Coexistence of helical morphologies in columnar stacks of star-shaped discotic hydrazones

Article abstract of DOI:10.1021/ja4029186

Discotic hydrazone molecules are of particular interest as they form discotic phases where the discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic behavior and solid-state organizations of three discotic hydrazone

Full text of DOI:10.1021/ja4029186

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Article
Coexistence of Helical Morphologies in Columnar
Stacks of Star-Shaped Discotic Hydrazones
Jie Shu, Dmytro V. Dudenko, Morteza Esmaeili, Jun Ha Park, Sreenivasa Reddy Puniredd,
Ji Young Chang, Dag Werner Breiby, Wojciech Pisula, and Michael Ryan Hansen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4029186 • Publication Date (Web): 05 Jul 2013
Downloaded from http://pubs.acs.org on July 14, 2013
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Coexistence of Helical Morphologies in Columnar Stacks of StarꢀShaped Discotic
Hydrazones
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Jie Shu,1 Dmytro Dudenko,1 Morteza Esmaeili,3 Jun Ha Park,2 Sreenivasa Reddy Puniredd,1 Ji
Young Chang,2 Dag Werner Breiby,3 Wojciech Pisula,1, * and Michael Ryan Hansen1,*
‡,
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1 Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
2 Department of Materials Science and Engineering, College of Engineering, Seoul National
University, Seoul 151-744, Korea
3 Department of Physics, Norwegian University of Science and Technology, Høgskoleringen 5,
7491 Trondheim, Norway
‡ Present address: Evonik Industries AG, Kirschenallee, 64293 Darmstadt, Germany
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
Corresponding authors: pisula@mpipꢀmainz.mpg.de, mrh@mpipꢀmainz.mpg.de
Abstract
Discotic hydrazone molecules are of particular interest as they form discotic phases where the
discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic
behavior and solidꢀstate organizations of three discotic hydrazone derivatives with dendritic
groups attached to their outer peripheries, containing six, eight and ten carbons of linear alkoxy
chains. Based on twoꢀdimensional Wide Angle Xꢀray Scattering (2DWAXS), the elevated
temperature liquid crystalline (LC) phases were assigned to a hexagonal columnar (Colh)
organization with nonꢀtilted hydrazone discs for all three compounds. Using WAXS, advanced
solidꢀstate nuclear magnetic resonance (SSNMR) techniques, and ab initio computations, the
compounds with six and ten carbons of achiral alkoxy side chains were further subjected to
studies at 25 °C, revealing complex crystalline phases with rigid columns and flexible side chains.
This combined approach led to models of coexisting helical columnar stacking morphologies for
both systems with two different tilt/pitch angles between successive hydrazone molecules. The
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differences in tilt/pitch angles between the two compounds illustrate that the columns with short
alkoxy chains (six carbons) are more influenced by the presence of other stacks in their vicinity,
while those with long side chains are less tilted due to a larger alkoxy (ten carbons) buffer zone.
The formation of different packing morphologies in the crystalline phase of a columnar LC has
rarely been reported so far, which suggests the possibility of complex stacking structures of
similar organic LC systems, utilizing small molecules as potential materials for applications in
organic electronics.
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Introduction
Soft matter comprised of spontaneously selfꢀassembled small molecular constituents establishes
wellꢀdefined mesoscopic supramolecular structures, which determine the macroscopic properties
of the material.1 Control of the selfꢀorganization is achieved by a number of different nonꢀ
covalent interactions between the single building blocks, including πꢀstacking,2 hydrogen bonds,3
and dipoleꢀdipole interactions, 4 which can be introduced chemically via specific functional
groups. Helical assemblies are one of the most complex systems that have been identified to
induce unique properties for a range of different classes of molecular systems such as polymers,5
dendrimers,6 and discꢀshaped molecules.7 Chemical modification of the design for single units
has a dramatic influence on the supramolecular array. Hereby, nature presents distinctive
examples and inspiration for chemists in the form of e.g. DNA.8
Thermotropic liquid crystals belong to an important category of soft matter whose molecular
order and dynamics are intermediate between the isotropic melt and that of a crystal.9 In this class,
discotic liquid crystals consisting of planar aromatic cores and peripheral flexible aliphatic
substituents create oneꢀdimensional columnar stacks.10 For polycyclic aromatic hydrocarbon
cores, chargeꢀcarrier transport occurs along the columnar structure, which depends on the πꢀ
orbital overlap.11 This orbital overlap, in turn, is strongly correlated to the rotational offset angle
between adjacent discs, a property that can be controlled via bulky rigid substituents,12 core
symmetry13 or amphiphilic peripheral14 interactions.
New types of core architectures for discotic molecules, as alternatives to the rigid aromatic
ones, are molecules stiffened by intramolecular nonꢀcovalent interactions such as hydrogen
bonding.15 This strategy allows achieving large discꢀshaped systems with efficiently promoted
molecular ordering and improved stability in the liquid crystalline phase. Examples of hydrogenꢀ
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bond driven assemblies are complexes of trisalkoxystilbazoles with trimesic acid 16 or C3ꢀ
symmetrical bipyridine. 17 The latter was found to organize in helically packed columnar
structures in the solid state due to rigidification, caused by strong in plane intramolecular
hydrogen bonds, and propeller formation after preorganisation by πꢀinteractions.18 We previously
demonstrated a facile way to synthesize discotic hydrazone compounds via the azoꢀcoupling
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19
reaction of 1,3,5ꢀtrisacetoacetamidobenzene with diazonium salts of aromatic amines.
Hydrazones have potential applications in organic photoconductors,20 nonꢀlinear optical,21 and
electroluminescence devices.22 Spectroscopic analysis shows that the discotic hydrazones are
stabilized through the formation of six intramolecular hydrogen bonds around the central benzene
ring, forming a discꢀlike shape. However, only little is known about their supramolecular
organization in the bulk and influence of alkyl substituents on their thermotropic behavior.
