Probing columnar discotic liquid crystals by EPR spectroscopy with a rigid-core nitroxide spin probe

Article abstract of DOI:10.1002/anie.201303194

Discotics studied by EPR: The application of EPR spectroscopy to columnar discotic liquid crystals using a novel rigid-core nitroxide spin probe (see picture) is possible. EPR spectra measured at different temperatures across three phases of hexakis(n-hexyloxy)triphenylene show a strong sensitivity to the phase composition, molecular rotational dynamics, and columnar order. Copyright

Full text of DOI:10.1002/anie.201303194

Angewandte
Chemie
Communications
Molecular Dynamics
Discotics studied by EPR: The first ap-
plication of EPR spectroscopy to colum-
nar discotic liquid crystal using a novel
rigid-core nitroxide spin probe (see pic-
ture) is reported. EPR spectra measured
at different temperatures across three
phases of hexakis(n-hexyloxy)tripheny-
lene show a strong sensitivity to the
phase composition, molecular rotational
dynamics, and columnar order.
H. Gopee, A. N. Cammidge,*
V. S. Oganesyan*
&&&& — &&&&
Probing Columnar Discotic Liquid
Crystals by EPR Spectroscopy with
a Rigid-Core Nitroxide Spin Probe
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Communications
DOI: 10.1002/anie.201303194
Molecular Dynamics
Probing Columnar Discotic Liquid Crystals by EPR Spectroscopy with
a Rigid-Core Nitroxide Spin Probe**
Hemant Gopee, Andrew N. Cammidge,* and Vasily S. Oganesyan*
Within the last decade columnar discotic liquid crystals
(DLCs) have attracted considerable interest, not least for
their potential technological applications as one-dimensional
conductors,[1] in sensors, field-effect transistors, and photo-
voltaic solar cells.[2–6] The coupling within stacked poly-
aromatic cores provides an efficient structure for charge
transport along the columns thus providing one-dimensional
pathways for charge and energy transfer with an efficiency
that depends on the extent and stability of the overlap of the
p-extended cores.[7] Understanding the dynamics and micro-
scopic phase behavior of columnar discotics at the molecular
scale is particularly challenging but fundamental for the link
between new systems and devices with desired functionalities.
The most extensively studied columnar LCs are based on
hexasubstituted triphenylenes, of which hexakis(n-hexyloxy)-
triphenylene (HAT6) is a representative example. HAT6 and
other members of the HATn series have been studied by
many methods including broad-band dielectric spectroscopy,
DSC, X-ray diffraction, Muon spectroscopy, deuteron NMR
spectroscopy, and quasi-electric neutron scattering.[8–11]
parameter) of the SP within the LC system. For instance,
rod-shaped EPR SPs such as cholestane derivatives have been
successfully applied to study calamitic 4-n-alkyl-4’-cyanobi-
phenyl (nCB) nematic liquid crystals.[23–28] However, currently
available spin probe molecules are incompatible with discotic
systems.
Here we report the first application of EPR spectroscopy
with a purpose-designed paramagnetic SP compatible with
columnar discotic LCs. The SP design requires two important
features. The probe itself should resemble the host matrix
molecules as closely as possible to favour intercalation and
cause minimal disruption to the phase, and the SP fragment
itself must be orientationally rigid with respect to the discotic
core. Discotic SP 1 was designed to meet these criteria and is
shown in Scheme 1 together with HAT6. Its synthesis is
shown in Scheme 2 and described in the Supporting Informa-
tion.
