Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance

Article abstract of DOI:10.1021/ol801135g

(Figure Presented) We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole (HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density in the colored state. The thermal bleaching rate (τ_1/2 = 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.

Full text of DOI:10.1021/ol801135g

ORGANIC
LETTERS
2008
Vol. 10, No. 14
3105-3108
Photochromism of a Radical
Diffusion-Inhibited Hexaarylbiimidazole
Derivative with Intense Coloration and
Fast Decoloration Performance
Kana Fujita, Sayaka Hatano, Daisuke Kato, and Jiro Abe*
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin
UniVersity, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan
jiro_abe@chem.aoyama.ac.jp
Received May 21, 2008
ABSTRACT
We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole
(HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density
in the colored state. The thermal bleaching rate (τ1/2 ) 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.
Switching of the physical and chemical properties of materi-
als by photochromic compounds has been the subject of
considerable research. There is increasing interest in the use
of organic photochromic compounds to modulate conductiv-
ity, fluorescence, magnetism, and shape at the bulk level.1
Another important application of photochromic compounds
lies in their use in photochromic lenses that darken in
sunlight.2 The ideal conditions for photochromic lenses are
satisfied with molecules that can be stimulated by sunlight,
rapidly developing an intense coloration in a wide range of
visible light and returning to the initial state with fast fading
kinetics in addition to fatigue resistivity for many coloring-
decoloring cycles. Herein, we describe the synthesis and the
photochromic behavior of a newly designed, photochromic,
radical diffusion-inhibited hexaarylbiimidazole (HABI) de-
rivative with markedly improved photochromic performance
in coloration and decoloration rates as well as greater optical
density in the colored state.
HABI is readily cleaved, both thermally and photochemi-
cally, into a pair of 2,4,5-triphenylimidazolyl radicals
(TPIRs), which thermally recombine to reproduce its dimer.3
The dimer is abbreviated as TPID and is identical to HABI.
The photochromic behavior of HABI derivatives can be
(1) (a) Kawai, T.; Kunitake, T.; Irie, M. Chem. Lett. 1999, 28, 905. (b)
Kawai, T.; Nakashima, Y.; Irie, M. AdV. Mater. 2005, 17, 309. (c)
Fernandez-Acebes, A.; Lehn, J.-M. AdV. Mater. 1998, 10, 1519. (d) Kawai,
T.; Sasaki, T.; Irie, M. Chem. Commun. 2001, 711. (e) Irie, M.; Fukaminato,
T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature 2002, 420, 759. (f) Lim, S.-J.;
Seo, J.; Park, S. Y. J. Am. Chem. Soc. 2006, 128, 14542. (g) Matsuda, K.;
Irie, M. J. Am. Chem. Soc. 2000, 122, 7195. (h) Molecular Switches; Feringa,
B. L., Ed.; Wiley-VCH: Weinheim, Germany, 2001. (i) Irie, M. Chem. ReV.
2000, 100, 1683. (j) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145.
(k) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature 2007,
446, 778.
(2) (a) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and
Thermochromic Compounds; Plenum Press: New York, 1999. (b) Duerr,
H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier:
Amsterdam, 2003.
10.1021/ol801135g CCC: $40.75
Published on Web 06/19/2008
2008 American Chemical Society

