Rational Design of Rod-Like Liquid Crystals Exhibiting Two Nematic Phases

Article abstract of DOI:10.1002/chem.201702742

Recently, a polar, rod-like liquid-crystalline material was reported to exhibit two distinct nematic mesophases (termed N and N_X) separated by a weakly first-order transition. Herein, we present our initial studies into the structure–property relationships that underpin the occurrence of the lower-temperature nematic phase, and report several new materials that exhibit this same transformation. We have prepared material with significantly enhanced temperature ranges, allowing us to perform a detailed study of both the upper- and lower-temperature nematic phases by using small-angle X-ray scattering. We observed a continuous change in d spacing rather than a sharp change at the phase transition, a result consistent with a transition between two nematic phases, structures of which are presumably degenerate.

Full text of DOI:10.1002/chem.201702742

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Accepted Article
Title: Rational Design of Rod-Like Liquid Crystals Exhibiting two
Nematic Phases.
Authors: Richard J Mandle, Stephen J Cowling, and John W Goodby
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10.1002/chem.201702742
Chemistry - A European Journal
Rational Design of Rod-Like Liquid Crystals Exhibiting two Nematic Phases.
R. J. Mandle *, S.J. Cowling and J. W. Goodby
Richard.mandle@york.ac.uk
Abstract
Recently a polar, rod-like liquid-crystalline material was reported to exhibit two distinct nematic mesophases
(termed N and NX) separated by a weakly first order transition. In this work we present our initial studies into the
structure-property relationships that underpin the occurrence of the lower temperature nematic phase, and in
doing so report several new materials that exhibit this same transformation. In doing so we have prepared
material with significantly enhanced temperature ranges, allowing us to perform a detailed study of both the
upper- and lower- temperature nematic phases using small angle X-ray scattering. We observe a continuous
change in d-spacing rather than a sharp change at the phase transition, a result consistent with a transition
between two nematic phases whose structures are presumably degenerate.
Introduction
The nematic liquid-crystalline state - as exhibited by low molar mass liquid crystals – is characterised by
relatively high fluidity, a lack of positional ordering of molecules, but with short-range orientational order.
Transitions from one nematic phase into another are rare, but also highly topical due in part to the recent
1-6
discovery of the twist-bend nematic phase. Several other nematic or nematic-like mesophases are known to
7
8
9, 10
exist (chiral nematic (N*), discotic nematic (ND), re-entrant nematic (NRE), biaxial nematic (NB),
blue
phases (BPI, BPII, BPIII)) or are either predicted to exist or have possibly been discovered (cubatic nematic
11
(Ncub), splay-bend nematic (NSB) 12, 13). Recently we reported a polar liquid-crystalline material (compound 1,
Table 1) that we observed to exhibit two distinct nematic mesophases, with a weak first-order transition between
14
the two phases. Other examples of nematic-to-nematic transitions have been observed in binary mixtures of
reentrant materials, 15 in frustrated chiral rod-like systems 16 and in main-chain liquid crystal polymers. 17 In terms
of response to applied electric fields the two nematic phases are similar, exhibiting a Fréedericksz transition with
threshold voltages of ~0.3 V and ~ 0.45 V in the N and NX phases respectively. Connoscopy demonstrates the
upper temperature nematic phase of 1 to be uniaxial and optically positive, however, following the NX-N transition
the homeotropic alignment is lost and a schlieren texture obtained – it is therefore unclear if this lower
temperature nematic phase is uniaxial or biaxial at this time. For compound 1 the NX-N transition occurs at 85.6
°C (determined by DSC); however this is below the melting point of the material and the mesophase is therefore
metastable; if the properties and local structure of the NX phase are to be understood it is important that materials
with superior working temperature ranges are developed.
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MP
NX-N N-Iso
T / °C
139.0 85.6 182.1
ΔH / kJ mol-1
34.8
10.2
0.2
0.1
0.6
0.2
ΔS/R
Table 1:
Transition temperatures, associated enthalpies of transition and dimensionless entropies of
transition for compound 1, as determined using DSC at a heat/cool rate of 10 °C min-1. 14
Given that only one material is known to exhibit this nematic-to-nematic transition a structure-property
relationship is presently absent. In this study we follow up on our earlier work by describing how the occurrence
or absence of the NX mesophase exhibited by compound 1 depends on molecular structure as shown in Figure
1, with the ultimate aim of preparing materials with superior working temperatures to that of the parent
compound, and which could be subjected to in-depth study across the NX-N phase transition.
