Novel biphenyl-substituted 1,2,4-oxadiazole ferroelectric liquid crystals: Synthesis and characterization

Article abstract of DOI:10.3762/bjoc.11.26

Two novel series of unsymmetrically substituted 1,2,4-oxadiazole viz., R.Ox.CC_n compounds are synthesized and characterized. An optically active, (S)-(+)-methyl 3-hydroxy-2-methylpropionate is used to introduce a chiral center in the molecule.

Full text of DOI:10.3762/bjoc.11.26

Novel biphenyl-substituted 1,2,4-oxadiazole ferroelectric
liquid crystals: synthesis and characterization
Mahabaleshwara Subrao1,2, Dakshina Murthy Potukuchi3,
Girish Sharada Ramachandra1, Poornima Bhagavath1, Sangeetha G. Bhat1
and Srinivasulu Maddasani*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 233–241.
1Department of Chemistry, Manipal Institute of Technology, Manipal
doi:10.3762/bjoc.11.26
University, Manipal - 576 104, India, 2Syngene International Ltd.,
Biocon Park, Bommasandra, Bangalore - 560 099, India and
Received: 10 October 2014
Accepted: 26 January 2015
Published: 11 February 2015
3
Department of Physics, University College of Engineering,
Jawaharlal Nehru Technological University: Kakinada, Kakinada - 533
03, India
0
Associate Editor: P. J. Skabara
Email:
Srinivasulu Maddasani* - s.maddasani@manipal.edu
© 2015 Subrao et al; licensee Beilstein-Institut.
License and terms: see end of document.
*
Corresponding author
Keywords:
,5-disubstituted-1,2,4-oxadiazoles; ferroelectric switching;
3
mesomorphism; optical textures; SmC* phase; spontaneous
polarization; Suzuki coupling
Abstract
Two novel series of unsymmetrically substituted 1,2,4-oxadiazole viz., R.Ox.C*Cn compounds are synthesized and characterized.
An optically active, (S)-(+)-methyl 3-hydroxy-2-methylpropionate is used to introduce a chiral center in the molecule. A biphenyl
moiety prepared by Suzuki coupling reaction is directly attached to the oxadiazole core at C-5 position. Investigations for the phase
behavior revealed that the series with a benzyl group on one end of the oxadiazole core exhibits an 1D orthogonal smectic-A phase
while the second series with dodecyl flexible end chain shows orthogonal smectic-A and tilted chiral smectic-C (SmC*) phases
over a wide range of temperatures. The smectic-C phase exhibits ferroelectric (FE) polarization switching. The mesomorphic
thermal stabilities of these compounds are discussed in the domain of the symmetry and the flexibility of the alkyloxy end chain
length attached to the chiral center.
Introduction
The 1,2,4-oxadiazole derivatives are prevalently reported com- decades in new drug invention programs. In recent years, these
pounds with promising biological [1-5] and physiological [6-8] compounds are also exploited for various technical applications
activity such as antiinflammatory, antibacterial, antimicrobial, [10-15] due to their electroluminescent, non-linear optical, elec-
antifungal, anticancer, anticonvulsant, growth hormone secreto- tron transport and liquid crystalline (LC) properties. Torgova et.
gogues, antispasmodics, antithrombotic, etc. Thus these com- al., have reported [16-19] 3,5-disubstituted-1,2,4-oxadiazoles
pounds received [9] considerable attention for the last two with LC phase behavior for the first time in the literature.
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Beilstein J. Org. Chem. 2015, 11, 233–241.
Subsequently, the derivatives of 1,2,4-oxadiazoles with under different reaction conditions. Treatment of benzyl
different configurations have been reported [11,20-22] 4-cyanobenzoate (2a) with hydroxylamine hydrochloride
which exhibited the thermotropic liquid crystalline yielded the corresponding benzyl 4-(N'-hydroxycarbamimido-
nematic (N), smectic and polar smectic (bent or banana) phase yl)benzoate (3a). This amidoxime (3a) is converted into benzyl
structures.
