Comprehensive understanding of structure- photosensitivity relationships of photochromic [2.2]paracyclophane-bridged imidazole dimers

Article abstract of DOI:10.1021/jp201969q

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. Experimental details for the enhancement of the photosensitivity and the unique photoisomerization of newly designed [2.2]paracyclophane-bridged imidazole dimers are demonstrated. We explored the structure-property relationships and demonstrated an efficient strategy for designing high-performance fast-photochromic molecules with increased photosensitivity to solar UVA radiation. The [2.2]paracyclophane-bridged imidazole dimer consists of two types of imidazole rings, Im1 and Im2. Im1 is characterized by a 6π electron system with an electron-donating characteristic, whereas Im2 is distinguished by a 4π electron system with an electron-withdrawing characteristic. The introduction of electron-donating substituents into the phenyl rings attached to the electron-withdrawing Im2 was proved to enhance the photosensitivity with the aid of the intramolecular charge transfer transitions. The unique photoisomerization resulting from the changes in the bonding manner between two imidazole rings was also investigated in detail.

Full text of DOI:10.1021/jp201969q

ARTICLE
pubs.acs.org/JPCA
Comprehensive Understanding of Structure- Photosensitivity
Relationships of Photochromic [2.2]Paracyclophane-Bridged
Imidazole Dimers
Katsuya Mutoh† and Jiro Abe†,‡,*
†Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara,
Kanagawa 252-5258, Japan
‡CREST, Japan Science and Technology Agency (JST), 5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan
S Supporting Information
b
ABSTRACT:
The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and
rapid fading in the dark. Experimental details for the enhancement of the photosensitivity and the unique photoisomerization of
newly designed [2.2]paracyclophane-bridged imidazole dimers are demonstrated. We explored the structureꢀproperty relation-
ships and demonstrated an efficient strategy for designing high-performance fast-photochromic molecules with increased
photosensitivity to solar UVA radiation. The [2.2]paracyclophane-bridged imidazole dimer consists of two types of imidazole
rings, Im1 and Im2. Im1 is characterized by a 6π electron system with an electron-donating characteristic, whereas Im2 is
distinguished by a 4π electron system with an electron-withdrawing characteristic. The introduction of electron-donating
substituents into the phenyl rings attached to the electron-withdrawing Im2 was proved to enhance the photosensitivity with
the aid of the intramolecular charge transfer transitions. The unique photoisomerization resulting from the changes in the bonding
manner between two imidazole rings was also investigated in detail.
homolytic bond cleavage of the CꢀN bond between the imidazole
1. INTRODUCTION
rings and successive fast CꢀN bond formation. Fast thermal-
Photochromic materials are a well-known class of molecules
bleaching kinetics enable a solution color change only where it is
that change their color upon UV light irradiation. The colored
irradiated with light, because the thermal-bleaching rate is much
species goes back to the original state either thermally or
faster than the diffusion rate of the colored species at room
photochemically. Thermally reversible photochromic molecules
offer the opportunity to change and reset the molecular proper-
ties by simply turning a light source on and off, and have attracted
temperature. Furthermore, we could achieve a remarkable accel-
eration for the thermal back-reaction (τ1/2 = 35 μs in the benzene
solution at room temperature) by applying Marcus theory.3b
great attention due to their potential applications in smart
Though the large optical density in the visible light region is
windows and ophthalmic glasses.1 The development of thermally
necessary when [2.2]PC-bridged imidazole dimers are applied to
reversible photochromic dyes with different switching kinetics,
thermal stabilities and coloration is important for the develop-
fast-color changing ophthalmic lenses, fast bleaching rate causes
the decrease in the optical density in the photostationary state.
ment of new switching devices and applications.2 We recently
One possible approach to increase the optical density for the
designed and synthesized a unique photochromic molecule,
colored state is to enhance the photosensitivity to solar radiation.
