Liquid crystals from shape-persistent porphyrin stars with intrinsic free space

Article abstract of DOI:10.1039/c9tc07086a

A small library of shape-persistent, conjugated star-shaped liquid crystals (LCs) with a zinc porphyrin core was successfully synthesized. Different conjugated arms are attached in themeso-positions. The self-assembly is at the limit between liquid crystals and soft crystal phases owing to the space-filling requirement. A model indicates the formation of dimers in double helices. A remarkable mesogen with thiophene linked directly to the porphyrin core generates J-aggregates in solution, which persist in the solid state. In the liquid crystal the J-aggregates disappear but can be reversibly reobtained by solvent treatment.

Full text of DOI:10.1039/c9tc07086a

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JPolueransael odfoMnaotetraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/C9TC07086A
ARTICLE
Liquid Crystals from Shape-Persistent Porphyrin Stars with
Intrinsic Free Space
Tapas Ghosh,a Lisa Gerbig,a Martin Lambov,a Moritz Dechant,a Matthias Lehmanna,b
*
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract: A small library of shape-persistent, conjugated star-shaped liquid crystals (LCs) with a zinc porphyrin core was
successfully synthesized. Different conjugated arms are attached in the meso-positions. The self-assembly is at the limit
between liquid crystals and soft crystal phases owing to the space-filling requirement. A model indicates the formation of
dimers in double helices. A remarkable mesogen with thiophene linked directly to the porphyrin core generates J-aggregates
in solution, which persist in the solid state. In the liquid crystal the J-aggregates disappear but can be reversibly reobtained
by solvent treatment.
DOI: 10.1039/x0xx00000x
chains do not lead to LC behaviour and also with
alkylcarbonyloxyphenyl groups no mesomorphism was
observed.[7] Only tetraphenyl derivatives with alkyl and alkoxy
chains in para positions afforded discotic lamellar phases.[7,10]
Introduction
Porphyrin derivatives are important natural products, which
take over essential functions like catalysis, oxygen transport or
light harvesting for photosynthesis.[1] For that reason,
porphyrins are also in the focus of materials research.[2] In
principle they can be decorated with up to eight substituents on
the beta or four substituents at the meso-positions.[3] Star-
shaped meso-substituted porphyrins are robust compounds
and are, therefore, successfully applied in organic
electronics.[4] Derivatives with conjugated arms have been
shown to be excellent film forming and were used as active films
in field-effect transistors.[5] Star-shaped porphyrins with
conjugated arms at the meso-positions showed a breakthrough
in photovoltaic application, since derivatives coupled to low
band-gap conjugated oligomers or polymers approached
reasonable high power conversion efficiencies (PCE) in bulk
hetero junction cells.[2,6] One general issue for the still too low
PCE (< 10 %) has been suggested to be the order and orientation
of the samples, which could not rigorously controlled for the
active layer of the mixture. One possible solution to this
problem is the control of the structure by the liquid crystal state
of matter. Owing to their essential disk-shape, porphyrin LCs
were in the focus of liquid crystal research on columnar liquid
crystal materials since 1980.[7,8] Mesogens are known for
derivatives bearing multiple chains in the eight beta or four
meso positions.[7-9] Owing to their stability and ease of
synthesis, most of the investigated LC materials are those with
substituents at the meso positions.[3,4] Here, simple aliphatic
Four additional alkoxy chains in meta position at each phenyl
group furnished columnar mesomorphism,[11] whereas 3,4,5-
trialkoxy-substituted porphyrins are only low melting liquids
owing to the twist out of the porphyrin plane.[12,13] The
porphyrin becomes again liquid-crystalline in non-symmetric
mesogens, if two trans-substituents are exchanged by chain
decorated phenylethynyl units, thus with groups which can be
in principle planar.[14] Symmetrically, substituted tetra-
arylethynyl derivatives are thus expected to be liquid-
crystalline.[15] Columnar liquid crystals are also found with
tetra(oxyphenyl)
and
tetra(aminophenyl)
substituted
porphyrins transformed with aliphatic or aromatic acids to
esters or amides.[16] These columnar LC materials were used in
low efficient OPV cells. In this design the favourable antenna
effect observed for star-shaped mesogens with conjugated
arms is absent.[17] To the best of our knowledge there are no
porphyrin liquid crystals known with more extended, shape-
persistent arms at the meso-position. Therefore, the aim of the
present study is to reveal the effect of different conjugated arm
scaffolds on the mesomorphic properties of porphyrin LC
molecules 1-4 (Figure 1). One important aspect in this research
is the intrinsic free space, which is generated in principle by the
molecular structure between the shape-persistent arms.[18] In
order to form a liquid crystal this has to be filled in the LC phase,
which was recently shown to be possible by double helix
formation.[17,19] Here, we will highlight that with a peripheral
tridodecyloxyphenyl group all materials with different nature
and lengths of conjugated arms and degrees of planarity
between core and arm possess a soft nature – either soft
crystalline or columnar liquid-crystalline (Col). While only two
materials possess truly enantiotropic columnar LC phases, just
slight changes result in a material with a monotropic Col phase,
which transforms to a soft crystal upon annealing. This latter
a.Institute of Organic Chemistry, University of Würzburg, Am Hubland, 97074
Würzburg, Germany.
