Photochromic Radical Complexes That Show Heterolytic Bond Dissociation

Article abstract of DOI:10.1021/jacs.0c02739

Photochromic materials have been widely used in various research fields because of their variety of photoswitching properties based on various molecular frameworks and bond breaking processes, such as homolysis and heterolysis. However, while a number of

Full text of DOI:10.1021/jacs.0c02739

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Article
Photochromic Radical Complexes that Show Heterolytic Bond Dissociation
Ryosuke Usui, Katsuya Yamamoto, Hajime Okajima, Katsuya
Mutoh, Akira Sakamoto, Jiro Abe, and Yoichi Kobayashi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 May 2020
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Photochromic Radical Complexes that Show Heterolytic Bond
Dissociation
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‡
Ryosuke Usui , Katsuya Yamamoto , Hajime Okajima , Katsuya Mutoh , Akira Sakamoto , Jiro
‡,*
†,*
Abe and Yoichi Kobayashi
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga
†
5
25-8577, Japan.
‡Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku,
Sagamihara, Kanagawa 252-5258, Japan.
ABSTRACT: Photochromic materials have been widely used in various research fields because of their variety of
photoswitching properties based on various molecular frameworks and bond breaking processes such as homolysis and
heterolysis. However, while a number of photochromic molecular frameworks have been reported so far, there are few
reports on photochromic molecular frameworks that show both homolysis and heterolysis depending on the substituents
with high durability. The biradicals and zwitterions generated by homolysis and heterolysis have different physical and
chemical properties, and different potential applications. Therefore, the rational photochromic molecular design to control
the bond dissociation in the excited state on demand expands the versatility for photoswitch materials beyond the
conventional photochromic molecular frameworks. In this study, we synthesized novel photochromic molecules based on the
framework of a radical dissociation-type photochromic molecule: phenoxyl-imidazolyl radical complex (PIC). While the
conventional PIC shows the photoinduced homolysis, the substitution of a strong electron-donating moiety to the phenoxyl
moiety enables to switch the bond dissociation process from homolysis to heterolysis. This study gives a strategy for
controlling the bond dissociation process of the excited state of photochromic systems, and the strategy enables to develop
further novel radical and zwitterionic photoswitches.
■
INTRODUCTION
irradiation. For example, hexaarylbiimidazole (HABI) is a
well-known photochromic compound which generates a
pair of triphenylimidazolyl radical upon UV light
Photoinduced bond breaking is one of the most important
processes in photochemistry because it strongly influences
the direction of the chemical reactions and the reaction
yields. Among various photochemical reactions,
6
–8
irradiation.
2,3,4,4-Tetrachloro-l-oxo-l,4-
dihydronaphthalene (β-TKN)9 and dimeric nitroso
1
0,11
compounds
also show the photoinduced homolysis and
photochromism, which is defined as
a reversible
generate radical pairs upon light irradiation. Because
radical species generated by the photoinduced homolysis
have relatively high reactivity, these photochromic
materials can be applied to the initiator for radical
transformation of a chemical species between two forms by
light, has been widely applied in biological and materials
sciences because of their variety of photoswitching
1
–4
properties based on various molecular frameworks.
polymerizations
materials.
and
rate-tunable
photoswitching
Photoinduced bond breaking and making processes of
photochromic compounds can be categorized by several
processes: pericyclic reactions, E-Z isomerizations,
1
2
While the homolytic bond dissociation is the major
process for photoinduced σ-bond dissociations, there are
several compounds that show the photoinduced
2
tautomerizations, σ-bond dissociations, and so on. For
examples, photochromic reactions of diarylethene, fulgide,
spiropyran, naphthopyran, and naphthooxazine are
induced by the 6π electrocyclization reactions. Because the
bond breaking and making processes of these reactions are
cooperative processes, no intermediate chemical species
are detected in ultrafast spectroscopy. These reactions
usually generate closed-shell colored species upon light
irradiation.
1
3–21
heterolysis.
Triarylmethanes and diphenylmethyl
halides are well-known examples to show both homolysis
and heterolysis depending on the environments and
1
4
13
substituents. Systematic investigations by Turro, Hertz,
5
15,17,18
and Peter groups
suggest that the photoinduced
heterolysis is likely to occur when the potential energy of
*
n
the S (σ, σ ) state, which has the zwitterionic character,
decreases by dissolving in polar solvents or by introducing
substituents stabilizing the zwitterion. If the activation
barrier for the heterolysis is sufficiently low, the heterolysis
σ-bond dissociations are also important bond breaking
processes for various photochromic reactions. σ-bond
dissociations are classified into two cases: homolysis and
heterolysis. Photochromic molecules that show homolysis
generate open-shell radical pairs (or biradicals) upon light
*
1
from the S (π, π ) state occurs by the predissociation
process. Recently, Winter group gives more detailed
explanations of the photoinduced heterolytic bond cleavage
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Scheme 1. (a) Oxidation reactions of phenol and
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phenothiazine moieties. (b) Photochromic reaction
schemes of PIC, 1, and 2.
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Figure 1. (a) ORTEP representations of the molecular
structures of 1 and 2 with thermal ellipsoids (50%
probability), where nitrogen, oxygen, and sulfur atoms are
highlighted in blue, red, and yellow, respectively. (b) Steady-
state absorption spectra of 1 and 2 in benzene at room
temperature. Vertical lines indicate the theoretical spectra of
each
molecule
(MPW1PW91/6-31+G(d)//M05-2X/6-
3
1G(d) level of the theory).
of carbocation systems using complete active space self-
consistent field (CASSCF) methods. They suggest that the
heterogeneous radical moieties: imidazolyl and phenoxyl
radical moieties. It is interesting whether PIC shows
homolysis or heterolysis during the photochromic reaction.
Moreover, it is expected that the substitution of further
electron donating or withdrawing groups to the PIC
framework may switch the bond dissociation process from
homolysis to heterolysis. The different bond dissociation
process results in the products whose photophysical and
photochemical properties are different. Therefore,
revealing and controlling the photodissociation process is
important not only for fundamental photochemistry but
also for expanding the versatility of photofunctions over
21
conical intersection plays a crucial role in the photoinduced
heterolysis. Because the zwitterionic species generated by
the photoinduced heterolysis have the large dipole
moments, these photochromic materials can show
2
2
photoinduced aggregations and phase separations. These
properties have been applied to ionic channels and mass
2
2
transportations. Therefore, the switching of the bond
dissociation process between homolysis and heterolysis
2
1
expands the potentials of photochromic molecules.
However, because the photoinduced reactions of
triarylmethanes and diphenylmethyl halides have very low
reversibility and some of them are irreversible processes,
there were almost no photochromic molecular frameworks
that show both homolysis and heterolysis upon light
irradiation depending on substituents and environments
with high durability.
3
1,32
conventional PIC frameworks.
In this study, we synthesized novel photochromic radical
complexes composed of an imidazolyl radical and a
phenothiazine or a phenoxazine radical (1 and 2,
respectively, Scheme 1b) and investigated the ultrafast
bond dissociation processes of these compounds and PIC.
Phenothiazine is well-known as a strong electron donor and
the highest occupied molecular orbital (HOMO) level of the
Recently, Abe group has developed a series of radical-
dissociation-type photochromic compounds based on and
12,23–30
over the HABI framework.
Among them, phenoxyl-
nonsubstituted
ferrocene/ferrocenium (Fc/Fc )) is higher than those of
phenothiazine
(0.32
V
vs
+
33
imidazolyl radical complex (PIC) is one of the recently
developed rate-tunable T-type photochromic compounds
which reversibly generate an imidazolyl radical and a
phenoxyl radical (ring-opening form) in a molecule
+
phenol and triphenylimidazole (0.64 and 0.95 V vs Fc/Fc ,
3
4,35
respectively).
It suggests that the phenothiazine moiety
may act as the electron donor to the imidazole moiety
during the bond dissociation process. Moreover,
phenothiazine shows the similar oxidation reaction to that
of phenol as shown in Scheme 1a. This similarity suggests
that phenothiazine can be incorporated to the PIC
framework as similar to phenol of PIC. The detailed analyses
2
7,29,30
(
Scheme 1b).
The great advantage of PIC is that the
half-life of the generated biradical form can be rationally
controlled from tens of nanoseconds to tens of seconds by
replacing the central phenylene ring to the thiophene ring.
