Enhancement of Negative Photochromic Properties of Naphthalene-Bridged Phenoxyl-Imidazolyl Radical Complex

Article abstract of DOI:10.1002/cphc.202000296

Negative photochromism has increased attention as a light-switch for functional materials. A development of fast photochromic molecules has been also expected because a rapid thermal back reaction within a millisecond time scale is useful for real-time switching. Herein, we synthesized the derivatives of the naphthalene-bridged phenoxyl-imidazolyl radical complex (Np?PIC) showing the negative photochromism to demonstrate the efficient strategy to increase the visible light sensitivity and to control the thermal back reaction rates. The distances of the C?C bond of the transient 2,4’-isomer shows good agreement with the thermodynamic stability, leading to the control of the thermal back reaction rate. We revealed the cyclic voltammetry and the DFT calculations are efficient to predict the characters of the HOMO and LUMO. The introduction of the electron-withdrawing dicyanoquinodimethane group is efficient to induce the photochromic reaction with increased visible-light sensitivity by the expansion of the π-conjugation. The results will give an important insight for the future development of fast-responsive negative photochromic molecules.

Full text of DOI:10.1002/cphc.202000296

Accepted Article
Title: Enhancement of Negative Photochromic Properties of
Naphthalene-Bridged Phenoxyl-Imidazolyl Radical Complex
Authors: Jiro Abe, Hiroki Ito, and Katsuya Mutoh
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.202000296
Link to VoR: https://doi.org/10.1002/cphc.202000296
01/2020

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
Enhancement of Negative Photochromic Properties of
Naphthalene-Bridged Phenoxyl-Imidazolyl Radical Complex
Hiroki Ito[a], Katsuya Mutoh[a] and Jiro Abe*[a]
[a]
H, Ito, Dr. K. Mutoh, Prof. J. Abe*
Department of Chemistry
School of Science and Engineering, Aoyama Gakuin University
5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan
E-mail: jiro_abe@chem.aoyama.ac.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: Negative photochromism has increased attention as a light-
switch for functional materials. A development of fast photochromic
molecules has been also expected because a rapid thermal back
reaction within a millisecond time scale is useful for real-time
switching. Herein, we synthesized the derivatives of the naphthalene-
bridged phenoxyl-imidazolyl radical complex (Np-PIC) showing the
negative photochromism to demonstrate the efficient strategy to
increase the visible light sensitivity and to control the thermal back
reaction rates. The distances of the C–C bond of the transient 2,4’-
isomer shows good agreement with the thermodynamic stability,
leading to the control of the thermal back reaction rate. We revealed
the cyclic voltammetry and the DFT calculations are efficient to predict
the characters of the HOMO and LUMO. The introduction of the
electron-withdrawing dicyanoquinodimethane group is efficient to
induce the photochromic reaction with increased visible-light
sensitivity by the expansion of the π-conjugation. The results will give
an important insight for the future development of fast-responsive
negative photochromic molecules.
control for photochromic lenses,[15,16] fluorescence property,[17,18]
holographic materials,[19,20] a development of fast-switchable
negative photochromic molecules will be important to open a new
area of photochromism. Recently, we developed negative
photochromic binaphthyl-bridged imidazole dimer (BN-ImD)
derivatives which show decoloration reaction upon visible light
irradiation.[21–24] Because the thermal back reaction of BN-ImD
does not depend on the solvent polarity, BN-ImD will be able to
work as a photoswitch in various mediums. The thermal back
reaction can be accelerated to the second time scale by replacing
one of the imidazole rings to the phenoxyl ring.[25] The
naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-
PIC, 1) also shows the fast thermal back reaction in the
millisecond time scale.[26] However, the efficient strategy to control
the photosensitivity and the thermal back reaction rate has not
been explored yet. Therefore, in this study, we investigated the
photochromic properties of Np-PIC derivatives to reveal the
relationship between the electronic structures and the
photochromic properties. We designed the Np-PIC derivatives
(Scheme 1), 2 in which an imidazole and a phenoxyl units are
bridged by 1,2-dihydroacenaphthylene, 3 in which a phenoxyl unit
of Np-PIC is replaced by a dicyanoquinodimethane unit, 4 in
which an imidazole and a phenoxyl units are bridged by
Introduction
Photochromic molecules are one of the important photoswitches
not only to control the color, but also to switch electronic and
magnetic properties, mechanical works, and biological
activities.[1–4] Though almost common photochromic molecules
respond to UV light in at least one way of the reversible
photoswitching reactions, increasing the visible light sensitivity
has been recently considered to avoid unexpected reactions and
degradations by harmful UV light. In addition, the excitation UV
light cannot penetrate into the inside of the photochormic
materials because the colored species re-absorbs the excitation
light on the surface.
acenaphthylene,
5
in which an imidazole and
a
dicyanoquinodimethane units are bridged by an acenaphthylene
unit, and 6 in which the π-conjugation of the imidazole unit is
extended by a pyreno-imidazole unit.
Negative photochromism, in which a thermally stable
colored isomer photoisomerizes to a transient colorless isomer
upon visible light irradiation, has increased much attention as a
novel photoswitch.[5–14] A negative photochromic reaction has
some advantages over a conventional positive photochromic
reaction: photosensitivity to energetically low toxic visible light and
deep penetration of excitation light inside materials. Although
several novel negative photochromic molecules have been
reported, the slow thermal back reaction in minutes and the strong
dependence on the solvent polarity limit the application and
capability of negative photochromic molecules. Because the
switching ability within milliseconds is attractive for a real-time
Scheme 1. (a) The scheme of the negative photochromic reaction of Np-PIC
(1). (b–f) The molecular structures of the Np-PIC derivatives in this study.
1
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
Results and Discussion
1,2-dihydroacenaphthylene bridge, the acenaphthylene bridge
effectively expands the π-conjugation, leading to the decrease in
the energy level of the π* orbital. In fact, 4 and 5 have intense
absorption bands reached to 550 nm in the visible light region as
shown in Figure 2. The TDDFT calculations of 4 and 5 indicate
these absorption bands can be assigned to the HOMO → LUMO
and the HOMO → LUMO+1 transitions, respectively (Figure 3).
