An unusual stacking transformation in liquid-crystalline columnar assemblies of clicked molecular propellers with tunable light emissions

Article abstract of DOI:10.1002/chem.201403297

The columnar liquid-crystalline (LC) and fluorescence properties of three-dimensional molecular propellers based on tetraphenylethylene is reported. X-ray scattering studies reveal an unusual transition from a rectangular (Col_rec) to a hexagona

Full text of DOI:10.1002/chem.201403297

DOI: 10.1002/chem.201403297
Communication
&
Liquid Crystal Transitions
An Unusual Stacking Transformation in Liquid-Crystalline
Columnar Assemblies of Clicked Molecular Propellers with
Tunable Light Emissions
Jinhee Kim,[a] Sung Cho,[b] and Byoung-Ki Cho*[a]
Some aromatic materials exhibit strongly emissive phenom-
Abstract: The columnar liquid-crystalline (LC) and fluores-
cence properties of three-dimensional molecular propel-
lers based on tetraphenylethylene is reported. X-ray scat-
tering studies reveal an unusual transition from a rectangu-
lar (Colrec) to a hexagonal columnar (Colhex) phase. In con-
trast to second-order intercolumnar transitions based on
a common tilt mechanism, the transition is first order and
involves an unprecedented zigzag stacking of aromatic
propellers in the Colrec phase. A sudden change in emis-
sion color from sky blue to green occurs rapidly and re-
versibly at this transition, which is due to the planarization
of the propeller mesogen.
ena in the aggregation or bulk states, which is referred to as
aggregation-induced emission (AIE).[9] Many AIE-active materi-
als can switch their luminescent color in response to external
stimuli such as temperature, pressure, and vapor, which ac-
companies a morphological variation. LC molecules have been
used to manufacture stimuli-responsive luminescent materials.
To date, some thermochromic materials have been reported to
reveal their fluorescence color change based on LC-to-liquid
transitions, but the number is still limited.[10]
The frustration of the molecular shape guideline for colum-
nar LCs can be achieved in an LC system with propeller-like
mesogens. Tetraphenylethylene (TPE) is an interesting meso-
genic candidate because it is not only an ideal rigid propeller
in molecular shape but also has a unique AIE property.[11] To
date, several research groups have reported a few LC examples
containing TPE moieties.[12] However, the intrinsic packing and
emission properties of propeller-like TPE mesogens in “colum-
nar LC phases” are not precisely understood. In this context,
we intuitively thought that the stacking structures of propel-
ler-like mesogens, including TPE derivatives, would be funda-
mentally distinct from classical planar discotic LCs. Considering
the 3D propeller-like shape, some degree of the interdigitation
between adjacent aromatic wings could be expected and
modulated by the variation of molecular design. Therefore, it is
worth investigating the assembling and photophysical behav-
ior of TPE-based LCs in order to understand propeller LC
systems.
Columnar liquid crystal (LC) assemblies are an attractive
research field because they can provide an anisotropic one-
dimensional (1D) conducting channel for electrons, holes, and
photons through the p-overlap of neighboring aromatic
cores.[1] The conductive properties enable columnar LCs to be
applied for optoelectric materials, such as thin-film transistors
and photovoltaics.[2] In general, columnar LC phases are
formed by the self-assembly of discotic molecules consisting of
a “flat” aromatic core and flexible peripheral chains, and their
packing symmetries are determined by the shape of the con-
stituent columns.[3] Although most columnar LCs rely on the
classical flat discotic mesogens, some exceptional examples
based on nonplanar cores such as dendritic,[4] bowl-like,[5] tetra-
hedral,[6] and octahedral[7] shapes have been reported to reveal
columnar LC phases. In several cases, three-dimensional (3D)
aromatic stacks are formed within a column, which showed
interesting material functions, for example, ferroelectric proper-
ties.[8]
To this end, we prepared TPE-based propeller-like molecules
1 and 2 (Figure 1a). In the molecular design, the 1,2,3-triazolyl
group was incorporated as a linkage group between the TPE
and benzenyl groups by click chemistry.[13] We considered that
the polar character of the triazolyl group may reinforce micro-
phase separation, which produces stable LC phases.[14] Indeed,
in comparison to TPE-based LC analogues with ether or ester
linkages,[12a,b] the isotropic transition temperatures of 1 and 2
are more than 808C higher. To manipulate the packing struc-
ture of the propeller-like mesogens, the number of peripheral
dodecyl chains was changed from eight (1) to twelve (2),
because the space-filling of alkyl chains could influence the
stacking of propeller mesogens.
