Unusual negative photochromism via a short-lived imidazolyl radical of 1,1′-binaphthyl-bridged imidazole dimer

Article abstract of DOI:10.1021/ja311344u

We have synthesized a new photochromic compound that exhibits unusual negative photochromism, in which the stable colored species photochemically converts into the metastable colorless species via a short-lived radical. This compound has a 1,1′-binaphthyl moiety bridging the two diphenylimidazole units. Its photochemical properties were investigated by nanosecond laser flash photolysis. The colored species isomerizes to the colorless species upon exposure to visible light and thermally returns to the original colored species within 20 min at room temperature. Moreover, the photodecoloration reaction proceeds via a short-lived radical with a half-life of 9.4 μs in benzene at room temperature. Both the colored and colorless species show the photoinduced homolytic bond cleavage reaction of the C-N bond between the nitrogen atom of the imidazole ring and the carbon atom of the 1-position of the 1,1′-binaphthyl moiety and that of the C-C bond between each of the carbon atoms of the 2-position of the imidazole ring, respectively, followed by their formation by rapid radical coupling.

Full text of DOI:10.1021/ja311344u

Article
pubs.acs.org/JACS
Unusual Negative Photochromism via a Short-Lived Imidazolyl
Radical of 1,1′-Binaphthyl-Bridged Imidazole Dimer
Sayaka Hatano,† Takeru Horino,‡ Atsuhiro Tokita,‡ Toyoji Oshima,‡ and Jiro Abe*,†,§
†Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara,
Kanagawa 252-5258, Japan
‡Tokyo Research Laboratory, Mitsubishi Gas Chemical Company, Inc., 6-1-1 Niijuku, Katsushika, Tokyo 125-0051, Japan
§CREST, Japan Science and Technology Agency, K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
S
* Supporting Information
ABSTRACT: We have synthesized a new photochromic
compound that exhibits unusual negative photochromism, in
which the stable colored species photochemically converts into
the metastable colorless species via a short-lived radical. This
compound has a 1,1′-binaphthyl moiety bridging the two
diphenylimidazole units. Its photochemical properties were
investigated by nanosecond laser flash photolysis. The colored
species isomerizes to the colorless species upon exposure to
visible light and thermally returns to the original colored
species within 20 min at room temperature. Moreover, the photodecoloration reaction proceeds via a short-lived radical with a
half-life of 9.4 μs in benzene at room temperature. Both the colored and colorless species show the photoinduced homolytic
bond cleavage reaction of the C−N bond between the nitrogen atom of the imidazole ring and the carbon atom of the 1-position
of the 1,1′-binaphthyl moiety and that of the C−C bond between each of the carbon atoms of the 2-position of the imidazole
ring, respectively, followed by their formation by rapid radical coupling.
Hence, it is important for the negative photochromism of
spiropyran to produce a suitable environment that stabilizes the
MC form. Thus, negative photochromism can be seen only
under the above-mentioned specific conditions. The develop-
ment of a unique photochromic compound with negative
photochromism would open a new avenue for the design of
advanced photoresponsive materials. Herein, we report a new
photochromic compound exhibiting unusual negative photo-
chromism, in which a stable colored species photochemically
converts to a metastable colorless species via a short-lived
radical. The isomerization mechanism from the colored to the
colorless species is completely different from those reported for
the negative photochromic compounds.
Hexaarylbiimidazole (HABI) is a photochromic compound
that shows photochemical color change from colorless
imidazole dimer to colored imidazolyl radical (Scheme 1a).4
This behavior of HABI can be attributed to the photogenerated
homolytic cleavage of the C−N bond between the imidazole
rings, resulting in the formation of the colored triphenylimid-
azolyl radical (TPIR). The decoloration reaction proceeds only
thermally by the radical recombination reaction of TPIRs to
form the original C−N bond. However, this reaction takes
several minutes at room temperature because of the TPIR
diffusion into the medium. To increase the thermal bleaching
rate, the TPIR diffusion should be suppressed. Thus, we have
INTRODUCTION
■
The reversible rearrangements of a chemical species between
two forms induced in one or both directions by light absorption
that causes changes in the absorption spectra is known as
photochromism.1 Photochromic materials change their color
upon irradiation; the photogenerated species can be reversed to
the original species either thermally or by subsequent
irradiation with a specific light wavelength. For example,
azobenzene shows a photochemically and thermally responsive
transformation between the cis and trans isomers. Diarylethene
and fulgide derivatives photochemically undergo reversible
cyclization and cycloreversion reactions. Most of the photo-
chromic compounds change from a thermally stable colorless
state to a metastable colored state upon UV light irradiation.
