Rational design of a new class of diffusion-inhibited HABI with fast back-reaction

Article abstract of DOI:10.1002/poc.1183

The fast reversible photochromic molecule was synthesized with the aid of triphenylimidazolyl radicals (TPI) and naphthalene linker. The crystal structure, photochemical properties, and kinetics of the target compound (1,8-TPID-naphthalene) were investiga

Full text of DOI:10.1002/poc.1183

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 857–863
Published online 29 April 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1183
Rational design of a new class of diffusion-inhibited
HABI with fast back-reaction
*
Fumiyasu Iwahori, Sayaka Hatano and Jiro Abe
Department of Chemistry, and The 21st Century COE Program, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa
229-8558, Japan
Received 26 December 2006; revised 5 March 2007; accepted 12 March 2007
ABSTRACT: The fast reversible photochromic molecule was synthesized with the aid of triphenylimidazolyl radicals
.
(TPI ) and naphthalene linker. The crystal structure, photochemical properties, and kinetics of the target compound
.
(1,8-TPID-naphthalene) were investigated. 1,8-TPID-naphthalene photochemically cleaved into 1,8-bisTPI -
naphthalene and the color of the solution changed from colorless to green. ESR spectroscopy detected the light-
induced triplet radical pair in frozen matrix. After the UV irradiation is ceased, the back-reaction of
.
1,8-bisTPI -naphthalene occurred by thermal conversion in the dark. The kinetic study on the back-reaction revealed
that the reaction obeys the first-order kinetics with 2.04 s of half-life time at 295 K. The activation energy and
frequency factor of the back-reaction were determined as 42.0 kJ/mol and 9.25 ꢀ 106 sꢁ1, respectively. Copyright #
2007 John Wiley & Sons, Ltd.
KEYWORDS: photochromism; Hexaarylbiimidazole (HABI); radical cleavage; first-order kinetics; fast back-reaction;
diffusion-inhibited; photoresponsive material; triplet radical pair
INTRODUCTION
co-workers postulated a complex kinetic behavior
involving ionic and radical intermediates,3 Hayashi and
.
The development of a fast reversible photochromic
material is a major challenge for the photochemistry and
material science because of their potential applications
including smart windows, solar protection lenses and
decorative objects. The photochromism of hexaarylbii-
midazoles (HABIs) was discovered by Hayashi and
Maeda in 1960s.1 HABI derivatives can be prepared by
oxidation of the corresponding triphenylimidazole
(lophine) derivatives. HABIs readily cleave into a pair
Maeda1h demonstrated that the kinetics of TPI clearly
obeys the second-order rate law. Recently, we have
reported the acceleration of the back-reaction of
bistpy-HABI (bis(terpyridyl)hexaarylbiimidazole).4 The
formation of iron(II) complex of bistpy-HABI resulted in
the acceleration of the back-reaction even though the
detailed mechanism is still obscure. This back-reaction
also obeyed the second-order kinetics, and the recombi-
nation rate constant drastically increased from 48.4 L
molꢁ1 sꢁ1 to 504.6 L molꢁ1 sꢁ1 (at 302 K) on the
coordination of iron(II). In this case, the lowering of the
activation energy barrier from 48.6 kJ/mol (free bistpy-
HABI) to 24.3 kJ/mol (metal-coordinated bistpy-HABI)
predominantly achieved the acceleration of the recombi-
nation rate.
.
of triphenylimidazolyl radicals (TPI ) both thermally and
.
photochemically, and TPI will thermally recombine to
.
reproduce the dimer of TPI s, which is abbreviated as
TPID in this manuscript. With respect to the molecular
structure, TPID is identical with HABI. There are a
number of spectroscopic studies on the photochemical
reaction of HABI and its derivatives.2 The typical
mechanism for the photochromism of HABIs consists
of two reactions as shown in Chart 1, namely light-
induced radical cleavage and thermal back-reactions.
At the earlier stage of the chemistry of HABIs, several
kinetic studies on the thermal back-reactions of
.
In our previous reports, thermal behavior of the TPI
derivative, tF-BDPI-2Y (1,4-bis-(4,5-diphenylimidazol-2-
ylidene)-2,3,5,6-tetrafluorocyclohexa-2,5-diene) have been
discussed (Chart 2).5 The dimerization reaction of tF-BDPI-
2Y was found in solution at 293 K, while tF- BDPI-2Y was
crystallized at 200 K because of the existence of the
activation energy barrier toward the recombination reac-
tion. On irradiation with UV light to the colorless
benzene solution of tF-BDPI-2YD at room temperature,
the dimer immediately cleaved into tF-BDPI-2Y and the
solution intensively colored. Here, an instantly reversible
.
TPI s have been reported. Although Willis and
*Correspondence to: J. Abe, Department of Chemistry, Aoyama
Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa
229-8558, Japan.
E-mail: jiro_abe@chem.aoyama.ac.jp
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 857–863