attributed to the photoinduced homolytic reversible cleavage
of the C-N bond between the imidazole rings. We have been
investigating the photochromism of various kinds of HABI
derivatives.4 Recently, we have developed a new class of
radical diffusion-inhibited HABI, 1,8-TPID-naphthalene, with
the aid of a naphthalene linker as a radical diffusion inhibition
unit, as shown in Scheme 1a.4a 1,8-TPID-naphthalene cleaves
Scheme 2. Synthesis of 1,8-NDPI-TPI-naphthalene
Scheme 1
.
Photochromism of (a) 1,8-TPID-naphthalene and (b)
1,8-NDPI-TPI-naphthalene
photochemically into 1,8-bisTPIR-naphthalene, and the solu-
tion changes from colorless to green. Unlike conventional
HABI derivatives, the photoinduced radical pair in 1,8-
bisTPIR-naphthalene cannot diffuse into the medium to yield
free radicals. A kinetic study of the thermal back-reaction
from the diradical to the dimerized product showed that the
reaction obeys first-order kinetics with a 730 ms half-lifetime
in degassed benzene at 298 K.
In this study, we have developed a new type of radical
diffusion-inhibited HABI derivative, 1,8-NDPI-TPI-naph-
thalene (Scheme 1b), which consists of two different TPIR
units, i.e., 2-(1-naphthyl)-4,5-diphenylimidazolyl radical
(NDPIR) and TPIR units. The colored species (1,8-NDPIR-
TPIR-naphthalene) of 1,8-NDPI-TPI-naphthalene can be
expected to give a superposed absorption spectrum in the
visible region for the corresponding NDPIR and TPIR. Thus,
a wide range of visible light would be absorbed by 1,8-
NDPIR-TPIR-naphthalene because TPIR and NDPIR absorb
light between 500 and 600 nm and 550 and 900 nm in
addition to a sharp band at around 460 nm, respectively
(Figure S1, Supporting Information).
The synthesis was performed according to Scheme 2. The
molecular structure revealed by X-ray crystallographic
analysis is shown in Figure 1. 1,8-NDPI-TPI-naphthalene
(3) (a) Hayashi, T.; Maeda, K. Bull. Chem. Soc. Jpn. 1960, 33, 565. (b)
White, D. M.; Sonnenberg, J. J. Am. Chem. Soc. 1966, 88, 3825. (c) Cohen,
R. J. Org. Chem. 1971, 36, 2280. (d) Riem, R. H.; MacLachlan, A.; Coraor,
G. R.; Urban, E. J. J. Org. Chem. 1971, 36, 2272. (e) Cescon, L. A.; Coraor,
G. R.; Dessauer, R.; Silversmith, E. F.; Urban, E. J. J. Org. Chem. 1971,
36, 2262.
(4) (a) Iwahori, F.; Hatano, S.; Abe, J. J. Phys. Org. Chem. 2007, 20,
857. (b) Satoh, Y.; Ishibashi, Y.; Ito, S.; Nagasawa, Y.; Miyasaka, H.;
Chosrowjan, H.; Taniguchi, S.; Mataga, N.; Kato, D.; Kikuchi, A.; Abe, J
Chem. Phys. Lett. 2007, 448, 228. (c) Miyamoto, Y.; Kikuchi, A.; Iwahori,
F.; Abe, J. J. Phys. Chem. A 2005, 109, 10183. (d) Nakahara, I.; Kikuchi,
A.; Iwahori, F.; Abe, J. Chem. Phys. Lett. 2005, 402, 107. (e) Kikuchi, A.;
Iwahori, F.; Abe, J. J. Am. Chem. Soc. 2004, 126, 6526. (f) Kikuchi, A.;
Iyoda, T.; Abe, J. Chem. Commun. 2002, 1484. (g) Abe, J.; Sano, T.;
Kawano, M.; Ohashi, Y.; Matsushita, M. M.; Iyoda, T. Angew. Chem., Int.
Ed. 2001, 40, 580. (h) Kawano, M.; Sano, T.; Abe, J.; Ohashi, Y. Chem.
Lett. 2000, 29, 1372. (i) Kawano, M.; Sano, T.; Abe, J.; Ohashi, Y. J. Am.
Chem. Soc. 1999, 121, 8106.
Figure 1. Crystal structure of 1,8-NDPI-TPI-naphthalene with
thermal ellipsoids (50% probability). The hydrogen atoms and the
solvents are omitted. Nitrogen atoms are highlighted in red.
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Org. Lett., Vol. 10, No. 14, 2008

