Photochromic phenoxyl-imidazolyl radical complexes with decoloration rates from tens of nanoseconds to seconds

Article abstract of DOI:10.1021/jacs.5b02353

We report a novel photochromic molecular system, phenoxyl-imidazolyl radical complex (PIC), in which both a phenoxyl radical site and an imidazolyl radical site are reversibly and simultaneously generated upon UV light irradiation. PIC consists of the three parts: an aromatic linker, a diarylimidazole moiety, and a 4H-cyclohexadienone ring. Upon UV light irradiation, the C-N bond between the 4H-cyclohexadienone ring and the imidazole ring in the colorless closed-ring isomer of PIC undergoes a homolytic cleavage, leading to the formation of the transient colored open-ring isomer. Based on the substituents on the imidazoyl/4H-cyclohexadienone rings and the nature of the aromatic linker, the half-life of the colored open-ring isomer can be varied between tens of nanoseconds and seconds. PIC derivatives containing a 1,2-phenylene linker exhibit high fatigue resistance toward repeated photochromic reactions. Analysis using laser flash photolysis reveals that the absorption spectra of the open-ring isomers are not readily rationalized by a straightforward superposition of the spectra of the two component radical fragments and the photogenerated radicals are electronically coupled through the aromatic linker. Furthermore, the open-ring isomer can be treated as a hybrid of the pure open-shell biradical and closed-shell quinoid resonance structures.

Full text of DOI:10.1021/jacs.5b02353

Communication
pubs.acs.org/JACS
Photochromic Phenoxyl-Imidazolyl Radical Complexes with
Decoloration Rates from Tens of Nanoseconds to Seconds
Hiroaki Yamashita,† Takahiro Ikezawa,† Yoichi Kobayashi,† 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
‡CREST, Japan Science and Technology Agency (JST), K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
S
* Supporting Information
Scheme 1. Photochromism of (a) HABI, (b) [2.2]PC-Bridged
Imidazole Dimer, (c) PABI, and (d) PIC
ABSTRACT: We report a novel photochromic molecular
system, phenoxyl-imidazolyl radical complex (PIC), in
which both a phenoxyl radical site and an imidazolyl
radical site are reversibly and simultaneously generated
upon UV light irradiation. PIC consists of the three parts:
an aromatic linker, a diarylimidazole moiety, and a 4H-
cyclohexadienone ring. Upon UV light irradiation, the C−
N bond between the 4H-cyclohexadienone ring and the
imidazole ring in the colorless closed-ring isomer of PIC
undergoes a homolytic cleavage, leading to the formation
of the transient colored open-ring isomer. Based on the
substituents on the imidazoyl/4H-cyclohexadienone rings
and the nature of the aromatic linker, the half-life of the
colored open-ring isomer can be varied between tens of
nanoseconds and seconds. PIC derivatives containing a
1,2-phenylene linker exhibit high fatigue resistance toward
repeated photochromic reactions. Analysis using laser flash
photolysis reveals that the absorption spectra of the open-
ring isomers are not readily rationalized by a straightfor-
ward superposition of the spectra of the two component
radical fragments and the photogenerated radicals are
electronically coupled through the aromatic linker.
Furthermore, the open-ring isomer can be treated as a
hybrid of the pure open-shell biradical and closed-shell
quinoid resonance structures.
the colorless HABI. Since the discovery of HABI and its
photoinduced chemical transformation, other photochromic
bridged imidazole dimers have been developed. Subsequent
generations of photochromic imidazole dimers have contained a
linker between the two imidazole rings capable of restricting the
diffusion of imidazolyl radicals and ensuring control over the rate
of decoloration, which varied between a few microseconds and
hundredsofmilliseconds.13−18 Previously, wedevelopedaunique
series of photochromic [2.2]PC-bridged imidazole dimers,
wherein the [2.2]paracyclophane ([2.2]PC) moiety linked two
diphenylimidazole groups (Scheme 1b).14,17 Solutions of these
dimers exhibit instantaneous coloration upon exposure to UV
light and the colors fade rapidly in the dark. The rapid response
exhibited by these photochromic imidazole dimers make them
suitable for use as excellent probes and triggers to reveal and
control phenomena that occur in similar time scales (tens of
milliseconds). However, previously reported radical-generating
photochromic molecules are dimers of two structurally identical
radicals such as substituted imidazoles or tetraphenylpyrryls.19
Except for the [2.2]PC-bridged imidazole dimers (where the
substitution patterns on the imidazole rings are different),
photochromic materials generating two structurally and
electronicallydifferentradicalshavenotbeenpreviouslyreported.
