Synthesis and thermal behavior of new liquid crystals arylaldoxime esters

Article abstract of DOI:10.1590/S0103-50532012000500013

We report the synthesis of a series of liquid-crystalline materials based on arylaldoxime esters and the characterization of these materials by ¹H, ¹³C NMR, ATR/FT-IR spectroscopy and elemental analysis. The chemical stability and liquid-crystalline as well as photophysical properties of the compounds are described being dependent on the heating/cooling cycles. The changes in chemical stability and the liquid crystals properties of arylaldoxime esters were observed during these thermal cycles by IR analysis. All samples underwent a thermal degradation yielding the corresponding nitriles and carboxylic acid as evidenced by IR and ¹H NMR spectra. Due to the decomposition process, information about the mesomorphic behaviour is lost. The UV-Vis absorption spectra in solution display three absorption bands between 230 and 340 nm. The fluorescence spectra exhibit a broad and structureless emission band located at 430 nm. Ab initio calculations were performed to obtain information on the molecular structure and properties of the title compounds.

Full text of DOI:10.1590/S0103-50532012000500013

J. Braz. Chem. Soc., Vol. 23, No. 5, 880-888, 2012.
Printed in Brazil - ©2012 Sociedade Brasileira de Química
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Article
A
Synthesis and Thermal Behavior of New Liquid Crystals Arylaldoxime Esters
Aline Tavares,a Bárbara C. Arruda,a Elvis S. Boes,a Valter Stefani,a Hubert K. Stassen,a
Leandra F. Campo,a Ivan H. Bechtoldb and Aloir A. Merlo*,a
aInstituto de Química, Universidade Federal do Rio Grande do Sul,
Av. Bento Gonçalves, 9500, 91501-970 Porto Alegre-RS, Brazil
bDepartamento de Física, Universidade Federal de Santa Catarina,
88040-900 Florianópolis-SC, Brazil
Uma série de cristais líquidos derivados de ésteres arilaldoximas foi sintetizada e caracterizada
1
via espectroscopia de H, 13C RMN, ATR/IV-FT e análise elementar. A estabilidade química e
fotofísica desses compostos foram dependentes dos ciclos de aquecimento e resfriamento, com
a decomposição sendo induzida pelo calor. As mudanças nas propriedades durante os ciclos de
aquecimento foram acompanhadas através dos respectivos espectros de infravermelho. Todas as
amostras se decompõem termicamente, gerando as respectivas nitrilas e ácidos carboxílicos como
evidenciado pelos espectros de infravermelho e de 1H NMR.As propriedades mesomórficas foram
avaliadas apenas no primeiro ciclo de aquecimento, devido à decomposição térmica observada
após a primeira transição para o estado isotrópico. Espectros de absorção em solventes orgânicos
apresentam três bandas entre 230 e 340 nm, e uma banda de emissão, larga e sem estrutura, em
430 nm. Cálculos teóricos ab initio foram realizados para a obtenção de informações estruturais
dos ésteres arilaldoximas.
We report the synthesis of a series of liquid-crystalline materials based on arylaldoxime esters
and the characterization of these materials by 1H, 13C NMR,ATR/FT-IR spectroscopy and elemental
analysis. The chemical stability and liquid-crystalline as well as photophysical properties of the
compounds are described being dependent on the heating/cooling cycles. The changes in chemical
stability and the liquid crystals properties of arylaldoxime esters were observed during these thermal
cycles by IR analysis. All samples underwent a thermal degradation yielding the corresponding
nitriles and carboxylic acid as evidenced by IR and 1H NMR spectra. Due to the decomposition
process, information about the mesomorphic behaviour is lost. The UV-Vis absorption spectra in
solution display three absorption bands between 230 and 340 nm. The fluorescence spectra exhibit
a broad and structureless emission band located at 430 nm. Ab initio calculations were performed
to obtain information on the molecular structure and properties of the title compounds.
Keywords: arylaldoxime esters, liquid crystals, infra-red analysis, isoxazolines
Introduction
found in many practical applications.13 For example,
(E)- and (Z)-oximes were used to elucidate action
mechanisms of their dehydration catalyzed by liver
cytochromes P450.14 Bisaryloxime ethers were found
to be active inhibitors in vitro of plasma protein
transthyretin (TTR) which is responsible of amyloid
fibril formation.15 Perillartine is the oxime derived from
perillaldehyde, found in Perilla frutescens (Lamiaceae).
