Breaking symmetry toward nonspherical janus particles based on polyhedral oligomeric silsesquioxanes: Molecular design, "click" synthesis, and hierarchical structure

Article abstract of DOI:10.1021/ja202906m

The design, synthesis, and self-assembly of a series of precisely defined, nonspherical, polyhedral oligomeric silsesquioxane (POSS)-based molecular Janus particles are reported. The synthesis aims to fulfill the "click" philosophy by using thiol-ene chemistry to efficiently install versatile functionalities on one of the POSS cages. In such a way, both the geometrical and chemical symmetries were broken to create the Janus feature. These particles self-organize into hierarchically ordered supramolecular structures in the bulk. For example, the Janus particle with isobutyl groups on one POSS and carboxylic groups on the other self-assembles into a bilayered structure with head-to-head, tail-to-tail arrangements of each particle, which further organize into a three-dimensional orthorhombic lattice. While the ordered structure in the layers was lost upon heating via a first-order transition, the bilayered structure persisted throughout. This study provides a model system of well-defined molecular Janus particles for the general understanding of their self-assembly and hierarchical structure formation in the condensed state.

Full text of DOI:10.1021/ja202906m

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Breaking Symmetry toward Nonspherical Janus Particles Based
on Polyhedral Oligomeric Silsesquioxanes: Molecular Design,
“Click” Synthesis, and Hierarchical Structure
Yiwen Li,† Wen-Bin Zhang,*,† I-Fan Hsieh,† Guoliang Zhang,† Yan Cao,† Xiaopeng Li,‡
Chrys Wesdemiotis,†,‡ Bernard Lotz,§ Huiming Xiong,*,|| and Stephen Z. D. Cheng*,†
†College of Polymer Science and Polymer Engineering, Department of Polymer Science, and ‡Department of Chemistry,
The University of Akron, Akron, Ohio 44325, United States
§Institut Charles Sadron (CNRS-Universitꢀe de Strasbourg), 23, Rue du Loess, F-67034 Strasbourg, France
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,
Shanghai, 200240, China
S Supporting Information
b
limits the synthetic versatility and the formation of 3D long-range
ABSTRACT: The design, synthesis, and self-assembly of a series
of precisely defined, nonspherical, polyhedral oligomeric silses-
quioxane (POSS)-based molecular Janus particles are reported.
The synthesis aims to fulfill the “click” philosophy by using
thiolÀene chemistry to efficiently install versatile functionalities
on one of the POSS cages. In such a way, both the geometrical and
chemical symmetries were broken to create the Janus feature.
These particles self-organize into hierarchically ordered supramo-
lecular structures in the bulk. For example, the Janus particle with
isobutyl groups on one POSS and carboxylic groups on the other
self-assembles into a bilayered structure with head-to-head, tail-to-
tail arrangements of each particle, which further organize into a
three-dimensional orthorhombic lattice. While the ordered struc-
ture in the layers was lost upon heating via a first-order transition,
the bilayered structure persisted throughout. This study provides a
model system of well-defined molecular Janus particles for the
general understanding of their self-assembly and hierarchical
structure formation in the condensed state.
ordered structures. In this respect, molecular Janus entities, such
as amphiphilic dendritic macromolecules,8 unimolecular micelles
of block copolymers,9 and polymer brushes,10 provide an intri-
guing alternative. Owing to their decreased sizes of typically only
a few nanometers and the precisely defined molecular structures,
they exhibit diverse self-assembled morphologies in the bulk,8a at
the interface,8c or in solution.8b,dÀf An extraordinary example was
demonstrated by Percec et al. in the development of amphiphilic
“Janus dendrimers” that spontaneously form various morpho-
logies in water.8b Nevertheless, unlike “hard” colloidal Janus
particles, the molecular Janus entities reported so far are often not
“volume-persistent” or “shape-persistent” due to the flexibility of
conformations. In addition, it requires multiple-step synthesis invol-
ving delicate control over reaction conditions and nontrivial purifica-
tions. Thus, it is of great interest to develop molecular Janus entities
that are readily available, only a few nanometers in size, 3D volume-
and shape-persistent, and that can self-assemble into hierarchical
structures of potential technological importance.
