Organogel formation and chromic behavior of hydrogen-bonding lophine derivatives

Article abstract of DOI:10.1246/cl.2009.1096

A new hydrogen-bonding lophine derivative and its dimeric compound have been prepared. The monomeric compound acts as a low molecular weight gelator for several organic solvents such as benzene and tetrachloromethane, while the dimeric compound does not.

Full text of DOI:10.1246/cl.2009.1096

1096
Chemistry Letters Vol.38, No.11 (2009)
Organogel Formation and Chromic Behavior of Hydrogen-bonding Lophine Derivatives
Kazuhiro Yabuuchi,ꢀ1;2 Naoki Oda,1 and Makoto Inokuchi1
1Faculty of Engineering, Tokyo University of Science, Yamaguchi, 1-1-1 Daigaku-dori, Sanyo-Onoda 756-0884
2Department of Applied Chemistry, College of Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501
(Received August 31, 2009; CL-090798; E-mail: yabuuchi@isc.chubu.ac.jp)
A new hydrogen-bonding lophine derivative and its dimeric
Table 1. Gelation properties of lophine derivativesa
compound have been prepared. The monomeric compound acts
as a low molecular weight gelator for several organic solvents
such as benzene and tetrachloromethane, while the dimeric com-
pound does not. However, the dimeric compound easily forms
thin films by casting from solutions, and the film exhibits chro-
mic behavior by both UV irradiation and strong shear stress.
Solvent
Hexane
Acetone
Ethyl acetate
Ethanol
Chloroform
Tetrachloromethane
Benzene
1
2
Gelation (9)
Precipitation
Viscous solution
Precipitation
Solution
Gelation (50)
Gelation (25)
Gelation (20)
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Materials which change their colors and luminescent behav-
ior have potential applications in a wide range of fields such as
optics, sensing, and data storage. Chromic molecules often
change their molecular structures along with their optical prop-
erties by applying external stimuli.1 When such chromic mole-
cules are introduced to self-assembled materials such as organo-
gelators,2 stimuli-induced structural changes on the molecular
scale often lead to drastic changes in the self-assembled structure
and even in the macroscopic structure of the materials due to the
dynamic nature of noncovalent interactions.3 Among chromic
molecules incorporated in self-assembled materials, mechano-
chromic molecules remain to be developed compared with pho-
to- and thermochromic molecules. Mechanochromic behavior is
induced by mechanical stress such as pressure and shear stress.4,5
Only a few examples of mechanochromic behavior of self-as-
sembled materials have been reported.6 In this study, we have
focused our attention on 2,4,5-triphenylimidazole (lophine) be-
cause its dimers exhibit chromic behavior with changing their
molecular structures and generating radical species induced by
mechanical stress as well as photoirradiation.5,7 Here, we report
the development of hydrogen-bonding lophine derivatives as
shown in Figure 1 aiming at stress-responsive organogelators.
Our new lophine derivative 1 has three (dodecylcarbamoyl)-
methoxy chains as self-assembling moieties attached to a lo-
phine core. As for the molecular design of low molecular weight
organogelators, introduction of hydrogen-bonding moieties with
long alkyl chains to functional cores is often reported.2d,2e Com-
pound 1 was prepared by the reaction of a corresponding benzil
derivative and a benzaldehyde derivative with ammonium ace-
tate in acetic acid under reflux for 24 h.5b,8 The dimeric product
2 was prepared by oxidation of 1 with K3[Fe(CN)6] in 10% etha-
nolic potassium hydroxide solution.9
Toluene
aMinimum gel concentrations are in parentheses (given in
mg mLꢁ1).
Figure 2. SEM image of xerogel of 1 formed in hexane.
The gelation properties of these lophine derivatives for com-
mon organic solvents are shown in Table 1. Monomeric com-
pound 1 forms organogels in hexane, tetrachloromethane, and
aromatic solvents. SEM observation reveals that 1 forms net-
work-structured aggregates in solvents as shown in Figure 2.