Here we report the photochromic behavior of a novel
photochromic compound, phenoxyl-imidazolyl radical complex
(PIC), which generates two different radical species when
irradiated with light (Scheme 1d). Though the radical dimers
connected between the carbon atoms at the 4-position of 2,6-di-
tert-butylphenoxyl radicals are reported,20,21 the radical complex
hotochromismissimplydefinedasalight-induced, reversible
Ptransformation of a chemical species between two forms that
have different absorption spectra. Photochromic materials are a
well-known class of molecules that change their color upon
irradiation with light. Fast photochromic molecules whose
coloration−decoloration cycle completes in microsecond to
millisecond time scales can be used as a trigger pulse to induce
unfolding of proteins,1,2 motions of protein motors,3 and
isomerization of DNAs4,5 in addition to switchable fluorophores
for super-resolution imaging6−8 and industrial applications such
as holographic displays9,10 and security-recognition devices.11
Organic radicals are usually encountered in the study of organic
photochromism because homolytic bond cleavage is a common
reaction pathway preferred by the excited state molecules. Upon
UV light irradiation, hexaarylbiimidazole (HABI) generates a pair
of 2,4,5-triphenylimidazolyl radicals (TPIRs) via the homolytic
cleavage of the C−N bond between the two imidazole rings
(Scheme 1a).12 In the dark, TPIRs slowly dimerize to regenerate
Received: March 5, 2015
© XXXX American Chemical Society
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consisting of a phenoxyl radical and another radical has not been
demonstrated. The basic concept of the molecular design of PIC
is inspired by the similarities in the reactivity of the oxidation
products between phenol and imidazole. This report presents the
first instance of the simultaneous generation of phenoxyl and
imidazolyl radicals upon light irradiation.
Our experience with the synthesis and study of the photo-
chromic pentaarylbiimidazole (PABI) (Scheme 1c) guided the
design of PIC (Scheme 1c). PABI is easy to synthesize, exhibits
rapid photochromic response (on the order of few micro-
seconds), and also exhibits high fatigue resistance toward
repeated photochromic reactions.22 Furthermore, even when
incorporated in a polymer matrix, PABI retains its rapid response
and exhibits no residual color when the light source is removed.
PIC can be considered as a PABI analogue, wherein the 2H-
imidazole motif is replaced with the 4H-cyclohexadienone motif.
PIC derivatives are readily prepared using readily accessible
reagents (Figure 1a). In the final step toward the synthesis of
between the C-4 of the 4H-cyclohexadienone ring and the N-1 of
the imidazole ring. Frontier molecular orbitals of the closed-ring
isomer of PIC are similar to those of PABI, suggesting that the
lowest excited states of PABI and PIC derivatives are similar
(Figure S64). DFT calculations of PIC derivatives provide further
support in favor of the homolytic cleavage on irradiation by light
since LUMOs of the molecules containing the relevant C−N
bond exhibit antibonding characteristics. Next, we measured the
electron spin resonance (ESR) spectra of the light-irradiated
benzene solutions of PIC derivatives to confirm the formation of
radical species. Though ESR signals were not detected for
solutions of PIC1, PIC2, and PIC4, which is attributed to the low
levels of open-ring isomers under continuous UV light irradiation
(see below), an ESR signal representing a paramagnetic species
with unpaired electrons was observed for the solution of PIC3 at
room temperature (Figure S63). When the irradiation was
stopped, the ESR signal instantaneously disappeared. This
observation is an evidence for the homolytic cleavage of the C−
N bond. While the ring-opening reaction of the five-membered
ring of PIC derivatives is reminiscent of the dihydroindolizines-
based photochromic molecules, it must be noted here that the
former generates radicals and that the latter affords a zwitterionic
species.28,29
Flash photolysis experiments were performed using a nano-
second laser pulse as an excitation light source to investigate the
photochromic behaviors of PIC derivatives. While the pulse
repetition rate for the measurements for PIC1, PIC2, and PIC3
were 10, 10, and 0.1 Hz, respectively, the data for PIC4 was