b.Center for Nanosystems Chemistry, University of Würzburg, 97074 Würzburg.
† matthias.lehmann@uni-wuerzburg.de
Electronic Supplementary Information (ESI) available: Synthesis, NMR spectra,
additional XRS, Modeling details. See DOI: 10.1039/x0xx00000x
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ARTICLE
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case is thought to be favourable for organic electronics as with p-chloranil [23] and subsequent Zn-metalation.[24] The
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shown recently for OPV or transistor applications [20, 21] – the bromomethylphenyl functionalized pDoOrpI:h1y0r.1i0n39/Ch9aTsC07b0e8e6nA
LC state is used for the materials alignment, while the soft obtained
by
the
same
protocol
using
4-
crystalline phase exhibit improved electronic properties.
bromomethylbenzaldehyde.[25] The subsequent Arbuzov
reaction with triethylphosphite under solvent-free conditions
furnished the core
The conjugated arm with thiophene 11 was prepared according
to known procedures starting from bromine (Scheme 1).[27]
7 in 21% total yield (two steps).[26]
9
Figure 1. Star-shaped porphyrin mesogens (R = C12H25); magenta: porphyrin core; blue:
conjugated scaffold; black: peripheral aryl group decorated with alkoxy chains.
Figure 2. Porphyrin cores used for the synthesis of shape-persistent star-mesogens.
The bromination and borylation of 11 furnished the boronic acid
functionalized arm 13. The formyl derivative 16 was prepared
via a Suzuki coupling of the boronic acid 14, produced analogous
to reference [28] and the 5-bromothiophene-2-carbaldehyde
15. The shape-persistent arms were subsequently attached
either by a fourfold Suzuki cross coupling or a Wittig-Horner
Results and Discussion
Synthesis
The target compounds have been prepared based on a
convergent strategy using suitable functionalized porphyrin
cores 5-7 (Figure 2) and the arm building blocks 13, 14, 16, 17
(Scheme 1). The central units 5-7 have been synthesized from
pyrrole and benzaldehyde using Lindsey’s method.[22]
protocol of the boronic acid derivatives 13
, 14 or the aldehyde
arms 16 17 to afford the target compounds in moderate yields.
,
All products were purified thoroughly by recycling GPC and
Porphyrin cores
5 and 6 were successfully obtained via the
1
characterized by H, 13C NMR spectroscopy and HRMS MALDI
condensation of pyrrole with either 5-bromothiophene-2-
carbaldehyde or 4-bromobenzaldehyde, followed by oxidation
mass spectrometry.
Scheme 1. Synthesis of thiophene arms and star mesogens 1-4.
