Implications of flexible spacer rotational processes on the liquid crystal behavior of 4,5-dihydroisoxazole benzoate dimers

Article abstract of DOI:10.1039/c5nj02199e

The synthesis of some novel non-symmetric liquid crystal dimers, {3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-(decyloxy)benzoates (5a-d) and 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-{[6-(octyloxy)naphthalen-2-yl]ethynyl}benzo

Full text of DOI:10.1039/c5nj02199e

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Implications of flexible spacer rotational
processes on the liquid crystal behavior of
Cite this:DOI: 10.1039/c5nj02199e
4,5-dihydroisoxazole benzoate dimers†
Aline Tavares,*a Josene M. Toldo,a Guilherme D. Vilela,a Paulo F. B. Gonçalves,a
Ivan H. Bechtold,b Stuart P. Kitney,c Stephen M. Kellyd and Aloir A. Merlo*a
The synthesis of some novel non-symmetric liquid crystal dimers, {3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-
5-yl}alkyl 4-(decyloxy)benzoates (5a–d) and 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl
4-{[6-(octyloxy)naphthalen-2-yl]ethynyl}benzoate (9a–d), are reported. The liquid-crystalline properties,
theoretical calculations based on the conformational aspects of the flexible alkyl spacer and X-ray
experiments are discussed. The syntheses of the key intermediates, 2-{3-[4-(octyloxy)phenyl]-4,5-
dihydroisoxazol-5-yl}alkanol (3a–d), presenting the flexible alkyl spacer were achieved through [3+2]
cycloaddition reactions between nitrile oxides, which were generated in situ by oxidation of the respective
aromatic oximes, and dipolarophile alkenols (CH2QCH(CH2)nOH, n = 1, 2, 3, and 4). The benzoates 5a–d
were synthesized through esterification of 3a–d and p-n-decyloxybenzoic acid (4). The esters 9a–d
were synthesized through derivatization of isoxazolines 3a–d into 4-{3-[4-(octyloxyphenyl)]-4,5-
dihydroisoxazol-5-yl}alkyl 4-bromobenzoate (7a–d) followed by a Sonogashira reaction with 2-ethynyl-
6-octyloxynaphthalene (8). 5a and 5b showed a monotropic smectic C phase. 9a/c displayed a enantiotropic
nematic (N) mesophase, whereas 9b/d showed a monotropic nematic mesophase. No mesophase was
observed for 7a–d. An odd–even effect was observed for 5a–d and 9a–d associated with the crystal to
isotropic phase transition and crystal to nematic phase, respectively, as the length of the spacer was
increased from 1 to 4 carbon atoms. The transitional properties were higher for odd-numbered members
(n = 1 and 3) for all of the series studied. The X-ray data of compounds 5a and 5b are in agreement with
polarizing optical microscopy observations with the assignment of an SmC mesophase. Density functional
theory calculations using the B3LYP hybrid functional with the level 6-311G(d,p) basis set were performed
for molecules 5a–d to correlate the conformation of the flexible spacer and the transitional properties.
The conformational analysis showed that the most stable conformation for 5a–d is one where all of
the carbon atoms of the flexible spacer are orientated at 1801 (antiperiplanar orientation) except for 5a
because the spacer is too short. The odd-numbered members have a more bent shape and are less
elongated molecules than the even-numbered members. Thus, mesomorphic behavior is dictated by
the conformational constraint imposed by the flexible spacer on the mesogenic groups.
Received (in Montpellier, France)
18th August 2015,
Accepted 16th October 2015
DOI: 10.1039/c5nj02199e
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is usually a mesogenic group and the simplest LCOs are termed
dimers which have just two mesogenic units linked chemically
Introduction
Liquid crystal oligomers (LCOs) are a special class of soft to a single methylene chain.2 The first reports about liquid
3
¨
materials formed of two or more anisotropic-shaped cores crystal dimers were by Vorlander at the beginning of the 1920s
connected by flexible spacers, normally alkyl chains.1 The core and by Rault4 some years later. However, these important reports
appear to have been a forgotten past until the early 1980s, when
they were rediscovered again and gained new interest.5 The vast
majority of LCOs are composed of two symmetric rod-like
mesogenic units,6 whereas their non-symmetrical analogues7 have
two different mesogenic groups connected by a flexible alkyl chain
and less frequently by oligo(ethyleneoxide),8 oligo(siloxane)9 and
sulfur–sulfur links10 in the chain. In this context, the length and
parity of the flexible spacer are important parameters that have
a Institute of Chemistry, UFRGS, Porto Alegre, RS, Brazil.