obtained by a single shot irradiation of the laser pulse. Transient
absorption spectra and decay profiles of the open-ring isomers of
PIC derivatives are shown in Figure 2a−e. Contribution to the
absorption spectra from any triplet−triplet absorption can be
excluded since these spectra were little affected by the presence of
molecular oxygen(FiguresS58−S61). Throughrationaldesignof
the molecules, half-lives of radical species can be varied over a
wide time range, from tens of nanoseconds to seconds. Moreover,
itisworthnotingthatPIC1readilyundergoesseveralcoloration−
decoloration cycles despite the absence of the bulky tert-butyl
groups on the carbons ortho to the phenolic carbon. No
significant differences are observed when the decay profile of a
freshly prepared solution of open-ring isomer of PIC1 is
compared to that measured for the sample after 13,000 exposures
to laser pulses (335 nm; 5 ns duration; and 4 mJ power). This
indicates that PIC1 can undergo fatigue resistant photochromic
reactions (Figure S54). Intriguingly, phenoxyl radicals unencum-
bered by bulky groups are highly reactive.30 However, in the case
of PIC1, it is very likely that the reaction of the phenoxyl radicals
with intramolecularly accessible diphenylimidazolyl radical takes
precedence over other potential intermolecular reactions; the
diphenylimidazolyl radical moiety sterically stabilizes the
phenoxyl radical. In addition, the spin delocalization through
the 1,2-phenylene linker contributes to the thermodynamic
stabilization of phenoxyl radical. It is likely that the two factors,
proximity to the diphenylimidazolyl radical and spin delocaliza-
tion, contribute to the observed stability of the open-ring isomer
of PIC1 in spite of the absence of bulky substituents that are
typicallyinstalledtopreventaccesstothehighlyreactivephenoxyl
radical.
Figure 1. (a) General synthetic scheme of PIC derivatives, where Bpin =
4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl, and (b) ORTEP representa-
tionsofthemolecularstructuresofPIC1−4withthermalellipsoids(50%
probability), where nitrogen, oxygen, and sulfur atoms are highlighted in
blue, red, and green, respectively.
PABI, oxidation of the precursor affords a colored bisimidazo-
lylradical23−27 (open-ring isomer), which readily undergoes an
intramolecular cyclization, resulting in the formation of the C−N
bond between the adjacent imidazole rings and affording the
colorless closed-ring isomer. Along similar lines, the colorless
closed-ring isomers of PIC derivatives are obtained as a result of
intramolecular recombination of the phenoxyl and imidazolyl
radicals. All PIC compounds consist of the three critical structural
motifs: an aromatic linker, the diarylimidazole moiety, and the
4H-cyclohexadienone ring. We synthesized four PIC derivatives,
PIC1−4 (Figure 1b). X-ray diffraction analyses of the crystals of
thePICderivativeshaveconfirmedthemolecularstructuresofthe
closed-ring isomers. The imidazole ring and the ring of the
aromatic linker occupy a single plane, while the plane of the 4H-
cyclohexadienone ring is oriented perpendicular to this plane.
The photochromic behavior of PIC is considered to be the
result of a homolytic cleavage of the C−N bond, i.e., the bond
Our studies reveal that the half-life of the open-ring isomer, i.e.,
the rate of decoloration, is predominantly dependent on the
molecular structure of the open-ring isomer. A possible structural
change in a coloration−decoloration cycle begins when the C−N
bond starts breaking on the excited state potential energy surface.
B
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closing reaction. The nature of thiophene, which is less aromatic
when compared with benzene, also influences the electronic
structure of the open-ring isomer (see below).31 Meanwhile,
replacement of the diphenylimidazole with a rigid phenanthroi-
midazole can create higher barriers for the adoption of a planar
conformation and further decrease the stability of the open-ring
isomer. Not surprisingly, the half-life of the open-ring isomer of
PIC4 (6.6 ms) is nearly 3 orders of magnitude shorter than that of
PIC3, indicating the decrease in the ΔG‡ of the ring-closing
reaction in the case of PIC4. Rate constants and activation
parameters for the ring-closing reactions, half-lives for the open-
ring isomers of PABI, and PIC derivatives are summarized in
Table 1.