pseudogem-bisDPI[2.2]PC, which is one of the [2.2]paracyclo-
Unfortunately the photosensitivity of the earliest [2.2]PC-bridged
phane-bridged imidazole dimers showing instantaneous colora-
tion upon UV light irradiation and rapid fading in the dark.3 The
[2.2]paracyclophane-bridged imidazole dimer has a [2.2]paracy-
clophane ([2.2]PC) moiety that tightly couples two imidazole
rings. The [2.2]PC-bridged imidazole dimer shows photoinduced
Received: March 1, 2011
Revised: April 4, 2011
Published: April 15, 2011
r
2011 American Chemical Society
4650
dx.doi.org/10.1021/jp201969q J. Phys. Chem. A 2011, 115, 4650–4656
|

The Journal of Physical Chemistry A
ARTICLE
Scheme 1. Photochromic Reaction of [2.2]Paracyclophane-
Bridged Imidazole Dimers
Scheme 2. Syntheses of pseudogem-DPI-TMDPI[2.2]PC (4)
and pseudogem-TMDPI-DPI[2.2]PC (5)
with distilled water and dried. Unless otherwise noted, all reagents
and reaction solvents except acetic acid were purchased from TCI,
Wako Co. Ltd., Aldrich Chemical Co., Inc., and ACROS Ora-
ganics, and were used without further purification. Acetic acid was
purified by adding 5 wt % KMnO4, boiling under reflux for 2 h and
then fractionally distilling to remove acetaldehyde.
pseudogem-DPI-TMDPI[2.2]PC (4) and pseudogem-TMDPI-
DPI[2.2]PC (5) were synthesized according to Scheme 2, using
pseudogem-[4-formyl-13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]-
paracyclophane as a starting material. Pseudogem-[4-formyl-
13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]paracyclophane3b (1)
and 3,30,4,40-Tetramethoxybenzil4 (2) were prepared according
to a literature procedure.
imidazole dimer is poor because it does not have an efficient
absorption band in the UVA radiation region. To improve the
photosensitivity to sunlight, we recently developed a novel [2.2]PC-
bridged imidazole dimer derivative, pseudogem-bisTMDPI[2.2]PC,
which shows excellent photochromic color change even under
sunlight.3d The [2.2]PC-bridged imidazole dimer has two types of
imidazole rings—Im1 and Im2—as shown in Scheme 1. It should
be noted that the two imidazole rings are not equivalent in their
electronic environment. Im1 is a resonant planar structure that has a
typical bond distance for a 6π electron system with an electron-
donating characteristic, whereas Im2 has two localized CdN double
bonds and one sp3 carbon connecting Im1, consistent with a 4π-
electron system with an electron-withdrawing characteristic. The
absorption band in the UVA radiation region of pseudogem-
bisTMDPI[2.2]PC was assigned to the intramolecular charge-
transfer (CT) transition from the electron-donating dimethoxy-
substituted phenyl rings to the electron-withdrawing Im2 based on
the TDDFT calculation. However, the presence of the intramole-
cular CT transition was not proved by the definitive experimental
evidence. In this study, we investigated the CT characteristics and
the photochromic properties of two novel photochromic molecules,
pseudogem-DPI-TMDPI[2.2]PC (4) and pseudogem-TMDPI-DPI-
[2.2]PC (5). Moreover, we revealed that 4 and 5 would isomerize
each other through an identical transient colored species upon UV
light irradiation.
pseudogem-(4,5-Diphenyl-1H-imidazol-2-yl-4,5-(3,4,-di-
methoxyphenyl)-1H-imidazol-2-yl)[2.2]paracyclophane (3).
pseudogem-[4-Formyl-13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]-
paracyclophane (176 mg, 0.387 mmol), 3,30,4,40-tetramethoxy-
benzil (153 mg, 0.463 mmol) and ammonium acetate (1.846 g,
23.95 mmol) were refluxed in acetic acid (9 mL) for 1 day. The
reaction mixture was cooled with an ice bath, and neutralized with
aqueous NH3, to form a yellow precipitate. The aqueous layer was
extracted with dichloromethane. The combined organic layer was
washed with water and brine, dried over Na2SO4, filtered, and
evaporated. The product was purified with silica gel column
chromatography twice, using hexane/THF = 1/1 and then
CH2Cl2/ethyl acetate = 2/1, as eluents respectively, to give a
brown amorphous solid (3), 165 mg (0.216 mmol, 55.7%).
