Multistate photochromism of 1-phenylnaphthalene-bridged imidazole dimer that has three colorless isomers and two colored isomers

Article abstract of DOI:10.1021/jp411190d

A new type of the bridged imidazole dimer with a 1-phenylnaphthalene moiety that bridges two diphenylimidazole units at the 2- and 2′-positions was synthesized and the photochemical and thermochemical properties were investigated. This molecule shows unique multistate photochromism, in which the stable colorless 1,2′-isomers A and B photochemically isomerize to the colorless 2,2′-isomer through the short-lived biradical with a half-life of 180 ns at 25 C. The 2,2′-isomer thermally returns to the 1,2′-isomers A and B through the colored isomer at elevated temperatures. The 1,2′-isomers A and B, the 2,2′-isomer, and the colored isomer were isolated, and their molecular structures were determined by X-ray crystallographic analysis. These isomers are stable at room temperature and can be almost fully converted to the 2,2′-isomer by light irradiation. This study serves the useful strategy for the molecular design of a new type of negative photochromic molecules applicable to switch molecular properties by visible light irradiation.

Full text of DOI:10.1021/jp411190d

Article
pubs.acs.org/JPCA
Multistate Photochromism of 1‑Phenylnaphthalene-Bridged
Imidazole Dimer That Has Three Colorless Isomers and Two Colored
Isomers
Tetsuo Yamaguchi,† Sayaka Hatano,‡ and Jiro Abe*,†,§
†Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara,
Kanagawa 252-5258, Japan
‡Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima
739-8526, Japan
§CREST, Japan Science and Technology Agency, K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
S
* Supporting Information
ABSTRACT: A new type of the bridged imidazole dimer with
a 1-phenylnaphthalene moiety that bridges two diphenylimi-
dazole units at the 2- and 2′-positions was synthesized and the
photochemical and thermochemical properties were inves-
tigated. This molecule shows unique multistate photochrom-
ism, in which the stable colorless 1,2′-isomers A and B
photochemically isomerize to the colorless 2,2′-isomer through
the short-lived biradical with a half-life of 180 ns at 25 °C. The
2,2′-isomer thermally returns to the 1,2′-isomers A and B
through the colored isomer at elevated temperatures. The 1,2′-
isomers A and B, the 2,2′-isomer, and the colored isomer were
isolated, and their molecular structures were determined by X-ray crystallographic analysis. These isomers are stable at room
temperature and can be almost fully converted to the 2,2′-isomer by light irradiation. This study serves the useful strategy for the
molecular design of a new type of negative photochromic molecules applicable to switch molecular properties by visible light
irradiation.
respectively, to accelerate the radical recombination reaction by
suppressing the diffusion of the imidazolyl radicals into the
medium. The colorless [2.2]PC-bridged imidazole dimers show
instantaneous coloration upon exposure to UV light and rapid
fading in the dark. Upon UV light irradiation, the C−N bond
between the two imidazole rings is homolytically cleaved to
give a pair of the imidazolyl radicals, and the solution of the
[2.2]PC-bridged imidazole dimer is colored. In contrast to any
other currently available photochromic molecules, the [2.2]PC-
bridged imidazole dimers have high quantum yields close to
unity for the bond-cleavage reactions, enabling the visible
inspection of the coloration upon UV light irradiation, even
with their fast thermal bleaching rate. The molecular structures
of the colorless form of the [2.2]PC-bridged imidazole dimers
were revealed by X-ray crystallographic analysis to have the C−
N bond between the two imidazole rings to form the 1,2′-
isomer.20,21 The formation of the 1,2′-isomer by the radical
recombination reaction between the imidazolyl radical is also
the case for the NP-bridged imidazole dimers except for the
unique NP-bridged imidazole dimer in which two imidazole
INTRODUCTION
■
Hexaarylbiimidazole (HABI) is a photochromic molecule that
shows photochemical color change from the colorless imidazole
dimer to the colored imidazolyl radical (Scheme 1a).1−19 This
coloration is attributed to the photogeneration of triphenyli-
midazolyl radical (TPIR) by homolytic cleavage of the C−N
bond between the imidazole rings. The decoloration reaction
proceeds only thermally by the radical recombination reaction
of TPIRs to form the imidazole dimer. However, the radical
recombination reaction forms several different kinds of
imidazole dimers with different binding manners between two
imidazole rings depending on temperature (Scheme 1b). The
diversity of the bond formation between the imidazole rings
makes the imidazole dimer attractive to develop a new type of
photoresponsive material. It was reported that the 1,2′-isomer
and the 1,4′-isomer show photochromism, whereas the 2,2′-
isomer and the 2,4′-isomer exhibit piezochromism and
thermochromism, respectively.2−8 The radical recombination
reaction of TPIRs takes several minutes at room temperature
due to the diffusion of TPIRs into the medium.