4-(5-(4-bromophenyl)-1,2,4-oxadiazole-3-yl)benzoate (5a) by
treating it with 4-bromobenzoyl chloride in the presence of
Due to the presence of three heteroatoms in the ring structure of pyridine. Oxadiazole 5a is borylated under Miyaura coupling
the oxadiazole, it possesses a high dipole moment [23,24] and conditions [31-34] with bis(pinacolato)diborane (pin2b2) to give
polarisability, which results for greater optical anisotropy and benzyl 4-(5-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
birefringence of the material. It is well established that the yl)phenyl)-1,2,4-oxadiazol-3-yl)benzoate (6a), which facili-
chiral center in a molecule renders it to exhibit ferroelectric tated the C–C bond formation of a biphenyl group. The same
(
FE) behavior due to its configuration for transverse dipole methodology is adopted to get the product 6b which contains a
moment (μt). Meyer et al. [25] demonstrated the FE nature in dodecyl chain instead of the benzyl moiety as in 6a. The
tilted smectic-C (SmC) phase of LC materials for the first time hydroxy group of the chiral ingredient, i.e., (S)-(+)-methyl
and then proved that the FELCs are potential candidates for EO 3-hydroxy-2-methylpropanoate (7) is protected by a benzyl
display devices owing to their large birefringence, fast response group by treating it with benzyl 2,2,2-trichloroacetimidate.
[
26] and large viewing angles. The ferroelectric materials pos- Hydrolysis of product 8 gives the carboxylic acid 9 which on
sess [27,28] spontaneous polarization due to the reduced esterification with different long chain alcohols gives various
symmetry of C2v caused by molecular polarity and thus make esters 10p–10s. The esters containing the benzyl ether group are
the phase as a polar phase. Thus, an oxadiazole derivative with deprotected to give the corresponding alcohols (11p–11s). Then
a chiral center is tantamount to exhibit FE properties.
they are treated with 4-bromobenzoic acid to give products
2p–12s with two ester groups and a chiral center. This bromo-
1
In order to explore these materials in devices, it is necessary substituted chiral molecule is treated with boronate esters 6a/6b
that they have a low melting point (at ambient temperatures) under Suzuki coupling conditions to get the final products with
and exhibit the LC phases over a wide range of temperatures. It a biphenyl moiety directly attached to the oxadiazole core at
is found in the literature [29] that the unsymmetrically substi- C-5 position, 13ap–13as (Ph.Ox.C*Cn) / 13bp–13bs
tuted benzene derivatives with a biphenyl moiety possess low (C12.Ox.C*Cn). The detailed procedures involved in various
melting points. Hence, presently we made a methodical attempt steps of the synthesis are explained in Supporting Information
to synthesize and characterize unsymmetrically substituted File 1. The final oxadiazole products are characterised by spec-
1
,2,4-oxadiazoles with a chiral substituted biphenyl moiety at troscopic techniques. The peaks at δC = 175.29 and 168.38 ppm
C-5 position of the ring and two different moieties at C-3 pos- in the 13C NMR spectrum confirm the formation of the 1,2,4-
ition of the oxadiazole core, which are exhibiting the FELC oxadiazole ring without effecting [35-37] the ester group
phase structures.
(δC = 165.76 ppm), attached to the phenyl ring under these
conditions.
Results and Discussion
Synthesis and characterization
Phase characterization
The synthesis of 1,2,4-oxadiazoles involves [30] a single stage The mesomorphism exhibited by the compounds is character-
dehydration of O-acylated amidoximes via a reaction between ized by a polarizing optical microscope (POM) equipped with a
amidoximes and derivatives of carbonyl compounds (esters, hot stage. The sample is sandwiched between a glass plate and a
amides, acids, acid chlorides, aldehydes, etc.). Otherwise, the cover slip and is subjected to heating followed by cooling scans.
reaction is based on the 1,3-dipolar cycloaddition of N-oxides to The observed textural changes through POM under crossed
azomethines, nitriles and iminoesters. In the present study, we polarizers are simultaneously recorded. On heating the sample
synthesized 1,2,4-oxadiazoles through amidoxime intermedi- under crossed polarizers, the solid sample is melted and a focal
ates with substitutions at C-3 and C-5 positions. A moiety with conic fan texture is observed. On further heating the focal conic
a chiral center is attached to the phenyl ring (at C-5 position) of fan texture disappeared and transformed into an isotropic liquid.
