Fullerene-Filled Liquid-Crystal Stars: A Supramolecular Click Mechanism for the Generation of Tailored Donor–Acceptor Assemblies

Article abstract of DOI:10.1002/anie.201812465

Two shape-persistent star mesogens with oligo(phenylene ethenylene) arms and a phthalocyanine core—one providing free space (2) and one sterically encumbered by four fullerenes attached through spacers (3)—have been successfully synthesized. In contrast to the smaller discotic derivative 1, mesogen 2 forms a columnar liquid crystal (LC), which can only be partially aligned without π-stacking, while 3 is not an LC. Exceptionally, the 1:1 mixture of 2 and 3 forms an alignable columnar LC with strong π-stacking and quadruply helically organized fullerenes by an unprecedented click process that is similar to a ball detent mechanism. The C₆₀ units also interconnect different columns. This is driven by nanosegregation and space-filling of the voids with fullerenes. Photophysical studies confirm the presence of a light-collecting system that generates charge-separated states in solution and in the solid state, which makes such highly organized materials attractive for the study of future photovoltaic devices.

Full text of DOI:10.1002/anie.201812465

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Accepted Article
Title: Fullerene-Filled LC Stars – A Supramolecular “Click” Mechanism
for the Generation of Tailored Donor- Acceptor Self-Assemblies
Authors: Matthias Lehmann, Moritz Dechant, Marco Holzapfel,
Alexander Schmiedel, and Christoph Lambert
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812465
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10.1002/anie.201812465
Angewandte Chemie International Edition
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Liquid Crystals
Fullerene-Filled LC Stars – A Supramolecular “Click” Mechanism for the
Generation of Tailored Donor- Acceptor Self-Assemblies
Matthias Lehmann*, Moritz Dechant, Marco Holzapfel, Alexander Schmiedel, Christoph Lambert
WAXS and SAXS, modeling with materials studio and fiber
diffraction simulation.[12] The present parent star-shaped target
ABSTRACT: Two shape-persistent star mesogens with oligo(phenyl-
eneethenylene) arms and a phthalocyanine core – one providing
free space (2) and one sterically overcrowded with four fullerenes
attached by spacers (3) – have been successfully synthesized. In
contrast to a smaller discotic derivative 1, mesogen 2 forms a
columnar liquid crystal (LC), which can only be partially aligned
without π-stacking, while 3 is not an LC. Exceptionally, the 1:1
mixture of 2 and 3 forms an alignable columnar LC with strong π-
stacking and quadruple helical organized fullerenes by an
unprecedented click process similar to a ball detent mechanism. The
C60 units interconnect also different columns. This is driven by
nanosegregation and space-filling of the voids with fullerenes.
Photophysical studies confirm the presence of a light collecting
system generating charge separated states in solution and solid
state, which makes such highly organized materials attractive for
the study in future photovoltaic devices.
molecules
1 and 2 (Figure 1) consists of a larger zinc
phthalocyanine (ZnPc) core decorated with oligo(phenyl-
enevinylene) arms. The derivative with extended arms 3 is equipped
with four fullerenes, attached via long spacers to the conjugated arm
scaffold. The periphery has been chosen to be oligo(ethyleneoxy)
chains, since they are known to lower the transition temperatures
and in case of future applications they would lead to higher relative
permittivities facilitating the charge separation.[13] We highlight that
extending the arm length from n = 0 in 1 to n = 1 in 2 the thermo-
tropic properties and packing changes dramatically owing to the
need of space filling. While in material 2 π-stacking is almost
absent, it is recovered when mixed with non-mesomorphic com-
pound 3, which is a consequence of the click process. The result is a
unique donor-acceptor nanostructure. Photophysical studies were
performed to confirm energy transfer processes and the formation of
charge separated states.
The coupling of building blocks of different nature in a final
synthetic step is an attractive task for the development of advanced
materials. In the last two decades the click concept, introduced by B.