Figure 1:
Proposed structural changes to compound 1: (a) terminal chains, (b) polar terminal group(s),
(c) lateral ‘bulky’ group, (d) linking unit orientation.
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Experimental
18
4-Hydroxy-4′-nitrobiphenyl was prepared as described previously. Chemical reagents were purchased from
commercial suppliers (Sigma Aldrich, TCI, Fluorochem and Apollo Scientific) and used without further
purification. Solvents were purchased from Fisher Scientific and dried by percolation through activated alumina
prior to use. Polarised optical microscopy was performed on a Zeiss Axioskop 40Pol microscope using a Mettler
FP82HT hotstage controlled by a Mettler FP90 central processor. Photomicrographs were captured via an
InfinityX-21 MP digital camera mounted atop the microscope. Differential scanning calorimetry was performed on
a Mettler DSC822e calibrated before use against indium and zinc standards under an atmosphere of dry nitrogen,
with DSC data then processed in Matlab. Computational chemistry was performed using Gaussian G09 revision
e01 19 on the York Advanced Research Computing Cluster (YARCC) as described in the text. Small angle X-ray
diffraction was performed using a Bruker D8 Discover equipped with a temperature controlled, bored graphite rod
furnace, custom built at the University of York. The radiation used was copper Kα (λ = 0.154056 nm) from a 1 μS
microfocus source. Diffraction patterns were recorded on a 2048x2048 pixel Bruker VANTEC 500 area detector
set at a distance of 121 mm from the sample, allowing simultaneous collection of small angle and wide angle
scattering data. Samples were filled into 0.9 mm O.D. capillary tubes and aligned with a pair of 1T magnets, with
the field direction being perpendicular to the incident X-ray beam. Diffraction patterns were collected as a
function of temperature and the data processed using custom Matlab scripts. Raw data are available upon
request from the University of York data catalogue. Full experimental details, including synthetic schemes and
chemical characterisation, are given in the ESI to this article.
Results
Initially we prepared a selection of compounds analogous in structure to 1 but with varying terminal chain
lengths. Transition temperatures and phase identification were determined with a combination of polarised-light
optical microscopy (POM) differential scanning calorimetry (DSC) and variable temperature small angle X-ray
scattering (VT-SAXS), as summarised below in Table 2.
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No.
1
R
Cr
NX
139.0 (●
[34.5]
N
Iso
●
85.6)
[0.2]
●
182.1
[0.6]
●
-C2H5
●
●
●
●
●
139.8 (●
132.7)
[0.2]
●
●
●
●
●
187.9
[0.5]
●
●
●
●
●
2
3
4
5
6
-CH3
-C3H7
-C4H9
-C5H11
-C6H13
[29.9]
134.7 (* 8.7 ±0.9)
[35.1]
161.2
[0.6]
111.4
[28.7]
-
-
-
-
-
-
160.3
[0.5]
98.8
150.3
[0.5]
[30.9]
109.6
[31.8]
148.5
[1.5]
Table 2:
Transition temperatures (°C) and associated enthalpies of transition [kJ mol-1] for compounds
-1
1- 6, as determined by DSC at a heat/cool rate of 10 °C min . * An extrapolated ‘virtual’
transition temperature was determined by linear fitting of TNX-N versus concentration (see
Figure 2b) however the material does not exhibit this phase in its neat state.
Increasing the length of the terminal chain from C2 in the parent compound led to the loss of the NX phase along
with modest reductions in melting point and clearing point, thereby indicating that the Nx phase is preferred when
potential nanosegregation is minimised. Thus, shortening the terminal chain to C1 (i.e. OMe) affords compound
2, and relative to the parent compound this structural change gives a large increase in the onset temperature of
the NX phase. Representative photomicrographs are shown in Figure 2c-e. The enthalpy associated with the NX-
N transition for both 1 and 2 is vanishingly small (0.2 kJ mol-1 for both) and this results in the associated entropy
of transition being extremely small for both materials (ΔSNx-N/R, 0.06 and 0.07 for 1 and 2 respectively). As was
noted by us previously, the small value of the enthalpy/entropy associated with the NX-N transition is consistent
with a transition between two phases with the same macroscopic symmetry. 14 A phase diagram was constructed
between compounds 1 and 2 as shown in Figure 2a. Both 1 and 2 were found to be miscible at all concentrations
with both the N-Iso and NX-N transition temperatures varying approximately linearly with concentration and
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therefore confirming the NX phase is indeed exhibited by both materials. Given that a linear relationship exists
between concentration and TNx-N we are able to obtain ‘virtual’ transition temperatures via extrapolation for
materials that do not exhibit this mesophase or exhibit it at temperatures that are experimentally inaccessible.