On the other hand, there are a few reports on “negative
photochromism” that is observed with several photochromic
molecules where the absorption spectrum of the compound
after irradiation is blue-shifted relative to that before
irradiation.2,3 The colored species of a negative photochromic
compound is more stable than the colorless species, and the
colored species becomes colorless upon exposure to visible
light. For example, in a highly polar environment, several
spiropyrans,2 especially those with free hydroxy, carboxy or
nitro groups, exhibit negative photochromism because of the
stabilization of the colored zwitterionic merocyanine (MC)
form. Factors such as hydrogen bonding,2n combination with
crown ether2f,o,q or a polypeptide,2e,k,p and complexation with a
metal2g−i,l can also lead to the stabilization of the MC form.
Received: November 19, 2012
© XXXX American Chemical Society
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Scheme 1. Photochromism of (a) HABI, (b) 1-NDPI-8-TPI-naphthalene, (c) pseudogem-bisDPI[2.2]PC, (d) pseudogem-DPI-
PI[2.2]PC, and (e) bisDMDPI-BN
Scheme 2. Preparations of (a) bisDMDPI-BN, (b) bisDMDPI-BP, and (c) pseudogem-bisDPI[2.2]PC
designed and synthesized several HABI derivatives with a
naphthalene or a [2.2]paracyclophane ([2.2]PC) moiety that
bridges two triphenylimidazole (TPI) or diphenylimidazole
(DPI) units.5,6 We have also succeeded in the acceleration of
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the thermal decoloration rate. The half-lives of the colored
species of naphthalene-bridged imidazole dimer, 1-NDPI-8-
TPI-naphthalene,5b and [2.2]PC-bridged imidazole dimer,
Pseudogem-bisDPI[2.2]PC,6a in solution are 170 and 33 ms at
298 K, respectively. pseudogem-DPI-PI[2.2]PC, containing a
phenanthroimidazole (PI) group, shows the fastest thermal
back-reaction, with a half-life of 35 μs in solution at 298 K.6b
The original colorless state is fully restored within 200 μs at 298
K.
In this study, we synthesized a novel HABI derivative, (2,2′-
dimethoxydiphenylimidazole)-1,1′-binaphthyl (bisDMDPI-
BN), with a flexible linker, 1,1′-binaphthyl (BN) moiety, that
bridges two DPI units (Scheme 1e). We found that this
molecule has a unique molecular structure compared with the
known photochromic imidazole dimers. This newly developed
imidazole dimer shows unusual negative photochromism with a
complicated photochemical reaction.
and the carbon atom C(8) at the 1-positon of the BN moiety.
This bond formation results in a five-membered ring structure.
The C−N bond length (1.487(2) Å) is approximately equal to
that of other bridged imidazole dimers. The colored species has
a spiro carbon at the BN moiety. The C(9)−C(10) bond
length (1.372(2) Å) is slightly longer than that typical CC
bond length (1.339 Å), and the N(2)−C(11) and N(3)−C(24)
bond lengths in the imidazole ring are 1.313(2) and 1.311(2)
Å, respectively. They are also shorter than the respective
C(10)−N(2) and C(10)−N(3) bond lengths of 1.394(2) and
1.413(2) Å. These results indicate that a part of the structure
including one imidazole ring forms the diazafulvene structure.6e
Recently, we reported a unique photochromic [2.2]PC-
bridged imidazole derivative with the C−N bond between the
[2.2]PC moiety and the imidazole ring, which yields a
monoradical on either UV or visible light irradiation.6e This
imidazolyl radical cannot form the C−N bond between the
imidazole rings because of the strong steric repulsion, and a
unique radical−radical coupling was observed. This exemplifies
the feasibility of the specific reaction of the imidazolyl radical.
Furthermore, the imidazolyl radical with the BN moiety also
reacts with the carbon atom of the 1-positon of the BN moiety.
In this molecule, due to the structural flexibility of the BN
moiety in arranging an orthogonal conformation, it is
considered that the two imidazole rings are very far apart to
form the C−N bond between the imidazole rings. As a result,
the carbon atom at the 1-position of the BN moiety is attacked
by the imidazolyl radical.
Molecular Structures of the Colorless Species. The
colored species was found to reversibly isomerize to the
colorless species upon exposure to visible light. Though the
details of the photochromic behavior of the colored species will
be discussed later, the negative photochromism of the colored
species was confirmed by measuring the UV−vis absorption
spectra. However, it is difficult to obtain crystals for the
colorless species of bisDMDPI-BN because of its thermal
instability. The colored species is restored from the colorless
species within 20 min at room temperature in the dark.
