Photochromic organogel based on [2.2]paracyclophane-bridged imidazole dimer with tetrapodal urea moieties

Article abstract of DOI:10.1016/j.dyepig.2010.03.019

A novel photochromic organogelator possessing [2.2]paracyclophane-bridged imidazole dimer unit with four hydrogen bonding urea arms was obtained. Irradiating UV light to benzene solution of the photochromic organogelator, rapid photochromism from colorles

Full text of DOI:10.1016/j.dyepig.2010.03.019

Dyes and Pigments 89 (2011) 254e259
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photochromic organogel based on [2.2]paracyclophane-bridged
imidazole dimer with tetrapodal urea moieties
*
Masahiro Takizawa, Atsushi Kimoto, Jiro Abe
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel photochromic organogelator possessing [2.2]paracyclophane-bridged imidazole dimer unit
with four hydrogen bonding urea arms was obtained. Irradiating UV light to benzene solution of the
photochromic organogelator, rapid photochromism from colorless to green is observed. From the tran-
sient viseNIR absorption measurement, a characteristic absorption around 400 nm and a broad
absorption from 500 to 1000 nm can be attributed to the colored species. The half-life of the colored
species is 243 ms at 298 K. In cyclohexane, the organogel was successfully formed, which was charac-
terized by SEM observation and IR spectroscopy and showed fast photochromism even in the gel phase.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 8 February 2010
Received in revised form
10 March 2010
Accepted 16 March 2010
Available online 24 March 2010
Keywords:
Hexaarylbiimidazole
Organogelator
Photochromism
Hydrogen bonding
Urea
[2.2]Paracyclophane
1. Introduction
with an unpaired electron [32e34], which could modulate the
magnetic property simply by turning the optical stimulation on
Photoresponsive organogels are recognized as a new class of
stimuli-responsive material [1e4], which enables photoinduced
morphological change, including shape deformation [5e11] and
solegel transition [12e18]. In the last decade, many systems [19e28]
with conventional photochromic moiety as photoreactive module
were proposed. However, of particular concern is that it takes over
a minute to exhibit the morphological change because the response
to light originates from photochromic reaction rate of the photo-
chrome. In order to realize real-time photoinduced morphological
control, which allows simultaneous deformation only under irradi-
ation, an inherently fast photochromic system in solution should be
applied into orgenogel systems.
and off. Our accumulated previous research has been focused on
the control of the simultaneous color change in solution, such
as photosensitivity, reaction kinetics, and controlling colors of the
photogenerated radical species by modifying the electronic structure.
Hence, we consider the fast photochromic [2.2]paracyclophane-
bridged imidazole dimer to be a promising switching component for
fast photoinduced morphological change; simultaneous morpho-
logical control only under light irradiation is expected to produce
a new class of photo-oriented intelligent materials system. Herein,
we focused on the unique photochromic behavior of the [2.2]para-
cyclophane-bridged imidazole dimer unit even in organogel.
We have recently developed a unique series of photochromic
molecules based on [2.2]paracyclophane-bridged imidazole dimer,
which allows instantaneous coloration upon exposure to UV light
and rapid fading in the dark [29e31]. The half-life of the colored
species derived from the homolytic photocleavage of the CeN
bond of imidazole dimer unit is within the order of milliseconds in
solution. Another remarkable aspect of the photochromism of such
imidazole dimer systems is photogeneration of imidazolyl radicals
2. Experimental
2.1. General
All reactions were monitored by thin-layer chromatography
carried out on 0.2 mm E. Merck silica gel plates (60F-254).
Column chromatography was performed on silica gel (WakogelÒ
C-300). All reagents were purchased from TCI, Wako Co. Ltd.,
Aldrich Chemical Company, Inc, and ACROS Organics, and were
used without further purification. All reaction solvents were
distilled on the appropriate drying reagents prior to use. The NMR
spectra were recorded on a JMN-ECP500A (JEOL) spectrometer,
* Corresponding author.
E-mail address: jiro_abe@chem.aoyama.ac.jp (J. Abe).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.019