We also constructed a phase diagram between compound 1 and 3 (Figure 2b), both materials are miscible
across all concentrations studied but the NX phase was found to decrease linearly with increasing concentration
of 3, disappearing at 43 wt%. Using a linear fit we obtain a virtual (i.e. extrapolated) NX-N transition temperature
of ~ 8.5 °C. As demonstrated by the materials in Table 2 the NX-N transition apparently displays no odd-even
effect with regards to the terminal chain length, again confirming that nanosegregation associated with the
aromatic to aliphatic proportions strongly influences mesophase formation.
Figure 2:
Gibbs phase diagram of binary mixtures (wt %) of compound 1 with (a) compound 2 and (b)
compound 3. Linear fitting of the NX-N transition temperature as a function of concentration for
the phase diagram between 1 and 3 gives a virtual transition temperature of 8.7 °C via the
equation TNX-N = -75.9(±16.1)wt% + 84.7(±4.0), R2 = 0.987. Mixtures containing more than ~
42 wt% of compound 3 crystallise prior to the onset of the NX phase. Photomicrographs of
compound 2 on untreated glass slides under crossed polarisers: in the nematic phase (c, 140
°C); at the NX-N transition (d, 132.6 °C); cooled deep into the NX phase (e, 110 °C).
We subjected compound 2 to analysis by SAXS with the intention of studying the change in scattering as a
function of temperature across both nematic phases. As with compound 1 we observed that the scattering of X-
rays by 2 was weak, and thus we required relatively long exposure times to obtain good signal to noise ratios.
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Despite the increase in the NX-N transition temperature afforded by compound 2, we observed that the material
was still prone to crystallisation during experimental studies, and so we were limited to collecting individual SAXS
frames at specific temperatures rather than across the entire phase range.
Figure 3:
Analysis of compound 2 by small angle X-ray scattering: (a) the two dimensional scattering
pattern obtained in the nematic phase at 140 °C; (b) the two dimensional scattering pattern
obtained in the NX phase at 129 °C; (c) plot of scattered intensity parallel to the director as a
function of Q (Å-1), obtained by radially averaging (0.05 ° steps) the upper and lower wedges
illustrated on the nematic SAXS pattern; (d) plot of scattered intensity perpendicular to the
director as a function of Q (Å-1), obtained by radially averaging (0.05 ° steps) the left and right
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wedges illustrated on the nematic SAXS pattern; (e) plot of scattered intensity obtained by
radially averaging the entire SAXS pattern (0.05 °) steps showing how information is ‘lost’ due
to the overlapping of small- and wide-angle scattering peaks.
In both the higher- and lower-temperature nematic phases the observed scattering was broadly similar to that
14
reported previously for compound 1; three diffuse peaks were seen at ‘small’ angles parallel to the external
aligning field (i.e. along the director, Figure 2c). The positions of each of the three peaks were determined by
deconvolution (see Figure 4) the results are presented in Table 3. Whereas the d-spacing of ‘peak 1’ (smallest
value of Q) is comparable, if notably larger than, the molecular length of 2 (calculated to be 20.5 Å at
DFT(B3LYP/6-31G(d)) the other two peaks occur at significantly smaller d-spacings.
Peak no.
N 140 °C
(Q, Å-1)
0.2937
0.6321
0.9292
N 140 °C
(d, Å)
21.82
9.94
NX 129 °C NX 129 °C
(Q, Å-1)
0.2978
0.6620
0.9244
(d, Å)
21.09
10.10
6.80
1
2
3
6.76
Table 3:
Peak positions (in Q / Å-1 and d / Å) obtained by fitting scattered X-ray intensity in parallel to
the aligning magnetic field (i.e. the director) for 2 in the nematic and NX phases at 140 °C and
129 °C respectively.