1H NMR (500 MHz, DMSO-d6) δ: 11.61 (s, 1H), 11.52 (s,
1H), 7.22 (d, J = 7.1 Hz, 2H), 7.18ꢀ7.13 (m, 3H), 7.08ꢀ6.98 (m,
8H), 6.87 (s, 1H), 6.79ꢀ6.77 (m, 3H), 6.71ꢀ6.69 (m, 2H),
6.64ꢀ6.61 (m, 3H), 4.58ꢀ4.52 (m, 2H), 3.74 (s, 3H), 3.67
(s, 3H), 3.44 (s, 3H), 3.14ꢀ3.04 (m, 6H); FABꢀMS: m/z 765
[M þ H]þ.
2. EXPERIMENTAL SECTION
2.1. Synthesis. 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
(Wakogel C-300). 1H NMR spectra were recorded at 500 MHz
on a JEOL JNM-ECP500A. DMSO-d6 and CDCl3 (1% TMS)
were used as deuterated solvents. Mass spectra (FABꢀMS) were
measured by using a JEOL MStation MS700. m-Dinitrobenzi-
lalcohol was used as a MASS matrix. All glasswares were washed
pseudogem-DPI-TMDPI[2.2]PC (4) and pseudogem-TMDPI-
DPI[2.2]PC (5). All manipulations were carried out with the
exclusion of light. Under nitrogen, to a solution of 3 (99 mg,
0.129 mmol) in benzene (10 mL) was added the solution of
potassium ferricyanide (2.270 g, 6.895 mmol) and KOH (0.78 g,
13.90 mmol) in water (20 mL), and the reaction mixture was
4651
dx.doi.org/10.1021/jp201969q |J. Phys. Chem. A 2011, 115, 4650–4656

The Journal of Physical Chemistry A
ARTICLE
vigorously stirred for 30 min at room temperature. The organic
layer was separated, exhaustively washed with water, and concen-
trated in vacuo. Then the crude mixture of 4 and 5 was separated
withsilica gel columnchromatography using hexane/AcOEt = 2/3
as eluent. The separated 4 was recrystallized from CH2Cl2/hexane
to give colorless crystals, 72 mg (0.0944 mmol, 72%). An 8 mg
(0.01 mmol, 8%) yield of yellow crystals of the separated 5 was
obtained from the recrystallization from CHCl3/hexane. As can be
foundfrommolecular structures, both of the [2.2]paracyclophane-
bridged imidazole dimers 4 and 5 could have diastereomers due to
hindered rotation about the CꢀC bond between the imidazole
ring and the dimethoxy-substituted phenyl ring. Though the
existence of diastereomers of 5 was confirmed by the NMR
spectrum at room temperature, 4 was found to have only one
diastereomeric conformer or the rate of interconversion between
the diastereomeric conformers of 4 is fast. Colorless crystals
Figure 1. ORTEP representation of the molecular structures of (a) 4
and (b) 5 with thermal ellipsoids (50% probability), where nitrogen and
oxygen atoms are highlighted in blue and red, respectively. The
hydrogen atoms and the solvent molecules are omitted for clarity.
1
(4). H NMR (500 MHz, CDCl3) δ: 7.51ꢀ7.48 (m, 1H),
3. RESULTS AND DISCUSSION
7.30ꢀ7.28 (m, 3H), 7.22 (d, J = 7.5 Hz, 1H), 7.13ꢀ7.03 (m,
6H), 6.93 (d, J = 1.3 Hz, 1H), 6.87ꢀ6.84 (m, 2H), 6.80ꢀ6.62 (m,
6H), 6.50(d, J = 7.7 Hz, 1H), 6.45 (dd, J = 7.7 Hz, 1H), 4.58ꢀ4.53
(m, 1H), 3.91 (s, 3H), 3.88 (s, 3H), 3.78 (s, 3H), 3.72 (s, 3H),
3.06ꢀ2.93 (m, 7H). FABꢀMS: m/z 763[M þ H]þ. Yellow
crystals (5). 1H NMR (500 MHz, CDCl3) mixture of diastereo-
mers δ: 7.71ꢀ7.69 (m, 1H one diastereomer), 7.53ꢀ7.43 (m, 3H,
two diastereomers), 7.41ꢀ7.29 (m, 11H, two diastereomers),
7.25ꢀ7.20 (m, 3H, two diastereomers), 7.13 (dd, J = 8.3, 1.9 Hz,
1H, one diastereomer), 7.09 (dd, J = 8.3, 1.3 Hz, 2H, one
diastereomer), 7.00ꢀ6.87 (m, 7H, two diastereomers), 6.81 (d,
J = 1.9 Hz, 1H, one diastereomer), 6.66ꢀ6.59 (m, 11H, two
diastereomers), 6.52ꢀ6.44 (m, 4H, two diastereomers), 4.60ꢀ4.50
(m, 1H, one diastereomers), 4.25ꢀ4.18 (m, 1H, one diastereomer),
3.78 (s, 6H, two diastereomers), 3.70 (s, 3H, one diastereomer),
3.65 (s, 3H, one diastereomer), 3.56 (d, J = 6.4 Hz, 6H, two
diastereomers), 3.37ꢀ2.92 (m, 20H, two diastereomers). FABꢀ
MS: m/z 763[MþH]þ.