E-mail: aloir.merlo@ufrgs.br
b
´
Department of Physics, UFSC, Florianopolis, Brazil
c Polar OLED, University of Hull, Hull, England, UK
d Department of Chemistry, University of Hull, Hull, UK
† Electronic supplementary information (ESI) available: Experimental procedure,
1H and 13C NMR spectrum and spectroscopic data final compounds, modeling
pictures of the rotamers. See DOI: 10.1039/c5nj02199e
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a great influence on the transitional properties.11 Dimers and
higher oligomers, such as trimers12–14 and tetramers,15,16 have
a special level of attention due to their ability to act as model
compounds for semi-flexible main-chain liquid crystal polymers.17,18
Also, from the academic point of view these oligomers are
interesting because they behave differently to conventional
liquid crystals of low molar mass.19,20
Mostly, LCOs have an aryl group as the mesogenic unit and a
few examples of LCOs incorporating 5- and 6-membered rings
such as 1,3,4-oxadiazoles and pyridyl-based dimers respectively,
have also been published, with their transitional properties having
been investigated.21,22 Compounds containing a 4,5-dihydro-
isoxazole moiety have been prepared recently and the liquid
crystalline behavior evaluated.23 To the best of our knowledge,
however, there have been no examples of non-symmetric
4,5-dihydroisoxazole-based liquid crystal dimers reported in
the literature.
Scheme 2 Preparation of 3,5-disubstituted 4,5-dihydroisoxazole benzo-
ate dimers 5a–d.
compounds 3a–d and 4-n-decyloxybenzoic acid 4 in the
presence of DCC and catalytic amounts of DMAP in a THF
solution at room temperature. The compounds 7a–d, prepared
from the esterification reaction between compounds 3a–d and
4-bromobenzoic acid 6 in the presence of DCC and catalytic
amounts of DMAP in THF, were precursors for the synthesis of
the liquid-crystalline materials 9a–d. The yields reported for the
compounds 5a–d and 7a–d refer to the pure compounds after
purification via recrystallization or column chromatography to
remove the undesirable byproduct urea.
We have previously reported24 the synthesis of liquid-crystalline
3,5-disubstituted isoxazolines, where the number of carbon atoms of
the terminal aliphatic chains exerts a strong influence on the
molecular shape and mesomorphic behavior. In this work, we report
the synthesis and transitional properties of two new homologous
series of non-symmetrical 3,5-disubstituted 4,5-dihydroisoxazole
benzoate liquid crystal dimers with emphasis on the dependence
of the liquid-crystalline transition temperatures on the number of
methylene carbon atoms of the flexible spacer. Density functional
theory (DFT) calculations combined with X-ray analysis provided
supporting information in this study.
The compounds 9a–d were prepared using a Sonogashira
cross-coupling reaction between the intermediates 7a–d and
the terminal alkyne 827 in the presence of a palladium catalyst,
i.e., (PPh3)2PdCl2, CuI, PPh3 in NEt3 (Chart 1).
The mesophase identification for the LC compounds 5a–d
as well as for 9a–d was made using polarizing optical micro-
scopy (POM). The smectic C mesophase (SmC) was assigned
through the observation of a typical broken fan texture (at the
top of Fig. 1(A) for 5a) and a schlieren texture (at the bottom of
Fig. 1(C) for 9b). For compound 5b, the mesophase SmC range
was very narrow and it appears very quickly before the crystal-
lization takes places [Fig. 1(B)]. The DSC traces for 5b display a
shoulder along a peak related to the transition temperature of
the crystal phase to the isotropic phase. The SmC mesophase
for 5c and 5d was detected and the texture persists for just a few
seconds followed by fast crystallization. No POM images or
Results and discussion
Synthesis and description of liquid-crystalline behavior
The synthetic route used for the preparation of the isoxazolines
3a–d is shown in Scheme 1. We selected the aldehyde 2 as a
precursor for the reactive arylnitrile oxide.25 Thus, the aldehyde 2
was synthesized through alkylation of 4-hydroxybenzaldehyde 1
with octylbromide in 78% yield. The isoxazolines were obtained
by a [3+2] 1,3-dipolar cycloaddition26 of nitrile oxide, formed by
the in situ oxidation reaction of 4-octyloxybenzaldehyde oxime
and four different dipolarophile alkenols – CH2QCH(CH2)nOH,
n = 1, 2, 3, and 4. The final key isoxazolines 3a–d were obtained
in low yields (33–36%).