We have developed two types of the bridged imidazole
dimers with a [2.2]paracyclophane ([2.2]PC) moiety20−32 and
a naphthalene (NP) moiety33−38 that bridge a set of
diphenylimidazole (DPI) and triphenylimidazole (TPI) units,
Received: November 14, 2013
Revised: December 19, 2013
Published: December 20, 2013
© 2013 American Chemical Society
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obtained as the colorless 1,2′-isomers by oxidizing the
corresponding lophine precursor, bisDMDPI-BN is obtained
as the colored isomer that has the C−N bond between the
nitrogen atom of the imidazole ring and the carbon atom at the
1-position of the 1,1′-binaphthyl moiety to have the
diazafulvene substructure. The thermally stable colored isomer
shows decoloration from reddish orange to colorless by visible
light irradiation and the 13C NMR spectral analysis revealed
that the colorless isomer is the 2,2′-isomer. The colorless 2,2′-
isomer in benzene thermally returns to the initial colored
isomer within 20 min in the dark at 25 °C. Both the 2,2′-isomer
and the colored isomer photochemically generate the common
biradical with a half-life of 9.4 μs in benzene at 25 °C. That is,
bisDMDPI-BN has three kinds of isomers, i.e., the 2,2′-isomer,
the colored isomer, and the biradical. On the other hand, the
biphenyl-bridged imidazole dimer (bisDMDPI-BP) is obtained
as the colorless photochromic 2,2′-isomer (Scheme 2b) that
does not show the photochromic color change at room
temperature. Moreover, the formation of the biradical of
bisDMDPI-BP could not be detected by using a nanosecond
leaser flash photolysis setup.67
Scheme 1. Photochromism of (a) HABI, (c) [2.2]PC-
Bridged Imidazole Dimer, and (d) NP-Bridged Imidazole
Dimer and (b) Molecular Structures for Possible HABI
Isomers
Thus we expected that negative photochromism of the biaryl-
bridged imidazole dimers could be controlled by changing the
bridging unit. Herein, we report the unique photochromic and
thermochromic behavior of the 1-phenylnaphthalene-bridged
imidazole dimer (bisDPI-PN) shown in Scheme 2c. We found
that this molecule has three kinds of colorless isomers and one
colored isomer other than the short-lived colored biradical and
successfully isolated four kinds of isomers, i.e., the 1,2′-isomer
A, the 1,2′-isomer B, the 2,2′-isomer, and the colored isomer,
leading to the determination of the molecular structures by X-
ray crystallographic analysis.
EXPERIMENTAL SECTION
■
Synthesis. Commercially available reagents and solvents for
syntheses were of reagent grade and used without further
purification. 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 (spherical, neutral), 40−50 μm, Kanto
1
Chemical Co., Inc.). H and 13C NMR spectra were recorded
rings are constrained to an anticonformation leading to the
formation of the 1,4′-isomer.38
with a NMR spectrometer (Bruker, Avance III 400 NanoBay).
DMSO-d6 (0.03% TMS) and CDCl3 (0.03% TMS) were used
as deuterated solvents. Chemical shifts are reported in parts per
million (ppm) relative to tetramethylsilane, and the coupling
constants (J) are reported in Hertz (Hz). ESI-TOF MS spectra
were recorded on a Bruker Daltonics micrOTOFII-AGA1
instrument using positive-ion mode.
Almost all photochromic molecules including HABI
derivatives change their molecular structures from a thermally
stable colorless isomer to a thermally metastable colored isomer
upon UV light irradiation. The metastable colored isomer can
be reversed to the original colorless isomer either by thermal
means or by subsequent irradiation with a specific light
wavelength. On the other hand, only a few molecules have been
reported to show negative photochromism that is observed
with several photochromic molecules where the absorption
spectrum of the molecule after irradiation is blue-shifted relative
to that before irradiation.39−66 The colored isomer of a negative
photochromic molecule is more stable than the colorless
isomer, and the colored isomer becomes colorless upon
exposure to visible light. By changing the bridging unit with a
1,1′-binaphthyl moiety, we have recently developed the 1,1′-
binaphthyl-bridged imidazole dimer (bisDMDPI-BN) that
shows unusual negative photochromism (Scheme 2a), in
which the stable colored isomer photochemically converts
into the metastable colorless isomer through the short-lived
biradical.67 Although the usual bridged imidazole dimers are
BisDPI-PN was prepared as shown in Scheme 3.