Table 1. Rate Constants and Activation Parameters for the
Ring-Closing Reaction, Half-Lives for the Open-Ring Isomers
of PABI1, PIC1−4 in Degassed Benzene at 25 °C; Molecular
Structure of PABI1 Is Shown in Figure 2e
k
τ1/2
[s]
ΔH‡
ΔS‡
ΔG‡
[s−1
]
[kJ/mol] [J/mol·K] [kJ/mol]
PABI1
PIC1
PIC2
PIC3
PIC4
3.5 × 105
2.8 × 106
2.6 × 107
6.4 × 10−1
2.2 × 102
2.0 × 10−5
2.5 × 10−7
2.6 × 10−8
1.1 × 100
35.4
31.7
26.3
66.7
55.5
−20.0
−14.9
−15.1
−26.1
−19.5
41.4
36.2
30.7
74.5
61.3
6.6 × 10−3
Figure 2. Decay profiles of the open-ring isomers of (a) PIC1, (b) PIC2,
(c) PIC3, and (d) PIC4 in degassed benzene at 25 °C (excitation
wavelength, 355 nm; pulse width, 5 ns; power, 4 mJ/pulse; the
concentrations of PIC1, PIC2, PIC3, and PIC4 are 1.0 × 10−3, 1.0 ×
10−3, 6.1 × 10−5, and 2.4 × 10−5 M, respectively). Insets of panels a−d
show transient absorption spectra of the open-ring isomers of PIC1−4.
(e) Plots of the half-lives of the open-ring isomers of PIC and PABI
derivatives on a logarithmic time scale.
Fromthevalencebondapproach, theelectronicstructureofthe
open-ring isomer has significant contributions from both open-
shell biradical and closed-shell quinoid resonance structures. The
quinoid resonance structure, with its ability to form the π-bond, is
more stable than the biradical structure; however, the quinoid
resonance structure loses the resonance energy. When the
resonance energy is large enough to compete with the
stabilization energy associated with the formation of the π-
bond, the contribution from the biradical resonance structure to
the electronic structure of the open-ring isomer is negligible. We
estimated the energies of the two resonance structures of o-
The planes containing the aromatic rings of the phenoxyl and
imidazolyl radicals, which are oriented perpendicular to each
other as the C−N bond is broken, are reoriented as the phenoxyl
radical rotates in an attempt to form the π-bond. However, the
steric repulsion between the phenoxyl and imidazolyl radicals
precludes the open-ring isomer from adopting the planar
quinoidal conformation. The conformation adopted by the
open-ring isomer is a key factor that regulates the ΔG‡ of the ring-
closing reaction (C−N bond formation) that leads to the
regeneration of the colorless closed-ring isomer. Open-ring
isomers of PIC1 and PIC2 have half-lives of 250 and 26 ns in
benzene at 298 K, respectively. The difference in the half-lives of
these molecules can be rationalized based on the effect of the
substituted tert-butyl groups. The presence of the bulky tert-butyl
groups on the carbons ortho to the phenolic carbon forces the
open-ring isomer to adopt a nonplanar twisted conformation. As
theenergyoftheopen-ringconformerofthePIC2isclosertothat
of the transition state in the ring-closing reaction, PIC2 has
shorter half-life than PIC1.
The open-ring isomer of PIC3 displays a half-life of 1.1 s in
benzeneatroomtemperature. Replacementofthephenylenering
linker by a thienyl ring effectively prolongs the half-life of the PIC.
It is likely that both geometric and electronic characteristics of the
thienyl ring significantly contribute to the elongation of the half-
life. The conformation of the open-ring isomer is likely to become
relatively more planar when the six-membered ring linker is
replaced by a five-membered ring, as the steric clash between the
phenoxyl and imidazolyl radicals is partially relieved. Further-
more, distance between the bond forming carbon and nitrogen
atoms increases, leading to an increase in the ΔG‡ of the ring-
quinodimethane and its thiophene analogue by the Huckel
̈
approximation. The difference in the energies of the biradical and
quinoid resonance structures (ΔEBR−Q) for o-quinoidmethane
and the thiophene analogue are 1.95|β| and 2.21|β|, respectively.
The larger ΔEBR−Q value in the case of the thiophene analogue
indicates that replacement of the phenylene ring with the thienyl
ring further minimizes the contribution of the biradical resonance
to the open-ring isomer. The quinoid resonance structure prefers
the planar conformation, thereby increasing the ΔG‡ of the ring-
closing reaction.
The transient absorption spectra of the open-ring isomers are
showninFigure2(seeinset). Theintenseabsorption bandsinthe
visible region (600−800 nm) can be ascribed to the radical−
radical interaction between the imidazolyl and phenoxy radicals.