In this work, we investigate the thermotropic properties and solidꢀstate organization for a
series of hydrazoneꢀbased compounds with (achiral) linear alkoxy chains of various lengths (see
Figure 1). We use a combination of twoꢀdimensional Wide Angle Xꢀray Scattering (2DWAXS)
and SolidꢀState Nuclear Magnetic Resonance (SSNMR) techniques. While 2DWAXS provides
spatially averaged information about both the overall packing and the molecular arrangement
within helical stacks, 23 SSNMR is sensitive to the local molecular features to determine
conformations as well as dynamic processes. 24 Thus, the combination of these techniques
provides information on both short and longꢀrange structural order and dynamical properties for
the achiral discꢀshaped hydrazone derivatives. In the present case, the molecules have been found
to form stacks with tendencies of complex helix formation. The pitch angle of the formed helices
depends on the bulkiness and steric demand of the peripheral alkoxy side chains. As
demonstrated previously for benzeneꢀ1,3,5ꢀtricarboxamides,25 the combination of WAXS and
SSNMR experiments26 with ab initio computations using Gaussian0327 and CPMD28 allows us to
identify the solidꢀstate packing structures. These studies demonstrate that a length variation of
alkoxyꢀchains can lead to significantly different columnar arrangements, which is reflected in
different relative orientations of πꢀelectron adjacent molecular discs and their overlap. To the best
of our knowledge, the structure investigation of coexisting helical packing morphologies in one
single columnar liquid crystal is a rare phenomenon and suggests the possibility of packing
complexity of similar small, functional molecules as employed in various electronic devices.
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Results and Discussion
Materials
We have previously demonstrated a facile way to synthesize discotic hydrazone compounds via
the azoꢀcoupling reaction of 1,3,5ꢀtrisacetoacetamidobenzene with diazonium salts of aromatic
amines (Figure 1, compounds 5ꢀ7).19 The synthesis route for the derivatives investigated in this
work is described in the supporting information. A hydrazone is a tautomer of an azo compound.
The coupling reaction of a diazonium salt of an aromatic amine with a nucleophile generally
gives a mixture of hydrazone and azo compounds. Tautomerization between two compounds
depends on their relative thermodynamic stabilities. The discotic hydrazones are stabilized by the
formation of six intramolecular hydrogen bonds around the benzene ring. A hydrazone tautomer
exists only at the cost of the resonance stabilization energy of an aromatic ring because an azo
group was directly linked to an aromatic ring. For this study, derivatives with three different
linear alkoxy side chains (Figure 1, compounds 5ꢀ7) were synthesized.
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Figure 1. Liquidꢀcrystalline hydrazone derivatives 5ꢀ7.
Phase Transitions
Differential scanning calorimetry (DSC) reveals an expected reduction of the phase transition
temperatures with longer alkoxy chains. While the isotropization temperature decreases only
slightly for longer side chains, the transition temperature to the columnar ordered (Colh) liquid
crystalline (LC) phase drops significantly by changing the chain length from 6 to 8 carbon atoms
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(Table 1). Further chain extension to 10 carbon atoms induces only a minor decrease of the LC
temperature. At ambient temperature (25 °C) the phase for all compounds is assigned to
columnar helical (H) as described in more detail in the structure analysis below. Interestingly, the
width of the Colh phase follows an identical trend as the phase transition temperature.
Compounds 6 and 7 carrying longer alkoxy tails form a Colh phase over a wider temperature
range than 5 (Table 1). On the other hand, the transition enthalpy for the Colh transition is
significantly higher for 5 (6.8 J/g) in comparison to 6 and 7 (values between 0.7ꢀ0.8 J/g) (Table
1). The small enthalpy values for 6 and 7 suggest only minor changes and similarities between
their Colh and H phases.
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Table 1. Phase Transition Temperatures and Corresponding Enthalpy Values for Compounds 5ꢀ7.a,b
T (oC) [ꢀHt, J/g]
Compound
H 120 [6.8], Colh 175 [1.1], I
H 49 [0.8], Colh 172 [2.2], I
H 34 [0.7], Colh 162 [1.8], I
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7
aAbbreviations: Colh: hexagonal columnar; H: helical columnar; I: isotropic melt.
bEnthalpy values are obtained during 2nd heating at a rate of 10°C/min.
In order to gain a deeper insight into the phase behavior, the optical textures were inspected
by polarized optical microscopy (POM). The samples were sandwiched as thin films between two
glass slides and cooled down at a rate of 0.1 °C/min from the isotropic melt. All three compounds
form films without birefringence in POM with crossꢀpolarizers (Figure 2a). Only few birefringent
defect structures appear in the image. This optical behavior of the film strongly suggests the
spontaneous alignment of the molecules into a homeotropic phase. During the ordering associated
with solidification, the discotic columns arrange with their stacking axis perpendicular to the
surface.29 The structure investigation of the homeotropically aligned thin films by 2DWAXS in
transmission confirms the arrangement of the stacks parallel to the surface normal.
Characteristically, the patterns show only the hexagonal lattice, which corresponds to
intercolumnar ordering (Figure 2b).30
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Figure 2. a) Polarized optical microscopy image of 5 obtained in the LC phase between crossꢀpolarizers
(scale bar corresponds to 100 ꢁm), b) Transmission 2DWAXS pattern of the corresponding film based on
5.