1-(Formylphenyl)-2-naphthaldehyde (1).72 Under a N2
atmosphere, 1-bromo-2-naphthaldehyde (202 mg, 0.86
mmol) and 2-formyl-1-phenylboronic acid (202 mg, 1.35
mmol) were added to degassed DME (3 mL), EtOH (2 mL),
and 2 M Na2CO3 aq (3.8 mL). The reaction mixture was stirred
at 60 °C, and tetrakis(triphenylphosphine)palladium (0) (112
mg, 0.097 mmol) was added. The mixture was stirred at 80 °C
for 28 h. After cooling to room temperature, the resulting
mixture was washed with water and extracted with CH2Cl2. The
crude sample was purified by silica gel column chromatography
using CH2Cl2/hexane = 2/1 as eluent to give a white powder of
1. 1H NMR (400 MHz, CDCl3), δ: 9.84 (s, 1H), 9.57 (s, 1H),
8.18 (d, 1H, J = 7.67 Hz), 8.12 (d, 1H, J = 8.52 Hz), 8.03 (d,
1H, J = 8.94 Hz), 7.97 (d, 1H, J = 8.52 Hz), 7.78 (t, 1H, J =
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Scheme 2. (a) Photochromism of bisDMDPI-BN and Molecular Structures of (b) bisDMDPI-BP and (c) bisDPI-PN
Scheme 3. Synthesis of bisDPI-PN
7.51 Hz), 7.72 (t, 1H, J = 7.51 Hz), 7.65 (t, 1H, J = 7.51 Hz),
7.51−7.42 (m, 2H), 7.38 (d, 1H, J = 8.51 Hz).
with water. The crude mixture was purified by silica gel column
chromatography using AcOEt/hexane = 1/2 as eluent to give
reddish white powder. The slow evaporation of an acetonitrile
solution of bisDPI-PN gave colorless, plate single crystal
suitable for X-ray crystallographic analysis. 1H NMR (400
MHz, CDCl3), δ: 8.79 (d, 1H, J = 8.42 Hz), 8.26 (d, 1H, J =
7.73 Hz), 7.86 (d, 1H, J = 7.73 Hz), 7.69 (d, 1H, J = 8.42 Hz),
7.56−7.38 (m, 14H), 7.38−7.14 (m, 11H), 7.00 (d, 1H, J =
8.42 Hz). HR-ESI-TOF-MS, m/z: calcd, 638.247 (C46H30N4);
found, 661.239 [M+Na]+.
HPLC Analysis. HPLC analysis was carried out on a ODS
reversed phase column (Mightysil RP-18 GP, Kanto Chemical
Co., Inc.). The HPLC system consists of a pump unit (PU-
2080 plus, JASCO), a photodiode array (PDA) detector (MD-
2018 plus, JASCO), and a control unit (LC-NetII/ADC,
JASCO).
bisDPIH-PN (2). Compound 1 (30 mg, 0.115 mmol), benzil
(54 mg, 0.260 mmol), and ammonium acetate (118 mg, 1.34
mmol) were dissolved in CHCl3 (0.6 mL). The mixture was
refluxed at 110 °C for 2 days. After cooling to room
temperature, the resulting mixture was extracted with CH2Cl2
and washed with water. The crude mixture was purified by silica
gel column chromatography using AcOEt/hexane = 1/2 as
eluent to give white powder of 2. 1H NMR (400 MHz, DMSO-
d6), δ: 13.83 (s, 1H), 13.66 (s, 1H), 8.07 (d, 1H, J = 7.99 Hz),
7.99 (d, 1H, J = 8.31 Hz), 7.92 (d, 1H, J = 8.63 Hz), 7.87 (d,
1H, J = 7.35 Hz), 7.61 (t, 1H, J = 7.70 Hz), 7.50 (d, 1H, J =
7.34 Hz), 7.43−7.06 (m, 23H). ESI-TOF-MS, m/z: calcd,
640.263 (C46H32N4); found, 663.3150 [M + Na]+.
bisDPI-PN. All manipulations were carried out with exclusion
of light. Compound 2 (25 mg, 0.039 mmol) was dissolved in
degassed benzene (12 mL) and K3[Fe(CN)6] (627 mg, 2.33
mmol) aqueous solution containing KOH bilayer, and the
solution was stirred for 18 h. After the reaction was completed,
the reaction product was extracted with benzene, and washed
X-ray Crystallographic Analysis. The diffraction data of
the single crystals were collected on the Bruker APEX II CCD
area detector (Mo Ka, λ = 0.710 73 nm). During the data
collection, the lead glass doors of the diffractometer were
covered to exclude the room light. The data refinement was
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carried out by the Bruker APEXII software package with
SHELXT program73,74 All non-hydrogen atoms were aniso-
tropically refined.
thermally returns to the colored isomer within 20 min in
benzene at room temperature. Though the 2,2′-isomers of the
classical HABI derivatives show piezochromism, the 2,2′-isomer
of bisDPI-PN does not show the color change by grinding at
room temperature The solution of the 2,2′-isomer of bisDPI-
PN changes its color from colorless to yellow by UV light
irradiation and returns to the original colorless state by visible
light irradiation. This reversible color change can be repeated
many times by alternating UV and visible light irradiation. This
photoisomerization behavior was followed by HPLC analysis.