3.1. X-ray Crystallographic Analysis. The molecular struc-
tures of 4 and 5 revealed by X-ray crystallographic analysis are
shown in Figure 1. As described before, while 4 has only one
diastereomeric conformer at room temperature, 5 has diastere-
omeric conformers due to hindered rotation about the CꢀC
bond between the imidazole ring and the dimethoxy-substituted
phenyl ring. Recrystallization from the dilute solution (CHCl3/
hexane) of 5 gave the single crystal composed of only one of the
diastereomeric conformers. Each CꢀN bond lengths connecting
two imidazole rings (4, 1.4851(18) Å; 5, 1.480(3) Å) are approxi-
mately equal to that of pseudogem-bisDPI[2.2]PC (1.4876(15) Å).
It can be found that the imidazole ring of 4 attached with the
dimethoxy-substituted phenyl rings is an electron-withdrawing
Im2, whereas the imidazole ring of 5 attached with the di-
methoxy-substituted phenyl rings is an electron-donating Im1.
Though we could not observe the significant difference in the
bond lengths of 5 compared with those of pseudogem-bisDPI-
[2.2]PC, the bond lengths of the CꢀC bond between the
imidazole ring and the dimethoxy-substituted phenyl rings of 4
are considerably different from those in pseudogem-bisDPI[2.2]PC.
These differences in the molecular geometries can be plausibly
explained by the contribution from the intramolecular CT reson-
ance between the electron-donating dimethoxy-substituted phenyl
rings and the electron-withdrawing imidazole ring (Figures S6 and
S7 and Tables S3 and S4, Supporting Information).
3.2. UVꢀVis Absorption Spectra. Figure 2 shows the UVꢀvis
absorption spectra of 4 and 5 in the acetonitrile solution, along with
those calculated by the TDDFT method (MPW1PW91/
6-31þG(d)//MPW1PW91/6-31G(d)) for the optimized molec-
ular geometries. The blue curves are the experimental spectra and
the calculated oscillator strengths are shown by the red perpendi-
cular lines. It should be noted that a clearly distinct absorption band
is found in the UVA radiation region for the UVꢀvis absorption
spectra of 4, whereas 5 does not show the similar absorption band
in the same region. The S0 f S2 transition at 372 nm (f = 0.159) of
4 demonstrated by the TDDFT calculation is described by
HOMO-1 f LUMO transition. This transition can be attributed
to the intramolecular CT transition from the dimethoxy-substi-
tuted phenyl rings to the electron-withdrawing Im2 (Figure 3).
Moreover, both of the S0 f S4 transition at 351 nm (f = 0.075) and
S0 f S5 transition at 348 nm (f = 0.066) are associated with
HOMO-3 f LUMO and HOMO-4 f LUMO transitions. These
excited states are also characterized by the same type of the
2.2. X-ray Crystallographic Analysis. The diffraction data of
the single crystal of 4 and 5 were collected on the Bruker APEX II
CCD area detector (Mo KR, λ = 0.71073 nm). During the data
collection, the lead glass 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.5
All non-hydrogen atoms were anisotropically refined.