The design and synthesis of the liquid crystals 5a–d and 9a–d
was based on creating the structural characteristics necessary
for the occurrence of a mesophase through a short and
quick synthesis. Thus, the first homologous series 5a–d, (see
reaction Scheme 2) was prepared through the esterification of
Scheme 1 Synthetic route used to prepare compounds 3a–d.
Chart 1 Intermediates and final compounds of the series 7a–d and 9a–d.
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on the ester side. The number of carbon atoms in the flexible
spacer was varied. So, compounds 5a–d have n = 1, 2, 3, and 4 in
the flexible spacer, respectively.
The transition temperatures shown in Table 1 were obtained
through a combination of POM and DSC analysis. Compounds
5a and 5b, upon a second cooling cycle, exhibit a monotropic
phase with two exothermic peaks at 83 1C and 61 1C, respectively.
These peaks were associated with the transition temperature when
the samples enter the SmC mesophase from the isotropic state.
Upon further cooling, 5a and 5b displayed a crystallization peak
from the SmC mesophase, at 74 1C and 54 1C, respectively. For the
higher homologues 5c and 5d, the DSC traces show only peaks
related to the crystal phase to isotropic phase transition. However,
samples of 5c and 5d when analyzed using POM upon fast cooling,
displayed a monotropic SmC phase before recrystallization. For
compounds that belong to the homologous series 9a–d the
terminal alkyl chain was fixed at eight carbon atoms (n-octyl).
The variation in the flexible spacer was made in the same way as
described for 5a–d. According to the DSC data [Table 1, Fig. 1(C)
and (D) and Fig. S31 (ESI†)], the 9a and 9c homologues of this
series exhibit an enantiotropic nematic mesophase, while 9b
and 9d display a monotropic nematic mesophase. When heated,
the temperature range decreased by increasing the number of
methylene units in the aliphatic chain, e.g., for 9a DT = 13 1C and
for 9c DT = 9 1C. For 9b and 9d, a monotropic nematic
Fig. 1 Photomicrographs of the textures obtained from optical microscopy
on cooling (Â10) of: (A) the broken fan texture (at the top) and schlieren
texture (at the bottom) of the SmC mesophase displayed by compound 5a
below 79 1C, (B) the coexistence of the broken fan texture of the SmC
mesophase and crystal phase displayed by compound 5b below 58 1C in
fast cooling, (C) the schlieren texture of the nematic mesophase displayed
by compound 9b below 129 1C, (D) the thread-like texture of the nematic
mesophase displayed by compound 9d below 120 1C.
shoulders in the DSC traces for these compounds could be mesophase was observed which was more persistent for 9d,
acquired. Identification of the nematic mesophase for com- whereas 9b displayed a nematic mesophase for a few seconds
pounds 9a–d was made from the observation of the typical through quick cooling from the isotropic state to under room
schlieren texture with two- and four-point brushes and a planar temperature. Under this circumstance the nematic mesophase
texture [Fig. 1(C) and (D)]. The low enthalpy values associated grew along with the crystal formation.
with the transition of the nematic mesophase to the isotropic
state corroborate this assignment (see Table 1).
Compounds 9a–d have a more pronounced rod-like or lath-like
structure than compounds 5a–d and 7a–d and, consequently, they
Compounds 5a–d are composed of two terminal alkyl chains – exhibit an enantiotropic mesophase at higher temperatures than
eight carbon atoms on the isoxazoline side and ten carbon atoms compounds 5a–d.