As shown in Figure 2, the UV light irradiation to the benzene
Laser Flash Photolysis. The laser flash photolysis
experiments were carried out with a time-resolved spectropho-
tometer (TSP-1000, Unisoku Co., Ltd.). A Continuum Minilite
II Q-switched Nd:YAG laser with the third harmonic at 355 nm
(pulse width, 5 ns; power, 3 mJ/pulse) was employed for the
excitation light. Excitation pulses at 510 and 550 nm (pulse
width, 5 ns; power, 3 mJ/pulse) were provided by a Continuum
Surelite II Q-Switched Nd:YAG coupled to a Continuum
Panther EX OPO. The probe beam from a halogen lamp
(HLX64623, OSRAM GmbH) was guided with an optical fiber
scope to be arranged in an orientation perpendicular to the
exciting laser beam. The probe beam was monitored with a
photomultiplier tube (R2929, Hamamatsu Photonics K.K.)
through a spectrometer (MD2000, Unisoku Co., Ltd.). Sample
solutions were deaerated by argon bubbling prior to the laser
flash photolysis experiments.
DFT Calculations. All DFT calculations were carried out
using the Gaussian 09 program (Revision D.01).75 The
molecular geometries were fully optimized at the
MPW1PW91/6-31G(d) level of the theory, and analytical
second derivatives were computed using vibrational analysis to
confirm each stationary point to be a minimum. Zero-point
energy (ZPE) corrections were adopted to compare the relative
energies for the isomers.
Figure 2. HPLC chromatograms of the benzene solution of the 2,2′-
isomer of bisDPI-PN (a) before and (b) after UV irradiation. The
HPLC analysis was performed using a reverse phase column (eluent,
H2O/acetonitrile = 1/9; flow rate, 1 mL/min; λobs = 344 nm).
solution of the 2,2′-isomer results in the appearance of a small
peak other than the peak of the 2,2′-isomer. Upon visible light
irradiation to the yellow solution, the small peak completely
disappears and the solution color returns to colorless. The
UV−vis absorption spectrum of the fraction of the small peak
has an intense absorption band in visible light region,
suggesting that the fraction of the small peak is the colored
isomer in analogy with bisDMDPI-BN. The colored fraction is
stable at room temperature to allow for recrystallization. The X-
ray crystallographic analysis of the single crystal of the colored
fraction revealed the molecular structure as shown in Figure 1b.
As would be expected, the colored isomer forms the structure
similar to the colored isomer of bisDMDPI-BN. The
photostationary state of the benzene solution was reached
after 1 min of irradiation at 365 nm at room temperature and
the fraction of the colored isomer at the photostationary state is
0.01, indicating low conversion efficiency from the 2,2′-isomer
to the colored isomer upon UV light irradiation.
RESULTS AND DISCUSSION
■
Photochromic and Thermochromic Behavior. Color-
less crystals are formed from the acetonitrile solution of the
reddish white powder of bisDPI-PN. Though the oxidation of
the lophine precursor of bisDMDPI-BN gives only the colored
isomer, the major oxidation product of the lophine precursor of
bisDPI-PN is colorless isomer as well as the oxidation of the
lophine precursor of bisDMDPI-BP. The molecular structure of
the colorless isomer of bisDPI-PN was determined by X-ray
crystallographic analysis as shown in Figure 1a. It is revealed
that the colorless isomer is the 2,2′-isomer with the C−C bond
(1.5775(18) Å) between the imidazole rings as well as
bisDMDPI-BP. The 2,2′-isomer of bisDPI-PN is stable at
room temperature contrary to that of bisDMDPI-BN that
The thermal stability at room temperature of the colored
isomer and the colorless 2,2′-isomer of bisDPI-PN makes it
possible to isolate these isomers, whereas the thermal
coloration of the 2,2′-isomer is observed at elevated temper-
atures. The heated colorless solution of the 2,2′-isomer once
becomes yellow, and the color of the solution subsequently
changes to colorless very slowly with time. These color changes
take longer in benzene as compared with those in cyclohexane
(Figure S7, Supporting Information). Therefore, we carried out
the thermal isomerization experiment for the cyclohexane
solution of the 2,2′-isomer. Figure 3b shows the HPLC
chromatograms for the cyclohexane solution of the 2,2′-isomer
kept at 70 °C for 3 days in the dark. The HPLC peak of the
2,2′-isomer decreases and that of the colored isomer increases
by heating. Thus the 2,2′-isomer is thermally converted to the
colored isomer. The rate of disappearance of the 2,2′-isomer
follows the first-order kinetics, and the rate constant is 2.70 ×
10−5 s−1 at 70 °C in cyclohexane (Figures S10 and S11,
Figure 1. ORTEP representations of the molecular structures of (a)
the 2,2′-isomer, (b) the colored isomer, (c) the 1,2′-isomer A, and (d)
the 1,2′-isomer B of bisDPI-PN with thermal ellipsoid. The hydrogen
atoms and the solvent molecule are omitted. Nitrogen atoms are
highlighted in blue.