2 | J. Name., 2012, 00, 1-3
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Photophysical Properties
This is completely different, when the thienVyilewuArntiictlesOnalirnee
connected directly with the porphyrin core in mesogens
DOI: 10.1039/C9TC10a70a8n6dA
1b Compound 1a exhibits Soret band, which is
bathochromically shifted by 5 nm compared to although the
conjugation length of the arm is shortest and the λmax of the arm
is located only at 308 nm. In addition, the extinction coefficient
The UV-Vis absorption and fluorescence spectra were recorded
in CHCl3 (Figure 3 and Table 1). Compounds
superposition of arm and porphyrin absorptions. At short
wavelengths the arm absorptions show bathochromically
shifted maxima according to the conjugated arm lengths
.
a
2-4 reveal as
3
1a
2,0
1,8
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
A
B
C
2
3
4
1b (6E-7 mol/l)
1b (4.5E-6 mol/l)
1a
1b
2
3
4
300
400
500
600
700
800
300
400
500
600
700
800
900
550
600
650
700
750
800
850
900
950 1000
 / nm
 [nm]
 [nm]
2 Col
3 Col
3 soft Cr
4 soft Cr
1a soft Cr
1b amorphous phase
1b Col
D
E
400
500
600
700
800
900
400
500
600
700
800
900
 / nm
Figure 3. Absorption (A, B) and emission spectra (C) in CHCl3 and absorption spe/ cntmra of different thin films in the various phases (D, E). Concentrations and
excitation wavelengths for fluorescence spectroscopy: Compound 1a 1.55*10-7 mol/l, λex = 445 nm, 1b 1.73*10-7 mol/l, λex = 433 nm, 2 0.91*10-7 mol/l, λex
435 nm, 3 0.84*10-7 mol/l, λex = 433 nm, 4 0.79*10-7 mol/l, λex = 433 nm.
=
(318 nm (
4
), 341 nm (
2
), 363 nm (
3
)) with typical extinction of the band at 445 nm increases by a factor of 1.6 compared
. The bathochromic shift is even stronger for the mesogen
coefficients between 30000-41000 lmol-1cm-1. This is followed with
4
by a very strong Soret band and a weak Q band with vibrational 1b with arms containing two thiophene repeating units. Here
progression. The intense Soret band is obviously affected by the λmax of the arm appears at the longest wavelength (365 nm) and
conjugated arm, since the shift of the maxima follow the same with the strongest absorption, while the Soret band is shifted
trend than the arm absorption maxima at shorter wavelengths further to 452 nm, but decreases with its absorption coefficient
(434 nm (4), 435 nm (2), 440 nm (3)). For the Q band the again by a factor of 2 compared with 1a.
absorption characteristics are not affected by the arm structure.
Table 1 Absorption, emission maxima and extinction coefficients.
compound
absorption
CHCl3
1a
1b
1b (aggregate)
2
3
4
λ / nm
308
57116
445
307628
567, 614
21675,
12447
365
64424
452
158962
588
356
57678
446
88719
504 (s575)
1147365
364
37799
435
341
40984
440
184602
558, 601
10122
7337
319
35911
434
196392
557, 600
9006
ε / lmol-1cm-1
λ / nm
ε / lmol-1cm-1
λ / nm
184743
518, 558, 600,
651
6535, 10467
6301, 1687
ε / lmol-1cm-1
11170
5776
λ / nm
826
90758
ε / lmol-1cm-1
emission
λmax / nm
662
742
-
615, 663, 725
617, 662
615, 664
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Interestingly, at higher concentration compound 1b exhibit shows the J-aggregate band with a filmD, OwI:h1i0c.h103w9a/Cs9oTCb0ta70in8e6dA
typical absorption spectra for J-aggregates with a broad intense from the material without thermal treatment (amorphous
band at 826 nm.[29] Such absorptions were recently of phase, see thermotropic property section). The J-band
enormous interest for the application in porphyrin containing disappears completely, when the film is thermally treated to
OPV bulk heterojunction photovoltaic cells.[2] The fluorescence obtain the columnar liquid crystal phase. Interestingly, if the
spectra in chloroform confirm the peculiar behaviour of film is dissolved in 50 mL of chloroform, the solvent is removed
compounds 1a and 1b. While compounds 2-4 exhibit emission and the material again deposited, it yields a film, for which the
spectra with defined vibrational progression, which are typically J-band reappears. However, if the film is dissolved only in a
observed for porphyrin chromophores, i.e., where the excited small amount of solvent (0.1 mL) and deposited by drop casting
state is located at the porphyrin cores,[30] compounds 1a and the J-band does not form. This point to stable columnar
1b exhibit only broad emission spectra with maxima at 662 nm aggregates, which are generated in the LC phase and are not
and 742 nm. This enormous influence of the absorption and easily disassembled. Only higher dilutions lead to the separation
emission maxima indicate that the oligothiophene arms are of the aggregates and to a reformation of J-aggregated units.
