The liquid crystal Click procedure for oligothiophene-Tethered phthalocyanines-self-Assembly, alignment and photocurrent

Article abstract of DOI:10.1039/d1tc00710f

A series of star-shaped liquid crystals (LCs) with a phthalocyanine donor core, oligothiophene antennae and fullerene acceptors have been successfully prepared. This hierarchical self-Assembly results in a nanosegregated helical donor-Acceptor-Antennae LC-system promoted by the recently discovered Click procedure. This model system reveals all photophysical prerequisites for energy conversion, charge generation and transport. Uniform amorphous thin films of 150 nm could be produced by bar-coating. Annealing did not only induce the formation of columns via the Click procedure but also partially homeotropically aligned the non-clearing sample in a sandwich geometry (ITO and Ag/MoO3). This has been confirmed by microscopic studies and the measurement of photocurrent, which increased by a factor of 300 after the annealing step.

Full text of DOI:10.1039/d1tc00710f

Journal of
Materials Chemistry C
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PAPER
The liquid crystal Click procedure for
oligothiophene-tethered phthalocyanines –
self-assembly, alignment and photocurrent†‡
Cite this: J. Mater. Chem. C, 2021,
9, 5689
ab
c
Moritz Dechant,a Matthias Lehmann,
Masanori Ozaki
*
Genya Uzurano,c Akihiko Fujii
and
c
A series of star-shaped liquid crystals (LCs) with a phthalocyanine donor core, oligothiophene antennae
and fullerene acceptors have been successfully prepared. This hierarchical self-assembly results in a
nanosegregated helical donor–acceptor–antennae LC-system promoted by the recently discovered
Click procedure. This model system reveals all photophysical prerequisites for energy conversion, charge
generation and transport. Uniform amorphous thin films of 150 nm could be produced by bar-coating.
Annealing did not only induce the formation of columns via the Click procedure but also partially
homeotropically aligned the non-clearing sample in a sandwich geometry (ITO and Ag/MoO3). This has
been confirmed by microscopic studies and the measurement of photocurrent, which increased by a
factor of 300 after the annealing step.
Received 12th February 2021,
Accepted 15th March 2021
DOI: 10.1039/d1tc00710f
rsc.li/materials-c
space between its arms and one is a shape-amphiphile, in which
the flat star mesogen is connected to four spherical fullerenes via
1. Introduction
Recently, phthalocyanines or porphyrins attached to conjugated flexible spacers.24–26 The latter compound is sterically over-
oligomers and polymers have attracted much attention owing to crowded existing only as an amorphous material. However, a
their promising properties for organic electronic applications.1,2 1 : 1 mixture of the two stars generates triple-nanosegregated
Dyads with fullerenes have also been prepared for photovoltaic helical columnar liquid-crystalline phases, in which the
applications.3–10 Long-lived charge carrier separation was fullerenes occupy the free space of the parent stars. This process
reported6 and the dyads were mixed with conjugated polymers is called the Click procedure, which is based on the shape-
realizing absorption over the full visible spectral range, which amphiphilicity and driven by the nanosegregation tendency of
added a light-harvesting effect.11,12 Many liquid-crystalline dyads the fullerenes and the filling of the void space. This phase realizes
have been synthesized in order to combine high order and the almost an ideal donor–acceptor double cable structure of phtha-
propensity for alignment – both properties are needed to locyanines locked in the center of the columns surrounded by a
improve the efficiency of organic electronic devices.13–23 quadruple helical fullerene structure. The conjugated oligomers
Although large ambipolar and balanced charge carrier mobilities in between act as additional light harvesters (antennae) transfer-
have been found for a columnar double cable structure, no ring the absorbed energy to the phthalocyanine center.
photovoltaic applications from such dyads are reported.16 All
The molecules were provided with peripheral oligo
these features have recently been combined in shape-persistent (ethyleneoxy) chains, which should lower the transition tem-
star mesogens. One parent star compound provides intrinsic free perature to the isotropic phase (clearing temperature). This is
needed for the conventional alignment of the material, in
which cooling from the isotropic phase between two substrates
a
¨
Institute of Organic Chemistry, University of Wurzburg, Am Hubland,
results in standing columns like molecular cables connecting
the electrodes (homeotropic alignment of a columnar phase).