Another important aspect is that PIC is composed of two
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results show that the S states of the closed forms of 1 and 2
do not have the charge transfer (CT) feature.
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To investigate the photochromic properties of 1 and 2,
the transient absorption measurements from nanosecond
to microsecond time scales were conducted using the
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randomly interleaved pulse train (RIPT) method. Figures
2a and 2c show the transient absorption spectra of 1 and 2
−
4
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in benzene (7.1 × 10 and 5.7 × 10 M, respectively) excited
at 355 nm of a picosecond laser pulse at room temperature
under argon atmosphere. A previous report showed that the
photoinduced absorption of PIC due to the ring-opening
form, which is the resonance hybrid of the biradical and
quinoidal forms, was located at 470 and 660 nm upon
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excitation with UV light. The photogenerated ring-opening
form of PIC thermally reverted to the initial closed form
with a half-life of 250 ns in benzene at room temperature.
On the other hand, both 1 and 2 show the quite broad
transient absorption spectra over the visible and the near
infrared (NIR) light regions (Figures 2a and 2c). The
transient absorption spectra of 1 and 2 are largely shifted to
the NIR light region as compared to that of PIC, while the
spectral shapes are qualitatively similar to that of PIC. The
absorption peaks are located at 460, 610, and 1300 nm for
1 (Figure 2a). 2 shows the similar broad transient
absorption spectra to those of 1 and the peaks are located
at 450, 600, and 1200 nm (Figure 2c). DFT calculations
suggest that the triplet biradical forms are slightly more
stable than the singlet biradical forms in 1 and 2 (the energy
differences for 1 and 2 are 1.6 kJ/mol). However, while the
simulated absorption spectra of the singlet biradical and the
quinoidal forms reproduce the characteristic absorption
bands at the NIR light regions, the simulated absorption
spectrum of the triplet states has the absorption bands only
shorter than 600 nm (Figures S50 and S52). It suggests that
the ring-opening forms of 1 and 2 exist as the singlet states.
TDDFT calculations suggest that the molecular orbitals
Figure 2. Nanosecond to microsecond transient absorption
spectra and dynamics of 1 (a and b) and 2 (c and d) in benzene
(7.1 × 10 and 5.7 × 10 M, respectively) excited with a 355-
nm picosecond laser pulse under argon atmosphere at room
temperature. The probe wavelengths are 620 and 600 nm for
−
4
−4
1
and 2, respectively. Insets show the semi-logarithmic plots
of these time profiles.
with time-resolved visible and infrared (IR) absorption
spectroscopies show that the bond dissociation processes
of these compounds are different from conventional
radical-dissociation-type photochromic compounds and
these photochromic reactions are durable.
■
RESULTS AND DISCUSSION
The syntheses of 1 and 2 are shown in the experimental
section. The molecular structures of 1 and 2 were
unambiguously determined by X-ray crystallographic
analyses. Figure 1a shows the ORTEP representations of
these compounds. It shows that the phenothiazine and the
phenoxazine units are arranged almost orthogonal to the
imidazole skeleton as similar to PICs. The C−N bond lengths
of 1 and 2 are 1.502 and 1.488 Å, respectively. These values
(
MOs) originating to the absorption band at the NIR light
region are delocalized over whole imidazole and
phenothiazine (phenoxazine) moieties (Figures S50 and
S52). The larger delocalizations of MOs of the ring-opening
forms of 1 and 2 than that of PIC would be the most
plausible reason why the absorption spectra of 1 and 2 are
largely redshifted as compared to that of PIC. The transient
absorption spectra of 1 and 2 do not change with time and
decay monotonically with a time scale of microseconds.
These results suggest that the ring-opening form is formed
in 1 and 2 by the C−N bond dissociation upon light
2
6
are longer than those of PABI (1.459 Å) and PIC (1.485
2
7
Å).
Figure 1b shows the steady-state absorption spectra of 1,
and PIC in benzene at room temperature. While the
2
absorption band of PIC only appears shorter than 360 nm,
the absorption spectra of both 1 and 2 have additional
absorption bands around 360-500 nm. The time-dependent
2
7
irradiation as similar to PIC.
Figures 2b and 2d show the decay profiles of the
transient absorbance of 1 and 2 at the same condition to
those in Figures 2a and 2c. The probe wavelengths are 620
nm for 1 and 600 nm for 2. The semi-logarithmic plots of the
decays (insets of Figures 2b and 2d) clearly show that the
decays of the generated transient species follow the first-
order kinetics. The half-lives of the transient species are 400
and 310 ns for 1 and 2, respectively. The decay profiles of
the transient species of 1 and 2 do not change under air and
argon atmospheres, indicating that the triplet excited states
are not involved in the transient absorption signals (Figure
S17). The photogenerated transient signals do not degrade
even by the irradiation of intense picosecond laser pulses
density
functional
theory
(TDDFT)
calculations
(
MPW1PW91/6-31+G(d)//M05-2X/6-31G(d) level of the
theory) suggest that the additional absorptions in the
visible region of 1 and 2 are ascribed to the ππ* transitions
of the phenothiazine and the phenoxazine moieties (Figures
S49 and S51). Absorption spectra are not shifted depending
on the solvents (benzene, dichloromethane, acetonitrile,
and dimethyl sulfoxide (DMSO), Figure S15). It is important
to note that the steady-state emission spectra are not
shifted as well depending on the solvents, although the
emission spectrum of 2 in benzene could not be measured
due to the low emission quantum yield (Figure S16). These
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for more than 1 hour (20 μJ/pulse, 1 kHz), indicating that
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these photochromic reactions are durable (Figure S18).
Moreover, the photochromic reactions can be induced by
even 510-nm light for 1 and 480-nm light for 2 (Figures
S19-S21). These results show that the ππ* transitions of the
phenothiazine and phenoxazine moieties act as an efficient
sensitizer for the photochromic reactions.
The generated transient absorption spectra of the ring-
opening form do not shift depending on the polarity of the
solvent (Figures S23 and S26), while the rates of the thermal
back reaction increase with the increase in the solvent
polarity (Figures S24 and S27). The acceleration of the
thermal back reactions of the photogenerated transient
species in polar solvents was also observed in conventional
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radical-dissociation-type
photochromic
compounds.
Figure 3. Cyclic voltammograms of 1 (blue line), and 2 (red
line) in dichloromethane containing 0.1 M (TBA)PF as the
supporting electrolyte. The concentrations are 1.1 mM and
Insensitivity of the absorption spectra to the solvent
polarity and the acceleration of the thermal back reaction in
polar solvents of the ring-opening form indicate that the
ring-opening form is a neutral species rather than the CT
state as discussed later.
6
1
.0 mM for 1 and 2, respectively. Potential scan rate was set
to 1 V/s.
return sweep are most probably assigned to the oxidation
of the imidazole anion to the imidazolyl radical because of
the similarity of the results of PIC. The first oxidation peaks
can be assigned to the reduction of the phenothiazine and
phenoxazine anion. The reduction potentials of the
phenothiazine and phenoxazine anions of 1 and 2 are much
lower than the phenolate and imidazolyl anions of PIC
probably because the phenothiazine and phenoxazine
moieties have high electron donating properties. The
reduction peaks coupled with the oxidation peaks of the
dianion are not observed, most probably due to the fast
thermal back reactions of 1 and 2 as compared to the
potential scan rate. These results show that the
electrochemical reductions of 1 and 2 generate the dianion
species and spontaneously cleave the C−N bonds as similar
to other imidazole-based photochromic compounds.
It is known that the C−N bond of imidazole-based
photochromic compounds (HABI, bridged imidazole dimers,
PABI, and PIC) is cleaved by electrochemical reductions. To
investigate the bond dissociation processes of 1 and 2 by
electrochemical reductions, cyclic voltammogram (CV)
measurements were performed. In the case of the CV for
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PIC, the irreversible reduction peak was observed at −2.03
V, which was attributed to the electron transfer–chemical
reaction–electron transfer (ECE) mechanism. That is, one
electron reduction of PIC produces the closed-ring radical
anion, which spontaneously shows the C–N bond
dissociation reaction. The subsequent reduction of the
radicals generated from the closed-ring radical anion
further produces the dianion (an imidazolyl radical anion
and a phenoxyl radical anion) at the same reduction
potential. The formation of the dianion was evident for the
emergence of two oxidation peaks on the return sweep at
To reveal the detail of the bond dissociation processes,
the transient absorption measurements from femtosecond
to nanosecond time scales were conducted using a UV
femtosecond laser pulse as an excitation light. Figure 4
shows the transient absorption spectra of PIC, 1, and 2 in
−
0.76 and −0.63 V, which were ascribed to the oxidation of
the phenolate anion and the imidazole anion of PIC,
respectively.