Therefore, we attempted to determine the molecular structure
of the colorless species using 13C NMR by comparison with
that of the colorless 1,1′-biphenyl-bridged imidazole dimer
(bisDMDPI-BP).
RESULTS AND DISCUSSION
■
Molecular Structure of the Colored Species. As shown
in Scheme 2c, generally, HABI derivatives can be obtained in a
colorless form with the C−N bond between the imidazole rings
by oxidizing the lophine precursor with potassium ferricyanide
under basic conditions. Upon the oxidation of the imidazole
anion, the radical species is generated and the C−N bond is
formed between the two imidazole rings by radical−radical
coupling, resulting in the formation of an imidazole dimer as
the colorless species. On the other hand, the oxidation of
bisDMDPIH-BN under basic conditions leads to the formation
of a deep-red-colored species in powder form.
The recrystallization of the compound with the exclusion of
light from a benzene/acetonitrile mixture gave deep-red blocks.
The molecular structure of the colored species revealed by X-
ray crystallographic analysis is shown in Figure 1. This structure
obtained by the oxidation of bisDMDPIH-BN is completely
different from that obtained by the oxidation of the
triarylimidazole derivatives. The colored species has the C−N
bond between the nitrogen atom N(1) of the imidazole ring
The preparation of bisDMDPI-BP is shown in Scheme 2b.
Following the oxidation of bisDMDPIH-BP with potassium
ferricyanide under basic conditions, bisDMDPI-BP is obtained
in a colorless form. The recrystallization of the compound from
an acetonitrile/ethanol mixture gave colorless blocks. The
molecular structure of bisDMDPI-BP was determined by X-ray
crystallographic analysis as shown in Figure 2. A remarkable fact
is the formation of a 2,2′-imidazole dimer with the C−C bond
(1.5619(16) Å) between the imidazole rings, whereas all of
other bridged imidazole dimers previously reported by our
group have the C−N bond between the imidazole rings. Thus,
bisDMDPI-BP has a couple of equivalent sp3 carbon atoms at
the 2-position of each imidazole ring. This compound does not
exhibit photochromic properties as observed in the 1,2′-
imidazole dimers. The colorless species of bisDMDPI-BN can
be also expected to have a molecular structure similar to
bisDMDPI-BP.
The 13C NMR spectra of the colored and colorless species of
bisDMDPI-BN and bisDMDPI-BP in CDCl3 are shown in
Figure 3. As previously described, the colorless species of
bisDMDPI-BN thermally isomerizes to the colored species,
Figure 1. ORTEP representations of the colored species of
bisDMDPI-BN with thermal ellipsoids (50% probability), where
nitrogen atoms are highlighted in blue and oxygen atoms are
highlighted in red. Hydrogen atoms are omitted for clarity.
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species of bisDMDPI-BN has also a symmetric structure similar
to that formed in bisDMDPI-BP. The symmetric feature of the
molecular structures for bisDMDPI-BN and bisDMDPI-BP is
1
also supported by the H NMR spectra (Figures S5 and S9).
Moreover, the colorless bisDMDPI-BP shows a single
characteristic peak at 105 ppm assignable to the sp3 carbon
of the imidazole ring. The spectrum of the colorless species of
bisDMDPI-BN also shows peaks with almost the same
chemical shifts of bisDMDPI-BP. Thus, from the analogy of
the molecular structure of bisDMDPI-BP, the molecular
structure of the colorless species of bisDMDPI-BN can be
unambiguously determined to be the 2,2′-isomer, where the
C−C bond is formed between the imidazole rings.
Photochromism. Generally, HABI derivatives show photo-
chromic behavior changing from the colorless imidazole dimer
to the colored imidazolyl radicals upon UV light irradiation.
The colored species recombines to form the original imidazole
dimer. Thus, the colorless species of HABI is more stable than
the colored species. In contrast, the oxidation of the precursor
bisDMDPIH-BN gives bisDMDPI-BN as the stable colored
species, as shown in Scheme 2a. The solution color of the
colored species changes from pale orange to colorless upon
visible light irradiation at room temperature, followed by the
back reaction to that thermally restores the colored species. By
taking advantage of 1,2-bis(2-methylbenzo[b]thiophene-3-yl)-
perfluorocyclopentene7 (DAE) as a standard, the photo-
chemical conversion efficiency from the colored species to
the colorless species was estimated by nanosecond laser flash
photolysis. The colorless open-ring isomer of DAE can undergo
cyclization in a conrotatory fashion to the colored closed-ring
isomer upon UV light irradiation. The thermal back-reaction is
symmetry-forbidden, giving excellent thermal stability to the
closed-ring isomer. The cycloreversion reaction from the
closed-ring isomer to the open-ring isomer occurs only by
irradiating with a visible light. Solutions of the colored species
of bisDMDPI-BN in benzene and the closed-ring isomer of
DAE in hexane with matched absorbances at the excitation
wavelength of 517 nm were irradiated at various laser energies.