Additionally, these polar flexible chains should increase the
dielectric constant of the material to promote charge separation and
prevent recombination. Unfortunately, the clearing temperature
remained above 300 1C and all the attempts to align the sample
failed.27 Here, we present phthalocyanine star mesogens equipped
with oligothiophene arms as excellent organic semiconductors28
and to prevent photoreactions known for the frequently applied
¨
97074 Wurzburg, Germany. E-mail: Matthias.Lehmann@uni-wuerzburg.de
b
¨
Center for Nanosystems Chemistry, University of Wurzburg, Theodor-Boveri-Weg,
¨
97074 Wurzburg, Germany
c Graduate School of Engineering, Osaka University, 2-1 Yamada-oka,
Suita, Osaka 565-0871, Japan
† In memory of Professor Siegfried Hu¨nig.
‡ Electronic supplementary information (ESI) available: Synthetic procedures,
molecule spectroscopy data, GPC elugrams, density measurements, X-ray data,
modelling, alignment, photoelectron yield spectroscopy data, and I–V measure-
ments. See DOI: 10.1039/d1tc00710f
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This group could be attached to the dicyano derivative 8a to
yield 16 and subsequently 17 by bromination. The following
Suzuki–Miyaura cross coupling with 9b furnished the arm 18.
Tetracyclomerization of 18 and acidic cleavage of the THP-
protective groups resulted in an insoluble material owing to
side reactions. Therefore, the THP-groups have been cleaved
prior to the tetracyclomerisation to yield 19 (91%), which gave
subsequently the fullerene derivative 5 in 24% yield in the last
two steps – tetracyclomerisation and fourfold esterification.24
All compounds were characterized by standard analytical
methods (NMR, mass spectrometry, elemental analysis; see the
ESI‡). The purity of target structures is demonstrated by a
single peak in the elugram after recycling GPC (see Fig. S10,
ESI‡). The characterization by 1H and 13C NMR spectroscopy is
challenging since they exhibit generally broad signals31,32
owing to aggregation and the presence of four regioisomers –
the (2,9,16,24)-, (2,9,17,24)-, (2,10,16,24)- and (2,9,16,23)-
isomers, which are typically obtained in a statistical ratio of
4 : 2 : 1 : 1 during the synthesis.31 The aggregation increased
Fig. 1 Target molecules 1–5.
oligo(phenylenevinylene) scaffolds (Fig. 1).29 The oligothiophene
arms were capped with phenyl rings decorated with aliphatic chains,
replacing the oligo(ethyleneoxy) chains of the previous system. Since
earlier results demonstrated that oligo(ethyleneoxy) chains have little
impact in bringing down the clearing temperatures to practical
values, this structural modification enabled us to reduce the com-
plexity of the synthesis. The free space providing LC star mesogens
1–4 and the sterically overcrowded non-LC fullerene derivative 5 have
been successfully prepared. Their thermotropic properties have been
studied and the self-assembly using the Click procedure for the 1:1
mixture of compounds 4 and 5 has been demonstrated. This
procedure resembles supramolecular recognition procedures, espe-
cially the CPI effect proposed by Boden and Bushby,30 which has
been previously summarized and discussed.25 The new 1 : 1 mixture
has been comprehensively studied according to its photophysical
properties. The LC could be partially aligned by annealing in a
sandwich geometry, which resulted in a significantly increased
photocurrent.
1
with the size of the stars. Thus resolved H NMR spectra were
obtained for compounds 1–3 in either CD2Cl2 or THF-d8 or by the
addition of some pyridine-d5 which revealed phthalocyanine
protons typically in the range of 7.7–9.5 ppm, thienyl protons
between 7.2 and 8.0 ppm and peripheral aromatic protons from
6.6 ppm to 7.3 ppm (Fig. S1–S3, ESI‡).30 The 1H NMR spectrum of
the largest compound 4 revealed distinct protons only at a high
temperature of 378 K in C2D2Cl4 (Fig. S4, ESI‡). The 13C NMR
spectra revealed exclusively broad signals.14 The analysis has been
especially challenging for compound 5, which exhibited poorly
resolved spectra even at higher temperatures (398 K, see Fig. S5–
S7, ESI‡). However, compounds 4 and 5 were unambiguously
characterized by HRMS mass spectrometry, for which the signal of
the product could clearly be observed and the experimental and
simulated isotopic patterns were in excellent agreement (Fig. 2).
Furthermore, the IR spectrum of compound 5 confirms the
absence of free OH groups and the strong peak at 1735 cmÀ1
corroborates that the ester group formed successfully during
the fourfold Steglich esterification (see Fig. S11, ESI‡). The
presence of the ester and the fullerenes is further verified by 13
NMR spectroscopy, which shows a broad signal at 172 ppm and
C
2. Synthesis
All target molecules 1–5 (Fig. 1) were synthesized by following a rather prominent, unresolved signals for the fullerene carbons
convergent route, in which the corresponding dicyano- between 140 and 155 ppm.
functionalized arms without fullerenes were condensed in a
tetracyclomerization to afford the phthalocyanines (Scheme 1).
