Structurally and electronically modulated spin interaction of transient biradicals in two photon-gated stepwise photochromism

Article abstract of DOI:10.1039/c7pp00420f

The development of two-photon induced photochromic compounds is important for advanced photoresponsive materials. The utilization of the long-lived transient states or species for two-photon absorption is one of the efficient strategies to realize the advanced photochemical behavior beyond a one-photon photochemical reaction. We have synthesized bi-photochromic compounds composed of two photochromic phenoxyl-imidazolyl radical complex units. The biphotochromic compounds generate two biradical units when the two photochromic units absorb photons with a stepwise manner. The interaction between the two biradicals through the central bridging phenyl ring is the key feature to control the stepwise photochromic reaction. Here, we introduced aromatic spacers in order to modulate the distance and the dihedral angle between the biradical units. The color and the rate of the thermal back reaction of the stepwise photochromism can be regulated by the control of the central bridging part. These results give important insights to develop desirable advanced photoresponsive compounds.

Full text of DOI:10.1039/c7pp00420f

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Structurally and electronically modulated spin
interaction of transient biradicals in two
photon-gated stepwise photochromism†
Cite this: DOI: 10.1039/c7pp00420f
a
Izumi Yonekawa,a Katsuya Mutoh, a Yoichi Kobayashi b and Jiro Abe
*
The development of two-photon induced photochromic compounds is important for advanced photo-
responsive materials. The utilization of the long-lived transient states or species for two-photon absorp-
tion is one of the efficient strategies to realize the advanced photochemical behavior beyond a one-
photon photochemical reaction. We have synthesized bi-photochromic compounds composed of two
photochromic phenoxyl-imidazolyl radical complex units. The biphotochromic compounds generate two
biradical units when the two photochromic units absorb photons with a stepwise manner. The interaction
between the two biradicals through the central bridging phenyl ring is the key feature to control the step-
wise photochromic reaction. Here, we introduced aromatic spacers in order to modulate the distance
and the dihedral angle between the biradical units. The color and the rate of the thermal back reaction of
the stepwise photochromism can be regulated by the control of the central bridging part. These results
give important insights to develop desirable advanced photoresponsive compounds.
Received 14th November 2017,
Accepted 2nd January 2018
DOI: 10.1039/c7pp00420f
rsc.li/pps
Recently, we have developed two-photon induced stepwise
photoresponsive systems involving two fast-photochromic
Introduction
Organic photoswitchable materials have been studied in units, an imidazole dimer or a phenoxyl–imidazolyl radical
various research fields aiming to develop ophthalmic lenses, complex (PIC), in a molecule (Scheme 1).18–20 The photochemi-
optical memory media, photomechanical materials and fluo- cal reactions of PIC derivatives lead to the generation of a
rescence switching dyes.1–6 The use of light as a stimulus for
chemical reactions enables us to control photofunctions
temporally and spatially. Recently, a basic principle and
system based on a synergetic response between molecules and
photons have received much attention for the construction of
advanced photoresponsive systems.7–11 The utilization of the
long-lived transient states or species as an intermediate state
for two-photon absorption is an efficient strategy to realize
advanced photochemical behavior beyond
a one-photon
photochemical reaction: the stepwise two-photon photochemi-
cal reaction with nonlinear photoresponse.12–17 To achieve
multiple photoresponses depending on the intensity and wave-
length of excitation light in bichromophoric systems, the
mutual electronic and structural couplings between the
chromophores is fundamentally required.11,13
aDepartment 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; Fax: +81-42-759-6225; Tel: +81-42-759-6225
bDepartment of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan
†Electronic supplementary information (ESI) available: NMR and MS spec-
troscopy and other experimental data. See DOI: 10.1039/c7pp00420f
Scheme 1 Photochromic reaction of PIC and two photon-gated
photochromic reaction of bisPIC.
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couple of imidazolyl and phenoxyl radicals with the half-lives
in the range of μs to ms. Because these time-scales of the life-
Results and discussion
time is suitable for additional photon absorption,20 the tran- The plausible two-photon induced photochromic reactions of
sient biradical species can be expected to behave as the inter- 1–3 are shown in Scheme 2. It was expected that a one-photon
mediate state for the two-photon absorption process such as absorption by the initial closed ring form produces the transi-
the excited triplet state of energy upconversion processes.15,16 ent BR, and the further photochemical reaction of BR gener-
BisPIC, in which two PIC units are combined in a molecule ates Q, as similar to our previous reports.20 Because the rela-
with a phenyl ring, generates two biradical units when both tive spatial arrangements between the imidazolyl and phenoxyl
the PIC units absorb photons with a stepwise manner. It radicals are different in 1 and 2, the expected two-photon pro-
means that the bisPIC derivatives undergo two-photon ducts (Q) of 1 and 2 have different conjugation paths between
induced photochromic reactions. After the two-photon reac- the central aromatic rings and the radical units: the closed-
tion, a couple of the imidazolyl radicals at the para-position shell structures between the two phenoxyl radicals (1Q) and
of the central bridging aromatic ring form a para-quinoid that between two imidazolyl radicals (2Q), respectively.
(p-quinoid) structure (such as BDPI-2Y)21,22 through an anti-
Fig. 2 shows the absorption spectra of 1–3 in benzene. The
ferromagnetic electronic interaction. Therefore, the photo- molar extinction coefficient of 1 is almost twice as large as that
chromic properties are nonlinearly regulated, depending on of PIC. On the other hand, the absorption spectra of 2 and 3
the excitation light intensity because of the drastic change in are largely red-shifted as compared to that of PIC. These
the electronic structures between the one-photon produced results indicate the extended π-conjugation between the two
transient biradical (BR) and the two-photon produced imidazole rings through the biphenyl ring in 2 and 3. The
quinoid (Q) states (Scheme 1). The electronic structure of Q is imidazole ring and the phenyl ring at the 2-position of the
mainly dictated by two types of radical–radical interactions at imidazole ring are coplanar and conjugated as revealed by the
the precursor tetraradical (TR) state: the interaction between X-ray crystallography of PIC.18 On the other hand, the phenoxyl
the radicals at the para-position of the central aromatic ring ring is arranged perpendicularly against the imidazole ring,
(p-interaction) and that at the ortho-position (o-interaction). and the imidazole ring is substituted at the meta-position
Therefore, the interaction between the two transient radical against the position where two PIC units are substituted.
units is the key feature to control the stepwise photochromic Therefore, the two imidazole rings of 1 are not involved in the
reaction.
