Spherical Supramolecular Structures Constructed via Chemically Symmetric Perylene Bisimides: Beyond Columnar Assembly

Article abstract of DOI:10.1002/anie.201914889

Like other discotic molecules, self-assembled supramolecular structures of perylene bisimides (PBIs) are commonly limited to columnar or lamellar structures due to their distinct π-conjugated scaffolds and unique rectangular shape of perylene cores. The discovery of PBIs with supramolecular structures beyond layers and columns may expand the scope of PBI-based materials. A series of unconventional spherical packing phases in PBIs, including A15 phase, σ phase, dodecagonal quasicrystalline (DQC) phase, and body-centered cubic (BCC) phase, is reported. A strategy involving functionalization of perylene core with several polyhedral oligomeric silsesquioxane (POSS) cages achieved spherical assemblies of PBIs, instead of columnar assemblies, due to the significantly increased steric hindrance at the periphery. This strategy may also be employed for the discovery of unconventional spherical assemblies in other related discotic molecules by the introduction of similar bulky functional groups at their periphery. An unusual inverse phase transition sequence from a BCC phase to a σ phase was observed by increasing annealing temperature.

Full text of DOI:10.1002/anie.201914889

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Spherical Supramolecular Structures Constructed via Chemically
Symmetric Perylene Bisimides: Beyond Columnar Assembly
Authors: Jiahao Huang, Zebin Su, Mingjun Huang, Rongchun Zhang,
Jian Wang, Xueyan Feng, Rui Zhang, Ruimeng Zhang,
Wenpeng Shan, Xiao-Yun Yan, Qing-Yun Guo, Tong Liu,
Yuchu Liu, Yunpeng Cui, Xiaopeng Li, An-Chang Shi, and
Stephen Z. D. Cheng
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914889
Link to VoR: https://doi.org/10.1002/anie.201914889

10.1002/anie.201914889
Angewandte Chemie International Edition
RESEARCH ARTICLE
Spherical Supramolecular Structures Constructed via Chemically
Symmetric Perylene Bisimides: Beyond Columnar Assembly
Jiahao Huang, † [a] [b] Zebin Su† [b], Mingjun Huang[a], Rongchun Zhang[a], Jian Wang[d], Xueyan Feng[b],
Rui Zhang[a], Ruimeng Zhang[b], Wenpeng Shan[b], Xiao-Yun Yan[b], Qing-Yun Guo[b], Tong Liu[b], Yuchu
Liu[b], Yunpeng Cui[e], Xiaopeng Li[e], An-Chang Shi*[c], and Stephen Z. D. Cheng*[a][b]
devices,[3] photonics,[4] and some biology related applications.[5] It
Abstract: Like other discotic molecules, self-assembled
supramolecular structures of perylene bisimides (PBIs) are commonly
limited to columnar or lamellar structures due to their distinct π-
conjugated scaffolds and unique rectangular shape of perylene cores.
The discovery of PBIs with supramolecular structures beyond layers
and columns may expand the scope of PBI-based materials. Herein,
we report a series of unconventional spherical packing phases in PBIs
including A15 phase, σ phase, dodecagonal quasicrystalline (DQC)
phase, and body-centered cubic (BCC) phase. By the
functionalization of perylene core with several polyhedral oligomeric
silsesquioxane (POSS) cages, our strategy successfully achieves
spherical assemblies of PBIs, instead of columnar assemblies, due to
the significantly increased steric hindrance at the periphery. This
strategy may also be employed for the discovery of unconventional
spherical assemblies in other related discotic molecules by the
introduction of similar bulky functional groups at their periphery. In
addition, an unusual inverse phase transition sequence from BCC
phase to σ phase by increasing annealing temperature is observed.
is well recognized that the properties of materials do not solely
depend on their primary chemical structures, but also rely on how
they are organized across multiple length scales.[6] A simple
example is that the optical properties of PBIs are largely affected
by their different packing arrangements.[7]Tailoring the properties
of PBI-based materials with different hierarchical structures may
help improve their performance and lead to new applications.