oxadiazole in the molecular structure. The synthetic route for This observation is noticed for both the 13a and 13b compound
the preparation of 3,5-disubstituted-1,2,4-oxadiazoles presently series. The phase observed both in heating and cooling cycles
adopted is presented in Scheme 1.
are called enantiotropic while that observed only in cooling is
termed monotropic. On cooling the isotropic liquid of the
4
-Cyanobenzoic acid is converted into its ester by treating with sample, batonnets are formed, which are found to grow and
two different moieties, i.e., benzyl bromide and 1-dodecanol coalesce to form a focal conic fan texture. It is also observed
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Beilstein J. Org. Chem. 2015, 11, 233–241.
Scheme 1: Synthetic route for the preparation of R.Ox.C*Cn compounds. (i) PhCH2Br, K2CO3, DMF/1-dodecanol, DCC, DMAP, DCM,
ii) NH2OH·HCl, NaOH, EtOH, 70 °C, (iii) pyridine, 80 °C, (iv) Pin2B2, KOAc, Pd(dppf)Cl2, dioxane, 110 °C, (v) benzyl 2,2,2-trichloroacetimidate,
(
CF3SO3H, n-hexane, rt, (vi) LiOH, MeOH, H2O, (vii) alkyl alcohol, DCC, DMAP, DCM, (viii) Pd/C, EtOAc, H2 (1 atm), (ix) 4-bromobenzoic acid, DCC,
DMAP, DCM, (x) Pd(PPh3)2, Na2CO3, 1,2-DME, H2O, MW, 120 °C, 20 min.
235

Beilstein J. Org. Chem. 2015, 11, 233–241.
that the sample exhibits a pseudo-isotropic texture in the that when the tilted phase is subjected to an input triangular ac
homeotropic regions simultaneously. This texture is similar to wave (20 Hz, 10 Vp-p bias field) (Figure 2), the smectic
the texture reported [38] for the calamitic SmA phase of N-(p-n- domains are started to switch and the output wave is found to
tridecyloxybenzylidene)-p-n-butylaniline. This confirms that develop a peak. The area under the peak is found to increase
the phase is an orthogonal smectic-A (SmA) phase. The optical with decreasing the temperature. This observation reflects on
texture of the SmA phase exhibited by 13ar (Ph.Ox.C*C10) is the temperature-dependent spontaneous polarisation (PS) in the
given in Figure 1a as a representative of the series. Similar tilted SmC* phase. The maximum spontaneous polarisation of
textural changes are observed in other compounds of the 13a about 65 nC/cm2 and 100 nC/cm2 are observed in 13bp
(
Ph.Ox.C*Cn) and the 13b (C12.Ox.C*Cn) series. The com- (C12.Ox.C*C10) and 13br, respectively. The temperature depen-
pounds of the 13a (Ph.Ox.C*Cn) series exhibiting a SmA phase dent PS of 13bp and 13br are given in Figure 3 as representa-
are found to get transformed into crystal phase upon further tives of the series. Hence, the tilted phase is confirmed as chiral
cooling. Since the SmA phase is observed in both heating and SmC phase, i.e., SmC*. The optical microscopic texture of
cooling cycles, it is an enantiotropic in both the 13a and 13b SmC* exhibited by 13br (C12.Ox.C*C10) at 65 °C is given in
series. But, in the compounds of the 13b (C12.Ox.C*Cn) series Figure 1b as a representative case of the series. It is noticed that
during heating, the sample is melted and exhibiting focal conic the compounds are transformed into a crystalline phase on
fan texture. On further heating it is transformed into an isotropic further cooling from their SmC* phase. All the compounds in
liquid. During cooling, the isotropic phase is found to trans- the 13b (C12.Ox.C*Cn) series are found to exhibit a similar
form into the SmA phase. Interestingly, the focal conic fan sequence of phase transitions, i.e., Iso. – SmA – SmC* – Cryst.
texture of the SmA phase is found to be transformed into a Hence, the LC phase variance exhibited by the 13a and 13b
broken focal conic fan texture on further cooling and the areas series is confirmed as SmA and SmA, SmC*, respectively.