[a] J. Kim, Prof. B.-K. Cho
Department of Chemistry, Dankook University
Jukjeon 126, Gyeonggi, 448-701 (Korea)
Fax: (+82)31-8005-3153
E-mail: chobk@dankook.ac.kr
[b] Prof. S. Cho
Molecular propellers 1 and 2 were synthesized, as outlined
in Scheme S1 (see the Supporting Information). The TPE
precursor with four ethynyl units, 1,1,2,2-tetrakis(4-ethynylphe-
nyl)ethene (A), was prepared by a reaction sequence from a
Sonogashira reaction to McMurry coupling.[15] The aromatic
Department of Chemistry
Chonnam National University
Gwangju, 500-757 (Korea)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403297.
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(Figure 1d). Consequently, mo-
lecular propellers
1
and
2
formed stable LC phases over
wide temperature ranges of RT
to more than 1308C. These LC
temperature windows mean en-
hanced microphase segregation
between the aromatic mesogens
and dodecyl chains.[18]
To characterize the microstruc-
tures, we performed tempera-
ture-variable small and wide
angle X-ray scattering (SAXS and
WAXS) experiments. The WAXS
data in all the LC phases exhibit-
ed diffuse halos, consistent with
the DSC and POM interpreta-
tions (Figures 3c and S4 in the
Supporting Information). The
Figure 1. (a) Molecular structures of 1 and 2, and the propeller-like TPE structure. The double-ended arrow indi-
o
cates the torsional variation by the rotation of the phenyl rotor. Temperatures are given in C. The values in paren-
theses are the enthalpy change (kJmolꢀ1) of each transition. Optical textures of (b) 1 at 1818C, (c) 1 at 1598C, and
(d) 2 at 1318C. Colrec, rectangular columnar; Colhex, hexagonal columnar; I, isotropic liquid phase.
SAXS pattern of
1 at 1808C
showed one strong and two
p
p
azide precursors (1-azide and 2-azide) were prepared by
etherification, and/or nitration, reduction, and azidation reac-
tions.[16] The final click reactions were performed by
using CuSO4·5H2O and sodium ascorbate as reagents,
producing the target molecular propellers (1 and 2).
All experimental data fitted well with the designed
molecular structures (see the Supporting Informa-
tion).
weak reflections, with q-spacing ratios of 1: 3: 4 (Figure 2a),
which could be indexed as the (10), (11), and (20) planes of
The thermal properties of molecular propellers
were analyzed by differential scanning calorimetry
(DSC) at a rate of 108Cminꢀ1 (Figure S3, see the Sup-
porting Information). Molecule 1, with eight dodecyl
chains, exhibited two first-order transitions at
173.48C and 1868C on the heating scan, which were
observed to be reversible on the cooling scan. The
small degrees of supercooling (DT<58C) for both
phase transitions in the DSC data suggest the exis-
tence of two different LC phases with temperature.
This thermal behavior was also examined by polar-
ized optical microscopy (POM). On cooling from the
isotropic liquid phase, the LC phase at 1818C dis-
played a fan-like texture, which is typically found in
the columnar LC phase of discotic LCs (Figure 1b).[17]
On further cooling to the other LC phase at 1598C,
the fan-like texture did not alter, instead persisting to
room temperature (Figure 1c). Only the texture color
changed, which is due to some variations in the
sample thickness as the temperature decreased. This
POM data implies that there is significant structural
change with the LC-to-LC transition of 1.
In contrast to 1, molecule 2, with twelve dodecyl
chains, showed only an LC phase. In the DSC data, an
Figure 2. (a) SAXS data at various temperatures, (b) the 2D SAXS data from the surface-
aligned sample at 308C, (c) the unit cells (blue lines) of the Colrec and Colhex phases of 1,
(d) models of “tilt” and “zigzag” stackings, (e) the molecular organization in the zigzag
stacking of the Colrec phase of 1, and (f) the schematics for the variation in the degree of
endothermic peak was observed at 139.48C on heat-
ing and it also appeared on cooling. Like 1, the POM
texture of this LC phase displayed a fan-like texture,
which indicates the formation of a columnar phase interdigitation between propeller-like mesogens at the intercolumnar transition.