From a physiochemical perspective, the most important factor
affecting the molecular packing is the π-π stacking interaction
between rigid polycyclic aromatic cores of PBIs. This relatively
strong interaction usually favors the formation of columnar
assemblies in solution,[8] organo-gels,[9] or liquid crystalline
phases.[10] In particular, Percec and co-workers have explored in
great detail the supramolecular organization of dendronized PBIs
and observed the formation of different columnar liquid-crystalline
phases.[11] Discotic and quasi-discotic molecules have already
employed in the design of spherical assemblies such as
triphenylenes and cyclotriveratrylenes.[12] However, observation
of spherical assembly of PBIs based on π-π stacking interaction
is scarce due presumably to the unique molecular geometry of
PBIs.[13]
Introduction
In the past decades, perylene bisimides (PBIs) have attracted
much research attention due to their self-assembled π-
conjugated scaffolds with outstanding stability as well as
conveniently tuned absorption/fluorescence properties.[1] These
features make PBI-based materials an ideal candidate for a wide
range of applications including organic electronics,[2] photovoltaic
Periodically or quasiperiodically ordered structures generated
from spherical motifs occur in many different classes of self-
assembling soft matter systems. The last two decades have
witnessed remarkable progresses in the discovery of a series of
highly ordered and complex Frank-Kasper (F-K) phases
composed of spherical motifs in soft matters, including liquid
crystals,[14] small-molecular surfactants,[15] dendrimers,[16] block
copolymers,[17] colloidal particles,[18] giant molecules,[19] and very
recently, sugar-polyolefin conjugates.[20] It should be mentioned
that the Frank-Kasper phases refer to a family of topologically
close-packed spherical packing phases originally observed in
metal alloys.[21] Furthermore, the dodecagonal quasicrystalline
phase (DQC) with 12-fold rotational symmetry but no long-range
translational periodicity has also been reported in soft matters.[22]
Aperiodic DQC phase is closely related to some Frank-Kasper
phases due to their similar local tetrahedral sphere packing.[22e, 23]
The discovery of these complex spherical phases in PBI-based
materials still remains a challenging topic.
[a]
[b]
[c]
J. Huang, Prof. M. Huang, Prof. R.-C. Zhang, Prof. R. Zhang, Prof.
S. Z. D. Cheng
South China Advanced Institute for Soft Matter Science and
Technology
School of Molecular Science and Engineering
South China University of Technology, Guangzhou 510640, China
J. Huang, Z. Su, X. Feng, R.-M. Zhang, W. Shan, X.-Y. Yan, Q.-Y.
Guo, T. Liu, Y. Liu, Prof. S. Z. D. Cheng
Department of Polymer Science
College of Polymer Science and Polymer Engineering
The University of Akron, Akron, Ohio 44325-3909, USA
E-mail: scheng@uakron.edu
Prof. A.-C. Shi
Department of Physics and Astronomy
McMaster University, Hamilton, Ontario L8S 4M1, Canada
E-mail: shi@mcmaster.ca
Here, we present an experimental study of a series of rationally
designed POSS-based PBIs constructed by attaching six cubic
polyhedral oligomeric silsesquioxane (POSS) cages to the planar
PBI core at its imide positions with tunable linkage lengths,
exhibiting features of shape amphiphiles (Figure 1).[24] These
molecules are distinguished from previously reported PBIs by the
introduction of the bulky and rigid peripheric BPOSS (POSS
functionalized with seven isobutyl groups, Figure 1) cage building
[d]
[e]
Prof. J. Wang
School of Life Science and Technology
ShanghaiTech University, Shanghai 201210, China
Y. Cui, Prof. X. Li
Department of Chemistry
University of South Florida, Tampa, Florida 33620, USA
J. Huang and Z. Su contribute equally to this work.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201914889
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 1. Molecular cartoon model and chemical structures of the PBI-Cn-6BP with linkage structure variations. For the naming of these molecules, PBI refers to
perylene bisimides, Cn refers to the number of methylene group in each linkage part between triazole group and one POSS cage, BP refers to BPOSS, and ISO
refers to isomer. (a) Simplified carton of POSS-based PBIs, exhibiting features of shape amphiphiles. Blue sphere represents BPOSS cages, red block represents
aromatic rings, and detailed linkage chemical structures are shown in the blue dashed box. (b) Chemical structure and molecular model (shown in shadow, hydrogen
atoms are omitted for clarity) of PBI-C1-6BP. Dimensions of perylene core (12.3 Å in length and 7.3 Å in width, including hydrogen atoms) and cross center distance
(13 Å) of cubic BPOSS cage are indicated.