−
4
−4
−4
benzene (3.4 × 10 , 6.7 × 10 , and 6.3 × 10 M,
respectively). The excitation wavelength was 330 nm for
PIC and 360 nm for 1 and 2. The obtained transient
absorption spectra were analyzed by the global analyses
with the singular value decomposition (SVD) with the
Figure 3 shows the CVs for 1 (1.1 mM, blue line) and 2
(1.0 mM, red line) in dichloromethane with 0.1 M TBAPF
a supporting electrolyte. The potential scan rate was set to
V/s to avoid the gradual decompositions of reacted
6
as
1
products. CVs of other potential scan rates are shown in
Figure S28. The irreversible reduction peaks of 1 and 2 are
observed at −1.92 and −1.99 V, respectively. The differential
electrochemical absorption spectra before and after the
constant potentials for 40 s are shown in Figure S29. The
applied constant potentials are −2.07 and −2.27 V versus
3
8
Glotaran program (http://glotaran.org). The details of the
analyses are shown in SI (Figures S34-S46). In the case of
PIC in benzene (Figure 4a), the excited-state absorption
instantaneously appeared at ~400 nm just after the
excitation at 330 nm (spectrum at 0.15 ps). This signal can
be assigned to the excited state absorption from the S
1
state
+
Fc/Fc for 1 and 2, respectively. The differential absorption
as similar to the previously reported radical-dissociation-
spectra are well explained by the superposition of the
absorption spectra of the dianion species generated by the
basic condition of the lophine forms (5 and 8 in Schemes 2
and 3) and the bleach of the absorption spectra of the closed
form of 1 and 2. Therefore, the spectroelectrochemistry
shows that the imidazole anion is formed under the
potential at −2.07 V for 1 and −2.27 V for 2. In the return
sweep after the reduction, two oxidation peaks are clearly
observed at −1.13 and −0.53 V for 1 and at −1.19 and −0.49
V for 2 as similar to PIC. The second oxidation peaks in the
3
9–41
type photochromic molecules.
The signal quickly
decays with a lifetime of 430 fs and a broad absorption band
assigned to the ring-opening form is alternatively
generated. It is noted that an isosbestic point was observed
at 483 nm until 1 ps (Figure S30a), indicating that the
ultrafast process does not have any intermediate species.
The possibility that the transient absorption band at ~400
nm is the CT state can be excluded because the decay of the
band becomes faster with the increase in the solvent
polarity (acetonitrile, Figure S46). Therefore, it suggests
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Figure 4. Femtosecond to sub-nanosecond transient absorption spectra of (a) PIC, (b) 1, and (c) 2 in benzene (3.4 × 10 , 6.7 ×
−
4
−4
1
2
0 , and 6.3 × 10 M, respectively) at room temperature. The excitation wavelength was 330 nm for PIC and 360 nm for 1 and
at room temperature. The excitation intensity was set to 36 nJ pulse−1 for PIC and 56 nJ pulse for 1 and 2.
−1
that the ring-opening form is instantaneously generated by
the photoinduced homolytic bond dissociation as similar to
dichloromethane, acetonitrile, and DMSO, respectively.
Although the decay components assigned to the
intermediate were small in 2, the intermediate signal of 2
also follows the similar behavior (Figure S33). The solvent
dependence of the intermediate lifetimes also suggests that
3
9–41
previously reported HABI derivatives and PABI.
The
transient absorption signal assigned to the ring-opening
form gradually grows with the lifetimes of 1.3 ps and 48 ps
most probably ascribable to the gradual increase in the
radical-radical interaction by the rotation of the two
radicals and vibrational cooling.
On the other hand, in the case of 1 in benzene, while the
transient absorption due to the S -S transition was
1 n
observed as similar to that of PIC, another transient
absorption band alternatively appeared at around 500 nm
at 1 ps with the decay of the S
1 n
-S absorption (Figure 4b).
The lifetime of the S state of 1 is 380 fs, which is almost
1
identical to that of PIC. The intermediate signal decayed
with a time constant of 1.1 ps and the broad transient
absorption band assigned to the ring-opening form was
alternatively observed at 450-620 nm. The transient
absorption band gradually grows with a time constant of 37
ps and the spectrum becomes almost identical to those
observed
at
nanosecond
transient
absorption
measurements. The similar intermediate spectrum was
observed in 2 at ~1 ps (Figure 4c). These results suggest
that the bond dissociation processes of these compounds
are different from those of conventional radical-
dissociation-type photochromic compounds, which show
the photoinduced homolysis.
To reveal the origin of the intermediate spectra, we
investigated the solvent dependence on the transient
absorption spectra. The details of the transient absorption
spectra in different solvents are shown in SI (Figures S30-
S32). Figure 5a shows the transient absorption dynamics at
Figure 5. (a) Transient absorption dynamics at 500 nm of 1
in different solvents and (b) the assignment of the
intermediate spectrum at 1 ps of 1 in benzene with the
TDDFT calculation of 1c shown in the inset
5
00 nm of
dichloromethane, acetonitrile, and DMSO. Except the sub-
picosecond decay of the S state, the picosecond decay
1
in different solvents: benzene,
1
(
MPW1PW91/6-31+G(d)//MPW1PW91/6-31G(d) level of
kinetics assigned to the intermediate state were
decelerated with the increase in the solvent dielectric
constant (Figure S47). The lifetimes of the intermediate
state of 1 are 1.1, 4.0, 7.2, and 19 ps for benzene,
theory). The gray, blue, and yellow atoms in the inset
indicate the carbon, nitrogen, and sulfur atoms,
respectively.
ACS Paragon Plus Environment

Journal of the American Chemical Society
the viscosity of the solvents is important factor as well as
Page 6 of 13
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
dielectric constants for these lifetimes because the
deceleration of the lifetime in DMSO is large as compared to
the increase in the dielectric constants. The small difference
of the lifetimes in dichloromethane and acetonitrile may be
due to the solvent viscosity. It is important to remind that
the S
1
state of the closed form does not have the CT feature
(
Figure S16). Moreover, because we observed one
intermediate species during the bond dissociation process
in the global analyses (Figure S44a), the stepwise process,
namely, electron transfer and then homolysis, is less
conceivable. In that case, two intermediate species should
be observed because a radical cation and a radical of
phenothiazine are formed in the stepwise manner.
Therefore, these results strongly suggest that the
intermediate state is the CT state generated by the
photoinduced heterolysis.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
If the CT state is formed, the cation of the phenothiazine
unit and the anion of the imidazole unit are formed. It has
been reported that the anion of the imidazole unit does not
have any absorption in the visible light region.37,42 It has
been also reported that the absorption spectrum of the
cation of the non-substituted phenothiazine shows a similar
absorption spectrum, although the absorption peak was
4
3
shifted to the shorter wavelength (417 nm). Moreover, the
observed transient absorption spectrum of the
intermediate of 1 is well explained by the simulated
absorption spectrum of the phenothiazine cation
substructure (1c) shown in the inset of Figure 5b by TDDFT
calculations. These results suggest that the observed
transient absorption spectrum is ascribable to the cation of
the phenothiazine moiety. It is noted that the imidazole and
phenothiazine moieties are orthogonally aligned before the
bond dissociation. The near-orthogonal alignment is
expected in the initial formation process of the CT state
because the CT state is formed in hundreds of femtoseconds
while the rotation of molecules often takes more than
Figure 6. Picosecond time-resolved IR (a) spectra and (c)
dynamics of 1 in CD Cl (~1.4 × 10 M) excited at 360 nm of
2 2
a femtosecond laser pulse at room temperature. (b) The
simulated IR spectrum of the singlet biradical, triplet
biradical, and quinoidal forms of the ring-opening form of 1
−
2
(
(U)CAM-B3LYP/6-31G(d) level of theory). The scalingꢀ
factor (0.955) was multiplied to the calculated IR
wavenumbers.