In both the nematic and NX phases we observe two peaks at ‘wide’ angles (i.e. perpendicular to the aligning field
and the director, Figure 3d); a broad peak at large values of Q, and a less intense broad peak at small values of
Q (Q = 0.3565 Å-1, d = 17.7 Å). The scattering perpendicular to the aligning field is a consequence of the average
lateral separation of the molecules, given that compound 1 has been demonstrated to form extensive antiparallel
pairs it is unsurprising that the wide-angle peak is so broad, as many different forms of pairing are likely to exist
(dimer, trimer… n-mer). The broad peak can be deconvoluted into two separate peaks (Figure 4, Q = 1.0018 Å-1
and Q = 1.2755 Å-1, equal to d = 6.3 Å and 4.9 Å respectively); 4.9 Å is close to the width of an individual
molecule and so we speculate that 6.3 Å is the width of a paired species. It is interesting to note that the position
of each individual peak is the same in both the N and NX phases; however their relative size changes with the
intensity of the peak at 6.3 Å increasing and that of the peak at 4.9 Å decreasing. If we assume that 4.9 Å and
6.3 Å are indeed the widths of an unpaired molecule and a paired species respectively then this suggests the
degree of dimerisation is inversely proportional to temperature and therefore higher in the NX phase than in the
nematic, a result consistent with measurement of the Kirkwood factor of compound 1. 14
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Figure 4:
(a) Fitting the scattered X-ray intensity perpendicular to the aligning magnetic field for
compound 2 in the nematic phase at 140 °C (R2>0.99), (b) the B3LYP/6-31G(d) minimised
geometry of 2 with the approximate molecular length and widths indicated.
Next we prepared a series of compounds with varying lateral group, allowing us to assess how variations to the
steric bulk of the lateral unit impact upon the NX-N transition. We also studied how moving this lateral group from
the 2- to the 3- position of the left-hand ring (compound 8) impacts on mesomorphic behaviour. The melting
properties for these compounds are given in Table 4 along with the parent material (1) for comparison.
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No.
X
Y
Cr
Nx
N
Iso
●
139.0
[34.5]
●
85.6)
[0.2]
●
182.1
[0.6]
●
1
7
H
OMe
●
155.2 (* -17.4 ±1.0)
[33.1]
●
274.8
[0.6]
●
H
H
●
●
●
●
●
171.9
[48.7]
-
-
●
●
●
●
●
181.9
[0.3]
●
●
●
●
●
8
OMe
H
H
164.7 (* 1.8 ± 2.1)
[28.8]
>240 #
9
OH
138.8 (* 32.7 ± 0.7)
[38.8]
186.2
[0.7]
10
11
12
H
CH3
143.3 (●
[36.1]
91.0
[0.2]
77.6
[0.6]
130.3)
[0.6]
H
OC2H5
OC3H7
143.2 (●
[40.6]
99.2)
[0.6]
H
Table 4:
Transition temperatures (°C) and associated enthalpies of transition [kJ mol-1] for compounds 7
– 12, in the case of compound 9 (denoted with a hash) the material begins to decompose
before the clearing point at 240 °C is reached. An extrapolated ‘virtual’ transition temperature
was determined by linear fitting of TNX-N versus concentration for compounds 7, 9 and 10 (see
Figure 6) however these materials does not exhibit the NX phase in their neat state.
Materials in which the lateral group is smaller than methoxy (compounds 7¸9 and 10) do not exhibit the NX phase,
whereas repositioning the methoxy group from the 2- position to the 3- position (compound 8) also leads to the
loss of the Nx phase. Increasing the length of the lateral alkyl chain, and hence the bulk volume, leads to a
reduction in clearing points (see Figure 5) and either a small increase (11) or decrease (12) in the temperature at
which the NX-N transition occurs. Conversely, melting points do not appear to exhibit a dependence on the size
of the lateral bulky group.
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300
250
200
150
100
50
-H
-OH
-CH3
-OCH3
-OC2H5
40
-OC3H7
60
0
0
10
20
30
50
70
Lateral Group Volume / Å3
20
Figure 5:
Plot of the N-I transition temperature vs the volume of the lateral group (values from ref.
with an exponential fit (Y = 288.37-0.019 X, R2 > 0.99) to guide the eye.