chemistry. The targeted POSS-based PBIs were then
synthesized via an imidation reaction between perylene-3,4,9,10-
tetracarboxylic dianhydride (PTCDA) and aromatic amines
attached with three BPOSS cages at elevated temperature. The
lengths and chemical structures of the linkages could be precisely
tuned, resulting in variations on the flexibility of the linkages, and,
thus, the steric effect of the peripheric BPOSS cages. The
detailed synthetic procedures of these PBIs are shown in the
Supporting Information (SI). Their precise chemical structures
and high purity were confirmed by mass spectroscopy, gel
permeation chromatography (GPC), and nuclear magnetic
resonance (NMR) spectroscopy (Figures S1, S2, and S3).
blocks. The self-assembly of these PBIs is dictated by the
interplay between the planar PBI cores and the cubic POSS
cages. On the one hand, the π-π stacking interaction between
PBIs combined with relative short linkages inhibits the
crystallization of BPOSS cages, which usually have strong
tendency to crystallize.[25] On the other hand, due to significant
steric hindrance of the rigid and bulky BPOSS cages, the
formation of columnar structures as commonly observed in PBI is
impeded. The unique nanometer size and shape persistency of
cubic POSS cages combined with flexible linkages provide
significant and tunable steric hindrance during PBI stacking
process, facilitating the spherical assemblies of PBIs and further
the formation of various spherical packing phases. Surprisingly,
diverse and highly ordered spherical supramolecular lattices,
including BCC phase, A15 phase, σ phase, and DQC phase, are
observed in the POSS-based PBIs. Moreover, an unusual inverse
phase transition sequence from a BCC phase to a σ phase by
increasing annealing temperature is observed.
Typical thermal properties of these POSS-based PBIs were
characterized by thermal gravimetric analysis (Figure S4) and
differential scanning calorimetric experiments (Figure S5). The
samples are thermally stable at temperatures higher than 250 C
(no weight loss up to 320 C at a heating rate of 10 C/min with
N2 flow). No crystallization peak of BPOSS cages was observed
in the DSC curves, which indicates that crystallization of BPOSS
cages is strongly suppressed. No visible phase transition peak
was observed in the DSC curves, either. We speculate that π-π
interaction between the PBI cores could induce stacking of the
entire molecules and result in disordered yet dense packing of the
BPOSS cages, which have relatively short linkages with the PBI
cores. Optical properties of the POSS-based PBIs were
investigated by ultraviolet-visible (UV-vis) and fluorescence
Results and Discussion
Synthesis and characterization. The synthesis of PBI-C1-6BP
is briefly shown in Scheme 1 as an example. In short, three
BPOSS cages were first connected simultaneously via copper-
catalyzed azide-alkyne [3+2] cycloaddition (CuAAC) “click”
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Scheme 1. Synthesis of PBI-C1-6BP
Reagents and conditions: (i) CuBr, PMDETA, THF, 25 C, 6h; (ii) DMAP, imidazole, ODCB, 125 C, 6h
spectroscopy (Figure S6). Taking PBI-C1-6BP as an example, in
dilute solution (10-5 M, monomeric state), the absorption bands
between 410 and 550 nm for the S0-S1 transition of PBIs with well-
resolved vibronic structure from the breathing vibrations of the
perylene skeleton, was observed in the corresponding UV-vis
spectrum (Figure S6a). In annealed thin film (aggregated state),
a shift of absorption maximum about 30 nm was observed
compared to in dilute solution, indicating the face-to-face stacking
of perylenes in the aggregated state.[10c] The fluorescence of the
POSS-based PBIs was observed in monomeric and aggregated
state by the fluorescence emission spectra in dilute solution and
in annealed thin film (Figure S6c). The photoluminescence in the
aggregated state suggests there is a relatively large rotational
displacement between perylenes in the π-stacked aggregates.[10e]
It’s worthy to note that previously reported tridodecyloxyphenyl-
substituted PBIs are not photoluminescent despite comparative
chemical structure to our POSS-based PBIs.[10b] The POSS-
based PBIs were annealed in a series of temperatures with 5 C
interval for 12 h with N2 flow. After the thermal annealing
processes, a variety of complex supramolecular phases were
obtained in the POSS-based PBIs. The formation of the highly
ordered supramolecular structures was demonstrated by a
combination of small angle X-ray scattering (SAXS) technique
and bright field (BF) transmission electron microscopy (TEM)
images.