2.3. Laser Flash Photolysis. The laser flash photolysis experi-
ments were carried out with a Unisoku TSP-1000 time-resolved
spectrophotometer. A Continuum Minilite II Nd:YAG (Q-switch-
ed) laser with the third harmonic at 355 nm (ca. 8 mJ per 5 ns
pulse) was employed for the excitation light. The probe beam from
an OSRAM HLX64623 halogen lamp 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
Hamamatsu R2949 photomultiplier tube through a spectrometer
(Unisoku MD200). Sample solutions were deaerated by argon
bubbling prior to the laser flash photolysis experiments.
2.4. DFT Calculations. All calculations were carried out using
the Gaussian 03 program (Revision E.01).6 The molecular
structures 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. The TDDFT calculations were per-
formed at the MPW1PW91/6-31þG(d) level of the theory for
the optimized geometries.
4652
dx.doi.org/10.1021/jp201969q |J. Phys. Chem. A 2011, 115, 4650–4656

The Journal of Physical Chemistry A
ARTICLE
Figure 2. UVꢀvis absorption spectra and TDDFT calculations for
(a) 4 and (b) 5. The calculated spectra (MPW1PW91/6-31þG(d)//
MPW1PW91/6-31G(d)) are shown by the red perpendicular lines.
intramolecular CT transition. On the other hand, the dimethoxy-
substituted phenyl rings are connected to the electron-donating
Im1 for 5 as shown in Figure 1. Thus, the intramolecular CT
transition from the electron-donating dimethoxy-substituted phen-
yl rings to the electron-withdrawing Im2 would have small
oscillator strength because of the small orbital overlap between
the molecular orbitals (MOs) delocalized over the dimethoxy-
substituted phenyl rings and the MOs delocalized over Im2.
Indeed, as shown in Figure 2b, significant absorption bands are
not found both in the experimental and theoretical absorption
spectra of 5 in the UVA radiation region. The solvent effects on the
absorption spectrum of 4 are also investigated. The maximum
wavelength of the absorption in the UVA radiation region is slightly
red-shifted depending on the increase in the permittivity of the
solvent (Figure 4). This result also supports the CT nature of the
absorption in the UVA radiation region. These structure-spectra
relationships are the definitive evidence for the presence of the
intramolecular CT band from the electron-donating substituents to
the electron-withdrawing Im2.
Thus, the photosensitivity of [2.2]PC-bridged imidazole di-
mers can be readily controlled through appropriate design of the
phenyl rings attached to the imidazole rings. The introduction of
electron-donating substituents into the phenyl rings attached to
the electron-withdrawing Im2 would enhance the photosensitiv-
ity with the aid of the intramolecular CT transitions.
3.3. Laser Flash Photolysis Measurement. Both of the
solutions of 4 and 5 undergo photochromic reaction involving a
color change from colorless to blue upon UV light irradiation, and
Figure 3. Relevant molecular orbitals of (a) 4 and (b) 5 obtained at the
MPW1PW91/6-31þG(d) level.
Figure 4. UVꢀvis absorption spectra of 4 in benzene, acetonitrile, and a
mixture of acetonitrile and water.
generate the identical colored species R as shown in Scheme 3. The
solutions reach the photostationary equilibrium very rapidly under
continuous irradiation, and after irradiation ceases, the absorption
decreases very quickly according to monoexponential thermal-
bleaching kinetics. As described in the Experimental Section, the
oxidation of the precursor 3 gives the crude mixture of 4 and 5, and
4653
dx.doi.org/10.1021/jp201969q |J. Phys. Chem. A 2011, 115, 4650–4656

The Journal of Physical Chemistry A
ARTICLE
Scheme 3. Photochromic Reactions of 4 and 5
Figure 5. Transient visꢀNIR absorption spectra of 4 in degassed
benzene at 25 °C (2.1 ꢁ 10ꢀ4 M, light path length: 10 mm). Each of
the spectra was recorded at 20 ms intervals after excitation with a
nanosecond laser pulse (excitation wavelength, 355 nm; pulse width,
5 ns; power 8 mJ/pulse).
Figure 6. Decay profiles of the colored species R monitored at 400 nm in
degassed benzene (4, 2.1 ꢁ 10ꢀ4 M; light path length, 10 mm). The
measurements were performed in the temperature range from 5 to 40 °C.