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from colorless to yellow, whereas the visible light irradiation
does not change the color of the solution and forms only the
2,2′- isomer, as shown in Figure 3d. As will be described below,
this behavior can be attributable to the photoconversion of the
colored isomer to the 2,2′-isomer through the biradical with the
irradiation of visible light that is not absorbed by the 2,2′-
isomer. As shown in Figure 4, the absorption tails of the 1,2′-
Figure 3. HPLC chromatograms of the cyclohexane solution of
bisDPI-PN: (a) the 2,2′-isomer, (b) the solution after kept 3 days at 70
°C, (c) the solution after kept 18 days at 70 °C, and (d) the solution
after visible light (400−700 nm) irradiation. HPLC analysis was
performed using reversed phase column (eluent, H2O/acetonitrile =
9/1; flow rate, 1 mL/min; λobs = 344 nm).
Figure 4. UV−vis absorption spectra of each isomer of bisDPI-PN in
benzene at 25 °C. The inset is the enlargement of the spectra.
Supporting Information). Other than the peak for the colored
isomer, new peaks (peak A and peak B) are observed with a
retention time of 17−19 min, as shown in Figure 3b. These
fractions are not detected for the light irradiated solution of the
2,2′-isomer or the colored isomer. After another 15 days at 70
°C, the peaks for the 2,2′-isomer and the colored isomer
completely disappeared and the peaks A and B increased
(Figure 3c). Taking into account the thermal conversion from
the colorless 2,2′-isomer to the colored isomer of bisDMDPI-
BN at room temperature, it is plausible to assume that the 2,2′-
isomer of bisDPI-PN is thermally converted to the colored
isomer and the resultant colored isomer also thermally
isomerizes to the two new constituents of peaks A and B.
These two constituents are also stable at room temperature and
can be isolated by preparative HPLC. The single crystals were
grown from the solution of the constituent of peak A in the
CH2Cl2/MeOH mixture and that of peak B in CH3CN. The X-
ray crystallographic analysis revealed that the constituents of
peak A and peak B are the 1,2′-isomer A and the 1,2′-isomer B,
respectively (Figure 1c,d). The 1,2′-isomer has two types of
imidazole rings, Im1 and Im2, as indicated in Figure 1c,d.23,26
Im1 is a resonant planar structure that has a typical bond length
for a 6π-electron system with an electron-donating character-
istic, whereas Im2 has two localized CN double bonds and
one sp3 carbon atom connecting Im1, consistent with a 4π-
electron system with an electron-withdrawing characteristic.
Thus the 1,2′-dimer A and the 1,2′-dimer B can be
distinguished in a bonding manner between the imidazole
ring and the aryl moiety. Im1 of the 1,2′-dimer A is bonded to a
naphthyl ring whereas that of the 1,2′-dimer B is bonded to a
phenyl ring. The bond lengths of the C−N bonds between two
imidazole rings of 1,2′-isomers A and B are 1.478(2) and
1.486(2) Å, respectively. These bond lengths are approximately
equal to that of 1,2′-isomers of the other bridged imidazole
dimers.
isomers A and B extending into the visible light region of the
spectra as well as the other 1,2′-dimers including the classical
HABI derivatives9 make it possible to form the biradical by
visible light irradiation. The mechanism of this unique
photoisomerization will be discussed later.
Laser Flash Photolysis. Because all bridged imidazole
dimers except for bisDMDPI-BP have been reported to
generate biradicals by light irradiation, the four kinds of
structural isomers of bisDPI-PN could be also expected to
generate the biradical by light irradiation. We investigated the
possible presence of the short-lived biradical in the photo-
chemical process of bisDPI-PN by nanosecond laser flash
photolysis. Figure 5a shows the transient vis−NIR absorption
Figure 5. (a) Transient vis−NIR absorption spectra of the 2,2′-isomer
of bisDPI-PN in deaerated benzene (1.1 × 10−4 M) at 25 °C. The
spectrum was recorded at 100 ns after excitation with a nanosecond
laser pulse (excitation wavelength, 355 nm; pulse width, 5 ns; power, 3
mJ/pulse). (b) Decay profile of the biradical monitored at 400 nm in
deaerated benzene (1.1 × 10−4 M) at 25 °C (excitation wavelength,
355 nm; pulse width, 5 ns; power, 3 mJ/pulse).
spectrum of the 2,2′-isomer of bisDPI-PN in benzene at room
temperature, excited with a nanosecond laser pulse at 355 nm.