It is known as perilla sugar being 2000 times sweeter
than sucrose.16 Pralidoxime is another example of an
oxime, which is able to bind to organophosphatate-
inactivated acetylcholinesterase.17 Benzisoxazoles
are present in a large number of important products
Carbonyl compound derivatives possessing a carbon-
nitrogen double bond have been recognized as attractive
synthetic targets due to their potential application in
medical and material chemistry.
Schiff bases, which are the most popular class
of theses carbonyl derivatives, have been reported
as important materials in coordination chemistry,1,2
analytical chemistry3,4 and liquid crystal science.5-12
Oximes are less popular than Schiff bases, but they are
*e-mail: aloir@iq.ufrgs.br

Vol. 23, No. 5, 2012
Tavares et al.
881
with pharmacological properties and are synthesized
by [3+2] cycloaddition reaction of a benzyne with
a nitrile oxide.18 Isatin is a natural product found in
plants of genus Isatis. Many of its derivatives, which
are used in drug synthesis are obtained from precursors
derived from oximes.19 Oxime-derived, chloro-
bridged palladacycles represent very efficient catalyst
precursors for the Heck olefination of haloarenes.20
Recently, oxime carbonates were introduced as novel
reagents for the instalation of Fmoc and Alloc protecting
groups, free of side reactions.21
As part of our continuing interest in the synthesis of
new liquid crystals (LC) derived from 3,5-disubstituted
isoxazolines, from [3+2] 1,3-dipolar cycloaddition reaction
between aryl nitrile oxide and alkene,22 and others23 we
explored the synthetic potential of the arylaldoximes,
as a precursor of aryl nitrile oxide, in the preparation
of new LC materials. The initial strategy utilizes
arylethynylbenzaldehyde oxime, an advanced intermediate
in our research program involving the preparation of novel
isoxazoline liquid crystals,24 as a key component for the
synthesis of mesogenic arylaldoxime esters.
Results and Discussion
Synthesis
Scheme 1 describes the intermediates and the final
liquid crystals synthesized in this work.A two-step process
was applied to synthesize the aryloxime 4. The route begins
with a Sonogashira cross-coupling reaction29 of the terminal
acetylene 128 and commercial p-bromobenzaldehyde (2) in
dry triethylamine. The following step produces the oxime 4
from a reaction of aldehyde 3 under mild conditions in an
aqueous ethanol solution of hydroxylamine hydrochloride
and sodium acetate.
The homologous series of arylaldoxime esters 6a-d was
preparedinordertoinvestigatetheeffectofthebenzoylgroup
on liquid crystals and photophysical properties. In addition,
the benzoyl group linked to the oxime allows evaluating the
conformational aspects related with rotamers around the
N-O bond that are possible in these compounds (see the topic
Ab initio calculations). Thus, four compounds 6a-d were
synthesizedbyesterificationofoxime4with4-alkoxybenzoic
acids 5a-d25,28,30 using DCC/DMAP protocol.31
In the present article, we report the synthesis and the
physical chemical behaviour of a series of arylaldoxime
esters 6a-d and 8 derivatives as well as the intermediates 3
and 4. The liquid-crystalline properties,ATR/FT-IR spectra,
differential scanning calorimetry (DSC) X-ray experiments,
absorption and fluorescence spectra are reported.
Considering our recent results using isoxazoline as
molecular kits for liquid-crystalline materials22 we are
reporting also the synthesis and the LC behaviour of a
single LC containing the isoxazoline ring. To compare, we
selected the acid 7 as a chemical partner of the oxime 4.
The final compound 8 was then synthesized by applyng
the same reaction protocol used for compounds 6a-d.
