Self-organization of star-shaped columnar liquid crystals with a coaxial nanophase segregation revealed by a combined experimental and simulation approach

Article abstract of DOI:10.1039/c4cc08602c

Fully consistent X-ray data and molecular dynamics simulations on new star-shaped liquid crystals yield two nanosegregated architectures with a coaxial stacking of two functional discotic units: tris(triazolyl)triazine and triphenylene. Analysis of lattice order along the principal axes reveals preferential staggered arrangement of the stacked molecules in the columnar assembly.

Full text of DOI:10.1039/c4cc08602c

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Self-organization of star-shaped columnar liquid
crystals with a coaxial nanophase segregation
revealed by a combined experimental and
simulation approach†
Cite this: Chem. Commun., 2015,
51, 1811
Received 30th October 2014,
Accepted 10th December 2014
Eduardo Beltran,a Matteo Garzoni,b Beatriz Feringan,a Alberto Vancheri,b
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DOI: 10.1039/c4cc08602c
Joaquın Barbera,a Jose Luis Serrano,c Giovanni M. Pavan,*b Raquel Gimenez*a and
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Teresa Sierra*a
www.rsc.org/chemcomm
Fully consistent X-ray data and molecular dynamics simulations on
new star-shaped liquid crystals yield two nanosegregated architec-
tures with a coaxial stacking of two functional discotic units:
tris(triazolyl)triazine and triphenylene. Analysis of lattice order
along the principal axes reveals preferential staggered arrangement
of the stacked molecules in the columnar assembly.
Molecularly engineered systems able to organize spontaneously in
domains with different functionalities at the nanoscale are promising
candidates for miniaturized optoelectronic devices.1 One strategy to
control phase separation at the nanometer scale is to rely on the self-
organization of columnar liquid crystals. These are systems able
to stack spontaneously into columns, leading to one-dimensional
materials.2 A conveniently designed columnar liquid crystal with
different functional units covalently grafted to their molecular
structure can lead to a coaxial assembly allowing, in principle, to
control the phase segregation with nanoscale precision.1d,3 How-
ever, this is not an easy task as high control over face stacking to
avoid mixed columns is needed for this purpose.
Fig. 1 Schematic drawing of the compounds studied in this work.
We have previously described a novel electron-deficient and functionalization with peripheral discotic electron-rich triphenylenes
luminescent star-shaped tris(triazolyl)triazine platform,4 which has (Fig. 1). In this way, two compounds have been prepared, i.e., T-TPC6,
a strong tendency to self-organize in hexagonal columnar meso- with terminal hexyloxy chains, and T-TPC10, with decyloxy chains.
phases with different arrangements as consequence of the flexibility In both cases the triphenylene units (red) are grafted to the
of the polar triazole-containing branches.4b The one-pot synthetic tris(triazolyl)triazine core (blue) by means of an hexamethylene
procedure used to build this tris(triazolyl)triazine core, which spacer (see synthetic details in the ESI†). These supermolecules
is based on click chemistry methods,5 makes straightforward its self-organize in two different ways, both based on coaxial columns
made of a single type of discotic unit. The self-assembled structure
of these materials has been characterized by the synergic use of
a
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Departamento de Quımica Organica, Instituto de Ciencia de Materiales de Aragon
(ICMA) - Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain.
E-mail: rgimenez@unizar.es, tsierra@unizar.es
X-ray experiments (XRD) and molecular dynamics (MD) simulations.
The study of the mesomorphic behavior concludes that both
compounds show a columnar liquid crystal phase (Table S1, ESI†).
The mesophase of T-TPC6 develops on cooling at 90 1C with a
homeotropic texture (black between cross-polarizers). The tendency
to spontaneous homeotropic alignment (columns normal to
surfaces) is an important property for some device implementation.
In contrast, compound T-TPC10 shows a birefringent texture on
cooling from 118 1C (Fig. S1, ESI†).
b Department of Innovative Technologies, University of Applied Sciences and Arts of
Southern Switzerland, Galleria 2, Manno 6928, Switzerland.
E-mail: giovanni.pavan@supsi.ch; Tel: +41 58 666 6676
c
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Departamento de Quımica Organica, Instituto de Nanociencia de Aragon.