Only in the subsequent step, an esterification with a fullerene
derivative results in the sterically overcrowded compound 5.
3. Thermotropic properties
The arms for compounds 1–4 were built by a series of Suzuki– The thermotropic properties of all compounds were studied by
Miyaura and bromination reactions beginning with precursor 6. polarized optical microscopy (POM), differential scanning
The 3,4-dicyano derivatives 11, 10a–c were tetracyclomerized in calorimetry (DSC) and X-ray scattering (XRS).
the last step to give the star-mesogens 1–4 in moderate yields
between 15% and 37%.
Star mesogen 1 clears into the isotropic phase over a broad
temperature range beginning at 249 1C. A typical texture for a
For the synthesis of the sterically overcrowded derivative 5, a columnar phase was obtained by slow cooling of the sample
hydroxymethyl function had to be introduced. Starting with from the isotropic phase (Fig. 3A). In contrast, compounds 2–4 do
the commercially available boronic acid 12, esterification, reduction not clear (see Fig. S9B–D, ESI‡), but decompose at temperatures
and a protection step with dihydropyran yielded the building block above 350 1C. This prevented the formation of characteristic
15 with the protected hydroxymethyl anchor (Scheme 1).
textures. The DSC results confirm the absence of phase transitions
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Scheme 1 Synthesis of the target compounds 1–5 (R = C12H25).
220 1C, furnishing a beautiful, planar aligned film (Fig. 3K). The
improved alignment of the mixture compared with the neat
compounds 4 and 5 hints at a special self-assembly, which
will be shown to originate from the Click procedure. Insertion
of the l-compensator reveals that the largest refractive indices of
the samples add to the slow axis of the compensator when the
shearing direction is orthogonal to the latter. This confirms that
the phthalocyanines are essentially oriented perpendicular to the
columnar structures (Fig. 3C, D, F, G, I, J, L and M).
Fig. 2 Comparison of the experimental (red) and simulated (green)
HRMS-MALDI spectra of compounds 4 (A) and 5 (B). For full range MALDI
spectra see the ESI.‡
3.1 X-Ray scattering and modelling
Detailed structural information of compounds 1–4 has been
derived for aligned fibers by temperature-dependent wide-angle
X-ray scattering (WAXS). Fig. 4 highlights all WAXS patterns.
They reveal signals typical for columnar liquid crystals – a series
of equatorial reflections reminiscent of the 2D lattice of
columns, a halo attributed to the average distance of the
hydrocarbons, namely the liquid-like chains, and signals
corresponding to 3.4–3.7 Å indicating p–p stacking of the
phthalocyanine discs. Only compound 1 forms a soft crystal
below 180 1C with a rather large c-parameter pointing to a
from ambient temperature up to 300 1C for 2–4. In the case of
these compounds, the liquid-crystallinity was confirmed by
shearing (2, 3 at 150 1C, 4 at 180 1C) resulting in planar aligned
mesophases (Fig. 3B, E and H). In contrast, the sterically over-
crowded fullerene derivative exhibits only an amorphous solid
without any birefringence. The mixtures of compounds 4 and 5
also do not clear and reveal only one phase over the whole
temperature range. The 1 : 1 mixture can be sheared above
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contains one molecule per column, confirmed by density
measurements (see the ESI‡), and thus these compounds are
rather classical discotics. Similar X-ray patterns but without
any splitting of the p–p-signals up to the highest accessible
temperature (250 1C) are obtained for compounds 3 and 4.
Interestingly, compound 4 improves its alignment after the
extrusion procedure only in the more fluid phase above 220 1C.
The core diameters of the molecules obtained from
molecular models increase by approximately 7 Å with the
addition of a thiophene repeating unit. The columnar diameters
for molecules 1–3 also increase by 6–7 Å, while there is evidently
a larger jump in the columnar diameter by 9 Å for compound 4
(see Table S2, ESI‡). In a columnar slice of only 3.4 Å thickness
for compound 4, density considerations predict two molecules
that obviously cannot stack on top of each other, and thus it is
reasonable that they arrange side by side in a dimer, in order to
compensate for the rather large intrinsic free space between the
arms. This also explains the rather large increase in the
columnar diameter.25 A possible model is a helical organization
of dimers, which was previously discussed for other derivatives
based on diffuse meridional scattering intensities, indicating
periodicity along the columns.24 For mesogen 4, the diffuse
intensity superimposed on the 21 reflections is discerned as
well. This points to helical stacks of a short correlation length.
The double helical stacking further explains the less efficient
p-stacking visualized by the low intensity of the wide angle
reflection.