M. Takizawa et al. / Dyes and Pigments 89 (2011) 254e259
255
and the chemical shifts are quoted in ppm relative to tetrame-
2.4.3. Synthesis of urea-substituted [2.2]paracyclophane-bridged
bisimidazole (1H)
thylsilane. The IR spectra were obtained by IRPrestige-21 (Shi-
madzu). The FAB mass spectra were measured with an MStation
MS-700 (JEOL) spectrometer by using 3-nitrobenzyl alcohol as
a matrix. Scanning electron microscopy (SEM) was performed
on a JEOL model JSM-6301F FE-SEM operating at 8 kV. Pt was
deposited on the sample before observation using a JFC 1300 Auto
Fine Coater (JEOL).
To a solution of 5 (210 mg, 0.144 mmol) was dissolved in THF
(3 mL) was added 75% hydrazine hydrate (1 mL) and heated at 60 ꢀC
for 24 h. After addition of water, a colorless precipitation was
formed and collected in vacuo to yield crude mixture of 6, which
was used without further purification because of its insolubility in
common organic solvents. To a crude mixture of 6 in chloroform
(50 mL) was added dodecyl isocyanate (0.500 g, 2.37 mmol) and
the reaction mixture was refluxed for 24 h. After the solvent
was removed in vacuo, the crude mixture was purified with silica
gel column chromatography using chloroform to chloroform/
methanol ¼ 9/1 as eluent and further purified by reprecipitation in
hexane to yield 1H as a pale brown powder (0.201 g, 77%) and was
used without further purification because of its large polarity. 1H
2.2. Experimental detail for laser flash photolysis
The laser flash photolysis experiments were carried out with a
Unisoku TSP-1000 time-resolved spectrophotometer. A Continuum
Minilite II Nd:YAG (Q-switched) laser with the third harmonic at
355 nm (ca. 8 mJ per 5 ns pulse) was employed for the excitation
light. The probe beam from an OSRAM HLX64623 halogen
lamp was guided with an optical fiber scope to be arranged in an
orientation perpendicular to the exciting laser beam. The probe
beam was monitored with a Hamamatsu R2949 photomultiplier
tube through a prior to the laser flash photolysis experiments.
NMR (500 MHz, DMSO-d6)
d
: 11.4 (s, 2H), 7.20 (d, J ¼ 8.6 Hz, 4H),
7.08 (m, 2H), 6.97 (t, J ¼ 8.6 Hz, 4H), 6.69e6.59 (m, 12H), 5.93e5.82
(m, 8H), 3.93e3.87 (m, 2H), 3.14 (m, 8H), 2.96 (m, 8H), 1.81 (m, 8H),
1.34 (d, J ¼ 4.9 Hz, 8H), 1.23 (b, 80H), 0.84 (t, J ¼ 6.7 Hz, 12H);
FAB-MS: m/z 1782 [M þ H]þ.
2.4.4. Synthesis of urea-substituted [2.2]paracyclophane-bridged
imidazole dimer (1)
2.3. Experimental detail for characterization of the organogel
All manipulations were carried out with the exclusion of light.
Under nitrogen, 1H (0.060 g, 0.058 mmol) and lead(IV) oxide was
refluxed in benzene (30 mL) and ethanol (5 mL) for 24 h. The reaction
mixture was filtered over Celite and the filtrate was evaporated to
dryness. The crude product was purified with reprecipitation in
The organogel in cyclohexane was deposited on the waveguide
surface by drop casting and covered with glass plate to prevent
evaporation of the solvent. Absorption spectrum of the organogel of
1 was measured by Fastevert S-2600 Soma Kougaku multichannel
spectrometer equipped with surface and interface stage (System
Instruments Co., Ltd.) under irradiating UV light with Keyence
UV-400 series UV-LED (UV-50H type), equipped with a UV-L6 lens
unit (365 nm, irradiation power 300 mW/cm2).
hexane to yield 1 (82.5 mg, 60%). 1H NMR (500 MHz, DMSO-d6)
d:
7.25 (t, J ¼ 7.3 Hz, 3H), 7.13 (t, J ¼ 8.2 Hz, 3H), 7.03 (d, J ¼ 8.6 Hz, 2H),
6.94 (d, J ¼ 8.6 Hz, 3H), 6.82 (d, J ¼ 8.6 Hz, 2H), 6.64 (m, 6H), 6.50 (m,
3H), 5.88e5.72 (m, 8H), 4.34 (m, 1H), 4.03e3.68 (m, 8H), 3.15e3.04
(m, 7H), 2.94 (m, 8H), 1.83e1.69 (m, 8H), 1.33e1.15 (br, 88H), 0.73
(t, J ¼ 6.1 Hz, 12H); 13C NMR (125 MHz, CDCl3):
d: 167.8, 164.3, 161.2,
2.4. Synthesis
161.1, 159.6, 159.5, 159.3, 159.1, 158.5, 157.5, 147.1, 143.6, 141.2, 141.0,
139.9, 137.8, 137.3, 137.202, 137.198, 136.1, 135.0, 134.9, 134.7, 134.2,
133.8, 133.3, 133.0, 132.9, 131.9, 131.5, 128.2, 127.3, 127.1, 124.1, 123.9,
123.3, 114.7, 114.0, 113.9, 113.5, 111.6, 65.9, 65.7, 65.6, 40.7, 40.4, 40.2,
40.0, 37.3, 37.2, 37.1, 37.0, 36.9, 35.4, 34.5, 34.1, 32.0, 30.6, 30.4, 29.8,
29.6, 27.2, 22.8, 22.8, 12.3, 12.4; FAB-MS: m/z 1780 [M þ H]þ.
2.4.1. Synthesis of propoxyphthalimide-substituted benzil (4)
4,40-dihydroxybenzil 3 (501 mg, 2.07 mmol), N-bromopro-
pylphthalimide (1.24 g, 4.62 mmol), and potassium carbonate
(1.54 g, 11.1 mmol) were added to anhydrous DMF (7 mL), and
the reaction mixture was heated at 65 ꢀC for 5 h. After cooling to r.t.,
the reaction mixture was extracted with chloroform; the combined
extracts were washed with water and brine. And the organic layer
was dried over Na2SO4, and the solvent was removed in vacuo. The
crude product was purified with silica gel column chromatography
using chloroform as eluent to obtain colorless powder (712 mg;
3. Results and discussion
We designed a novel photochromic organogelator with [2.2]
paracyclophane-bridged imidazole dimer (1), possessing tetrapo-
dal urea arms at its termini (Scheme 1). Compound 1 was synthe-
sized in five steps from 4,40-dihydroxybenzil (3) (Scheme 2). After
introduction of N-propylphthalimide unit via Williamson reaction
in 56% yield, the phthalimide-substituted benzil (4) was reacted
yield: 56.0%). 1H NMR (500 MHz, CDCl3)
d: 7.87 (m, 8H), 7.73 (m,
4H), 6.84 (d, J ¼ 8.6 Hz, 4H), 4.11 (t, J ¼ 5.5 Hz, 4H), 3.92 (t, J ¼ 6.4 Hz,
4H), 2.22 (t, J ¼ 6.1 Hz, 4H); FAB-MS: m/z 617 [M þ H]þ.
2.4.2. Synthesis of propoxyphthalimide-substituted bisimidazole (5)
4 (2.27 g, 1.9 mmol), [2.2]paracyclophane-4,13-dicarbaldehyde
(502 mg, 1.90 mmol) and ammonium acetate (2.16 g, 28.0 mmol)
were refluxed in acetic acid (40 mL) for 24 h. After adding
ammonium acetate (2.00 g, 25.9 mmol), the reaction mixture was
heated for additional 24 h and was cooled to room temperature
and was neutralized with aqueous NH3. The resulting precipitate
was filtered and washed with water. The crude mixture was
purified with silica gel column chromatography using THF/
hexane ¼ 3/1 to 1/1 as eluent to give a yellow powder (1.35 g,
48.7%). 1H NMR (500 MHz, DMSO-d6)
d: 11.4 (s, 2H), 7.77 (m, 16H),
7.10 (m, 5H), 6.89 (d, J ¼ 9.2 Hz, 2H), 6.67 (d, J ¼ 7.9 Hz, 2H), 6.57
(m, 6H), 6.50 (d, J ¼ 9.2 Hz, 4H), 4.51 (m, 2H), 3.98e3.87 (m, 8H),
3.73 (m, 8H), 3.06 (m, 6H), 2.03 (m, 8H); FAB-MS: m/z 1458
[M þ H]þ.
Scheme 1.