)
Compounds 7, 9 and 10 do not exhibit the N-NX transition in their neat state, and so virtual transition
temperatures were obtained by constructing phase diagrams between these materials and the standard
compound 1. Linear fitting of TNx-N as a function of concentration affords the virtual transition temperature. Phase
diagrams are presented in Figure 6, the virtual TNx-N values were found to be respectively -23.8 °C, 1.8 °C and
32.7 °C for 7, 9 and 10.
Figure 6:
Gibbs phase diagram of binary mixtures (wt %) of compound 1 with (a) compound 7, (b)
compound 9 and (c) compound 10. Linear fitting of the NX-N transition temperature as a
function of concentration for each phase diagram gives a virtual NX-N transition temperatures
of -23.8 °C, 1.8 °C and 32.7 °C for 7, 9 and 10 respectively. Equations of fit for 7 TNX-N = -
102.57 + 85.168, R2 > 0.99, for 9: TNX-N = -78.611x +80.436, R2 >0.90, and for 10: TNX-N = -
50.442x + 82.088, R2 > 0.99. Mixtures containing more than ~ 10 wt % of compound 7, ~ 10
wt% of 9 or ~ 20 wt% of 10 recrystallize prior to the NX-N phase transition.
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We next explored how the magnitude of the lateral dipole moment influenced the NX-N transition by preparing
compounds 13 and 14 (Table 5). Positioning a fluorine atom ortho to the nitro group in compound 1 afforded
compound 13, which compared to the parent material exhibits a significant increase in the NX-N transition
temperature as well as a significantly reduced clearing point. Conversely, if a terminal nitrile is used in place of
the nitro group the NX phase is suppressed. In an attempt to rationalise this we calculated dipole moments at the
B3LYP/6-31G(d) level of DFT. The dipole moment of 13 is, predictably, larger (12.5 Debye) than that of both 1
(11.7 Debye) and 14 (11.5 Debye), suggesting that the magnitude of the dipole moment is important to the
formation of this phase. Phase diagrams were constructed for binary mixtures of compound 1 with 13 and 14
(Figure 7). Across both phase diagrams the clearing point was observed to vary linearly with concentration. For
mixtures of 1 and 13 the NX-N transition was found to vary linearly with concentration (TNX-N = 30.393x+80.043,
R2 >0.96) indicating that the lower temperature mesophase exhibited by these two materials is the same. For
mixtures of 1 and 14 however the Nx phase was not observed even at low concentrations of 14 (~ 10 wt %).
No.
1
X
Y
Cr
NX
N
Iso
●
139.0 (● 85.6)
●
182.1
[0.6]
●
H
NO2
[34.5]
[0.2]
●
●
141.8 (● 117.1) ● 149.6
●
●
13
14
F
NO2
CN
[43.7]
117.1
[41.2]
[0.4]
-
[0.3]
204.2
[1.1]
-
●
H
Table 5:
Transition temperatures (°C) and associated enthalpies of transition [kJ mol-1] for compounds
13 and 14. Phase transitions in parenthesis () are monotropic, i.e. they occur below the melting
point of the sample.
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Figure 7:
Gibbs phase diagram of binary mixtures (wt %) of compound 1 with (a) compound 13 and (b)
compound 14. For binary mixtures of 1/13 (a) the NX-N transition temperature varies
approximately linearly with concentration, however for mixtures of 1/14 (b) the addition of just
10 wt% of 14 suppresses the NX-N transition entirely.
Studying compounds 1, 13 and 14 it is tempting to hypothesise about the role of the electric dipole moment: 13
has the largest dipole moment and the highest Nx-N transition temperature; 1 has a smaller electric dipole
moment than 13 and a lower NX-N transition temperature; 14 has a smaller dipole moment than 1 and does not
exhibit the NX phase. Although a small sample size, this hints that reducing the magnitude of the molecular
electric dipole moment also serves to reduce the thermal stability of the NX phase. As a test to this hypothesis we
prepared two materials, one in which a carboxylate ester was removed (15) and another in which a single
carboxylate ester had its orientation ‘reversed’ relative to that of the parent compound (16). Both structural
modifications would be expected to reduce the molecular dipole moment, and we confirmed this with
DFT(B3LYP/6-31G(d)) calculated dipole moments. In the case of the modulated twist-bend phase this reversal of
carboxylate esters was found to impact on the thermal stability of the TB phase. 21 As will be discussed shortly,
we also prepared several materials with larger dipole moments to test this hypothesis.