Universidad de Zaragoza, C/ Mariano Esquillor, 50018 Zaragoza, Spain
† Electronic supplementary information (ESI) available: Synthesis and character-
ization data, computational methods and additional material from the simula-
tions. See DOI: 10.1039/c4cc08602c
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diffractogram containing a set of reflections in the low angle region
that can be indexed to a rectangular network with parameters
a = 80.2 Å and b = 58.0 Å (Fig. 2c and d). The rectangular symmetry
can be assigned to a distorted hexagonal lattice. The distortion
could be associated to a tilting of the discotic units; however no
information about this can be drawn from the X-ray measure-
ments. In the wide angle region the broad halo corresponding to
the terminal chains in a disordered conformational state and an
additional halo at c = 3.5 Å (intracolumnar stacking order) are
clearly visible. The first, third and fourth reflections of the low angle
region are strong, which indicates a reinforcement of the electronic
density along the planes (110), (220) and (400). The first corre-
sponds to the main nodes of the network, but the other ones reveal
existence of a strong modulation of the electronic density with
wavelengths corresponding to the intercolumnar distances between
triphenylene moieties. This suggests that, in addition to the main
rectangular network formed by the supermolecule (green rectangle
in Fig. 2e), the peripheral triphenylenes pack to generate a sub-
Fig. 2 (a) XRD diffractogram of T-TPC6, inset shows a magnification of
the low-angle region. (b) Self-assembly model for T-TPC6. (c) XRD
diffractogram of T-TPC10 and (d) low-angle region of the XRD diffracto-
gram of T-TPC10; (e) self-assembly model for T-TPC10. In (b) and (e), red lattice. Taking into account the measured parameters of the rectan-
and blue dots represent the centers of the columns (see ESI†).
gular columnar mesophase and assuming that the density of the
material is close to 0.9–1 g cmꢀ3, in this case we obtain that each
node of the main rectangular lattice contains a single super-
molecule. Moreover, the fact that reflection (220) is strong suggests
that the triphenylene columns lie on these planes, and that each
T-TPC10 supermolecule has its three triphenylenes radiating at
XRD diffractograms of T-TPC6 (Fig. 2a) display two reflections in
pffiffi
the low-angle region which are in a relationship 1 : 1 7. In addition,
a broad, diffuse halo at 4.6 Å is observed in the high-angle region,
typical of the conformational disorder of the aliphatic chains of the
E 1201 and interdigitating with the triphenylenes from the neigh-
liquid crystalline state, with an intensity reinforcement in the outer
bouring supermolecules (Fig. 2e). In fact the (220) is equivalent to the
border at c = 4.1 Å, which corresponds to the intracolumnar stacking
(110) reflection of a sublattice with a0 = a/2 and b0 = b/2 containing
order. The two low-angle reflections can be assigned to the reflec-
locally packed columns in the centre and in the corner (purple
tions (100) and (210) of a two-dimensional hexagonal network with a
rectangle). Similar reasoning can be made about the (400) reflection,
parameter a = 52.6 Å. The intensity of both reflections is almost the
same, but the latter is unusually intense. This means that there is
which is equivalent to the (200) reflection of the sublattice. Both
families of planes (220) and (400) are generated by close-packed
pffiffi
a strong modulation of the electronic density with a period of
7
columns and thus they contain a high electronic density (Fig. 2e).
All obtained X-ray patterns show that the first reflection is
the strongest one, indicating that for both compounds the super-
molecule is the main unit in the lattice rather than the constituent
triphenylene or tris(triazolyl)triazine disks. This is in contrast with
previously reported columnar mesophases of star-shaped super-
molecules with peripheral triphenylenes. For example, in the case of
supermolecules with porphyrin7 or 1,3,5-benzenetrisamide8 cores it
was demonstrated that the main lattice is formed by the surrounding
triphenylene units. In both the T-TPC6 and T-TPC10 cases, the super-
molecules are stacked along the z-axis and the triphenylene units
form a sublattice around the cores on the xy plane, which is
consistent with a nanophase segregation in columns.