EPR spectroscopy of nitroxide spin probes (SPs) is
a valuable method for the study of both structure and
dynamics of complex partially disordered systems such as
proteins and their complexes, DNA/RNA, biological mem-
branes, nanoparticles, and soft matter.[12–22] EPR spectroscopy
has the ability to resolve directly molecular re-orientational
dynamics over time scales of 10ꢀ11–10ꢀ7 s through the
variation in spectral line shapes.[12] Because of the high
sensitivity of the EPR technique only very low concentrations
of the probe, about 100 mm, are required experimentally so
that the host system is essentially unperturbed. Continuous-
wave (CW) EPR spectra provide three types of important
information about the partially ordered fluid state, namely,
molecular dynamics, local order of molecules averaged over
a small volume, and global or long-range order in a multi-
domain system. The spectra are very sensitive to changes in
both dynamics (correlation times) and the order (order
[*] Dr. H. Gopee, Prof. A. N. Cammidge, Dr. V. S. Oganesyan
School of Chemistry, University of East Anglia
Norwich Research Park, Norwich, NR4 7TJ (UK)
E-mail: a.cammidge@uea.ac.uk
Scheme 1. a and b) Structures of HAT6 and the discotic rigid-core
nitroxide spin probe 1. Magnetic axes of the nitroxide head group are
ꢀ
indicated by arrows, where x of the magnetic frame lies along the N O
direction and the z axis is perpendicular to the nitroxide plane.
c) A schematic diagram of the columnar domain distribution of HAT6
molecules in the presence of a magnetic field, B. Spin probes are
shown in red.
v.oganesyan@uea.ac.uk
[**] V.S.O. thanks the EPSRC (grant number GR/H020411/1) for
financial support of this work. We gratefully acknowledge further
support from the Leverhulme Trust and the EPSRC Mass Spec-
trometry Service Centre (Swansea). We thank Prof. A. J. Thomson
for helpful comments and discussions.
We demonstrate herein that the novel probe, when
combined with variable-temperature CW EPR, is a sensitive
reporter of the changes of molecular dynamics and order
across the phase transitions in HAT6 thus allowing quantita-
Supporting information for this article, including details of the EPR
measurements and calculation of EPR spectra, is available on the
WWW under http://dx.doi.org/10.1002/anie.201303194.
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Chemie
with the external magnetic field, and A, the 14N nuclear
hyperfine coupling to the electron spin, are both anisotropic
leading to a strong dependence of the EPR resonances on the
direction of the applied magnetic field relative to the principle
magnetic axes. At X-band the spectrum is dominated by the
anisotropic A tensor resulting in three hyperfine coupling
lines.[16,26] The EPR line shapes can range from three narrow
lines due to averaging of the A tensor in the case of fast
isotropic motion to broad complex asymmetric features in the
case of the so-called “rigid limit”. A variety of shapes can be
observed between these two limiting cases depending on the
level of motional constraint and the ratio between rotational
correlation of the re-orientational dynamics of the SP and the
hyperfine splitting.
Scheme 2. Synthesis of the discotic spin probe 1 (Hx=n-C6H13).
Figure 1a and 1b show experimental EPR spectra at
selected temperatures of HAT6 doped with the SP 1 mea-
sured on cooling across the I-Col and Col-Cr phase tran-
tive determination of
the order parameter,
rotational correlation
times, and columnar
distribution in HAT6
at different tempera-
tures. HAT6 can adopt
three phases, namely,
the isotropic (I), hexag-
onal columnar (Col)
and crystalline (Cr),
with temperatures of
373 and 335 K corre-
sponding to I-Col and
Col-Cr phase transi-
tions, respectively.[10]
In DLCs molecular
alignment occurs in
macroscopic domains.
Due to the large dia-
magnetic anisotropy of
the polyaromatic core
for most DLCs, includ-
ing HAT6, the director
(perpendicular to the
aromatic planes, along
the column axis) aligns
normal to the applied
magnetic field.[30] In the
Col phase the magni-
tude of the resonant
magnetic field of
a
g ꢁ 2 paramagnet at
X-band (9.5 GHz) of
about 3400 G provides
partial alignment of the
columns of HAT6 as
shown in Scheme 1c.