Here, we report our first effort toward this goal by the design
and synthesis of polyhedral oligomeric silsesquioxane (POSS)-
based molecular Janus particles and the elucidation of their self-
assembled, hierarchical structures in the bulk. POSS, perhaps the
smallest silicon nanoparticles with cage diameter around 1.0 nm, are
versatile nanobuilding blocks that are conformationally rigid and
shape-persistent with diverse periphery functionalities.11 As a route to
“Janus silsesquioxanes”, selective/nonselective functionalization of the
POSS cage has been proposed in order to break the high symmetry
of the cage and create the Janus feature.12 Though fundamentally
interesting, nontrivial optimization is required to promote both the
selectivity of the process and the purity of the final product. Instead,
the current molecular design chose to connect two POSS of distinct
surface chemistry to create the Janus nature (Scheme 1). By mono-
functionalization, the symmetry of POSS cage is reduced from Oh to
C3v; by coupling of two POSSs, the overall molecular shape changes
from spherical to dumbbell- or snowman-like. Despite the same “bulk”
composition for both halves, the difference in surface chemistry com-
prises the major driving force for self-assembly. Facilitated by “click”
n analogy to the double-faced Roman god Janus, De Gennes
formally introduced in his Nobel lecture the concept of “Janus
I
grains” to describe grains with two distinct sides.1 The essential
idea is that a three-dimensionally dissymmetrical distribution
of certain physical/chemical features, such as size, shape,
bulk composition, and surface chemistry, shall lead to the for-
mation of specific hierarchical structures with novel properties.2
It has since thrived into a dynamic research area and the “Janus”
term has been extended to describe the “dissymmetrical entities”
in general,3 including macromolecules, supramolecular assem-
blies, nanoparticles, and colloidal particles. In synthesis, it
requires the break of symmetry in the first place to create the
“Janus” feature. So far, the synthesis of Janus particles based on
inorganic nanoparticles and colloidal particles has been achieved
by a number of methods, such as partial contact with reactive
media,4 directed fluxes and fields,5 spontaneous assembly,6 and
controlled surface nucleation,7 with a plethora of variations in
sizes, shapes, and chemical compositions. Yet it remains a
challenge to synthesize readily scalable, well-defined Janus
particles with molecular precision and high uniformity, which
Received: March 30, 2011
Published: June 16, 2011
r
2011 American Chemical Society
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Scheme 1. General Synthetic Strategy for BPOSS-XPOSS
Janus Particlesa
Figure 1. MALDI-TOF mass spectra of BPOSS-XPOSS.
BPOSS-HPOSS in methanol and ethanol are much increased com-
pared to BPOSS-VPOSS. Under basic conditions, BPOSS-APOSS
can dissolve in water. Their dual solubility reflects the amphilicity and
the Janus feature. Notably, the reaction can be readily scaled up,
facilitating preparation of gram quantities of samples and the study
of their physics. The exemplary elucidation of the self-assembled
structure of BPOSS-APOSS in the bulk is shown below.
a (i) Phthalic anhydride, DMAP, triethylamine, THF, rt, 90%; (ii)
PSS-(3-hydroxyethyl)-heptavinyl substituted (VPOSS-OH), DMAP, DIPC,
dry CH2Cl2, 0 °C, 93%; (iii) R2SH, DMPA, THF, rt, 15 min, 77À83%.
The phase behavior of BPOSS-APOSS was first examined by dif-
ferential scanning calorimetry (DSC) at a rate of 10 °C/min, as shown
in Figure 2a. The first cooling thermal diagram exhibits an exothermic
transition peaked at 141 °C with a latent heat of 18.6 kJ/mol. The
subsequent heating shows an endothermic transition peaked at 182 °C
with an essentially identical latent heat (18.8 kJ/mol). It is therefore a
first-order transition in the condensed state associated with the ordered
structure formation under supercooling. The temperature-dependent
simultaneous SAXS-WAXD experiments on a BPOSS-APOSS sample
were then performed (Figures S4 and S5). At room temperature, the
patterns suggest the formation of a hierarchically ordered structure in
the bulk (Figure 2c,d) and the diffraction patterns cover the length
scale from a few angstroms to several nanometers. In the SAXS pattern
(Figure 2c), two distinct peaks appear with a q ratio of 1:2, indicating a
layered supramolecular structure with a spacing of 4.62 nm. This is close
to twice the length of the long axis of BPOSS-APOSS (4.78 nm),
suggesting a bilayered structure. The halo near q1 peak may be
attributed to the disordered aggregates and, thus, a correlation hole
scattering.18 In the WAXD pattern (Figure 2d), several sharp
diffraction peaks represent the existence of an ordered structure
on the angstrom scale, which are largely attributed to the ordered
packing within the bilayers with segmental order of the molecules.