times shorter than the (100) spacing and suggests the existence
of an additional network or sublattice formed by locally packed
triphenylene columns with a hexagonal symmetry (Fig. 2b).6 Taking
into account the cell volume of the hexagonal network, and assu-
ming that the density of the material is close to 0.9–1 g cmꢀ3, the cell
unit should contain two molecules of T-TPC6 to achieve an efficient
space filling. Following this data, we can propose a model for the
organization of T-TPC6 in the mesophase (Fig. 2b) in which two
antiparallel supermolecules in a non-star conformation can accom-
modate in a columnar stratum and stack constituting the main
hexagonal network (green hexagon). In this situation, six triphenylene
moieties must be arranged at the periphery of the main column in
such a way that they form a hexagonal sublattice (purple hexagon)
MD simulations were carried out to further investigate the
proposed self-assembly models, and to study the levels of order
pffiffi
ꢀ
which is in relationship a0 ¼ a 7. The sublattice parameter a0 fits
the hexagonal cell parameter found for the columnar mesophase inside the lattice on the xy plane and along the z axis (stacking). To
generated by hexahexyloxytriphenylene (20.0 Å). Nevertheless, the tackle these points, two different symmetric self-assembled configu-
sublattice is not homogeneous as it is constituted by six triphenylene rations were considered to build the starting atomistic models of
columns (represented by red dots) around one column of tris- each compound, differing in the relative orientation of the mole-
(triazolyl)triazine dimers (represented by two blue half-disks), or by cules in the columns (rotation around the z axis). The two starting
five triphenylene columns and one column of tris(triazolyl)triazine configurations for the T-TPC10 system, both consistent with the
dimer around a triphenylene column.
rectangular lattice symmetry reported by the XRD data (Fig. 2e), are
Compound T-TPC10, with the same spacer but with decyloxy T-TPC10–08 (Fig. 3a), having all molecules perfectly superimposed
terminal chains in the triphenylene unit, shows a different inside the columns (01 rotation along the z axis), and T-TPC10–1808,
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Fig. 4 MD simulations of the T-TPC6 systems. Top and side views: ideal
starting (left) and equilibrated configurations (right) of the T-TPC6–08 (a)
and T-TPC6–608 (b) systems. (c) Gaussian distributions of blue and red
groups (CM) on the xy plane. (d) Radial distribution functions (g(r)) of the
molecular cores used to assess stacking order along the z axis.
superimposed systems allows understanding which assembly
scheme is more stable during the simulations. Namely, if during
the MD one system evolves to an equilibrated configuration that
strongly deviates from the starting one, this means that the ideal
configuration initially proposed was unrealistic for that system.
The simulation approach adopted herein is similar to that used
to study similar complex self-assembled nanostructures.9 All simu-
lated molecular systems consist of four pre-assembled columns
each containing 72 T-TPC6 (two in each node) and 36 T-TPC10 pre-
stacked and initially extended supermolecules arranged in space
according to the lattice parameters (a and c for the hexagonal lattice,
or a, b and c for the rectangular one) from the XRD experiments
(Fig. 3a, c and 4a, b). All MD simulations were conducted in periodic
boundary conditions, so that the molecular systems replicated in
space are representative of the bulk ordered phase of the liquid
crystals. Analysis of the lattice parameters (a, b and c) of the
molecular systems obtained from MD at the equilibrium demon-
strates very good consistency between the models and the experi-
ments (extensive computational details are provided in the ESI†).
For the best clarity, the results obtained from the MD simulations
Fig. 3 MD simulations of the T-TPC10 systems. Top and side views: starting
(a) and equilibrated configurations (b) of the T-TPC10–08 system. (c) Starting and
equilibrated (d) configurations of the T-TPC10–1808 system. (e) Gaussian
distributions of blue and red groups (CM) on the xy plane. (f) Radial distribution
functions (g(r)) of the cores used to assess stacking order along the z axis.
with molecules rotated by 1801, leading to a staggered arrangement
along the z axis (Fig. 3c). For T-TPC6, the two selected configurations will be presented first for T-TPC10 and then for the T-TPC6 system.