electronically more strongly coupled to the porphyrin core.[31]
Pryce et al. attribute the different behaviour to charge transfer Molecular Structure
bands between porphyrin and thiophene. Such intramolecular
In order to understand these photophysical phenomena, force
charge transfer bands were made responsible for long-wave-
field geometry optimization (Materials Studio, Forcite,
length absorptions in various porphyrin-acceptor systems,[32]
COMPASS II) were performed for all stars and DFT calculations
although in the essential flat systems the aggregation to J-type
have been subsequently carried out on compounds 1b and 4. A
aggregates should impact most severely on the absorption
maximum. The thienyl and the phenyl substituted Zn-
porphyrins possess essentially flat cores with out of plane
rotated aryl substituents.[31, 33] The unique presence of the J-
aggregates for the thiophene derivative 1b indicates that this
derivative may be able to adopt a more planar conformation,
which would favour aggregation owing to the lower steric
repulsion of the twisted aryl rings.
number of meso-aryl-substituted porphyrins were previously
studied and usually a planar porphyrin core with out-of-plane
rotated aryl rings are observed.[31] However, also saddle-
shaped conformers are known.[34] Figure 4A highlights the
results of the force field geometry optimization. While
compounds 2-4 possess all a flat zinc porphyrin and out-of-
plane turned phenyl rings as it would be expected, COMPASS II
geometry optimization predicted for the derivatives 1a and 1b
A
1a
1b
2
3
B
4
HOMO 1b
LUMO 1b
Figure 4. Forcite (Force-Field) geometry minimized structures 1-4 and DFT geometry optimized molecule 1b. A: Forcite (Force field COMPASS II) geometry optimized
structures of compounds 1-4. B: Optimized structure of 1b by using the Gaussian 16 with the 6-31G(D) basis set.
The thin film absorption spectra substantiate these saddle conformers, in which the arms are planar with the
characteristics. Compounds 1a, 2-4 exhibit very similar spectra porphyrin system and the latter is only slightly deformed. DFT
with slightly broader and bathochromically shifted absorption geometry optimization and calculation of the frontier orbitals
maxima compared with the solution in CHCl3, which almost do anticipate that the mesogen 1b has a flat geometry with out-of-
not change in the different phases at 25 °C. Only mesogen 1b plane twisted arms (Figure 4B, Figure S3). However, the frontier
4 | J. Name., 2012, 00, 1-3
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orbitals for 1b are delocalized over the whole system,(Figure material was first shearable but completely non-birefringent
View Article Online
DOI: 10.1039/C9TC07086A
4B) while for compound 4, they are localized at the porphyrin when observed between crossed polarisers (Figure 5). X-ray on
core (Figure S4). These results may be attributed among others an extruded, thus, aligned fibre indeed confirmed an
to the smaller torsional angle (61.6 °) between the porphyrin amorphous structure (Figure 6). At 100 °C the whole sample
and the conjugated oligothiophene arms in 1b compared with became strongly birefringent. The shearable phase exhibited a
the stronger out-of-plane turned phenyl rings (69.3 °) in
explains the different photophysical results.
4
and typical diffraction pattern of an oblique columnar structure with
a strong meridional signal. This phase started to melt slowly
above 130 °C and was completely isotropic at 190 °C. Since
86 °C
75 °C
111 °C
C
A
B
D
E
100 mm
100 mm
50 mm
50 mm
F
138 °C
137 °C
137 °C
L
G
H
homeo-
tropic
alignment
within the
droplet
planar
alignment
at the
interface
with air
130 °C
30 °C
112 °C
I
J
K
Figure 5 . Polarized optical microscope images of the LC phases of the star-shaped mesogens 1-4. A-E: Textures of the phases of compound 1b. Sheared amorphous phase (A);
Birefringent soft crystal (B); Droplet between two glass slides (C), rotated by 45 ° (D) and with a λ compensator (E). The yellow arrows indicate the direction of planar growth of the
columns. F: Model of the planar and homeotropic domains in plates C-E. G,H: Textures of 2 with planar and homeotropic domains. I-K: Textures of the phases of compound 3.