The expansion of the π-conjugation of the naphthalene unit
largely decreases the energy level of the π*-orbital, lowering the
photo-excitation energy. Therefore, the double-bond bridge for
the naphthalene unit effectively extends the π-conjugation and to
enhance the visible light sensitivity. The pyreno-imidazole unit is
also efficient to increase the visible light sensitivity because of the
planar structure as shown in Figure 1. The absorption band at
around 420 nm can be assigned to the charge transfer (CT)
transition from the electron-donating pyrenyl unit to the electron-
withdrawing naphthalene unit (HOMO → LUMO+1).
UV–vis Absorption Spectroscopy
The molecular structures of 2–6 were revealed unambiguously by
the X-ray crystallographic analyses of the single crystals (Figure
1). The all derivatives are the 1,4’-isomers that is the most
thermally stable structural isomer. The imidazole ring and the
electron
accepter
units
(the
phenoxyl
or
the
dicyanoquinodimethane unit) of the all derivatives are
perpendicularly oriented as same as that of 1. The distances of
the C–N bond are almost similar with that of 1 (ca. 1.49 Å).[26]
Figure 2. UV-vis absorption spectra of 1–6 in benzene at 298 K.
DFT calculation for the Np-PIC derivatives
Figure 1. ORTEP representations of the molecular structures of 2–6 with
thermal ellipsoids (50 % probability), where the nitrogen atoms are highlighted
in blue and the oxygen atom is highlighted in red. The hydrogen atoms are
omitted.
The femtosecond time-resolved absorption spectroscopy
and the electrochemical measurement of the photochromic
imidazole dimer and the phenoxyl-imidazolyl radical complex
derivatives have revealed that the C–N bond breaking reaction
proceeds from the S1 state to generate the biradical species
because the LUMO has the anti-bonding character in relation to
the C–N bond (C–N*).[27–30] The 1,4’-isomer of Np-PIC is also
expected that the biradical species which thermally isomerizes to
the 2,4’-isomer is generated upon UV light irradiation.[26] As
shown in Figure 3, the LUMOs of 1, 2 and 6 are similar to that of
the typical PIC derivative having the anti-bonding character in
relation to the C–N bond. However, it is presumed that 4 does not
show the photochromic reaction from the S1 state because the
energy level of the π*-orbital is lower than that of the C–N* orbital.
The introduction of the electron-accepter group to the phenoxyl
unit of Np-PIC will be efficient to reduce the energy level of the C–
N* orbital because the C–N* orbital is localized on the phenoxyl
unit. The dicyanomethylene group has known as a strong
electron-withdrawing group and it is easy to substitute a carbonyl
group with a dicyanomethylene group. The energy level of the C–
N* orbital of 3 is largely decreased by the introduction of a
dicyanomethylene group with small change in the energy level of
the π*-orbital. Based on the theoretical calculations, we expected
5 would show the photochromic reaction with enhanced visible-
The UV–vis absorption spectra of 1–6 in benzene are shown
in Figure 2a. Figure 3 shows the energy levels of the molecular
orbitals calculated by the DFT calculations (MPW1PW91/6-
31+G(d)//MPW1PW91/6-31G(d)). Compound
1
has the
absorption band at around 370 nm which is attributable to the
transition from the orbital delocalized over the bridging
naphthalene and the imidazole units (HOMO) to the π*-orbital on
the naphthalene unit.[26] Compound
2 shows the similar
absorption spectrum with that of 1 because the single-bond bridge
of the 1,2-dihydroacenaphthylene unit does not affect the π-
conjugation of the naphthalene unit, that is consistent with the
results of the DFT calculations. Compound 3 has a small
absorption band at 450 nm in addition to the absorption band at
370 nm. The absorption band at 450 nm of 3 is attributable to the
HOMO−2 to the LUMO transition (Figure S31). The TDDFT
calculation revealed that the S1←S0 transition of Np-PIC is
assigned to the HOMO → LUMO transition. However, the
transition probability is almost zero due to the small overlap
integral between the perpendicularly oriented HOMO and LUMO.
Thus, the reduction of the energy level of the π* orbital is efficient
to increase the photosensitivity to visible light. In contrast to the
2
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
Figure 3. The relevant molecular orbitals of 1–6 (MPW1PW91/6-31+G(d)//MPW1PW91/6-31G(d)).
light photosensitivity because of the significantly reduced energy
level of the C–N* orbital lower than the π*-orbital.
Cyclic Voltammetry
The cyclic voltammetry (CV) is one of the efficient methods to
predict the HOMO and LUMO levels of molecules. It has been
revealed by the detail investigations of the CV measurement of
imidazole dimer and PIC derivatives that an electron injection to
the LUMO leads to the generation of the radical anion species in
which the C–N bond spontaneously breaks to form the radical and
the anion species (the ECE process as shown in Scheme 2).[29–
31] The subsequent reduction of the generated radical species to
generate the dianion species is simultaneously proceeded at the
same reduction potential of the initial dimer. Figure 4 shows the
cyclic voltammograms of the Np-PIC derivatives. The cyclic
voltammograms are similar to those of the previously reported
PIC derivatives. Thus, it can be considered that Np-PIC
derivatives show the same electrochemical reaction with those of
the PIC derivatives. Table 1 summarizes the reduction and
oxidation peaks and the estimated energy gaps between the
HOMO and LUMO of 1−6. The HOMO–LUMO gaps were
approximately estimated by using the redox peaks due to the
irreversibility. The irreversible reduction peaks of the 1,4’-isomers
1, 2, 4 and 6 were observed at −1.37–−1.58 V, which can be
assigned to the ECE process to produce the dianion species. The
reduction peaks of 3 and 5 are shifted to more positive potentials
(−0.85 and −0.81 V, respectively) by introducing the electron-
withdrawing dicyanomethylene group. These results are
consistent with the DFT calculations (Figure 3). The
electrochemical oxidation of 1–6 would produce the radical cation
species as similar with the oxidation of the PIC derivatives.[32] The
oxidation potential of 6 is shifted to more negative potential,
compared with those of 1–5 because of the electron-donating
ability of the pyrenyl unit. The order of the HOMO−LUMO gaps
calculated by the DFT calculations (MPW1PW91/6-
31+G(d)//MPW1PW91/6-31G(d)) is in good agreement with that
estimated by the CV measurements.