4
4,45
picoseconds.
The near-orthogonal alignment of the
imidazole and phenothiazine moieties is most probably the
reason why the intermediate transient signal is well
explained simply by the phenothiazine cation substructure.
These results clearly show that the photoinduced
heterolysis occurs and the CT state is formed in 1. The decay
process of the CT state with a time scale of picoseconds is
most probably due to the back electron transfer from the
imidazole anion moiety to the phenothiazine cation moiety
and the formation of the ring-opening form.
(~1.4 × 10−2 M) excited with a 360-nm femtosecond laser
pulse at room temperature. A broad absorption band is
−1
observed around 1280-1350 cm just after the excitation
(
0 ps). The transient band is most probably ascribable to the
state of the ring-closing form of 1 as similar to those of
S
1
41
PABI. At 2.5 ps after the excitation, an intense absorption
−1
peak is observed at 1350 cm . This signal is most probably
ascribable to the CT state as was observed in the visible
transient absorption spectroscopy. Because it is difficult to
simulate the CT state by DFT calculations, we tentatively
simulated the radical anion of a derivative of 1 (1a), where
the nitrogen atom of the phenothiazine moiety is capped
with a methyl group (Figure S48). The peaks at 1350 and
After the decay of the CT state, the transient absorption
bands at 621 nm for 1 and at 600 nm for 2 in benzene
gradually increase with the lifetimes of 37 ps and 51 ps. This
behavior was also observed in PABI and the time constant
4
1
−1
was ~10 ps. The gradual growth was attributed to the
thermal equilibrium between two transient isomers
differing in the contributions of biradical character. It is
expected that 1 and 2 show the similar behavior to PABI.
This will be discussed in detail in time-resolved IR
spectroscopy.
1330 cm could be assigned to the C−N stretching modes of
the imidazole anion moiety as shown in Figure S48.
Therefore, time-resolved IR spectroscopy also supports the
photoinduced heterolysis of 1. The peak gradually decays
with a time scale of picoseconds and the peaks at 1260,
−
1
1290, 1325, and 1365 cm alternatively appeared as
shown in Figure 6a. The transient absorption dynamics at
To further investigate the bond dissociation process and
the ring-opening form in detail, we measured picosecond
−
1
1
351 and 1262 cm are well correlated with the decay
kinetics of the CT state and the rise kinetics of the ring-
opening form (Figure 6c). The similarity of the dynamics
time-resolved IR spectra of 1 in CD
time evolution of the time-resolved IR spectra of 1 in CD
2
Cl
2
. Figure 6a shows the
Cl
2
2
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Journal of the American Chemical Society
+
−
between visible and IR spectroscopies suggests that the
r and r are the radii of the anion and cation, respectively.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
decay and the rise kinetics of the IR signals indicate the
transformation from the CT state to the ring-opening form.
In the actual estimation, the r and r values were
tentatively set to be 0.5 and 0.6 nm based on the optimized
molecular structures of the DFT calculations, respectively.
+
−
−
1
The peaks at 1262 and 1325 cm gradually increase with
the time scale of tens-to-hundreds of picoseconds (Figure
r
ε is the relative dielectric constant of the solvent employed
6
a). These peaks can be attributable to the closed-shell
for the electrochemical measurements and ε is the relative
dielectric constant of benzene, 2.28. The details of the
calculations are shown in the supporting information. The
estimated ΔGCT values of PABI, PIC, 1, and 2 in benzene are
0.20, 0.07, −0.18, and −0.19 eV, respectively. The simple
calculation suggests that the formations of the CT states of
1 and 2 are energetically possible, while those of PABI and
PIC are not. The stabilization of the CT state in 1 and 2 will
lower the activation barrier for the photoinduced
heterolysis. Thus, the pathway of the photoinduced
heterolysis could be the dominant process for the bond
dissociation over the pathway of the photoinduced
homolysis. Although it is difficult to directly compare the
activation barriers for the photoinduced homolysis and
heterolysis by this calculation, the stabilization of the CT
state by introducing the phenothiazine and the phenoxazine
moieties is the most plausible reason why 1 and 2 show the
photoinduced heterolysis while PABI and PIC show the
photoinduced homolysis.
quinoidal form of the ring-opening form of 1 as shown in
Figure 6b. The simulated IR spectrum of the triplet biradical
form is almost identical to that of the singlet biradical form
in these regions. However, we tentatively excluded the
contribution of the triplet biradical form based on the
results for the nanosecond visible transient absorption
measurements as shown in Figure S49. The transient IR
spectrum at 1 ns can be assigned to the superposition of the
simulated IR spectra of the singlet biradical and quinoidal
forms. Therefore, time-resolved IR spectroscopy also
suggests the dynamic evolution of the biradical character of
the ring-opening form as similar to PABI. However, more
sophisticated quantum chemical calculations such as the
extended multistate complete active space second-order
perturbation theory (XMS-CASPT2) are necessary to
discuss the details of the open-shell characteristics of the
ring-opening form of 1.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
1
A possible reason why 1 and 2 show the photoinduced
heterolysis is probably the decrease in the energy level of
the CT state generated by the photoinduced heterolysis in
these compounds. The stabilization of the CT state results in
the decrease in the activation barrier of the photoinduced
heterolysis. When the activation barrier of the
photoinduced heterolysis becomes lower than that of the
photoinduced homolysis, the photoinduced heterolysis
preferentially occurs. To verify the hypothesis, we estimate
The fact that the photoinduced heterolysis preferentially
occurs than the photoinduced homolysis in 1 and 2 suggests
that the activation barrier for the photoinduced heterolysis
is lower than that of the photoinduced homolysis in the
closed form, and may suggest that the pathway of the
heterolytic bond dissociation is an adiabatic process. It
suggests that the CT state generated by the heterolysis is
more stable than the biradical generated by the homolysis
around the closed form geometry. However, after the
formation of the CT states of 1 and 2 generated by the
photoinduced heterolysis, the back electron transfer occurs
and the neutral ring-opening form is generated. This result
indicates that the neutral ring-opening form becomes more
stable than the CT state in the ring-opening form geometry.
In the PIC and PABI frameworks, the spin-spin interaction
between two radical sites gradually increases after the bond
dissociation by the rotation of these moieties from the
1
the relative energy level of the CT state to the S state (ΔGCT)
of PABI, PIC, 1, and 2 by assuming that the imidazole moiety
acts as the acceptor while the other moiety (imidazole,
phenol, phenothiazine, and phenoxazine for PABI, PIC, 1,
and 2, respectively) acts as the donor with the Rehm-Weller
4
4,46
equation as below.
퐺CT = 퐹 퐸ox ― 퐸red
, ε, ε
2
푁 푒
A
∆
(
)
― ∆퐺D
∗
―
(1)
4
휋휀 휀푟
0 CT
4
1
F, Eox, Ered, ΔGD*, N
A
0
, rCT, and e indicate the Faraday
initial orthogonal geometry. The increased spin-spin
interaction dynamically stabilizes the biradical and the
quinoidal forms during the formation of the ring-opening
forms. The gradual increase in the spin-spin interaction
would be the most plausible reason why the CT state formed
by the photoinduced heterolysis is subsequently converted
to the neutral ring-opening form in 1 and 2.
constant, the oxidation potential of the electron donor, the
reduction potential of the electron acceptor, the energy of
the S -S transition of the donor, the Avogadro constant, the
0 1
relative dielectric constant of the medium, the vacuum
dielectric constant, the distance between the donor and the
acceptor, and the elementary charge, respectively. We used
the reduction potentials of PABI, PIC, 1, and 2 for Ered, which
is the reduction potential of these imidazole moieties
■
CONCLUSION
In summary, we developed novel photochromic
(
acceptors), because all reduction reactions produced the
compounds composed of an imidazolyl radical and a
phenothiazine or a phenoxazine radical. These compounds
show the photoinduced heterolysis in spite that the
molecular framework is based on the radical-dissociation-
type photochromic molecule. These compounds show
visible-sensitized and durable photochromic reactions
which induce the absorption change up to the NIR light
region. The lifetime of the current CT state is at most tens of
picoseconds in the polar solvent. However, further
improvements of the molecular structures such as the
combination of donor and acceptor type moieties would
imidazole anion. On the other hand, because it was difficult
to measure Eox of the donor moieties of these compounds,
we tentatively used the oxidation potentials of
nonsubstituted triphenylimidazole, phenol, phenothiazine,
3
3–35
and phenoxazine for Eox
.