For example, a solution of the open-ring isomer of PIC2 can be
considered as a mixture of two independent radical species, 2,4,5-
triphenylimidazolyl radical and 2,6-di-tert-butyl-4-phenylphenox-
yl radical. The 2,4,5-triphenylimidazolyl radical absorbs light
between 500 and 600 nm in addition to a sharp band at ∼460 nm,
whereas 2,6-di-tert-butyl-4-phenylphenoxyl radical has weak
absorption peaks at ∼348, ∼418, ∼498, and ∼534 nm.32 It is
evident from the absorption spectrum of the open-ring isomer of
PIC2 that it is not simply a superposition of spectra of the
imidazolyl and phenoxyl radicals. While the open-shell biradical
and the closed-shell quinoid resonance structures contribute to
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the electronic structure of the open-ring isomer, the transient
absorption spectra of the open-ring isomer of PIC derivatives
indicate the existence of a strong through-bond electron spin
exchange coupling. Thus, the wave function of the open-ring
isomer of PIC can be described neither by an open-shell biradical
state nor a closed-shell quinoidal state; these factors indicate the
challenges associated with obtaining the correct wave function by
using a simple DFT calculation based on a single Slater
determinant. Generally, the theoretical calculation for a
biradicaloid species33 requires the multireference SCF ap-
proaches34 such as CASSCF (complete active space SCF) and
CASPT2 (CASSCF with second order MP perturbation theory),
which are too expensive for straightforward application to large
conjugated systems as PIC derivatives.
In conclusion, we report a novel photochromic molecular
system based on phenoxyl and imidazolyl structural motifs. The
molecules disclosed here represent the first example of a family of
photochromic compounds that generate two structurally and
electronically different stable radicals upon UV light irradiation. It
should be emphasized here that the colored open-ring isomer of
PIC1 is stable despite the absence of tert-butyl groups on the
carbons ortho to the phenolic carbon. Moreover, we have
developed a strategy for controlling the ΔG‡ of the ring-closing
reaction, allowing for anability totune andpredict the half-lives of
the open-ring isomers. The half-lives of the colored open-ring
isomersofPICderivativesspanawiderangeofhalf-lives, i.e., from
tens of nanoseconds to seconds. A rigorous analysis of the
absorption spectra of the open-ring isomer of a PIC derivative is a
nontrivial task and is considerably more complex than the
straightforward superposition of the spectra of its structural
fragments. The design of PIC, two structurally and electronically
distinct moieties connected through a linker, the modifications of
which can regulate the rate of photochromic reactions, provides a
unique platform to develop and study various radical complexes
leading to a new direction in radical chemistry.
REFERENCES
■
(1) Muramatsu, S.; Kinbara, K.; Taguchi, H.; Ishii, N.; Aida, T. J. Am.
Chem. Soc. 2006, 128, 3764.
(2) Lindorff-Larsen, K.; Piana, S.; Dror, R. O.; Shaw, D. E. Science 2011,
334, 517.
(3) Hoersch, D.; Roh, S.-H.; Chiu, W.; Kortemme, T. Nat. Nanotechnol.
2013, 8, 928.
(4) Blanco-Lomas, M.; Samanta, S.; Campos, P. J.; Woolley, G. A.;
Sampedro, D. J. Am. Chem. Soc. 2012, 134, 6960.
(5) Samanta, S.; Beharry, A. A.; Sadovski, O.; McCormick, T. M.;
Babalhavaeji, A.;Tropepe, V.; Woolley, G. A. J. Am. Chem. Soc. 2013, 135,
9777.
(6) Deniz, E.; Tomasulo, M.; Cusido, J.; Sortino, S.; Raymo, F. M.
Langmuir 2011, 27, 11773.
(7)Deniz, E.;Tomasulo, M.;Cusido,J.;Yildiz,I.;Petriella, M.;Bossi, M.
L.; Sortino, S.; Raymo, F. M. J. Phys. Chem. C 2012, 116, 6058.
(8) Mutoh, K.; Sliwa, M.; Abe, J. J. Phys. Chem. C 2013, 117, 4808.
(9) Ishii, N.; Kato, T.; Abe, J. Sci. Rep. 2012, 2, 819.
(10) Ishii, N.; Abe, J. Appl. Phys. Lett. 2013, 102, 163301.
(11) Iftime, G.; Breton, M. P.; Lee, F. P.-H.; Valeriu, A. M.; Chopra, N.;
Odell, P. G.; Moorlag, C. Photochromic security enabled ink for digital
offset printing applications. U.S. Patent 20130305947 A1, 2012.