Scheme 2. The redox behaviour of Np-PIC.
Figure 4. Cyclic voltammograms of 1 (red line), 2 (orange line), 3 (purple line),
4 (green line), 5 (blue line), 6 (light blue line) in CH2Cl2 (1.0 mM) containing 0.1
M (TBA)PF6 as a supporting electrolyte. Potential scan rate = 500 mV/s.
3
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
reason why the negative photochromism of 5 could not be
observed would be due to the destabilization of the 2,4’-isomer by
the C=C bond as discussed in the next section.
Table 1. Electrochemical Reduction (Ered.), Oxidation (Eox.) Peaks, and the
HOMO−LUMO Gaps (Eg,exp) Estimated by Electrochemistry and the Excitation
Energies (Eex.) Estimated by TDDFT Calculations (MPW1PW91/6-31+G(d)//
MPW1PW91/6- 31G(d))
compounds
Ered. (V)
−1.51
−1.48
−0.85
−1.37
−0.81
−1.58
Eox. (V)
1.30
1.16
1.30
1.24
1.28
0.87
Eg,exp (eV)
2.81
Eex. (eV)
2.80
Table 2. The C–C Bond Distances of 2,4’-isomers of 1–6 and the ΔG0 between
the 1,4’-isomer and the 2,4’-isomer Calculated by the DFT (MPW1PW91/6-
31G(d) level of the theory)
1
2
3
4
5
6
2.64
2.63
1
2
3
4
5
6
2.15
2.07
dc-c (Å)
1.610
84.1
1.633
112.7
1.613
77.8
1.636
117.5
1.641
113.4
1.611
85.2
2.61
2.56
ΔG0
(kJ/mol)
2.09
2.00
2.45
2.37
Although the negative photochromic reactions of 4 and 5
were not observed from nanosecond to second time scales, the
positive transient absorption bands were observed upon UV light
irradiation to 4 and 5 in the picosecond and nanosecond time
scale, respectively (Figure 6). The time-resolved absorption
spectroscopic measuments of 4 and 5 were carried out by the
time-resolved absorption spectroscopy from picosecond to
nanosecond time scales by using the randomly interleaved pulse
train (RIPT) method (Unisoku, picoTAS).[34] Since the LUMO of 4
is localized on the acenaphthylene unit (Figure 3), the
photoirradiation to 4 does not promote the C–N bond breaking
reaction based on Kasha’s rule. The fluorescence lifetime of 4
was estimated to be 0.26 ns in benzene at 298 K. This is
consistent with the lifetime estimated from the time-resolved
absorption spectroscopy (0.29 ns). That is, the transient
absorption spectrum of 4 can be assigned to the excited singlet
state, and therefore 4 does not show the photochromic reaction.
In contrast, the broad absorption band from 400 nm to 1000 nm
with a lifetime of 100 ns was observed by the irradiation to 5 at
298 K. The fluorescence lifetime of 5 is much faster than the
lifetime of the transient absorption spectrum, indicating that 5
generates a transient species upon UV light irradiation. The most
plausible assignment of the transient species is the biradical
generated by the C–N bond breaking reaction because the
TDDFT calculation result of the biradical of 5 is in good agreement
with the transient absorption spectrum (Figure S35). The
stabilization of the biradical by the delocalization over the
dicyanoquinodimethane unit would be the reason for the
deceleration of the lifetime. There are two relaxation pathways
after the generation of the biradical species, (i) the thermal back
reaction to the initial 1,4’-isomer and (ii) the isomerization to the
thermally unstable 2,4’-isomer. However, the biradical of 5 would
not isomerize to the 2,4’-isomer because we could not observe
the negative photochromic reaction. It can be considered that the
destabilization of the 2,4’-isomer by bridging the naphthalene unit
with a C=C bond also increases the activation energy barrier from
the biradical to the 2,4’-isomer.
Time-Resolved Absorption Spectroscopy
Figure 5 shows the time-resolved absorption spectra and the
temporal changes of the transient absorbance of 2, 3 and 6
measured by laser flash photolysis exciting with a 355-nm
nanosecond laser pulse. The negative bleaching signals were
observed upon UV light irradiation. On the other hand, 4 and 5 did
not show such negative absorption changes from the picosecond
to the second time region. The reason on this point will be
discussed in detail later. The bleach signals of 2, 3 and 6
monotonically recovered to the initial absorption spectra with the
half-lives of 5.8 μs, 0.26 ms and 5.8 ms at 298 K, respectively, in
the whole spectral region. The spectral shapes of the bleach
signals are similar with the absorption spectra of the initial
thermally stable 1,4’-isomers. It is suggested from the similarity of
the spectral change with that of 1 that the negative photochromic
reaction from the 1,4’-isomer to the 2,4’-isomer is observed upon
UV light irradiation.[26] Although the generation of the short-lived
biradical species was suggested in the previous study of 1, we
could not observe clear absorption spectra of the biradical in a
few hundreds of picosecond time region due to the degradation
of the compounds during the accumulation of the time-resolved
absorption spectra.
The thermal back reaction rate of 2 is much accelerated by
bridging with a 1,2-dihydroacenaphthylene unit, compared with
those of the other derivatives. It is known as the linear free-energy
relationship that the change in the Gibbs free energy (ΔΔG0)
between the initial state and the thermally metastable state is
proportional to the change in the activation energy barrier (ΔΔG‡)
between the transition state and the thermally metastable state.33
Table 2 summarizes the ΔG0 values calculated by the DFT
(MPW1PW91/6-31G(d)) between the 2,4’-isomer and the 1,4’-
isomer, and the calculated bond distances of the C–C bond (dc–c
)
between the 2-position of the imidazole ring and 4’-position of the
phenoxyl or the dicyanoquinodimethane unit of 2,4’-isomer. The
large ΔG0 value of 2 leads to the acceleration of the thermal back
reaction of the 2,4’-isomer. Because the magnitude relation of dc–
c and the ΔG0 values show similar tendency, the increment of the
dc–c largely contributes to the destabilization of the 2,4’-isomer.
