EPR spectra of a mono- and a hetero-diradical in nematic and isotropic phases

Article abstract of DOI:10.1080/15421406.2012.676834

A hybrid hydrazine-nitroxide monoradical and the corresponding heterohydrazyl- nitroxide diradical were used as probes in 4-cyano-4- pentylbiphenyl liquid crystal. EPR data (hyperfine constants, rotational time, and anisotropy parameter values) are reported in the range of 25C-55C. It was shown by EPR spectrometry that the nitroxide moiety gives an anisotropic triplet line, while the hydrazyl moiety cannot be detected under these conditions. Taylor and Francis Group, LLC.

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EPR Spectra of a Mono- and a Hetero-
Diradical in Nematic and Isotropic Phases
G. Ionita a , I. Zarafu b , A. Paun b & P. Ionita a b
a Institute of Physical Chemistry , Bucharest , Romania
b Department of Organic Chemistry, Biochemistry and Catalysis ,
University of Bucharest , Bucharest , Romania
Published online: 30 Jul 2012.
To cite this article: G. Ionita , I. Zarafu , A. Paun & P. Ionita (2012) EPR Spectra of a Mono- and a
Hetero-Diradical in Nematic and Isotropic Phases, Molecular Crystals and Liquid Crystals, 562:1,
141-146, DOI: 10.1080/15421406.2012.676834
To link to this article: http://dx.doi.org/10.1080/15421406.2012.676834
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Mol. Cryst. Liq. Cryst., Vol. 562: pp. 141–146, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.676834
EPR Spectra of a Mono- and a Hetero-Diradical
in Nematic and Isotropic Phases
G. IONITA,1 I. ZARAFU,2 A. PAUN,2 AND P. IONITA1,2,∗
1Institute of Physical Chemistry, Bucharest, Romania
2Department of Organic Chemistry, Biochemistry and Catalysis, University of
Bucharest, Bucharest, Romania
A hybrid hydrazine–nitroxide monoradical and the corresponding heterohydrazyl–
nitroxide diradical were used as probes in 4-cyano-4ꢀ-pentylbiphenyl liquid crystal.
EPR data (hyperfine constants, rotational time, and anisotropy parameter values) are
reported in the range of 25◦C–55◦C. It was shown by EPR spectrometry that the nitrox-
ide moiety gives an anisotropic triplet line, while the hydrazyl moiety cannot be detected
under these conditions.
Keywords Diradical; EPR; hydrazyl; liquid crystal; synthesis
Introduction
Liquid crystals are molecular materials with very high impact in many fields of science; for
example, they have anisotropic properties and can be orientated under magnetic or electric
fields, making them extremely attractive compounds for different types of applications
(most known being the light modulating properties used in displays) [1–4].
Among these materials, 4-cyano-4ꢀ-pentylbiphenyl (5CB, Fig. 1) is a thermotropic
liquid crystal with a nematic phase close to room temperature between 21◦C and 35◦C [5].
Ordered structures like liquid crystals can be studied by electronic paramagnetic resonance
(EPR), if the system contains a paramagnetic probe; thus, the dynamics of stable free
radicals in such oriented systems have been reported previously [6–10].
Stable organic mono- and polyradicals have been also used as novel molecule-based
organic magnetic materials with multifunctionality; spin systems with magnetic properties
depending on external stimuli such as heat and light are also present in literature [9–12].
Di- and polyradicals were studied as well; for example, the EPR of ground state triplets
in liquid crystals solutions has been studied since 1960s [12]. Those studies showed that
it is possible to align planar molecules with their plans parallel to the magnetic field using
a nematic liquid crystals as solvent [1,2,13]. The use of a liquid crystalline spin-probe in
diamagnetic liquid crystals is also reported [14,15].
In a previous work, we synthesized a stable hybrid hydrazine–nitroxide monoradical 1
and the corresponding heterohydrazyl–nitroxide diradical 2 and characterized their structure
(Fig. 1) [16].
∗Address correspondence to P. Ionita, Department of Organic Chemistry, Biochemistry and
Catalysis, University of Bucharest, Bucharest 060021, Romania. Fax: +40213121147. E-mail:
pionita@icf.ro
141