π-conjugation pathway and no shift of the absorption spec-
In this study, we developed novel photochromic compounds trum was observed.
based on bisPIC to investigate and to regulate the interaction
The transient absorption spectra of 1, 2 and 3 upon 355 nm
between the two transient biradical units (Fig. 1). The balance laser irradiation in benzene are shown in Fig. 3a. The broad
of p- and o-interactions determines the stability of the Q state transient absorption spectra can be assigned to BRs which are
and affects the two-photon induced photochromic properties. generated by the one-photon absorption of 1, 2 and 3 (1BR,
To modulate the interactions, we introduced a phenylene 2BR and 3BR, respectively). The transient absorption spectrum
spacer to the central aromatic ring of bisPIC (1 and 2). The of 1 is almost identical to that of PIC, indicating that the
relatively large distance and dihedral angle between the two bi- biradical and PIC units of 1BR are independent. On the other
radical units will reduce the contribution of the p-interaction, hand, the transient absorption spectra of 2 and 3 are different
leading to the destabilization of the Q state. Moreover, we from those of both PIC and bisPIC, indicating that the conju-
investigated the effect of the aromaticity of the central phenyl gations of 2BR and 3BR are extended over the biradical and
ring on the radical–radical interaction through the measure- PIC units. The slight shift of the absorption spectrum of 3BR
ment of the photochromic property of 3.
would be due to the conjugation through the phenanthrene
unit. These transient absorption spectra monotonically decay
in the whole visible regions. The rates of the thermal back
reaction of the transient BR show first order reaction kinetics
(Fig. 3b). The half-lives of transient 1BR, 2BR and 3BR were
estimated to be 5.0 × 102, 3.6 × 102 and 16 ns at room tempera-
ture, respectively. As described in a previous report,23 the stabi-
lization of the phenoxyl radical is efficient in reducing the rate
of the thermal back reaction more than the stabilization of the
imidazolyl radical. According to this insight, the slight decel-
eration of the thermal back reaction of 1BR, compared with
that of 2BR, would be due to the stabilization of the phenoxyl
radical by the delocalization of the π-electron over the phe-
noxyl radical and the other PIC unit. The fast thermal back
reaction of 3BR will be derived from the large steric hindrance
between the biradical unit and the phenanthrene unit. To
investigate the two-photon induced photochromism of 1 and
Fig. 1 Molecular designs of the bisPIC derivatives 1, 2 and 3 to control
the radical–radical interaction of the transient biradicals.
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Scheme 2 Stepwise photochromic reaction scheme.
2, the excitation light intensity dependence on the transient
absorption spectra was investigated. While significant change
in the transient absorption spectra of 1 was not observed
depending on the excitation intensity, a new transient absorp-
tion band at around 600–650 nm was observed by increasing
the excitation light intensity in the photochromism of 2
(Fig. 4). This new transient absorption band can be tentatively
assigned to 2Q, which has the p-quinoidal structure between
the imidazolyl radicals. It was reported that the p-quinoidal
structure such as BDPI-2Y had the characteristic absorption
band at around 600–650 nm because of the interaction
between the two imidazolyl radicals leading to the formation
of a closed-shell quinoid structure.21,22 This absorption band
Fig. 2 Steady-state absorption spectra of 1, 2, 3 and PIC in benzene at
298 K.
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p-interaction in 1TR, the dominant o-interaction would de-
stabilize the p-quinoidal structure, leading to the similar elec-
tronic structure of the two-photon product of 1 with that of
1BR. However, the difference in the absorption spectra
between 2BR and 2Q is relatively small compared with that of
bisPIC,19 indicating the lower contribution of the p-quinoidal
structure in 2Q (Scheme 2). It was revealed by Montgomery in
1986 that the bond lengths in the central section of
Chichibabin’s hydrocarbons are slightly longer than the usual
CvC double bond. They concluded that the longer bond
length came from the contribution of the biradical character
due to the phenylene spacers in Chichibabin’s
hydrocarbon.24–26
To reveal the origin of the less conjugation between the
imidazolyl radicals through the central biphenyl ring in 2Q, we
synthesized the biphenyl-bridged bis(imidazolyl radical)
derivatives R1, R2 and R3 as reference compounds for the tran-
sient absorption spectroscopy of 2. The introduction of methyl
substituents to the biphenyl unit varies the dihedral angle
between the two adjacent phenyl rings, which directly varies
the radial–radical interaction between the imidazolyl radicals.
Fig. 5 shows the absorption spectra of R1, R2 and R3 in
benzene at room temperature. Compound R1 has the absorp-
tion maximum at 710 nm, indicating the extended conjugation
and the planar quinoidal structure of R1. The absorption
bands of R2 and R3 are blue-shifted by the introduction of the
methyl groups. This shift clearly indicates that the increase of
the dihedral angle between the two central phenyl rings
reduces the conjugation between the imidazolyl radicals,
leading to the gradual blue shift of the absorption maximum
from 710 nm to 590 nm. The increased transient absorption
band of 2 after the intense light excitation has the maximum
at around 625 nm. Therefore, this result suggests the relatively
large dihedral angle between the two central phenyl rings
resulting in the weak p-interaction of 2Q. Because the imid-
azolyl radicals of 2Q also interact with the phenoxyl radical
(o-interaction), the central C–C bond of the p-phenylene struc-
ture of 2Q would relatively possess a single bond feature.
Therefore, the two PIC units can rotate around the central C–C
bond and the contribution of the p-interaction is relatively
small in the 2Q state, compared with planar R1. Since the pla-
Fig. 3 (a) Transient vis–NIR absorption spectra of 1 (7.2 × 10−5 M), 2
(1.6 × 10−5 M) and 3 (1.4 × 10−5 M) in degassed benzene at 298 K (λex.
=
355 nm, 2 mJ mm−2, pulse width = 5 ns). (b) Time profiles of the transi-
ent absorbance at 600 nm in degassed benzene at 298 K. The inset
shows the logarithmic plots for the time profiles of the transient
absorbance.
Fig. 4 Dependence of the transient vis–NIR absorption spectra of 2
(1.6 × 10−5 M) on the excitation light intensity in degassed benzene (λex.
= 355 nm, pulse width = 5 ns). The spectra were normalized at 700 nm.
of 2Q disappeared with the half-life of 1.5 × 102 ns at 298 K
(Fig. S38, ESI†). The dependences of the transient absorption
spectra of 1 and 2 on the laser excitation intensity indicate
that the p-interaction is a key feature to modulate the two-
photon photochromic properties. That is, the p-interaction
between the imidazolyl radicals is more efficient in stabilizing
the p-quinoidal structure, compared with that between the
phenoxyl radicals. It is presumably because the five-membered
ring of the imidazole ring avoids the steric repulsion with the
central biphenyl unit. Therefore, the small dihedral angle
between the central biphenyl ring and the adjacent imidazole
ring in 2Q is suitable for the efficient conjugation to form the
Fig. 5 Steady-state absorption spectra of R1, R2 and R3 in benzene at
p-quinoidal structure. On the other hand, because of the weak 298 K.