Long-Range Ordering from 2DWAXS Analysis
The bulk supramolecular organization was investigated by 2DWAXS of extruded fibers. Already
at ambient temperatures, the materials are soft and waxy with a storage modulus of only around
107 Pa (Figure S1 for 7) that allows the sample extrusion at 25 °C. As mentioned this technique
provides valuable information about the molecular arrangement within superstructures possessing
longꢀrange order. The fiber samples were mounted vertically, and perpendicular to the incoming
beam. The equatorial plane (hk0) is of particular interest, as it contains smallꢀangle reflections
with information about the intercolumnar assemblies (columns) that are aligned in the extrusion
direction of the fiber. For discotic fibers, the meridional plane contains wideꢀangle reflections
arising mainly from intracolumnar arrangements, often soꢀcalled πꢀstacking. Reflections that are
located off the meridional and off the equator are the signature of a 3D crystalline phase, and in
the case of reduced registry between the columns, these reflections will be the first to fade.
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The scattering patterns can be interpreted in terms of the molecular form factor (of a single
discotic molecule) and the structure factor describing the tendencies of 3D packing, including
any registry parallel to the columnar direction. As we shall demonstrate, both are important to
understand the scattering patterns observed here. For helical structures, the scattering pattern
tends to have the intensity distributed on evenly spaced layer lines that are parallel to the
equatorial plane.31 To simulate the WAXS patterns of various columnar assemblies Cerius2 32 and
SimDiffraction 33 have been applied. Throughout in the simulations, the side chains are neglected
for simplicity, even though they could contribute to the scattering patterns.34
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High-Temperature Liquid-Crystalline Phases
Scattering patterns collected at elevated temperatures are presented in Figure 3, for 5 and 7 (for 6
see Figure S2a), showing that the hexagonal ordering typical for discotic columnar LC phases
was found with the columns being highly collinear with the fiber axis. This columnar orientation
in the fibers is in accordance with the Xꢀray data for the homeotropicallyꢀaligned film (Figure
2b), and reports for other columnar discotics.35 As expected, the packing parameter increases
with the length of the alkoxy side chains, resulting in ahex = 30.5 Å for 5, ahex = 33.1 Å for 6 and
ahex = 34.4 Å for 7 (Table 2). In the meridional plane a blurred wideꢀangle reflection appears
which we attribute to the intracolumnar organization. The maximum intensity of the meridional
scattering feature is found to be directly on the meridional axis, which we interpret as signifying
nonꢀtilted hydrazone discs. From the maximum of the meridional scattering, a predominant πꢀ
stacking distance between adjacent discs of 3.5 Å for 5 and 6 and 3.6 Å for 7 was derived. The
broad shape of the reflection indicates a rather low degree of intracolumnar order in this phase.
We ascribe this lower order in the stacks due to the steric demand of the trialkoxy wedge, which
might possess increased molecular dynamics at higher temperatures, resulting in poorer
interactions between individual discs. Especially for 7, carrying the longest substituents, the
liquid crystalline meridional scattering is blurred out (Figure 3b). In summary, the high
temperature phase is assigned to hexagonal columnar (Colh) for all three compounds.
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Table 2. 2D unit cell parameters for the intercolumnar arrangement of 5ꢀ7 in different phases as derived
from WAXS.
Compound
Temperature
25 °C
Phase
Unit cell
Unit cell parameter / Å
a = 29.2
b = 18.0
H
Rectangular
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130 °C
25 °C
Colh
H
Hexagonal
a = 30.5
a = 33.5
b = 64.0
Rectangular
6
7
a = 33.1
130 °C
Colh
Hexagonal
a = 33.1
a = 34.4
25 °C
H
Hexagonal
Hexagonal
120 °C
Colh
Low-Temperature Crystalline Phases
Slow cooling of the samples to 25 °C leads to a significantly enhanced supramolecular order, as
compared to both the initial state directly after fiber extrusion and the high temperature Colh
phase. A considerable number of spotꢀlike reflections emerge as shown in Figure 4a for 5 and
Figure 6a for 7 (Figure S2b for 6), which implies a pronounced longꢀrange supramolecular order
that can be attributed to the annealing effect of visiting the liquid crystalline Colh phase. Based on
the crystal structures suggested by 2DWAXS and the SSNMR results showing higher molecular
mobilities than normally expected for a crystal (vide infra), the low temperature crystalline phase
is assigned for to all three compounds 5, 6 and 7.36 The unit cell parameters for the intercolumnar
organization for the low temperature phases are summarized in Table 2. Derivatives 5 (Figure 4a)
and 6 (Figure S1b) show similar distribution of reflections suggesting closely related molecular
organization. After cooling, the hexagonal arrangement of the columns tunes to a rectangular
lattice for 5 and 6 due to a change of the columnar symmetry, while the high temperature unit cell
is maintained for 7. The columnar asymmetry for 5 and 6 is related to the molecular packing,
which is described in detail below. It should be noted that the large number of higher order
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reflections indicates a pronounced longꢀrange supramolecular order that can be attributed to the
annealing effect in the liquid crystalline Colh phase.
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Figure 3. 2DWAXS patterns of a) 5 at 160 °C and b) 7 at 140 °C in their LC phases.
Figure 4. a) 2DWAXS pattern of 5 at 25 °C, simulations of b) 5i and c) 5ii. The scattering lines
are assigned by Miller indices and indicate a characteristic helical intracolumnar organization.
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Compound 5 and 6
5
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Due to the similarities in organization between 5 and 6, the further analysis is only focused on the
first compound. The repetition distance along the columnar axis can be determined from the
meridional spacing of the hkl layer lines, corresponding to 20.5 Å for 5 (Figure 4a). With the
diffraction peaks in the present datasets, there is a certain ambiguity in assigning the number of
units to the helical structures. For 5, we thus discuss two alternative molecular packing models
which yield fair fits to the Xꢀray patterns and are also consistent with the later discussed SSNMR
results. The first model 5i is derived from the reflections located on the hk6 line, which are
attributed to a small tilt angle of ~12° of the molecules towards the columnar axis (see illustration
in Figure 5). Taking into account the intracolumnar period of 3.5 Å, which results from the small
tilt angle (πꢀstacking distance of 3.4 Å), and a helical pitch length of 20.5 Å, 6 molecules are
necessary for one complete helical winding. Since a full rotation of 120° is necessary to reach an
identical lateral positional order of the C3ꢀsymmetric hydrazone molecules within the column,
adjacent 5 and 6 molecules are rotated by 20° (=120°/6) with respect to each other (Figure 5).