Experimental
Liquid-crystalline behaviour
Instruments, techniques, spectrum data are available in
Supplementary Information. The experimental conditions
were adapted from literature.25-28
The LC properties were investigated by polarizing
optical microscopy (POM) and differential scanning
Y
H
H
C8H17O
C8H17O
1
3 Y = O
4 Y = NOH
O
N O
H
CO2H
OR
RO
5a-d
R = CnH2n+1
R = CnH2n+1
C8H17O
OC8H17
6a n = 7 b n = 8
c n = 9 d n = 10
N
O
N O
H
O
C8H17O
N
O
OH
7
O
C8H17O
8
Scheme 1. The chemical intermediates and the final liquid crystals 6a-d and 8.

882
Synthesis and Thermal Behavior of New Liquid Crystals Arylaldoxime Esters
J. Braz. Chem. Soc.
calorimetry (DSC). The transition temperatures and
enthalpics of compounds 3, 4, 6a-d and 8 are summarized
in Table 1. As expected, compounds 3 and 4 exhibited
the enantiotropic nematic mesophase. On cooling, 3
and 4, showed birefringent droplets followed by large
areas of planar orientation, schlieren texture which are
the characteristic features of the nematic phase. The
transformation of 3 into the oxime 4 indicates that the
presence of the C=NOH group in substitution of C=O
furnishes a stable mesophase probably due to the stronger
dipole of the oxime and the possibility of association by
intermolecular hydrogen bonds. This is also corroborated
by the larger enthalpy of 4 for the transition of the nematic
to the isotropic phase obtained from DSC traces.
heated at first time presented irreversible endothermic
transitions. For example, the selected peak at 155 °C, as
shown in Figure 1, is associated with the transition crystal
phase Cr2 to SmC mesophase, which disappeared after the
first heating. On cooling, the new and low intensity broad
peak that arises around 139-143 °C which is attributed
to unknown concentrations of starting oxime ester and
decomposition products, probably the corresponding
alkyloxybenzoic acid and the related nitrile.32
Table 1. Transition temperatures (°C)a and enthalpy values (kcal mol-1,
in brackets) for compounds 3, 4, 6a-d and 8
LC First heating cycle
T / oC (ΔH / (kcal mol-1))
Liquid-crystalline aldoximesters and ketoximesters
have been published first by Weissflog et al.32 They founded
that some of these oximesters are unstable under heating
decomposing above the clearing temperatures. Compounds
6a-d and 8 synthetized in this work display similar
thermal unstability when heated. Thus, when the samples
were subjected to the heating/cooling cycles, the DSC
profiles exhibited different behaviours. As an example,
the modification of the DSC traces in the heating/
cooling cycles of the samples 6c and 8 are represented
in Figure 1. The increased length-to-breadth ratio of
the arylnaphthylacetylene group bonded to the oxime
ester is not sufficient to maintain the thermal stability
of these new LC aryloxime esters. The data in Table 1
were collected only in the first cycle due to the chemical
decomposition observed above the clearing temperature.
It can be seen that original LC properties of these
compounds is vanished after the first heating cycle. The
thermally induced degradations were more intense after
the samples entered the isotropic phase during the first
heating run. The DSC traces show that all samples when
3
Cr 107 (5.5) N 151 (0.04)I
4
Cr 151 (4.1) N 176 (0.7) I
6a
6b
6c
6d
Cr1 96 (1.1) Cr2 139 (4.7) SmC 153 (0.1) N 175 (0.1) I
Cr1 99 (6.3) Cr2 133 (5.7) SmC 170 (0.06) N 173 (0.04) I
Cr1 102 (4.2) Cr2 155 (10.1) SmC 170 (0.07) N 184 (0.2) I
Cr1 78 (0.4) Cr2 108 (1.2) Cr3 135 (6.2) SmC 170 (0.04)
N 174 (0.1) I
8
Cr109 (2.4) SmXb 146 (0.7) I
aPeak temperature; scan rate: 10 °C min-1; Cr = crystal phase;
SmX = smectic X phase, SmC = smectic C phase and N = nematic
phase; bref. 33.
It is almost impossible to determine the mesophase
behavior after the first heating cycle. The compounds 6a-d
decompose thermally forming a mixture of unknown ratio
composition of the nitrile and 4-n-alkyloxybenzoic acid.