142
G. Ionita et al.
O2N
O2N
CN
N
N
N
H
N O
N
N
H
N
H
N
O
O
O
O2N
O2N
5CB
1
2
Figure 1. Chemical structure of 5CB, monoradical 1 and diradical 2.
This paper deals with the improved synthesis of compounds 1 and 2 and their uses as
probes in 5CB. Variable temperature EPR spectroscopy has been used as main investigation
tool.
Results and Discussion
Synthesis
Although the synthesis of the desired compounds 1 and 2 was already reported by us [16], in
the present work we have improved the procedure for all steps starting from commercially
available materials (Fig. 2).
The reaction of the 1,1-diphenylhydrazine with the activated chloro-derivative
4-chloro-3,5-dinitrobenzoic acid led to the corresponding hydrazine, namely 4-(Nꢀ,Nꢀ-
diphenylhydrazine)-3,5-dinitrobenzoic acid, which is subsequently derivatized with
4-aminotempo in the presence of the coupling agent N-ethoxycarbonyl-2-ethoxy-1,2-
dihydroquinoline (EEDQ), yielding the nitroxide monoradical 1 in over 60% yields. In
the previous paper [16], the yield of the monoradical 1 in this step, using dicyclohexylcar-
bodiimide (DCC), reached 40%, while the use of carbonyldiimidazole (CDI) or the acyl
chloride led to 25% yield at most.
O2N
O2N
O2N
N
NH2
N
N
OH
Cl
OH
+
H
-HCl
O
O
O2N
H2N
N O
EEDQ
O2N
O2N
ox
N
N
N
H
N O
N
N
H
N
N
O
O
H
O
O2N
O2N
2
1
Figure 2. Synthesis of the stable free radicals 1 and 2.

EPR Spectra in Liquid Crystal
143
3280
3300
3320
3340
3360
3280
3300
3320
3340
3360
G
G
Figure 3. EPR spectra of the monoradical 1 (left) and the diradical 2 (right).
The oxidation of the monoradical 1 to the corresponding diradical 2 (Fig. 2) was
achieved in DCM using lead dioxide; the yield is almost quantitative.
EPR Spectroscopy of the Monoradical 1 and the Hetero-diradical 2
EPR spectra of these compounds showed the expected sharp triplet lines of a nitroxide
radical (Fig. 3). For the nitroxide 1, the triplet lines due to the coupling of the free electron
with the nitrogen atom have a hyperfine coupling constant (aN) of 1.6 mT (in DCM), while
in the case of the nitroxide–hydrazyl diradical 2, supplementary broad lines are noticed
between the main triplet lines due to the hydrazyl moiety (aN1 = 1.2 mT; aN2 = 0.5 mT).
EPR Spectroscopy of the Mono- and Diradical in 5CB
5CB contains a CN group, which is a strong dipolar head and may interact with compounds
1 and 2, which are aromatic polynitro-derivatives. The well-known bulkiness of the free
radical moieties (nitroxide or diphenylhydrazyl), which stabilize the free radicals centers,
are detrimental of the mesophase stability [1,2]. Our aim was to study how the 5CB liquid
crystal, used as solvent, affects the shape of the EPR spectra. Variable temperature EPR
spectra were recorded in the range of 25◦C–55◦C.
The motion of a spin label can be characterized quantitatively by the rotational cor-
relation time τ, which is a reciprocal of the frequency of rotation (Eq. 1). In the same
way, the relative intensities of the EPR triplet lines can also provide information about the
anisotropy of the radical motion. Thus, the values obtained from the EPR spectra were the
hyperfine coupling constant of the nitroxide free radical (aN), the h−1, h0, and h values,
+1
represented by the heights of the low field (Fig. 4), middle and high field lines of the 14N
hyperfine components, and the anisotropy parameter ε values (Eq. 2) [17]
ꢁ
ꢀ
ꢁ
ꢂ
h0
h0
τ = 6.5ꢀH
+
− 2 10−10
,
(1)
h
h
−1
+1

144
G. Ionita et al.
55 oC
45 oC
35 oC
25 oC
monoradical
diradical
Figure 4. EPR spectra of the monoradical 1 and the diradical 2 in 5CB at selected temperatures.
ꢃ
h0
− 1
h
+1
ε = ꢃ
.
(2)
h0
− 1
h
−1
At the first sight, both types of spectra look very similar. The first important observation
is that the broad lines showed by the diradical 2 in DCM (Fig. 3) are not visible, while
the second important observation is that for the EPR spectra of the free radicals 1 and 2
recorded in 5CB the intensities and the broadness of triplet lines changed dramatically
(Fig. 4). These facts are due to the change in the motion of the free radical moiety from fast
in DCM to slow in 5CB (liquid crystal).
Tables 1 and 2 show the earlier enumerated values (hyperfine coupling constant, heights
and widths of the triplet lines, τ and ε).
As a general rule, in both cases, the aN values and the τ values are shrinking with the
temperature (with about 1 G for 30◦C). These may be explained by the reduced viscosity
of the liquid crystal at higher temperature, meaning that the free radical moiety is rotating
with a faster frequency. The ε values are always negative, meaning that the line heights
are decreasing monotonically with increasing field (easily noticed in Fig. 4). These values