Sharpless, was successfully applied for this purpose, however,
thereby the compounds are joint irreversibly by covalent bonds.[1]
Click chemistry with dynamic covalent bonds and even
supramolecular click chemistry were later presented, which widen
the scope to temporary connections.[2-4] However, these concepts do
still not model macroscopic inventions that can be opened and
closed reversibly, namely velcro fastener, snap fit buckels, snap
fasters and ball detents. These mechanisms rely exclusively on the
shape of the two different components – the “host” providing space
for the “guest” counterpart linking these units mechanically. Here,
we present a supramolecular “Click” procedure reminiscent of the
ball detent mechanism fixing the position of star-shaped zinc
phthalocyanins (ZnPc) 2 and 3 (Figure 1) with regular π-π distances
to the center of liquid-crystalline (LC) columns. The mesogens 2
with long oligo(phenyleneethenylene) arms do not naturally
assemble by π-π stacks and compensate the intrinsic free space by
dimer propeller formation.[5] In the mixture, C60 fills the free space
between the conjugated arms, producing a filled LC phase with a
ZnPc donor stack surrounded by a fullerene acceptor quadruple
helix. Filled mesophases were first reported by Pelzl and Weißflog
and space-filling has been shown also to be important in the CPI
concept or the LC shuttlecocks.[6-8] Recently even an empty
macrocycle has been reported to form a LC pseudorotaxane with a
linear dicyano derivative.[9] The dyad combination of fullerenes and
phthalocyanines or porphyrines have been previously intensely
studied as promising materials for future organic photovoltaic
devices owing to their charge formation and transport
properties.[10,11] These shape-amphiphiles segregate because of their
large differences in morphology between the spherical C60 and the
large flat phthalocyanines and helical arrangements of the fullerenes
were frequently suggested. The helical stacking in triple helices has
been recently substantiated in star-shaped oligo(phenylenevinylene)
mesogens bearing fullerene bound by a short spacer by means of
Figure 1. Target star mesogens.
The synthesis of the new ZnPc compounds 1-3 was performed by a
convergent strategy with a final fourfold condensation of dicyano-
benzene derivatives 6a-b and 11 (Scheme 1). The stilbenoid arms
were assembled by Wittig-Horner reactions and functional group
interconversions using the intermediates 4,5,7,8 (Scheme 1 and
Supporting Information), which were synthesized analogous to the
known derivatives with aliphatic chains.[12,14] Compounds 6a-b were
condensed to obtain the ZnPc derivatives 1 and 2 in moderate yield
between 22-27 %. The fourfold condensation of compound 11, the
deprotection and fourfold esterification yielded the fullerene
derivative 3 in 6 % yield. All materials were analyzed compre-
hensively by standard analytical techniques (NMR, mass spectro-
metry and elemental analysis see supporting information). The
target molecules were obtained as a mixture of four regioisomers,
namely (2,9,16,24)-, (2,9,17,24)-, (2,10,16,24)- and (2,9,16,23)-
1
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10.1002/anie.201812465
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temperature (see SI). While for 1 the density could be only estimat-
ed in non-polar solvents to be in the range of 1.04 to 1.29 gcm-3
owing to solubility reasons, the value for 2 were determined to be
1.18 gcm-3 using aqueous CaCl2 solutions. Combining XRS and
density data the number of molecules 1 in a unit cell (a = 38.0 Å,
c = 3.3 Å) can be estimated to be one at a density of 1.20 gcm-3,
whereas there are two molecules 2 in the columnar stratum (a = 50.1
Å, h = 4.6 Å, δ = 1.18 gcm-3). Compound 1 forms a rather
conventional columnar stack of discotic molecules, which is
visualized in Figure 2I for the geometry optimized columnar stratum
of six mesogens, modeled using Materials Studio using the major
regioisomer.
tetrasubstituted derivatives. In theory, they are statistically formed in
the ratio of 4 : 2 : 1 : 1. [10a] The regioisomers could not be analyzed
in detail by NMR spectroscopy because of broad, overlapping sig-
nals for the ZnPc protons (see Figures S1-4). Especially, the
strongly aggregating compound 3 furnished only moderately re-
solved 1H NMR spectra even at temperatures above 90 °C in
C2D2Cl4 (see Figures S3,4). Interestingly, compounds 1 and 2 re-
vealed a completely different solubility. While 1 is soluble in aque-
ous solution and all common organic solvents except completely
nonpolar liquids such as hexane, compound 2 is insoluble in hexane
and aqueous solutions. The latter points to the increased hydro-
phobicity of the core and the larger hydrophobic pockets.
Scheme 1. Synthesis of the target compounds 1-3 (R = (C2H4O)3C2H5); DCM
dichloromethane, DIC diisopropylcarbodiimide, DMAE dimethylaminoethanol,
DPTS dimethylaminopyridinium toluenesulfonate, PPTS pyridinium toluene-
sulfonate, THF tetrahydrofuran, TFA trifluoro acetic acid.