Such degradation products have already been characterized
treating aldoxime esters under basic conditions.34 The
following mesophase sequence is assigned to pure nitrile 10:
Cr 95 N 157 I (DSC)35 and acid 5a: Cr 101 SmC 107
N 145 I,36 respectively (see Supplementary Information
Figure 1. DSC thermograms of 6c and 8 on 1st heating/cooling stages at rate 10 oC min-1. The peak temperature is given.

Vol. 23, No. 5, 2012
Tavares et al.
883
for the synthesis). The degradation of these compounds
was also observed in the ATR/FT-IR analysis. Figure 2
describes four ATR/FT-IR spectra of the sample 6c under
four different conditions: (a) before heating (no thermal
history); (b) spectrum collected from the sample that
underwent many heating/cooling cycles, (c) spectrum
recorded for the sample that remained overnight at room
temperature; (d) ATR/FT-IR spectrum obtained from the
batches of crystals (20 mg) of 6c that were heated at 185 °C
for 15 min and cooled to room temperature. After thermal
treatment, 6c was dissolved again in CHCl3 and then the
solvent was evaporated. The pale yellow solid obtained was
washed with ethanol and dried. In (e) the spectrum of the
synthesized nitrile-LC 10 is presented.35
The set of the ATR/FT-IR spectrum shows clearly a
change in the region of 1600-1750 cm-1. In (a), 6c displays
a strong absorption at 1733 cm-1 which corresponds to the
carbonyl group stretching. However, this absorption peak
is shifted to lower frequency after heating/cooling cycles
in (b) and (c) in Figure 2. The main modification is the
appearance of a large peak at 1671 cm-1. In (d) we can see
that the 6c does not return to the original condition. The
absorption peak at 1671 cm-1 is accompanied by another
peak at 2223 cm-1 which is an indication of the C≡N group
resulting from the thermal decomposition.A broadening of
peak at the left side in (d) is seen associated with formation
of carboxylic group. The absorption band at 2207 cm-1 is
related to the C≡C group. The peak at 1090 cm-1 is assigned
to the N-O-C group. The N-O-C stretching vibration
appears in the recorded spectrum of the first heating (a) and
it is absent in the other spectra (b-d). The changes observed
in the peaks at 1733 cm-1 and 1090 cm-1 clearly indicate that
the decomposition is being induced continuously during the
cycles of heating and cooling. The decomposition is indeed
an elimination reaction induced by heat.
The nature of the mesophase of these esters observed
during the first heating was studied and characterized by
POM and X-ray diffraction. So, the sequence smectic C
and nematic was observed for oxime ester 6c. When the
samples were slowly heated, the schlieren texture appeared
showing singularity lines with four dark brushes in some
domains.At SmC → N transition, the samples become more
fluid and schlieren texture exhibit singularity lines with
two and four dark brushes and homeotropic texture-dark
region.37 The mesophase identification for 8 was critical
due to the decomposition of the samples when heated. We
tried to determine the nature of this mesophase combining
the X-ray data and POM analysis, but our attempts
failed. The texture of the mesophase of 8 during the first
heating is mosaic-like texture, (Figures S40 and S41 in
Supplementary Information)38 and it was named as SmX.
In this sense, LC 8 present the following phase sequence
taken from the first heating: crystal phase Cr → SmX → I.
The X-ray diffraction experiments were carried out with
compounds 6a-d. For compound 6a the spectrum recorded
during the first heating cycle at 150 °C in the SmC phase
contain a sharp peak in the low-angle region, arising from
the reflection of the X-ray beam on the smectic layers, and
its corresponding second order peak (Figure 3). The inter-
layer spacing was obtained by applying Bragg’s law to the
first maximum. The smectic order was confirmed by the
ratio of the value obtained for the first peak with respect
to the value obtained for the second order peak. It is well
known that this ratio must obey the relation 1:1/2:1/3:1/4…
in smectic phases.38 The obtained inter-layer spacing was
37.3 0.1Å, which is shorter than the calculated molecular
length considering the distances between the far atoms
with the addition of the van der Waals radii from hydrogen
atoms bonded to the first and last carbon atoms in the
chain (at level RHF/6-311G(d,p), see discussion below).