The second model 5ii is based on the offꢀmeridional reflections with maximum intensity located
on the hk5 line ascribed to a molecular tilting of ~32° towards the columnar axis (Figure 5).37 The
intracolumnar spacing of 4.1 Å parallel to the columnar axis for such tilted discs is determined
from the meridional reflections located also on this layer line. This means that every 5th molecule
is in identical positional order within the 20.5 Å long helix pitch. In this model, both 5 and 6 are
rotated by 24° (= 120°/5) with respect to each other (Figure 5).
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The molecular rotation angles of 20° and 24° would originate from the steric hindrance of the
outꢀofꢀplane rotation of the substituents towards the molecular plane. This molecular
configuration is in agreement with the previously reported propellerꢀlike structure of bipyridineꢀ
based discotics.17a However, in the case of 5 and 6, the propeller arms might be arranged in at
least two different conformations, leading to two distinct packing motifs within the columnar
stacks. Both discussed models, 5i and 5ii, were simulated in Cerius2, which has been recently
applied successfully for various complex columnar assemblies, and yield patterns with scattering
lines of resembled intensity distributions and number as observed for the experimental data
(Figure 4b and 4c). The fact that several qualitatively different models contain features found in
the scattering patterns suggests the proposed coexistence motif of several intracolumnar
arrangements. It has to be emphasized that the distinct meridional reflection in the middleꢀangle
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range on the hk2 scattering line related to a dꢀspacing of 10.25 Å does not appear in the simulated
patterns. Such scattering intensity can be triggered e.g. by the formation of dimers packed in a
helical manner as recently reported for liquid crystalline perylene bisimides.32a That work also
demonstrated that the intensity of these reflections is highly sensitive to the configuration of the
substituents towards the aromatic core. In our study, various molecular conformations (see
examples in Figure S3) concerning the disc planarity and arrangement of the hydrazone side
arms, as well as dimer and trimer models with different configurations have been evaluated
without fully accounting for the experimental intensity patterns. Despite the immediate
appearance of being very similar to 5, compound 6 deserves a few more comments. In our study,
both are found to be consistent with rectangular unit cells. For 5, the equatorial diffraction peaks
can be indexed using a singleꢀstem rectangular unit cell. A higherꢀresolution scattering pattern
obtained at longer sampleꢀdetector distance demonstrates that, contrarily to 5 and 7, the
equatorial reflections for 6 are in fact split (Figure S4). The scattering patterns for 6 are consistent
with a doubleꢀstem rectangular lattice, described by a unit cell having a = 33.5 Å and b = 64.0 Å,
as firmly supported by WAXS simulations. This structure can be considered deformed by about
18% from the closely related hexagonal structure (Figure S4).
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Figure 5. Top and side views of the two packing models for 5 with two adjacent discs suggested
by 2DWAXS. For simplicity, the periphery phenyl moieties are omitted. The pitch angles are
shown in the top view illustration, while the molecular tilting angles and intracolumnar spacings
parallel to the stack axis are given in the side view.
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Compound 7
5
6
7
8
The 2DWAXS pattern of 7 shows a qualitatively different organization from that observed for 5
and 6. For 7, certain gross features of the highꢀtemperature ordering are maintained, in the sense
that the intercolumnar packing stays hexagonal and the discs remain nonꢀtilted, while an
unambiguous signature of helices develops with an intensity cross. A pronounced meridional
reflection points towards no molecular tilting and a πꢀstacking distance of 3.4 Å (Figure 6a).
However, as the 2D 13C{1H} HETCOR spectrum indicates (Figures 10a and 11a), the coexistence
of two packing morphologies with tilt angles of 0° and 15° can be derived. Cerius2 simulations
confirm that these small differences in intracolumnar packing are not well distinguishable in
2DWAXS of such extruded fibers. The simulated 2DWAXS patterns (Figure 6b and c) display
qualitatively similar features for both models, where both position and intensity of the reflections
are in accordance with the experimental pattern (Figure 6a). Information about the helical
organization is obtained again from reflections on layer lines. These suggest a change in the
helical packing in comparison to 5 and 6. From the position of the hk1 layer line a helical pitch of
34.0 Å is determined, which includes 10 molecules and thus is larger than for the other two
hydrazones. Interestingly, the calculated rotation angle (120°/10) of 12° does not correlate well
with the SSNMR data and chemical shift calculations (see below). Due to more bulky decyloxy
side chains, the angle for 7 is expected to be larger than for 5 and 6. Therefore, a full molecular
rotation of 360° within one helical pitch is assumed for 7, resulting in a molecular twist of 36°
valid for both models (0° and 15°).
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Figure 6. a) 2DWAXS pattern of 7 at 25 °C after annealing and the corresponding simulated patterns for
molecular tilting of b) 0° and c) 15°. Note the clear tendency of an intensity cross at the origin, being
strong evidence for a helical structure.
