Photochromism of a water-soluble vesicular [2.2]paracyclophane-bridged imidazole dimer

Article abstract of DOI:10.1039/c1cc12640g

Here we report the first photochromism of a newly designed [2.2]paracyclophane-bridged imidazole dimer in water. The photochromic dye with a hydrophilic and a hydrophobic substituent forms vesicles in water and shows instantaneous colouration upon UV ligh

Full text of DOI:10.1039/c1cc12640g

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COMMUNICATION
Photochromism of a water-soluble vesicular [2.2]paracyclophane-bridged
imidazole dimerw
Katsuya Mutoha and Jiro Abe*ab
Received 5th May 2011, Accepted 26th May 2011
DOI: 10.1039/c1cc12640g
Here we report the first photochromism of a newly designed
[2.2]paracyclophane-bridged imidazole dimer in water. The photo-
chromic dye with a hydrophilic and a hydrophobic substituent
forms vesicles in water and shows instantaneous colouration
upon UV light irradiation and successive rapid fading in
the dark.
([2.2]PC) moiety that couples two diphenylimidazole groups,
that shows instantaneous colouration upon exposure to UV light
and rapid fading in the dark (Scheme 1).5 The [2.2]PC-bridged
imidazole dimer also shows photoinduced homolytic bond
cleavage of the C–N bond between the imidazole rings and
successive fast C–N bond formation. Therefore, the solution
of the [2.2]PC-bridged imidazole dimer changes its colour only
when the solution is irradiated with UV light. The strategies to
improve the photochromic properties of the [2.2]PC-bridged
imidazole dimer are established in our recent studies6–8 and the
photochromic properties can be controlled to meet various
requirements. The [2.2]PC-bridged imidazole dimers have
attractive potential to apply to excellent switching devices
and photoresponsive materials. We have also demonstrated
the polymerization and the gelation of [2.2]PC-bridged imidazole
dimers.9 The diversity of the molecular design and the ease of
synthesis make this class of photochromic molecules highly
attractive for applications in smart windows, solar protection
lenses, decorative objects, and molecular switching devices.
In this study, we have newly designed a [2.2]PC-bridged
imidazole dimer derivative 1 which has both alkoxy chains
as hydrophobic substituents and poly(ethylene glycol) (PEG)
chains as hydrophilic substituents, expecting to form vesicles
in water. To the best of our knowledge, photochromism of the
HABI derivatives in water and the formation of vesicles have
never been reported.
Photochromic materials are a well-known class of molecules
which change their colour upon light irradiation. The coloured
species can switch back to the initial state either thermally or
photochemically. Photochromic materials are applied widely
to the field of not only ophthalmic lenses but also switching
devices.1 Photochromic molecules also have attractive potential as
the photoswitches of fluorescence in vivo and the trigger to
control living systems, and recently a lot of research has been
reported.2 Hexaarylbiimidazole (HABI) is one of the photo-
chromic molecules discovered by Hayashi and Maeda in
1960.3 HABIs are readily cleaved, both thermally and photo-
chemically, into a pair of 2,4,5-triphenylimidazolyl radicals
(TPIRs), which thermally recombine to reproduce its dimer. In
general, the thermal back-reactions of TPIRs are slow in
solution due to the diffusion of TPIRs. Thus, the photochromic
behaviour of HABIs can be attributable to the photoinduced
homolytic reversible cleavage of the C–N bond between the
imidazole rings. Though HABI derivatives have been used as
polymerization initiators and photosensitizers in polymer
matrices because of the low sensitivity of TPIRs to the
presence of oxygen,4 no photochromic reactions in water have
been demonstrated.