EPR Spectra in Liquid Crystal
145
Table 1. EPR parameters at selected temperatures recorded for compound 1 in 5CB
T (◦C)
aN (G)
h
−1
(a.u.)
h0 (a.u.)
h
+1
(a.u.)
τ (s × 1010)
ε
25
30
35
40
45
50
55
16.33
16.10
16.01
15.92
15.82
15.73
15.56
47 (3.36)
52 (2.75)
66 (2.53)
83 (2.43)
110 (2.33)
130 (2.20)
184 (2.05)
36 (4.12)
40 (3.72)
50 (3.38)
61 (2.95)
75 (2.55)
100 (2.43)
122 (2.32)
4 (6.34)
5 (6.11)
6 (5.78)
10 (5.25)
14 (5.01)
22 (4.41)
30 (3.63)
50.21
41.22
38.57
25.44
18.88
15.93
12.53
−16.00
−14.86
−14.50
−10.27
−7.50
−9.20
−5.49
are related to the anisotropy of rotation. Thus, if the long molecular axis is parallel to the
N O bond of the nitroxide radical, ε becomes increasingly negative with the increased
anisotropy of rotation [17]. Therefore, for the ε values compiled in Table 1, the variation is
easily explained (for the monoradical 1). A more confusing case noticed in Table 2 (for the
diradical 2), where those values do not follow a pattern. For the diradical 2, the molecular
structure determined by semiempirical calculations indicates a significant torsion angle of
76◦ (between the dinitrobenzene ring and the N–O bond), suggesting that the benzene ring
is nearly coplanar with the nitroxide bond and that the interspin distance is about 1 nm [16].
As a conclusion, it was shown that 5CB liquid crystal dramatically affects the EPR
spectra of a mono- and a diradical, and, moreover, the hydrazyl moiety cannot be detected
under these conditions in the EPR spectra.
Experimental
Chemicals and materials were purchased from Sigma-Aldrich. EPR spectra were recorded
either on a Jeol JES FA100 (at ambient temperature in DCM) or a Bruker ESP 300
spectrometer (at variable temperature) in 5CB (concentration about 10−4 M).
Synthesis of the stable radicals 1 and 2 was performed by modifying the previously
reported procedure [16]; thus, 368 mg 1,1-diphenylhydrazine (2 mmol) were refluxed in
chloroform for 2 h with 245 mg 4-chloro-3,5-dinitrobenzoic acid (1 mmol), filtered off,
and the solvent removed. Purification of the product was achieved by triturating with
hexane. In the next step, 40 mg of the acid (0.1 mmol) dissolved in DCM was reacted with
20 mg 4-aminotempo (0.12 mmol) in the presence 40 mg of EEDQ (0.16 mmol). After
Table 2. EPR parameters at selected temperatures recorded for compound 2 in 5CB
T (◦C)
aN (G)
h
−1
(a.u.)
h0 (a.u.)
h
+1
(a.u.)
τ (s × 1010)
ε
25
30
35
40
45
50
55
16.49
16.23
15.76
15.66
15.58
15.50
15.46
35 (2.86)
55 (2.55)
70 (2.31)
100 (2.11)
220 (2.00)
150 (1.97)
170 (1.82)
25 (3.86)
36 (3.60)
52 (2.66)
70 (2.32)
112 (2.28)
115 (2.11)
143 (2.00)
5 (7.00)
6 (6.07)
26.97
29.46
19.74
12.18
7.42
−7.93
−7.63
−8.84
−5.92
−2.74
−5.56
−9.02
10 (4.55)
18 (4.40)
35 (4.21)
40 (3.22)
50 (2.83)
7.81
8.65

146
G. Ionita et al.
2 days, the mixture was extracted with an aqueous solution of hydrochloric acid (1 M),
saturated aqueous sodium hydrogen carbonate, and water. After solvent removal, the solid
was purified on preparative TLC plates of silica gel, using ethyl acetate as eluent, yielding
the monoradical 1 as pure red–brown solid. Oxidation of 1 in DCM with lead dioxide
affords 2 in almost quantitative yields. All analytical data correspond with the literature
values [16].
Acknowledgments
This work was supported by a grant of the Romanian National Authority for Scientific
Research, CNCS–UEFISCDI, project number PN-II-ID-PCE-2011-3-0408.
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