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ring with a biphenyl and a phenanthrene unit. The long dis-
tance between the imidazolyl radicals increases the o-inter-
action which induces the large dihedral angle of the central
two phenyl rings. This significantly decreases the p-interaction
and destabilizes the p-quinoidal structure in Q. We also
revealed that the configuration of the imidazolyl and phenoxyl
radicals modulates the radical–radical interaction; the p-inter-
action between two imidazolyl radicals is efficient because of
the less steric repulsion between the imidazole ring and the
central aromatic ring. In addition to the above spatial control
of the arrangement of the radicals, the aromaticity of the
central phenyl ring determines the preferential interaction of
radicals. Therefore, the color and the rate of the thermal back
reaction of the stepwise photochromism can be regulated by
the control of the central bridging part. These results will give
useful insight to design and to develop attractive photorespon-
sive materials.
Fig. 6 Transient absorption spectra of 3 at 20 ns after irradiation by a
355 nm (5 ns) laser pulse. The power density was 2.2 mJ mm−2 and
0.5 mJ mm−2
.
Scheme 3 Resonance structures of the phenanthrene bridge of 3.
Experimental section
Laser flash photolysis
narity of the diphenyl ring strongly affects the interaction
between the radicals, we investigated the photochromic prop-
erty of the phenanthrene derivative 3, which possesses a copla-
nar arrangement around the central spacer; the increase in the
p-interaction is expected. However, the excitation power depen-
dence of the transient absorption spectra was not observed
(Fig. 6). Although the absorption band of 3 is slightly shifted
by the conjugation through the phenanthrene unit, 3 shows a
photochromic reaction upon 355 nm light irradiation as
shown in Fig. 3. That is, the excitation energy of 355 nm light
is sufficient for the photochromic reaction of 3. The absorp-
tion spectrum of 3BR is also similar to that of 2BR, suggesting
that the electronic structure of 3BR is similar to that of 2BR.
Therefore, the possibility of the quenching of the bond-break-
ing process of the C–N bonds of 3 and 3BR due to the energy
transfer to the phenanthrene unit is excluded. Thus no exci-
tation intensity dependence of the transient absorption
spectra of 3 indicates the small difference in the electronic
structures between 3BR and 3TR (or 3Q). When the two imid-
azolyl radicals of 3Q are conjugated through the phenanthrene
ring, the phenanthrene ring loses the aromaticity to form the
p-quinoidal structure. Hence, no change in the transient
absorption spectra would be attributable to the less stabiliz-
ation energy to form the p-quinoidal structure than the aro-
matic stabilization energy of the phenanthrene ring
(Scheme 3). This result clearly suggests that the aromaticity of
the central bridging part is one of the key factors controlling
the two-photon induced stepwise photochromism.
Laser flash photolysis experiments were 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 (time duration: 5 ns) was employed
for the excitation light. The probe beam from a halogen lamp
(OSRAM HLX64623) was irradiated to the sample solution
arranged in an orientation perpendicular to the exciting laser
beam. The transmitted probe beam was monitored with a
photomultiplier tube (Hamamatsu R2949) through a spectro-
meter (Unisoku MD200) for the transient absorption spectra
and the decay profiles of the open-ring isomers of 1, 2 and 3.
The excitation intensity was estimated by an energy detector
(Gentec Electro-Optics QE12LP-S-MB) with an energy monitor
(Gentec Electro-Optics MAESTRO). Optical grade solvents were
used for all measurements.
Synthetic procedure
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 Co., Inc.). 1H NMR spectra were
recorded at 400 MHz on a Bruker AVANCE III 400 NanoBay and
at 500 MHz on a JEOL JNM-ECP500A. DMSO-d6 and CDCl3
were used as deuterated solvents. ESI–TOF–MS spectra were
recorded on a Bruker micrOTOF II-AGA1 system. FAB-MS
spectra were measured by using a JEOL MStation MS700. All
glassware was washed with distilled water and dried. Unless
otherwise noted, all reagents and reaction solvents were pur-
chased from Tokyo Chemical Industry Co., Ltd, Wako Pure
Chemical Industries, Ltd, Sigma-Aldrich Inc. and Kanto
Chemical Co., Inc. and were used without further purification.
4,4′-Dibromo-3,3′-dimethyl-1,1′-biphenyl (4). 50% aqueous
Conclusions
We modulated the radical–radical interaction between the
imidazolyl and phenoxyl radicals in the two-photon generated H2SO4 (20 mL) was added to o-tolidine (3.00 g, 14.1 mmol).
products of bisPIC derivatives by replacing the bridging phenyl 45% aqueous NaNO2 (3 mL) and CuBr (17.1 g, 118.7 mmol) in
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47% aqueous HBr (39 mL) were added dropwise to the mixture temperature, neutralized by aqueous ammonia and filtered.
at 0 °C and the mixture was warmed to room temperature. The crude product was washed with water and CH2Cl2 to give 8
1
After gently stirring for 2.5 h at 100 °C, CH2Cl2 and water were as a pale yellow solid (86 mg, yield: 43%). H NMR (400 MHz,
added to the reaction mixture. The crude product was filtered DMSO-d6): δ 12.12 (s, 2H), 9.46 (s, 2H), 8.12 (s, 2H), 7.95 (d, J =
through a Celite pad to remove insoluble solid. The filtrate was 8.4 Hz, 2H), 7.56–7.19 (m, 26H), 6.75 (d, J = 8.6 Hz, 4H); HRMS
extracted with CH2Cl2, and the organic layer was washed with (ESI-TOF) calcd for C54H38N4O2 [M + H]+, 775.3068; found,
water and brine. The solvent was removed by evaporation. The 775.3085.
product was purified by silica gel column chromatography
Compound 1. A solution of potassium ferricyanide (1.64 g,
with hexane as an eluent, to give 4 as a white solid (1.17 g, 29.3 mmol) and aqueous 0.3 M KOH (20 mL) was added to a
1
yield: 24%). H NMR (400 MHz, CDCl3): δ 7.57 (d, J = 8.0 Hz, solution of 8 (42 mg, 53 µmol) and benzene (22.4 mL). After
2H), 7.41 (s, 2H), 7.22 (d, J = 8.0 Hz, 2H), 2.46 (s, 6H).