Figure 2. ORTEP representations of the colorless species of
bisDMDPI-BP with thermal ellipsoids (50% probability), where
nitrogen atoms are highlighted in blue and oxygen atoms are
highlighted in red. Hydrogen atoms and solvent molecules are
omitted for clarity.
which is completely restored within 20 min in the dark at room
temperature. To decelerate the rate of the thermal back
reaction of the colorless species, the NMR measurements of the
colorless species were carried out at 223 K after forming
sufficient amounts of the colorless species with visible light
irradiation. The spectrum of the colorless species (Figure 3b) is
similar to that of bisDMDPI-BP (Figure 3c). The NMR
spectrum of the colored species of bisDMDPI-BN exhibits four
characteristic peaks assignable to four different methoxy groups,
whereas only two peaks are observed around 55 ppm in the
spectrum of bisDMDPI-BP. The colorless species of bis-
DMDPI-BN also shows only two peaks corresponding to
methoxy groups. The NMR spectra suggest that the colorless
Figure 3. 13C NMR spectra of (a) the colored species of bisDMDPI-BN, (b) the colorless species of bisDMDPI-BN, and (c) the colorless species of
bisDMDPI-BP in CDCl3 at 223 K (* = solvent peaks).
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The absorbance changes, ΔOD(DAE), before and after a laser
pulse irradiation at 517 nm for the closed-ring isomer of DAE
were plotted against those for the colored species of
bisDMDPI-BN, ΔOD(s) (Figure S15). The conversion
efficiency was estimated from the slope of the fit of the data
to the following equation,8
state at 298 K. The UV−vis absorption spectrum before the
visible light irradiation clearly corresponds to that of the
colored species with a characteristic absorption band ranging
from 400 to 550 nm. As it can be seen in Figure 4a, the
colorless species possesses no absorption bands at wavelengths
longer than 400 nm. The intensity of the characteristic of the
colored species absorption band at 490 nm gradually increases
in association with the spectrum change and isosbestic points at
300 and 355 nm after ceasing the visible light irradiation.
Finally, the UV−vis absorption spectrum becomes identical to
the spectrum of the colored species. From the time profile of
the absorbance at 490 nm in the dark, shown in Figure 4b, it is
found that the time taken to thermally restore the colored
species is approximately 20 min at 298 K. This coloration
process obeys first-order kinetics with a rate constant of 5.16 ×
10−3 s−1 at 298 K. The first-order rate constant, k, was
determined by fitting the experimental data to the following
equation,10
φDAE
ε
ΔOD(DAE) =
DAE ΔOD(s)
φεs
(1)
s
where φDAE (0.35) is the quantum yield of cycloreversion
reaction of DAE excited at 517 nm,9 φs is the conversion
efficiency from the colored species to the colorless species of
bisDMDPI-BN, and εDAE (0.91 × 104 M−1 cm−1) and εs (2.53
× 104 M−1 cm−1) are the absorptivity coefficients of DAE9 and
the colored species of bisDMDPI-BN at 517 nm, respectively.
As shown in Figure S15, φs is obtained from the slope of the fit
of the data to eq 1 and was determined to be 0.03. The full
experimental data are shown in the Supporting Information.
As will be discussed below, this photodecoloration reaction
of the colored species proceeds via the unstable radical species.
That is, the C−N bond of the colored species is homolytically
cleaved to generate the imidazolyl radical upon visible light
irradiation, followed by the radical coupling to form the
colorless species with the C−C bond between the imidazole
rings in the dark. Finally, the colorless species thermally
changes to the colored species.
⎛
⎜
⎝
⎞
⎟
⎠
A∞ − At
ln
= −kt
A
− A0
(2)
∞
where At, A0, and A∞ are the absorbances at time t, time zero,
and infinite time, respectively. As shown in the inset of Figure
4b, the first-order plot according to eq 2 gives a good straight
line. The rate constant was obtained from the slope of the first-
order plot.