Adding one methyl group between the phenyl group and the
triazole group in the linkage (sample PBI-C1-6BP) slightly
increases the flexibility of the linkage and results in a completely
different self-assembly behavior. After annealing at 200 C for 12
h, PBI-C1-6BP powder sample exhibited sharp diffraction peaks
in the SAXS pattern with a set of scattering vector (q) ratios of
√4: √5: √6: √8 (Figure 2a). These scattering peaks could be
assigned as an F-K A15 cubic phase[25a] (space group ꢀꢁ3ꢃ)
ꢂ
with unit cell parameter a = 7.8 nm. This structural assignment
was further confirmed by a representative 44 tiling pattern along
the [001] direction in the BF TEM images (Figures 2g and 2h after
Fourier filtering) and the corresponding fast Fourier transformed
(FFT) diffraction pattern (Figure 2g, inset).[16e] The formation of
the spherical packing phases is also demonstrated by unit cell
electron density reconstruction (Figure S10). Increasing the
flexibility of this linkage enhances the mobility of the BPOSS cage
shells, facilitating the formation of supramolecular spherical motifs
and further assembling into an A15 phase. The cubic unit cell of
this phase is composed of eight supramolecular spheres (two with
coordination number (CN) = 12 and six with CN = 14).[26] On the
basis of the lattice parameter and experimentally measured
density, the average diameter of the supramolecular spheres
(<dsphere>) and the average number of molecules in each
supramolecular sphere (<μ>) can be estimated as 4.84 nm and
6.3, respectively (Table 1). The crystallization of BPOSS cages is
inhibited in this A15 phase, demonstrated by the observation of
one broad scattering peak corresponding to the average size of
BPOSS cages in the wide-angle X-ray diffraction (WAXD) pattern
(Figure S8a).
Analysis of self-assembly behavior and phase transition. For
PBI-C0-6BP with the shortest and most rigid linkage, no ordered
structures could be observed based on SAXS result (Figure S7),
despite extensive thermal and solvent annealing efforts. As
expected, the strong steric hindrance of the BPOSS cages
prevents the common columnar assembly of the PBIs. However,
the mobility of the BPOSS cages is also severely constrained by
the rigid linkage, inducing significant steric hindrance from
BPOSS cages, as well as inhibiting the formation of suitable
supramolecular spheres and ordered phases in the bulk state.
On the basis of PBI-C1-6BP, we further tune the linkage structure
by adding a short amide linkage with different methylene number
between BPOSS cage and phenyl group (Figure 1a). After
thermal treatment of PBI-C3-6BP at 220 C for 12 h, a complex
SAXS pattern with multiple diffraction peaks was recorded (Figure
2b). We could assign this SAXS pattern as an F-K σ phase (space
⁄
group ꢀ4ꢄ ꢁꢃꢁ) with a tetragonal unit cell and unit cell
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Figure 2. Self-assembled supramolecular structures of some POSS-based PBIs. PBI-C1-6BP shows an A15 phase in the SAXS pattern (a) and BF TEM images (g
and h after Fourier filtering). PBI-C3-6BP shows a σ phase in the SAXS pattern (b) and BF TEM images (k and l after Fourier filtering). PBI-C4-6BP and PBI-C5-
6BP exhibit BCC phases in the SAXS patterns (d and e) at 215 C, and these BCC phases transfer to the σ phase in the SAXS pattern (d) and the DQC phase in
the SAXS pattern (e) for PBI-C4-6BP and PBI-C5-6BP at 225 C and 230 C, respectively. And BF TEM images (m and n after Fourier filtering) further demonstrate
the DQC phase of PBI-C5-6BP at 230 C. PBI-C6-6BP shows a BCC phase in the SAXS pattern (c) and BF TEM images (i and j after Fourier filtering). TEM images
are taken along the [001] direction of the A15 phase, the [001] direction of the σ phase, the [001] direction of the BCC phase, and the [00001] direction of the DQC
phase. The upper left insets of the TEM images are corresponding FFT diffraction patterns. The packing models of the A15 phase, the σ phase, the BCC phase
and two-dimensional tiling pattern of DQC phase are illustrated in f.