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Molecular Fingerprints of Stacking Structures from Solid-State NMR
5
6
7
8
To complement the 2DWAXS results and gain a deeper insight into the molecular level
organization of the discotic hydrazones (Figure 1), we have employed solidꢀstate NMR as a
powerful technique to reveal such structural features.26b,38 This includes notably the sensitivity of
1H NMR towards hydrogen bonding and πꢀπ stacking.39 According to 2DWAXS both compounds
5 and 6 showed similar molecular organizations. We have for this reason chosen to focus only on
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1
5 and 7 in the following. Figure 7a shows the H MAS NMR spectra of 5 and 7, where the peak
assignments are illustrated using the color scheme given in Figure 7b. A comparison of these
spectra shows that 5 has better resolved 1H resonances than 7, notably for the hydrazone protons
in the range from 10.0 ppm to 14.5 ppm. Here, at least three signals for each ꢀNHꢀ site are
observed. In contrast, the 1H MAS NMR spectrum of 7 only includes a single, albeit broad signal
for each of these positions. The 1H chemical shifts of the resolved signals for 5 and 7 are listed in
1
Table 3, including their H chemical shifts in solution (CDCl3). By comparing these data, solid
and solution, it is clear that that the dominating hydrazone peaks of 5 and 7 in the solid phases are
shifted to highꢀfield by at more than 1.1 ppm as compared to those in solution. This is a clear
indication of a columnar stacking of both compounds. However, the specific stacking structures
1
for 5 and 7 are expected to be different due to the different line shapes observed in the H MAS
1
NMR spectra. Moreover, the weak H signals at 14.2 and 11.7 ppm from the two different
hydrazone protons in 5 are very similar to those in solution NMR of 14.7 and 11.6 ppm,
respectively. Therefore, these signals most likely originate from hydrazone molecules in the nonꢀ
stacked regions of 5, which could include similar dynamics as in solution. For 7, the broad line
shape of the hydrazone protons does not include a signal at higher frequency, illustrating that no
unpacked molecules are present.
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Figure 7. a) Solidꢀstate 1H MAS NMR spectra of 5 (black) and 7 (gray) with assignments given according
to the color code shown in b).
Table 3. Solidꢀ and SolutionꢀState 1H Chemical Shifts (in ppm) of the Hydrazone Protons in 5 and 7.
Compound 5
Compound 7
Solidb
Solutionc
Solidb
Solution
14.8
Assignmenta
●
14.2
14.7
13.4
13.6
13.3
11.7
●
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10.2
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11.0
10.6
aSee Figure 7b. bDetermined from 1H MAS NMR experiments. cDetermined from 1H NMR in CDCl3
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Figure 8. 2D solidꢀstate NMR correlation spectra and molecular dynamic studies of 5 in the lowꢀ
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temperature crystalline phase recorded at 45 °C. a) Rotorꢀsynchronized 2D Hꢀ1H DQꢀSQ NMR
correlation spectrum and b) 2D 13C{1H} REPTꢀHSQC NMR spectrum. c) Experimental and simulated 1Hꢀ
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13C dipoleꢀdipole sideband patterns extracted from 2D C{1H} REPTꢀHDOR NMR experiments (Figure
S5) for the resolved peaks at sites a and h.
To further characterize the stacking structure of 5 and 7 we have employed 2D rotorꢀ
1
synchronized Hꢀ1H double quantumꢀsingle quantum (DQꢀSQ) correlation spectroscopy to map
out the spatially close protons. Such correlations are very helpful in establishing packing models,
since these can be either intramolecular or intermolecular contacts.40 Figure 8a shows the results
of this approach for 5, where we have used a short dipoleꢀdipole recoupling period of one rotor
period to only detect protonꢀproton proximities less than 4 Å.41 All correlation peaks are assigned
using the colorꢀlabeling scheme of Figure 7b. Besides the expected intramolecular correlations,
the 2D spectrum also includes remarkable autoꢀcorrelation peaks between the two different –NH–
groups as well as crossꢀcorrelation peaks between them. This demonstrates a spatial proximity of
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less than 4 Å between these chemical moieties that can only come as a result of intermolecular
contacts between adjacent molecules in the columnar stacks. In addition, the distance between the
two –NH– groups of a single molecule is significantly longer, approximately 4.4 Å. Therefore,
the cross correlation peaks between the two different –NH– sites can also be assigned to
intermolecular contacts. These findings are in accordance with the 2DWAXS studies at 25 °C,
which showed that 5 assembles into helical columnar stacks. Based on the stacking parameters
derived from 2DWAXS, the detected intermolecular distance between –NH– groups of less than
4 Å is in agreement with the πꢀstacking distance of 3.4 Å. However, for the weak signals at 11.7
and 14.2 ppm, neither auto nor cross correlation signals between them are observed. This
observation further supports the assumption that these signals are associated with nonꢀstacked
molecules.
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Since the transition temperature of 7 from Colh to H phase is at ~34 °C (see Table 1), all
solidꢀstate NMR experiments for this compound were conducted using a cooled gas flow of ꢀ16
°C to compensate for the heating effects due to MAS, and thereby maintain a real sample
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temperature of ~20 °C. Figure 9a shows the resulting Hꢀ1H DQꢀSQ correlation spectrum of 7.
Here, all NHꢀNH correlations, which were present for 5 are absent, indicating that the
intermolecular distances between the two hydrazone –NH– moieties of each arm for 7 are more
than 4 Å away. Since we were not able to distinguish between molecular rotation angles of 12°
and 36° from 2DWAXS for 7 this result nicely illustrates the power of SSNMR to resolve such
molecular packing ambiguities. Thus, the differences in 1Hꢀ1H DQꢀSQ spectra between 5 and 7
indicate that the pitch angle between neighboring molecules of 7 must be larger than that of 5,
namely 36°, since both packing organizations for 5 have pitch angles of 20° and 24°. Moreover,
as demonstrated below, the hydrazone moieties in 7 adopt a 40° outꢀofꢀplane conformation,
which would be sterically impossible with a molecular rotation angle of 12°.