with pseudo-isotropic texture (in homeotropically configured
regions) are found to develop schlieren threads. This simulta- The phase transition studies of all the compounds, i.e., the 13a
neous observation of threads and broken focal conic fan texture (PhOx.C*Cn) and the 13b (C12.Ox.C*Cn) series are carried out
infers [39] that the LC phase is a tilted smectic phase. This by a Shimadzu (DSC-60) differential scanning calorimeter
texture is found to be similar to the texture reported [40] for the (DSC). The heating and cooling rates applied for the thermal
polar smectic-C (SmC*) phase of octyl 4-(4-(3-(4'-(undec-10- studies are 10 °C/min. The thermograms of 13ap and 13bp are
en-1-ylcarbonyloxy)biphenyl-4-carbonyloxy)benzoyloxy)ben- given in Figure 4 as representatives of the series. Compound
zoyloxy)benzoate. The same textural observations are noticed in 13ap in heating cycle shows a sharp peak at 105.7 °C with a
all the compounds of the 13b series. The broken focal conic fan shoulder at 107.8 °C (i.e., not well resolved) corresponding to
texture is observed only in cooling cycle and hence it is a melting and isotropic liquid transitions, respectively. But the
monotropic in this 13b series of compounds. It is also noticed peaks are well resolved in cooling cycle.
Figure 1: (a) Focal conic fan texture exhibited by 13ar (Ph.Ox.C*C10) at 120 °C. (b) Broken focal conic fan texture exhibited by 13br (C12.Ox.C*C10)
at 65 °C.
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Beilstein J. Org. Chem. 2015, 11, 233–241.
Figure 2: Polarization profile of 13br (C12.Ox.C*C10) compound at 65 °C.
In case of 13bp, the peaks at 76.1 °C and 131.4 °C are due to
melting and isotropic liquid transitions, respectively. The SmC*
phase is not noticed in POM studies during the heating cycle,
but the characteristic phase textural changes are noticed in the
cooling cycle, i.e., it is monotropic in nature. This is observed in
other compounds of the 13b series too. From the POM observa-
tions, it is expected that compound 13bp should show a peak
corresponding to SmA – SmC* during the cooling cycle in
thermal studies along with the Iso. – SmA and crystallisation
peaks. But, the SmA – SmC* transition is noticed as a very
small peak (not a prominent) with a very low enthalpy change
value. In anticipation of the evolving SmA – SmC* transition,
the sample is subjected to heating and cooling at low scan rates,
i.e., at 5 °C/min. Even at this low scan rate, the SmA – SmC*
transition is not evolved clearly in the 13b series. It is expected
that the SmA – SmC* transition is of a weak first order or
second order transition due to the changeover of the orthogonal
to tilted phase structure with less tilt (about the normal). The
minimal ∆H values noticed can be attributed to the second order
phase transition in the 13b series. The thermograms obtained
during the thermal studies at 5 °C/min are given in Supporting
Information File 1. It is noticed that the mesomorphic thermal
range is almost the same in both cases. The data of transition
temperatures (TC), the enthalpy changes (∆H) across the transi-
tions and entropy changes (∆S) of 13a (Ph.Ox.C*Cn) and 13b
Figure 3: Temperature dependant spontaneous polarisation in 13bp
and 13br.
(
C12.Ox.C*Cn) are presented in Table 1 and Table 2, respective-
ly.
The phase diagrams are constructed with DSC cooling cycle
data of the 13a (Ph.Ox.C*Cn) and the 13b (C12.Ox.C*Cn) series
and are presented in Figure 5 and Figure 6, respectively. It is
realized that the compounds of the 13a (Ph.Ox.C*Cn) series
contain a rigid phenyl moiety on one side and a flexible alkyl
Figure 4: DSC thermograms of 13ap and 13bp.
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Beilstein J. Org. Chem. 2015, 11, 233–241.
Table 1: Phase transition temperatures (°C), enthalpy changes (kJ/mol) and entropy changes (J/mol/K) of the 13a (Ph.Ox.C*Cn) series.