Table 1 Liquid-crystalline transition temperatures (1C),a and enthalpy and entropy values (kcal molÀ1) for the homologous series 5a–d, 7a–d, and 9a–d
Transition phase temperatures
Enthalpy, DH
DT,e 1C
Entropy, DS/Rg
Heating
Cooling
Melt f
I/phase/Cr
Entry
5a
5b
5c
5d
Cr 103 I
Cr 72 I
Cr 85 I
Cr 68 I
I 83 SmC 74 Cr
I 61 SmC 54 Cr
I 66 Crc
9
7
—
—
17.9
8.7
23.0
13.4
I/3.1 SmC/10.4 Cr
24.0
12.5
32.4
19.5
—
—
—
I 57 Crc
7a
7b
7c
7d
Cr 115 Ib
Cr 90 Ib
Cr 99 Ib
Cr 73 Ib
I 104 Cr
I 77 Cr
I 81 Cr
I 61 Cr
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
9a
9b
9c
9d
Cr 150 N 163 I
Cr 130 I
Cr 136 N 145 I
Cr 130 I
I 162 N 136 Cr
I Nd 120 Cr
I 144 N 126 Cr1 116 Cr2
I 122 N 112 Cr
13
—
9
13.0
10.0
9.3
I/0.4 N/13.2 Cr
I/—/10.5 Cr
I/0.9 N/0.8 Cr1 8.5 Cr2
I/0.5 N/11.5 Cr
15.4
12.4
11.5
15.8
10
12.7
a
Onset temperatures (Tonset). Data obtained from DSC (2nd cycle) with a rate of heating and cooling of 10 1C minÀ1; Cr = crystal phase, SmC =
b
c
smectic C mesophase, N = nematic mesophase, and I = isotropic liquid. Temperatures were obtained through optical microscopy. SmC or
d
e
N mesophases were observed only on fast cooling – samples were exposed to room temperature. Mesophase range upon cooling for compounds
f
5a–b and 9d and upon heating for 9a and 9c. For 9b the temperature range was too small to be measured. Enthalpy values (second cycle heating/
g
cooling) obtained in the Cr–I transition for the series 5a–d and the Cr–N transition for the series 9a–d. Values of melting entropy obtained in the
Cr–I transition for the series 5a–d and 9b/d and Cr–N for 9a/c. R = 8.31 J KÀ1 molÀ1
.
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properties analysis in this article suffers a drawback because
the nematic phase behavior is only stable for 9a and 9c.
However, upon heating or cooling the melting points exhibit
an even–odd effect as shown in Fig. 2 for the 5a–d and 9a–d
series. The odd-numbered members have the highest values (9a
and 9c) and the even-numbered members have the lowest values
(9b and 9d). The odd-numbered members displayed higher
values of melting point than the even-numbered members.
The dependence of the entropy changes associated with the
nematic crystal-isotropic transition on the number of methy-
lene units (n) present in the flexible spacer of non-dimeric LC
9a–d does not follow a regular tendency due to the nature of the
nematic phase observed in this series of liquid crystals (Fig. 2,
bottom). Anyway, it is possible to state that the differences
between odd and even membered dimers reflect, at least, the
difference in their average molecular shapes, which are governed
to a large extent by the parity of the flexible spacer.1
Fig. 2 Bar graphs for 5a–d and 9a–d. Plots of temperatures (1C) upon
heating (top) and entropy values for Cr - I for 5a–b and Cr - N for 9a–d
(DS/R) (bottom).
Transitions from a more ordered mesophase to the isotropic
state have higher entropy than transitions between a disordered
phase to the isotropic state. Less ordered phases, such as the
nematic phase, display lower entropy values and consequently,
The melting points for 5a–d, 7a–d and 9a–d displayed an when they undergo a transition to the isotropic liquid state are
odd–even effect as the length and parity of the flexible spacer less sensitive to structural parameters, such as variation in the
was varied. The values of the melting point alternate as the flexible spacer, due to the absence of short- or long-positional
length of the flexible spacer chain increase with odd-numbered order of the molecules.
members exhibiting higher values. The alternation is attenuated
Bar graphs are also presented for all of the compounds in
by increasing the spacer length. Values of the entropy associated this study, 5a–d and 9a–d, to get a better view of the odd–even
with the melting points of 5a–d displayed the odd–even effect effect. Fig. 2, at the top, represents the melting points for 5a–d
while those for 9a–d did not follow the tendency observed for 5a–d, and the transition temperatures upon heating for 9a–d. At the
probably due to the nature of the mesophase accompanying the bottom of Fig. 2, plots of the entropy values for Cr - I for 5a–b
crystal phase (Fig. 2). The melting processes followed the same and Cr - N for 9a–d (DS/R) are shown.
tendency as noted for the melting points of the compounds.