4,4′-Dibromo-3,3′-bis(dibromomethyl)-1,1′-biphenyl
stirring for 2.5 h at room temperature, the resultant solution
(5). was diluted with AcOEt. The organic layer was washed with
N-Bromosuccinimide (2.78 g, 15.6 mmol) and azobis(isobutyr- water and brine. The solvent was evaporated under reduced
onitrile) (172 mg, 1.05 mmol) were added to 4 (1.10 g, pressure. The residual solid was suspended in hexane, filtered,
3.24 mmol) in CCl4 (22 mL). After stirring for 13 h at 75 °C, and washed with hexane : CH2Cl2 = 1 : 1 to give 1 as a milky
the reaction mixture was diluted with CH2Cl2, succinimide was white solid (24 mg, yield: 58%). 1H NMR (400 MHz, DMSO-d6):
filtered off, and washed with aqueous Na2S2O3. The organic δ 8.32 (s, 2H), 7.89 (d, J = 8.0 Hz, 2H), 7.53 (d, J = 7.2 Hz, 4H),
layer was washed with water and brine, and passed through a 7.44–7.17 (m, 22H), 7.02 (d, J = 10 Hz, 2H), 6.26 (d, J = 9.6 Hz,
phase separator paper. The solvent was removed by evapor- 4H); HRMS (ESI-TOF) calcd for C54H34N4O2 [M
ation and the crude mixture was purified by silica gel column 771.2755; found, 771.2793 (Scheme 4).
+
H]+,
chromatography with hexane : CH2Cl2 = 2 : 1 as an eluent, to
2-Bromo-4-iodo-1-methylbenzene (9). 1N aqueous HCl
give 5 as a pale yellow solid (1.69 g, yield: 80%). 1H NMR (34 mL) was added to 3-bromo-4-methylaniline (25 g,
(400 MHz, CDCl3): δ 8.20 (s, 2H), 7.61 (d, J = 8.4 Hz, 2H), 7.41 0.134 mmol) in water (34 mL). The mixture was heated to
(d, J = 8.2 Hz, 2H), 7.13 (s, 6H).
60 °C and stirred for 1 h. After cooling to 0 °C, 32% aqueous
4,4′-Dibromo-[1,1′-biphenyl]-3,3′-dicarbaldehyde
(6). NaNO2 (21.5 mL) and 50% aqueous KI (23.5 mL) were added
NaHCO3 (1.78 g, 21.1 mmol) was added to the DMSO solution dropwise to the mixture at 0 °C. The mixture was vigorously
(20 mL) of 5 (1.66 g, 2.54 mmol). The mixture was stirred for stirred for 30 minutes at 0 °C. After further stirring for 1 h at
18 h at 100 °C. After cooling to room temperature, the reaction 60 °C, the mixture was cooled to room temperature and
was quenched with water. The organic layer was extracted with extracted with diethyl ether. The organic layer was washed
CH2Cl2, washed with water and brine, and passed through a with aqueous Na2S2O3, water and brine, passed through a
phase separator paper. The solvent was removed under phase separator paper, and the solvent was evaporated under
vacuum to give 6 as a pale yellow solid (775 mg, yield: 83%). reduced pressure. The product was purified by silica gel
1H NMR (400 MHz, CDCl3): δ 10.42 (s, 2H), 8.14 (s, 2H), 7.77 column chromatography (hexane) to give 10 as a pale pink
1
(d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.4 Hz, 2H), 7.67 (d, J = 8.4 Hz, liquid (28.6 g, yield: 72%). H NMR (400 MHz, CDCl3): δ 7.86
2H).
4,4′′′-Dihydroxy-[1,1′:4′,1″:4″,1′′′-quaterphenyl]-2′,3″-dicarbal- (s, 3H).
dehyde (7). Compound (196 mg, 0.532 mmol) and (3-Bromo-4-methylphenyl)boronic acid (10). To a solution of
(s, 1H), 7.51 (d, J = 8.0 Hz, 1H), 6.96 (d, J = 8.0 Hz, 1H), 2.34
6
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenol (544 mg, 9 (500 mg, 1.65 mmol) in dry THF (3.12 mL) was added drop-
2.47 mmol) were dissolved in THF (7.9 mL) and aqueous 1.3 M wise a solution of isopropylmagnesium chloride (192 mg,
Na2CO3 (2.2 mL). The mixture was degassed by freeze pump 1.87 mmol) in THF (0.925 mL) at −78 °C. After stirring at
thaw cycles. Tetrakis(triphenylphosphine)palladium(0) (33 mg, −78 °C for 30 minutes, the mixture was warmed slowly to
28.3 µmol) was added to the solution and the solution was −20 °C and stirred for 1 h. After cooling again to −78 °C, tri-
stirred for 22 h at 85 °C. The reaction mixture was extracted methylborate (350 mg, 3.37 mmol) was added dropwise. After
with AcOEt. The organic layer was washed with water and warming to 0 °C, the mixture was stirred for 2 h, treated with
brine, and passed through a phase separator paper. The 1N aqueous HCl (1.46 mL), and extracted with ethyl acetate.
solvent was removed by evaporation, and the product was puri- The organic layer was washed with water and brine, passed
fied by silica gel column chromatography (hexane : CH2Cl2
=
through a phase separator paper, and the solvent was evapor-
1
1 : 1) to give 7 as a yellow solid (186 mg, yield: 87%). H NMR ated under vacuum. The residual solid was suspended in
(400 MHz, DMSO-d6): δ 9.98 (s, 2H), 9.82 (s, 2H), 8.18 (s, 2H), hexane, filtered, and washed with hexane to provide 11 as a
8.13 (d, J = 8.0 Hz, 2H) 7.65 (d, J = 8.0 Hz, 2H), 7.33 (d, J = white solid (273 mg, yield: 75%). 1H NMR (400 MHz, DMSO-
8.4 Hz, 4H), 6.93 (d, J = 8.8 Hz, 4H). d6): δ 8.17 (s, 2H), 7.95 (s, 1H), 7.66 (d, J = 7.6 Hz, 1H), 7.32 (d,
2′,3″-Bis(4,5-diphenyl-1H-imidazol-2-yl)-[1,1′:4′,1″:4″,1′′′-quater- J = 7.6 Hz, 1H), 2.34 (s, 3H).
phenyl]-4,4′′′-diol (8). Compound 7 (100 mg, 0.436 mmol),
3,3′-Dibromo-4,4′-dimethyl-1,1′-biphenyl (11). Compound 10
benzil (172 mg, 0.819 mmol) and ammonium acetate (590 mg, (2.50 g, 11.6 mmol) was dissolved in methanol (30 mL). After
7.65 mmol) in acetic acid (1.7 mL) were stirred for 2 d at the addition of copper(I) chloride (0.058 g, 0.058 mmol), the
110 °C. The reaction mixture was allowed to cool to room mixture was stirred for 4 h at room temperature under air. The
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Scheme 4 Synthesis of 1.