Compound
Phase transition
Phase transition temperatures
ΔH
ΔS
(ΔΤ)LC
1
1
1
1
3ap
3aq
3ar
Cryst. – SmA
SmA – Iso.
105.7
107.8a
105.3
58.0
28.52b
1.88
75.3
4.97
48.6
Iso. – SmA
SmA – Cryst.
Cryst. – SmA
SmA – Iso.
16.09
47.3
45.3
47.0
51.7
106.4
108.5a
103.4
58.1
59.58b
3.87
157.04
10.28
88.01
52.57
3.18
Iso. – SmA
SmA – Cryst.
Cryst. – SmA
SmA – Iso.
29.14
19.55
1.20
98.9
104.6
109.0
62.0
Iso. – SmA
1.85
4.84
SmA – Cryst.
Cryst. – SmA
SmA – Iso.
17.32
22.67
2.39
51.70
61.59
6.31
3as
95.1
106.0
107.6
55.9
Iso. – SmA
2.43
6.38
SmA – Cryst.
11.77
35.78
aNot well resolved, bcombined enthalpy change.
Table 2: Phase transition temperatures (°C), enthalpy changes (kJ/mol) and entropy changes (J/mol/K) of the 13b (C12Ox.C*Cn) series.
Compound
Phase transition
Phase transition
temperatures
ΔH
ΔS
(ΔΤ)LC
(ΔΤ)orthog
(ΔΤ)tilted
13bp
13bq
13br
13bs
Cryst. – SmA
SmA – Iso.
76.1
39.42
2.34
112.9
5.8
131.3
128.2
83.6
Iso. – SmA
2.57
6.4
SmA – SmC*
SmC* – Cryst.
Cryst. – SmA
SmA – Iso.
0.34
0.95
78.1
116.5
6.2
51.7
25.37
40.70
2.51
76.5
74.7
72.6
89.2
44.6
49.3
49.5
62.0
31.9
25.4
23.1
27.2
76.4
133.7
128.7
79.4
Iso. – SmA
2.76
6.9
SmA – SmC*
SmC* – Cryst.
Cryst. – SmA
SmA – Iso.
0.28
0.79
36.4
160.0
6.1
54.0
11.92
56.31
2.44
79.0
128.2
127.2
77.7
Iso. – SmA
3.40
8.5
SmA – SmC*
SmC* – Cryst.
Cryst. – SmA
SmA – Iso.
0.54
1.5
54.6
30.82
48.06
2.50
94.1
135.9
6.1
80.7
137.1
137.0
75.0
Iso. – SmA
2.51
6.1
SmA – SmC*
SmC* – Cryst.
0.21
0.60
88.7
47.8
28.45
chain on the other side of the oxadiazole central core. They are changing with increasing the chain length (Figure 5). The
found to exhibit a 1D orthogonal SmA mesophase only. average mesomorphic thermal range of the compounds in this
Further, the mesomorphic thermal range is not appreciably series (13a) is found to be about 48 °C.
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Beilstein J. Org. Chem. 2015, 11, 233–241.
chains on both sides of the oxadiazole core, the molecules are
argued to favour [44-46] the orientational disorder, which leads
to the stabilization of tilt layered structures. The mesomorphic
thermal range of the SmC* phase is found to be less than the
SmA phase exhibited by both series. The average mesomorphic
thermal range of an orthogonal SmA phase is almost the same
in both series, i.e., (∆T)SmA is about 48 °C and 51 °C in the 13a
and the 13b series, respectively. The average mesomorphic
thermal range of the tilted phase (SmC*) exhibited by the 13b
(
C12.Ox.C*Cn) series is found to be about 27 °C.
The mesomorphic thermal range is found to be effected margin-
ally with increasing the end chain length in both series. The
mesomorphic thermal range is found to increase greatly in the
Figure 5: Phase diagram of the 13a (Ph.Ox.C*Cn) series.