Theoretical calculations
For the homologous series 9a–d the odd–even effect based
on the mesomorphic behavior is also barely observed upon Theoretical calculations were performed to evaluate the influ-
cooling the samples from the isotropic phase. A stable nematic ence of the number of methylene carbon atoms in the flexible
mesophase was observed for 9a and 9c, however, for 9b and 9d spacer chain on the most stable conformation of the com-
the nematic mesophase was only visible during the cooling pounds of the series 5a–d. DFT calculations were carried out
cycle. 9b displayed the N mesophase upon cooling with a very in order to obtain the optimized geometries and conforma-
short period of time [Fig. 1(C)]. Upon cooling, all samples tional distributions for the molecules 5a–d. All of the calcula-
showed a nematic mesophase. The correlation between the tions were performed with the Gaussian 0328 computational
length and parity of the flexible spacer is not an easy task to package and the geometries were optimized under vacuum
achieve considering that upon heating 9b and 9d did not display using the B3LYP29 hybrid functional with the level 6-311G(d,p)
mesomorphic behavior. The influence of the flexible spacer for basis set. In order to compare the energies, geometries and
compounds 9a–d is more visible and more pronounced due dipoles moments with the ones obtained in the gas phase, the
to the more anisotropic shape of the molecules of this series structure of 5a was calculated using the PCM model for
than the series 5a–d. The naphthyl group connected by triple the solvent effect and water and cyclohexane as solvents with
bond to the benzoate moiety induces the formation of a stable high and low dielectric constants, respectively.30 Calculations
mesophase when compared to the series 5a–d. However, a con- were performed considering that the molecular shape has a
formational issue associated with the methylene chain flexible prominent effect in liquid crystal behavior and is primarily
spacer alters the molecular packing in the mesophase, and determined by the rotational process.31 The study here is
consequently induces a dependence of the intermolecular inter- guided by the fact that 5a–d and 9a–d containing a five-
action between the aromatic ring and the size of the flexible membered heterocyclic ring, named D2-isoxazolines, are con-
spacer. When we move along the carbon skeleton of the flexible nected to a benzoate moiety core by a flexible aliphatic spacer.
spacer, the aromatic rings rotate around the last carbon bond The spacers in liquid crystal dimers contribute to the molecular
to produce a set of distinct conformations for 9a/c and 9b/d (see anisotropy and exert control over the relative orientation of the
the Theoretical calculations section). Obviously, the transitional two mesogenic groups.
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The D2-isoxazolines present a non-traditional bent-shape as bonds O1–C5 and C6–O2 are oriented at an angle of 173.31 to
a consequence of the tetrahedral carbon atoms C4 and C5 in the minimize the electrostatic repulsion that occurs in the gauche
heterocyclic ring. Deviations from linearity as well as the non- rotamer, where the f1 angle is equal to 61.81.
coplanarity of the aryl groups bonded to C3 and C5 of isoxazoline
This conformational preference for 5a can be explained by a
have a pronounced influence on the mesophase formation as destabilizing interaction that occurs when the O1–C5 bond of
well as on the liquid crystal mesophase stability. To compensate the heterocyclic ring and C6–O2 of the ester group are in the
these geometrical constraints, the elongating molecular strategy gauche position. In the gauche arrangement (inset at the top of
is applied for. Previous results have shown that stable meso- Fig. 3), despite O1 being on the opposite side to the carbonyl
phase formation is reached when highly anisotropic groups are oxygen O3, O1 and O2 are closer than in the anti conformation.
linked to the C3 and C5 isoxazoline carbon atoms.24
To avoid this unfavorable electrostatic repulsion, 5a shows a
For mesogens composed of flexible molecules a large num- more bent-shape form where the O2 and O1 oxygen atoms have
ber of conformational states can be obtained due to being adopted an antiperiplanar disposition. However when the size of
arranged antiparallel or inclined for odd and even dimers and the flexible spacer increases through the addition of methylene
degrees of freedom related to the alkyl chains. In fact, calculations carbon atoms, this destabilization is no longer observed and
performed in this work showed a set of lowest energy conforma- now it is its preference for all trans conformations for the flexible
tions for 5a–d with energy barriers lower than 1.0 kcal molÀ1 in spacers which is prevailing.
the gas phase as well as in the condensed phase.