Fig. 3 Texture of 1 after cooling from the isotropic phase at 130 1C (A),
POM texture of shear aligned compounds 2 (B–D), 3 (E–G), 4 (H–J) and
the 1 : 1 mixture of 4 and 5 (K–M). (S: shearing direction, P: polarizer,
A: analyser, l: direction of the slow axis of the compensator plate.)
The l-compensator confirms the optical negative nature of columnar
phases (C, D, F–J, L and M).
The obvious dimer formation of compound 4, and thus the
compensation of the intrinsic free space, makes it a suitable
system to test a different compensation mode like the recently
discovered Click mechanism with a sterically overcrowded
fullerene star.24 The XRS results for different mixtures of 4
and 5 are highlighted in comparison with the neat samples 4
helical arrangement (Fig. S20, ESI‡). At higher temperatures, and 5 in Fig. 4G–M. Note that the sterically overcrowded
the large number of meridional small angle signals disappears compound 5 exhibits only an X-ray pattern of an amorphous
and remained absent even upon cooling to 25 1C. Only the phase. All mixtures were extruded at 180 1C and the virgin
annealing at 160 1C resulted in the reformation of the soft fibers reveal only broad reflections with some preorganization
crystal. The liquid crystal phase does not exhibit any sign of 3D and almost no p–p-stacking signals. Only heating to the more
correlations. The equatorial signals might be indexed to a fluid phase at 220 1C furnished patterns pointing to a higher
hexagonal phase; however, the splitted p–p signals at all organization, indicating the Click procedure. The most
temperatures indicate the tilt of the discs between 27 and pronounced p–p-stacking is observed for the 1 : 1 mixture. A
351, and thus the phase is better described as a centered slight access of the sterically overcrowded component 5 results
rectangular phase which is related to the hexagonal phase with directly in a much lower level of organization. Yet, also a lower
brec = ahex and arec = 2ahex sin 601. Moreover, a slight deviation content of star 5 affords a reduced degree of order of the self-
from this exact relationship would explain the rather broad assembly, demonstrated by the weak p–p-stacking signal. The
signals and result in better fitting. Compound 1 is a compact equatorial reflections of the 1 : 1 mixture correspond to a
disc with a large number of chains. The difference in packing hexagonal lattice with an a-parameter of 54.7 Å at 25 1C. This
distance of the chains and aromatic building block results in enormous decrease of the columnar diameter compared with
the optimization of the stacking distance by tilting and in the the dimer phase of compound 4 by more than 6 Å combined
formation of the rectangular phase, especially when the with the strong p–p-stacking signal is clear evidence for the loss
mobile flexible chains occupy a growing volume with increasing of the lateral packed homodimer phase in favor of coplanar
temperature. This can also be observed for the next larger stacked heterodimers of 4 and 5. This demonstrates the opera-
molecule 2, which possesses a hexagonal columnar phase at tion of the Click procedure,24–26 in which fullerenes fill the
ambient temperature, but the increasing splitting of the p–p intrinsic free space between arms and the phthalocyanines are
signals above 150 1C indicates the formation of a centered locked in the middle of the column. The p–p-stacking distance
rectangular phase. For both compounds, the repeating unit at 25 1C amounts to 3.3 Å, and thus one mesogen 4 and one
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Fig. 4 2D WAXS patterns of the centred Colr-phase after cooling of 1 (A, 25 1C) and the Colh-phases of 2 (B, 150 1C), 3 (C, 250 1C), and 4 (D, 250 1C, the
diffuse meridional intensities are highlighted with white ellipses). (E) Integrated intensity along the equator of the patterns with indexation confirming the
different columnar structures. (F) Integrated intensity along the meridian showing the halo (average chain distance) and the variation of the intensity for
the signal attributed to the p–p-interaction at room temperature. WAXS patterns (25 1C) of 4 (G) and the mixtures of 4 and 5 with increasing mole fraction
of compound 5: (H) 0.40, (I) 0.50, (J) 0.59, and (K) 1. (L) Integrated intensity along the equator of the patterns with indexation confirming the Colh
structure for mixtures with increasing mole fraction of compound 5. (M) Comparison of the WAXS region (meridian) for mixtures with increasing mole
fraction of compound 5. (N) Model of the Click procedure and the formation of short helical columns of the 1 : 1 mixture of 4 and 5.