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No.
1
X
Cr
NX
139.0 (●
[34.5]
N
Iso
●
85.6)
[0.2]
●
182.1
[0.6]
●
●
●
151.6 (* -11.6 ±7.0)
[40.9]
●
●
165.9
[0.24]
190.1
[0.40]
●
●
15
16
180.8
[45.8]
-
-
Table 6:
Transition temperatures (°C) and associated enthalpies of transition [kJ mol-1] for compounds
15 and 16. An extrapolated ‘virtual’ transition temperature (*) was determined by linear fitting of
TNX-N versus concentration for compound 15 (see Figure 6) however this material does not
exhibit the NX phase in its neat state.
With regards to the ester unit, its removal (to afford the biphenyl benzoate 15) or reversal (to afford the phenyl 4-
nitrobenzoate 16) results in the loss of the NX-N phase transformation and yields materials that are only
nematogenic, with both 15 and 16 having a higher melting point than the parent 1. Dipole moments were
calculated at the B3LYP/6-31G(dp) level to be 10.04 and 8.26 Debye for 15 and 16 respectively. Both materials
exhibit a nematic phase, however neither exhibits the NX-N transition. Construction of a phase diagram between
1 and 15 and linear fitting of TNx-N versus concentration affords the extrapolated value shown in Table 6, with the
phase diagram presented in Figure 8.
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Figure 8:
Gibbs phase diagram of binary mixtures (wt %) of compound 1 with compound 15. Linear
fitting of the NX-N transition temperature as a function of concentration (via the equation TNx-N
=
-95.9x + 84.354, R2 > 0.96) gives the virtual transition temperature of -11.6 °C. Mixtures
containing more than ~ 10 wt % of compound 15 recrystallize prior to the NX-N phase
transition; the error in this extrapolated value is therefore higher than for others in this work and
is estimated to be ±7 °C
Most structural modifications to the molecular structure of compound 1 were found to suppress the formation of
the NX phase, with the exception of reducing the length of the terminal ethoxy chain to methoxy (compound 2)
and increasing the dipole moment by incorporating a fluoro-substituent ortho to the terminal nitro (compound 13).
Combining these two features affords 17. Compounds 18, 19 and 20 were prepared to further study how the
molecular dipole moment and terminal groups influences the nematic and NX phases. Transition temperatures
and associated enthalpies of transition are given in Table 7.
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No. X= R1 R2 Cr
NX
N
Iso
●
139.8 (● 132.7)
●
187.9
[0.5]
●
2
NO2
H
H
H
F
H
F
[29.9]
[0.2]
●
●
●
●
●
165.2 (● 139.6
●
●
155.2)
[0.5]
●
●
●
●
●
17 NO2
18 CN
19 CN
20 CN
21 SF5
[50.1]
[1.6]
-
173.2
[41.0]
-
-
-
-
200.4
[0.7]
H
H
F
197.3
[64.7]
-
-
-
(● 169.4)
[0.5]
162.6
[41.4]
(● 153.4)
F
[0.6]
172.0
[40.4]
(● 93.1)
H
H
[0.2]
Table 7:
Transition temperatures (°C) and associated enthalpies of transition [kJ mol-1] for compounds
17 – 21.