The calculated value for the fully extended all trans-
conformation is l = 43.1 Å. The reduction of about 5.8 Å
is consistent with the conformational disorder of the chains
in the mesophase and due to the molecules inclination in
the SmC phase.38 This observation is consistent with the
POM texture observed for 6a on the first heating which was
assigned as SmC phase. From the collected X-ray spectrum
of the compound 6a powder, before the first heating
cycle, we obtained also a smectic order with a spacing
layer of 39.3 0.1 Å, but the additional peaks around
2q = 20 degrees indicate a crystalline structure (Figure 3).
The compounds 6b-d presented similar behavior.
Figure 2. Portion of the ATR/FT-IR spectra: (a) original sample of
compound 6c without heating/cooling cycles; (b) spectrum obtained of 6c
after many heating/cooling cycles; (c) spectrum of the same sample 6c in
(b) after overnight at room temperature; (d) spectrum of 6c after thermal
and solvent treatment (see text for discussion); (e) compound 10. The
absorption band assigned to the N-O-C group appears at 1090 cm-1 in (a).

884
Synthesis and Thermal Behavior of New Liquid Crystals Arylaldoxime Esters
J. Braz. Chem. Soc.
7.4 kcal mol-1 (31.0 kJ mol-1). This is the barrier height for
the rotation of the dihedral angle C-N-O-C, corresponding
to the difference in the energies calculated for the cis and
trans conformer of this molecule.
The electrostatic analysis also provides us the dipole
momentum for this molecule which is 4.23 D (debyes) for
the trans conformer and 2.69 D for the cis conformer. The
dipole vector points along the shortest molecular axis or in
the direction orthogonal to the longest molecular axis. The
high energy calculated for this rotational barrier prevents
the rotation of the groups around the N-O bond at ordinary
or room temperatures and contributes to enhance even more
the structural rigidity of this molecule.
H
Figure 3. X-ray diffraction patterns of compound 6a.
Ab initio calculations
R
N O
R'
trans
O
Ab initio calculations were performed in order to obtain
information about the molecular structure and properties
such as relative energies of molecular conformations,
rotations barriers, molecular dimensions and electrostatic
properties. The calculations were carried out with the package
Gaussian 98, RevisionA.939 at the RHF/6-311G(d,p) level of
theory. We selected the LC compounds 6a as a representative
of this study to correlate the phase behavior and structural
features. We performed the geometry optimization of the
molecular structure and found a structure corresponding
to a point of minimum energy confirmed by the absence
of imaginary frequencies in the normal mode analysis
calculation. The molecular size, calculated for this optimized
structure as the distance between the hydrogen atoms located
at opposite ends of the alkyl chains, is 43.1 Å (Figure 4).
This is the length of the largest molecular axis. In addition
we have studied the barrier height for the conformers cis and
trans of this molecule corresponding to the rotation of the
dihedral angle C-N-O-C generated by the rotation around
the N-O bond. Therefore, we have optimized the molecule
with dihedral angles C-N-O-C of 0.0 and 180.0 degrees
corresponding to the cis and trans conformers respectively.
We have found that the cis conformer is not a stationary
point for this structure and we have performed the geometry
optimization of this conformer keeping the dihedral angle
C-N-O-C fixed at an angle of zero degrees while allowing
the optimization of all the other angles and bond lengths
in the structure of the molecule in this conformation. In
this case, we optimized the molecular structure with the
molecule constrained to the cis conformer for the dihedral
angle C-N-O-C.
7.4 kcal mol-1
cis
O
O
H
N
R'
R
Figure 4. Trans and cis conformation for 6a - the alkyloxy chains and
naphtylacetilenic group have been omitted for clarity.
The most stable conformation obtained theoretically
in this work is in agreement with previous work published
by Altinbas et al.40 describing the crystal structure of
the E-benzaldehyde O-benzoyloxime. They found that
the crystalline structure of this aromatic oxime is better
described by the trans conformation where the C-N-O-C
dihedral angle is approximately equal to 180.0 degrees.