reaction mixture was filtered and washed with ethyl acetate. Na2CO3 (1.5 mL). The solution was degassed by freeze pump
The solvent was evaporated under reduced pressure and the thaw cycles. Tetrakis(triphenylphosphine)palladium(0) (24 mg,
crude product was purified by silica gel column chromato- 21 µmol) was added and the mixture was stirred at 75 °C for
graphy (hexane) to afford 12 as a white solid (1.04 g, yield: 17.5 h. The resultant mixture was extracted with AcOEt. The
52%). 1H NMR (400 MHz, CDCl3): δ 7.72 (s, 2H), 7.38 (d, J = organic layer was washed with water and brine, passed
7.8 Hz, 2H), 7.28 (d, J = 8.0 Hz, 2H), 2.43 (s, 6H).
3,3′-Dibromo-4,4′-bis(dibromomethyl)-1,1′-biphenyl
through a phase separator paper, and the solvent was removed
(12). under reduced pressure. The product was purified by silica gel
N-Bromosuccinimide (2.26 mg, 12.7 mmol) was added to 11 column chromatography (hexane : CH2Cl2 = 1 : 1) to give 14 as
(900 mg, 2.65 mmol) in CCl4 (18 mL). Azobis(isobutyronitrile) a pale yellow solid (141 mg, yield: 98%). 1H NMR (400 MHz,
(150 mg, 0.913 mmol) was added to the solution and the DMSO-d6): δ 9.93 (s, 2H), 9.78 (s, 2H), 8.00–7.94 (m, 4H), 7.89
mixture was stirred for 22 h at 75 °C. After cooling to room (s, 2H), 7.38 (d, J = 8.4 Hz, 4H), 6.92 (d, J = 8.8 Hz, 4H).
temperature, the reaction mixture was diluted with CH2Cl2 and
4″,6′-Bis(4,5-diphenyl-1H-imidazol-2-yl)-[1,1′:3′,1″:3″,1′′′-
filtered to remove succinimide. The organic layer was washed quaterphenyl]-4,4′′′-diol (15). Compound 14 (100 mg,
with aqueous Na2S2O3, water and brine, and passed through a 0.253 mmol), benzil (161 mg, 0.766 mmol) and ammonium
phase separator paper. The solvent was evaporated under acetate (599 mg, 7.77 mmol) in acetic acid (1.7 mL) were
reduced pressure. The product was purified with silica gel stirred for 23 h at 110 °C. After cooling to room temperature,
column chromatography (hexane) to give 12 as a pale yellow the reaction mixture was neutralized by aqueous ammonia and
1
solid (933 mg, yield: 53%). H NMR (400 MHz, CDCl3): δ 8.10 extracted with CH2Cl2. The organic layer was washed by water
(d, J = 8.4 Hz, 2H), 7.69 (s, 2H), 7.60 (d, J = 8.4 Hz, 2H), 7.10 (s, and brine, and the solvent was evaporated under reduced
2H).
3,3′-Dibromo-[1,1′-biphenyl]-4,4′-dicarbaldehyde
pressure. The crude product was filtered and washed with
(13). CH2Cl2 to give 15 as a pale yellow solid (136 mg, yield: 69%).
NaHCO3 (840 mg, 10.0 mmol) was added to 12 (800 mg, 1H NMR (400 MHz, DMSO-d6): δ 11.85 (s, 2H), 9.46 (s, 2H),
1.22 mmol) in DMSO (9.7 mL). The mixture was stirred for 7.86–7.79 (m, 6H), 7.48 (d, J = 7.2 Hz, 4H), 7.39–7.18 (m, 10H),
16 h at 100 °C. After cooling to room temperature, water was 6.77 (d, J = 8.4 Hz, 4H); HRMS (ESI-TOF) calcd for C54H38N4O2
added to the reaction mixture. The organic layer was extracted [M + Na]+, 797.2887; found, 797.2909.
with CH2Cl2, washed with water and brine, passed through a
Compound 2. A solution of potassium ferricyanide (2.02 g,
phase separator paper, and evaporated, to give 13 as a milky 53 mmol) and 1.2 M aqueous KOH (24 mL) was added to a
white solid (1.09 g, yield: 89%). 1H NMR (400 MHz, CDCl3): solution of 15 (50 mg, 500 µmol) in benzene (22 mL) and
δ 10.41 (s, 2H), 8.03 (d, J = 8.0 Hz, 2H), 7.89 (s, 2H), 7.67 (d, J = EtOH (6 mL). After stirring for 2 h at room temperature, the
8.0 Hz, 2H).
resultant mixture was extracted with CH2Cl2, washed with
4,4′′′-Dihydroxy-[1,1′:3′,1″:3″,1′′′-quaterphenyl]-4″,6′-dicarbal- water and brine, and the solvent was evaporated under
dehyde (14). Compound 13 (150 mg, 0.362 mmol) and vacuum. The residual solid was suspended in hexane, filtered,
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenol (243 mg, and washed with hexane as a yellow solid (46 mg, yield: 71%).
1.10 mmol) were dissolved in THF (5.3 mL) and 1.2 M aqueous 1H NMR (400 MHz, DMSO-d6): δ 7.98–7.93 (m, 4H), 7.60 (s,
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2H), 7.47 (d, J = 7.2 Hz, 4H), 7.43–7.17 (m, 18H), 6.95 (d, J = 75 °C for 15 h and cooled to room temperature. The reaction
9.6 Hz, 4H), 6.23 (d, J = 9.6 Hz, 4H). HRMS (ESI-TOF) calcd for mixture was then poured into ice-cold 1 M aqueous HCl
C54H34N4O2 [M + H]+, 771.2755; found, 771.2768 (Scheme 5).
(15 mL) and extracted with CH2Cl2. The organic layer was
(2-Bromo-3-methylphenyl)methanol (16). 2-Bromo-3-methyl- washed with brine, passed through a phase separator paper
benzoic acid (3.00 g, 13.9 mmol) was dissolved in dry THF and concentrated under reduced pressure. The residue was
(18 mL). BH3-THF (30 mL, 1.0 M in THF) was added dropwise purified by recrystallization from MeOH to yield 18 (0.543 g,
to the reaction mixture and stirred at room temperature for yield: 59%) as a white solid. 1H NMR (400 MHz, CDCl3): δ 7.55
14 h. The reaction mixture was cooled to 0 °C and the reaction (d, J = 6.8 Hz, 2H), 7.43 (s, 2H), 7.25–7.17 (m, 4H), 2.45 (s, 6H).
was slowly quenched with MeOH. The solution was poured
1,8-Dibromo-2,7-dimethylphenanthrene (19). Compound 18
into 45 mL of saturated aqueous NaHCO3 and extracted with (0.604 g, 1.65 mmol) and iodine (0.471 g, 1.87 mmol) were
AcOEt. The organic layer was washed with water and brine, added to toluene (402 mL). The solution was irradiated with a
and passed through a phase separator paper. The material was 400 W high pressure Hg-lamp for 9 h. The reaction mixture
concentrated under reduced pressure to give 16 as a white was diluted with CH2Cl2 and washed with sodium dithionite
solid (2.19 g, yield: 75%). 1H NMR (400 MHz, CDCl3): δ 7.36 (d, in water. The organic layer was washed with water and concen-
J = 7.2 Hz, 1H), 7.29 (t, J = 7.7 Hz, 1H), 7.24 (d, J = 7.2 Hz, 1H), trated under reduced pressure. The crude product was purified
5.41 (t, J = 5.6 Hz 1H), 4.51 (d, J = 5.6 Hz, 2H), 2.36 (s, 3H).