Table 1. Variation of the Ratio of 4 and 5 for the Solution
Standing in the Dark after the Oxidation Reaction of the
Precursor 3 in the Temperature Range from 10 to 40 °C
consequently the photochromic reaction of 4 leads to the formation
of the mixture of 4 and 5 via the colored species R. The formation of
4 by the photochromic reaction of 5 is also true. Thus the repeated
irradiation of UV light to the solution of 4would gradually change to
the mixture of thermal equilibrium of 4 and 5.
isomer ratio
T/°C
4
5
Figure 5 shows the transient visꢀNIR absorption spectra of 4
in benzene at 25 °C measured by a nanosecond laser flash
photolysis experiment. A sharp absorption band at 400 nm and a
broad absorption band ranging from 500 to 1000 nm can be
ascribed to the colored species R. All of the absorption bands
decay with the same time constant, indicating the presence of a
single conformation of the colored species R. The thermal
bleaching process obeys first-order kinetics, and the half-life of
the colored species is 55 ms at 25 °C. Figure 6 shows the decay
profiles of the transient absorbance at 400 nm of the colored
species R, measured over the temperature range from 5 to 40 °C.
As described above, we can consider two kinds of reaction paths
for the radical dimerization reaction to form the imidazole dimer.
The reaction path 1 forms 4 with the reaction rate constant k1,
and 5 is formed with the reaction rate constant k2 along the
reaction path 2 (Scheme 3). Therefore, the rate constant
obtained from the first order plots of the time profile of the
transient absorbance for the colored species R shown in Figure 6
should be a sum of k1 and k2 (Table S5, Supporting Information).
Because the ratio of k1 and k2 can be estimated from the
concentration ratio of 4 and 5 after the thermal back reaction,
we measured the concentration ratio of 4 and 5 from HPLC
10
20
30
40
1
1
1
1
0.103
0.119
0.134
0.153
chromatograms after the oxidation reaction of the precursor 3
over the temperature range from 10 to 40 °C in the dark
(Table 1). The activation parameters for the thermal back-
reactions via the path 1 and path 2 were estimated from the
Eyring plots (Figure S9, Supporting Information). The Eyring
plots produce an excellent straight line, and the ΔH‡ and ΔS‡
values are estimated from the standard least-squares analysis of
the Eyring plots. The free energy barriers (ΔG‡ = ΔH‡ ꢀ TΔS‡)
at 25 °C of the path 1 and path 2 are 67.0 kJ molꢀ1 and 72.1 kJ
molꢀ1, respectively.
The difference in the radical recombination reaction rate for
the path 1 and path 2 can be explained by the Marcus theory7 as
described in our previous paper.3b Following standard Marcus
theory, an increase in the change in Gibbs energy, ΔG0, between
the reactant and the product would lead to a decrease in the free
4654
dx.doi.org/10.1021/jp201969q |J. Phys. Chem. A 2011, 115, 4650–4656

The Journal of Physical Chemistry A
ARTICLE
Table 2. Kinetic Parameters Associated with the Thermal
Back Reaction at 298 K
ΔH‡/kJ
molꢀ1
ΔS‡/J
molꢀ1 Kꢀ1
ΔG‡/kJ
molꢀ1
τ1/2/ms
reaction path 1
reaction path 2
57.5
65.1
ꢀ31.9
ꢀ23.5
67.0
72.1
60
485
energy of activation, ΔG‡, and consequently, the rate constant for
the reaction would be accelerated. We have considered that the
ΔG0 of the thermal-back reaction could be enlarged by destabiliz-
ing the colored species, and designed pseudogem-DPI-PI[2.2]PC,3b
with a [2.2]PC moiety that couples diphenylimidazole and phe-
nanthroimidazole groups. The steric repulsion between the rigid
phenanthroimidazole group and the phenyl rings facing each other
should destabilize the biradical state, whereas the rotational mo-
tion along the CꢀC single bond between the imidazole group and
the phenyl ring in pseudogem-bisDPIR[2.2]PC should relax the
steric hindrance between the phenyl rings facing each other. The
thermal back-reaction of the colored species of pseudogem-DPI-
PI[2.2]PC is actually accelerated about 1000 times compared with
that of pseudogem-bisDPIR[2.2]PC. As can be found from Table 2,
the free energy of activation for the path 1, ΔG‡ , is smaller than
1
that for the path 2, ΔG‡ . Therefore, it is suggested that the Gibbs
2
energy of 4, ΔG0 , is presumed to be larger than that of 5, ΔG0 . In
1
2
other words, 4 is more stable than 5 because 4 has a resonance
stabilization energy attributable to the intramolecular CT between
the electron-donating dimethoxy-substituted phenyl rings and the
electron-withdrawing Im2. The sum of the electronic and zero-
point energies of 4 calculated by the DFT method (MPW1PW91/
6-31G(d)) is 6.2 kJ molꢀ1 smaller than that of 5. Thus, the X-ray
crystallographic analysis and the DFT calculations support the
resonance stabilization of 4.