The reduction and oxidation
potentials measured in different solvents (ΔsolvE,
acetonitrile and dichloromethane) were corrected for the
benzene solution by using the Born equation as below.
2
푁 푒
1
+
1
푟 ―
1
1
휀)
A
∆solv퐸 = ―
+
―
(2)
(
)
(
8
휋휀
0 푟
휀r
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Journal of the American Chemical Society
Page 8 of 13
Scheme 2. Synthetic procedure for 1.
was added to the reaction mixture and the solution was
extracted with water. The organic layers were dried with
anhydrous Na SO , the solvent was removed in vacuo and
the residue was purified by column chromatography on
silica gel (dichloromethane/hexane 5:1) to give
compound 4 (255 mg, 62 %) as yellow solid. H NMR
DMSO-d , 400 MHz) δ: 9.91 (s, 1H), 7.86 (dd, J = 8.0 Hz, 1.6
Hz, 1H), 7.70 (ddd, J = 7.8 Hz, 1.6 Hz, 1H), 7.52 (t, J = 7.6 Hz,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
4
=
1
(
6
1
8
3
H), 7.47 (d, J = 8.0 Hz, 1H), 7.05-6.99 (m, 3H), 6.94 (d, J =
.0 Hz, 1H), MARDI-TOFMS: m/z 303 [M + H] .
-[2-(4,5-diphenyl-1H-imidazol-2-yl)phenyl]-10H-
+
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
phenothiazine (5): Compound 4 (196 mg, 1.0 equiv.),
benzil (218 mg, 1.6 equiv.), and ammonium acetate (900 mg,
1
8 equiv.) were stirred at 110 °C in acetic acid (2 mL) in a
sealed tube for 15 h. The reaction mixture was cooled to
3
room temperature and neutralized by aqueous NH . The
extend the lifetimes of the CT state and zwitterion-type fast
photoswitches could be developed based on the PIC
framework. The fast photoswitching property will increase
the photostability of zwitterion-type photochromic
Scheme 3. Synthetic procedure for 2.
3
0
molecules. Moreover, the fast and large modulation of the
dipole moment may be also advantageous for the temporal
controls of photoinduced aggregations and phase
separations. The different bond dissociation process results
in the different photoproducts, and will expand the
versatility for the photochromic materials.
■
EXPERIMENTAL SECTION
Synthesis. 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 (Wakogel® C-300). All reagents and solvents
were obtained from commercial suppliers (Sigma-Aldrich,
Wako Chemicals, TCI) and the reagents were used without
further purification.
precipitate was filtered and washed with water. The crude
product was purified by silica gel column chromatography
(AcOEt/hexane = 1:1), to give compound 5 as a pale-yellow
3
-bromo-10H-phenothiazine (3): 10H-phenothiazine
1
(
2.1 g, 1.0 equiv.) was dissolved in dry THF (20 mL). The
powder, (248 mg 78%). H NMR (DMSO-d , 400 MHz) δ:
6
solution was deaerated by a constant stream of nitrogen
through a syringe for 30 min and cooled to 0 °C (ice
bath/water). N-Bromosuccinimide (NBS, 1.9 g, 1.0 equiv.)
was dissolved in dry and degassed THF (20 mL) and the NBS
solution was slowly dropped to the cooled solution over 1
h. Then the solution was stirred for 12 h at 0 °C (ice
bath/water) and allowed to room temperature. A saturated
aqueous solution of sodium sulfite was added to the
solution and the solution was extracted several times with
dichloromethane. The organic layer was dried with
11.96 (s, 1H), 8.67 (s, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.52-7.42
(m, 5H), 7.33 (s, 5H), 7.26-7.15 (m, 3H), 7.06 (t, J = 7.8 Hz,
1H), 6.92 (d, J = 8.0 Hz, 1H), 6.86 (dd, J = 10.4 Hz, 2.4 Hz, 1H),
6.82 (d, J = 2.4 Hz, 1H), 6.76 (t, J = 7.8 Hz, 1H), 6.70 (d, J = 8.0
Hz, 1H), 6.65 (d, J = 8.4 Hz, 1H), MARDI-TOFMS: m/z 494 [M
+
+ H] .
2,3-diphenylspiro[imidazo[2,1-a]isoindole-5,3'-
phenothiazine] (1): A solution of potassium ferricyanide
(3.0 g, 36 equiv.), KOH (1.6 g, 112 equiv.) in water (40 mL)
was added to a suspension of compound 5 (125 mg, 1.0
equiv.) in benzene (100 mL). After stirring for 6 h at room
temperature, the resultant mixture was then extracted with
benzene. The organic extract was washed with water and
dried in vacuo. After removal of the solvents, the crude
product was purified by gel permeation chromatography
(GPC). The yellow powder was obtained, 103.8 mg (84%).
2 4
anhydrous Na SO and filtered. The solvent was removed in
vacuo and the residue was purified by chromatography on
silica gel (hexane/ethyl acetate = 4/1) to give compound 3
1
(
1.0 g, 31%) as a colorless solid. H NMR (DMSO-d
6
, 400
MHz) δ: 8.73 (s, 1H), 7.16-7.11 (m, 2H), 7.00 (ddd, J = 8.0,
2
7
8
.0 Hz, 3H), 6.91 (dd, J = 8.0 Hz, 2.4 Hz, 1H), 6.77 (ddd, J =
.8 Hz, 2.0 Hz, 1H), 6.66 (dd, J = 8.4, 2.0 Hz, 1H), 6.60 (d, J =
.8 Hz, 1H).
1
H NMR (DMSO-d , 400 MHz) δ:, 7.87 (d, J = 7.6 Hz, 1H), 7.55
6
(t, J = 7.4 Hz, 1H), 7.49 (d, J = 8.0 Hz, 2H), 7.42 (t, J = 7.6 Hz,
1
1
6
C
H), 7.39-7.26 (m, 7H), 7.25-7.20 (m, 5H), 7.15 (t, J = 7.4 Hz,
H), 6.46 (d, J = 9.6 Hz, 1H), 6.32 (dd, J = 5 Hz, 2.8 Hz, 1H),
.15 (d, J = 2.4 Hz, 1H), HRMS (ESI-TOF) calculated for
2
-(10H-phenothiazin-3-yl)benzaldehyde
Compound 3 (386 mg, 1.0 equiv.), 2-formylphenylboronic
acid (230 mg, 1.1 equiv.), Pd(PPh (83.6 mg, 0.03 equiv.)
and K CO (364 mg, 1.9 equiv.) were added to degassed
(4)
:
3 4
)
+
33 21 3
H N S [M+H] : 492.1529, found: 492.1524.
2
3
3-bromo-10H-phenoxazine (6): 10H-phenoxazine (2.52 g,
1.0 equiv.) was dissolved in dry THF (20 mL). The solution
was deaerated by a constant stream of nitrogen through a
DME (30 mL) and water (10 mL). The light brown
suspension was heated to reflux under nitrogen for 80
hours. After cooling to room temperature, dichloromethane
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Journal of the American Chemical Society
syringe for 30 min and cooled to 0 °C (ice bath/water). NBS
(2.20 g, 1.0 equiv.) was dissolved dry and degassed THF (20
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Setups for Material Characterization and Steady-
mL) and the solution was slowly dropped to the cooled
phenoxazine solution over 1 h, then stirred for 2 h at 0 °C
(ice bath/water) and allowed to come to room temperature.
A saturated aqueous solution of sodium sulfite and the
aqueous layer was added to the solution and the solution
was extracted several times with dichloromethane. The
combined organic layers were dried with anhydrous Na SO
2 4
and filtered. The solvents were removed in vacuo and the
residue was purified by chromatography on silica gel
State Absorption Measurements. Proton nuclear
1
magnetic resonance ( H NMR) spectra were recorded at
400 MHz by JNM-ECS 400 MHz (JEOL). Matrix-assisted laser
desorption ionization time-of-flight mass spectrometry
(MALDI-TOF-MS) were recorded on Axima-CFRplus
(Shimadzu). High resolution (HR) ESI–TOF–MS spectra
were recorded on a Bruker micrOTOF II-AGA1 (Bruker).