2-Bromo-3-methylbenzaldehyde (17). PCC (1.54
by recrystallization from toluene to give 19 as a yellow crystal
g, (0.404 g, yield: 36%). H NMR (400 MHz, CDCl3): δ 8.52 (d, J =
1
7.16 mmol) and a molecular sieve (MS-4A, 5 g) were added to a 8.8 Hz, 2H), 8.37 (s, 2H), 7.54 (d, J = 8.8 Hz, 2H), 2.67 (s, 6H).
solution of 16 (1.09 g, 5.21 mmol) in dry CH2Cl2 (16 mL). The 3,3′-Dibromo-[1,1′-biphenyl]-4,4′-dicarbaldehyde (20). The
mixture was stirred at room temperature for 2.5 h. Then, reaction mixture of 20 (0.308 g, 0.845 mmol), NBS (0.308 g,
MS-4A was filtered off and the crude mixture was purified by 1.73 mmol) and CCl4 (2.67 mL) was refluxed at 80 °C for 1.5 h
silica gel column chromatography with CH2Cl2 as an eluent, to under irradiation with a 100 W high pressure Hg-lamp before
give 17 as a white solid (0.846 g, yield: 82%). 1H NMR adding NBS (0.321 g, 1.80 mmol) and CCl4 (2 mL). The result-
(400 MHz, CDCl3): δ 10.46 (s, 1H), 7.75 (d, J = 7.6 Hz, 1H), 7.48 ing solution was refluxed at 80 °C for 1.5 h under irradiation
(d, J = 7.6 Hz,1H), 7.32 (t, J = 7.6 Hz, 1H), 2.49 (s, 3H).
1,2-Bis(2-bromo-3-methylphenyl)ethane (18).
with the high pressure mercury lamp and NBS (0.303 g,
TiCl4 1.79 mmol) was added to the solution. The reaction mixture
(0.615 mL) was slowly added dropwise to a suspension of zinc was refluxed for 17.5 h under irradiation, and cooled to room
powder (0.630 g, 9.63 mmol) and dry THF (21 mL) at room temperature. The product was filtered and washed with water
temperature. The resulting mixture was refluxed at 70 °C for and CHCl3. The organic phase was washed with saturated
1 h and cooled to room temperature. The THF solution aqueous NaHSO3, passed through a phase separator paper and
(2.9 mL) of 17 (1.002 g, 5.03 mmol) was slowly added dropwise concentrated under reduced pressure. The crude product was
to the resulting mixture at 0 °C. The mixture was refluxed at washed with hexane to give 20 as a yellow solid (0.264 g, yield:
Scheme 5 Synthesis of 2.
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46%). 1H NMR (400 MHz, CDCl3): δ 8.75 (d, J = 8.8 Hz, 2H),
8.44 (s, 2H), 8.33 (d, J = 8.8 Hz, 2H), 7.47(s, 2H).
4,4′-(2,7-Bis(4,5-diphenyl-1H-imidazol-2-yl)phenanthrene-
1,8-diyl)diphenol (23). Compound 22 (23.7 mg, 56.6 µmol),
benzil (60.5 mg, 288 µmol) and ammonium acetate (156 mg,
1,8-Dibromophenanthrene-2,7-dicarbaldehyde (21). To
a
solution of 20 (50 mg, 72 µmol) in EtOH (3.5 mL) was added a 2.02 mmol) in acetic acid (0.7 mL) were stirred at 110 °C for
solution of AgNO3 (68 mg, 400 mmol) in water (0.7 mL), and 21 h. The reaction mixture was allowed to cool to room temp-
the mixture was refluxed under a N2 atmosphere for 1 h at erature, neutralized by aqueous ammonia and filtered. The
110 °C. The reaction mixture was allowed to cool to room mixture was washed with water and CH2Cl2 to give 23 as a pale
1
temperature, filtered to remove AgBr and washed with hot yellow solid (28.0 mg, yield: 62%). H NMR (400 MHz, DMSO-
CH2Cl2. The combined filtrate was washed with water and d6): δ 11.27 (s, 2H), 9.59 (s, 2H), 9.10 (d, J = 9.2 Hz, 2H), 8.22
passed through a phase separator paper. The solvent was (d, J = 8.4 Hz, 2H), 7.52–7.16 (m, 26H), 6.89 (d, J = 8.4 Hz, 4H);
removed under reduced pressure to give 21 as a yellow solid HRMS (ESI-TOF) calcd for C56H38N4O2 [M + H]+, 799.3068;
(14 mg, yield: 47%). 1H NMR (400 MHz, CDCl3): δ 10.72 (s, found, 799.3092.
2H), 8.79 (d, J = 8.8 Hz, 2H), 8.64 (s, 2H), 8.21 (d, J = 8.8 Hz,
Compound 3. A solution of potassium ferricyanide (404 mg,
2H).
1.23 mmol) and 1.6 M aqueous KOH (4.8 mL) was added to a
1,8-Bis(4-hydroxyphenyl)phenanthrene-2,7-dicarbaldehyde solution of 23 (10.6 mg, 13.3 µmol) in benzene (5.4 mL). After
(22). Compound 21 (8.9 mg, 23 µmol) and 4-(4,4,5,5-tetra- stirring for 2 h at room temperature, the resultant mixture was
methyl-1,3,2-dioxaborolan-2-yl)phenol (17 mg, 74 µmol) were extracted with CH2Cl2, and the organic layer was washed with
dissolved in THF (3 mL). 0.12 M aqueous Na2CO3 (2 mL) was water and brine. The solvent was removed under reduced
added to the solution and the mixture was degassed by freeze pressure and the residual solid was suspended in cold CH2Cl2,
pump thaw cycles. Tetrakis(triphenylphosphine)palladium(0) filtered, and washed with cold CH2Cl2 to give compound 3 as a
(3.7 mg, 3.2 µmol) was added to the solution and the solution yellow solid (62 mg, yield: 63%). 1H NMR (400 MHz, DMSO-
was refluxed at 75 °C for 12 h. The mixture was filtered d6): δ 9.28 (d, J = 8.8 Hz, 2H), 8.30 (d, J = 8.8 Hz, 2H), 7.57–7.15
through a pad of Celite and extracted with AcOEt. The organic (m, 11H), 7.00 (d, J = 9.6 Hz, 4H), 6.32 (d, J = 10.0 Hz, 4H);
layer was washed with water and brine, passed through a HRMS (ESI-TOF) calcd for C56H34N4O2 [M + H]+, 795.2754;
phase separator paper, and the solvent was evaporated under found, 795.2793 (Scheme 6).