Such aggregates are believed to form through one-dimensional
molecular assembly. As for the nonsubstituted lophine, single
crystalline nanowires were fabricated by physical vapor deposi-
tion,10 which indicates a lophine scaffold is suitable for one-di-
mensional molecular assembly. Therefore, compound 1 is ex-
pected to assemble one-dimensionally with the assistance of in-
termolecular hydrogen bonding and long alkyl chains without
crystallization. Infrared spectra indicate the formation of hydro-
gen bonding, since the N–H stretching band at 3431 cmꢁ1 in so-
lution shifts to 3296 cmꢁ1 in the gel state. On the other hand, di-
meric compound 2 is readily soluble in various solvents and it
does not form organogels. It seems steric hindrance of six phenyl
rings in a molecule of 2 disturbs elongated one-dimensional ag-
gregation. However, it should be noted that 2 forms thin films
without crystallization by casting from solution due to the exis-
tence of long alkyl chains and amide groups. The formation of
hydrogen bonding in the film of 2 is also indicated, since the
RO
RO
OR
RO
RO
N
OR
N
H
N
N
N
N
N–H stretching band of infrared spectra appears at 3302 cmꢁ1
.
RO
1
OR
Nonsubstituted lophine dimers are known to respond to both
photoirradiation and mechanical stress as mentioned above.5,7
As for dimeric compound 2, it exhibits photochromic behavior
from yellow to green both in solutions and a solid state by UV
H
N
R:
CH2
C
C12H25
RO
2
O
Figure 1. Molecular structures of lophine derivatives 1 and 2.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.11 (2009)
1097
moieties. The existence of hydrogen-bonding moieties enables
the formation of organogels and thin films without crystalliza-
tion. Though efficient use of structural changes along with chro-
mic behavior for the modulation of molecular assemblies has not
been achieved so far, we are now exploring better molecular de-
signs serving our purpose.
We are grateful to financial supports of Grant-in-Aids for
Young Scientists (B) (Nos. 17750138 and 20750116 to KY)
and Scientific Research (C) (No. 18550134 to MI) from the
MEXT of Japan. We also thank Mr. Ryosuke Sakai for his help-
ful discussion and technical assistance.
Figure 3. Comparison of absorption spectra of 2 in benzene be-
fore (dashed line) and after (solid line) UV irradiation.
References and Notes
1
Chromic Phenomena, ed. by P. Bamfield, Royal Society of Chemistry,
Cambridge, 2001.
2
a) Molecular Gels, ed. by R. G. Weiss, P. Terech, Kluwer Academic
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Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. d) M. Shirakawa, S.
Kawano, N. Fujita, K. Sada, S. Shinkai, J. Org. Chem. 2003, 68,
5037. e) M. Ikeda, M. Takeuchi, S. Shinkai, Chem. Commun. 2003,
1354.
Figure 4. Photographs of the thin film of 2 (a) at ambient pres-
sure, (b) under shear stress, and (c) at ambient pressure after re-
leasing the stress.
3
4
a) K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T. Komori,
F. Ohseto, K. Ueda, S. Shinkai, J. Am. Chem. Soc. 1994, 116, 6664.
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M. T. Ong, P. V. Braun, T. J. Martınez, S. R. White, J. S. Moore,
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irradiation. Figure 3 shows spectral changes on UV irradiation.
New absorption bands around 380 and 630 nm appear on UV
irradiation. These bands gradually decrease in the dark. Prelimi-
nary ESR measurements of 2 in solid states indicate the genera-
tion of radical species along with photochromic behavior.
It should be noted that dimeric compound 2 also exhibits
mechanochromic behavior. As for mechanochromic behavior
of a nonsubstituted lophine dimer, it changes its color only by
grinding in a mortar.5 Though no color change was observed
for 2 with a mortar, a thin cast film of 2 exhibits reversible color
changes by applying strong shear stress with a sapphire anvil
cell11 as shown in Figure 4. A vertically pressured thin film be-
tween two sapphire anvils is sheared by the rotation of the lower
anvil and immediately the color of the film changes from yellow
to green. The color returns to yellow by releasing the stress.