The identical transient absorption spectra were also obtained
for the other isomers. A sharp absorption band around at 400
nm and a broad absorption band ranging from 500 to 800 nm
can be ascribed to the biradical because its spectrum shape is
similar to those of the colored biradicals observed for the other
bridged imidazole dimers. Figure 5b shows the decay profile of
Moreover, we found that the 1,2′-isomers A and B are
converted to the 2,2′-isomer not only by UV light irradiation
but also by visible light irradiation. The UV light irradiation to
the solution of 1,2′-isomers forms both the 2,2′-isomer and the
colored isomer and causes the color change of the solution
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the transient absorbance at 400 nm of the biradical. The decay
profile of the biradical obeys first-order kinetics with a half-life
of 180 ns at 25 °C, which is the fastest among the reported half-
lives of the imidazolyl radicals of HABI derivatives. We also
confirmed that the shape of the transient absorption spectra
and the decay kinetics are not affected by the presence of
molecular oxygen as shown in Figure S12 (Supporting
Information).
Radical Recombination Reaction of the Biradical. The
significant question to be solved as to the photochemical
isomerizations is to elucidate the products formed by the
radical recombination reaction of the photogenerated biradical.
As already mentioned, the 2,2′-isomer is converted to the
colored isomer by UV light irradiation, and the colored isomer
mainly returns to the 2,2′-isomer by visible light irradiation.
The 1,2′-isomers A and B mainly return to the 2,2′-isomer by
UV or visible light irradiation. These observations can raise the
assumption that the major product by the radical recombina-
tion reaction of the biradical is the 2,2′-isomer. The
photochemical conversion of the colored isomer was
investigated by HPLC analysis for the benzene solution
irradiated with the laser pulses of the wavelength of 550 nm
at 70 °C, at which temperature the experiment of the thermal
conversion of the 2,2′-isomer shown in Figure 3 was conducted.
As shown in Figure 6, the peak intensity of the colored
isomer with a retention time around 9 min decreases with
using 1,2-bis(2-methylbenzo[b]thiophen-3-yl)per-
fluorocyclopentene (DAE) as a standard.67,68 The colorless
open-ring isomer of DAE can undergo cyclization in a
conrotatory fashion to the colored closed-ring isomer upon
UV light irradiation. The thermal back-reaction is symmetry-
forbidden, giving excellent thermal stability to the closed-ring
isomer. The cycloreversion reaction from the closed-ring
isomer to the open-ring isomer occurs only by irradiating
with a visible light. The solutions of the colored isomer of
bisDPI-PN in benzene and the closed-ring isomer of DAE in
hexane with matched absorbances at the excitation wavelength
of 517 nm were irradiated at various laser energies (Figure S13,
Supporting Information). The absorbance changes, ΔOD-
(DAE), before and after a laser pulse irradiation at 517 nm for
the closed-ring isomer of DAE were plotted against those,
ΔOD(S), for the colored isomer of bisDPI-PN. The conversion
efficiency was estimated from the slope of the fit of the data to
the following equation,69,70
φDAE
ε
ΔOD(DAE) =
DAE ΔOD(S)
φεs
(1)
s
where φDAE (0.35) is the quantum yield of cycloreversion
reaction of DAE excited at 517 nm,71 φs is the conversion
efficiency from the colored isomer to the 2,2′-isomer of bisDPI-
PN, and εDAE (0.91 × 104 M−1 cm−1) and εs (0.98 × 104 M−1
cm−1) are the absorptivity coefficients of DAE71 and the
colored isomer of bisDPI-PN at 517 nm, respectively. As shown
in Figure S15 (Supporting Information), φs was obtained from
the slope of the fit of the data to eq 1 and was determined to be
0.14, which is about 5 times larger than that of bisDMDPI-BN
reported in our previous paper.67
DFT Calculations for the Isomers. By comparing the
photochemical and thermochemical properties of bisDMDPI-
BP, bisDMDPI-BN, and bisDPI-PN, we found that the biaryl
moiety that bridges a set of DPIs would have a significant effect
on the stability of the isomers. BisDPI- PN has four kinds of
stable isomers, i.e., the 2,2′-isomer, the colored isomer, and the
1,2′-isomers A and B, and a short-lived biradical, whereas
bisDMDPI-BN has two kinds of stable isomers, i.e., the 2,2′-
isomer and the colored isomer, and a short-lived biradical. On
the other hand, only the 2,2′-isomer is the stable form of
bisDMDPI-BP. We carried out the DFT calculations to
investigate the relative energies for the isomers of bisDMDPI-
BP, bisDMDPI-BN, and bisDPI-PN. As shown in Figure 7, the
most stable isomer for these biaryl-bridged imidazole dimers is
the 1,2′-isomer. However, the 1,2′-isomers are not formed from
the biradicals that is generated by the oxidation of the lophine
precursors or by the photochemical reaction in the case of
bisDMDPI-BN and bisDPI-PN. This incomprehensible behav-
ior leads to the idea that the activation energy barriers toward
the formation of the 1,2′-isomers from the biradicals are
significantly higher than those toward the formation of the 2,2′-
isomers and the colored isomers. We considered that the major
products of the radical recombination reaction of the biradicals
are not the colored isomers but the 2,2′-isomers. This idea can
be supported by the two experimental results. The first basis is
that the 1,2′-isomers A and B of bisDPI-PN are converted to
the 2,2′-isomer by UV light irradiation, and the second basis is
that the oxidation of the lophine precursor of bisDMDPI-BN at
room temperature initially forms the colorless solution that
gradually changes to the red solution by forming the colored
isomer.