The bridge of the C–C in 1,2-dihydroacenaphthylene or the C=C
bond in acenaphthylene increases the distance between the 1-
and 8-positions of the naphthalene unit due to the large steric
distortion. This distortion of the naphthalene bridge hinders the
formation of the five-membered ring of the 2,4’-isomer, resulting
in the acceleration of the thermal back reaction of 2. The plausible
4
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
We investigated the strategy to improve the photochromic
properties of the Np-PIC system by the combination of the DFT
calculation, electrochemistry and time-resolved absorption
spectroscopy. The extension of the π-conjugation of the imidazole
unit is efficient to increase the visible light sensitivity. The bridge
of an acenaphthylene unit instead of the naphthalene unit or the
introduction of polycyclic aromatic hydrocarbon to the imidazole
unit is suitable for Np-PIC derivatives. However, the replacement
of a dicyanomethylene group to the phenoxyl group of Np-PIC is
required to induce the photochromic reaction for the
acenaphthylene-bridged PIC system by decreasing the energy
level of the C–N* orbital possessing the anti-bonding character of
the C–N bond. The thermal back reaction from the colorless 2,4’-
isomer to the initial 1,4’-isomer can be accelerated by
destabilizing the 2,4’-isomer with a 1,2-dihydroacenaphthylene
bridge. The destabilization energy depends on the distance of the
C–C bond between the imidazole and the phenoxyl units of Np-
PIC. These insights will be important for the future development
of fast switchable negative photochromic molecules.
Experimental Section
Voltammetry Measurements.
All cyclic voltammetry (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 counter electrode, and an Ag/Ag+ reference
Figure 5. Transient absorption spectra and the time profiles of the transient
absorbance of (a,b) 2, (c, d) 3 and (e, f) 6 upon 355-nm nanosecond laser pulse
irradiation (4 mJ) at 298 K in benzene (2: 2.6×10−5 M, 3: 1.4×10−5 M, 6: 1.7×10−5
M).
electrode (Ag wire,
1 mM AgNO3, 0.1 M tetrabutylammonium
hexafluorophosphate in acetonitrile) was employed. All CV measurements
were achieved from 0.05 to 1 V/s in solutions of 0.1 M (TBA)PF6 in
acetonitrile or dichloromethane at room 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 formal potential
for the ferrocene/ferrocenium (Fc/Fc+) couple. μAutolab III potentiostat/
galvanostat (Metrohm Autolab B. V.) under computer control (General
Purpose Electrochemical System software) was used for the CV
measurement.
Time-Resolved Absorption Spectroscopy
The time-resolved absorption spectroscopy on microsecond to millisecond
time scale was carried out with
a
TSP-2000 time-resolved
spectrophotometer (Unisoku). A 10 Hz Q-switched Nd:YAG (Continuum
Minilite II) laser with the third harmonic at 355 nm (5 ns pulse) was
employed for the excitation light. A halogen lamp (OSRAM HLX64623)
was used as a probe beam arranged in an orientation perpendicular to the
exciting laser beam. The probe beam was monitored with a photomultiplier
tube (Hamamatsu R2949) through a spectrometer (Unisoku MD200) for
the decay profile of the colored species. The excitation intensity of one
pulse was estimated by an energy monitor (Gentec Electro-Optics
MAESTRO). Optical-grade solvents were used for all measurements. The
measurements were performed in a benzene solution placed in a 1-cm
quartz cell at 298 K. The time-resolved absorption spectroscopy on
picosecond to nanosecond time scale was conducted by the randomly
interleaved pulse train (RIPT) method.[34] A picosecond laser, PL2210A
(EKSPLA, 1 kHz, 25 ps, 8 mW 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 measurements were
Figure 6. (a, b) Transient absorption spectra and the time profiles of the
transient absorbance at 500 nm of 4 (1.7×10−4 M, in benzene) upon 355-nm
picosecond laser pulse irradiation (1 kHz, 8 mW) at 298 K. (c, d) Transient
absorption spectra and the time profiles of the transient absorbance at 500 nm
of 5 (2.7×10−5 M, in benzene) upon 355-nm nanosecond laser pulse irradiation
(4 mJ) at 298 K.
Conclusion
5
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
performed in a benzene solution placed in a 2-mm quartz cell under stirring
at room temperature.
7.47 (d, J = 7.2 Hz, 1H), 7.41–7.37 (m, 2H), 7.32–7.30 (m, 2H), 6.93–6.91
(m,2H), 5.11 (s,1H), 3.47 (s,4H).
Synthesis.
4-(6-(4,5-Diphenyl-1H-imidazol-2-yl)-1,2-dihydroacenaphthylen-5-
yl)phenol
(S4).
6-(4-hydroxyphenyl)-1,2-dihydroacenaphthylene-5-
carbaldehyde (S3) (280 mg, 1.02 mmol), benzil (404 mg, 1.92 mmol) and
ammonium acetate (985 mg, 12.8 mmol) were dissolved in acetic acid 13.6
mL. The mixture was stirred at 110 °C for 24 hours. After cooling to room
temperature, the reaction mixture was neutralized with aqueous NH3, and
the aqueous layer was extracted with AcOEt. The combined organic layer
was washed with water and brine, filtered through a phase separator paper.
and evaporated. After removal of the solvents, the crude product was
purified by silica gel column chromatography (hexane/AcOEt = 1/1), to give
desired product as a yellow solid (233 mg, 50 %). 1H NMR (400 MHz,
DMSO-d6) δ: 11.58 (s, 1H), 9.01 (s, 1H), 7.67 (d, J = 6.8 Hz, H), 7.45–7.14
(m, 12H), 6.92 (d, J = 7.2 Hz, 2H), 6.40 (d, J = 8.0 Hz, 2H), 3.45 (s, 4H).
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 (Silica gel 60N, Kanto Chemical). 1H NMR and
13C NMR spectra were recorded at 400 MHz on a Bruker AVANCE III 400
NanoBay. DMSO-d6 and CDCl3 were used as deuterated solvent. All
glassware was washed with distilled water and dried. Unless otherwise
noted, all reagents and reaction solvents were purchased from TCI, Wako
Co. Ltd., Aldrich Chemical Co., Inc. and Kanto Chemical Co., Inc. and were
used without further purification.