Pseudo-focal-conic textures of the Colh phase at 130 °C and 30 °C (I, J). The homeotropic domains are highlighted with white arrows. K: Texture of the soft crystal after short annealing
at 112 °C. The homeotropic domain (green arrow) becomes birefringent. L: Texture of the soft crystal phase of compound 4.
Thermotropic properties
these two phases did not reappear in subsequent heating,
cooling cycles and annealing steps, we assigned them as
metastable soft phases. Interestingly, after cooling the isotropic
liquid a columnar liquid crystal phase was reproducibly found
with a transition at 96 °C, thus this phase is enantiotropic. POM
displayed mosaic, pseudo focal conic and also homeotropic
textures, which are characteristic for hexagonal columnar LCs
(Figure 5C-F). These transitions are silent in DSC. Fibre XRS is
consistent with a Colh phase (Figure 6). Substitution of the inner
thiophene ring by a phenylene group leads to a tremendous
The thermotropic properties of the target compounds have
been investigated by polarized optical microscopy (POM),
differential scanning calorimetry (DSC) and X-ray scattering
(XRS). The results for compounds 1-4 are summarized in table
2. Compounds 1a and 1b with thiophene directly connected to
the core seemed not to produce LC phases in a preliminary
study. A comprehensive investigation revealed that 1a forms
only one soft crystalline mesophase demonstrated by a large
hysteresis, a slow transition and an X-ray pattern with diffuse
signals over the whole angular range (Figure S1). Compound 1b
displayed a rather peculiar behaviour. The freshly isolated
increase in liquid crystal stability for compound 2 by 65 K.
This journal is © The Royal Society of Chemistry 20xx
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Table 1. Thermotropic properties of star compounds 1-4 as observed by differential scanning calorimetry at a heating rate of 10 K/min and results of the X-ray diffraction.
View Article Online
DOI:ρ10/.1g0c3m9/-C3 9TC07086A
e
Compound
T (onset) / °C
a, b, c and γ d
Z
(ΔH / kJmol-1) a
1a
1b
1. H: Cr 74 I
-
-
-
-
1. C: - c
1. H: X (amorphous solid) 100 Crsoft 189 I b
1. C: I 92 Colh
-
-
Colh
2. H Colh 96 I
a = 47.6 Å (25°C)
Colh
a = 46.8 Å (25 °C)
Colh
a = 49.1 Å (25 °C)
a = 48.1 Å (130 °C)
Crsoft (25°C)
2
2
2
1.038 (ρ rs) f
2
3
2. H: Colh161 / 1.3 I
2. C: I 154.4 /-2.33 Colh
2. H: Colh 100/-36.7 Crsoft 147/33.8 I
1. C: I 136 / -1.8 Colh
f
1.080 (ρ xrs
1.010 (ρ xrs
)
)
f
f
a = 50.8 Å, b = 42.8 Å, c = 8.6 Å, γ = 108.5°
2
1.024 (ρ xrs
)
g
1.033 (ρ min,exp
1.099 (ρ xrs)
)
f
4
1. H: Cr 97/132.7 Crsoft 138.9/17.5 I
1.C: I 121.2 /-16.5 Crsoft
Crsoft (25°C)
a = 83.5 Å, b = 48.0 Å,
c = 69.6 Å,
56
g
1.039 (ρ min,exp
)
a) H Heating, C Cooling, Cr crystal, X unknown soft phase, Crsoft soft crystal / CONDIS crystal, Colh hexagonal columnar liquid crystal. b) POM observation, no transition in
DSC. c) variable transition appearing often at room temperature after shearing. d) unit cell parameters obtained from XRS. e) Z number of molecules within a
columnar height of 5.7 Å (1b-3). For compound 4, number of molecules in the repeat c. f) ρ xrs calculated X-ray density based on the number of molecules Z. g) ρ
min,exp density determined by the buoyancy method; air inclusion cannot be excluded therefore the values are the minimum values. The highest measured numbers are
given.