Figure 4a shows the transient UV−vis absorption spectrum
of the benzene solution of the colorless species of bisDMDPI-
BN standing at 298 K in the dark. The measurement of the
transient UV−vis absorption spectrum started immediately
after ceasing the visible light irradiation at the photo-stationary
Two different thermal reaction paths can be considered for
the coloration process as indicated in Scheme 1e in broken
lines: stepwise isomerization via the radical species formed by
the homolytic bond cleavage of the C−C bond between the
imidazole rings and the one-step rearrangement. Although an
electron spin resonance (ESR) measurement was conducted to
detect the radical species in the course of the thermal
transformation from the colorless species to the colored
species, no significant ESR signal could be detected for the
solution of the colorless species. This result never neglects the
possibility for the stepwise isomerization path via the radical
species in view of the fact that the radical species with an
extremely short half-life would not be accumulated enough to
be detected by a conventional continuous-wave ESR apparatus.
Though the mechanism for the thermal back reaction to form
the colored species from the colorless species is not presently
clear, the photodecoloration phenomenon of the colored
species of bisDMDPI-BN is exactly negative photochromism.
Laser Flash Photolysis. As described above, the colorless
species of bisDMDPI-BN thermally isomerizes to the colored
species, whereas the colored species photochemically isomer-
izes to the colorless species upon visible light irradiation. We
have investigated the possible presence of the unstable
imidazolyl radical in the photochemical process of bis-
DMDPI-BN by nanosecond laser flash photolysis. Figure 5a
shows the transient vis−NIR absorption spectra of the colored
species of bisDMDPI-BN in benzene at 298 K, excited by a
nanosecond laser pulse at 355 nm. The identical transient
absorption spectrum was also obtained by using a 500 nm laser
pulse as the excitation light source. A sharp absorption band at
400 nm and a broad absorption band ranging from 500 to 1000
nm can be ascribed to the radical species with two parts of the
imidazolyl radicals because its spectrum shape is similar to that
of the colored radical species observed for the bridged
imidazole dimers. Figure 5b shows the decay profile of the
transient absorbance of the radical species in benzene at 750
Figure 4. (a) Transient UV−vis absorption spectra of bisDMDPI-BN
in deaerated benzene (2.9 × 10−5 M) at 298 K. Each of the spectra was
recorded at 1 min intervals after irradiation with visible light. The
spectrum of the colored species is shown in red and that of the
colorless species is shown in gray. (b) Time profile of the colorless
species generated from the colored species upon irradiation with
visible light (400−700 nm), monitored at 490 nm in deaerated
benzene at 298 K. The inset is a first-order plot according to eq 2.
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Figure 5. (a) Transient vis−NIR absorption spectra of the colored bisDMDPI-BN in deaerated benzene (1.1 × 10−4 M) at 298 K. Each of the
spectra was recorded at 5 μs intervals after excitation with a nanosecond laser pulse (excitation wavelength, 355 nm; pulse width, 5 ns; power, 4 mJ/
pulse). (b) Decay profile of the radical species generated from the colored species of bisDMDPI-BN monitored at 750 nm in deaerated benzene (6.0
× 10−5 M) at 298 K (excitation wavelength, 500 nm; pulse width, 5 ns; power, 4 mJ/pulse). (c) Decay profiles of the radical species generated from
the colored species (red line) and the colorless species (gray line) of bisDMDPI-BN monitored at 750 nm in deaerated benzene (3.4 × 10−5 M) at
281 K (excitation wavelength, 355 nm; pulse width, 5 ns; power, 4 mJ/pulse).
nm measured at 298 K with excitation light of 500 nm. The
decay profile of the radical species obeys first-order kinetics
with a half-life of 9.4 μs at 298 K, which is the fastest among the
reported half-lives of the radicals of HABI derivatives.
the colored species and the C−C bond of the colorless species,
because the molar absorptivity at 355 nm of these species is
almost similar to that seen in Figure 4a.
The decay profiles of the absorbance in Figure 5c were
monitored at 750 nm, at which wavelength only the radical
species absorbs. On the other hand, as shown in Figure 6, the
As described above, the half-life of the colored species of
pseudogem-DPI-PI[2.2]PC, shown in Scheme 1d, is 35 μs at 298
K.6b We considered that the change in Gibbs energy (ΔG0) of
the thermal back reaction would increase by destabilizing the
colored species. According to Marcus theory, the thermal back
reaction would accelerate with increasing ΔG0, which is closely
linked to decreasing the free energy of activation ΔG⧧. The
steric repulsion between the rigid phenanthroimidazole group
and the phenyl rings facing each other should destabilize the
radical, whereas the rotational motion along the C−C single
bond between the imidazole group and the phenyl ring in the
radical of pseudogem-bisDPI[2.2]PC should relax the steric
hindrance between the phenyl rings facing each other.