858
F. IWAHORI, S. HATANO AND J. ABE
introduce the strategy with the aim of accelerating the
back-reaction rate of HABIs based on the rational
molecular designing. The synthesis, crystal structure, and
photochemical properties will be described.
EXPERIMENTS
Synthesis
Chart 1. Photochromism of conventional HABI
photochromic molecule can be designed if one imagines the
one-photon reaction of tF-BDPI-2YD. The strategy for the
molecular design begins with the virtual one-photon
reaction depicted in Chart 2. The light-induced fluorinated
1,8-TPID-naphthalene was synthesized as shown in
Scheme 1. All reagents except benzyl (recrystallized from
ethanol) were used as commercially supplied. Although it is
commercially available, p-formylphenylboronic acid can be
synthesized from p-bromobenzaldehyde. Benzene, acetic
acid, and toluene were distilled prior to use.
.
TPI pair will immediately recombine because the radical
pair is restricted in their diffusion and the reaction centers
are held in close proximity even in solution. Hence, this
class of structure is assumed to be optimum for the
achievement of very fast back-reaction of HABIs. In the
restricted field such as a crystalline phase, it is known that
4,5-diphenyl-2-(4(-
phenylboronicacid)imidazole (1)
.
the back-reaction of TPI pair instantly undergoes. For
example, the recombination reaction of chlorine-substituted
p-formylphenylboronic acid (1.00 g, 6.67 mmol), benzyl
(1.47 g, 6.99 mmol), and ammonium acetate (25.7 g,
0.333 mol) were gently refluxed in acetic acid (150 ml) for
20 h then allowed to cool to room temperature. The white
slurry that precipitated by the neutralization by
aqueous NH3 was filtered off. The crude slurry was
.
TPI pair completes within several milliseconds in the
crystal at room temperature. We have directed toward, with
the assistance of naphthalene linker as a diffusion inhibition
unit, to fabricate a fast reversible photochromic molecule
with HABI (Scheme 1). In this paper, we would like to
Chart 2. Photochromism and virtual one-photon reaction of tF-BDPI-2Y
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 857–863
DOI: 10.1002/poc