The enantiotropic mesophase clears at 161 °C and exhibit a and XRS signals over the complete angular range (Figure 7, Top).
rather small hysteresis of 6 °C at a cooling rate of 10 K/min, A liquid-crystalline phase is completely absent.
confirming the LC nature. The pseudo-focal conic textures with
meridian
meridian
Colh
planar and homeotropic domains support a Colh phase (Figure
5G,H). The equatorial reflections of the X-ray pattern can be
indexed perfectly according to a hexagonal structure (Figure 7).
The strong major signals are superimposed by diffuse intensities
along the meridian. Upon cooling, these signals become
stronger and new diffuse reflections appear. Although these
intensities could not be further evaluated, they indicate an
additional periodicity along the LC columns. The material did
not crystallize after the first heating. This situation changes
significantly when a double bond is introduced between the
amorphous
Crsoft
equator
equator
10
1b 30 °C amorphous
1b 130 °C (Crsoft
1b 81 °C (Colh)
1b 25 °C (Colh)
)
equator
meridian
20
phenyl and the thienyl ring in compound 3. Now DSC shows a
π- π
melting transition with a large enthalpy to the isotropic phase
indicating a soft crystal behaviour (table 2). Cooling revealed a
transition to a phase with characteristic Colh texture and a
typical low transition enthalpy. Heating and cooling between
120 °C and 160 °C confirmed the reversibility of this LC
transition. However, after decreasing the temperature
to 25 °C and reheating to 110 °C the mesophase returns to the
soft crystalline phase. This can be optically pursued by POM in
the homeotropically aligned area of the sample, which becomes
birefringent (Figure 5I-K). Consequently, this Colh phase is only
monotropic.
5
10
15
2 / °
20
25
Figure 6. Top: XRS patterns of the compound 1b in its amorphous phase
(30 °C), soft crystal phase (130 °C) and the Colh phase (25 °C). Yellow arrows
indicate the fibre direction. Bottom: Integrated intensity along the equator
(2 = 1-10°) and the meridian (2 = 10-27°) of the 2D patterns.
Molecule
4 connects the peripheral aromatic unit only with a
double bond, thus the molecule lacks the thiophene building
blocks. The material is still soft and can be sheared or extruded
to a fibre. However, POM, DSC and XRS are consistent with a
soft columnar crystalline, highly viscous shearable material,
with its largest polarizability orthogonal to the shearing
direction, large hysteresis (> 20 °C) and transition enthalpies,
6 | J. Name., 2012, 00, 1-3
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Colh 136 °C, (2)
ARTICLE
Crsoft 25 °C, (3)
View Article Online
DOI: 10.1039/C9TC07086A
Colh 25 °C, (2)
Colh 130 °C, (3)
Crsoft 25 °C, (4)
L = 10
meridian
meridian
equator
meridian
L = 4
L = 2
equator
equator
3 25 °C
3 130 °C
2 136 °C
2 100 °C
2 25 °C
10
10
equator
meridian
equator
meridian
Colh
Colh
11
11
21
Colh
Colh
Crsoft
5
10
15
20
25
5
10
15
20
25
2 / °
2 / °
Figure 7. Top: XRS patterns of compounds 2-4 in their Colh and soft crystalline phases. The pattern of the Colh phase of 3 was taken from an unaligned
sample, since extrusion did result in the crystallization of the material. In the pattern of the soft crystal of 4 the layer lines L = 2, 4, 10 are assigned for an
orthorhombic cell with c = 69.6 Å. Bottom left: Integrated intensity along the equator (2 = 1-10°) and the meridian (2 = 10-27°) of the Colh phase of 2
with the indexation of the small angle reflections. Bottom right: Integrated intensity along the equator (2 = 1-10°) and the meridian (2 = 10-27°) of the
soft Cr and Colh phases of 3 with the indexation of the small angle reflections (Colh).
of the out-of-plane phenyl rings will certainly additionally
hamper such a packing. In addition, X-ray patterns reveal no
Columnar Assembly in the Colh LC Structure and the Soft Crystal
significant π-stacking and additionally a dense coplanar self-
assembly would lead to H-aggregates, which should show a
hypsochromic shift in UV-Vis absorption spectroscopy.[35]
Instead, the absorption maxima are shifted slightly
bathochromically and are broadened, which does neither
indicate H- nor J-aggregation in the LC phases. Nevertheless, the
XRS for all mesogens reveal a broad second signal at 3.9 Å,
beside the halo reminiscent for the liquid-like aliphatic chains.