Compared to the parent material compound 17 has only a modest increase in the onset temperature for the NX to
nematic phase transition, with significantly reduced isotropisation temperature and a higher melting point. As
expected, the cyano- terminated material 18 does not exhibit the NX phase (mirroring the behaviour of compound
14). Compounds 19 and 20 were prepared to determine if the incidence of the NX phase is specific to nitro-
terminated materials, or if it can be formed by compounds with other suitably polar groups. There are several
18, 22-26
reports of liquid-crystalline materials incorporating the pentafluorosulphanyl (-SF5) group;
given the large
dipole moment we considered that a material analogous to 1/17 but with this terminal group in lieu of the nitro
group may yield the NX-N polymorphism, however compound 21 exhibited only a monotropic nematic phase, but
did not exhibit a NX-N transition. As both materials exhibit only a nematic phase it would appear at present that a
nitro group is essential, but given the limited set of compounds known to exhibit this phase it is perhaps too early
to draw firm property-structure correlations. Dipole moments for each of the materials in Table 5 were calculated
at the B3LYP/6-31G(dp) level of DFT: 11.37 Debye for 2; 12.14 Debye for 17; 11.13 Debye for 18; 12.00 Debye
for 19; 12.84 Debye for 20; 10.53 Debye for 21. For the nitrile-terminated compound 18 the calculated dipole
moment is intermediate between the two nitro-terminated materials 2 and 18, both of which exhibit the lower
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10.1002/chem.201702742
Chemistry - A European Journal
temperature nematic phase. The present results suggest that it is not the magnitude of the dipole moment of an
individual molecule that dictates the incidence of the lower temperature nematic phase, but rather some property
inherent to the nitro group, for instance the extent of the delocalization of the electrons, and hence polarizability,
over a broader functional group than nitrile.
One of our objectives was to obtain materials exhibiting an enantiotropic NX phase, rendering them amenable to
further study, however this was not met. The thermal stability of the NX-N transition is highest when the material
features short terminal chains and a highly polar terminal nitro group, however these conditions also (predictably)
lead to high melting points. Using transition temperatures and enthalpy data obtained by DSC, we used the
modification to the Schroder van Laar equation reported by Raynes to predict both the composition and transition
27
temperatures of the eutectic blends of a number of possible binary mixtures. This method predicts that the
eutectic mixture of 2 and 17 should exhibit an enantiotropic NX transition (NX-N 134.5 °C, with a melt of 126.0
°C).
N-Iso
Nx-N
MP
200
190
180
170
160
150
140
130
120
SVL N-Iso
SVL NX
SVL MP
0%
20%
40%
60%
80%
100%
wt % Compound 17
Figure 9:
Gibbs phase diagram of binary mixtures (wt %) of compound 2 with compound 17. The mixture
with 25 wt% of compound 17 exhibits an enantiotropic Nx-N transition over less than 0.5 °C.
Predictions made by the SVL equation using transitional data for 2 and 17 (obtained by DSC at
a heat/cool rate of 10 °C min-1) predicts a eutectic mixture at 25.9 wt % of 17 in 2; the
predicted melting point (126.0 °C), NX-N transition temperature (134.5 °C) and clearing point
(179.4 °C) are indicated in the phase diagram by dashed symbols.
The phase diagram of binary mixtures of 2/17 is shown in Figure 9. Although it was found that the experimental
melting points were somewhat higher than predicted, the mixture containing ~ 24 wt% of 17 exhibits an
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10.1002/chem.201702742
Chemistry - A European Journal
enantiotropic NX phase. Given the propensity of nitro-terminated materials to form antiparallel pairs it may be that
considering the phase diagram as being ‘binary’ is misleading due to the formation of AA, AB, and BB pairs in
addition to the unpaired species (where A/B = 2/17), and this may be account for the underestimation of the
melting point. As the NX-N and N-Iso transition temperatures are simply a weighted average of the two pure
components we observe predicted values to be reasonably close to those determined experimentally.
As an initial study we performed small angle X-ray scattering as a function of temperature (3 °C steps), and
using the radial averaging procedure outlined in Figure 3 we separately obtained scattered intensity parallel and
perpendicular to the director. Scattering data for the eutectic blend of 2/17 is presented as heatmap plots in
Figure 10, for which the material was studied across a range of temperatures in 3 °C steps from the isotropic
liquid (186 °C) deep into the NX phase (91 °C). The temperatures at which phase transitions occurred are
marked on the heatmap plots with dashed lines; on cooling from the isotropic liquid into the nematic phase there
is a significant change in the scattering pattern, however, at the NX-N transition there is little if any change. There
is, however, a continual reduction in the intensity of the small angle peaks as a function of temperature. The
relatively weak scattering at small angles indicates that there is no build-up of pretransitional cybotactic smectic
domains within either the nematic or NX phases. 28, 29 Thus, the nematic-to-nematic transition described herein is
15
presumably distinct from those observed in reentrant systems such as in ref Fitting the scattered intensity at
each temperature allows us to obtain the position of both small- and wide-angle peaks. There is little change in
peak positions at small angles (i.e. parallel to the director, Figure 10c) on cooling from the upper temperature
nematic into the lower temperature ‘NX’. At wide angles (i.e. perpendicular to the director) the two peaks overlap
significantly, and although the deconvoluted peaks do not shift in position (Figure 10d) we observe that the
intensity of peak #1 (Q ~ 1.1 Å-1, d ~ 5.7 Å ) increases relative to that of peak #2 (Q ~ 1.3 Å-1, d ~ 4.8 Å), leading
to a temperature dependent reduction in the Q value of the concurrent peak. Assuming that the earlier
hypothesis that peak #2 is the width of a single molecule and that peak #1 is the intermolecular spacing of a
dimer pair then the this change in intensity can be understood as being a consequence of the increase in pairing
of molecules as a function of temperature. This interpretation of the SAXS data is consistent with the
measurement of the Kirkwood factor of compound 1.