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Figure 9. 2D solidꢀstate NMR correlation spectra and molecular dynamic studies of 7 recorded in the lowꢀ
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temperature crystalline phase at 20 °C. a) Rotorꢀsynchronized 2D Hꢀ1H DQꢀSQ NMR correlation
spectrum and b) 2D 13C{1H} REPTꢀHSQC NMR spectrum. c) Experimental and simulated 1Hꢀ13C dipoleꢀ
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dipole sideband patterns extracted from 2D C{1H} REPTꢀHDOR NMR experiments (Figure S5) for the
resolved peaks at sites a and h.
To reveal details about the local molecular conformations of the hydrazone discs in 5 and
7 we have recorded 2D 13C{1H} REPTꢀHSQC spectra as shown in Figures 8b and 9b. From these
spectra it is apparent that site a, corresponding to the core phenyl ring, see Figure 7b, includes
two overlapping signals in the 13C dimension located at 106.7 and 105.6 ppm for 5 and 108.0 and
106.4 ppm for 7. The appearance of two 13C signals indicates that two different chemical
environments for the core phenyl protons are coexisting in both compounds. Moreover, both
13
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compounds show unequal intensities for these C signals along with differences in H linewidth
13
of their attached protons. These observations suggest that both position and linewidth of C and
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1H as revealed by the 2D C{1H} REPTꢀHSQC spectra in Figure 8b and 9b are sensitive to the
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specific packing organization, as confirmed below using Nucleus Independent Chemical Shift
calculations (NICS) calculations. Specifically, for the hydrazone compounds studied in this work,
1
we can use the H chemical shifts in terms of their positions and associated line widths to
characterize the columnar tilt angle and if the hydrazone moieties are outꢀofꢀplane with respect to
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the hydrazone disc (see Figure 1). On this basis, the splitting of the H signal observed for 5 is
expected to originate from two different packing morphologies with quite different tilt angles.
For 7, the difference in splitting is lower, indicating that the tilt angles for the two packing
morphologies are smaller.
Column Stability via Molecular Dynamics Characterization from Solid-State NMR
An important aspect to address before combining the static structural results from 2DWAXS and
SSNMR with NICS calculations is the stability of the formed columns in 5 and 7, and if these are
influenced by local molecular dynamics of the hydrazone discs. To evaluate this aspect we have
recorded siteꢀspecific heteronuclear H−13C dipoleꢀdipole couplings (DDCs) using the C{1H}
REPTꢀHDOR and REREDOR techniques.42 Both techniques capture the effective heteronuclear
1H−13C DDCs in the indirect dimension of a 2D experiment, while maintaining the highꢀ
resolution conditions offered by MAS in the direct dimension that is needed for 13C resolution. In
this manner quantitative information about molecular dynamics is available.43 Recent examples
utilizing this kind of NMR techniques have been very useful for elucidating complex molecular
dynamics in other types of discotic liquid crystals.24 The siteꢀspecific dynamical order parameter
1
13
eff
CH
rigid
CH
D
1
eff
CH
rigid
is defined as SCH
=
, where
is the effective H−13C DDC and
is that of a rigid
D
D
CH
D
CH segment, corresponding to 21.0 kHz in our cases.42 Note that the order parameter SCH is given
in the range from 0 to 1, corresponding to complete isotropic and rigid conditions, respectively.
Figures 8c and 9c illustrate the resulting H−13C sidebands patterns for 5 and 7, which belong to
the wellꢀresolved 13C signals extracted for sites a and h of both compounds. These sites
correspond to the core and outer phenyl rings, respectively, see Figure 7b, for both stacking
morphologies discussed above on the basis of solidꢀstate NMR measurements. Figure S5 shows
the 2D C{1H} REPTꢀHDOR spectra of all aromatic resonances. By fitting these H−13C DDC
sideband patterns, given using the color code of Figure 7b, the effective H−13C DDCs were
1
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determined and listed next to each sideband pattern in Figures 8c and 9c. The results obtained for
the side chains of 5 and 7 are summarized in Tables S1 and S2. Converting these values into siteꢀ
specific order parameters using the relationship given above shows that these, for both types of
phenyl sites (core and periphery phenyl moieties) in both stacking morphologies in 5 and 7, are
all very close to 1. In the LC phase, these are reduced to S~0.4 for the aromatic core and to S~0.1
for the periphery phenyl rings with very flexible side chains (Figure S8). In the lowꢀtemperature
crystalline phase, the siteꢀspecific order parameters close to 1 indicates a high rigidity of the
hydrazone discs and a high stability of the formed stacks on the ~ms timescale. The attached side
chains are on the other hand quite flexible. For 5 this comes as a result of the larger pitch angle
between successive molecules allowing the side chains to fill the space. In compound 7, with a
lower pitch angle, the dynamical order parameters (Table S2) suggest a more folded state of the
side chains, which might be a result of intercolumnar interactions. Meanwhile, the finding that
the formed stacks show high stability also indicates that no exchange between two packing
morphologies is occurring on the ~ms timescale. Another piece of information that can be
acquired from the 2D 13C{1H} REPTꢀHDOR experiments is that the nonꢀstacked hydrazone discs
observed in compound 5 also behave rather rigid. This illustrates that even though the nonꢀ
stacked hydrazone molecules do not participate in columnar stacking, they do adopt an
amorphous rigid structure rather than a molecular bulk with flexible chemical groups.