reduced pressure. The product was purified by silica gel
4,4′-Bis(4,5-diphenyl-1H-imidazol-2-yl)-1,1′-biphenyl
(24).
column chromatography (CH2Cl2 : AcOEt = 5 : 1) to give 22 as a (1,1′-Biphenyl)-4,4′-dicarbaldehyde (200 mg, 0.95 mmol),
1
yellow solid (6.9 mg, yield: 73%). H NMR (400 MHz, DMSO- benzil (400 mg, 1.90 mmol) and ammonium acetate (1.46 g,
d6): δ 9.86–9.85 (m, 4H), 9.16 (d, J = 9.2 Hz, 2H), 8.17 (d, J = 19.0 mmol) in acetic acid (15 mL) were stirred at 110 °C over-
8.4 Hz, 2H), 7.63 (s, 2H), 7.27 (d, J = 8.4 Hz, 4H), 6.97 (d, J = night. After cooling to room temperature, the solution was
8.4 Hz, 4H); HRMS (ESI-TOF) calcd for C28H18O4 [M − H]−, neutralized by aqueous ammonia. The suspension was filtered
417.1121; found, 417.1134.
and the collected solid was washed with ethanol and recrystal-
Scheme 6 Synthesis of 3.
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lized from pyridine to give 24 as a pale yellow solid (229 mg, rated aqueous NaHCO3, water and brine and dried with
yield: 40%). 1H NMR (500 MHz, DMSO-d6): δ 12.78 (s, 2H), Na2SO4. The solvent was evaporated under reduced pressure.
8.20 (d, J = 8.0 Hz, 2H), 7.91 (d, J = 8.0 Hz, 2H), 7.57 (d, J = The product was purified by silica gel column chromatography
8.0 Hz, 2H), 7.53 (d, J = 8.0 Hz, 2H), 7.46 (t, J = 7.6 Hz, 4H), (CH2Cl2 : hexane = 2 : 1) to give 28 as a transparent oil (118 mg,
7.39 (t, J = 7.3 Hz, 2H), 7.36 (s, 2H), 7.32 (t, J = 7.3 Hz, 4H), yield: 66%). 1H NMR (500 MHz, CDCl3): δ 10.01 (s, 2H), 7.99 (s,
7.24 (t, J = 7.3 Hz, 4H). FAB–MS: m/z 603[M + H]+.
2H), 7.94 (d, J = 7.8 Hz, 2H), 7.25 (d, J = 7.8 Hz, 2H), 2.10 (s,
2,2′-Dimethyl-[1,1′-biphenyl]-4,4′-diamine (26). 3-Methyl- 6H).
nitrobenzene (5.0 g, 36.5 mmol) and zinc powder (13.8 mg,
4,4′-Bis(4,5-diphenyl-1H-imidazol-2-yl)-2,2′-dimethyl-1,1′-
21.1 mmol) were added to ethanol under a N2 atmosphere and biphenyl (29). Compound 28 (40 mg, 0.17 mmol), benzil
the mixture was refluxed. 30% aqueous NaOH (25 mL) was (88 mg, 0.42 mmol) and ammonium acetate (259 mg,
added dropwise to the mixture and refluxed for 4 h. After the 3.36 mmol) in acetic acid (4.0 mL) were stirred at 110 °C over-
filtration of the resultant mixture, the filtrate was added to the night. The reaction mixture was allowed to cool to room temp-
mixture of acetic acid (22.5 mL) and 0.5 M aqueous Na2S2O5 erature, neutralized by aqueous ammonia and filtered. The
(50 mL). The precipitate was filtered off and the filtrate was mixture was washed with water and ethanol to give 29 as a
1
extracted with Et2O. The organic phase was washed with satu- white powder (68 mg, yield: 65%). H NMR (500 MHz, DMSO-
rated aqueous NaHCO3 and brine and dried with Na2SO4. After d6): δ 12.69 (s, 2H), 7.97 (d, J = 8.0 Hz, 2H), 7.57 (d, J = 8.0 Hz,
evaporation of solvent, a mixture of 1,2-bis(3-methylphenyl) 2H), 7.52 (d, J = 7.3 Hz, 4H), 7.45 (t, J = 7.3 Hz, 4H), 7.38 (t, J =
hydrazine (25) (72%) and 3-methylnitrobenzene was obtained. 7.3 Hz, 4H), 7.24 (t, J = 7.3 Hz, 4H), 2.13 (s, 6H). FAB–MS: m/z
This mixture was used in the next step without further 618[M + H]+.
purification.
To the degassed aqueous HCl under reflux, the mixture of nitrobenzene (9.98 g, 66.0 mmol) and zinc powder (25.0 g,
1,2-bis(3-methylphenyl)hydrazine (72%) and 3-methyl- 15.3 mmol) were added to ethanol (40 mL) under a N2 atmo-
1,2′-Bis(3,5′-dimethylphenyl)hydrazine (30). 3,5′-Dimethyl-
nitrobenzene (1.0 g) was added and refluxed for 5 h under a N2 sphere and the mixture was refluxed. 30% aqueous NaOH
atmosphere. After filtration of the resultant mixture, the fil- (50 mL) was added dropwise to the mixture and refluxed for
trate was heated with activated charcoal for 30 min. 4 h. After the filtration of the resultant mixture, the filtrate was
After removal of the activated charcoal, 20% aqueous added to the mixture of 30% acetic acid aq. and 0.5 M
NaOH was added to the solution. The mixture was aqueous Na2S2O5 (150 mL). The precipitate was filtered and
extracted with Et2O, and the organic phase was washed with extracted with hot methanol. The extract was added to acetic
water and brine and dried with Na2SO4. The solvent was acid and cooled to 10 °C. The precipitate was filtered and
removed under reduced pressure to give 26 as a brown oil extracted with Et2O. The organic phase was washed with satu-
(691 mg, 31%). This was used in the next step without further rated aqueous NaHCO3 and brine and dried with Na2SO4. The
purification.
organic solvent was evaporated and dried in vacuo to give the
4,4′-Dibromo-2,2′-dimethyl-1,1′-biphenyl (27). 2,2′-Dimethyl- desired product (5.52 g, 35%). 1H NMR (500 MHz, CDCl3): δ
[1,1′-biphenyl]-4,4′-diamine (691 mg, 3.26 mmol) was dissolved 6.64 (s, 6H), 5.42 (s, 2H), 2.20 (s, 12H).