mesogen 5 assemble in a stack of 6.6 Å along the columnar axis. fullerenes and oligothiophenes. The residual volume of the
This leads to a reasonable density of 1.142 Æ 0.040 g cmÀ3
,
unit cell is filled by the additionally nanosegregated aliphatic
which is in acceptable agreement with the experimentally chains. The model further indicates that the fullerenes of
observed density of the material (1.186 Æ 0.006 g cmÀ3). The different columns approach each other and the formation of
difference in the height of the heterodimer between 4 and 5 interwoven fullerene networks is feasible (Fig. S19, ESI‡).34
(h = 6.7 Å) and the diameter of a fullerene (10.3 Å) reveal that in a
model of this columnar phase the fullerenes need to be inter-
digitated along the column, which is best realized with a helical
4. Photophysical properties
displacement of the fullerenes highlighted previously.24,33,34 The
correlation length of the p–p-stacking along the c-axis is 26.9 Å, Fig. 5 highlights the photophysical behavior of the series of
and thus only eight molecules are correlated in a stack. This compounds 1–5 in THF and thin films. Fig. 5A shows the
explains naturally why the X-ray intensity attributed to the UV-Vis spectra in solution, which appear as superpositions of
periodical, helical packing could not be observed, since only the different building blocks – oligothiophene bands with
very short helical stacks are formed.
maxima at 356 nm (2), 379 (3), 418 (4) and 405 nm (5);
Fig. 4N highlights schematically the Click process and the phthalocyanine Soret bands with a maximum at 362 nm (1)
formation of the helical column with a short correlation length. and superimposed maxima for 2–5; Q bands with maxima at
A geometry optimized model of such helical fragments with 688 nm (1), 710 nm (2), 715 nm (3), 717 (4) and 725 nm (5)) and
seven heterodimers is visualized in Fig. S19 (ESI‡). The model in the case of molecule 5 the additional fullerene absorption
(Fig. 4N and Fig. S19, ESI‡) reveals a triple nanosegregated with maxima at 253 nm and 323 nm. The progression of the
donor–acceptor quadruple helix. This comprises the nano- oligothiophene absorption maxima with increasing arm length
segregation of the aromatic part in a zinc phthalocyanine is comparable to that of the neat oligothiophenes containing
center, which possesses
a
shell of helically segregated three to five aromatic building blocks35 and superimposes with
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materials possess absorption maxima at approximately
736 nm. Annealing leads in general to more defined narrow
Q-bands and to an intensity increase for the hypsochromically
shifted maximum at 650 nm for compound 2 and the mixture of
4 and 5, while for compounds 3 and 4 both absorption bands at
650 nm and 736 nm are almost identical in peak height. At room
temperature, the XRS results confirm for both materials 2 and
the 1 : 1 mixture of 4 and 5 that the mesogens are coplanarily
stacked, and thus the pronounced hypsochromic intensity might
indicate preferred H-aggregates.37 For compound 4 a double
helical arrangement has been proposed by XRS with a lower
organization degree of the zinc phthalocyanines. The deviation
of the absorption spectrum of the annealed film of 4 compared
with compound 2 reflects the difference in the molecular self-
assembly. The analogous thin film spectrum of 3 points to a
similar arrangement compared with material 4. The most
pronounced annealing effect for the 1 : 1 mixture of 4 and 5
can be attributed to the locking procedure in the middle of the
column and is another experimental result pointing to the
favorable aggregation by the Click process.24
Fig. 5 UV-Vis spectra of 1 (cyan blue), 2 (red), 3 (green), 4 (blue) and 5
(purple) in THF (A). Concentration-dependent UV/Vis studies of
4
in THF (B). (C) Concentration corrected emission spectra of 1 (cyan
blue, c = 1.2 Â 10À7 mol LÀ1), 2 (red, c = 2.4 Â 10À7 mol LÀ1), 3 (green,
c = 3.2 Â 10À7 mol LÀ1), 4 (blue, c = 3.6 Â 10À7 mol LÀ1) and 5 (purple,
c
=
2.6
Â
10À7 mol LÀ1
) in THF. Dotted lines indicate excitation
wavelengths (lEx). (D) Solid state UV-Vis spectra of 2 (red), 3 (green),
4 (blue) and the mixture of 4 and 5 (magenta) before (solid lines) and
after annealing (dashed lines). (E) Magnification of the Q-band before
annealing. (F) Amplification after annealing.
Fig. 5C highlights the emission spectra of compounds 1–5
recorded at similar concentrations and obtained by excitation
in the absorption maximum of the conjugated oligothiophene.