Compound 2. To a solution of 4-(6-(4,5-diphenyl-1H-imidazol-2-yl)-1,2-
dihydroacenaphthylen-5-yl)phenol (S4) (200 mg, 0.430 mmol) in benzene
13 mL and ethanol 1.5 mL was added KOH (2.04 g, 35.6 mmol) and
K3[Fe(CN)6] (1.54 g, 4.56 mmol) in water 13 mL. The mixture was
vigorously stirred for 2 hours at room temperature. The reaction mixture
was washed with water and the solvent was filtered through a phase
separator paper. After removal of the solvents, the crude product was
purified by silica gel column chromatography (hexane/AcOEt = 5/2), to give
desired product as a pale yellow solid (25 mg, 25 %). 1H NMR (400 MHz,
DMSO-d6) δ: 8.25 (d, 7.2 Hz, 1H), 7.54 (d, J = 7.6 Hz, 1H), 7.45–7.27 (m,
9H), 7.22–7.12 (m, 4H), 5.78–5.75 (m, 2H), 3.46–3.38 (m, 4H). 13C NMR
(400 MHz, CDCl3) δ: 184.76, 149.37, 147.08, 146.86, 141.07, 139.61,
139.16, 134.09, 132.86, 130.83, 130.10, 129.61, 128.31, 128.13, 126.69,
126.65, 126.35, 125.30, 123.01, 122.43, 120.73, 120.21, 118.03, 63.15,
31.08, 30.63. HRMS (ESI-TOF) calculated for C33H22N2O [M+H]+:
463.11804, found: 463.1826.
Scheme 3. Synthetic Scheme of 2.
5,6-Dibromo-1,2-dihydroacenaphthylene (S1) was prepared according to
a literature procedure.[35]
6-Bromo-1,2-dihydroacenaphthylene-5-carbaldehyde (S2). 1.50 mL (2.32
mmol) of n-butyl lithium solution (1.55 M in hexane) was added into a
solution of 600 mg (1.93 mmol) of 5,6-dibromoacenaphthylene in 6 mL of
anhydrous THF at −78 °C under N2 atmosphere. The resulting reaction
mixture was stirred at −78 °C for 20 minutes, treated with 1.2 mL (15.4
mmol) of anhydrous DMF, and then kept stirred at −78 °C for 30 minutes.
The reaction mixture was slowly warmed to room temperature, and stirred
for 30 minutes at room temperature. After the quenching with water, the
reaction mixture was extracted with CH2Cl2. The organic extract was
washed with water and brine, and filtered through a phase separator paper.
After removal of the solvents, the crude product was purified by silica gel
column chromatography (CH2Cl2/hexane = 2/1), to give desired product as
a white solid (438.4 g, 86 %). 1H NMR (400 MHz, CDCl3) δ: 11.65 (s, 1H),
8.14 (d, J = 7.2 Hz, 1H), 7.87 (d, J = 7.2 Hz, 1H), 7.41 (d, J = 7.2 Hz, 1H),
7.23 (d, J = 7.6 Hz, 1H), 3.45-7.21 (m, 4H).
Scheme 4. Synthetic Scheme of 3.
Np-PIC (1) was prepared according to a literature procedure.[26]
Compound 3. 0.1 mL (0.1 mmol) of thionyl chloride (1 M in CH2Cl2) was
added into a solution of 1 (10 mg, 0.023 mmol), malononitrile (9 mg, 0.14
mmol) and anhydrous pyridine (0.010 mL, 0.14 mmol) in 2.0 mL of
anhydrous CHCl3 at 0 °C under N2 atmosphere. The mixture is refluxed at
60 °C for 2 days. After the quenching with water, the reaction mixture was
extracted with CH2Cl2. The organic extract was washed with water and
brine, and filtered through a phase separator paper. After removal of the
solvents, the crude product was purified by silica gel column
chromatography (AcOEt/hexane = 1/3), to give desired product as a
orange solid (7.0 mg, 63 %). 1H NMR (400 MHz, DMSO-d6) δ: 8.44 (d, J =
6.4 Hz, 1H), 8.07 (t, J = 8.4 Hz, 2H), 7.78 (t, J = 7.2 Hz, 1H), 7.58–7.36
(m,8H), 7.24–7.08 (m, 6H), 6.46 (d, J = 9.6 Hz, 2H). 13C NMR (400 MHz,
CDCl3) δ: 153.89, 145.77, 140.78, 133.46, 132.93, 130.80, 129.76, 129.22,
128.74, 128.59, 128.21, 127.41, 126.93, 126.69, 126.25, 126.16, 126.17,
124.01, 121.67, 121.53, 119.74, 111.69, 82.09, 63.12. HRMS (ESI-TOF)
calculated for C34H20N4 [M+H]+: 485.1761, found: 485.1747.
6-(4-Hydroxyphenyl)-1,2-dihydroacenaphthylene-5-carbaldehyde
(S3).
K2CO3 (147 mg, 1.06 mmol), 6-bromo-1,2-dihydroacenaphthylene-5-
carbaldehyde (S2) (400 mg, 1.54 mmol), and 4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenol (400 mg, 1.85 mmol) were added to DME (7.2
mL) and water (4.0 mL) and the mixture was purged with N2 gas.
Pd(PPh3)4 (50 mg, 0.0432 mmol) was added to the solution and the
solution was refluxed for 11 hours. The reaction mixture was extracted with
CH2Cl2, and the organic extract was washed with water and brine, and
filtered through a phase separator paper. After removal of the solvents, the
crude product was purified by silica gel column chromatography
(hexane/AcOEt = 5/1), to give desired product as a yellow solid (292 mg,
69 %). 1H NMR (400 MHz, CDCl3) δ: 9.65 (s, 1H), 8.08 (d, J = 7.2 Hz, 1H),
6
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
solid (102 mg, 37 %). 1H NMR (400 MHz, DMSO-d6) δ: 11.75 (s, 1H), 9.14
(s, 1H), 7.90 (d, J = 6.8 Hz, 1H), 7.84 (d, J = 7.2 Hz, 1H), 7.80 (d, J = 7.2
Hz, 1H), 7.46 (d, J = 6.8 Hz, 1H), 7.39 (d, J = 8.4 Hz, 2H), 7.31–7.13 (m,
10H), 7.02 (d, J = 8.0 Hz, 2H), 6.46 (d, J = 8.8 Hz, 1H).