DIFFUSION-INHIBITED HABI WITH FAST BACK-REACTION
859
Scheme 1. Synthesis of 1,8-TPID-naphthalene
dissolved in ethyl acetate and the solution was frequently
washed with brine then dried over Na2SO4. The solvent
was removed under reduced pressure and the residue was
treated by silica-gel column chromatography (CHCl3:
MeOH ¼ 5:1). 4,5-diphenyl-2-(40-phenylboronic acid)i-
midazole was obtained as white powder (1.10 g; yield
48.7%). 1H NMR (500 MHz, DMSO-d6): d (ppm) 7.17 ꢂ
8.12 (m, 16 H), 12.63 (s, 1 H). Anal. Calcd for C21H17BN2
O2 ꢃ 2.5H2O ꢃ 0.25AcOEt: C, 64.88; H, 5.94; N, 6.88.
Found: C, 64.50; H, 5.09; N 7.07.
for C52H36N4 ꢃ 2(C4H8O2): C, 80.69; H, 5.87; N, 6.27.
Found: C, 80.10; H, 6.01; N 6.32.
1,8-TPID-naphthalene
All manipulations include recrystallization were carried
out with exclusion of the UV light. To the benzene
(9 ml) solution of 1,8-bisLophine-naphthalene ꢃ 3AcOEt
(188 mg, 0.192 mmol), the solution of potassium ferri-
cyanide (1.9 g, 5.8 mmol) and KOH (1.3 g, 23 mmol)
in H2O (9 ml) was added. The mixture was vigorously
stirred for 1.5 h, then the organic layer was separated and
exhaustively washed with deionized water. Though the
oxidation of 1,8-bisLophine-naphthalene gave the corre-
1,8-bisLophine-naphthalene
To
a solution of 1,8-diiodonaphthalene (260 mg,
.
0.684 mmol)6 in benzene (27 ml) and ethanol (27 ml),
680 mg (2.00 mmol) of 1 and Pd(PPh3)4 (111 mg,
96.1 mmol) was added. Sodium carbonate (438 mg,
4.13 mmol) in water (18 ml) was added, and the reaction
mixture was refluxed overnight with exclusion of the
light. After cooling to room temperature, the mixture was
poured into CHCl3 (150 ml) and brine (150 ml). The
organic layer was separated and the aqueous layer was
extracted three times with CHCl3. Combined organic
layer was washed with brine (ꢀ1) and water (ꢀ1) then
dried over Na2SO4. The solvent was removed in vacuo
and the residue was treated by silica-gel column
chromatography (CHCl3:AcOEt ¼ 5:1). 1,8-bisLophine-
naphthalene was obtained as pale yellow powder (420 mg;
yield 85.7%). Recrystallization from hot AcOEt gave
colorless, block crystal of 1,8-bisLophine-naphthalene ꢃ
2AcOEt. 1H NMR (500 MHz, DMSO-d6): d (ppm)
6.60 ꢂ 7.65 (m, 34 H), 11.85 (s, 2 H). Anal. Calcd
sponding 1,8-bisTPI -naphthalene, the intramolecular
.
dimerization reaction of the TPI moieties lead quickly
to the formation of 1,8-TPID-naphthalene. The solvent
was removed under reduced pressure to give a crude
product of 1,8-TPID-naphthalene. The purification was
carried out by silica-gel column chromatography
(CHCl3:AcOEt ¼ 20:1). The solvent was removed and
the residue was washed with acetonitrile. 1,8-TPID-
naphthalene was isolated as slightly green-colored
1
powder (83.3 mg; yield 60.9%). H NMR (500 MHz,
DMSO-d6): d (ppm) 6.69 ꢂ 8.14 (m). Anal. Calcd
for C52H34N4 ꢃ 0.4MeCN ꢃ 0.4H2O: C, 85.87; H, 4.91;
N, 8.35. Found: C, 85.89; H, 5.09; N 8.12. The slow
evaporation of dilute CH3CN solution of 1,8-TPID-
naphthalene under nitrogen stream gave transparent,
thin-plate single-crystal suitable for the X-ray crystal-
lographic analysis (crystal A in Table 1). The recrys-
tallization from hot CH3CN/benzene (8:1) mixture is also
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 857–863
DOI: 10.1002/poc