The thermotropic properties have been studied by polarized optical
microscopy (POM), differential scanning calorimetry (DSC) and X-
ray scattering (XRS) on aligned fibers. POM photographs of com-
pounds 1 and 2 at 220 °C of the sheared samples demonstrate their
birefringent and fluid nature and thus confirm the LC properties
(Figures 2A, S7). Unexpectedly, the star mesogens did not clear into
an isotropic phase but decomposed at temperatures above 300 °C,
which prevented the formation of characteristic LC textures.
Compound 3 is found to be an amorphous solid. DSC confirm for all
materials the absence of phase transitions from ambient temperature
to 300 °C (Figure S8). Detailed structural information have been
obtained for 1 and 2 from temperature-dependent wide-angle XRS
(WAXS) (Figure 2). The first tremendous difference between
compounds 1 and 2 is observed in the alignment of the two fibers.
While the XRS pattern of 1 shows typical features of an oriented
columnar phase of discotic molecules the pattern of 2 exhibits only
weak or almost no alignment (Figures 2D,E). The small angle
reflections for both compounds can be indexed according to a
hexagonal 2D positional order. The cell parameters increase as
expected from 38.1 Å (1) to 50.1 Å (2) at room temperature for the
shape-persistent molecules, which is in good agreement with the
molecular size (Figure S14). The halo at 4.5 Å and the signal
corresponding to 3.4 Å can be attributed to the average distance of
hydrocarbons including the liquid order of aliphatic chains and the
π-π distance of the ZnPc cores. While for the essential discotic
mesogens 1 a clear reflection for π-π distance of the ZnPc cores is
discerned, for the larger derivative 2 this signal is almost absent
indicating the low order of the ZnPc cores along the columns. In
order to clarify the structural differences, the density has been
measured by the buoyancy method for extruded fibers at room
Figure 2. POM textures of shear aligned compounds 2 (A) and mixture 2+3
(B,C) (S shearing direction, P Polarizer, A Analyzer, λ direction of compensator
plate). The mixture 2+3 reveals much better alignment (B) and the λ-
compensator confirms an optical negative columnar phase (C). WAXS pattern
of the hexagonal phases of 1, 2 and the 1 : 1 mixture of 2 + 3 at 25 °C (D - F).
Inset in F: Fiber diffraction simulation based on the model (K). G: Integrated
intensity along the equator of the patterns with indexation confirming the Colh
structure. The broad intensity H is attributed to the helical packing of 2. Inset: χ-
scan of the pattern of 2+3 between 2θ = 5-6° revealing the splitted intensities
along the meridian. H: Integrated intensity along the meridian demonstrating the
LC nature of the phases, with the broad halo and the intense signal for the π-π
distance in case of compound 1 and the mixture 2+3. I-K: models of the
columnar phases of 1, 2 and the mixture 2+3. The ZnPc cores are colored in
light blue, the stilbenoide arms are highlighted in orange and green. For the
model of the mixture, the chains have been omitted for clarity; the yellow arrows
show the interaction points of fullerenes from different columns.
2
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10.1002/anie.201812465
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Figure 3 shows the XRS results from extruded fibers of a series of
mixtures containing different mole fractions of 3 (0.00, 0.33, 0.40,
0.50, 0.59). The WAXS diffraction patterns at 220 °C clearly
highlight the strong π-stacking signal and the rather good alignment
along the fiber for the intermediate mole fractions (0.33-0.50,
Figure 3B, Figure S10). The parent compound 2 (Figure 3B) and an
excess of the sterically overcrowded compound 3 (Figure 3D) result
in the loss of alignment and the longer π-stacked aggregates. This
confirms clearly the concept of the supramolecular “click
mechanism”. The temperature-dependent XRS of the 1 : 1 mixture
(Figure S11) reveals that the molecules in the highly viscous fibers
extruded at 170 °C are still not fixed together, but transform to the
highly ordered and aligned columnar phase only above 190 °C. This
“Click” process in the mesophase can be followed by DSC
(Figure S8). Obviously the increasing mobility starting at 190 °C
allows the molecules to arrange in the ordered LC phase
accompanied by a characteristic weak endothermic signal. The
alignment and order do not change after the first heating process and
the DSC remains silent in the following cycles. This process is
reversible, since the self-assembled mixture can be separated in its
components by chromatography (Figure S9). The diffraction
patterns of the materials with intermediate mole fractions consist of
the equatorial reflections, which can be attributed to the 2D
hexagonal packing of columns. The a parameters are increasing
from 48.2 Å to 50.0 Å with the increasing mole fraction of 3 from
33-50 mol% (Figure 3E). This is in good agreement with fullerenes
filling the free space between the arms, displacing other molecular
building blocks to the periphery. The halo at 4.5 Å can be attributed