This can be attributed to a disordered and weak π- π stacking
along the columns owing to the steric constrains of the discs. A
disordered stacking without J- or H-aggregates would be
possible, when the mesogens form side by side dimers, as
previously suggested for phthalocyanines (Figure 8A). It is worth
We previously revealed that shape-persistent, star-shaped
mesogens need to efficiently fill their intrinsic free space in the
soft matter self-assembly.[18] In order to combine molecular
mobility and nanosegregation, the aromatic and aliphatic
building blocks need to be separated on large scale. A
segregation in interdigitating small space units would rather
result in a crystal. Two possible modes of self-assembly has
been presented, (i) the dense helical packing for stars with a
benzene core [19] and (ii) the double helical stacking for
phthalocyanines.[17] In order to unravel the structure for the
meso-substituted zinc porphyrins the diameters of the core can
be compared with the columnar diameter. Except for
compound
columnar diameter increases with the increasing aromatic core
diameter, i.e., 32.9 Å (core diameter) / b (Crsoft) = 48.0 Å ( ),
33.7 Å / a (Colh) = 46.8 Å ( ), 34.3 Å / a (Colh) = 47.6 Å (1b),
39.1 Å / a (Colh) = 49.1 Å, a (Crsoft) = 50.8 Å ( ). The differences
4, for which only a soft crystal is formed the
to note, that for compounds
2 and 3 a rather diffuse intensity
with a maximum at 5.2 ° corresponding to a distance of 17 Å
reveals in the hexagonal liquid crystal phases with a liquid like
correlation length of 1-2 repeating units (Figure 7). In case of
4
2
3
the oriented fibre of compound
2, this intensity could be
between the aromatic core and the columnar diameters are 10-
13.3 Å in the Colh phases, which is rather small compared to the
length of the peripheral dodecyloxy chains of approximately
16.5 Å. Consequently, the aliphatic peripheral building blocks
must be folded and significantly interdigitated. Although these
considerations, would point to a simple stacking of discs, the
following results lead to a different model. The minimum
identified to be at the equator. Therefore, it can be attributed
to the formation of a dimer of star-shaped mesogens, since the
van der Waals diameter of a porphyrin core amounts to
approximately 13 Å and the distance between two porphyrin
metal centres in the dimer will increase owing to steric
constrains and molecular mobility.
For the soft crystal of 4 we could set up a reasonable model with
density ρmin,exp of
3 and 4 in their soft crystal phase were
the program Materials Studio. In the orthorhombic unit cell 56
mesogens can be placed to possess a calculated density ρxrs of
1.099 gcm-1. The density is a bit higher than the minimum
density measured by the buoyancy method. However, for the
latter we cannot exclude the inclusion of air during the sample
determined by the buoyancy method to be 1.033 gcm-1 and
1.039 gcm-1. This agrees with two molecules in a columnar
repeat of 5.7 Å. A coplanar stack of two aromatic discs would
need at least a columnar height of 6.4 Å. The steric congestion
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DOI: 10.1039/C9TC07086A
A
C
D
Dimer
B
Figure 8. Models of the soft crystal phase of 4 (A-C) and the columnar LC phase of 1b (D). A: Schematic representation of a dimer of two mesogens compensation intrinsic
free space and the orthorhombic cell with helical columnar stacks of dimers without visualization of the aliphatic chains. In one column the cores are colored in green
and orange to visualize the helical strands. B: Top view on four unit cells with the aliphatic chains. The dimer is completely colored, highlighting the interdigitation of the
chains, but not entering to the core of the helix. C: Side view without chains. D: Model of 1b in its hexagonal columnar phase with helical packed dimers analogous to 4.
processing, thus, this relatively small difference is reasonable. Colh phase. All other mesogens do not exhibit higher
In the orthorhombic unit cell there are two columns with 28 intracolumnar order.
molecules each. A simple planar cofacial set-up would lead at
least to a stack of 90 Å (28 * 3.2 Å) instead of the experimental Experimental Section
height of 69.6 Å. Therefore, we assumed a propeller-like dimer
All commercial materials employed were used as received from
and packed this dimer in a helical arrangement. Double helices
Sigma Aldrich, abcr, and Acros Organics, without further
possess their strongest meridional signals at the second layer
purification, with the exceptions of pyrrole and P(OEt)3, which
line as experimentally observed (Figure 7). The second column
were freshly distilled under reduced pressure. Solvents were
was further shifted along the c-axis by c/2 to avoid steric
distilled and dried via standard procedures. Silica gel (60-120
mesh) and (230-400 mesh) were used for chromatographic
interaction. Geometry optimization of this model (see Figure
8A-C) indeed affords large non-bonding interaction energies in
separation.