Compound 1 was synthesized according to Schemes S1 and
S2 (ESIw). The introduction of the alkoxy and PEG chains was
confirmed by H NMR spectroscopy. The molecular formula
1
was determined by HR–ESI–TOF–MS and the purity was
checked by HPLC. Compound 1 has two structural isomers
due to the difference in the electronic environment of the two
imidazole rings. Because the photochromic reaction leads to
the isomerization between the two structural isomers,8 the
measurements were carried out using the solution of the
mixture of the structural isomers except for the UV–vis
absorption spectroscopy. Compound 1 undergoes a photo-
chromic reaction showing a colour change from colourless to
green upon UV light irradiation. Fig. S12 (ESIw) shows the
UV-vis absorption spectra of 1. The absorption band of 1 in
the UVA radiation region can be assigned to the intramolecular
charge transfer (CT) transition from the electron-donating
phenyl rings to the electron-withdrawing imidazole ring.7,8
Thus, 1 is sensitive to solar UVA radiation. Fig. 1a shows
We have recently developed a unique series of photochromic
[2.2]PC-bridged imidazole dimers, with a [2.2]paracyclophane
a Department of Chemistry, School of Science and Engineering,
Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku,
Sagamihara, Kanagawa 252-5258, Japan.
E-mail: jiro_abe@chem.aoyama.ac.jp; Fax: +81-42-759-6225;
Tel: +81-42-759-6225
b CREST, Japan Science and Technology Agency (JST),
5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan
w Electronic supplementary information (ESI) available: Details of
the syntheses, 1H NMR spectra, HR–ESI–TOF–MS spectra, HPLC
chromatogram, UV–vis absorption spectroscopy, details of optical
microscopy, details of laser flash photolysis measurements, kinetics of
the thermal back-reaction in benzene and water, decay profile of the
thermal back-reaction of amorphous film of 1 and details of cryogenic
transmission electron microscopy. See DOI: 10.1039/c1cc12640g
c
8868 Chem. Commun., 2011, 47, 8868–8870
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Scheme 1 Photochromism of HABI, pseudogem-bisDPI[2.2]PC and 1.
the transient vis-NIR spectra of 1 in benzene at 25 1C measured
by a nanosecond laser flash photolysis experiment. The very
similar absorption band to pseudogem-bisTMDPI[2.2]PC7 can
be ascribed to the coloured species, 1R. All of the absorption
bands decay with the same time constant, indicating the
presence of one conformation of the coloured species. The
half-life of the coloured species is 86 ms at 25 1C in benzene.
Fig. 1b shows the decay profiles of the transient absorbance at
400 nm of the coloured species, measured over the temperature
range from 5 to 40 1C. The thermal bleaching process obeys
first order kinetics. The activation parameters are estimated
from the Eyring plot (Fig. S16, ESIw). The Eyring plot
produces an excellent straight line, and the DHz and DSz
values are estimated from the standard least-squares analysis
of the Eyring plot. The free energy barrier (DGz = DHz ꢀ TDSz) at
25 1C is 67.8 kJ molꢀ1. The thermal back-reaction rate is acceler-
ated compared with that of pseudogem-bisTMDPI[2.2]PC,
DGz = 69.6 kJ molꢀ1. The electron-donating nature of both
long alkoxy and PEG chains is similar to that of the methoxy
substituent because the maximum wavelength of the intra-
molecular CT band is almost the same as shown in Fig. S12
(ESIw). Therefore, the origin of the acceleration of the thermal
back reaction would be the steric effect derived from the long
substituent groups. The acceleration of the thermal back
reaction rate can be explained by the Marcus theory10 as
described in our previous paper.6 In the Marcus theory, the
energy of activation (DGz) is decreased with the increase of the
Gibbs energy (DG0), between the reactant and the product.
Long alkoxy and PEG chains would cover the two imidazole
rings which change the conformation with the photochromism.
It decreases the free volume around two imidazole rings, and
the steric repulsion of 1R between the imidazole rings and long
Fig. 1 (a) Transient vis-NIR absorption spectra of 1 in degassed
benzene at 25 1C (2.1 ꢁ 10ꢀ4 M, 10 mm light path length). Each of the
spectra was recorded at 40 ms intervals after excitation with a
nanosecond laser pulse (excitation wavelength, 355 nm; pulse width,
5 ns; power, 8 mJ pulseꢀ1). (b) Decay profiles of the coloured
species generated from 1, monitored at 400 nm in degassed benzene
(2.1 ꢁ 10ꢀ4 M).
substituents would be increased. Therefore, the destabilization
of 1R leads to the increase in the thermal back reaction rate.