to the average distance of hydrocarbons and the strong signal at the
meridian corresponds to the π-π distance (3.4 Å). A set of two
diffuse off-meridional signals points to an additional periodicity
along the columns (Figures 2F,G inset). The latter intensities are
most prominent for the 1 : 1 mixture. Analysis of the off-meridional
reflections revealed that these signals are positioned at a layer line
corresponding to a periodicity of 27.2 Å along the c-axis. With an
experimental density of 1.28 gcm-3 four molecules of each 2 and 3
fill this columnar stratum. Thus 4 x 4 fullerenes must be assembled
and owing to the molecular structure, it is reasonable to assume four
strands of four fullerenes each. With a van der Waals diameter of a
single fullerene of 10.3 Å the total length of four fullerenes in a
chain is 41.2 Å. Consequently, the fullerenes must be strongly
intercalated in the columnar repeat of 27.2 Å. In order to generate
this periodicity continuously along the column, optimize the
nanosegregation and avoid steric repulsion a helical arrangement is
the most reasonable assumption. Dense helical packing of fullerenes
depends on various geometrical parameters, e.g. the number of
fullerenes in the helical pitch and thus the height h available for a
single sphere along c and the rotation about the helical axis without
losing the contact to the neighboring fullerene. These parameters
determine the radius of the helical strand, which must be larger than
the radius of the ZnPc core (approx. 8 Å) and cannot strongly
exceed the radius of the column (approx. 25 Å). Furthermore, the
symmetry of compound 3 allows only for the generation of a
quadruple helix, thus the strongest X-ray intensities are expected to
be on the fourth or eighth layer line.[17] The best agreement with the
experimental diffraction pattern was obtained assuming a helical
pitch and repeat along the c-axis of 217.8 Å. Then 64
phthalocyanines (128 fullerenes) pack in this giant unit cell with a
distance of 3.4 Å and four helical fullerene strands lock (“click”) the
aromatic discs to the column center. For steric reasons only 25
fullerenes from 32 can be assembled in a single helical strand while
seven fullerenes are distributed randomly to vacancies in the interior
of the column. After geometry optimization a 251 quadruple helix
emerges (Figure 2K), which can be regarded as a snapshot of the LC
assembly. The radius of the ideal 251 fullerene helix (26.2 Å)
without steric congestions would be slightly larger than the
experimental columnar diameter. As a consequence, the helices of
different columns intersect (Figure 3I yellow arrows) and the helices
In order to pack two molecules 2 in its columnar stratum the mole-
cules can be neither side by side nor on top of each other for steric
reasons. The most convenient packing is achieved by the propeller-
like intercalation of two mesogens, thus filling almost three of the
four intrinsic free spaces between the conjugated arms, similar to the
model of Maier et al..[5] In order to avoid steric congestions along
the column, the neighboring dimer has to be turned by at least 40°
about the columnar axis, which is reminiscent of the stacking of
octahedral metallomesogens (Figure S15).[15] The subsequent
assembly affords a complete double helix with a pitch of 41.1 Å
after nine dimers. The broad intensity H at 16.0 Å (2θ = 5.5°) in
Figure 2E,G indicates indeed the presence of a periodicity along the
column axis, which might be explained by such a helix. Owing to
the missing alignment, we lack information about the position of
this intensity along the meridian and thus about the details of the
intra columnar structure. Nevertheless, the described double helical
stack of 2 has been geometry optimized until large non-covalent
energies were obtained (Figure 2J), substantiating the stability of
this model assembly. The correlation length corresponding to the H
signal is calculated with the full-width at half maximum according
the Scherrer formula and amounts to approx. 80 Å, which is two
helical pitches of our model structure.[16,17] The low shape
anisotropy of this columnar assembly (diameter 50 Å, length 80 Å)
provides a natural reason why the material cannot be easily oriented
by extrusion. Thus the model confirms the nanosegregation and the
efficient space-filling of the intrinsic void leading to the absence of
π-π-stacking and alignment. In principle this is a lucky case. If the
dimer formation can be prevented by any other space-filling
procedure, then the ZnPc cores should generate alignable π-stacks of
longer correlation length. Therefore, we assumed that mixtures of
compound 2 and 3 would lock the ZnPcs in the center of the
columns analogous to a ball detent mechanism, used macroscop-
ically to fix metal pieces reversibly to defined positions (Figure 3A).
Figure 3. A: Click process by a ball detent mechanism (chains are omitted for
clarity). B-F: XRS study. WAXS pattern at 220 °C of 2 (B), mixture with 50 mol%
3 (C) and 59 mol% 3 (D). Comparison of the positions of 10 reflections (E) and
the WAXS region (F) for mixtures with increasing mole fraction of compound 3
given in mol%.
