystallization of a D4d-CDMB-8-containing species
occurred within 5 h. However, a corresponding solution
containing D4d-CDMB-8 and Cora (1:1 by concentration)
required nearly 10 h to achieve an equal level of crystallization
(cf. Figure 3a, Supporting Information Figure S18d). On this
basis, we conclude that even if little direct evidence of
interaction between D4d-CDMB-8 and Cora was observed in
CYH, the presence of Cora has some impact on the kinetics of
D4d-CDMB-8 crystallization.
In contrast to what was seen for D4d-CDMB-8, alone or in
conjunction with Cora, self-assembly leading to crystallization
of the iodine capsule complex, ((D4 d -CDMB-
8)3⊃(Cora)2)⊃I2, proved quite slow. Specifically, near-
complete crystallization of the three-component capsule from
a 1:1:1 (concentration basis) solution of D4d-CDMB-8, Cora,
and I2 is seen only after 30 h (cf. Figure 3b, Supporting
Information Figure S18). The composition of the resulting
crystalline form was found to match what would be expected
on the basis of the single-crystal X-ray diffraction analyses
discussed above.
Assuming that the products of crystallization are under
thermodynamic control allowed an analysis of the relative
energetics for the formation of [D4d-CDMB-8⊃CYH·
(CYH)4.5] and [(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·
5CYH] to be carried out. According to the standard
equilibrium (eq 1) and using the percentage of the total
material crystallized to deduce the free concentration of D4d-
CDMB-8 (Figure 3a), it proved possible to approximate the
equilibrium constant (KΘS1) and Gibbs free energy (ΔG1Θ
)
(298 K)
corresponding to the formation of [D4d-CDMB-8⊃CYH·
(CYH)4.5] as 1.9 × 102 and −13 kcal·M−1, respectively. A
similar analysis using the relative percentages of each
component (i.e., D4d-CDMB-8, Cora, and I2) in the
[(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·5CYH] product
(Figure 3b) and the equilibrium expression in eq 2 gave KSΘ2
= 5.5 × 104 and ΔG2Θ(298 K) = −27 kcal·M−1 for the formation
of [(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·5CYH]. We thus
conclude that the emergent behavior that leads to the
formation of [(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·
5CYH] is energetically favorable. A comparison of the
(ΔGΘ2(298 K)) values also provides support for the observation
that under crystallization conditions where all three compo-
nents are present [(((D4d-CDMB-8)3⊃(Cora)2·CYH)⊃I2)·
5CYH] is formed in preference to [D4d-CDMB-8⊃CYH·
(CYH)4.5].
D
https://dx.doi.org/10.1021/jacs.0c11838
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