Compound 4. To
a
solution of 4-(6-(4,5-diphenyl-1H-imidazol-2-
yl)acenaphthylen-5-yl)phenol (S9) (5 mg, 0.011 mmol) in benzene 2 mL
and ethanol 0.02 mL was added KOH (82 mg, 1.5 mmol) and K3[Fe(CN)6]
(48 g, 0.15 mmol) in water 2 mL. The mixture was vigorously stirred for 2
hours at room temperature. The reaction mixture was washed with water
and the solvent was filtered through a phase separator paper. After
removal of the solvents, the crude product was purified by silica gel column
chromatography (CH2Cl2/AcOEt = 60/1), to give desired product as an
orange solid (1 mg, 20 %). 1H NMR (400 MHz, DMSO-d6) δ: 8.30 (d, J =
7.2 Hz, 1H), 8.02 (d, J = 7.2 Hz, 1H), 7.84 (d, J = 7.6 Hz, 1H), 7.47–7.18
(m, 15H), 5.91 (d, J = 10.0 Hz, 1H). 13C NMR (400 MHz, CDCl3) δ: 183.52,
147.57, 138.69, 138.27, 132.79, 131.81, 129.86, 129.76, 129.67, 129.47,
129.25, 128.31, 127.31, 127.23, 127.17, 126.93, 126.47, 125.87, 125.64,
124.68, 123.43, 121.62, 120.41, 118.95. HRMS (ESI-TOF) calculated for
C33H20N2O [M+H]+: 461.1648, found: 461.1650.
Scheme 5. Synthetic Scheme of 4.
1,2,5,6-Tetrabromo-1,2-dihydroacenaphthylene
(S5)
and
5,6-
dibromoacenaphthylene (S6) were prepared according to a literature
procedure.[36]
6-Bromoacenaphthylene-5-carbaldehyde (S7). 0.75 mL (1.2 mmol) of n-
butyl lithium solution (1.55 M in hexane) was added into a solution of 300
mg (0.967 mmol) of 5,6-dibromoacenaphthylene (S6) in 3 mL of anhydrous
THF at −78 °C under N2 atmosphere. The resulting reaction mixture was
stirred at −78 °C for 20 minutes, treated with 0.60 mL (7.7 mmol) of
anhydrous DMF, and then kept stirred at −78 °C for 50 minutes. The
reaction mixture was slowly warmed to room temperature, and stirred for
30 minutes at room temperature. After the quenching with water, the
reaction mixture was extracted with CH2Cl2. The organic extract was
washed with water and brine, and filtered through a phase separator paper.
After removal of the solvents, the crude product was purified by silica gel
column chromatography (AcOEt/hexane = 1/10), to give desired product
as a yellow solid (189.6 mg, 76 %). 1H NMR (400 MHz, CDCl3) δ: 11.61 (s,
1H), 8.12 (d, J = 6.8 Hz, 1H), 7.87 (7.2 Hz, 1H), 7.73 (d, J = 7.2 Hz, 1H),
7.47 (d, J = 7.2 Hz, 1H), 7.10 (d, J = 5.2 Hz, 1H), 6.98 (d, J = 5.6 Hz, 1H).
Scheme 6. Synthetic Scheme of 5.
Compound 5. 0.5 mL (0.5 mmol) of thionyl chloride (1 M in CH2Cl2) was
added into a solution of compound 4 (47 mg, 0.10 mmol), malononitrile (39
mg, 0.59 mmol) and anhydrous pyridine (0.050 mL, 0.62 mmol) in 3.4 mL
of anhydrous CHCl3 at 0 °C under N2 atmosphere. The mixture is refluxed
at 60 °C for 26 hours. After the quenching with water, the reaction mixture
was extracted with CH2Cl2. The organic extract was washed with water
and brine, and filtered through a phase separator paper. After removal of
the solvents, the crude product was purified by silica gel column
chromatography (AcOEt/hexane = 1/4), to give desired product as an
orange solid (28 mg, 54 %). 1H NMR (400 MHz, DMSO-d6) δ: 8.30 (d, J =
7.2 Hz, 1H), 8.02 (d, J = 7.2 Hz, 1H), 7.81 (d, J = 7.2 Hz, 1H), 7.52–7.49
(m, 1H), 7.47 (d, J = 7.2 Hz, 2H), 7.40 (d, J = 4.4 Hz, 4H), 7.27–7.14 (m,
6H), 7.04 (d, J = 9.6 Hz, 2H), 6.54 (d, J = 9.6 Hz, 2H). 13C NMR (400 MHz,
CDCl3) δ: 153.89, 145.77, 140.78, 133.75, 132.93, 130.80, 129.76, 129.22,
128.74, 128.59, 128.21, 127.41, 126.93, 126.69, 126.25, 125.17, 124.01,
121.67, 121.53, 119.74, 111.69, 82.09, 63.12. HRMS (ESI-TOF)
calculated for C36H20N4 [M+H]+: 509.1760, found: 509.1767.
6-(4-Hydroxyphenyl)acenaphthylene-5-carbaldehyde (S8). K2CO3 (71 mg,
0.52 mmol), 6-bromoacenaphthylene-5-carbaldehyde (S6) (190 mg, 0.732
mmol), and 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenol (195 mg,
0.887 mmol) were added to DME (6.0 mL) and water (2.0 mL) and the
mixture was purged with N2 gas. Pd(PPh3)4 (23 mg, 0.020 mmol) was
added to the solution and the solution was refluxed for 6 hours. The
reaction mixture was extracted with CH2Cl2, and the organic extract was
washed with water and brine, and filtered through a phase separator paper.