Therefore, the radicals of bisDMDPI-BN and pseudogem-DPI-
PI[2.2]PC are anticipated to be less labile.
Though no transient species of the colorless bisDMDPI-BP
could be detected by laser flash photolysis excited by a
nanosecond laser pulse at 355 nm, the laser excitation of the
colorless species of bisDMDPI-BN was found to give a colored
transient species. We compared the photochemical reaction
between the colored and colorless species in benzene solutions
with same concentration of bisDMDPI-BN at 281 K. First, the
laser flash photolysis was carried out for the solution of the
colored species. Then, the measurement for the colorless
species was performed immediately after the colored species
fully converted to the colorless species by prolonged irradiation
with visible light at 281 K. The negligibly low concentration of
the colored species for the light-irradiated solution was
confirmed by the UV−vis absorption spectrum, where the
absorbance at 490 nm attributed to the colored species was
almost negligible. As shown in Figure 5c, the decay profile of
the transient species formed from the colorless species is almost
identical to that of the radical species formed from the
photolysis of the colored species, except the magnitude of the
ΔOD value immediately after laser excitation. Thus, the
transient species formed by the photolysis of the colorless
species could be assigned to the identical radical species formed
from the colored species. The difference in the ΔOD value in
Figure 5c can be attributed to the difference in the quantum
yield for the bond cleavage reaction between the C−N bond of
Figure 6. Decay profiles of transient absorbance of the colorless
species of bisDMDPI-BN in deaerated benzene (1.6 × 10−5 M) at 281
K monitored at 490 and 750 nm in wide time region (excitation
wavelength, 355 nm; pulse width, 5 ns; power, 4 mJ/pulse).
remaining absorbance attributable to the formation of the
colored species is confirmed by the photolysis of the colorless
species by monitoring at 490 nm, at which wavelength both the
colored species and the radical species absorb. Thus the
colorless species is also converted to the colored species via the
short-lived radical species upon UV light irradiation at room
temperature. That is, the C−C bond between the imidazole
rings of the colorless species would be homolytically cleaved
into the imidazolyl radicals upon UV light irradiation as well as
the photochemical bond cleavage of the C−N bond between
the imidazole rings of the colored bisDMDPI-BN. The
photochemical properties of HABI derivatives have been
extensively investigated since the discovery of the photo-
chromic behavior of HABI. It is noteworthy that no reports
have suggested the photochemical C−C bond cleavage between
two imidazole rings for the photochromism of imidazole
dimers.
CONCLUSION
We developed a novel photochromic compound, bisDMDPI-
BN, that exhibits unusual negative photochromism, in which
■
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Scheme 3. Synthesis of (a) (2,2′-bisDMDPI)-1,1′-BN (3) and (b) (2,2′-bisDMDPI)-1,1′-BP (7)
DMSO-d6): δ 3.67 (s, 3H × 2, −OMe), 3.71 (s, 3H × 2, −OMe), 6.72
(d, J = 8.0 Hz, 2H × 2), 6.83 (d, J = 8.0 Hz, 2H × 2), 6.89 (d, J = 10.0
Hz, 1H × 2), 7.10 (d, J = 8.0 Hz, 2H × 2), 7.14 (d, J = 8.0 Hz, 2H ×
2), 7.26 (t, J = 10.0 Hz, 1H × 2), 7.46 (t, J = 10.0 Hz, 1H × 2), 7.99
(d, J = 10.0 Hz, 1H × 2), 8.02 (d, J = 10.0 Hz, 1H × 2), 8.17 (d, J =
10.0 Hz, 1H × 2), 13.82 (s, 1H × 2, −NH). 13C NMR (100 MHz, 296
K, DMSO-d6): δ 54.97, 55.11, 113.62, 114.14, 123.08, 125.66, 126.50,
126.93, 127.13, 127.93, 128.22, 128.69, 130.21, 132.47, 133.01, 134.38,
135.26, 146.46, 157.92, 158.58. ESI-MS (m/z): [M+H]+ 811.35, [M
+Na]+ 833.33.