Compound 1 can form vesicles in water because 1 has two
types of substituents, hydrophilic PEG chains and hydrophobic
alkoxy chains. The vesicle formation and morphology of 1 in
water are confirmed by cryogenic transmission electron micro-
scopy (cryo-TEM) as shown in Fig. 2. The spherical giant
vesicles formed by a single bilayer of diameter about 1100 nm
and the large uni- or oligo-lamellar vesicles of diameter about
120–260 nm are observed. The other TEM pictures of the
vesicles of 1 are included in ESI.w We also observed the
Fig. 2 Cryo-TEM images of 1 in water (0.5 mM).
Chem. Commun., 2011, 47, 8868–8870 8869
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procedure as in the case of the benzene solution of 1. The free
energy barrier at 25 1C is 60.7 kJ molꢀ1 and the half-life of the
coloured species is 4.8 ms at 25 1C.
In summary, we demonstrated the first vesicle formation of the
HABI derivative by the introduction of hydrophilic PEG chains
and hydrophobic alkyl chains into the [2.2]paracyclophane-
bridged imidazole dimer. The vesicles show excellent photo-
chromism upon the UV light irradiation and the rapid
thermal bleaching in water. The applications of the photo-
chromism of the vesicle to the rapid switching materials in
water and the biochemical field should be the focus of the
future studies.
We acknowledge collaboration of Prof. Takuya Nakashima
and Prof. Tsuyoshi Kawai in Nara Institute of Science and
Technology (NAIST) on the Cryo-TEM observations under
support of Kyoto-Advanced Nanotechnology Network in
NAIST. This work was partially supported by a Grant-in-Aid
for Scientific Research (A) (No. 22245025) 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.
Fig. 3 Decay profiles of the coloured species generated from 1,
monitored at 700 nm in water (1.9 ꢁ 10ꢀ4 M).
photochromism of the aggregates of the vesicles with the laser
excitation (excitation wavelength is 405 nm) by optical micro-
scopy. The aggregates of the vesicles change their colour from
colourless to green with the excitation, which is the same
colour change of the benzene solution of 1, and after irradiation
ceases, the aggregates decolour within 300 ms at 5 1C. The
time profiles of the thermal back reaction of 1 in water were
measured by a nanosecond laser flash photolysis measurement
monitored at 700 nm because the transient absorbance at
400 nm cannot be observed due to the scattering of the
monitor light. Fig. 3 shows the time profile of the transient
absorbance at 700 nm measured over the temperature range
from 5 to 40 1C. As shown in Fig. S13 (ESIw), only the
aggregates of the vesicles change their colour with laser pulse
excitation. Therefore, the transient absorbance at 700 nm is
attributed to the coloured species of the aggregates. The
absorbance decreases according to the monoexponential
kinetics though it is expected that the intermolecular interactions
or the differences in the local environment around 1 affect the
thermal back-reaction rate of the aggregates. The thermal-
bleaching rate of the polymer films of the [2.2]paracyclophane-
bridged imidazole dimer does not obey first-order kinetics due
to the fact that the reaction rate of photochromism is generally
influenced by the environment around the photochrome.9
However, the thermal-bleaching rate of the amorphous film
of 1 obeys first-order kinetics (Fig. S19, ESIw), suggesting the
uniformity for the molecular environment around the photo-
chromic dyes in the amorphous film. Though we could not
confirm the change of the colour of the vesicles directly
because the vesicles are too small to be observed by optical
microscopy, the first-order kinetics of the thermal back reaction
in water suggests that the molecular environment around the
photochromic dyes in the aggregates and vesicles is almost
uniform as in the amorphous film and the photochromic
properties would not be influenced by the morphology. Therefore,
we concluded that the vesicles of the [2.2]paracyclophane-
bridged imidazole dimer also have the rapid photochromic
property even in water. The activation parameters of the thermal
back-reaction in water could be estimated by the same
Notes and references
1 J. C. Crano and R. J. Guglielmetti, Organic Photochromic and
Thermochromic Compounds, Plenum Press, New York, 1999;