860
F. IWAHORI, S. HATANO AND J. ABE
Table 1. Crystallographic parameters of 1,8-TPID-naphthalenes
Crystal A
Crystal B
Crystal preparation
Formula
Slow evap of CH3CN
C52H34N4
714.83
CH3CN:C6H6 ¼ 8:1
(C52H34N4)2 ꢃ 2C6H6 ꢃ CH3CN
1626.93
Formula weight
Temperature/K
Crystal system
Space group
90
Triclinic
90
Triclinic
P1
0.15 ꢀ 0.03 ꢀ 0.01
P1
0.35 ꢀ 0.20 ꢀ 0.10
Crystal dimension/mm3
˚
a/A
9.902 (9)
9.7666 (5)
˚
b/A
14.179 (5)
15.231 (5)
117.357 (4)
103.858 (6)
92.981 (6)
1811.12 (18)
2
21.1009 (10)
22.2607 (11)
106.569 (3)
99.922 (3)
94.037 (3)
4296.4 (4)
˚
c/A
a/deg
b/deg
g/deg
3
˚
V/A
Z
2
R1(wR) for I > 2s(I)
R(wR) for all data
G.O.F.
0.0571 (0.0751)
0.1658 (0.0938)
1.001
0.0626 (0.1419)
0.1501 (0.1894)
1.002
acceptable. This method afforded solvated compound,
crystal B as listed in Table 1.
series UV-LED lamp (UV-50H type) equipped with a
UV-L6 lens unit (365 nm, ca. 800 mW/cm2).
X-ray crystallographic analysis
RESULTS AND DISCUSSION
The diffraction data of the single crystal of
1,8-TPID-naphthalene was collected on the Bruler APEX
II CCD area detector (MoKa, l ¼ 0.71073 nm). During
the data collection, the lead grass doors of the
diffractometer were covered to exclude the room light.
The data refinement was carried out by the Bruker
APEXII software package with SHELXT program.7 All
non-hydrogen atoms were anisotropically refined. The
crystallographic parameters are listed in Table 1.
The molecular structure of 1,8-TPID-naphthalene was
unambiguously determined by X-ray crystallography as
.
the ORTEP drawing shown in Fig. 1.8 Two TPI moieties
afforded a photochromic 1,20-biimidazole dimer by
intramolecular radical recombination. The bond length
˚
between C1 and N1 (1.494 A) was almost identical with
˚
that found in tF-BDPI-2Y dimer (1.499 A). The other
bond lengths are reasonable as compared with conven-
tional HABIs. The bridging benzene rings are held in
close proximity to each other and their mean planes were
positioned at a slight tilt angle of 3.2 degree. The contact
distances between the bridging benzenes are also shown
in Figure 1. The torsion angle between the mean planes of
naphthalene and benzene rings are 54.2 and 52.0 degree,
respectively.
Photochemical studies
The ESR signal of the light-induced radical pair of
.
1,8-bisTPI -naphthalene in frozen toluene matrix was
.
recorded on a JEOL JES-TE200 spectrometer operating
at the X-band equipped with a digital temperature
controller model 9650 (Scientific Instruments, Inc.). The
sample solution in a quartz tube (f 3.2 mm) was degassed
by the freeze-pump-thaw method.
The time-resolved visible-NIR absorption spectra were
measured on a S-2600 multi-channel spectrometer (Soma
optics Ltd.). The time-course spectra at 620 nm were
recorded on an UV-3100 spectrophotometer equipped
with a TCC-260 temperature controller (Shimadzu
corporation).
Light from a xenon lamp passed thorough a band-pass
filter (360 nm, ca. 0.2 mW/cmꢁ1; LAX 100, Asahi bunko
Ltd.) was used for the ESR and time-resolved absorption
spectra measurements. The light irradiation for the
time-course study was performed by Keyence UV-400
The light-induced radical pair of 1,8-bisTPI -naphtha-
lene was investigated by ESR spectroscopy. The frozen
toluene solution of 1,8-TPID-naphthalene (1.61 ꢀ
10ꢁ3 M) was irradiated with 360 nm of UV light for
5 min at 84 K. As it is always observed for the HABI
.
family, the formation of triplet TPI pair was confirmed by
a characteristic fine structure as shown in Fig. 2. The
zero-field splitting parameter was obtained as 11.8 mT
(j2D/hcj ¼ 0.0110 cmꢁ1). An estimation by means of
point-dipole approximation revealed that the distance
˚
between spin-density centers is 6.18 A. This distance is
.
comparable with those found in other light-induced TPI
pairs in frozen matrices or crystal.9 The time-dependent
signal decay arose from the back-reaction or dispropor-
tionation was not observed at 84 K, but the ESR signal
.
disappeared due to the recombination of TPI pairs as the
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 857–863
DOI: 10.1002/poc

DIFFUSION-INHIBITED HABI WITH FAST BACK-REACTION
861
Figure 1. ORTEP drawing for 1,8-TPID-naphthalene with 50 % probability thermal ellipsoid
.
temperature increased to room temperature region. At a
temperature above the melting point of toluene, light-
induced ESR signal showed time decay with exclusion of
the light.
bisTPI -naphthalene appeared and the color of the
solution changed from colorless to green. Absorption
.
spectral change of the light-induced 1,8-bisTPI -
naphthalene measured at 295 K is shown in Fig. 3. The
absorption band immediately began to decrease as the
light irradiation was turned off. A thing worthy to note is
the existence of the shoulder band whose absorption
maximum at approximately 620 nm and broad absorption
band at longer wavelength (>750 nm). Interestingly,
The kinetics of the back-reaction was investigated by
means of vis-NR absorption spectra. The photoreaction of
1,8-TPID-naphthalene in degassed toluene (3.53 ꢀ
10ꢁ4 M) was performed by irradiation with 365 nm
of UV light in a well-stirred quartz cell (light-path
length ¼ 10 mm) equipped with a thermostat. Upon
irradiation, an intense absorption band with lmax
585 nm that attributable to the absorption of 1,8-
¼
Figure 3. Absorption spectral change of 1,8-bisTPI.-
naphthalene in degassed toluene at 295 K with the time
interval of 1.5 s. The broken line at 600 nm is eye-guide. The
line with circles shows the initial spectrum
Figure 2. ESR signal of the light-induced triplet radical pair
of 1,8-bisTPI.-naphthalene in frozen toluene
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 857–863
DOI: 10.1002/poc