This preference can be explained by minimization of the
Fig. 3 represents the lowest energy conformations obtained destabilizing electrostatic interaction created by the gauche con-
for 5a–d. It is interesting to notice that the favored geometry for formations, thus avoiding the syn-pentane effect. A destabilizing
5a is slightly different to 5c and quite different to 5b and 5d syn-pentane interaction is created when a hydrocarbon chain is
because of the dihedral angle f1. For 5a, the dihedral angle f1 folded such that a g+ dihedral angle is followed by one gÀ along
is related to the O1–C5–C6–O2 atoms, whereas for 5b–d the the backbone.32 This effect, primarily of steric origin, results in
dihedral angle f1 is defined between the C4–C5–C6–C7 carbon conformers substantially higher in energy and, for this reason,
atoms. In this way, the antiperiplanar arrangement of the carbon linear hydrocarbon chains in alkanes adopt conformations that
skeleton of the flexible spacer is obtained with different atoms as the are free of syn-pentane interactions.33 This effect can be clearly
length of spacer increases. Upon increasing from 1 to 4 methylene seen in the even-numbered members 5b and 5d, and the odd-
units in the flexible spacer the preferred rotamers are those that numbered member 5c.
contain all of the methylene units in a trans conformation,
Statistically, there are a lot of rotamers that contribute to the
except for 5a. 5a prefers a conformation where the two polar equilibrium of 5a–d because of the great number of internal
Fig. 3 The most stable 5a–d molecular structures calculated at the B3LYP/6-311G(d,p) level, in the gas phase. For 5a, the gauche rotamer is shown in
the inset at the top and the anti rotamer (the most stable) is shown below. 5b–d all showed the trans conformation for the flexible spacer.
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degrees of freedom for these molecules. For 5a, considering the
The values of dihedral angles f2, defined by N1–C3 and
terminal alkyl chains are all trans, there are two conformations O5–C2, and f3, defined by O2–C10–O4–C1, ensure that the alkyl
with a small energy difference between them. One of them is chains and aromatic rings are in the same molecular plane on
more elongated (the gauche conformer) and the other, the most each side of the molecules. The aliphatic side chains all have
stable, is more bent-shaped and less elongated (the anti rota- trans conformations. The carbonyl oxygen, O3, is always on the
mer in Fig. 3 – 5a). The energy barrier to convert the most stable opposite side to the O1–N1 polar bond of the isoxazoline ring,
rotamer obtained for 5a into the gauche rotamer is 0.67 kcal molÀ1
.
except for 5a. In this structure, the carbonyl oxygen is on the
The molecular length of the gauche rotamer is 34.8 Å which is same side of the O1–N1 polar bond as a consequence of the
about 6 Å bigger than the anti rotamer. We can overlook the antiperiplanar arrangement.
contribution of all of the conformers to the equilibrium if we
The main feature of this conformational analysis is related
restrict the conformers to a discrete number such as the anti to changes in the dihedral angle related to the flexible spacer
and gauche conformers.34 In doing so the populations in the (f1). The f2 and f3 dihedral angles are about 180.01 for 5a, 5b,
equilibrium are roughly estimated to be in favor of the more and 5d and about 01 for 5c. Thus, the 5a, 5b, and 5d molecules
bent-shaped conformer (anti) than the less bent-shaped con- display an antiperiplanar orientation of the imine group (CQN)
former (gauche).
and O5–C2 bonds. The same antiperiplanar orientation is observed
The relative orientation of the two molecular planes depends for the acyl linkage (OC10–O2) and O4–C1 bonds. A synperiplanar
on the conformation of the flexible spacer (see Fig. S33, ESI†). orientation was observed for the dihedral angles f2 and f3 for 5c
The most stable conformer of 5a presents two slightly collinear only. This is reflected in a lowering of the dipole moment shown
and twisted planes composed of the aromatic rings, isoxazoline by this molecule.