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10.1002/anie.201914889
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parameters a = b = 16.85 nm and c = 8.87 nm.[14b] The
corresponding BF TEM images along the [001] direction (Figures
2k and 2l after Fourier filtering) display a typical 32.4.3.4 tiling
pattern. The corresponding FFT diffraction pattern (Figure 2k,
inset) suggests the tetragonal lattice of the σ phase.[17a] A typical
tetragonal unit cell in the σ phase consists of thirty supramolecular
spheres in five different coordination environments. The average
<dsphere> and <μ> values are derived as 5.43 nm and 8.4,
respectively (Table 1). The self-assembled σ phase remains
thermodynamically stable up to around 230 C and then, this
phase transfers to the disordered melts state at higher
temperature (Figure S9a). It should be noted that here, hydrogen
bonding could be observed by Fourier-transform infrared (FT-IR)
spectroscopy due to the introduction of amide groups in the
linkages (Figure S12), which may assist the stacking of the
POSS-based PBIs and facilitate the phase formation process.
transition phenomenon has been observed after thermal
annealing in a wide range of temperatures (Figure S9d).
Thermodynamic analysis of self-assembly behavior. To better
illustrate the rich variety of spherical packing phases formed by
our POSS-based PBIs, a simplified description of this self-
assembly process is shown in Figure 3. The π-π stacking
interaction between the PBI cores enables the segregation
between the inner aromatic regions and the peripheric BPOSS
cages. Without those BPOSS cages at the periphery, the PBI
chromophores would stack to form a normal columnar structure.
However, the significant steric hindrance of those BPOSS cages
at the periphery breaks down the column and each broken
fragment that contains a few PBIs forms a supramolecular
spherical motif, in which the perylene cores are face-to-face π-
stacked together with rotational displacement (See additional
discussion in the SI). The self-assembled supramolecular
spheres sequentially pack into different superlattices. The
thermodynamic principle governing the structure formation of self-
assembled soft spheres is in generally dictated by maximizing
entropy and minimizing surface area. From an energy perspective,
the self-assembled soft spheres tend to keep their spherical
shape thus minimizing the surface area. On the other hand, when
packed to form an ordered crystalline phase, the supramolecular
spherical motifs must deform their shapes to polyhedra,
corresponding to the Winger-Seitz Cells or Voronoi Cells, so that
the system would maintain a uniform space filling.[27] The
competition of these two opposing trends leads to the formation
of complex spherical packing phases. Due to the bulky and rigid
nature of those BPOSS cages, it is reasonable that the
deformability of supramolecular spheres in our POSS-based PBIs
system is mainly coordinated by the flexible linkages in those
molecules. Therefore, the occurrence of different spherical
packing phases could be regulated by varying the length of the
linkage.
PBI-C4-6BP and PBI-C5-6BP contain longer and more flexible
linkages, exhibiting distinct phase structures and transition
behaviors in a relatively wide range of temperatures. Annealing
these two molecules at 215 C first, both molecules formed BCC
ꢂ
phases (space group ꢅꢁ3ꢁ), which were identified by two sharp
peaks in their SAXS patterns with characteristic q ratios of 1: √2
(Figures 2d and 2e). The dsphere and μ value (note that here these
two values are not average) are 5.80 nm and 10, respectively, in
the BCC phase of PBI-C4-6BP. The dsphere and μ value of PBI-C5-
6BP in the BCC phase are 5.99 nm and 11, respectively. As the
annealing temperatures of the two molecules increase, two
different SAXS patterns were recorded. The PBI-C4-6BP exhibits
the formation of a σ phase with a tetragonal unit cell (a = b = 17.56
nm, c = 9.24 nm) after thermal annealing at 225 C for a few hours
with average values of <dsphere> and <μ> of 5.66 nm and 9.4 in the
σ phase (Figure 2d). These numbers change between two phases
may imply the exchange and redistribution of the POSS-based
PBIs among the supramolecular spheres during the phase
transition from the BCC phase to the σ phase. A clearly indexed
σ phase is shown in Figure S11 and Table S1.
Deforming a spherical domain would break its spherical symmetry.
The extent of spherical symmetry breaking of a deformed
On the other hand, for PBI-C5-6BP, the SAXS pattern at 230 C
is in good accordance with a DQC phase, exhibiting identical
features and q ratios of the reported DQC phase (with typical
diffraction peaks of 00002, 12100, 10102, and 12101 in Figure
2e).[19d, 22a] From the BF TEM images along the [00001] direction
(Figures 2m and 2n after Fourier filtering), representative mixed
tiling patterns including 32.4.3.4, 33.42, and 36 can be identified,
further supporting the formation of an aperiodic DQC phase.