H. Duerr and H. Bouas-Laurent, Photochromism: Molecules and
Systems, Elsevier, Amsterdam, 2003; K. Amimoto and T. Kawato,
J. Photochem. Photobiol., C, 2005, 6, 207.
2 A. A. Beharry, L. Wong, V. Tropepe and G. A. Woolley, Angew.
Chem., Int. Ed., 2011, 50, 1325; I. Yildiz, S. Impellizzeri, E. Deniz,
B. McGaughan, J. F. Callan and F. M. Raymo, J. Am. Chem. Soc.,
2011, 133, 871; R. M. Uda, E. Hiraishi, R. Ohnishi, Y. Nakahara
and K. Kimura, Langmuir, 2010, 26, 5444; M. D. Yamada,
Y. Nakajima, H. Maeda and S. Maruta, J. Biochem., 2007,
142, 691; K. Fujimoto, M. Amano, Y. Horibe and M. Inouye,
Org. Lett., 2006, 8, 285; G. A. Woolley, Acc. Chem. Res., 2005,
38, 486.
3 T. Hayashi and K. Maeda, Bull. Chem. Soc. Jpn., 1960, 33, 565.
4 T. Hayashi and K. Maeda, Bull. Chem. Soc. Jpn., 1970, 43, 429;
X. Z. Qin, A. Liu, A. D. Trifunac and V. V. Krongauz, J. Phys.
Chem., 1991, 95, 5822; A. Liu, A. D. Trifunac and V. V. Krongauz,
J. Phys. Chem., 1992, 96, 207; J. V. Caspar, I. V. Khudyakov,
N. J. Turro and G. C. Weed, Macromolecules, 1995, 28, 636;
D. Lavabre, G. Levy, J. P. Laplante and J. C. Micheau, J. Phys.
Chem., 1998, 92, 16; Y. Lin, A. Liu, A. D. Trifunac and
V. V. Krongauz, Chem. Phys. Lett., 1992, 198, 200.
5 Y. Kishimoto and J. Abe, J. Am. Chem. Soc., 2009, 131, 4227.
6 Y. Harada, S. Hatano, A. Kimoto and J. Abe, J. Phys. Chem.
Lett., 2010, 1, 1112.
7 K. Mutoh, S. Hatano and J. Abe, J. Photopolym. Sci. Technol.,
2010, 23, 301.
8 K. Mutoh and J. Abe, J. Phys. Chem. A, 2011, 115, 4650.
9 A. Kimoto, A. Tokita, T. Horino, T. Oshima and J. Abe,
Macromolecules, 2010, 43, 3764; M. Takizawa, A. Kimoto and
J. Abe, Dyes Pigm., 2011, 89, 254.
10 R. A. Marcus, J. Chem. Phys., 1956, 24, 966; R. A. Marcus, Annu.
Rev. Phys. Chem., 1964, 15, 155; R. A. Marcus, J. Phys. Chem.,
1968, 72, 891; R. A. Marcus, J. Am. Chem. Soc., 1969, 91, 7224;
A. O. Cohen and R. A. Marcus, J. Phys. Chem., 1968, 72, 4249;
I. V. Alabugin, M. Manoharan, B. Breiner and F. D. Lewis, J. Am.
Chem. Soc., 2003, 125, 9329; X. Chen and J. I. Brauman, J. Am.
Chem. Soc., 2008, 130, 15038.
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8870 Chem. Commun., 2011, 47, 8868–8870
This journal is The Royal Society of Chemistry 2011