ring and terminal alkyl chains. The estimated angle between
Theoretical calculations are a powerful tool that helps us to
these two planes is about 32.51 from the side view (see the ESI†). understand the observed dependence of the thermal behavior
5c has similar planes which are shifted by about 601 when on the length and the parity of the spacer for 5a–d and 9a–d,
compared to the planes of 5a. These two planes are now and even for 7a–d which are definitely not liquid crystals.35
separated by 21.51 from the side view. Individually, each rotamer The even–odd effect of the melting point, and less visibly the
of 5a has two independent planes which are organized in an enthalpy and entropy changes as the length of the spacer
edge-to-edge manner. From the top view it is possible to see that increases, can be explained by the shape of the molecules when
the planes are nearly flattened to the normal. For the even- considering the conformational issues of the flexible spacer.36
numbered members 5b and 5d these planes are bent. The Despite the small differences in the energy barriers for the
rotamers of 5b and 5d are organized in a face-to-face manner conformers observed in this study it is possible to assume that
with an estimated angle between the planes of 98.51 and 1021 for the conformers shown in Fig. 3 are at least responsible for the
5b and 5d, respectively. The top view of these planes shows that observation of the odd–even effects or partially responsible for
they are tilted to the normal.
the observed behavior. We are assuming that, for example,
Table 2 shows the data for the most stable conformations compounds that have one and three carbon atoms in the
of 5a–d. The dipole moments, molecular lengths and some flexible spacer (odd-numbered members) are more anisotropic
dihedral angles obtained for the anti conformation of 5a in the in their V-shape than the even-numbered members, and there-
gas phase, in water and cyclohexane are shown. For 5b–d the fore their enthalpy and entropy values are higher than the even-
data were collected only in the gas phase. Both the gas phase numbered members.37 Under these circumstances, the melting
and condensed phase calculations for 5a pointed to same points of the odd-numbered members are higher than the even-
conformation as being the most stable conformer, but when numbered members for all of the compounds listed in Table 1
solvent effects were taken into account the dipole moment as a consequence of the better packing for the odd-numbered
increases with the increase of the solvent dielectric constant.
members. We consider that the molecular packing for the odd-
numbered members in an intermolecular V-fashion is face to
face where the two molecular planes are nearly flattened to the
normal. Even-numbered members may be packed one by one in
V-fashion similar to a chevron structure (see Fig. S44, ESI†)
considering that the planes are tilted to normal. So the odd-
numbered members in this study can absorb more energy
without causing disintegration of the crystal lattice until the
melting point is reached.
The dependence of the transitional properties of LCs 5a–d
and 9a–d in relation to the flexible spacer is better seen when
observing the melting point. The irregular behavior of the meso-
phase makes the analysis more complex due to the enantiotropic
or monotropic behavior observed for these LCs (Table 1). The
enthalpy values also show a similar effect being more pronounced
for the 5a–d series. The entropy values follow the same tendency
Table 2 Dipole moments (D), dihedral angles (degrees) and molecular
lengths (Å) for 5a–d. For 5a, data in water and cyclohexane are also shown.
Molecular lengths are measured from carbon to carbon
Dihedral
Dihedral
Dihedral
Molecular
e
f
Entry m (Debye) angle, f1 (1) angle, f2 (1) angle, f3 (1) length (Å)
5a
7.7
8.5
9.8
5.4
3.0
5.0
173.3c
174.7c
177.0c
175.6d
176.7d
176.9d
179.4
176.2
175.1
179.8
0.660
176.5
176.5
176.9
176.8
178.0
0.86
178.6
28.96
29.70
29.80
33.25
36.93
37.45
5aa
5ab
5b
5c
5d
a
b
c
Cyclohexane. Water. Dihedral angle between the O1–C5–C6–O2
d
e
atoms. Dihedral angle between the C4–C5–C6–C7 atoms. Dihedral
angle N1–C3–O5–C2. Dihedral angle O2–C10–O4–C1.
f
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as observed for compounds 5a–d and 9a–d. However, the values character of the phase. The calculated molecular length (L) for
for 5a–d are higher due to transitions occuring from the compound 5a in the lower energetic conformation between the
ordered crystalline state to the directly disordered liquid state. external H atoms is 29.95 Å. Considering that the molecules
LCs which have an enantiotropic mesophase such as 9a or 9c adopt the most extended form of the aliphatic chains in the
have an enhanced anisotropic-shape which allows molecules to mesophase, the tilt angle (y) of the molecules in the SmC phase
pack more efficiently in the mesophase resulting in higher can be determined using the expression y = cosÀ1(d001/L) =
transition temperatures and entropy changes.2 It is possible to 19 degrees. Despite the fact that this value is relatively low,
associate to these LCs the synergy between the conformational the same has been previously obtained for another SmC
distribution and the orientational order of the nematic phase.33 compound.36 The inset in Fig. 4 shows a schematic representa-
The mesomorphic behavior found for 9a–d in this work is due tion of the molecular packing according to the optimized
to the presence of the long aromatic moiety terminally bonded molecular structure obtained from the theoretical analysis.