Although lacking long-range translational order in two-
dimensional (2D) plane, this morphology contains 12-fold
rotational symmetry as demonstrated by the FFT pattern (Figure
2m, inset). The DQC phase was able to maintain after thermal
annealing for prolonged time (3 days at 230 C, Figure S9b).
Annealing at higher temperature led to the disordered melts state
(12 hours at 235 C, Figure S9c). Finally, PBI-C6-6BP with the
longest linkage forms a BCC phase after thermal annealing at 230
C, as indicated by the q ratios of 1: √2: √3 in the SAXS pattern
(a = 6.2 nm, Figure 2c) and the BF images with the FFT diffraction
pattern (Figures 2i and 2j after Fourier filtering). No phase
supramolecular sphere could be quantified by using the
ꢊ
isoperimetric quotient (ꢅꢆ = 36ꢇꢈꢄ ꢉ ), where ꢈ and ꢉ represent
⁄
the volume and surface area of deformed spheres, based on
Voronoi cells).[17b, 27c] For the superlattice consisting of more than
ꢂꢂꢂ
one type of Voronoi cells, the average value of ꢅꢆ (ꢅꢆ) of all
Voronoi cells is used to quantify the average sphericity. The ꢅꢆ of
a perfect sphere is 1, whereas ꢅꢆ < 1 for non-spherical domains.
ꢂꢂꢂ
As reported in the literature, the ꢅꢆ values of an A15 phase, a σ
phase, and a BCC phase are 0.7618, 0.7624, and 0.7534,
respectively, and the sphericity of a DQC phase is expected to be
similar to the σ phase.[22e] The complex spherical packing phases,
ꢂꢂꢂ
i.e. A15 phase, σ phase, and DQC phase, have a larger ꢅꢆ, thus
they are less deformed away from sphericity than the domains in
the BCC phase. When the spherical domains are with a higher
ꢂꢂꢂ
deformation energy, the spherical motifs with higher ꢅꢆ, such as
those in both A15 and σ phases, would be favored. On the other
hand, when the spherical domains are easily deformable, the
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Figure 3. Schematic illustration of the hierarchical self-assembly mechanism of the POSS-based PBIs. The central red blocks represent perylene cores, the gray
shell of the supramolecular sphere represent amorphous BPOSS cages.
BCC phase is favored because it could accommodate more
deformed spherical motifs and realized more uniform space filling.
For PBI-C1-6BP and PBI-C3-6BP, they have short and relatively
rigid linkages, resulting in less deformed self-assembled
supramolecular spheres. Therefore, the A15 phase and the σ
phase would be favored for these samples. However, for PBI-C4-
6BP and PBI-C5-6BP with longer linkage lengths, the relatively
more flexible linkages induce higher degree of deformability in
their self-assembled supramolecular spheres. Thus the BCC
phase would become thermodynamically stable at lower
temperature because it could accommodate more gauche
molecules to be even less deformed. Therefore, a transition from
the BCC phase to the σ phase would occur as the temperature is
increased. Similarly, for PBI-C5-6BP, with the redistribution ability
of the POSS-based PBIs among the spherical motifs, the BCC
phase transfer to the DQC phase with higher sphericity at higher
temperature. For PBI-C6-6BP with the longest linkage length, the
high degree of deformability is largely kept, and the BCC phase
structure is retained in the entire temperature range studied.
Different self-assembly behaviors with subtle variations in
chemical structures of linkages. To further elucidate the effects
of linkage structure on the supramolecular self-assembly, we
specifically designed and examined a set of constitutional
isomers: PBI-C5-6BP, ISO1-PBI-C5-6BP, and ISO2-PBI-C5-6BP
(Figure 4a). Their linkages are divided into two parts: the alkyl
amide between the POSS cage and the phenyl group and the
methylene between the phenyl group and the triazole group. With
identical chemical composition and extended linkage chain length,
different self-assembly behaviors are observed among these
three isomers due to the subtle differences in their linkage
structures. As described above, PBI-C5-6BP exhibits a phase
transition from a BCC phase at low temperature to a DQC phase
at high temperature. However, ISO1-PBI-C5-6BP only forms a
BCC phase after thermal annealing in the entire temperature
range studied (Figure 4b), while ISO2-PBI-C5-6BP shows a phase
transition from a BCC phase to a σ phase at 230 C (Figure 4c).