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M. Takizawa et al. / Dyes and Pigments 89 (2011) 254e259
Scheme 2.
with [2.2]paracyclophane-4,13-dicarbaldehyde to form bisimida-
zole (5) in 49% yield. The four phthalimide moieties were cleaved
by hydrazinolysis to yield the tetraamine as insoluble powders (6),
which was reacted with an excess amount of dodecyl isocyanate to
give the urea-substituted [2.2]paracyclophane-bridged bisimida-
zole (1H) in 77% yield. The precursor,1H was oxidized with lead (IV)
oxide to afford the target urea-substituted [2.2]paracyclophane-
bridged imidazole dimer (1) in 60% yield.
Irradiating UV light to benzene solution of 1, rapid photochro-
mism from colorless to green is observed. Under continuous irra-
diation, the benzene solution of 1 reaches the photostationary
equilibrium, and the absorption decays rapidly following mono-
exponential thermal bleaching kinetics after ceasing the irradiation.
The complete bleaching is achieved within a few seconds which
makes it possible to see the color change of the solution only where
UV light is irradiated. Fig. 1a shows the transient viseNIR absorption
spectra of 1 in degassed benzene solution at 298 K measured
by a nanosecond laser flash photolysis experiment. A characteristic
absorption around 400 nm and a broad absorption from 500 to
1000 nm can be attributed to the colored species, 1R. The half-life of
the colored species is 243 ms at 298 K. The thermal recombination
obeys first-order kinetics over the temperature range from 278 to
328 K as shown in Fig. 1b.
least-square analysis of the Eyring plots are 55.1 kJ molꢁ1 and
ꢁ55.3 J Kꢁ1 molꢁ1, respectively. On the other hand, the
D D
Hz and Sz
values for the recombination of 2R are 63.5 kJ molꢁ1 and
ꢁ22.4 J Kꢁ1 molꢁ1, respectively. It should be noted that the smaller
D
Sz value for recombination of 1R than that of 2R is derived from
introduction of the bulky urea arms, which brings steric hindrance
between the imidazolyl radicals. Remarkably, below 288 K, the half-
life of 1R becomes smaller than that of 2R (Fig. 4). For example, at
278 K, the half-life of 1R is 1.14 s, which is smaller than that of 2R
(1.42 s). This is probably due to formation of inter/intramolecular
hydrogen bonding network at low temperatures, resulting in the
rigid environment around the photogenerated imidazolyl radicals.
The entangled hydrogen bonding network as rigid environment
gives restriction of molecular structural change for photochromic
reaction; upon UV irradiation to generate the imidazolyl radical,
one of the imidazole rings (Im1, Scheme 1) rotates to form face-to-
face geometry with the other imidazole ring (Im2, Scheme 1) as the
formation process of the imidazolyl radical colored species [29].
As a result, the closer distance between two imidazole rings allows
accelerated recombination of the radical species to give the shorter
half-life of 1R than that of 2R.
We investigated the gelation behavior of 1 for various common
organic solvents by first dissolving 1 by heating, followed by cooling
to room temperature. It can form a stable turbid gel in cyclohexane
with more than 2.2 wt% (9.7 mM) (Fig. S6). The organogel dissolves
into solution over 328 K to be transparent. Using other solvents
such as acetonitrile, acetone, toluene, benzene and so on, no orga-
nogel formation is observed after heatingecooling cycles. The
morphology of the xerogel prepared from 1 was investigated by field
emission scanning electron microscopy (FE-SEM). From the FE-SEM
observation, the cyclohexane gel forms three-dimensional entan-
gled network structures consisting from sub-micrometer scale
nanofibrous structures (Fig. 5). The fibers have several micrometers
length and approximately 200 nm width, which can similarly be
observed for xerogels composed of other urea-based organogelators
[35e38]. The formation of the hydrogen bonding network through
urea moieties is characterized by IR absorption bands which corre-
sponds to the NeH stretching and the amide bands of urea unit
In order to study influence of the urea arms on the photochromic
properties in solution, the identical unsubstituted segment 2, which
has four methoxy substituents, was prepared (Scheme S1) and its
photochromic properties were also investigated. The half-life of the
colored species is 220 ms at 298 K, which is comparable to that of 1
(243 ms) (Fig. 2). The absorption of the colored species, 2R, gener-
ated by UV irradiation also gives similar spectrum with a broad
absorption from 500 to 1000 nm to the absorption spectrum of 1R.
These results indicate that there are no substitution effects of the
urea arms on the half-life and absorption spectrum of the colored
species at 298 K (Figs. S3 and S4). The enthalpies and entropies of
activation (
D D
Hz and Sz, respectively) for the thermal recombination
of 1R and 2R were estimated from Eyring plots over the temperature
range from 278 to 328 K. The Eyring plot gives a good straight line
(Fig. 3 and Fig. S5), and the
D D
Hz and Sz values determined by