the stable colored species converts to the metastable colorless
species via short-lived radical species. The colored species has
the C−N bond between the nitrogen atom of the imidazole
ring and the carbon atom of the 1-positon of the 1,1′-
binaphthyl moiety. The C−N bond formation gives a five-
membered ring structure, whereas the other imidazole ring
forms a diazafulvene structure. The colored species isomerizes
to the colorless species upon visible light irradiation at room
temperature, followed by back reaction to thermally restore the
colored species. This photodecoloration reaction of the colored
species upon visible light irradiation is negative photo-
chromism. The colorless species forms the 2,2′-imidazole
dimer with the C−C bond between each of the carbon atoms of
the 2-position of the imidazole ring. Both the colored and
colorless species show the photoinduced homolytic bond
cleavage reaction of the C−N and C−C bonds, respectively,
that produces the short-lived radical species with a half-life of
9.4 μs at 298 K. The rapid radical coupling reactions lead to the
formation of the colored and colorless species. This unusual
negative photochromism has not been reported to date and
may open new avenues in the development of advanced
photoresponsive materials.
bisDMDPI-BN Colored Species (3). All manipulations were carried
out with the exclusion of light. Under nitrogen, to a solution of 2 (1.55
g, 1.91 mmol) in benzene (200 mL) was added a solution of potassium
ferricyanide (36.2 g, 144 mmol) and KOH (16.1 g, 287 mmol) in
water (150 mL), and the reaction mixture was vigorously stirred for 2
h. The organic layer was separated, exhaustively washed with water,
and concentrated; 1.48 g of a dark red powder was obtained. 80 mg of
this residue was purified by recrystallization from CH2Cl2/hexane to
give dark red crystals (59.5 mg, 71.4%). Slow evaporation of benzene/
CH3CN of bisDMDPI-BN gave deep red, block single crystals suitable
1
for the X-ray crystallographic analysis. H NMR (400 MHz, 296 K,
CD2Cl2): δ 3.64 (s, 3H, −OMe), 3.66 (s, 3H, −OMe), 3.72 (s, 3H,
−OMe), 3.74 (s, 3H, −OMe), 6.37 (d, J = 8.9 Hz, 2H), 6.50 (d, J =
8.9 Hz, 2H), 6.64 (d, J = 8.9 Hz, 2H), 6.75 (d, J = 8.9 Hz, 2H), 6.78
(d, J = 8.9 Hz, 2H) 6.82 (d, J = 7.9 Hz, 1H), 6.85 (d, J = 9.8 Hz, 1H),
7.01 (ddd, J = 7.9, 6.1, 2.1 Hz, 1H), 7.13−7.25 (m, 4H), 7.33 (d, J =
8.9 Hz, 2H), 7.34−7.39 (m, 1H), 7.38 (d, J = 8.9 Hz, 2H), 7.41 (d, J =
8.9 Hz, 2H), 7.74 (d, J = 7.9 Hz, 1H), 7.82 (d, J = 9.8 Hz, 1H), 7.88
(d, J = 8.4 Hz, 1H), 8.10 (d, J = 8.4 Hz, 1H). 13C NMR (100 MHz,
223 K, CDCl3): δ 55.05, 55.13, 55.39, 55.42, 65.74, 113.12, 113.22,
113.32, 113.46, 117.90, 122.10, 122.32, 124.62, 125.39, 125.46, 125.59,
127.03, 127.25, 127.49, 128.53, 128.64, 129.11, 129.58, 129.95, 130.80,
130.90, 131.03, 131.55, 133.03, 136.36, 137.45, 140.78, 151.64, 152.99,
157.19, 158.39, 160.52, 160.72, 161.69, 163.92, 164.19. ESI-MS (m/z):
[M+H]+ 809.33.
EXPERIMENTAL SECTION
■
Synthesis. Commercially available reagents and solvents for
syntheses were of reagent grade and used without further purification.
1H and 13C NMR spectra were recorded with a NMR spectrometer
(Bruker, Avance III 400NanoBay). DMSO-d6 (0.03% TMS), CD2Cl2,
and CDCl3 (0.03% TMS) were used as deuterated solvents. Chemical
shifts are reported in parts per million (ppm) relative to
tetramethylsilane, and the coupling constants (J) are reported in
Hertz. ESI-TOF MS spectra were recorded on a Bruker Daltonics
micrOTOFII-AGA1 instrument using positive-ion mode.
1
bisDMDPI-BN Colorless Species (4). H NMR (400 MHz, 223 K,
bisDMDPI-BN (3) and bisDMDPI-BP (6) were prepared as shown
in Scheme 3.
