Glass-forming organic radical compounds with cholesterol and benzylideneamine cores

Article abstract of DOI:10.1016/j.tetlet.2005.07.132

Several organic radical compounds based on TEMPO radical with cholesterol and benzylideneamine cores (3a-c) were prepared. The radical compounds were found to freeze into glassy state when cooled from their isotropic liquid state and characteristic heat-r

Full text of DOI:10.1016/j.tetlet.2005.07.132

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6701–6703
Glass-forming organic radical compounds with cholesterol and
benzylideneamine cores
Hidetoshi Kinoshita,a Masayuki Hata,a Ammathnadu S. Achalkumar,b
Channabasaveswar V. Yelamaggad,b Hiroki Akutsu,a Jun-ichi Yamadaa and
ShinÕichi Nakatsujia,*
aGraduate of Science, University of Hyogo, 3-2-1 Kouto, Kamigori, Hyogo 678-1297, Japan
bCentre for Liquid Crystal Research, Jalahalli, Bangalore 560013, India
Received 30 June 2005; revised 25 July 2005; accepted 26 July 2005
Available online 10 August 2005
Abstract—Several organic radical compounds based on TEMPO radical with cholesterol and benzylideneamine cores (3a–c) were
prepared. The radical compounds were found to freeze into glassy state when cooled from their isotropic liquid state and charac-
teristic heat-responsive magnetic properties were observed in the radicals due to the phase transitions.
Ó 2005 Elsevier Ltd. All rights reserved.
To the development of novel organic spin systems with
multi-property significant attention is paid in the field
of materials chemistry and among them spin systems
with photo-functionality have recently been explored
in this respect rather extensively.1 The preparation of
organic spin system with liquid crystalline2 and/or
heat-responsive property is also attractive because of
the possibility of the alternation of magnetic property
through the phase transition and we hence, prepared
so far several organic radical compounds like 1 or 2
and found significant changes of the magnetic properties
in a couple of compounds (1: R = CH3(CH2)6 and 2b)
owing to their phase transitions.3 In this letter, we wish
to report the preparation of glass-forming organic
radical compounds based on TEMPO (2,2,6,6-tetra-
methylpiperidinyl-1-oxy) with cholesterol and benzyl-
ideneamine cores (3a–c) (Chart 1) and the changes of
their intermolecular magnetic interactions through the
thermal phase transitions. Since glassy materials offer
some advantage in processing into films without grain
boundaries as often observed in crystalline materials,
they are thought to have potentials for advanced tech-
nology4 and, to our knowledge, no distinctly glass-form-
ing organic radical has so far been reported.
The preparation of the radical compounds 3a–c was car-
ried out as shown in Scheme 1. Cholesteryl bromoalk-
anoates (4a–c)5 prepared by treating commercial
optically pure cholesterol with bromoalkanoyl chlorides
were reacted with 4-hydroxybenzaldehyde under a mild
basic reaction condition to give the corresponding benz-
aldehyde derivatives with a cholesterol unit (5a–c) in 71–
77% yield. After several unsuccessful trials, presumably
because of the gel formation with the solvents used, the
aldehydes could be converted to the corresponding
Schiff bases 3a–c by heating with 4-amino-TEMPO in
benzene or toluene for prolonged time (2–4 days) with
the yields (after recrystallization from appropriate sol-
vents) shown in Scheme 1.6
The liquid crystal property of aldehyde derivatives 5a–c
was evidenced with the help of optical polarizing micro-
scopic and calorimetric studies. These compounds were
found to exhibit smectic A (SmA) and/or chiral nematic
(N*) phases.7 On the contrary, surprisingly, no meso-
morphic behavior could be discerned for the radical
compounds 3a–c. However, interestingly, these crystal-
line materials were found to form glassy films when
cooled from their isotropic liquid state that was ascer-
tained based on optical and calorimetric studies. Radical
compounds 3a and 3b form the glassy films (vitrifica-
tion) either by rapid (quenching at 5–10 °C) or by slow
cooling at a rate of 2–10 °C/min, whereas for 3c
vitrification was achieved only by rapid cooling from
Keywords: Organic radical; TEMPO radical; Magnetic property;
Cholesterol derivative; Mesogenic core.
*
Corresponding author. Fax: +81 791 58 0164; e-mail: nakatuji@
sci.u-hyogo.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.132