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Disc Co-Planarity and Columnar Tilt Angles from Ab Initio Calculations
Based on the 2DWAXS analysis and SSNMR results discussed above we have constructed model
stacks for 5 and 7 to determine the molecular packing in terms of pitch and molecular tilt angles
within the columnar stacks (see Figure 5) for the two different morphologies observed for each
compound. We have adopted the nomenclature i and ii for the models with small and large tilt
angles, respectively. For 5, the first model (5i) consisted of six helically organized hydrazone
molecules with a pitch angle of 20° and an intracolumnar tilt of 12°, while the second model (5ii)
had a larger intracolumnar tilt of 32° and, hence, a pitch angle of 24° with only five molecules
per complete winding. The second pair of models for 7 had ten molecules per complete winding,
a pitch angle of 36°, and an intracolumnar tilt of 0° and 15° for the models 7i and 7ii, respectively.
The constructed models were subjected to NICS calculations. Figure S9 shows tilt angles for both
compounds in the range from 0° to 45°. The final tilt angles were derived on the basis of Figure
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S9 as shown in Figures 10 and 11. We estimate the uncertainty for the tilt angles to be in the
range of ±5° for 5, while the smaller tilt angles for 7 have larger uncertainties of ±10°. Further
details about the NICS calculations are described in the Supporting Information.
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1
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Figure 10. a) Expanded region of the aromatic signals for 5 illustrating the sensitivity of the H and C
resonances at sites a (core) and h (periphery) toward the columnar tilt angle and hydrazone disc coꢀ
planarity, respectively. (b and c) Packing models derived for 5i and 5ii with columnar tilt angles of 12°
and 32° and their corresponding NICS maps shown in d) and e), respectively. The NICS color bar
quantifies the NMR chemical shift offset of the core phenyl nuclei (at site a) induced by the aromatic ring
currents of neighboring hydrazone discs. Details relating the disc coꢀplanarity in terms of the arm
conformations are given in Figure 12.
1
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Figure 11. a) Expanded region of the aromatic signals for 7 illustrating the sensitivity of the H and C
resonances of sites a (core) and h (periphery) toward the columnar tilt angle and hydrazone disc coꢀ
planarity. (b and c) Packing models derived for 7i and 7ii with columnar tilt angles of 0° and 15°, and their
corresponding NICS maps shown in d) and e), respectively. The NICS color bar quantifies the NMR
chemical shift offset of the core phenyl nuclei (site a) induced by the aromatic ring currents of
neighboring hydrazone discs. Details relating the disc coꢀplanarity in terms of the arm conformations are
given in Figure 12.
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From an analysis of the resulting NICS maps for 5 (Figures S9, 10d and 10e) and a
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comparison of these with the expansion of the 2D C{1H} REPTꢀHSQC spectrum (Figure 10a),
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the signal for site a, resonating at 106.7 ppm with a narrow line width in the H dimension, can
be assigned to the stacking morphology of 5i with a small columnar tilt angle of 12°. In contrast,
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a larger tilt angle of 32° results in the breaking of the C3ꢀsymmetry for the stacked hydrazone
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discs and leads to the C signal at 105.6 ppm with a broader H line width for site a, which is
associated with the structure 5ii. Compound 7 demonstrates the same trends as 5. However, due
to the longer alkoxy side chains, the columnar stacks in 7 experience less intercolumnar
interactions and are for this reason only subject to small tilt angles. We note that control of pitch
and tilt angles by varying the chemical nature of the attached side chains have been reported for
other discotic systems.44 Thus, the signals at 108.0 ppm and 106.4 ppm observed for 7 (Figure
11a) are assigned to the stacking morphologies 7i and 7ii with the tilt angles of 0° and 15°,
respectively. We note that our approach of evaluating NICS maps in combination with
experimental NMR chemical shifts will be even more useful for revealing tilt and pitch angles in
larger discotic systems. This relies on the fact that larger πꢀconjugated systems, like those based
on hexaꢀperiꢀhexaꢀbenzocoronenes (HBCs), show much larger magnetic screening effects as
observed here for a rather small discotic system based on a triꢀsubstituted benzene core.45
In addition to site a, site h (see structure in Figure 7b) also includes two dominating peaks
located at 93.5 and 95.4 ppm for 5 (Figure 10a) while these signals appear at 89.2 and 93.3 ppm
for 7 (Figure 11a). The resonance at 89.5 ppm observed in 5 is assigned to the nonꢀstacked,
amorphous hydrazone discs (see above). From the (planar) chemical structure shown in Figure
1
7b, it is apparent that unequal H/13C chemical shifts for the two sites associated with h can only
be caused by a difference in their chemical environments. However, if the outer phenyl rings
bearing site h are rotated to have a torsional angle of 90° with respect to the amide plane, the h
protons will experience equal environments, resulting in a single 1H/13C chemical shift for site h.
13
Therefore, the experimentally observed C chemical shift difference between the two signals of
site h can be used to reveal the torsional angle of the outer periphery phenyl groups with respect
to the disc plane, i.e., the hydrazone disc coꢀplanarity.
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Figure 12. Torsional angle dependencies for H and 13C chemical shifts of the core benzene ring and
periphery phenyl rings of a hydrazone disc. The inserts illustrate the two different groups and the
definition of their respective torsional angles, whereas the nomenclature cis and trans designate the
relative position of the 1H/13C sites with respect to the NH group of the periphery phenyl rings.