The thermal behaviour of the series 6a-d displayed
distinct behavior to 8. These data suggested that the
non-colinearity of the two planes of the isoxazoline ring
(about 135.9 degrees)41 cause an influence on the stability of
the mesophase. Due to the introduction of isoxazoline ring,
the molecular symmetry is lowered changing the mesophase
nature found in this work. The isoxazolinic system disfavours
the interactions between the planar cores reducing the
intermolecular forces responsabile for packing arrangements
in the mesophase. It should be noted that the presence of
the naphthalene ring induces the formation of more stable
mesophase due to increased molecular interactions.42
Photophysical properties
Then we compared the energies calculated for the trans
and cis conformers and we have found the difference of
The photophysical properties of compounds 3, 4, 6c and
8 in dioxane, chloroform, ethanol and acetonitrile are listed

Vol. 23, No. 5, 2012
Tavares et al.
885
in Table 2, and their UV spectra are shown in Figure 5. The
spectra of all compounds clearly exhibit the two bands of
the naphthalene ring,43 a major band at short wavelengths
(220-260 nm), resulting from excitation along the major
axis of the molecule (the b-band), and a two-peak band
(p-band) between 250-300 nm. As has been observed by
Marsh et al.,43 the addition of a ethynyl group preserves the
major features of the absorption bands of naphthalene. On
the other hand, the long-wavelength band around 340 nm
can be attributed to the charge-transfer transition. In fact, a
third band in these systems is evidently different from the
spectra of naphthalene or ethynylnaphthalene43,44 indicating
that conjugation effects of the aryl group and the ethynyl-
naphthalene are pronounced.
Although the free rotation of the ethynyl group may
result in a steric effect that minimizes the conjugation
effect of these groups,45 the electronic interaction between
the electrons of the aryl substituents and the 2-ethynyl-
6-(octyloxy)-naphthalene ring plays an important role
in activating the p-conjugation.46 Thus, we assumed that
the ground-state absorption can be attributed to the local
excitation of the 2-ethynyl-6-(octyloxy)naphthalene
chromophore. The absorption peaks of compounds 3, 4,
6c and 8, show little dependence on the solvent polarity
which implies that the solvent stabilization of the ground
state species is not significant.
The fluorescence emission spectra of the compounds 3,
4, 6c and 8 (Figure S44) were recorded in the same solvents
(dioxane, chloroform, ethanol and acetonitrile) and in
the solid-state using an excitation wavelength at 343 nm.
Table 2 summarizes the emission maxima, Stokes’shift and
quantum yield (f) in the studied solvents and in the solid-
state. The results show that all compounds emitted violet-
blue light in solution and in the solid state. We observed
constant emission band profiles in the 290-350 nm range
of excitation wavelength. The Stokes shift observed is very
small (40-98 nm) suggesting similar geometries for the S0
and S1 states. All the fluorescence spectra are quite similar
and the peaks appear at 350-550 nm with a hypsochromic
shift produced in dioxane. The lem was shifted to longer
wavelength (ca. 40 nm) when the aldehyde is the bearing
group of the aryl ring. The spectra of the compounds in
solution lose the characteristic of naphthalene fluorescence
emission and show a broad and structureless emission
Figure 5. Absorption spectra recorded in different solvents. Compounds: (A) aldehyde 3, (B) oxime 4, (C) esters 6c and (D) 8.

886
Synthesis and Thermal Behavior of New Liquid Crystals Arylaldoxime Esters
J. Braz. Chem. Soc.
Table 2. Absorption and emission data of compounds 3, 4, 6c and 8 in different solvents and in the solid-state (S-s)
b
Entry
Solvent
Dioxane
CHCl3
MeCN
EtOH
S-sc
Absorption peaks / nm
243.5, 289.0, 343.5
246.5, 292.0, 349.5
242.0, 288.0, 343.4
237.0, 288.5, 342.0
403.8
lema / nm
384.4
440.6
441.6
423.6
466.8
380.2
398.0
397.6
417.0
446.0
398.8
418.4
412.8
434.6
436.6
405.6
412.2
409.0
418.8
432.2
Stokes’ shift / nm
40.9
f
3
0.20
0.11
0.17
91.1
93.7
98.2
63.0
-
4
Dioxane
CHCl3
MeCN
EtOH
S-sc
239.0, 288.5, 329.5
245.5, 290.5, 333.0
244.5, 290.0, 332.2
221.0, 287.5, 327.5
396.8
50.7
0.30
0.29
0.21
0.25
-
65.0
65.4
89.5
49.2
6c
Dioxane
CHCl3
MeCN
EtOH
S-sc
243.5, 293.5, 339.5
245.5, 296.0, 344.0
238.5, 284.0, 341.5
246.6, 294.0, 346.5
353.4
59.3
0.70
0.10
0.27
0.25
-
74.4
71.3
88.1
83.2
8
Dioxane
CHCl3
MeCN
EtOH
S-sc
242.0, 291.0, 340.0
245.0, 289.0, 346.5
246.5, 294.0, 346.5
285.0, 345.0
65.6
0.47
0.17
0.18
0.40
65.7
62.5
73.8
373.8
58.4
a
l
ex = 343 nm. bRelative to quinine sulfate in 0.1 mol L-1 H2SO4 as a standard (lex = 343 nm, f = 0.55). cSolid-state.