1
3b series possessing a chiral center and an aliphatic flexible
end chain with 12 carbons (i.e., 89.2 °C) against the corres-
ponding member of the 13a series (51.7 °C). Both of the series
seem to follow the odd-even effect [47] regarding clearing
(
isotropic) temperatures to infer the impact of axial polarizabili-
ties. An overview of tilted and orthogonal LC phase abundance
exhibited by the 13a and 13b series (Table 1 and Table 2)
reveal that the later series with flexible end chain yielded
approximately 56% of tilted phase structures and is argued due
to the relative strength of orientational disorder in chiral LCs
with flexible end chain on both sides.
It is noticed that the isotropic temperatures are higher for the
1
3b (C12.Ox.C*Cn) series than the other phenyl substituted 13a
Figure 6: Phase diagram of the 13b (C12.Ox.C*Cn) series.
homologs. Higher clearing temperatures are argued due to the
chain geometry and close packing of the molecules in the 13b
On the other hand, the 13b (C12.Ox.C*Cn) series comprises of (C12.Ox.C*Cn) series. The compounds in the 13a (Ph.Ox.C*Cn)
the flexible alkyl end chains on both sides of the central oxadia- series consist of a rigid and bulky phenyl moiety on one end,
zole core. This series of compounds are found to exhibit both which tends to keep the molecules away due to the repulsive
orthogonal and tilted phases viz., SmA and SmC*. However, forces. Also, the crystallization temperatures (i.e., melting
the mesomorphic thermal ranges of the compounds are found to temperatures) of the 13a (Ph.Ox.C*Cn) series are found to be
be higher than the other series (13a) of the present study.
higher than those of the 13b (C12.Ox.C*Cn) series.
We can recall [41-43] that the mesomorphic thermal ranges and Conclusion
the stabilities of the compounds are affected by the polarity, Two novel series of ferroelectric liquid crystalline materials
polarizability and intermolecular interactions between the mole- with 1,2,4-oxadiazole central core are synthesized and charac-
cules. Comparison between the orthogonal mesomorphic terized. The phenyl moiety at the C-5 position of the oxadia-
thermal ranges of both of the series, the series of the com- zole core is converted into a biphenyl moiety by Suzuki
pounds with flexible chains on both sides, i.e., 13b are found to coupling reaction. Both the series of compounds have the same
exhibit a higher mesomorphic thermal range than the other molecular skeleton except the end group on one side of the
series, i.e., 13a with phenyl end group. The large mesomorphic central core. The series with the bulky phenyl end group on one
thermal range in the 13b (C12.Ox.C*Cn) series are due to the side of the oxadiazole core is found to exhibit a 1D orthogonal
higher number of methylene units in the alkyl end chain, which SmA mesophase only whereas the other series with flexible
results in the higher surface area and higher London dispersion alkyl end chain are found to exhibit chiral SmC phase along
forces between the molecules. Interestingly, a tilted chiral SmC with the SmA mesophase. The compounds with flexible end
phase is also noticed in the 13b (C12.Ox.C*Cn) series along chains on both sides (13b or C12.Ox.C*Cn) of the oxadiazole
with the 1D orthogonal SmA phase. Due to the presence of long core are found to exhibit a higher mesomorphic thermal range
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Beilstein J. Org. Chem. 2015, 11, 233–241.
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1
1
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Supporting Information File 1
1
Experimental and analytical data.
doi:10.1080/10587259908023785
[
http://www.beilstein-journals.org/bjoc/content/
2
0.Parra, M. L.; Hidalgo, P. I.; Elgueta, E. Y. Liq. Cryst. 2008, 35,
supplementary/1860-5397-11-26-S1.pdf]
823–832. doi:10.1080/02678290802211114
2
1.Parra, M. L.; Hidalgo, P. I.; Soto-Bustamante, E. A.; Barbera, J.;
Elgueta, E. Y.; Trujillo-Rojo, V. H. Liq. Cryst. 2008, 35, 1251–1262.
doi:10.1080/02678290802513790
Acknowledgements
2
2.Parra, M. L.; Saavedra, C. G.; Hidalgo, P. I.; Elgueta, E. Y. Liq. Cryst.
The authors are grateful to the management of the Manipal
2008, 35, 55–64. doi:10.1080/02678290701751434
University for providing infrastructural facilities.
2
3.Madsen, L. A.; Dingemans, T. J.; Nakata, M.; Samulski, E. T.
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