Figure 7. (a) UVꢀvis absorption spectra for the degassed acetonitrile
solution (20 °C, 1.2 ꢁ 10ꢀ5 M) of 4 measured after the irradiation with
UV laser pulses. (b) HPLC chromatograms for the UV laser pulse-
irradiated acetonitrile solution of 4 measured at 20 °C. The mobile phase
was water:CH3CN = 1:9 with a flow rate of 1.0 mL/min, detection
wavelength = 267 nm (isosbestic point).
3.4. Photoisomerization between 4 and 5. As described
above, both of the photochromic reaction of 4 and the oxidation
of the precursor 3 lead to the formation of the mixture of 4 and 5
via the colored species R. The photoisomerization between 4
and 5 was investigated by UVꢀvis absorption spectroscopy and
HPLC analysis. We confirmed that while the isomerization
would not proceed in the dark over the temperature range from
10 to 50 °C (Figure S11, Supporting Information), the UVꢀvis
absorption spectra of the acetonitrile solution of 4 changed with
isosbestic points after the irradiation with UV laser pulses at
20 °C (Figure 7a). The absorption band around 350 nm of 4 can
be assigned to the intramolecular CT transition from the
dimethoxy-substituted phenyl rings to the electron-withdrawing
Im2. On the other hand, 5 does not have significant absorption
bands in the UVA radiation region as shown in Figure 2. Thus,
the spectral changes induced by the UV light irradiation can be
tentatively attributable to the photoisomerization of 4 yielding 5.
Actually, as shown in Figure 7b, the HPLC analysis for the UV
laser pulse-irradiated solution of 4 revealed the increase in the
concentration of 5 with increasing the number of irradiation
cycles. Figure 8 shows the ratio of isomers against the number of
irradiation cycles of the laser pulse, determined by the HPLC
analysis. The system reached the photostationary state after
irradiating the laser pulses more than 500 times. As can be found
from Figure 2, the absorption coefficient of 5 at 355 nm is
apparently smaller than that of 4 due to the absence of the
intramolecular CT transition. Therefore, the excitation efficiency
of 5 should be poor under this experimental condition.
Figure 8. Variation of the ratio of 4 and 5 with the number of irradiation
cycles of laser pulse determined by the HPLC analysis.
Therefore, as shown in Figure 9, the ΔOD values immediately
after laser excitation for the UV laser pulse-irradiated solutions,
which are the mixture of 4 and 5, were smaller than that of the
solution of 4. The fatigue resistance of the photochromic cycle
was evaluated in benzene at 25 °C. The time profiles for the
transient absorbance at 400 nm for the solution of the photo-
stationary state after irradiating the laser pulses about 500 times
and the solution after irradiation with 20000 laser pulses are
4655
dx.doi.org/10.1021/jp201969q |J. Phys. Chem. A 2011, 115, 4650–4656

The Journal of Physical Chemistry A
ARTICLE
(No. 471)” from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan and by a High-Tech
Research Center project for private universities with the match-
ing fund subsidy from MEXT.
’ REFERENCES
(1) (a) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and
Thermochromic Compounds; Plenum Press: New York, 1999. (b) Duerr,
H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier:
Amsterdam, The Netherlands, 2003.