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a two-dimensional (2D) hexagonal structure (Colhex) with the
intercolumnar distance (a) of 3.86 nm. On cooling to 1708C,
several new reflections appeared, together with the three re-
flections in the Colhex (Figure 2a). For further analysis, 2D SAXS
measurements with surface-aligned samples were conducted.
The aligned samples were made by casting the chloroform so-
lution of 1 (~8 wt%) on 3-aminopropyltriethoxysilane (APS)-
coated Si wafers, and were slowly cooled from the liquid
phase. Owing to the hydrophobic nature of the APS-coated
surface, the columns aligned parallel to the substrate. The 2D
SAXS data revealed many spot-like reflections, which could be
assigned as the lattice planes of a 2D primitive rectangular
structure in a multi-domain sample with four distinct orienta-
tions (Figure 2b and Table S1 in the Supporting Information).
The lattice constants were determined to be a=6.95 nm and
b=3.87 nm (a>b; Figure 2c).
Colrec and Colhex phases were calculated to be 1.31 and 1.33, re-
spectively (Table S2 in the Supporting Information). As the cal-
culation for Colhex used the identical 1 value, the N value for
Colhex may be slightly overestimated. These non-unit values in-
dicate that wings of aromatic propellers stack in an interdigi-
tated mode (Figure 2 f).
Along with this interdigitated stacking, another factor
should be considered to understand the intercolumnar transi-
tion from Colhex to Colrec. Taking into account the anisotropic
rectangular symmetry of the Colrec phase, the rotational mo-
tions of propeller-like mesogens along the column axis are
more restricted in Colrec than Colhex (Figure 2e). To gain a clue
as to the difference in the rotational barriers between the two
LC phases, the core distance (hcore =h/N), which is defined as
the distance between neighboring TPE ethylenic groups, was
calculated by dividing the thickness (h) of a columnar cross
section by the number of molecules (N) in a columnar cross
section. The hcore values were found to be 3.40 and 3.56 ꢁ for
Colrec and Colhex, respectively (Table S2). In contrast to the N
value, the hcore (3.56 ꢁ) of Colhex may be slightly underestimat-
ed. The smaller hcore value of Colrec means that propeller-like
mesogens are closer at the intercolumnar transition (Figure 2 f).
Therefore, the aromatic wings are more strongly interdigitated,
which hinders the rotational motions of mesogens, leading to
an anisotropic aromatic cross section with a zigzag stacking in
Colrec.
Similar intercolumnar transitions from a rectangular to hex-
agonal lattice have been observed in a few discotic and poly-
catenar LCs.[19] In these previous cases, the transition occurred
by the tilting of rigid cores along the column axis (Figure 2d).
According to the tilt mechanism, the primary d spacing of the
rectangular phase should be smaller than that of the hexago-
nal phase at the transition. However, the (10) reflection of the
Colhex phase of 1 divided into the (11) and (20) reflections of
the Colrec phase by keeping the dimension nearly constant
(Figure 2a). This suggests that the transition of 1 does not con-
form to the tilt mechanism. The deviation from the tilt mecha-
nism can be corroborated by examining the columnar cross-
sectional area (S) calculated from the SAXS data. The S value of
Colrec was calculated to be 1345 ꢁ2, which is even greater than
that of Colhex (1289 ꢁ2; Table S2 in the Supporting Information).
Therefore, from these X-ray analyses, we conclude that the in-
tercolumnar transition of 1 is not driven by the known “tilt”
mechanism. Instead, a translational displacement of aromatic
propellers, referred to as “zigzag” stacking, can be proposed as
a plausible mechanism (Figure 2d). In the Colhex phase, propel-
ler mesogens stack orthogonally on top of each other. In this
situation, the mesogens rotate along the column axis to form
the circular cross section (Figure 2c). In contrast, to cover the
expanded columnar area, the aromatic cores of the Colrec
phase should stack up in a zigzag manner (Figure 2e).