Figure 6. HPLC chromatograms of the benzene solutions of the
colored isomer of bisDPI-PN. The solution was irradiated with the
laser pulses (3 mJ/pulse) of the wavelength of 550 nm at 70 °C.
HPLC analysis was performed using reverse phase column (eluent,
H2O/acetonitrile = 9/1; flow rate, 1 mL/min; λobs = 344 nm).
increasing the irradiation times of the visible laser pulses,
whereas that of the 2,2′-isomer with a retention time around 8
min increases. It is worth noting that the HPLC peaks
attributable to the 1,2′-isomers A and B do not appear after the
light irradiation. Thus, it can be concluded that the radical
recombination reaction of the biradical affords the 2,2′-isomer
as a major product and the colored isomer as a minor product.
The observation of the photochemical conversion from the
2,2′-isomer to the colored isomer by UV light irradiation as
shown in Figure 2 would be the direct evidence for the
formation of the colored isomer by the radical recombination
reaction of the biradical. It is considered that the activation
energy barriers toward the formation of the 2,2′-isomer is
smaller than those of the colored isomer and the 1,2′-isomers A
and B.
Photochemical Conversion Efficiency. The photochem-
ical conversion efficiency from the colored isomer to the 2,2′-
isomer was investigated with the intention to evaluate the
negative photochromic property of bisDPI-PN. The conversion
efficiency was estimated by nanosecond laser flash photolysis by
139
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The Journal of Physical Chemistry A
Article
Figure 7. Schematic drawing of the energy level diagrams of the isomers of bisDMDPI-BP, bisDMDPI-BN, and bisDPI-PN calculated by the DFT
MPW1PW91/6-31G(d) level of the theory. The energy levels of the 1,2′-isomers of all the molecules are set to zero for comparing the energy
differences for each molecule.
The stability of the colored isomers can be accounted for by
the resonance energy of the phenyl ring in the biaryl-bridging
moiety. Both the colored isomer of bisDMDPI-BP and the
colored isomer B of bisDPI-PN described in Figure 7 are
unstable due to the loss of aromaticity of the phenyl ring in the
biaryl-bridging moiety. On the other hand, the colored isomer
of bisDMDPI-BN and the colored isomer A of bisDPI-PN,
which is identical to the colored isomer isolated in this work,
have the same diazafulvene substructure connected to the
naphthyl moiety. Thus it is considered that these colored
isomers are relatively stable compared with the colored isomers
that lose the aromaticity of the phenyl rings.