Except for the first diffraction peak that can be assigned as the (004)
diffraction of the layered structure in Figure 2d (d-spacing of
1.16 nm), the rest of the diffractions should be attributed to the
3D ordered structure within the bilayers. Note that, in the wide angle
region, there are also two correlation hole scatterings (one is located at
∼6°, and the other at ∼14°). Although the diffractions at the wide
angle region, except the (004) diffraction, disappear above 182 °C
upon heating and reappear at 140 °C during cooling (Figure S5), the
diffractions associated with the bilayer structure at the low angle
region persist throughout (Figure S4). Nevertheless, there is a
discontinuous change in the spacing of this bilayered structure from
4.80 to 4.70 nm at 140 °C during cooling (Figure 2b). This is another
manifestation of a thermodynamic first-order transition. From the
structural point of view, this transition is associated with the order-
ing of the supramolecular bilayered liquid crystal phase into the
crystalline phase.
chemistry, the simultaneous control over size, shape, and surface
chemistry provides a series of precisely defined, model molecular Janus
particles for a systematic study of their self-assembly and properties.
The synthetic strategy is outlined in Scheme 1, where we strive to
fulfill the “click” philosophy by modular construction of diverse
materials from a set of simple building blocks and efficient chemical
reactions. In literature, various well-defined functional POSS cages
with distinct periphery functionalities, such as carboxylic acids,
hydroxyls,13 perfluorinated chains,14 and bioactive moieties,15 have
been reported. The synthetic methods include metal-catalyzed
cross-coupling reactions,16 olefin metathesis,17 Cu(I)-catalyzed
Huisgen [3 + 2] cycloadditions,15a and thiolÀene “click” chemi-
stry.13,14,15b Here, the common precursor BPOSS-VPOSS was
prepared from commercially available BPOSS-OH in two steps by
alcoholysis of phthalic anhydride and esterification with VPOSS-
OH in excellent yields (>90%). The functionalization of BPOSS-
VPOSS using thiolÀene chemistry proceeds in high efficiency
under mild conditions (∼100% conversion), as evidenced by the
1
complete disappearance of signals of vinyl protons in H NMR
spectra of the crude product, which is remarkable in view of multiple
reactive sites and short reaction time involved. Specifically,
carboxylic acid groups, hydroxyl groups, and short perfluorinated
chains have been successfully installed on VPOSS to generate
APOSS, HPOSS, and FPOSS, respectively, in ∼80% yield after
workup and purification. These nonspherical molecular Janus
1
particles (BPOSS-XPOSS) were thoroughly characterized by H
NMR, 13C NMR, FT-IR, and MALDI-TOF mass spectrometry to
confirm their identity (see Supporting Information). The most
convincing evidence was provided by the MALDI-TOF mass
spectra (Figure 1) where only one strong peak matching the
proposed structures of the targeted molecules was observed. For
example, the observed peak at m/z 2321.44 of [BPOSS-APOSS
3
Na]+ agrees well with the calculated monoisotopic molecular mass
(2321.21 Da) (Figures 1 and S3a). Physically, all the BPOSS-
XPOSS are powders readily soluble in common organic solvents
such as THF and CHCl3. The solubilities of BPOSS-APOSS and
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Figure 3. (a) TEM bright field image of the thermal annealed BPOSS-
APOSS film; (b) SAED pattern of the [001] zone without tilting
obtained from the sample in (a). SAED patterns of (c) and (d) were
obtained by tilting the sample by 46° and À45° from the [001] zone
around a*-axis, respectively.
Figure 2. DSC heating and subsequent cooling diagrams (a), the
change of layer spacing with temperature during the cooling process
(b), SAXS pattern (c), and WAXD pattern (d) of thermally annealed
BPOSS-APOSS.
by the spacing filling model as shown in Figure S8. Along the c-axis,
the bilayered structure is unambiguous with a head-to-head, tail-to-tail
packing orientation of BPOSS-APOSS parallel to the layer normal.
The ab-crystalline planes are thus alternatively arranged with BPOSS
or APOSS bilayers. In addition, the geometrical symmetry breaking
induced by the covalent linkage actually leads to a small tilt of one set
of POSS edges away from the c-axis toward the b-axis and the other set
of POSS edges titling less toward the a-axis. To achieve the minimum
free energy of crystal packing in our simulation, the tilt angle is
alternatively positive (clockwise) and negative (anticlockwise) with
respect to the c-axis along the layer normal (Figure 4a). This causes
the slight difference between the aand bdimensions (Figure 4b). The
computer-simulated ED patterns along the [001], [04À1] and [041]
zones (Figures 4d and S7) are in quantitative agreement with
the experimental ones in terms of both position and intensity
(Figure 3bÀd), which indicates that the proposed overall symmetry
and molecular packing are correct reflections of the reality.