The proposed zigzag stacking model with the smaller hcore
value may agree well with the first-order character of the
Colrec-to-Colhex transition in the DSC results because of the
compact zigzag packing (Figure S3 in the Supporting Informa-
tion). The compact zigzag packing can increase the degree of
mesogenic interaction in comparison to the loose packing in
the Colhex phase. Notably, this first-order transition is an unusu-
al phenomenon. To date, symmetry-breaking columnar LC
transformations revealed a second-order character because
their tilting mechanism of “flat” discogens does not require
a significant change in the mesogenic interaction.[20]
Molecular propeller 2 revealed a columnar phase. Like with
the Colhex phase of 1, the SAXS data at 308C exhibited a 2D
hexagonal lattice with the lattice parameter (a) of 3.92 nm
(Figure 3a and Table S1 in the Supporting Information). This
lattice parameter is similar to that of the Colhex of 1 (3.86 nm),
although the number of dodecyl chains has been increased
from eight to twelve. From this, it can be speculated that the
increased alkyl peripheries contribute to the expansion of the
intercore distance owing to the elevated in-plane steric repul-
sion by the bulky alkyl peripheries of 2. Indeed, the hcore value
of the Colhex phase of 2 was calculated to be 4.01 ꢁ, which is
larger than that of 1 (Figure 3b and Table S2). For 2, 2D X-ray
diffraction data from a surface-aligned sample was obtained
(Figure 3c). In the small angle region, arced reflections corre-
sponding to the hexagonal symmetry were observed, consis-
tent with the 1D SAXS result. On the other hand, the WAXS
data showed two broad isotropic reflections. An intense reflec-
tion at a larger q spacing of 14.2 nmꢀ1 is a typical halo signal,
indicating the mean distance (h=4.43 ꢁ) of liquid-like alkyl
chains. Additionally, a less intense halo signal appeared near
This unusual rearrangement of the aromatic cores at the in-
tercolumnar transition of 1 can be understood in terms of the
mesogenic structure. In contrast to conventional “flat” disco-
gens, the TPE-based mesogen has a propeller-like structure. To
fill the columnar space efficiently, the molecules should interdi-
gitate to some extent on top of each other (Figure 2 f). This
could be rationalized by checking the number of molecules (N)
in a columnar cross section. The N value can be obtained by
dividing the volume (Vcs) of a columnar cross section by the
molecular volume (Vmol). The density (1=0.861 gcmꢀ3) of
1 measured at RT was employed for the calculation of both
the Colrec and Colhex phases. Assuming that the thickness (h) of
a columnar cross section is equal to the mean distance of
liquid-like alkyl chains, the h values were obtained from the
maxima of the halo signals in the WAXS data (Table S2 and
Figure S4 in the Supporting Information). The N values for the
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survived for a long time, indicat-
ing that there are at least two
emissive states in 1 and 2. These
trends are consistent with the
short fluorescence lifetime of
0.85 or 1.37 ns at 450 nm and
the relatively long lifetime of
2.94 or 3.81 ns at 555 nm for
1 and 2, respectively (Figure S6
in the Supporting Information).
These spectroscopic results indi-
cate that there are two kinds of
distinct
local
environments
caused by the structural hetero-
geneity of 1 and 2. As the p-
conjugation length is a major
factor to determine the emission
wavelength of 1 and 2, we sug-
gest that the narrow blue-edge
emission with a shorter fluores-
Figure 3. (a) SAXS data, (b) the schematic of the intercore distance, (c) the 2D WAXS data at 308C, (d) the vertical
cut of the 2D WAXS data, and (e) the packing structure in the Colhex phase of 2. In (c), the spots in the small-angle
region represent the reflections from the hexagonal symmetry.
the q spacing of 8.0 nmꢀ1 (Figure 3c and d). Its d spacing is ap-
proximately 8.0 ꢁ, which is nearly twice the distance of the
hcore of 4.01 ꢁ. From this, it can be said that aromatic mesogens
stack with a mutual rotating angle between adjacent mole-
cules of 90o (Figure 3e), which leads to efficient space filling of
the C2-symmetric 2.[21]
The thermochromic properties were examined under
365 nm UV light upon heating. Both molecular propellers dis-
played emission color variation
cence lifetime mainly originates from a local environment with
a shorter p-conjugation length and that the broad emission
around 530 nm with a longer fluorescence lifetime is due to
a different local environment with a longer p-conjugation
length.