which is solvent dependent. In the solid state the spectra
are red-shifted and show some vibrational fine structure for
isoxazoline 8. The bands at 410 and 455 nm (Figure S44
(D)) are assigned as solid-state bands and belong to crystal
packing absent in solution.
LC and photophysical were evaluated. The heat-induced
decomposition of these arylaldoxime esters was observed
above the clearing temperature. The mesomorphic
behaviour was analysed from the first heating run, and
it is nearly impossible to determine the mesophase
type after first heating because the composition is
presumably changed during the acquisition of the data.
The thermal decomposition was also analysed using
IR data. The stability of these compounds was also
checked by measuring the decrease in absorption of the
O-benzoyloximes at 230-340 nm. For aryloxime esters
6a-d, the smectic C and nematic phase were observed
during the first heating. The ester 8 exhibits the SmX
mesophase. The heat-induced decomposition of these
compounds probably produces the correspondent nitriles
as evidenced by IR spectra. In this sense, compound 10
was prepared and compared with the mixture produced
during the heat-induced aging.
Quantum yields have been measured in all solvents
relative to quinine sulfate showing that efficiency is
comparable to this standard.47 As expected, the quantum
yields of the esters 6c and 8 were significantly larger than
the analogous aldehyde 3 and oxime 4. The f of ester
6c (70%) is very high in dioxane. The photostability of
these compounds was determined using the protocol
established by Cho et al.48 The control experiments
were performed using a CHCl3 solution of ester 6c. The
mixture was monitorated at 296 nm and 343 nm during
six months maintaning the sample at 0 °C in a amber
bottle. After this period, solution of 6c became unstable
as seen clearly by the changes in the UV-Vis absorption
spectra of these compound in different period of the time
(Figure S45) which indicates that side reactions such as
hydrolysis can occur. This photochemical behaviour was
found for all the esters 6a-d and 8.
Acknowledgments
This work was supported by MCT/CNPq Universal
n. 471194/2008-5, PROCAD-CAPES, INCT-Catálise.
B. C. A. is an undergraduate student and thanks to CNPq-
PIBIC-UFRGS for her fellowship. We acknowledge
FAPESC and Laboratório de Difração de Raios-X (LDRX-
DF/UFSC) for the X-ray diffraction experiments.
Conclusions
A novel series of aryl ethynylbenzaldehyde
O-benzoyloximes 6a-d and 8 was synthesized and their

Vol. 23, No. 5, 2012
Tavares et al.
887
Supplementary Information
16. Bassoli, A.; Borgonovo, G.; Caimi, S.; Scaglioni, L.;
Morini, G.; Moriello, A. S.; Di Marzo, V.; De Petrocellis, L.;
Bioorg. Med. Chem. 2009, 17, 1636; Gamba, D.; Pisoni, D. S.;
da Costa, J. S.; Petzhold, C. L.; Borges, A. C. A.; Ceschi,
M. A.; J. Braz. Chem. Soc. 2008, 19, 1270; de Almeida,
Q. A. R.; Pereira, M. L. O.; Coelho, R. B.; de Carvalho, E. M.;
Kaiser, C. R.; Jones Jr., J.; da Silva, F. M.; J. Braz. Chem.
Soc. 2008, 19, 894.
Supplementary information (spectra data and spectra
of synthesized compounds) is available free of charge at
http://jbcs.sbq.org.br as PDF file.
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Submitted: October 19, 2011
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Published online: April 17, 2012