After removal of the solvents, the crude product was purified by silica gel
column chromatography (CH2Cl2/AcOEt = 15/1), to give desired product
as a orange solid (161 mg, 81 %). 1H NMR (400 MHz, DMSO-d6) δ: 9.81
(br, 1H), 9.42 (s, 1H), 8.03 (d, J = 7.2 Hz, 1H), 7.96 (d, J = 7.2 Hz, 1H),
7.88 (d, J = 6.8 Hz, 1H), 7.56 (d, J = 7.2 Hz, 1H), 7.37 (d, J = 5.2 Hz, 1H),
7.25 (d, J = 8.4 Hz, 2H), 7.22 (d, J = 5.2 Hz, 1H), 6.92 (d, J = 8.4 Hz, 2H).
4-(6-(4,5-Diphenyl-1H-imidazol-2-yl)acenaphthylen-5-yl)phenol (S9). 6-(4-
hydroxyphenyl)acenaphthylene-5-carbaldehyde (S8) (161 mg, 0.592
mmol), benzil (229 mg, 1.09 mmol) and ammonium acetate (586 mg, 7.60
mmol) were dissolved in acetic acid 7.5 mL. The mixture was stirred at
110 °C for 11 hours. After cooling to room temperature, the reaction
mixture was neutralized with aqueous NH3, and the aqueous layer was
extracted with AcOEt. The combined organic layer was washed with water
and brine, filtered through a phase separator paper, and evaporated. After
removal of the solvents, the crude product was purified by silica gel column
chromatography (CH2Cl2/AcOEt = 2/1), to give desired product as a yellow
Scheme 7. Synthetic Scheme of 6.
7
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
8-(4-Hydroxyphenyl)-1-naphthaldehyde (S10) was prepared according to
a literature procedure.[26]
[9]
H.-R. Blattmann, D. Meuche, E. Heilbronner, R. J. Molyneux, V.
Boekelheide, J. Am. Chem. Soc. 1965, 87, 130-131.
[10] R. H. Mitchell, Eur. J. Org. Chem. 1999, 2695-2703.
[11] K. Honda, H. Komizu, M. Kawasaki, J. Chem. Soc., Chem. Commun.
1982, 253-254.
2,7-di-tert-butylpyrene-4,5-dione (S11) was prepared according to
literature procedure.[37]
a
[12] S. Helmy, F. A. Leibfarth, S. Oh, J. E. Poelma, C. J. Hawker, J. Read de
Alaniz, J. Am. Chem. Soc. 2014, 136, 8169-8172.
(1s,4s)-4-(8-(2,7-di-tert-butyl-9H-pyreno[4,5-d]imidazol-10-yl)naphthalen-
1-yl)cyclohexa-2,5-dien-1-ol (S12). 8-(4-hydroxyphenyl)-1-
[13] J. R. Hemmer, S. O. Poelma, N. Treat, Z. A. Page, N. D. Dolinski, Y. J.
Diaz, W. Tomlinson, K. D. Clark, J. P. Hooper, C. Hawker, J. Read De
Alaniz, J. Am. Chem. Soc. 2016, 138, 13960-13966.
naphthaldehyde (S11) (25 mg, 0.10 mmol), 2,7-di-tert-butylpyrene-4,5-
dione (40 mg, 0.12 mmol) and ammonium acetate (60 mg, 0.78 mmol)
were dissolved in acetic acid 1 mL. The mixture was stirred at 110 °C for
17 hours. After cooling to room temperature, the reaction mixture was
neutralized with aqueous NH3, and the aqueous layer was extracted with
AcOEt. The combined organic layer was washed with water and brine,
filtered through a phase separator paper, and evaporated. After removal
of the solvents, the crude product was purified by silica gel column
chromatography (CH2Cl2/AcOEt = 10/1), to give desired product as a pale
yellow solid (43 mg, 75 %). 1H NMR (400 MHz, CDCl3) δ: 8.97 (s, 1H), 8.86
(d, J = 1.6 Hz, 1H), 8.23 (d, J = 6.8 Hz, 1H), 8.08–7.95 (m, 6 H), 7.76 (s,
1H), 7.66 (dd, J = 7.2, 11.2 Hz, 2H), 7.57 (d, 0.8 Hz, 1H), 7.10 (d, 7.6 Hz,
2H), 6.12 (d, 8.0 Hz, 2H), 4.79 (s, 1H), 1.59 (s, 1H), 1.55(s, 1H).
[14] J. R. Hemmer, Z. A. Page, K. D. Clark, F. Stricker, N. D. Dolinski, C. J.
Hawker, J. Read De Alaniz, J. Am. Chem. Soc. 2018, 140, 10425-10429.
[15] J. C. Crano, R. J. Guglielmetti, Organic Photochromic and
Thermochromic Compounds, Plenum Press, New York, 1999.
[16] H. Kuroiwa, Y. Inagaki, K. Mutoh, J. Abe, Adv. Mater. 2019, 31, 1805661.
[17] E. Deniz, M. Tomasulo, J. Cusido, S. Sortino, F. M. Raymo, Langmuir
2011, 27, 11773-11783.
[18] K. Mutoh, M. Sliwa, J. Abe, J. Phys. Chem. C 2013, 117, 4808-4814.
[19] N. Ishii. T. Kato, J. Abe, Sci. Rep. 2012, 2, 819.
[20] Y. Kobayashi, J. Abe, Adv. Optical Mater. 2016, 4, 1354-1357.
[21] S. Hatano, T. Horino, A. Tokita, T. Oshima, J. Abe, J. Am. Chem. Soc.
2013, 135, 3164-3172.
[22] I. Yonekawa, K. Mutoh, Y. Kobayashi, J. Abe, J. Am. Chem. Soc. 2018,
140, 1091-1097.