In addition, we examined the temperature dependency of
the emission bands with short and long wavelengths
(Figure 4c). For 1, the intensity of the band at 475 nm continu-
with temperature. Among them,
a
sudden fluorescence color
change was observed at the in-
tercolumnar transition of 1. As
shown in Figure 4a, the emission
color is sky blue for the Colrec
phase, which changed to emer-
ald at the transition temperature
of 1708C. Finally, the Colhex
phase displayed a green color.
The green emission persisted in
the liquid phase but became
dimmer as the temperature in-
creased. For molecular propeller
2, on the other hand, the emis-
sion color change from sky blue
to green occurred continuously
in the Colhex phase (Figure S5 in
the Supporting Information).
To obtain more information
about the fluorescence behavior,
time-resolved
measurements for 1 and 2 were
performed. As shown in
fluorescence
Figure 4b, the narrow emission
bands at 475 nm for 1 and 2
rapidly disappeared and broad
Figure 4. (a) Emission color change of 1 as a function of temperature under 365 nm UV light at a heating rate of
108Cminꢀ1. (b) Transient emission spectra at 258C at several time delays from 0 to 7 ns, and (c) the normalized
fluorescence intensity change of two emission bands at 475 nm (450 nm) and 575 nm (555 nm) of 1 (2) as a
emission bands around 530 nm function of temperature after photoexcitation at 375 nm.
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ously decreased on going from the LC to the liquid phase. On
the other hand, the band at 575 nm for 1 became weakened
in the Colrec phase but became stronger upon entering the
Colhex phase. This result means that the relative contribution of
the more conjugated component is significant in Colhex. In con-
trast to 1, the intensities of both bands at 450 nm and 555 nm
for 2 simultaneously decreased upon heating, although the
band at the longer wavelength decreased more gradually.
These observed spectroscopic data are identical to the emis-
sion color change. During the LC transition to Colhex in 1, the
contribution of the more conjugated component considerably
increases, resulting in the sudden bathochromic shift to green.
Based on the structural analysis of 1 and 2 and several previ-
ous studies about crystalline TPEs and dibenzofulvenes, the
emission color change is associated with the conformational
variation of the propeller-like aromatic mesogen.[22] In the
Colrec phase of 1, the rotational motions of the aromatic wings
are significantly hindered owing to the smaller intercore dis-
tance (Figure 2 f). This may lead to the greater twist angle with
respect to the ethylenic stator, by which the p-conjugation
length reduces, resulting in the sky-blue emission. However,
after the transition into the Colhex phase upon heating, the ro-
tational motions of the aromatic wings with respect to the eth-
ylenic stator become vigorous. This thermal transition produ-
ces more planar conformations of the propeller mesogens. The
conformational difference at the intercolumnar transition dis-
tinguishes the fluorescence color between Colrec and Colhex of
1. In contrast to 1, the conformational freedom of 2 does not
increase discontinuously, but continuously upon heating be-
cause the rotations of the aromatic wings are not strictly hin-
dered in the Colhex phase. Consequently, a continuous change
in emission color was observed in the Colhex phase of 2.
lar propeller 1 is a unique example because it is based on the
intercolumnar LC phase transition. The propeller-like LC ap-
proach suggested in this study would be useful for the devel-
opment of stimuli-responsive functional materials.
Acknowledgements
This work was supported by the National Research Foundation
(NRF) of Korea grant funded by the Ministry of Education, Sci-
ence and Technology (MEST), Republic of Korea (No.
2012R1A2A2A01045017). We acknowledge the Pohang Accel-
erator Laboratory (Beamline 9A), Korea.
Keywords: click chemistry · fluorescence · liquid crystals ·
liquid crystalline propeller
thermochromism
·
propeller mesogen
·
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Received: April 28, 2014
Published online on August 14, 2014
Chem. Eur. J. 2014, 20, 12734 – 12739
www.chemeurj.org
12739
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