By combining the above-described considerations, we assume
the thermal conversion from the 2,2′-isomers to the colored
proceeds by the thermally generated biradical. The 2,2′-isomer
of classical HABI is known to show thermochromism that gives
triphenylimidazolyl radicals by homolytic cleavage of the C−C
bond between the imidazole rings.3,4,7,12 The oxidation of
triphenylimidazole at lower than 5 °C gives the 2,2′-isomer,
whereas the 1,2′-isomer is obtained at room temperature. This
behavior can be accounted for by the difference in the
activation energy barriers toward the formation of the imidazole
dimers. That is, the activation energy barrier for the
dimerization path to form the 2,2′-isomer is lower than that
for the 1,2′-isomer. However, the 2,2′-isomer consisting of a
couple of 4π-electron imidazole rings is less stable than the 1,2′-
isomer consisted of a 6π-electron imidazole ring and a 4π-
electron imidazole ring because of the lack of resonance effect
in imidazole ring for the 2,2′-isomer. Thus the thermochemical
instability of the 2,2′-isomer causes the thermochromism by
forming the thermally stable 1,2′-isomer through the biradical
by increasing the temperature. We already reported the in situ
direct observation of the thermal isomerization from the 2,2′-
isomer to the 1,2′-isomer of HABI in crystal by X-ray
crystallographic analysis.73,74 Therefore, it seems reasonable
to assume that the 2,2′-isomer of bisDPI-PN also shows
thermochromism at elevated temperature to form the biradical
that would dimerize to form the more stable colored isomer A
140
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The Journal of Physical Chemistry A
Article
than the 2,2′-isomer by overcoming the activation energy
barrier toward the formation of the colored isomer A. However,
an attempt to detect the biradical of bisDPI-PN at elevated
temperature by EPR measurement was unsuccessful due to the
low concentration of the biradical with extremely short half-life
(180 ns at 25 °C) as described above.
On the other hand, it can be considered that the colored
isomer of bisDPI-PN isomerizes to the 1,2-isomers A and B by
a rearrangement reaction, whereas the large activation energy
barrier for the rearrangement reaction of bisDMDPI-BN is
presumed to prevent the thermal isomerization to form the
1,2′-isomer. If the biradical is formed in the thermal
isomerization process, the radical recombination reaction of
the biradical should give the 2,2′-isomer besides a small amount
of the colored isomer, and the 1,2′-isomers will not be formed.
As a whole, the photochemical and thermochemical trans-
formations between the isomers of bisDPI-PN can be
summarized as shown in Figure 8.
2′-position and the carbon atom at the 2-position of the 1-
phenylnaphthalene moiety. The molecular structures of these
four kinds of structural isomers were determined by X-ray
crystallographic analysis. It should be emphasized that these
isomers are stable at room temperature and can be almost fully
converted to the 2,2′-isomer by light irradiation. The
photochemical conversion can be accounted for by the radical
recombination reaction of the short-lived biradical with a half-
life of 180 ns at 25 °C that is generated from all isomers by light
irradiation. The reason the 2,2′-isomer is mainly formed from
the biradical would be attributable to the low activation energy
barrier toward the formation of the 2,2′-isomer by comparing
the activation energy barriers toward the formation of the other
isomers. The theoretical considerations of the mechanisms for
the thermal isomerization and the radical recombination
reaction are important problems left for future study.
Thus the aryl-bridged imidazole dimers are attractive
multistate photochromic molecules with negative and positive
photochromic properties. As described at the beginning, only a
few molecules have been reported to show negative photo-
chromism.39−66 The development of unique photochromic
molecules with negative photochromism would open a new
field in the photoresponsive materials. This study serves the
useful strategy for the molecular design of a new type of
negative photochromic molecules applicable to switch molec-
ular properties by visible light irradiation. The rational
molecular design to prevent the formation of the colorless
1,2′-dimer from the colored isomer and to destabilize the
colorless 2,2′-isomer will lead to the development of an
unknown fast negative photochromic molecule that enables us
to change and reset the molecular properties by simply turning
a visible light source on and off.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra, MS spectra, X-ray crystallographic
analysis data, HPLC chromatograms, decay profiles, transient
vis−NIR absorption spectra, conversion efficiency measure-
ment (including laser intensity dependence of the time profiles
and laser power dependence of the absorbance change), and
DFT calculations (atomic coordinates and optimized struc-
tures). Complete refs 17 and 75. CIF files. This information is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*J. Abe: e-mail, jiro_abe@chem.aoyama.ac.jp.
Notes
■
Figure 8. Schematic drawing of the potential energy surfaces for the
excited state (upper) and the ground state (lower) of bisDPI-PN
accounting for the photochemical and thermochemical transformation
between the isomers.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CONCLUSION
■
■
This work was partially supported by the Core Research for
Evolutional Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST), a Grant-in-Aid
for Scientific Research (A) (22245025) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan. T. Yamaguchi appreciates the Research Fellowships of
JSPS for Young Scientists.
We investigated the photochemical and the thermochemical
behavior of bisDPI-PN with a 1-phenylnaphthalene moiety that
bridges two diphenylimidazole units at the 2- and 2′-positions.
This molecule shows unique multistate photochromism, in
which the stable colorless 1,2′-isomers A and B photochemi-
cally isomerize to the colorless 2,2′-isomer. Although the 2,2′-
isomer is stable at room temperature, it thermally returns to the
initial 1,2′-isomers A and B through the biradical and the
colored isomer at elevated temperatures. The colored isomer
consists of the diazafulvene structure formed by the C−N bond
between the nitrogen atom of the imidazole ring bonded at the
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