are T-TPC6–08, having all molecules perfectly superimposed along The MD simulations of T-TPC10 show that at the equilibrium the
the z axis (01 rotation), and T-TPC6–608, where molecules are rotated T-TPC10–08system deviates from the initial (ideal) configuration much
by 601 in the columns. Importantly, in the T-TPC6 system only the more than the T-TPC10–1808 staggered one. The T-TPC10–08distortion
stacking of the dimeric cores is different between the 01 and 601 is evident by looking at the columnar stacking of the blue cores
configurations (Fig. 4a and b: left), while both dispositions are (Fig. 3b). Conversely, the ideal lattice columnar assembly is much
consistent with the hexagonal symmetry of the lattice reported by more preserved at the equilibrium in the T-TPC10–1808 case (Fig. 3d).
the XRD experiments (Fig. 2b).
This indicates that the initial superimposed (01) assembled configu-
Since each simulated system starts from an ideal configuration ration is less stable than the staggered one (1801) for the T-TPC10
consistent with the experiments and evolves toward the equilibrium system, which is consistent with the less uniform atom density
during the MD run, the comparison between the rotated and the initially present in the T-TPC10–08 lattice compared to T-TPC10–1808.
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From the equilibrated phase MD trajectories, we extracted respect to each other rather than perfectly superimposed along
information on the relative space displacement (distributions on the z axis.
the xy plane and along the z axis) of the blue and red groups centers
In conclusion, a new class of self-organized systems leading to
of mass (CM) of the T-TPC10 molecules in the two systems. Shown coaxial nanosegregation have been described. These are based on
in Fig. 3e, the Gaussian distributions of blue and red groups on the two chemically different discotic units with a tris(triazolyl)triazine
xy plane are clearly broader for T-TPC10–08 than in the case of star-shaped core linked by a flexible spacer to three pentaalkoxy-
T-TPC10–1808. The positions of the Gaussian peaks indicate the triphenylenes. The length of the terminal alkoxy chains plays an
most probable distances between blue and red group centers on the important role in the supramolecular organization leading to
xy plane in equilibrium conditions, which are well consistent with different self-organization modes.
the lattice results from the XRD experiments (see ESI†). The height
MD simulations of the self-assembled systems starting from
of the Gaussians is indicative of the relative probability of finding two possible ideal lattices consistent with the experiments,
blue or red groups in the experimental positions on the xy plane where T-TPC6 and T-TPC10 units are perfectly superimposed
rather than far from them. Thus, these data quantify how well the (01) or symmetrically rotated along the z axis (601 or 1801,
two models reproduce the experimental (XRD) lattice of Fig. 2e on respectively), show that in both cases the supermolecules are
the xy plane. The red and blue Gaussian peaks are almost twice likely self-assembled by rotated stacking along the z axis.
higher when the T-TPC10 molecules are rotated by 1801 (solid lines)
The synergy between XRD experiments and MD simulations
rather than perfectly superimposed (01: dotted lines) in the columns, reported herein allows for a unique and comprehensive charac-
demonstrating that the T-TPC10–1808 assembly is much more terization of these self-assembled structures, proposing a new
consistent with the XRD experiments than T-TPC10–08.
In addition, information on the stacking order along the zaxis can design of new self-assembling functional nanomaterials.
be inferred from the radial distribution functions (g(r)) of the cores. This work was financially supported by the MINECO-FEDER
Analysis of the g(r) of T-TPC10–1808 (Fig. 3f, black curve) shows a clear funds (projects MAT2012-38538-CO3-01, CTQ2012-35692, FPI
combined theoretical-experimental approach for the rational
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high first peak at distance consistent with the experimental stacking fellowship to E.B.), and the Gobierno de Aragon-FSE (E04
one (c = 3.5 Å), and successive peaks with reduced height at intercore research group, fellowship to B.F.). Authors would like to
´
distances multiple of c. Since g(r) data are indicative of the relative acknowledge the Servicio General de Apoyo a la Investigacion-
probability of finding cores in space as a function of the intercore SAI and CEQMA, Universidad de Zaragoza –CSIC.
distance, these results reveal the presence of short-range regular
stacking in the T-TPC10–1808 staggered system consistent with the
XRD experiments. On the contrary, the same evidence of regular
stacking is not present in the case of the superimposed T-TPC10–08
system (Fig. 3f, green curve). Altogether, these results demonstrate
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