Preparative
recycling
gel
permeation
a nanosegregated system (see ESI), which would agree with
most of the experimental observation. An analogous model has
been set-up for compound 1b, which is highlighted in Figure 8D
and rationalizes the absence of π-π distances, J- or H-aggregates
in the liquid crystal, and the density. To summarize, it should be
emphasized that in principle the columnar and molecular
diameters of the porphyrin stars are in good agreement with
each other, however, the larger number of molecules in the
stack evidenced by density measurements, the missing regular
π-π -stacking and the absence of H- or J-aggregates clearly
confirm a more sophisticated dimer model for all of the
compounds. The modelling program demonstrates the
feasibility by producing large negative non-bonding interaction
energies after geometry optimization for the helical dimer
model. Eventually, the helical model rationalizes a three-
dimensional correlation, when the correlation length along the
c-axis of the unit cell is sufficiently high. This can be observed by
chromatography was performed with the liquid chromatograph
LC-20A (Shimadzu). The column set (PSS SDV 50 Å, 20∙600 mm;
PSS SDV 500 Å, 20∙600 mm) was eluted with HPLC-grade CHCl3
at a flow rate of 4.0 mLmin-1.
NMR spectra were recorded on a Bruker-Daltonics Avance-400
spectrometer operating at 400 MHz (1H) or 100 MHz (13C) with
the residual protic solvent used as the internal standard. Mass
spectra were recorded on a Bruker-Daltonics autflex II (MALDI).
The studies of optical textures of the mesophases were realized
with a Nikon Eclipse LV100Pol optical polarizing microscope
equipped with a Linkam LTS420 heating stage and a Linkam T95-
HS system controller. Thin film fluorescence spectra were
recorded on
a
StellarNet BLACK-Comet CXR-100
spectrophotometer equipped with a Nikon Intensilight C-HGFI
illuminator. UV-Vis-absorption studies in solution were
performed with a PerkinElmer Lambda 35. Emission studies in
solution were realized with a PTI QM-4/2003. The temperature
dependent WAXS X-ray investigations were performed on a
XRS for compound 4 in its soft crystal and for compound 2 in its
8 | J. Name., 2012, 00, 1-3
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Bruker Nanostar (Detector Vantec2000, Microfocus copper
anode X-ray tube Incoatec). The samples were prepared by fibre
extrusion using a home-made mini-extruder – fibre diameter:
0.7 mm. The measurements were carried out in Mark capillaries
(Hilgenberg) positioned perpendicular to the incident X-ray
beam. The modelling of the columnar phase has been
performed with the program suit BIOVIA Materials Studio 2017
R2. Density Functional Theory (DFT) structural optimization
have been performed on molecules, in which the aliphatic
chains were reduced to methyl groups, with the Gaussian 16
software.[36]
Nguyen and Pachter could show only minor differences
between 6-31G(D), 6-3+G(D), 6-311+G(D) in their studies on Zn
porphyrin systems.[37] Therefore, the calculations were
performed using the B3LYP functional and the 6-31G(D) basis
set. Electron density difference maps (EDDMs) for the HOMOs
and LUMOs frontier orbitals were constructed using a Avogadro
program and displayed using an isosurface value of 0.02
electrons per Å3.[38]
Acknowledgements
We are grateful to the DFG for the financial support in the frame
of the project LE1571/8-1. T.G. acknowledges the Alexander
von Humboldt Foundation for the postdoctoral fellowship.
View Article Online
DOI: 10.1039/C9TC07086A
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View Article Online
DOI: 10.1039/C9TC07086A
Shape-persistent porphyrin
star mesogens form thin
oriented films of columnar
liquid crystal and soft crystal
phases as a function of the
conjugated arm scaffold.
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