862
F. IWAHORI, S. HATANO AND J. ABE
these two absorption bands were affected by a bubbling of
oxygen gas to the optical cell. As shown in Fig. 4, the
absorption decay at 580 nm accelerated and longer
wavelength absorption disappeared in the presence of
oxygen. The initial spectral behavior recovered on the
removal of oxygen by continuous bubbling of argon gas.
These phenomena strongly support the existence of
the contribution from the triplet state to the absorption
spectra. The absorption band at 620 nm may be
.
presumably assignable to that of singlet TPI pair that
led to the radical recombination. To our knowledge, the
.
spectral observation of the triplet state of TPI pair in
solution has not been reported previously. The detailed
mechanism of the recombination process is currently
under investigation.
Taking into account above insight, the time course
spectra for the radical recombination reactions of 1,8-
.
bisTPI -naphthalene were probed at 620 nm of wavelength
after 10 s of light irradiation followed by a few seconds of
delay-time after the irradiation was ceased. The first-order
kinetic plot is depicted in Fig. 5. The rate constant was
obtained as 8.46 ꢀ 10ꢁ2 sꢁ1 at 273 K and was found to
increase with temperature. The temperature dependence of
the kinetic constant of the recombination reaction followed
an Arrhenius kinetics (Fig. 6). The activation energy barrier
and the frequency factor determined from the plot are
42.0 kJ/mol and 9.25 ꢀ 106 sꢁ1, respectively. The activation
energy is comparable to those of conventional HABI family
that obey the second-order kinetics. On the basis of the
kinetic parameters, the rate constant and half-life time at
295 K (ln2/k295K) was estimated by the extrapolation to be
0.339 sꢁ1 and 2.04 s. It is noteworthy that this recombina-
tion reaction is extremely fast, and the kinetics clearly
obeys the first-order as shown in Fig. 5, whereas several
kinetic studies of HABIs based on the second-order have
Figure 5. The first-order kinetic plot for 1,8-bisTPI.-
naphthalene in toluene. For the graphical clarity, results at
four temperatures are represented
been reported. The intramolecular recombination of
.
diffusion-inhibited two radical centers of 1,8-bisTPI -
naphthalene allowed the kinetics to be the first-order. It
is known that HABIs occur the solid state photoreaction as
well as in the solution, but the back-reaction in the solid
state does not obey the second-order law.1(e) The detailed
investigation for the mechanism and kinetics of the
back-reaction in the solid state have not been explored.
In conclusion, the fast reversible HABI derivative,
1,8-TPID-naphthalene, was designed with the aid of
naphthalene linker, and a significant acceleration of the
recombination reaction of light-induced radical pairs was
.
achieved. The kinetic study of 1,8-bisTPI -naphthalene
revealed an unequivocal first-order behavior whose
Figure 4. Absorption spectral change of 1,8-bisTPI.-
naphthalene in toluene with existence of oxygen with the
time interval of 1.5 s. The broken line at 600 nm is eye-guide
Figure 6. Arrhenius plots for the first-order kinetics of
1,8-bisTPI.-naphthalene
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 857–863
DOI: 10.1002/poc

DIFFUSION-INHIBITED HABI WITH FAST BACK-REACTION
863
half-life time is 2 s at room temperature. These results
established a new paradigm of the molecular designing
for the photochemistry and functional material science.
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Acknowledgements
This work was partially supported by a Grant-in-Aid for
the 21st Century COE program from the Ministry of
Education, Culture, Sports, Science and Technology,
Japan. We thank Professor Jean-Claude Micheau of Paul
Sabatier University for the fruitful discussions on the
kinetics of radical recombination reaction.
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DOI: 10.1002/poc