6702
H. Kinoshita et al. / Tetrahedron Letters 46 (2005) 6701–6703
O
R'
N
O
N O
NC
O
O
n
NC
N
O
1 : R = CH (CH ) , CH (CH ) O
2a : n = 10
2b : n = 11
2c : n = 12
3
2 6
3
2 9
R' = H, CH , CH (CH )
3
3
2 11
O
O
O
N
N
O
n
3a : n = 3
3b : n = 5
3c : n = 7
Chart 1.
O
O
H
O
(i)
O
Br
O
O
n
n
5a : n = 3 (71%)
5b : n = 5 (77%)
5c : n = 7 (72%)
4a : n = 3
4b : n = 5
4c : n = 7
O
(ii)
O
O
N
N O
n
3a : n = 3 (44%)
3b : n = 5 (41%)
3c : n = 7 (31%)
Scheme 1. Reagents and conditions: (i) anhyd K2CO3, DMF, 85 °C, 16 h; (ii) 4-aminoTEMPO, benzene, or toluene, reflux, 2–4 days.
its isotropic liquid state, which otherwise reverts to crys-
0.39
talline state when cooled slowly. In other words, these
compounds while cooling from their liquid state (melt-
ing point) fail to show the transition to crystal state,
instead, they freeze into glassy state that is characterized
by their superior optical quality (transparent) over a
large domain with no grain boundary and therefore, in
our opinion have potentials for advanced technology.4
This behavior is quite remarkable given the fact that
these systems are low molar mass magnetically active
compounds with glassy film forming capabilities, which
otherwise is ubiquitous among polymeric systems. The
lack of liquid crystalline nature in the radicals 3a–c
may be attributed to the existence of bulky tetra-methyl
groups around the spin center, being detrimental for the
formation of ordered molecular aggregates. Since appar-
ent phase transitions into the glassy states were observed
in the radicals, the temperature dependence of their
magnetic susceptibilities was measured from 2 to ca.
400 K (over their melting points) by using SQUID sus-
ceptometer. As a typical example, the vT–T profile for
3b is shown in Figure 1 and the data for heating/cooling
processes of the radicals 3a–c are summarized in Table
1.8 Thus, the vT values of the radical 3b deviate gradu-
ally from Curie–Weiss law increasing over 350 K, which
in turn decrease abruptly over the phase transition tem-
perature at around 388 K (115 °C). Although the reason
is not clear yet, antiferromagnetic interaction is appar-
ently intensified in the glassy state of this radical during
Heating
0.37
0.35
0.33
0.31
0.29
0.27
0.25
Cooling
0
100
200
T (K)
300
400
Figure 1. vT–T profile (heating/cooling) of the radical 3b between 2
and 400 K.
the cooling process. It is generally seen within the three
radicals that the Curie constants as well as Weiss tem-
peratures decrease in the cooling process after melting
to the glassy state (Table 1).8 Among them, weak ferro-
magnetic interaction observed in the heating process of
the compound 3a was found to turn to antiferromag-
netic one in the glassy state, while the antiferromagnetic
nature is preserved on the whole for the other two rad-
icals 3b,c with the longer paraffinic central spacer in
spite of the difference of their Curie constants and Weiss
temperatures. Thus, characteristic heat-responsive mag-

H. Kinoshita et al. / Tetrahedron Letters 46 (2005) 6701–6703
Table 1. Magnetic data, phase transition temperatures (°C) and enthalpies (J/g) of 3a–c
6703
Compds
Process
Magnetic interactiona
Cb/emu K molÀ1
hc/K
Phase transitiond
3a
3a
3b
3b
3c
3c
Heating
Cooling
Heating
Cooling
Heating
Cooling
Ferro
0.29
0.26
0.37
0.32
0.27
0.25
0.09
Cr 100.5 (25.1) Iso
Iso 56e Tgf
Antiferro
Antiferro
Antiferro
Antiferro
Antiferro
À0.27
À0.90
À0.37
À0.86
À0.40
Cr 115.5 (33.9) Iso
Iso 82e Tgf
Crg 107.3 (28.6) Iso
Iso 45.8 (0.5) Cr or Iso 5 Tgh
Cr = Crystal; Iso = isotropic phase; Tg = glassy state.
a Fitted by using the Curie–Weiss law.
b Curie constant.
c Weiss temperature.
d Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at a rate of 5 °C/min.
e Observed under microscope and too weak to get recognized in DSC.
f Exists till À60 °C in which the Iso state is freezed.
g An additional Cr–Cr transition was observed at 91.5 °C (3.3 J/g).
h Vitrification occurs upon rapid cooling.
3. (a) Nakatsuji, S.; Mizumoto, M.; Ikemoto, H.; Akutsu,
netic properties were observed in the radical compounds
due to the phase transitions.
H.; Yamada, J. Eur. J. Org. Chem. 2002, 1869; (b)
Nakatsuji, S.; Ikemoto, H.; Akutsu, H.; Yamada, J.; Mori,
A. J. Org. Chem. 2003, 68, 1708.
We are currently making efforts to prepare related com-
pounds, such as the ones with salicylideneamine9 instead
of benzylideneamine or Cu(hfac)2 complexes with two
radical units10 and the details of the experimental results
will be reported in future.
4. Cf. Chen, S. H.; Chen, H. M. P.; Geng, Y.; Jacobs, S. D.;
Marshall, K. L.; Blanton, T. N. Adv. Mater. 2003, 15,
1061.
5. (a) Surendranath, V. Mol. Cryst. Liq. Cryst. 1999, 332,
135; (b) Yelamaggad, C. V.; Nagamani, S. A.; Hiremath,
U. S.; Nair, G. G. Liq. Cryst. 2001, 28, 1009.
6. The radical compounds 3a–c were characterized by
elemental analysis and spectroscopic data.
Acknowledgements
7. Details of the liquid crystalline properties of 5a–c will be
reported elsewhere.
This work was partially supported by a special Grant-in-
Aid for Scientific Research from University of Hyogo,
which is gratefully acknowledged.
8. Although we have no concrete evidence to explain the
smaller values of the Curie constants of the radicals than
the theoretical ones (0.375 emu K molÀ1, g = 2.0033), it
might be due to a partial decomposition of the radicals
during handlings and/or contamination of diamagnetic
impurities.
References and notes
1. Cf. Nakatsuji, S. Chem. Soc. Rev. 2004, 348.
2. Cf. Ikuma, N.; Tamura, R.; Shimono, S.; Kawame, N.;
Tamada, O.; Sakai, N.; Yamauchi, J.; Yamamoto, Y.
Angew. Chem., Int. Ed. 2004, 43, 3677.
9. Hata, M.; Akutsu, H.; Yamada, J.; Nakatsuji, S. Mole-
cules 2004, 9, 746.
10. Griesar, K.; Haase, W.; Svoboda, I.; Fuess, H. Inorg.
Chim. Acta 1999, 287, 181.