To characterize the torsional angle dependency of each hydrazone fragment we have performed
a Potential Energy Surface (PES) scan for a single hydrazone molecule (see Figure S10),
according to the rotational scheme for the benzene core and outer periphery phenyl fragments
shown in the inserts of Figure 12. These were conducted at different levels of theory and showed
the same trend for the two different methods (MP2 and B97D). Moreover, the PES scans reveal
that the amide groups attached to the benzene core can have torsional angles of up to 40° with
respect to the molecular plane caused by thermal motions and packing effects. Obviously, the
torsional angle may be even higher in a columnar stack due to the presence of neighboring
molecules, leading to more energetically favorable assemblies. It is well known that the chemical
shifts of aromatic protons and carbons can serve as reliable sensors towards this kind of
conformational freedom.46 For this reason we have computed the H and 13C chemical shift
1
dependencies as a function of the torsional angles shown in Figure 12. These show that a
1
13
variation in torsional angle leads to smooth decreasing and increasing curves for the H and C
chemical shifts of both the core benzene and outer phenyl rings. Specifically, it leads to distinct
13C and H chemical shifts that enable us to distinguish these two different aromatic moieties
1
(core and periphery) and to evaluate the coꢀplanarity of the hydrazone discs. The latter relies on
13
the fact that C chemical shifts for the periphery carbons in trans position with respect to the
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amide groups (see insert of Figure 12) have a stronger dependency towards the torsional angle,
which eventually leads to identical 13C chemical shifts at higher torsional angles.
Taking into account the magnetic screening effects as derived from the NICS maps in Figures
10 and 11, which is on the order of ꢀ1.6 ppm for both columnar structures of 5 and 7, it is clear
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that the difference in solid H chemical shift between 5 and 7 can only originate from unequal
conformations of the hydrazone groups attached to the core phenyl ring. By subtracting the
screening effect from 7.4 and 6.5 ppm for 5 and 7, respectively, one obtains 1H chemical shifts of
the free hydrazone discs in their respective conformations of ~9.0 and ~8.1 ppm. These values
indicate that phenyl core of 5 is planar, whereas compound 7 is adopting an outꢀofꢀplane
conformation for the hydrazone groups with respect to benzene core moiety (see Figure 1),
leading to a lower 1H chemical shift. This difference in hydrazone conformation is also reflected
in the average 13C positions of the core benzene ring of 106.2 and 107.2 ppm for 5 and 7, i.e., the
13
1
higher the C chemical shift and the lower the H chemical shift, the larger the torsional angle
(see Figure 12).
13
The coꢀplanarity of the peripheral phenyl groups can be evaluated on the basis of the C and
1H chemical shift differences observed in Figures 10a and 11a. Here, the splitting differences in
13C and H chemical shifts are 1.9 and 0.7 ppm for 5 and 4.1 and 0.2 ppm for 7, respectively.
1
These chemical shift differences show the opposite trend as for the core benzene ring, since the
outer phenyl groups are close to being coꢀplanar with the hydrazone groups in 7 and outꢀofꢀplane
in 5. Thus, the above calculations of H and 13C NMR chemical shifts are very useful for
1
quantifying the disc coꢀplanarity in terms of the torsional angle between the outer periphery
phenyl groups and the inner aromatic core. Such calculations can also be useful when
characterizing the molecular conformations of larger dendritic structures.47
Conclusions
In this work, three discotic hydrazone compounds with increasing length of linear and achiral
alkoxy chains attached to their outer dendritic groups were investigated in terms of their stacking
morphologies and thermotropic behavior. The hydrazone molecules are stabilized and rigidified
by six intramolecular hydrogen bonds to form stable discotic entities, which selfꢀassemble into
columnar structures with three distinct thermodynamic phases. All compounds showed nonꢀtilted
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hexagonal columnar packing (Colh) in their liquidꢀcrystalline phases with only minor differences
in their intermolecular πꢀstacking distances. Using a multiꢀtechnique strategy combining
2DWAXS, SSNMR, and DFT calculations we were able to show that the columnar structures
show remarkable differences in their solid phase packing by extending the peripheryꢀattached
alkoxy side chains by only two methylene groups. The models found to give the best fits to our
experimental data showed that the discotic hydrazone with the shortest alkoxy chains of six
carbons included two morphologies with pitch angles of 20° and 24° between neighboring
molecules and intracolumnar tilt angles of 12° and 32°, respectively. Similarly, for the longest
alkoxy chains with ten carbons, the columnar stacks had a larger pitch angle of 36° and smaller
intracolumnar molecular tilt angles of 0° and 15°. Both molecules were found by solidꢀstate
NMR to have by rigid columns and flexible side chains. Considering that the hydrazone
compounds studied here are rather small discotic systems, it can be concluded that the peripheryꢀ
attached alkoxy chains mainly acts as soft buffer regions between adjacent columns, rendering
space for the molecular cores to adopt appropriate tilts. Thus, the specific length of linear alkoxy
chains critically determines the molecular helical stacking structure with unexpected complexity.
The reason for this complexity most likely has its origin in the energetic balance between flexible
side chains and π−π stacking interactions between the benzene cores, driving the selfꢀassembly.
Compared to other discotic systems with significantly larger aromatic cores, the energetic balance
is typically in favor of π−π interactions. For this reason, such systems with larger polycyclic
aromatic hydrocarbon cores have not revealed such a packing complexity due to dominant π−π
interactions between the aromatic cores.
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To the best of our knowledge the formation of several different packing morphologies in
columnar liquid crystals is a rare phenomenon and would be hard to detect using a single
technique alone. For this reason, we anticipate that our taken strategy of combining Xꢀray
diffraction and solidꢀstate NMR with DFT calculations of NMR chemical shifts represent a
unique possibility to reveal such structural complexities. This kind of information will obviously
be crucial for identifying the optimal packing organization and possible stacking defects for small
molecules, oligomers, and larger polycyclic aromatic hydrocarbons i.e., when characterizing
chargeꢀcarrier transport and other physical phenomena that occur on the molecularꢀ and mesoꢀ
length scales.
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Supporting Information. Material synthesis, characterization and experimental details, and
additional WAXS and solidꢀstate NMR experiments. This information is available free of charge
via the Internet at http://pubs.acs.org.
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Acknowledgment. J. S. and D. D. thank the Max Planck Society for postdoctoral stipends. S.R.P
acknowledges the ERC Advanced Grant NANOGRAPH (AdGꢀ2010ꢀ267160).
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