Although the conjugated arms are highly emissive (see Fig. S12,
ESI‡), their fluorescence is almost completely quenched by the
ZnPc moiety pointing to efficient energy transfer. In contrast to
the previously described oligo(phenylenevinylene) derivatives
for which the Q-band emission of the ZnPcs increases in
intensity when extending the arm size,24 the oligothiophene
stars 1–4 exhibit an opposite behavior. This can be in part
explained by the reduced emission of the arm derivatives with
an increase in size. The principle reason for the largest break-
down in Q-band fluorescence of compound 4 is, however, the
formation of H-type aggregates even at concentrations as low as
2 Â 10À7 mol LÀ1 (see Fig. 5B). Such aggregates are non-
emissive.38,39 For the fullerene derivative 5, the emission of the
ZnPc-core is also completely quenched, which demonstrates the
efficient energy transfer cascade from the conjugated oligomers
over the phthalocyanine cores to the fullerene. Usually an
electron transfer results between the latter, separating positive
and negative charges, which was previously confirmed for a
similar mesogen by femto-second spectroscopy.24,40–42
Therefore, the present system is an attractive material for OPV
applications.
the Soret band of the phthalocyanine core. Interestingly, for
compound 5, this oligothiophene band shifts hypsochromically
by 13 nm compared with 4 owing to the functionalization of the
scaffold to attach the fullerene. With increasing length of the
conjugated oligomer, the Q-band of the ZnPc-stars, which
corresponds to the p–p* transition, moves slightly bathochro-
mically. This clearly indicates that the conjugated arm and the
core are interacting with each other. Even a minor change in
the substitution pattern of 5 results in a bathochromic shift of
8 nm compared with 4. In addition, the increasing arm length
seems to influence dramatically the intensity of the maximum
of the Q-band which decreases from 1 to 4. The integral of the
band between 600 and 850 nm remains almost constant for
compounds 1–3, indicating that the transition dipole moment
does not change significantly. However, for compound 4, the
integral decreases to 60% compared with compounds 1–3
revealing a considerable change in the transition dipole. This
can be attributed to the aggregation of 4 even at concentrations
as low as 4.9 Â 10À6 mol LÀ1. Fig. 5B demonstrates that the
intensity of the Q-band increases when the concentration
decreases and this is limited only by the increasing noise at a
concentration of 2.7 Â 10À7 mol LÀ1. The absorption maximum
of 4 shifts hypsochromically to almost 715 nm, which equals the
absorption maximum of compound 3, indicating that this con-
jugated scaffold has already achieved the effective conjugation
length.36 Fig. 5D–F show the normalized absorption spectra of
2–4 and the 1 : 1 mixture in their thin films. The continuous lines
correspond to spectra recorded before and the dotted lines
correlate with spectra recorded after annealing for five minutes
at 220 1C. The typical broadening of solid state spectra can be
discerned. The Q-band exhibits now the low bathochromically
shifted maximum for all materials pointing to aggregation,
expected in the thin solid film. Except for compound 3, all
5. Electronic properties
The HOMO levels of 4, 5 and the 1 : 1 mixture (before and
after annealing to 220 1C for five minutes) were studied by
photoelectron yield spectroscopy. HOMO–LUMO gaps were
determined by absorption spectroscopy (see Table S4 and
Fig. S27, ESI‡). Metal phthalocyanine HOMO and LUMO
energies may vary between À(4.4–5.0) eV and À(3.1–4.4) eV
depending on substitution patterns and metals.43–45 The
determined energy of 5.2 eV for ZnPc 4 is slightly above these
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HOMO values and fits perfectly the ITO work function, which is system only amorphous films are generated. In order to
5.2 eV.46 On the other hand, the LUMO level of 5 is identical to improve the alignment of the films, they have been annealed
the value for the fullerene derivative PCBM (À3.8 eV).47 This either before vacuum deposition of a thin MoO3/Ag electrode
indicates that the energy levels would fit well for a possible (pre-annealing) or after deposition of the electrode (post-
application in photovoltaic cells. The most important annealing). A series of annealing procedures have been
additional precondition for engineering an acceptable LC performed on different cells, followed by POM studies and
device is the correct alignment of the columnar complex the measurement of the current density profile, to qualitatively
structure. For the use of the present columnar liquid crystal prove the correct homeotropic alignment. Pre- or post-
as photovoltaics or photodetectors, a homeotropic alignment is annealing processes at temperatures r220 1C changed neither
mandatory, in which the columns stand orthogonal between the texture nor the current density profile, indicating that the
the two electrodes and connect them like molecular cables. films remained in their amorphous state. Pre- or post-
Unfortunately, this cannot be conventionally achieved by annealing the samples at 280 1C for 5 minutes initiated
simple cooling from the isotropic liquid, owing to the extremely important changes. The texture became birefringent, however,
high clearing point of the material. Recently, E. Grelet et al. without preferred orientation. Dark spots indicate that some
found that discotics can be oriented also by annealing in the homeotropically aligned domains are present. Nevertheless, the
columnar liquid crystal when sandwiched between a glass current density profile remained at low values for dark current
substrate and a vacuum deposited metal film (Ag).27 The speed and the current upon AM1.5G illumination. This changed
of the homeotropic alignment depended strongly on the thickness significantly when the samples were post-annealed at 200 1C
of the spin-coated material. This process is only geared by the for 24 hours. As a result, the dark currents became low but the
surface tension and was successfully applied in several currents at illumination increased by a factor of up to 300
studies.27,48,49
(Fig. 6D), although the texture did not change significantly.