in 10% aqueous H2SO4 (7.0 mL) at 10 °C and NaNO2 (492 mg,
2,2′,6,6′-Tetramethyl-[1,1′-biphenyl]-4,4′-diamine (31). To
7.16 mmol) in H2O (4.0 mL) was added dropwise to the solu- degassed aqueous HCl (260 mL) under reflux, compound 30
tion at 10 °C. After stirring for 30 minutes, CuBr (4.67 g, (5.52 g, 15.06 mmol) was added and refluxed for 5 h under a
32.6 mmol) in 48% aqueous HBr (45.0 mL) was added to the N2 atmosphere. After filtration of the resultant mixture, the fil-
reaction mixture and the mixture was stirred at 50 °C for 3 h. trate was heated with activated charcoal for 30 min. After the
After cooling to room temperature, the solution was extracted removal of the activated charcoal, 20% aqueous NaOH was
with Et2O. The organic phase was washed with 3 M aqueous added to the solution. The aqueous solution of saturated
HCl and water and dried with Na2SO4. The solvent was evapor- aqueous AcONa was added to the solution and the mixture
ated under reduced pressure. The product was purified by was extracted with Et2O. The organic phase was dried with
silica gel column chromatography (hexane) to give 27 as a Na2SO4. The solvent was removed under reduced pressure and
transparent oil (393 mg, yield: 55%). 1H NMR (500 MHz, the residue was washed with hot benzene/hexane to give com-
1
CDCl3): δ 7.40 (s, 2H), 7.32 (d, J = 7.8 Hz, 2H), 6.90 (d, J = pound 31 (3.55 g, 64%). H NMR (500 MHz, CDCl3): δ 6.67 (s,
7.8 Hz, 2H), 1.98 (s, 6H).
6H), 3.58 (s, 4H), 1.79 (s, 12H).
2,2′-Dimethyl-[1,1′-biphenyl]-4,4′-dicarbaldehyde
(28).
4,4′-Dibromo-2,2′,6,6′-tetramethyl-1,1′-biphenyl
(32).
Compound 27 (256 mg, 0.75 mmol) was dissolved in dry THF 2,2′,6,6′-Tetramethyl-[1,1′-biphenyl]-4,4′-diamine
(3.42
g,
(3.0 mL) and cooled to −78 °C. 1.55 M n-BuLi in hexane 14.2 mmol) was dissolved in 10% aqueous H2SO4 (31.4 mL) at
(1.16 mL) was added dropwise and the solution was stirred for 10 °C and NaNO2 (2.14 g, 31.1 mmol) in H2O (20 mL) was
30 min at −78 °C. Dry DMF (0.14 mL) and dry THF (1 mL) added dropwise to the solution at 10 °C. After stirring for
were added dropwise and the reaction mixture was warmed to 30 minutes, CuBr (21.4 g, 149 mmol) in 48% aqueous HBr
room temperature and stirred for 3 h. The reaction was (214 mL) was added to the reaction mixture and the mixture
quenched with 3 M aqueous HCl and the solution was was stirred at 50 °C for 3 h. After cooling to room temperature,
extracted with Et2O. The organic phase was washed with satu- the solution was extracted with Et2O. The organic phase was
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washed with 3 M aqueous HCl and water and dried with
Na2SO4. The solvent was evaporated under reduced pressure.
The product was purified by alumina gel column chromato-
graphy (Et2O) to give 32 (2.23 g, yield: 43%). 1H NMR
(500 MHz, CDCl3): δ 7.28 (s, 4H), 1.82 (s, 12H).
2,2′,6,6′-Tetramethyl-[1,1′-biphenyl]-4,4′-dicarbaldehyde (33).
Compound 32 (2.12 g, 5.98 mmol) was dissolved in dry THF
3.0 mL and cooled to −78 °C. 1.55 M n-BuLi in hexane
(8.17 mL) was added dropwise and the solution was stirred for
30 min at −78 °C. The mixture of dry DMF (1.38 mL) and dry
THF (50 mL) was added dropwise and the reaction mixture
was warmed to room temperature and stirred for 3 h. The reac-
tion was quenched with 3 M aqueous HCl and the solution
was extracted with Et2O. The organic phase was washed with
saturated aqueous NaHCO3, water and brine and dried with
Na2SO4. The solvent was evaporated under reduced pressure.
The product was purified by silica gel column chromatography
(CH2Cl2 : hexane = 2 : 1) to give 33 as a transparent oil (437 mg,
yield: 28%). 1H NMR (500 MHz, CDCl3): δ 9.96 (s, 2H), 7.64
(s, 4H), 1.93 (s, 12H).
Scheme 9 Synthesis of R3.
6.5 Hz, 2H), 7.31 (t, J = 6.5 Hz, 4H), 7.29 (t, J = 6.5 Hz, 2H), 1.96
(s, 12H). FAB–MS: m/z 646[M + H]+.
General procedure for the syntheses of R1, R2 and R3. To a
solution of the precursor lophine derivative (0.04 mmol) in
40 mL degassed benzene was added 15% aqueous KOH
(10 mL).
A solution of potassium ferricyanide (1.0 g,
3.30 mmol) was added to the solution and dried with Na2SO4.
The solvent was removed under reduced pressure to give a
photochromic solid. This was used for absorption spec-
troscopy after UV light irradiation in benzene (Schemes 7–9).
4,4′-Bis(4,5-diphenyl-1H-imidazol-2-yl)-2,2′,6,6′-tetramethyl-
1,1′-biphenyl (34). Compound 33 (40 mg, 0.17 mmol), benzil
(88 mg, 0.42 mmol) and ammonium acetate (259 mg,
3.36 mmol) in acetic acid (4.0 mL) were stirred at 110 °C over-
night. The reaction mixture was allowed to cool to room temp-
erature, neutralized by aqueous ammonia and filtered. The
mixture was washed with water and ethanol to give 34 as a
Conflicts of interest
1
white powder (71 mg, yield: 64%). H NMR (500 MHz, DMSO-
d6): δ 12.65 (s, 2H), 7.92 (d, J = 8.0 Hz, 4H), 7.56 (t, J = 6.7 Hz,
4H), 7.51 (t, J = 6.7 Hz, 4H), 7.45 (t, J = 6.5 Hz, 4H), 7.37 (t, J =
There are no conflicts to declare.
Acknowledgements
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 JSPS
KAKENHI Grant Number JP26107010 in Scientific Research on
Innovative Areas “photosynergetics” and MEXT KAKENHI
Grant Number JP17K14475 for K. M. Financial assistance for
this research was also provided by the MEXT-Supported
Program for the Strategic Research Foundation at Private
Universities, 2013–2017.
Scheme 7 Synthesis of R1.
Notes and references
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on photoirradiation, Nature, 2007, 446, 778–781.
Scheme 8 Synthesis of R2.
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