These color changes are almost the same as those induced by
photoirradiation. Therefore, compound 2 is expected to be
cleaved to radical species by strong shear stress. As for the
nonsubstituted lophine, it has been reported that molecular
structures of the photochromic dimer and the mechanochromic
dimer are slightly different in the way of connection of two
lophine units.5 However, both dimers are cleaved to the same
radical species on photoirradiation or mechanical stress. In our
studies, an individual dimeric compound 2 exhibits both photo-
and mechanochromic behavior. It is still unclear whether our re-
sults can be explained by the effects of the introduction of self-
assembling moieties to lophine molecules. One possible reason
is the strength of mechanical stress. Our apparatus can generate
shear stress equivalent to more than 1 GPa of hydrostatic pres-
sure,11b,11c which is much larger than the stress generated by
grinding with a mortar. Moreover, we have recently reported
chromic behavior of photochromic spiropyran induced by shear
stress with our apparatus.11b,12
5
6
7
8
1H NMR (d6-DMSO, 270 MHz): ꢀ 12.43 (br, 1H), 8.03 (t, J ¼ 3:6 Hz,
3H), 7.99 (d, J ¼ 8:9 Hz, 2H), 7.43 (d, J ¼ 8:6 Hz, 4H), 7.04 (d,
J ¼ 8:9 Hz, 2H), 6.95 (d, J ¼ 8:6 Hz, 4H), 4.52 (s, 2H), 4.47 (s, 4H),
3.12 (q, J ¼ 6:5 Hz, 6H), 1.44–1.40 (m, 6H), 1.28–1.21 (m, 54H),
0.83 (t, J ¼ 6:8 Hz, 9H).
9
1H NMR (d6-DMSO, 400 MHz): ꢀ 8.09–8.02 (m, 6H), 7.98 (d,
J ¼ 8:8 Hz, 4H), 7.45 (d, J ¼ 8:3 Hz, 4H), 7.39 (d, J ¼ 8:8 Hz, 4H),
7.03 (d, J ¼ 8:8 Hz, 4H), 7.00 (d, J ¼ 9:3 Hz, 4H), 6.88 (d, J ¼ 8:3
Hz, 4H), 4.51 (s, 8H), 4.44 (s, 4H), 3.14–3.09 (m, 12H), 1.42–1.39
(m, 12H), 1.24–1.21 (m, 108H), 0.83 (t, J ¼ 6:6 Hz, 18H).
10 Y. S. Zhao, D. Xiao, W. Yang, A. Peng, J. Yao, Chem. Mater. 2006, 18,
2302.
11 For a detailed description of the sapphire anvil cell, see: a) I. Shirotani,
J. Hayashi, K. Hirano, H. Kawamura, M. Inokuchi, H. Inokuchi, Proc.
Jpn. Acad., Ser. B 2003, 79B, 267. b) M. Inokuchi, D. Kawamura, Y.
Sakka, K. Yabuuchi, K. Yakushi, I. Shirotani, J. Hayashi, H.
Kawamura, H. Inokuchi, J. Low Temp. Phys. 2006, 142, 211. c) K.
Yabuuchi, D. Kawamura, M. Inokuchi, I. Shirotani, J. Hayashi, K.
Yakushi, H. Kawamura, H. Inokuchi, Mol. Cryst. Liq. Cryst. 2006,
455, 81.
In conclusion, we have demonstrated emerging uses of
lophine-based compounds as organogelators and multistimuli
responsive materials by the introduction of hydrogen-bonding
12 Our preliminary experiments indicate a nonsubstituted photochromic
lophine dimer can also change its color by applying strong shear stress
with our apparatus.
Published on the web (Advance View) October 17, 2009; doi:10.1246/cl.2009.1096