has the chirality resulting from the 1,1′-binaphthyl unit, and
the two enantiomeric forms of the molecule are found to
make up the racemic crystal. The C-N bond connecting two
imidazolyl rings is formed between N1 of the NDPI unit
and C2′ of the TPI unit (1,2′-isomer), and the bond length
(1.488(2) Å) is approximately equal to that (1.494(3) Å) of
1,8-TPID-naphthalene. Another possibility for the C-N bond
formation between N1′ of the TPI unit and C2 of the NDPI
unit (2,1′-isomer) was suggested by the B3LYP/6-31G(d)
level density functional theory (DFT) calculations (Figure
2). However, the zero-point corrected electronic energy of
Figure 3. Transient vis-NIR absorption spectra of 1,8-NDPI-TPI-
naphthalene in degassed toluene solution (1.2 × 10-4 M) at 253 K
after irradiation with 365 nm of UV light in a well-stirred quartz
cell (light-path length: 10 mm). Each of the spectra was recorded
at 3 s intervals over a 1 min period.
Figure 2. Optimized structures of the 1,2′-isomer (left) and the
2,1′-isomer (right) by the DFT Beck3LYP/6-31G(d) method.
Nitrogen atoms are highlighted in red.
the latter conformation is 2.2 kcal/mol higher than that
corresponding to the observed conformation. This energy
difference is attributable to the steric repulsion between the
H atom in the naphthalene ring of NDPI unit and the N atom
in the later conformation. The 2,1′-isomer is not formed even
after photolysis. This may be due to the difference in the
reaction rate to form the imidazole dimer from the radical
species. What this means is that the free energy barrier (∆G‡
) ∆H‡ - T∆S‡) for the radical-radical reaction leading to
the 2,1′-isomer is larger than that for the 1,2′-isomer.
1,8-NDPI-TPI-naphthalene shows photochromic reaction
changing its color from colorless to moss green on UV
irradiation in both solid and solution. Under continuous
irradiation, the degassed toluene solution of 1,8-NDPI-TPI-
naphthalene reaches the photostationary equilibrium very
quickly, and the absorption decreases very rapidly following
monoexponential thermal bleaching kinetics after ceasing the
irradiation. The complete bleaching is achieved within 1 s
at 295 K. Figure 3 shows the transient vis-NIR absorption
spectra of 1,8-NDPI-TPI-naphthalene in degassed toluene
solution at 253 K. The measurement of the transient
absorption spectra was started immediately after ceasing the
irradiation when a photostationary state was reached on UV
irradiation. The colored species absorbs the whole range of
visible light, which is attributed to the NDPIR and TPIR
chromophores of 1,8-NDPIR-TPIR-naphthalene. Figure 4a
shows the transient absorption decay at 450 nm measured
by the nanosecond laser flash photolysis experiment (355
nm excitation, 5 ns pulsewidth). The thermal back-reaction
obeys first-order kinetics, and the half-lifetime at 295 K is
260 ms. This thermal bleaching rate is the fastest among
the reported one for HABI derivatives. The fast fading
kinetics makes it possible to change the color of the solution
Figure 4. (a) Decay profiles of the colored species generated from
1,8-NDPI-TPI-naphthalene monitored at 450 nm in degassed toluene
solution (1.2 × 10-4 M). The measurements were performed in
the temperature range from 298 to 325 K. (b) Eyring plots for the
thermal back-reaction over a temperature ranging from 298 to 325
K.
Org. Lett., Vol. 10, No. 14, 2008
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only where UV light is irradiated, because the diffusion rate
of colored species is slower than the decoloration rate at room
temperature (Movie S1, Supporting Information). The en-
thalpies and entropies of activation (∆H‡ and ∆S‡, respec-
tively) for the thermal back-reaction were estimated from
Eyring plots over a temperature ranging from 298 to 325 K.
The Eyring plots give an excellent straight line (Figure 4b),
and the ∆H‡ and ∆S‡ values estimated from standard least-
squares analysis of the Eyring plots are 53.0 kJ mol-1 and
-57.1 J K-1 mol-1, respectively.
In conclusion, we have developed a new type of radical
diffusion-inhibited HABI derivative with intense coloration
and fast decoloration performance. In the case of conven-
tional HABI derivatives, the photoinduced homolytic cleav-
age of the C-N bond of a heterodimer of imidazolyl radicals
yields a mixture of the homo- and heterodimers. However,
the combination of different imidazolyl radicals is kept for
the radical diffusion-inhibited HABI after many coloring-
decoloring cycles, and the chemical and physical properties
of each imidazolyl radical can be utilized to enhance the
photochromic performance. Though the reason why the
thermal bleaching rate of 1,8-NDPIR-TPIR-naphthalene is
drastically accelerated compared to that of 1,8-bisTPIR-
naphthalene is not clarified at the present stage, the steric or
electronic effects resulting from the combination of different
imidazolyl radicals probably play an important role in the
photochromic performance. Thus, the present study demon-
strated the advantage of the heterodimer of the imidazolyl
radical incorporated in the radical diffusion-inhibited HABIs.
We thus believe that this work may open an exciting new
avenue for future development of the photochromic dyes.
Acknowledgment. This work was supported by a Grant-
in-Aid for Science Research in a Priority Area “New
Frontiers in Photochromism” (471) (No. 19050011) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
Supporting Information Available: Synthesis of 1,8-
NDPI-TPI-naphthalene, experimental details of spectroscopic
measurements, crystallographic data in CIF format, transient
absorption spectra of the HABI and the NDPI dimer, DFT
calculations, and movies for the fading process in MOV
format. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL801135G
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Org. Lett., Vol. 10, No. 14, 2008