[1,1′-Binaphthalene]-2,2′-dicarboxaldehyde (1). [1,1′-Binaphth-
alene]-2,2′-dicarboxaldehyde (1) was prepared according to a
literature procedure11a
CDCl3): δ 3.79 (s, 3H × 2, −OMe), 3.88 (s, 3H × 2, −OMe), 6.74 (d,
J = 8.8 Hz, 2H × 2), 6.90 (d, J = 8.8 Hz, 2H × 2), 7.24 (d, J = 8.1 Hz,
1H × 2), 7.39 (t, J = 8.1 Hz, 1H × 2), 7.42 (d, J = 8.8 Hz, 2H × 2),
7.46 (d, J = 8.8 Hz, 2H × 2), 7.57 (t, J = 10.0 Hz, 1H × 2), 7.82 (d, J =
10.0 Hz, 1H × 2), 7.88 (d, J = 10.0 Hz, 1H × 2), 7.98 (d, J = 10.0 Hz,
1H × 2). 13C NMR (100 MHz, 223 K, CDCl3): δ 55.31, 55.42, 106.38,
112.77, 113.28, 122.50, 123.75, 124.46, 124.68, 125.39, 128.10, 128.22,
128.34, 130.54, 130.66, 131.43, 132.49, 134.08, 160.65, 160.76, 165.49,
170.58.
bisDMDPIH-BN (2). 1 (940 mg, 3.03 mmol), 4,4′-dimethoxybenzil
(1.77 g, 6.55 mmol), and ammonium acetate (6.88 g, 89.2 mmol) were
refluxed in acetic acid (30 mL) for 18 h. After cooling to room
temperature, the reaction mixture was neutralized with aqueous NH3.
The slurry precipitate formed by neutralization was filtered off, washed
with water, and then dried. This residue was purified with silica gel
column chromatography using AcOEt/hexane =1/2 to 1/1 as eluent
[1,1′-Biphenyl]-2,2′-dicarboxaldehyde (5). [1,1′-Biphenyl]-2,2′-
dicarboxaldehyde (5) was prepared according to a literature
procedure11b
1
to give a yellow powder (1.72 g, 70.0%). H NMR (400 MHz, 296 K,
G
dx.doi.org/10.1021/ja311344u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
bisDMDPIH-BP (6). 5 (49.5 mg, 0.235 mmol), 4,4′-dimethoxybenzil
(174 mg, 0.643 mmol), and ammonium acetate (662 mg, 8.59 mmol)
were refluxed in acetic acid (3 mL) for 24 h. After cooling to room
temperature, the reaction mixture was neutralized with aqueous NH3.
The slurry precipitate formed by neutralization was filtered off, washed
with water, and then dried. This residue was purified with silica gel
column chromatography using AcOEt/hexane = 1/3 to 1/1 as eluent
to give a yellow powder, 136 mg (80.4%). 1H NMR (400 MHz, 296 K,
DMSO-d6): δ 3.69 (s, 3H × 2, −OMe), 3.73 (s, 3H × 2, −OMe), 6.75
(d, J = 8.4 Hz, 2H × 2), 6.85 (d, J = 8.8 Hz, 2H × 2), 7.05 (dd, J = 7.6,
1.0 Hz, 1H × 2), 7.13 (d, J = 8.8 Hz, 2H × 2), 7.19 (d, J = 8.4 Hz, 2H
× 2), 7.38 (td, J = 7.6, 1.0 Hz, 1H × 2), 7.47 (td, J = 7.6, 1.0 Hz, 1H ×
2), 7.72 (td, J = 7.6, 1.0 Hz, 1H × 2), 13.23 (s, 1H × 2, −NH). 13C
NMR (100 MHz, 296 K, DMSO-d6): δ 54.98, 55.11, 113.62, 114.12,
123.27, 126.20, 127.36, 127.42, 127.96, 128.23, 128.76, 129.39, 130.30,
130.64, 135.20, 140.86, 146.04, 157.88, 158.54. ESI-MS (m/z): [M
+H]+ 711.31.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic analysis data, HPLC chromatograms, and
1H and 13C NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jiro_abe@chem.aoyama.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the Core Research for
Evolutional Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST), a Grant-in-Aid
for Scientific Research (A) (22245025) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan, and NAIST Advanced Research Partnership Project.
bisDMDPI-BP (7). All manipulations were carried out with the
exclusion of light. Under nitrogen, to a solution of 6 (40.0 mg, 0.0563
mmol) in benzene (15 mL) was added the solution of potassium
ferricyanide (1.53 g, 6.07 mmol) and KOH (650 mg, 11.6 mmol) in
water (10 mL), and the reaction mixture was vigorously stirred for 2 h.
The organic layer was separated, exhaustively washed with water, and
concentrated; 27.8 mg of a white powder was obtained. This residue
was purified by recrystallization from CH3CN/EtOH to give colorless,
block crystals suitable for the X-ray crystallographic analysis (19.8 mg ,
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49.6%). H NMR (400 MHz, 223 K, CDCl3): δ 3.82 (s, 3H × 2,
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