A SAXS pattern with multiple sharp diffraction peaks illustrates
the formation of the σ phase (a = b = 18.3 nm, c = 9.63 nm) in
ISO2-PBI-C5-6BP at 230 C, in contrast to the DQC phase
observed in PBI-C5-6BP and the BCC phase in ISO1-PBI-C5-6BP.
These results suggest that self-assembled supramolecular
structures of POSS-based PBIs are highly sensitive to the
chemical structures of the linkage. Detailed supramolecular
structure analysis of all samples is summarized in Table 1.
conformation and better packing for the flexible linkages.[28]
.
When the temperature is increased for PBI-C4-6BP, the σ phase
overtakes BCC phase and becomes stable at higher temperature.
It’s worthy to note that this phase transition sequence is inverse
to that observed or predicted in the other soft matter systems
17b]
including block copolymers and dendrimers,[14b,
where the
BCC phase is more stable at higher temperature. In the diblock
copolymer system, maximizing conformational entropy
(minimizing the stretching energy) of soft polymer chains
dominates over reducing interfacial tension during heating,
leading to the phase transition from a σ phase to a BCC phase.[29]
However, in the POSS-based PBIs system, the effect of
conformation entropy is relatively weak. Instead, we speculate
that the orientational entropy (ꢉꢋꢌ) of the whole molecule would
play a significant role in their self-assembly process. At lower
temperature, the BCC phase is preferred due to better packing of
flexible and deformable linkages. In addition, each spherical motif
in the BCC phase contains more molecules than in the σ phase
(Table 1). In other words, the BCC phase exhibits higher degree
of π-π stacking than the σ phase, indicating a smaller enthalpy
(ꢍꢎꢏ) in the BCC phase. At high temperature, the degree of π-π
stacking would become less due to thermal fluctuations. At the
same time, the orientational entropy of the whole molecule
becomes significant so that each molecule would tend to occupy
larger area on the surface of the domain. This could be achieved
by a more regular arrangement of the molecules, which in turn
would drive the spherical motifs constructed by PBI-C4-6BP
We speculate that the length of alkyl amide linkage connecting
the POSS cage and the phenyl group plays a major role in
increasing the flexibility of the PBIs compared with the methylene
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Figure 4. Linkage chemical structures of the isomers are magnified in the blue dashed box (a). For the naming of these three isomers, prefix ISO refers to isomer.
ISO1-PBI-C5-6BP shows BCC phase in a wide range of temperature in the SAXS pattern (b). ISO2-PBI-C5-6BP forms a BCC phase in low temperature and transfers
to a σ phase at high temperature in the SAXS pattern (c). Solid-state 1H NMR spectra of the annealed isomers under the 700 MHz magnetic field at a MAS rate of
100 kHz. The samples for Solid-state 1H NMR were annealed at 230 C for 12 h.
linkage connecting the phenyl group and the triazole group,
considering the hydrogen bonding between amide groups. The
increased flexibility of linkage would help reduce steric hindrance
of the BPOSS cages and improve the extent of molecular
aggregation. The extent of molecular aggregation could be
qualitatively estimated by 1H solid-state NMR spectroscopy. The
1D proton NMR spectra at 100 kHz magic angle spinning (MAS)
for the three annealed samples (annealing at 230 C for 12 h) are
shown in Figure 4d. The chemical shifts of aromatic region
between 7 and 10 ppm vary among these three samples. Such
broad signals are resulted from the strong π-π stacking
interaction between PBIs. In fact, stronger molecular aggregation
will lead to stronger 1H-1H dipolar couplings as well as the
increase of anisotropic bulk magnetic susceptibility (ABMS), both
of which will lead to the broadening of aromatic proton peaks.[30]
Therefore, based on the comparison of the full width at half
maximum (FWHM) of the aromatic peaks (around 1.8 kHz, 1.3
kHz and 1.2 kHz for ISO1-PBI-C5-6BP, PBI-C5-6BP, and ISO2-
PBI-C5-6BP, respectively), it could be concluded that the highest
extent of molecular aggregation occurs in ISO1-PBI-C5-6BP
sample, and the PBI-C5-6BP has slightly higher extent of
molecular aggregation than that of ISO2-PBI-C5-6BP. Therefore,
we could deduce that ISO1-PBI-C5-6BP possesses the most
flexible linkage among the isomers and thus the largest
deformability while ISO2-PBI-C5-6BP’s linkage is relatively most
rigid. The differences in the flexibility of linkages among isomers
is in line with their different phase structures at high temperature,
ranging from σ phase in ISO2-PBI-C5-6BP, DQC phase in PBI-
C5-6BP to BCC phase in ISO1-PBI-C5-6BP.