In the present case, a 1 : 1 mixture of 4 and 5 has been Textural changes have been observed, when the samples were
deposited on an ITO substrate by bar-coating as a 150 nm thick, annealed for a longer period of time at 280 1C (Fig. 6C).
uniform film, which is non-birefringent between crossed polarizers The images became slowly less birefringent pointing to
(Fig. 6A). In contrast to conventional discotics for which bar-coating homeotropic alignment. However, the cells prepared by post-
produces planar aligned columnar LC samples,50 in the present annealing at 280 1C for 30 minutes showed high leak currents,
which indicates that the cell structure or the material has been
damaged.
Fig. 7 highlights a schematic model of the processes explaining
these experimental observations. Bar-coating of the mixture in
chlorobenzene at 60 1C produces a uniform but amorphous thin
film. The Click procedure proceeds only at high temperatures,
and thus no columnar aggregates are present at that step (Fig. 7,
left). Cautious annealing to up to 220 1C to prevent decomposition
resulted neither in any birefringence nor in changes of the very
low current densities. Only at 280 1C the molecules in the thin
film became mobile enough for the formation of columnar
aggregates via the Click procedure. Subsequent self-assembly
of the columns in small mm size domains (Fig. 7, right)
affords a grainy birefringent texture. The post-annealing process
did improve the contact to the electrodes and resulted in low
dark currents and the 300 times higher current density upon
illumination. We assume that this is a consequence of the low
number of dark domains, which are homeotropically or almost
Fig. 6 Bar-coated film of the 1 : 1 mixture of 4 and 5 with a thickness
of 150 nm. Uniform film at room temperature (A) and after 5 minutes of
pre-annealing at 280 1C (B). (C) Annealing study: sandwiched bar-coated
film and post-annealed film at 280 1C after 0, 5, 10 and 60 minutes.
Decreasing birefringence point to slow homeotropic alignment. (D) I–V
measurements with (red line) and without (black line) illumination of a
sandwiched film treated with 5 minutes pre-annealing at 280 1C and 24 h
post-annealing at 200 1C. The photocurrent increases by a factor of
Fig. 7 Self-assembly processes observed after bar coating, pre- and
300 after the annealing procedures.
post-annealing.
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homeotropically aligned, while the majority of the domains are oriented soft matter the photocurrent increases by a factor of
highly birefringent and do not possess the desired orientation. 300. This implies the tremendous significance of the correct
Nevertheless, the photocurrent increases by a factor of 300 and alignment of donor–acceptor materials for future applications.
this was reproducibly measured with different cells. The present Consequently, the present outstanding nanostructured material
photocurrent is rather low compared to what is needed for real will be extremely promising for organic photovoltaics or photo-
photovoltaic devices, but this is a consequence of the very limited detectors, if the alignment procedures can be optimized. These
homeotropic alignment. Nevertheless, these results are highly investigations are currently in progress.
encouraging since they demonstrate the crucial role of the
6.1 XRS experiments
morphology and alignment of the materials. They indicate that
improved properties can be expected for a material, for which
the Click process and the alignment procedure start at lower
temperatures and, consequently, the thermal procedure does not
damage the material or the cell. Based on these findings, which
reveal the promising potential of the tailor-made donor–acceptor
materials, further work is currently in progress to bring down the
clearing temperatures and viscosities in order to establish the
optimal alignment.