3
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10.1002/anie.201812465
Angewandte Chemie International Edition
aggregates.[18] This is most pronounced for the 1 : 1 mixture, which
supports the favorable aggregation procedure by the Click process.
The concentration corrected emission spectra in Figure 4B reveal
exclusively the emission of the ZnPc Q-band, which is increasing
with the increasing extinction coefficient of the conjugated arms and
the efficient overlap of their emission spectra with the Soret band of
the phthalocyanines. The frequently intense emission of the
conjugated stilbenoid oligomers is completely quenched pointing to
an effective energy transfer to the ZnPc moiety. Consequently, the
molecules exhibit all features of an antenna system, which will be
favorable for future photovoltaic materials. For the fullerene
derivative also the ZnPc emission is efficiently quenched.
Fluorescence spectroscopy with the thin film of the mixture reveals
a complete absence of light emission pointing to the permanent
presence of fullerenes close to the ZnPc in the LC state and a
are slightly deformed. This results in an additional nanosegregation
and the formation of a fullerene network. All ZnPcs are forced to be
in the center of the column in the regular π-stack, the shape-
persistent oligo(phenyleneethenylene), defining the size of intrinsic
free space, are placed helically in between the fullerenes, while the
spacers, oligo(ethyleneoxy) chains and in total 4 x 7 = 28 residual
fullerenes fill the remaining void. This model is confirmed by the
simulated 2D fiber diffraction pattern showing all main features
which are in agreement with the experimental results (Figure 2F
inset).
quantitative quenching process.
A
transient femtosecond
spectroscopy study in solution and thin film revealed that fullerene
anions and ZnPc cations are generated with a life time in the
nanosecond range (Figure S17).[19]
In conclusion, the design of oligo(phenyleneethenylene) ZnPc star
mesogens was highlighted – one which provide intrinsic void space
and one which is sterically overcrowded with fullerene guests.
While the mesogen without fullerenes is able to compensate the void
by dimer formation the fullerene derivative is not liquid-crystalline.
However, the 1 : 1 mixture of both reveal the formation of a highly
ordered LC donor-acceptor system by a kind of ball detent
mechanism, which we call a mechanical supramolecular “CLICK”
procedure. Processes using the filling of voids are omnipresent in
nature (e.g. function of enzymes) but here this strategy is used for
the first time intentional to stabilize and organize a donor-acceptor
system side by side in a single column. The conjugated arms are not
only steric parts creating space but are building blocks of an antenna
system transferring energy to the central phthalocyanine. This leads
eventually to a charge separation providing ZnPc radical cations and
fullerene radical anions. Thus this molecular system may become a
promising organic photovoltaic material, if the columnar
mesophases can be oriented homeotropically between two
electrodes. These investigations are currently in progress.
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: Liquid Crystal • Donor-Acceptor Dyad • Supramolecular
Click procedure• Columnar Phase • Helical packing
Figure 4. UV/vis absorption (A) and fluorescence spectra (B) of 1 (red), 2 (blue)
and 3 (magenta) in THF. The inset in (A) shows the absorption spectra of spin-
coated thin films as prepared (dashed line) and annealed at 220 °C solid line,
normalized to the long wavelength absorption maxima. For comparison the
spectra of the 1 : 1 mixture have been visualized.
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[]
Prof. Dr. M. Lehmann, M. Dechant, Dr. M. Holzapfel, A. Schmiedel,
Prof. Dr. Ch. Lambert
Institut für Organische Chemie & Center for Nanosystems Chemistry
Julius Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
Fax: (+) +49 931 31 87350
E-mail: Matthias.Lehmann@uni-wuerzburg.de
Homepage: https://www.chemie.uni-wuerzburg.de/oc/m-lehmann-
group/home/
[+]
Footnote: M. L. and M. D. are responsible for synthesis, LC studies,
absorption, emission spectroscopy and writing of the manuscript. M.
H., A. S. and Ch. L. are responsible for the measurement, analysis
and interpretation of femtosecond spectroscopy.
[]
Acknowledgements. We are grateful to the DFG for the financial
support (LE 1571/8-1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201xxxxxx.
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10.1002/anie.201812465
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Only a Click Away: A supramolecular CLICK procedure leads to a helical columnar liquid-crystalline Donor-Acceptor
separated antenna system by a molecular ball detent mechanism.
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This article is protected by copyright. All rights reserved.