In a nitroxide SP the
Figure 1. EPR spectra of HAT6 doped with the discotic rigid-core SP. The I phase at 420 K, Col phase at 360 K, and
Cr phase at 320 K are shown as green, red, and purple lines, respectively. In panels (a) and (c) experimental and
simulated spectra are shown at 420, 411, 399, 378, 372, 369, and 360 K representing most characteristic line
shapes. The phase transition critical point is shown by blue lines. In panels (b) and (d) experimental and
simulated EPR spectra are compared, respectively, across the Col-Cr phase transition. Overlapping black lines in
(b) correspond to EPR spectra at several intermediate temperatures and a simulation for the Col phase at 339 K is
shown by a black line in (d). Vertical lines in (a) and (b) indicate the resonance positions, which are inner and
outer edges of left and right hyperfine coupling lines, corresponding to orientations of the magnetic field along x,
paramagnetic tensors g,
defining the interaction
of the spin of the probe y, and z axes of the SP.
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Table 1: Parameters derived from the simulation and analysis of EPR
lineshapes.
sitions, respectively. A full set of EPR spectra measured every
3 K along with the integrated spectra are given in the
Supporting Information in Figures S1–S4. For temperatures
378 K < T< 420 K of the isotropic phase EPR spectra have
characteristic line shapes corresponding to slow rotational
diffusion motion. Upon approaching the critical point the
spectra undergo dramatic change showing the emerging
contribution from the Col phase in the sample. The line
shape corresponding to a critical point at T= 372 K is shown
by a blue line in Figure 1a. In the Col phase the magnetic field
on average is lying in the plane of the nitroxide ring (see
Scheme 1). As a result the line shape at 360 K of a pure Col
phase is characterized by a substantial increase of the
resonance intensity corresponding to the orientation of the
magnetic field in the xy plane. For such orientations the
distance between the resonance field positions (the inner
edges of the hyperfine coupling lines shown as vertical lines in
Figure 1a) approaches 2Axx/yy (about 12 G). The shift in
intensity towards in-plane orientations of the field are
particularly prominent in the EPR absorption profiles
obtained by integrating the spectra (Figure S3).
Spectra measured upon further cooling of the sample
through the Col-Cr phase transition are shown in Figure 1b.
The Col-Cr transition is characterized by an abrupt change of
the EPR lineshape which occurs within a narrow temperature
interval (< 3 K). This behavior is caused by the change in the
molecular distribution upon going from Col (columnar) to
powder Cr (random) when the number of resonances is
decreased in the xy plane and increased in the z orientations
(outer edges of the lines).
Figure 1c and 1d present EPR spectra calculated using
a Brownian Dynamics (BD) simulation model for rotational
diffusion of the SP in the presence of an ordering poten-
tial.[16,31,32] This model has been used here to simulate EPR
line shapes corresponding to different phases of HAT6,
namely, I, Col, and Cr. Isotropic contributions have been
calculated as follows. First, two experimental EPR spectra at
420 and 399 K of purely isotropic phases were fitted by
varying the adjustable parameters DIx=y=z, principle compo-
nents of the rotational diffusion tensor of the probe. These
temperatures and adjusted DIi values were then used as
reference values to calculate the activation energy and
temperature dependences of DIx=y=zaccording to the relation-
ship DIi ¼ Ai expðꢀEiI=kTÞ.[8,24] The isotropic contributions
for the rest of the spectra shown in Figure 1b were sub-
sequently predicted. Application of isotropic DIi proves
inadequate to provide a satisfactory fit of the 420 K spectrum,
particularly the intensity ratio between the low- and high-field
hyperfine coupling lines. However, EPR spectra can be fitted
well using the model of axial rotational diffusion. Parameters
derived from the simulations are presented in Table 1.
Overall, for isotropic states of HAT6 at temperatures
< 420 K EPR spectra have characteristic line shapes corre-
sponding to the slow motional regime of the axially sym-
metric rotational diffusion. Contributions in the EPR spectra
from the columnar phase were simulated using a simple
model of BD rotational diffusion of molecular disks in the
presence of the axially symmetric ordering potential
UðqðtÞÞ ¼ kTC20ð3 cos2 qðtÞ ꢀ 1Þ=2, the main axis of which is
T [K]
LC states
Dk [sꢀ1]/
k
D? [sꢀ1]/
? [ns]
Contrib.