The next question is the rationale for the formation of such a
bilayered structure and its implications on the self-assembly of Janus
particles in the bulk. The covalent linkage between two distinct POSS
is responsible for the symmetry breaking in shape, while the dissym-
metry in surface chemistry on POSS is the driving force for the self-
assembly due to the specific interactions such as hydrogen bonding
(Figure S9). It should be noted that, if surface functional groups on
two POSSs are not completely immiscible such as BPOSS-VPOSS, the
hierarchical bilayer structure was not observed (Figure S10). Only
when both POSSs are commensurate in shape and volume but
immiscible with respect to surface functionalities, their rigid, large
building blocks will lead to a nanoscale segregation of POSS to form
bilayered structure. In addition, the functional groups within each layer
can further self-organize into crystalline order at the angstrom level. The
structure is thus hierarchical by design. If the volumes are incommen-
surate as in the case of “snowman-like” Janus particles, the formation of
ordered structures would require more delicate balance in molecular
design among the volume ratios, chemical interactions, and packing
considerations. The relevant study is currently ongoing in our group
using these model molecular Janus particles.
To determine the detailed lattice parameters and symmetry group
of the ordered structure of BPOSS-APOSS, selected area electron
diffraction (SAED) was performed in transmission electron micro-
scopy (TEM). The detailed procedure for TEM sample preparation
is included in the Supporting Information. Figure 3a shows a typical
TEM bright field image of the BPOSS-APOSS film, which reveals
stacked lamellar morphology with a flat-on arrangement. The
presence of granular structures is due probably to the fast solvent
removal during the film preparation. With increasing the annealing
time at 140 °C, substantial decrease of the granular structures in the
lamellar morphology was observed. We speculate that the correlation
hole scattering observed in Figure 2c may be associated with this
granular structure. In Figure 3b, the SAED pattern was obtained along
the [001] zone and the extinct spots on the SAED pattern suggest a
2D rectangular symmetry (p2gg). The dimensions of the unit cell can
be determined as a = 1.53 nm, b = 1.43 nm, and γ = 90°. The
determination of c-axis dimension and R and β angles rely on the
tilted ED experiments. Panels c and d of Figure 3 show two ED
patterns at tilting angles of 46° and À45° around the a*-axis,
respectively, giving the c-axis dimension as 4.62 nm, which is identical
to the layer spacing determined by SAXS (Figure 2b). Both R and
β are determined to be 90°. It is thus an orthorhombic unit cell with a
symmetry group of Pna21. In addition, the calculated crystallographic
density is 1.51 g/cm3 based on four BPOSS-APOSS molecules within
each unit cell, which matches well with the experimental value of
1.49 g/cm3. The lower experimental value is due to the fact that the
bulk sample is not composed of 100% crystalline structures (see
Figure 2c,d for the correlation hole scatterings). The thickness of each
lamella shown in Figure 3a can be measured using atomic force
microscopy (AFM). Indeed, the multilayer lamellar structure was also
observed in the phase mode of AFM (Figure S6a). The vertical height
scanning profiles (Figure S6b,d) reveal that the thickness of each layer
is between 4.6 and 4.9 nm. This is in a good agreement with the SAXS
and SAED results, suggesting that each lamella only has the dimension
of one single unit cell along the c-axis in a flat-on arrangement.
The most probable molecular packing of BPOSS-APOSS in the
orthorhombic unit cell was illustrated via computer simulation by the
Accelrys Cerius2 package using the universal force field. The results
are shown in Figure 4. The atomic packing can be better represented
In summary, a series of precisely defined “dumbbell-like” or
“snowman-like” POSS-based molecular Janus particles has been
synthesized by covalently linking together two POSS cages with
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Supporting Information. Synthesis, characterization,
b
and simulation data. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(13) (a) Yu, X.; Zhong, S.; Li, X.; Tu, Y.; Yang, S.; Van Horn, R. M.;
Ni, C.; Pochan, D. J.; Quirk, R. P.; Wesdemiotis, C.; Zhang, W.-B.;
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Corresponding Authors
wz8@uakron.edu; hmxiong@stju.edu.cn; scheng@uakron.edu
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(DMR-0906898).
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