to the isoxazoline ring. However, in some cases such as 5a–d or Below the SmC–Cr transition temperature at 70 1C, additional
7a–d the molecular dimensions (length-to-breadth ratio) of the peaks appear at the high angle region between 15 and
aromatic moiety are not sufficient to overcome the non- 27 degrees, which are characteristic of the sample crystal-
coplanarity of the isoxazoline ring and the conformational lization. However, in the low angle region an intense peak is
issues of the flexible spacer. In this situation no mesophase still observed with an associated distance of 22.8 Å. It suggests
or unstable mesophase (i.e., monotropic behavior) appears.
that the smectic order is preserved, where the reduction of the
interlayer distance compared to the one in the SmC phase can
be associated to an increase of the tilt angle, to a reduction of
the molecular length, or both.
X-ray experiments
In Fig. 4 we present the X-ray results obtained for compound 5a
through varying the temperature of the sample. The spectra
were collected during cooling from the isotropic phase, where
a broad halo is observed in the isotropic phase at around 2y E
201, which according to Bragg’s law corresponds to a distance
of about 4.5 Å. This distance is related to the short length
correlations between neighboring molecules. It is worth empha-
sizing that the SmA and SmC phases are almost indistinguish-
able from X-ray experiments, where in both cases an intense
peak appears in the low angle region as a result of the X-ray
beam diffraction by the smectic layers. In this case, the assign-
ment is possible with additional techniques such as polarizing
optical microscopy.38
For compound 5b it was not possible to obtain a clear
spectrum of the SmC phase, where the crystallization and the
mesophase occur simultaneously for the sample, which was
observed right below the isotropic transition. This was due to
the narrow temperature region of the SmC phase, but it was
also an indication that the SmC phase is very unstable.
Conclusion
Two new series of non-symmetric liquid-crystalline dimers with
3,5-disubstituted 4,5-dihydroisoxazole benzoates and non sym-
metric isoxazoline derivatives, 5a–d and 9a–d, have been
synthesized. 5a and 5b displayed a monotropic SmC phase
whereas 9a–d displayed an enantiotropic nematic phase for 9a
and 9c. Monotropic behaviour was observed for 9b and 9d. The
mesomorphic behavior observed for 9a–d in this work is due to
the presence of the naphthyl aromatic moiety laterally bonded
to the isoxazoline ring. A dependence of the melting point
thermal behavior on the length and the parity of the spacer was
observed for 5a–d and 9a–d, and even to 7a–d which are not
definitely not liquid crystals. Transition temperatures, and
enthalpy and entropy values for odd-numbered members were
higher than those for even-numbered members, reflecting that
they are more shape-anisotropic. Density functional theory
calculations were performed to evaluate the conformational
issues of the flexible spacers and how these influence the
mesomorphic behaviour of the dimers. An antiperiplanar
arrangement was found for all of the carbon atoms of the
flexible spacer including the C4 carbon atom of the isoxazoline
ring, except for 5a. For 5a, an antiperiplanar arrangement was
observed when considering the oxygen atom O1 of the isoxazo-
line ring. Otherwise, a less stable gauche rotamer for 5a could
be observed taking into account the alignment of the carbon
atom of the flexible spacer with the heterocyclic C4 carbon
atom. Compounds that have one and three carbon atoms in the
By applying Bragg’s law to the position of the first intense
peak in the SmC phase at 80 1C it is possible to obtain an
interlayer spacing of 28.3 Å, as well as, the second ordered peak
at 14.6 Å. The ratio of d001/d002 E 2 confirms the smectic
Fig. 4 X-ray spectra of compound 5a for different temperatures. The
inset shows a schematic representation of the molecules in the layered
structure.
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Acknowledgements
This work was supported by MCT/CNPq, Fapergs-Edital PqG
002/2014. A. A. M. thanks MCT/CNPq for his postdoctoral
studies at the University of Hull, Hull, UK (grant no. 201116/
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Department of Chemistry, University of Hull, Hull, UK for the
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