M. Takizawa et al. / Dyes and Pigments 89 (2011) 254e259
257
Fig. 3. Eyring plots for the thermal back-reaction of 1 in the temperature range from
278 to 328 K.
(Fig. 6). The solution of 1 in chloroform (8.4 mM) shows character-
istic absorption bands for free amide moiety at 3409 and 1671 cmꢁ1
attributed to the NeH stretching and C]O stretching band,
respectively. After transition to gel phase, the NeH stretching and
C]O stretching bands are shifted to lower wavenumbers (3337 and
1611 cmꢁ1, respectively). According to the literature, this IR spectral
shift is indicative for formation of the hydrogen bonds of adjacent
urea groups of 1 [36e38].
Upon illumination at 365 nm, the organogel of 1 rapidly colorizes
on the surface where UV light is illuminated and keeps the gel phase
(Fig. S6). No significant phase transitions of the organogel during
photoreaction are mainly due to internal filter effect with turbidity to
inhibit total conversion; absorption and scattering of the UV light by
1 to prevent the photochromic reaction inside the organogel. When
the light irradiation ceases, the colored part rapidly disappears to
Fig. 1. (a) Transient viseNIR absorption spectra of 1 in degassed benzene solution
(9.7 ꢂ 10ꢁ5 M, 10 mm light-path length) at 298 K. Each of the spectra was recorded at
80 ms intervals after excitation with a nanosecond laser pulse (excitation wavelength,
355 nm; pulse width, 5 ns; power, 8 mJ/pulse). (b) Decay profiles of the colored species
generated from 1 monitored at 400 nm in degassed benzene solution. The measure-
ments were performed in the temperature range from 278 to 328 K.
Fig. 4. Temperature dependence of the half-life of the colored species of 1 and 2 in
degassed benzene determined from the measurements of the nanosecond laser flash
photolysis experiment.
Fig. 2. Decay profiles of the colored species generated from 1 to 2 monitored at
400 nm in degassed benzene solution at 298 K.