High performance liquid chromatography (HPLC) was
conducted with Chromaster (Hitachi High-Technologies)
equipped with a reverse phase analytical column (Mightysil
RP-18GP II, 25 cm × 4.6 mm, 5 μm particle) from Kanto
Chemical Industries, a linear photodiode array (PDA)
detector. GPC was conducted with a recycling preparative
HPLC (Japan Analytical Industry Co., Ltd.) series equipped
with two GPC columns (JAIGEL-1H-A and JAIGEL-2H-A) and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
hexane/ethyl acetate 4:1) to give compound 6 (38%) as a
1
colorless solid. H NMR (CDCl
3 6
-d , 400 MHz) δ: 6.88-6.68 (m,
7
2
H).
-(10H-phenoxazin-3-yl)benzaldehyde (7) : Compound
6 (573 mg, 1.0 equiv.), 2-formylphenylboronic acid (387 mg,
1.2 equiv.), Pd(PPh (82.1 mg, 0.03 equiv.) and Na CO
3
)
3 4
2
(433mg, 1.8 equiv.) were added degassed toluene (30 mL)
and ethanol (10 mL). The light brown suspension was
heated to reflux under nitrogen for 18h. After cooling to
room temperature, the reaction mixture was extracted with
dichloromethane and the organic layer was washed with
water. The organic layers were dried with anhydrous
3
UV detector. CHCl was used as an eluent with the flow rate
of 3.5 mL/min. The diffraction data of the single crystal
were collected on Rigaku XtaLAB mini II with Mo Kα
radiation (λ = 0.71073 Å) at 100 K. Data integration and
Pro
reduction were undertaken with Rigaku CrysAlis . The
2
data refinement was carried out by Olex software package
4
7–49
2
Na SO
4
, the solvents were removed in vacuo, and the
with SHELXL program.
All non-hydrogen atoms were
residue was purified by column chromatography on silica
anisotropically refined. Steady-state absorption spectra
were measured with UV-3600 (Shimadzu).
gel (dichloromethane/hexane =5:1) to give compound 7 (42
1
%
) as orange solid. H NMR (DMSO-d
6
, 400 MHz) δ: 9.94 (s,
Voltammetry Measurements. All CV measurements
were performed in a conventional three-electrode cell. A
glassy carbon electrode (0.6 cm in diameter) was employed
as a working electrode after polishing with 1 μm diamond
on a diamond polishing pad and then with 0.05 μm alumina
on an alumina polishing pad attached to a glass plate (ALS
Co., Ltd.). The electrode was rinsed with pure acetone and
dried in air before use. A platinum wire was used as a
1
1
H), 8.47 (s, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.69 (t, J = 7.6 Hz,
H), 7.52 (t, J = 7.6 Hz, 1H), 7.5 (dd, J = 13.6 Hz, 8.0 Hz, 2H),
6.78-6.72 (m, 3H), 6.65-6.54 (m, 2H),6.49 (d, J = 8.0 Hz, 1H),
MARDI-TOFMS: m/z 287 [M + H]
+
3-[2-(4,5-diphenyl-1H-imidazol-2-yl)phenyl]-10H-
phenoxazine (8): Compound 7 (111.7 mg, 1 equiv.), benzil
(
147 mg, 1.8 equiv.), and ammonium acetate (830 mg, 28
equiv.) were stirred at 110 °C in acetic acid (2 mL) in a
sealed tube for 18 h. The reaction mixture was cooled to
+
counter electrode, and an Ag/Ag reference electrode (Ag
wire,
1
mM AgNO
3
,
0.1
M
tetrabutylammonium
3
room temperature and neutralized by aqueous NH . The
precipitate was filtered and washed with water. The crude
product was purified by silica gel column chromatography
hexafluorophosphate in acetonitrile) was employed. All CV
measurements were achieved from 0.05 to 1 V/s in
solutions of 0.1 M (TBA)PF in dichloromethane at room
6
(
AcOEt/hexane = 1:1), to give compound 8 as a pale-peach
temperature. Prior to each experiment, the solutions were
deoxygenated by bubbling with nitrogen, and the nitrogen
atmosphere was maintained throughout the course of the
experiments. All potentials are referenced to the reversible
1
powder, (78%). H NMR (DMSO-d
1
7
6
, 400 MHz) δ: 11.99 (s,
H), 8.27 (s, 1H), 7.68 (d, J = 8.0 Hz, 1H), 7.52-7.46 (m, 3H),
.46-7.40 (m, 3H), 7.33 (s, 5H), 7.25 (t, J = 8.0 Hz, 2H), 7.18
(t, J = 8.0 Hz, 1H), 6.73 (t, J = 8.0 Hz, 1H), 7.46-7.40 (m, 4H),
6.46 (d, J = 8.0 Hz, 1H), 6.40 (d, J = 8.0 Hz, 1H), MARDI-
+
formal potential for the Fc/Fc couple. μAutolab III
potentiostat/ galvanostat (MetrohmAutolab B. V.) under
computer control (General Purpose Electrochemical System
software) was used for the CV measurement.
+
TOFMS: m/z 478 [M + H]
2,3-diphenylspiro[imidazo[2,1-a]isoindole-5,3'-
phenoxazine] (2) : A solution of potassium ferricyanide
Spectroelectrochemical Measurements. Quartz glass
cells with a 0.5 mm path length and a 1.0 cm path length
were used for the spectroelectrochemistry for all the
compounds. A standard three-electrode arrangement with
a Pt mesh as a working electrode, a platinum wire as a
(
623 mg, 34 equiv.), KOH (350 g, 123 equiv.) in water (30
mL) was added to a suspension of compound 8 (35.6 mg, 1.0
equiv.) in benzene (50 mL). After stirring for 90 min at room
temperature, the resultant mixture was then extracted with
benzene. The organic extract was washed with water and
dried in vacuo. After removal of the solvents, the crude
product was purified by GPC. The ocher powder was
+
counter electrode, and Ag/Ag as a reference electrode was
employed. A spectrophotometer (Ocean-FX, Ocean Optics,
Inc.) was used to record the absorption spectra in the range
from 350 to 900 nm. All measurements were performed
1
obtained, 21.6 mg (61%). H NMR (400 MHz, DMSO-d6) δ:
.87 (d, J = 8.0 Hz, 1H), 7.55 (t, J = 7.6 Hz, 1H), 7.48 (d, J = 7.6
7
2
using N bubbled solutions.
Hz, 1H), 7.45–7.34 (m, 4H), 7.31– 7.19 (m, 5H), 7.17– 7.08
(m, 2H), 7.01 (d, J = 8.0 Hz, 1H), 6.44 (d, J = 9.6 Hz, 1H), 6.33
(d, J = 4.8 Hz, 2.0 Hz, 1H), 5.54 (d, J = 9.6 Hz, 1H), HRMS (ESI-
Nanosecond to Microsecond Transient Absorption
Measurements. Nanosecond to microsecond transient
absorption measurements were conducted by the
21 3
TOF) calculated for C33H N O [M+H]+: 476.1757, found:
3
6
randomly-interleaved-pulse-train (RIPT) method.
A
4
76.1756.
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Journal of the American Chemical Society
Page 10 of 13
picosecond laser, PL2210A (EKSPLA, 1 kHz, 25 ps, 3.4 was estimated to be ~0.5 ps. To remove the artificial signal
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mJ/pulse for 355 nm), and a supercontinuum (SC) radiation
source (SC-450, Fianium, 20 MHz, pulse width: 50–100 ps
depending on the wavelength, 450–2000 nm), were
employed as the pump–pulse and probe sources,
respectively. The wavelength of the excitation pulse was
altered from 355 nm to 510 nm depending on the
experiments. The measurements were performed in a
benzene solution placed in a 2-mm quartz cell under argon
condition with stirring at room temperature.
that appears in transient IR spectra for all time delay, the
spectrum at −400 ps, when the probe pulse comes before
the pump pulse, was subtracted from the all transient IR
spectra.