The temperature dependent WAXS investigations were performed
on a Bruker NANOSTAR (Detector Vantec 2000, Incoatec
Microfocus copper anode X-ray tube). The fibers were produced
by extrusion from the liquid crystal state using a home-made
miniextruder. The aligned fibers were transferred to Mark
capillaries, which were sealed and glued into the metal sample
holder, with the fiber direction parallel to the tilt direction of
the detector (tilted by 141). The XRS heating system was
calibrated by liquid crystal standard compounds. The XRS data
were evaluated using the program Datasqueeze with silver
behenate as a calibration standard.51 The correlation lengths
were estimated using the Scherrer formula.52
6. Conclusion
In conclusion, we have synthesized successfully new shape-
persistent star-mesogens with oligothiophene arms and zinc
phthalocyanine cores. With increasing arm length, the intrinsic
free space between the arms increases. While the first members
of the series form conventional hexagonal and rectangular
columnar liquid-crystalline phases, the star mesogen with three
thiophene repeating units clearly packs in a double helical
structure with a short correlation length, to compensate for the
large void between the arms. A different compensation pathway
could be uncovered by mixing the sterically overcrowded and
non-mesogenic compound 5 in a ratio of 1 : 1 with the largest
star 4. In this mixture, the fullerenes fill the space between the
arms and lock the zinc phthalocyanines to the center of the
column. The strong segregation tendency of the fullerenes
leads to short helical interdigitated fullerene strands along
the column. As a result, the mesogenic core is triple nano-
segregated – the zinc phthalocyanine in the center, which is
helically surrounded by a nanosegregated shell of fullerenes
and oligothiophenes. This is a result of the previously described
Click procedure, which is working also for the present type of
stars. The absorption spectroscopy supports the different
packing modes in the liquid crystal systems. Photophysical
studies reveal further that energy transfer as in an antenna
system and aggregate formation even at very low concentrations
dominate the fluorescence quenching. The most efficient
quenching is observed for the fullerene containing star in
which the energy cascade transfers the energy to the fullerene.
HOMO–LUMO levels have been determined to be promising
for application. An alignment study clearly shows that uniform
150 nm thick films can be produced by bar coating. At 25 1C,
before the Click procedure, these films are amorphous.
6.2 Photoelectron spectroscopy
The photoelectron spectra were measured by using photoelectron
yield spectroscopy (Sumitomo Heavy Industries, PYS-202-H) to
determine the HOMO level. The D2 lamp light passing through a
monochromator as a UV light source, the intensity of which was
calibrated using a Si photo-diode, was irradiated onto the sample
surface under a vacuum of 10À3 Pa, and the generated photo-
electrons collected by the ring electrode were measured using a
sub-femtoamp remote source meter (Keithley, 6430).
6.3 Bar-coating and cell production
The cleaned indium-tin-oxide (ITO)-coated glass substrates were
placed on a hotplate and the substrate temperature was set at
60 1C, and a coating-bar with a 0.1 mm interval and 9 mm-depth
grooves (OSG system products OSP-03) was set onto the substrate.
The sample solution of 10 g LÀ1 with chlorobenzene, which was
prepared by ultrasonic stirring and filtered through a PTFE
membrane, was dropped onto the coating-bar, and then the
coating-bar was moved in the horizontal direction at a speed of
33 mm sÀ1 by a brushless motor (Oriental Motor, BXS230A-10S). Thus
150 nm-thick uniform films were successfully obtained. The film
fabrication was carried out in atmospheric air. Subsequently, a 3 nm-
thick MoO3 layer and a 150 nm-thick Ag electrode were deposited
onto the bar-coated film of the mixture of 4 and 5 at approximately
1.0 nm sÀ1 under 10À5 Pa in an vacuum evaporator (Eiko Engineer-
ing, EO-5) to complete the device structure of ITO/4 + 5 mixture/
MoO3/Ag. The active area of the sandwich device was 2 Â 2 mm2.
6.4 J–V measurements
The sandwiched film with a Ag/MoO3 top electrode could be Current density–voltage (J–V) characteristics of the device were
partially aligned using an annealing protocol, which initiated measured using a source meter (KEITHLEY, 2400) without/with
the Click process in the material to form the birefringent illumination of 100 mW cmÀ2 from a solar simulator equipped
columnar liquid crystal. Despite the low fraction of correctly with an AM1.5G filter (Bunkoukeiki, OTENTO-SUN III).
5696 | J. Mater. Chem. C, 2021, 9, 5689–5698
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Paper
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Author contributions
The manuscript was written by MD and ML through contributions
from all authors. MD synthesized all the materials and studied the
LC and optical properties. Models were generated by MD and ML.
GU, AF and MO recorded the film data. All authors have given
approval to the final version of the manuscript.
¨ ¨
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´
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CPI
DCM
DIC
DMAE
DPTS
NBS
Complementary polytopic interaction
Dichloromethane
Diisopropylcarbodiimide
Dimethylaminoethanol
Dimethylaminopyridinium toluene sulfonate
N-Bromosuccinimide
´
´
´
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´
´
´
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Pd(dppf)Cl2
(1,1-Bis(diphenylphosphino)-
ferrocene)palladium(II) dichloride
Pyridinium toluene sulfonate
p-Toluenesulfonic acid
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PPTS
pTsOH
THF
Tetrahydrofuran
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There are no conflicts to declare.
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