Table 1. Summary of supramolecular structures of the POSS-based PBIs.
[b]
[
Molecules
phase
a (nm)[a]
Mwt
dsphere
c]
μ[d]
PBI-C0-6BP
PBI-C1-6BP
PBI-C3-6BP
PBI-C4-6BP
Disorder
A15
-
6474.4
6558.6
6985.1
-
-
7.8
4.84
6.3
σ
16.852×8.87
5.43
5.8
8.4
BCC
5.89
10.1 (10)
7069.2
7153.4
σ
17.562×9.24
5.66
5.99
9.4
11
BCC
6.08
PBI-C5-6BP
DQC
BCC
-
-
-
ISO1-PBI-C5-
6BP
6.23
7153.4
7153.4
6.13
11.9 (12)
BCC
σ
6.04
5.95
5.9
10.8 (11)
10.5
ISO2-PBI-C5-
6BP
18.32×9.63
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BCC
6.2
6.1
11.6 (12)
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PBI-C6-6BP
7237.5
[a] Lattice parameters derived from corresponding SAXS results.
[b] Average molecular weights of POSS-based PBIs. [c]
Diameters of supramolecular spheres. [d] Number of molecules
in each supramolecular sphere based on experimental measured
density. The number of molecules in each supramolecular sphere
in the BCC phase should be an integer, which is indicated in the
parentheses.
[3]
Conclusion
In summary, we have designed and synthesized a series of
precisely defined POSS-based PBIs by tuning the linkage
structures. The incorporation of the peripheric BPOSS in our
molecular design successfully achieved the unconventional
spherical assemblies in PBIs instead of columnar assemblies due
to significantly increased steric hindrance at the periphery.
Furthermore, the deformability of the supramolecular spheres can
be precisely regulated by carefully tuning linkage structures
(linkage flexibility), resulting in diverse unconventional spherical
packing phases in PBIs including BCC phase, A15 phase, σ
phase, and DQC phase. By the introduction of similar bulky
functional groups at the periphery, this strategy may also be
employed for the achievement of unconventional spherical
assemblies in other related discotic or rod-like molecules. The
discovery of a series of spherical packing phases in PBIs may
offer opportunities for understanding their packing-function
relationships, expanding the scope of PBI-based materials.
[4]
[5]
[6]
[7]
Acknowledgements
This work was supported by National Science Foundation (DMR-
1408872 to S.Z.D.C.) and the Program for Guangdong
Introducing Innovative and Entrepreneurial Teams (no.
2016ZT06C322). Mingjun Huang thanks the support from the
Fundamental Research Funds for the Central Universities (no.
2019JQ05). The research of A.-C.S. is supported by the Natural
Science and Research Council (NSERC) of Canada. We thank
technical support from Shanghai Synchrotron Radiation Facility
(SSRF) and mass spectrometry characterization by Molecular
Scale Lab (X. Li).
[8]
Keywords: supramolecular chemistry • nanostructures • perylene
bisimides
•
discotic molecules
•
Frank-Kasper phases
•quasicrystalline phase
[9]
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10.1002/anie.201914889
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RESEARCH ARTICLE
The unconventional spherical
assembly of discotic PBIs were
successfully achieved, instead of
common columnar assembly, by the
incorporation of bulky units at the
periphery. The peripheric steric
hindrance can be tuned by varying the
linkage structures, resulting in diverse
spherical supramolecular structures in
PBIs.
Jiahao Huang, Zebin Su, Mingjun
Huang, Rongchun Zhang, Jian Wang,
Xueyan Feng, Rui Zhang, Ruimeng
Zhang, Wenpeng Shan, Xiao-Yun Yan,
Qing-Yun Guo, Tong Liu, Yuchu Liu,
Yunpeng Cui, Xiaopeng Li, An-Chang
Shi*, and Stephen Z. D. Cheng*
Page No. – Page No.
Spherical Supramolecular Structures
Constructed via Chemically
Symmetric Perylene Bisimides:
Beyond Columnar Assembly
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