2 2 2
1 in CD Cl solution filled in a BaF cell (optical path
length; 200 μm) was measured. The accumulation time was
1 hour for each time delay and totally 18 hours for all time
delays. Since the sample stage was moved right and left
(
velocity; 1 mm/s) during the measurement, transient and
Femtosecond to Nanosecond Transient Absorption
Measurements. Transient absorption measurements on
the femtosecond to nanosecond time scale were conducted
by a homemade pump-probe system. An amplified
femtosecond laser, Spirit One 1040-8 (Spectra-Physics,
steady-state IR spectra of the sample was not changed even
after the 18-hours experiment.
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Supporting Information
Details of characterizations, spectroscopic analyses, and DFT
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
040 nm, the pulse width: ~270 fs), was split into two
beams with a ratio of 1:9. The stronger beam was directed
to a noncollinear optical parametric amplifier (NOPA),
Spirit-NOPA-3H (Spectra-Physics) to generate the 330- or
360-nm femtosecond laser pulse for the pump beam. The
pump beam was chopped prior to the sample at 500 Hz for
signal differencing. The other weaker beam was focused to
a deuterated water placed in a 10-mm quartz cuvette to
generate the white light continuum for the probe beam.
Both pump and probe beams were focused to the sample
solution placed in the 2-mm quartz cuvette. The excitation
beam was focused to a spot with the full width at the half
maximum (FWHM) of ~70 μm. The spot size was measured
by knife-edge method assuming a Gaussian laser beam. The
polarization between the pump and probe pulses was set at
magic angle. The intensity of the pump pulse was set to 36
nJ pulse for PIC and 56 nJ pulse for 1 and 2. The
transmitted probe beam was detected with multichannel
detection system, PK120-C-RK (UNISOKU), composed of a
CMOS linear image sensor and a polychromator. The
obtained spectra were calibrated for group velocity
dispersion using the data obtained by the optical Kerr signal
AUTHOR INFORMATION
Corresponding Author
*
*
ykobayas@fc.ritsumei.ac.jp
jiro_abe@chem.aoyama.ac.jp
ACKNOWLEDGMENTS
This work was supported partly by JSPS KAKENHI Grant
Numbers JP18K14194, JP18H05263. The authors acknowledge
Dr. Tatsuo Nakagawa and Mr. Hiroaki Hanada, UNISOKU Co.,
Ltd., for nanosecond time-resolved transient absorption
measurements, Prof. H. Maeda at Ritsumeikan University for
MALDI-TOF MS and gel permeation chromatography, and Prof.
Y. Shigeta for valuable comments for quantum chemical
calculations.
−1
−1
REFERENCES
(
1)
Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S.
Photochromism of Diarylethene Molecules and
Crystals: Memories, Switches, and Actuators. Chem.
Rev. 2014, 114 (24), 12174–12277.
2 2
of CH Cl between the pump pulse and the white-light
continuum. The instrumental response function was
shorter than approximately 100 fs. The sample solution was
stirred with a stirrer during the experiments and the
measurements were performed at room temperature.
(2)
Dürr, H.; Bouas-Laurent, H. Photochromism: Molecules
and Systems; 2003.
(
(
3)
4)
Photon-Working Switches; Yokoyama, Y., Nakatani, K.,
Eds.; Springer Japan: Tokyo, 2017.
Organic Photochromic and Thermochromic
Compounds Volume 1 Main Photochromic Families;
Crano, J. C., Guglielmetti, R. J., Eds.; Plenum: New York,
1999.
Picosecond
Time-Resolved
IR
Absorption
Measurements. The detail of the experimental setup was
described in the previous study of the excitation dynamics
4
1
of PABI. The fundamental output from a Ti:sapphire
regenerative amplifier (Solstice Ace, Spectra Physics,
wavelength; 800 nm, repetition rate; 1 kHz, pulse duration;
(5)
Tamai, N.; Miyasaka, H. Ultrafast Dynamics of
Photochromic Systems. Chem. Rev. 2000, 100 (5),
1
90 fs) was used to excite two optical parametric amplifiers
1
875–1890.
(
TOPAS-C and TOPAS-prime, Light Conversion) and
(
(
6)
7)
Hayashi, T.; Maeda, K. Preparation of a New
Phototropic Substance. Bull. Chem. Soc. Jpn. 1960, 33,
generate pump UV pulse (wavelength; 360 nm, energy at
the sample point; 1 μJ/pulse) and probe IR pulse (spectral
5
65–566.
−
1
coverage; 1250~1520 cm , < 0.4 μJ/pulse). The pump UV
pulse was modulated at half the repetition rate by a
mechanical chopper and focused onto the sample non-
collinearly against the probe pulse. The probe IR pulse was
passed through the photo-excited sample and detected with
a 19-cm spectrograph (TRIAX190, HORIBA JOBIN YVON), a
multichannel liquid-nitrogen-cooled HgCdTe detector array
and box-car integrators (IR-12-128, InfraRed Associates).
The cross-correlation between the pump and probe pulses
Hayashi, T.; Maeda, K.; Kanaji, T. The Mechanism of
the Photochromism and Thermochromism of
2,2’4,4’5,5’-Hexaphenyl-1,1’-Biimidazolyl in a Solid
State. Bull. Chem. Soc. Jpn. 1965, 38 (5), 857.
Hayashi, T.; Maeda, K. Photochromic Color Change of
the Dimer of Triphenylimidazolyl at Low
Temperatures. Bull. Chem. Soc. Jpn. 1969, 42, 3509–
3514.
(8)
(9)
Scheibe, G.; Feichtmayr, F. The Photochemistry of
Tetrachloroketodihydronaphthalene as a Reversible
ACS Paragon Plus Environment

Page 11 of 13
1
Journal of the American Chemical Society
Photochemical Reaction. J. Phys. Chem. 1962, 66 (12),
Imidazolyl Radical of 1,1’-Binaphthyl-Bridged
Imidazole Dimer. J. Am. Chem. Soc. 2013, 135 (8),
3164–3172.
2449–2455.
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(10)
Mackor, A.; Wajer, T. A. J. W.; de Boer, T. J.; van Voorst,
J. D. W. C-Nitroso Compounds. Part I. The Formation
of Nitroxides by Photolysis of Nitroso Compounds as
Studied by Electron Spin Resonance. Tetrahedron Lett.
(26)
(27)
Yamashita, H.; Abe, J. Pentaarylbiimidazole, PABI: An
Easily Synthesized Fast Photochromic Molecule with
Superior Durability. Chem. Commun. 2014, 50 (c),
8468–8471.
1
966, No. 19, 2115–2123.
(
11)
Bluhm, A. L.; Weinstein, J. Photodissociation of
Nitroso Dimers. Nature 1967, 215, 1478–1479.
Shima, K.; Mutoh, K.; Kobayashi, Y.; Abe, J. Enhancing
the Versatility and Functionality of Fast Photochromic
Bridged Imidazole Dimers by Flipping Imidazole
Rings. J. Am. Chem. Soc. 2014, 136 (10), 3796–3799.
Herz, M. L. The Photochemical Ionization of the
Triarylmethane Leuconitriles. J. Am. Chem. Soc. 1975,
Yamashita, H.; Ikezawa, T.; Kobayashi, Y.; Abe, J.
Photochromic Phenoxyl-Imidazolyl Radical
Complexes with Decoloration Rates from Tens of
Nanoseconds to Seconds. J. Am. Chem. Soc. 2015, 137,
4952–4955.
Yamaguchi, T.; Kobayashi, Y.; Abe, J. Fast Negative
Photochromism of 1,1′-Binaphthyl-Bridged Phenoxyl
−Imidazolyl Radical Complex. J. Am. Chem. Soc. 2016,
138, 906–913.
Ikezawa, T.; Mutoh, K.; Kobayashi, Y.; Abe, J.
Thiophene-Substituted Phenoxyl-Imidazolyl Radical
Complexes with High Photosensitivity. Chem.
Commun. 2016, 52 (12), 2465–2468.
Kobayashi, Y.; Mishima, Y.; Mutoh, K.; Abe, J. Highly
Durable Photochromic Radical Complexes Having No
Steric Protections of Radicals. Chem. Commun. 2017,
53 (31), 4315–4318.
Kobayashi, Y.; Mutoh, K.; Abe, J. Fast Photochromic
Molecules toward Realization of Photosynergetic
Effects. J. Phys. Chem. Lett. 2016, 7 (18), 3666–3675.