Compound 6. To
a
solution of (1s,4s)-4-(6-(2,7-di-tert-butyl-9H-
pyreno[4,5-d]imidazol-10-yl)acenaphthylen-5-yl)cyclohexa-2,5-dien-1-ol
(S11) (20 mg, 0.035 mmol) in benzene 12 mL was added KOH (170 mg,
3.03 mmol) and K3[Fe(CN)6] (793 mg, 2.41 mmol) in water 20 mL. The
mixture was vigorously stirred for 2 hours at room temperature. The
reaction mixture was washed with water and the solvent was filtered
through a phase separator paper. After removal of the solvents, the crude
product was purified by silica gel column chromatography (CH2Cl2/AcOEt
= 1/1), to give desired product as an orange solid (6 mg, 30 %). 1H NMR
(400 MHz, CDCl3) δ: 9.17 (d, J = 6.4 Hz, 1H), 8.96 (dd, J = 0.8, 7.2 Hz,
1H), 8.53 (d, 2.0, 1H), 8.20 (s,1H), 8.19 (s, 1H), 8.12 (dd, J = 0.8, 6.8 Hz,
2H), 7.93 (d, J = 0.8 Hz, 1H), 7.91 (d, J =0.8 Hz, 1H ), 7.72 (t, 7.2 Hz, 1H),
7.68–7.57 (m, 2H), 7.54–7.50 (m, 2H), 1.67 (s, 1H), 1.49(s, 1H). 13C NMR
(400 MHz, CDCl3) δ: 152.74, 149.27, 148.21, 144.81, 141.27, 132.94,
131.66, 131.22, 129.29, 129.14, 128.46, 128.34, 128.08, 127.63, 127.56,
127.48, 126.48, 125.95, 125.74, 124.81, 123.30, 122.08, 121.98, 121.94,
121.87, 121.80, 121.35, 119.85, 1175.05. HRMS (ESI-TOF) calculated for
C41H34N2O [M+H]+: 571.2743, found: 571.2743.
[23] I. Yonekawa, K. Mutoh, J. Abe, Chem. Commun. 2019, 55, 1221-1224.
[24] K. Mutoh, N. Miyashita, K. Arai, J. Abe, J. Am. Chem. Soc. 2019, 141,
5650-5654.
[25] T. Yamaguchi, Y. Kobayashi, J. Abe, J. Am. Chem. Soc. 2016, 138, 906-
913.
[26] K. Mutoh, Y. Kobayashi, Y. Hirao, T. Kubo, J. Abe, Chem. Commun.
2016, 52, 6797-6800.
[27] Y. Satoh, Y. Ishibashi, S. Ito, Y. Nagasawa, H. Miyasaka, H. Chosrowjan,
S. Taniguchi, N. Mataga, D. Kato, A. Kikuchi, J. Abe, Chem. Phys. Lett.
2007, 448, 228-231.
[28] H. Miyasaka, Y. Satoh, Y. Ishibashi, S. Ito, S. Taniguchi, H. Chosrowjan,
N. Mataga, D. Kato, A. Kikuchi, J. Abe, J. Am. Chem. Soc. 2009, 131,
7256-7263.
[29] K. Mutoh, E. Nakano, J. Abe, J. Phys. Chem. A 2012, 116, 6792-6797.
[30] K. Yamamoto, K. Mutoh, J. Abe, J. Phys. Chem. A 2019, 123, 1945-1952.
[31] E. Nakano, K. Mutoh, Y. Kobayashi, J. Abe, J. Phys. Chem. A 2014, 118,
2288-2297.
[32] K. Yamamoto, I. Gomita, H. Okajima, A. Sakamoto, K. Mutoh, J. Abe,
Chem. Commun. 2019, 55, 4917-4920.
[33] F. A. Carey, R. J. Sundberg, Advanced Organic Chemistry: Part A:
Structure and Mechanisms, Springer, 2007.
Acknowledgements
[34] T. Nakagawa, K. Okamoto, H. Hanada, R. Katoh, Opt. Lett. 2016, 41,
1498-1501.
This work was supported by JSPS KAKENHI Grant Number
JP18H05263. This work was also supported by Aoyama Gakuin
University Research Institute “Early Eagle” grant program for
promotion of research by early career researchers.
[35] M. A. Niyas, R. Ramakrishnan, V. Vijay, E. Sebastian, M. Hariharan, J.
Am. Chem. Soc. 2019, 141, 4536-4540.
[36] L. M. Diamond, F. R. Knight, K. S. A. Arachchige, R. A. M. Randall, M.
Bühl, A. M. Z. Slawin, J. D. Woollins, Eur. J. Inorg. Chem. 2014, 9, 1512-
1523.
Keywords: photochromism • visible light • biradical • imidazole
[37] P.-Y. Gu, Z. Wang, G. Liu, H. Yao, Z. Wang, Y. Li, J. Zhu, S. Li, Q. Zhang,
Chem. Mater. 2017, 29, 4172-4175.
[1]
[2]
[3]
[4]
M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Chem. Rev. 2014, 114,
12174-12277.
V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, Angew. Chem., Int. Ed.
2000, 39, 3348-3391.
Molecular Switches (Eds.: B. L. Feringa, W. R. Browne), Wiley-VCH,
Weinheim, 2011.
Photon-Working Switches (Eds.: Y. Yokoyama, K. Nakatani), Springer:
Tokyo, 2017.
[5]
[6]
[7]
[8]
I. Shimizu, H. Kokado, E. Inoue, Bull. Chem. Soc. Jpn. 1969, 42, 1730-
1734.
J. Sunamoto, K. Iwamoto, M. Akutagawa, M. Nagase, H. Kondo, J. Am.
Chem. Soc. 1982, 104, 4904-4907.
F. Ciardelli, D. Fabbri, O. Pieroni, A. Fissi, J. Am. Chem. Soc. 1989, 111,
3470-3472.
M. Tanaka, M. Nakamura, M. A. A. Salhin, T. Ikeda, K. Kamada, H. Ando,
Y. Shibutani, K. Kimura, J. Org. Chem. 2001, 66, 1533-1537.
8
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000296
ChemPhysChem
ARTICLE
Entry for the Table of Contents
Insert graphic for Table of Contents here.
A fast switchable negative photochromic molecule is useful as a photo-switch aiming to the application to functional materials. Herein,
we succeed in the control of the thermal back reaction rates and the enhancement of the visible-light sensitivity of the naphthalene-
bridged phenoxyl-imidazolyl radical complex (Np-PIC). These insights will be important for the future development of fast switchable
negative photochromic molecules.
9
This article is protected by copyright. All rights reserved.