258
M. Takizawa et al. / Dyes and Pigments 89 (2011) 254e259
Fig. 7. Decay profile of the colored species of the organogel of 1 after UV light irradiation
(365 nm). Inset shows viseNIR absorption spectra of the colored species of organogel 1
normalized at 625 nm. The measurements were performed at room temperature with
slab optical waveguide technique.
Fig. 5. SEM image of the xerogel of 1 prepared by dropping a cyclohexane gel (2.2 wt%)
onto a silicon substrate, and then air-drying the organogel. Pt was deposited on the
sample before observation.
recover the colorless organogels. The absorption spectrum of
the colored species of the turbid gel 1 was measured under UV light
irradiation with slab optical waveguide technique instead of the
conventional transmission absorption spectroscopy. In the gel phase,
a similar absorption spectrum of the colored species to that in solu-
tion is observed with characteristic absorption bands around 600 and
800 nm (Fig. 7, inset). This indicates that the coloration under UV light
irradiation is derived from the photogenerated imidazolyl radicals
and there are small effects on the electronic state of the imidazolyl
radical unit upon gelation. On the other hand, the thermal bleaching
reaction does not obey first-order kinetics and half-life of the colored
[39e43]. In other conventional photochromic gel systems, it takes
over a minute to complete the photochromic reaction because the
slower response to light originates from photochromic reaction rate
of the photochrome. On the other hand, applying an inherently
fast photochromic unit into organogel system, the photochromic
reaction with orders of seconds even in organogel can be expected as
a promising candidate for simultaneous photoinduced morpholog-
ical transition only under light irradiation.
4. Conclusion
species of 1, defined as the time with 50% of the normalized
D O.D.
(optical density), was estimated to be approximately 1.4 s at room
temperature (Fig. 7). Considering that the half-life in benzene solu-
tion is 243 ms at 298 K, this deceleration of the recombination
reaction is due to the contribution of the slow component, which
can generally be observed in photochromic reaction in bulk level
In conclusion, we have prepared a novel organogelator pos-
sessing [2.2]paracyclophane-bridged imidazole dimer as a photo-
chromic unit. We demonstrated the inherently fast photochromic
reactions, homolytic photocleavage of the CeN bond of imidazole
dimer unit and recombination between the photogenerated
imidazolyl radicals, took place even in the gel phase. We believe
that the modification of this system makes it possible to realize
simultaneous morphological/physicochemical control only under
light irradiation, which will create a new class of photo-oriented
intelligent materials system.
Acknowledgement
This work was supported by a Grant-in-Aid for Science Research
in a Priority Area “New Frontiers in Photochromism (No. 471)” from
the Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan and by a High-Tech Research Center project for
private universities with the matching fund subsidy from MEXT.
Appendix. Supplementary data
Synthesis and photochromic property of 2, NMR chart of 1 and 2
and the photograph of the organogel of 1 were included in the
Supplementary materials. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/j.dyepig.
200.03.019.
Fig. 6. FTeIR spectra of the organogel (solid lines, 9.7 mM in cyclohexane) and solu-
tion (dashed lines, 8.4 mM in chloroform) of 1.

M. Takizawa et al. / Dyes and Pigments 89 (2011) 254e259
259
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