(2) (a) Silvi, S.; Arduini, A.; Pochini, A.; Secchi, Andrea.; Tomasulo,
M.; Raymo, F. M.; Baroncini, M.; Credi, A. J. Am. Chem. Soc. 2007,
129, 13378–13379. (b) Raboin, L.; Matheron, M.; Biteau, J.; Gacoina,
T.; Boilot, J. P. J. Mater. Chem. 2008, 18, 3242–3248.
(3) (a) Kishimoto, Y.; Abe, J. J. Am. Chem. Soc. 2009, 131, 4227–4229.
(b) Harada, Y.; Hatano, S.; Kimoto, A.; Abe, J. J. Phys. Chem. Lett. 2010,
1, 1112–1115. (c) Kimoto, A.; Tokita, A.; Horino, T.; Oshima, T.; Abe, J.
Macromolecules 2010, 43, 3764–3769. (d) Mutoh, K.; Hatano, S.; Abe, J.
J. Photopolym. Sci. Technol. 2010, 23, 301–306. (e) Takizawa, M.; Kimoto,
A.; Abe, J. Dyes Pigm. 2011, 89, 254–259.
Figure 9. Decay profiles of the coloredspecies Rgenerated in the UV laser
pulse-irradiated acetonitrile solution of 4 monitored at 400 nm (20 °C).
almost indistinguishable, indicating excellent durability for ap-
plications in ophthalmic lenses.
(4) Sengul, A.; Arslan, H.; Bayari, H. S.; Buyukgungor, O. Struct.
Chem. 2008, 19, 467–476.
4. CONCLUSIONS
(5) (a) Sheldrick, G. M. SHELXS-97 and SHELXL-97; University of
Gottingen, Germany, 1997. (b) Sheldrick, G. M. SADABS; University of
Gottingen, Germany, 1996.
The present study demonstrated the structureꢀproperty
relationships of newly designed [2.2]PC-bridged imidazole
dimers, and an efficient strategy for designing high-performance
fast-photochromic molecules with increased photosensitivity to
solar UVA radiation. The [2.2]PC-bridged imidazole dimer
consists of two types of imidazole rings, Im1 and Im2. Im1 is
characterized by a 6π electron system with an electron-donating
characteristic, whereas Im2 is distinguished by a 4π electron
system with an electron-withdrawing characteristic. The differ-
ence in the electronic environment between these two imidazole
rings is extremely important to control the photosensitivity of
[2.2]PC-bridged imidazole dimers. An appropriate design of the
phenyl rings attached to the imidazole rings makes it possible to
enhance the photosensitivity in the UVA radiation region.
Indeed, we can demonstrate that the introduction of electron-
donating substituents into the phenyl rings attached to the
electron-withdrawing Im2 would enhance the photosensitivity
with the aid of the intramolecular CT transitions. The diversity of
the molecular design and the ease of synthesis make this class of
photochromic molecules highly attractive for revolutionary
optical switching devices.
(6) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.;
Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford CT, 2004.
(7) (a) Marcus, R. A. J. Chem. Phys. 1956, 24, 966–978. (b) Marcus,
R. A. Annu. Rev. Phys. Chem. 1964, 15, 155–196. (c) Marcus, R. A. J. Phys.
Chem. 1968, 72, 891–899. (d) Marcus, R. A. J. Am. Chem. Soc. 1969,
91, 7224–7225. (e) Cohen, A. O.; Marcus, R. A. J. Phys. Chem. 1968,
72, 4249–4256. (f) Alabugin, I. V.; Manoharan, M.; Breiner, B.; Lewis,
F. D. J. Am. Chem. Soc. 2003, 125, 9329–9342. (g) Chen, X.; Brauman,
J. I. J. Am. Chem. Soc. 2008, 130, 15038–15046.
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra of pseudogem-
S
b
(4,5-diphenyl-1H-imidazol-2-yl-4,5-(3,4,-dimethoxyphenyl)-1H-imi-
dazol-2-yl)[2.2]paracyclophane, 4 and 5. HPLC chromatograms,
X-ray crystallographic analysis data (including separate cif format
files), kinetics for the thermal back-reaction, thermal stability,
and details of the DFT calculations. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jiro_abe@chem.aoyama.ac.jp.
’ ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research in a Priority Area “New Frontiers in Photochromism
4656
dx.doi.org/10.1021/jp201969q |J. Phys. Chem. A 2011, 115, 4650–4656