[%]
S
t [ns][a]
t
420
411
399
378
372
I
I
I
I
1.60/6.22
1.32/7.52
1.02/9.77
0.62/16.10
0.53/18.80
0.85/11.70
0.49/20.11
0.81/12.34
0.65/15.32
0.20/45.00
0.12/80
5.80/1.72
4.68/2.13
3.60/2.77
2.18/4.57
1.86/5.35
2.23/4.48
1.34/5.75
2.22/4.50
1.74/5.74
0.47/21.07
0.12/80
100
100
100
100
50
50
40
60
0
0
0
0
0
0.78
0
0.79
0.82
0.83
0
I
Col
I
Col
Col
Col
Cr
369
360
339
320
100
100
100
[a] ti are calculated using the relationship adapted for anisotropic
[33]
diffusion ti =1/D
i.
directed along the columnar director, where q is the angle
between the z axis and director.[34] In addition, the effect of
the director distribution of the columns was modelled using
a normal distribution with a bandwidth of about 420. This
distribution is responsible for the spread of the observed
resonances and, in particular, the positions of the outer edges
of the left and right hyperfine coupling lines in Col. The best
simulations were achieved with C20 = 3.00 and the values for
DCi ol which are given in Table 1 along with corresponding
correlation times and relevant values of the order parame-
Col
ter S. The adjusted values D and DC?ol for the columnar
jj
phase are in good agreement with those previously reported[8]
confirming that the tumbling motion of a disc is slightly faster
than its spinning motion. Also the calculated value of the
order parameter of the probe is in agreement with the
columnar order parameter of the HAT6 molecule obtained
from NMR studies.[8] A peak at about 3375 Gauss in the
simulated Col EPR spectra appears to be stronger and
narrower compared to the experimental one suggesting that
a normal distribution of the director combined with the
simple form of ordering potential employed by the model
overestimate slightly the xy-plane resonances. Both DCi ol and
S indicate that the probe is a true mimic of HAT6 in terms of
dynamics and order. Numerical analysis shows that the I-Col
transition occurs in the 369 K < T< 378 K temperature inter-
val with the critical point at 372 K which has equal contribu-
tions from both phases of HAT6.
The line shape corresponding to a “powder” type
distribution of Cr has been simulated using an even distribu-
tion of the molecules. The best fit is obtained using slow
correlation times exceeding 80 ns which corresponds to the
“rigid” limit in X-band EPR.
In addition, experimental line shapes corresponding to
both the Col and Cr states of HAT6 show little dependence on
the temperature at T< 360 K (Figure 1b and Figure S2). This
is confirmed by the variation of the values of rotational
diffusion coefficients in the simulation of Col EPR lineshapes.
For instance, Figure 1d shows only a small difference between
the simulated spectra corresponding to 360 and 339 K of Col
phase although their relevant DCi ol values differ by a factor of
about 3.7.
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Figure 2. Comparison between EPR spectra simulated for the Col
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To demonstrate the sensitivity of EPR spectral lineshapes
to both the ordering potential for HAT6 molecules in the
column and the director distribution of columns we have
performed simulations of EPR lineshapes, shown in Figure 2,
when the director order was excluded (even distribution of
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(blue line). The resulting EPR spectra are compared in
Figure 2 demonstrating that the columnar phase in HAT6 has
significant impact on the EPR line shapes. In particular,
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In conclusion, we have demonstrated the first application
of EPR spectroscopy to the discotic columnar liquid crystal
HAT6 using a rigid-core nitroxide SP designed and synthe-
sized for this purpose. The probe intercalates within the
columns of HAT6. EPR spectra measured at different
temperatures across three phases show a strong sensitivity
to the HAT6 phase composition, molecular rotational
dynamics, and columnar order as well as the director
distribution. Simulation of the EPR line shapes using a BD
model gives a numerical estimate of these parameters at
different temperatures along both I-Col and Col-Cr phase
transitions. This study opens promising prospects for the
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chromonic phases of discotic LCs for following their self-
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Received: April 16, 2013
Revised: June 25, 2013
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Published online: && &&, &&&&
Keywords: liquid crystals · EPR spectrocopy ·
.
molecular dynamics · phase transitions · spin probes
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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