(12) Hayashi, T.; Maeda, K. Bull. Chem. Soc. Jpn. 1960, 33, 565.
(13) Fujita, K.; Hatano, S.; Kato, D.; Abe, J. Org. Lett. 2008, 10, 3105.
(14) Kishimoto, Y.; Abe, J. J. Am. Chem. Soc. 2009, 131, 4227.
(15) Mutoh, K.; Shima, K.; Yamaguchi, T.; Kobayashi, Y.; Abe, J. Org.
Lett. 2013, 15, 2938.
(16) Hatano, S.; Horino, T.; Tokita, A.; Oshima, T.; Abe, J. J. Am. Chem.
Soc. 2013, 135, 3164.
(17)Shima, K.;Mutoh, K.;Kobayashi, Y.;Abe, J. J. Am. Chem. Soc. 2014,
136, 3796.
(18) Iwasaki, T.; Kato, T.; Kobayashi, Y.; Abe, J. Chem. Commun. 2014,
50, 7481.
(19) Blinder, S. M.; Peller, M. L.; Lord, N. W.; Aamodt, L. C.;
Ivanchukov, N. S. J. Chem. Phys. 1962, 36, 540.
(20) Nakatuji, K.; Oda, M.; Kozaki, M.; Morimoto, Y.; Okada, K. Chem.
Lett. 1998, 27, 845.
(21)Wittman, J. M.;Hayoun, R.;Kaminsky, W.;Coggins, M. K.;Mayer,
J. M. J. Am. Chem. Soc. 2013, 135, 12956.
(22) Yamashita, H.; Abe, J. Chem. Commun. 2014, 50, 8468.
ASSOCIATED CONTENT
■
(23) Mayer, U.; Baumgartel, H.; Zimmermann, H. Angew. Chem. 1966,
̈
S
* Supporting Information
78, 303.
Synthesis, 1H NMR, HR-ESI−TOF−MS spectra, HPLC
chromatogram, X-ray crystallographic analysis, UV−vis absorp-
tion spectra, ESR spectrum, experimental detail for laser flash
photolysis measurements, kinetics for the ring-closing reaction in
benzene, DFT calculations, and the movie for the photochromic
behavior of PIC3. This material is available free of charge via the
Internet at http://pubs.acs.org.
(24) Okada, K.; Imamura, K.; Oda, M.; Kozaki, M.; Morimoto, Y.;
Ishino, K.; Tashiro, K. Chem. Lett. 1998, 27, 891.
(25) Okada, K.; Imamura, K.; Oda, M.; Kajiwara, A.; Kamachi, M.;
Ishino, K.; Tashiro, K.; Kozaki, M.; Sato, K.; Takui, T. Synth. Met. 1999,
103, 2308.
(26) Kikuchi, A.; Iwahori, F.; Abe, J. J. Am. Chem. Soc. 2004, 126, 6526.
(27) Kikuchi, A.; Ito, H.; Abe, J. J. Phys. Chem. B 2005, 109, 19448.
(28) Hauck, G.; Durr, H. Angew. Chem., Int. Ed. 1979, 18, 945.
̈
(29) Bleisinger, H.; Scheidhauer, P.; Durr, H.; Wintgens, V.; Valaat, P.;
̈
Kossanyi, J. J. Org. Chem. 1998, 63, 990.
AUTHOR INFORMATION
■
(30) Foti, M.; Ingold, K. U.; Lusztyk, J. J. Am. Chem. Soc. 1994, 116,
9440.
(31) Irie, M. Chem. Rev. 2000, 100, 1685.
Corresponding Author
*jiro_abe@chem.aoyama.ac.jp
(32) Morita, Y.; Ueda, A.; Nishida, S.; Fukui, K.; Ise, T.; Shiomi, D.;
Sato, K.; Takui, T.; Nakasuji, K. Angew. Chem., Int. Ed. 2008, 47, 2035.
(33) Abe, M. Chem. Rev. 2013, 113, 7011.
Notes
The authors declare no competing financial interest.
(34) Snyder, G. J. J. Phys. Chem. A 2012, 116, 5272.
ACKNOWLEDGMENTS
■
This work was supported partly by the Core Research for
Evolutionary Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST) and a Grant-in-Aid
for Scientific Research on Innovative Areas “Photosynergetics”
(No. 26107010) from MEXT, Japan. Financial assistance for this
research was also provided by the MEXT-Supported Program for
the Strategic Research Foundation at Private Universities, 2013−
2017.
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