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10.1002/chem.201702742
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Figure 10:
SAXS data for the eutectic mixture of 2/17: (a) heatmap plot of scattered intensity parallel to
the director as a function of temperature (°C) and Q (Å-1); (b) heatmap plot of scattered
intensity perpendicular to the director as a function of temperature (°C) and Q (Å-1); (c) plot of
the Q (Å-1) value of the two major diffuse peaks in the small angle region as a function of
temperature; plot of the Q (Å-1) value of the two major diffuse peaks in the wide angle region as
a function of temperature. Dashed lines correspond to phase transitions.
In our initial study on 1 we noted that the NX-N transition could either occur as a consequence of a continual
change in the concentration of dimers to unpaired molecules, or alternatively at the transition there may be a
discontinuous change in this concentration. The present SAXS data supports the former; the degree of pairing
increases continually with reducing temperature, at some critical concentration the degree of pairing is sufficient
to lead to the NX phase, and a first order phase transition occurs. In this sense the formation of the NX phase
proceeds via a comparable mechanism to that described by Cladis for the reentrant nOCB materials, although
there is no intervening smectic mesophase in the present case. To date we have only observed the NX phase in
nitro terminated materials, presumably some property of the nitro group leads to a specific pairing which is crucial
to the incidence of this phase.
Conclusions
Compound 1 was previously demonstrated to exhibit two nematic mesophases (NX and N) separated by a first
order phase transition of small enthalpy; however as the NX-N transition is monotropic - occurring ~ 50 °C below
the melting point – detailed study is complicated by crystallisation of the sample. In this study we have
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10.1002/chem.201702742
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investigated the molecular features that give rise to this phase sequence with a view to producing materials with
superior working temperatures to compound 1.
Using the onset temperature of the NX-N phase as a measure of the thermal stability of the lower temperature
nematic we find the following property-structure correlations: (i) the NX mesophase is promoted by a short
terminal chain (ethoxy or preferably methoxy); (ii) a terminal nitro group is essential; (iii) the thermal stability can
be increased by positioning additional fluoro- groups to enhance the molecular dipole moment; (iv) the use of
other terminal polar groups (nitrile, pentafluorosulphanyl) or removal/reversal of carboxylate esters (which reduce
the dipole moment) is detrimental to Nx phase formation, and (v) a lateral ‘bulky’ group is required for a material
to exhibit the NX phase.
Using each of these correlations we prepared a material designed to exhibit a high NX-N onset temperature
(compound 17), and although we do observe significantly enhanced thermal stability of the Nx phase relative to
that of parent compound (1) the material also exhibits a high melting point, which is perhaps unsurprising given
the large dipole moment and short terminal chains. However, binary mixtures of 17 with 2 have afforded a
eutectic mixture that exhibits an enantiotropic NX phase. Studies of this material by X-ray scattering confirm the
identity of both mesophases, and allow us to present SAXS and WAXS data across the entire temperature
range, and results suggests a continuous change in the degree of pairing rather than a jump at the NX-N phase
transition.
Acknowledgements
The authors thank QinetiQ for funding an ICASE studentship for RJM. EPSRC grant EP/M020584/1 for the
development of dyes for liquid crystal applications, EPSRC Platform Grant EP/D055261/1, EPSRC grant
EP/J007714/1 for the development of liquid crystals for displays and lastly EPSRC core capabilities grant
EP/K039660/1. We thank the referees for helpful and constructive comments during the peer review process.
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10.1002/chem.201702742
Chemistry - A European Journal
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