Kobayashi, Y.; Mutoh, K.; Abe, J. Stepwise Two-Photon
Absorption Processes Utilizing Photochromic
Reactions. J. Photochem. Photobiol. C Photochem. Rev.
2018, 34, 2–28.
Kistenmacher, A.; Baumgarten, M.; Enkelmann, V.;
Pawlik, J.; Mullen, K. Novel Donor 1,4-
Benzothiazino[2,3-b]Phenothiazine. J. Org. Chem.
1994, 59 (4), 2743–2747.
(12)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(28)
(29)
(30)
(
(
13)
14)
9
7 (23), 6777–6785.
Geiger, M. E.; Turro, N. J.; Waddell, W. H.
Triarylmethane Dye Photochromism: Spectroscopy
and Photochemistry of Brilliant Green Leucocyanide.
Photochem. Photobiol. 1977, 25, 15–20.
Peters, K. Picosecond Organic Photochemistry. Annu.
Rev. Phys. Chem. 1987, 38 (1), 253–270.
Bartl, J.; Mayr, H.; Steenken, S.; McClelland, R. A.
Photo-Heterolysis and -Homolysis of Substituted
Diphenylmethyl Halides, Acetates, and Phenyl Ethers
in Acetonitrile: Characterization of Diphenylmethyl
Cations and Radicals Generated by 248-Nm Laser
Flash Photolysis. J. Am. Chem. Soc. 1990, 112 (19),
(
(
15)
16)
(31)
(32)
6
918–6928.
(
17)
Dreyer, J.; Peters, K. S. Picosecond Kinetic Study of the
Photoinduced Homolysis and Heterolysis of
Diphenymethyl Bromide. 1. The Nature of the
Conversion from Radical Pairs to Ion Pairs. J. Phys.
Chem. 1996, 100 (37), 15156–15161.
(33)
(18)
Lipson, M.; Deniz, A. A.; Peters, K. S. Nature of the
Potential Energy Surfaces for the SN1 Reaction: A
Picosecond Kinetic Study of Homolysis and
Heterolysis for Diphenylmethyl Chlorides. J. Am.
Chem. Soc. 1996, 118 (12), 2992–2997.
Baciocchi, E.; Del Giacco, T.; Elisei, F.; Lanzalunga, O.
Homolytic vs Heterolytic C-H Bond Cleavage in
Alkylaromatic Radical Cations. Formation of
Diarylmethyl Cation in the Photoinduced Electron
Transfer Reaction of Bis(4-Methoxyphenyl)Methane
Sensitized by Chloranil [3]. J. Am. Chem. Soc. 1998,
(34)
(35)
Ulrike, L.; Baumgartel, H. Electrochemical Formation
and Cleavage of Bis-Imidazolyls. J. Electroanal. Chem.
1977, 78, 133–143.
Pavitt, A. S.; Bylaska, E. J.; Tratnyek, P. G. Oxidation
Potentials of Phenols and Anilines: Correlation
Analysis of Electrochemical and Theoretical Values.
Environ. Sci. Process. Impacts 2017, 19 (3), 339–349.
Nakagawa, T.; Okamoto, K.; Hanada, H.; Katoh, R.
Probing with Randomly Interleaved Pulse Train
Bridges the Gap between Ultrafast Pump-Probe and
Nanosecond Flash Photolysis. Opt. Lett. 2016, 41 (7),
1498.
Yamamoto, K.; Mutoh, K.; Abe, J. Photo- and Electro-
Driven Molecular Switching System of Aryl-Bridged
Photochromic Radical Complexes. J. Phys. Chem. A
2019, 123 (10), 1945–1952.
Snellenburg, J. J.; Laptenok, S. P.; Seger, R.; Mullen, K.
M.; van Stokkum, I. H. M. Glotaran: A Java-Based
Graphical User Interface for the R Package TIMP. J.
Stat. Softw. 2012, 49 (3), 1–22.
Satoh, Y.; Ishibashi, Y.; Ito, S.; Nagasawa, Y.; Miyasaka,
H.; Chosrowjan, H.; Taniguchi, S.; Mataga, N.; Kato, D.;
Kikuchi, A.; Abe, J. Ultrafast Laser Photolysis Study on
Photodissociation Dynamics of a Hexaarylbiimidazole
Derivative. Chem. Phys. Lett. 2007, 448 (4–6), 228–
231.
(
19)
(36)
1
20 (45), 11800–11801.
(
(
20)
21)
Jarikov, V. V.; Neckers, D. C. Photochemistry and
Photophysics of Triarylmethane Dye Leuconitriles. J.
Org. Chem. 2001, 66 (3), 659–671.
(37)
(38)
(39)
Buck, A. T.; Beck, C. L.; Winter, A. H. Inverted
Substrate Preferences for Photochemical Heterolysis
Arise from Conical Intersection Control. J. Am. Chem.
Soc. 2014, 136 (25), 8933–8940.
Klajn, R. Spiropyran-Based Dynamic Materials. Chem.
Soc. Rev. 2014, 43 (1), 148–184.
Fujita, K.; Hatano, S.; Kato, D.; Abe, J. Photochromism
of a Radical Diffusion-Inhibited Hexaarylbiimidazole
Derivative with Intense Coloration and Fast
Decoloration Performance. Org. Lett. 2008, 10 (14),
3105–3108.
Kishimoto, Y.; Abe, J. A Fast Photochromic Molecule
That Colors Only under UV Light. J. Am. Chem. Soc.
(
(
22)
23)
(24)
(40)
Miyasaka, H.; Satoh, Y.; Ishibashi, Y.; Ito, S.; Nagasawa,
Y.; Taniguchi, S.; Chosrowjan, H.; Mataga, N.; Kato, D.;
Kikuchi, A.; Abe, J. Ultrafast Photodissociation
Dynamics of a Hexaarylbiimidazole Derivative with
2
009, 131 (12), 4227–4229.
(
25)
Hatano, S.; Horino, T.; Tokita, A.; Oshima, T.; Abe, J.
Unusual Negative Photochromism via a Short-Lived
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Pyrenyl Groups: Dispersive Reaction from
Femtosecond to 10 Ns Time Regions. J. Am. Chem. Soc.
2009, 131 (21), 7256–7263.
Kobayashi, Y.; Okajima, H.; Sotome, H.; Yanai, T.;
Mutoh, K.; Yoneda, Y.; Shigeta, Y.; Sakamoto, A.;
Miyasaka, H.; Abe, J. Direct Observation of the
Ultrafast Evolution of Open-Shell Biradical in
Photochromic Radical Dimer. J. Am. Chem. Soc. 2017,
(44)
(45)
Lakowicz, J. R. Principles of Fluorescence Spectroscopy,
3rd ed.; Springer Science & Business Media, 2006.
Turro, N. J.; Ramamurthy, V. V.; Scaiano, J. C. Principles
of Molecular Photochemistry: An Introduction;
University Science Books, 2009.
Rehm, D.; Weller, A. Kinetics of Fluorescence
Quenching by Electron and H-Atom Transfer. Isr. J.
Chem. 1970, 8 (2), 259–271.
Sheldrick, G. M. A Short History of SHELX. Acta
Crystallogr. Sect. A Found. Crystallogr. 2008, 64 (1),
112–122.
Sheldrick, G. M. Crystal Structure Refinement with
SHELXL. Acta Crystallogr. Sect. C Struct. Chem. 2015,
71 (Md), 3–8.
Sheldrick, G. M. SHELXT - Integrated Space-Group and
Crystal-Structure Determination. Acta Crystallogr.
Sect. A Found. Crystallogr. 2015, 71 (1), 3–8.
1
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5
6
(
41)
(46)
(47)
(48)
(49)
1
39 (18), 6382–6389.
(42)
Mutoh, K.; Nakano, E.; Abe, J. Spectroelectrochemistry
of a Photochromic [2.2]Paracyclophane-Bridged
Imidazole Dimer: Clarification of the Electrochemical
Behavior of HABI. J. Phys. Chem. A 2012, 116 (25),
0
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3
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7
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792–6797.
(
43)
Hanson, P.; Norman, O. C. Heterocyclic Free Radicals.
Part IV. Some Reactions of Phenothiazine, Two
Derived Radicals, and Phenothiazin-5-Ium Ion. J.
Chem. Soc. Perkin Trans. 2 1973, 264–271.
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