Photochromic properties of [2.2]paracyclophane-bridged imidazole dimer with increased photosensitivity by introducing pyrenyl moiety

Article abstract of DOI:10.1021/jp204440s

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. A new [2.2]paracyclophane-bridged imidazole dimer, pseudogem-PPI-DPI[2.2]PC, with high photosensitivity to

Full text of DOI:10.1021/jp204440s

ARTICLE
pubs.acs.org/JPCA
Photochromic Properties of [2.2]Paracyclophane-Bridged Imidazole
Dimer with Increased Photosensitivity by Introducing Pyrenyl Moiety
Hiroaki Yamashita† 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
‡Core Research for Evolutional Science and Technology (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. A new [2.2]
paracyclophane-bridged imidazole dimer, pseudogem-PPI-DPI-
[2.2]PC, with high photosensitivity to UVA radiation was
developed. To enhance the photosensitivity, we introduced
pyrenyl moieties to the [2.2]paracyclophane-bridged imidazole
dimer. The localized πÀπ* transition of pyrenyl moieties
appears in the UVA radiation region by introducing a pyrenyl
moietiy on the 4-position of the imidazole rings. The expansion
of the π-electron system also affects the absorption spectrum of the colored species. The broad absorption band of the colored
species covers the whole range of visible light region and its absorbance is approximately equal throughout the visible light region.
Thus, pseudogem-PPI-DPI[2.2]PC shows the photochromic reaction coloring black upon light irradiation and successive fast
thermal bleaching following the monoexponential kinetics with a time constant of 12 ms at room temperature.
The increase in the photosensitivity to solar ultraviolet A
(UVA) radiation region is essential for the applications in smart
windows and ophthalmic glasses. HABIs can be sensitized by a
large number of sensitizing dyes. HABI-based initiator systems,
which are sensitive to visible light, have been developed for laser
imaging and other radiation sources with high visible emission.
The sensitization reaches out into the near-infrared region. For
example, aryl ketones and p-(dialkylamino)aryl aldehydes are
known as efficient HABI sensitizers.4 On the other hand, we have
successfully achieved the enhancement of the photosensitivity to
UVA light with the help of the charge-transfer (CT) transition.2d,g
The [2.2]PC-bridged imidazole dimer has two types of imidazole
rings (Im1 and Im2) asshown 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. By introducing
electron-donating dimethoxy groups on the phenyl rings attached
to the electron-withdrawing Im2, we confirmed the appearance of
the CT transition band from the electron-donating dimethoxy-
substituted phenyl rings to Im2 by the TDDFT calculation.2g This
strategy of the molecular design for the enhancement of the
1. INTRODUCTION
Considerable attention has been paid to the organic photo-
chromic materials that change their color upon light irradiation.
The photogenerated colored-species can be reversed to the initial
colorless-species either by thermal means or by subsequent
irradiation with a specific wavelength of light. The thermally
reversible photochromic molecules are allowed to change and
reset the molecular properties by simply turning a light source on
and off.1 The development of fast photochromic materials is a
major challenge for the photochemistry and material science
because of their potential applications including smart windows,
solar protection lenses and decorative objects. We have recently
developed a unique series of photochromic [2.2]PC-bridged
imidazole dimer, with a [2.2]paracyclophane ([2.2]PC) moiety
that couples two diphenylimidazole groups, that shows instanta-
neous coloration upon exposure to UV light and rapid fading in
the dark.2 Upon UV light irradiation, the CÀN bond between
two imidazole rings of the [2.2]PC-bridged imidazole dimer is
homolytically cleaved to give a pair of imidazolyl radicals and the
color of the solution changes from colorless to blue. The blue color
fades very rapidly following monoexponential thermal bleaching
kinetics after ceasing the light irradiation. Thus, the molecular
design based on the photochromism of hexaarylbiimidazole
(HABI)3 would be a very promising candidate for the develop-
ment of a new family of photochromic compounds with unprece-
dented switching speeds, which could eventually evolve into solid
state photonic materials with unique photoresponsive characters.
Received: May 12, 2011
Revised:
September 19, 2011
Published: October 08, 2011
r
2011 American Chemical Society
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Scheme 1. Photochromic Reaction of
[2.2]Paracyclophane-Bridged Imidazole Dimers
1-Phenylethynylpyrene (1). 1-Bromopyrene (3.67 g, 13.1
mmol), ethynylbenzene (1.43 mL, 13.0 mmol), Pd(PPh3)4
(0.274 mg, 0.237 mmol), and copper iodide (0.306 mg, 1.61
mmol) were refluxed in Et3N (60 mL) for 48 h. After cooling to
room temperature, the reaction mixture was concentrated in
vacuo. The crude mixture was purified with silica gel column
chromatography using hexane/CHCl3 = 5/1 as eluent to give a
1
yellow powder (1), 3.85 g (12.7 mmol, 98%). H NMR (500
MHz, DMSO-d6), δ: 8.68 (d, J = 9.0 Hz, 1H), 8.25À8.19 (m,
4H), 8.15 (d, J = 6.0 Hz, 1H), 8.11 (d, J = 8.5 Hz, 1H), 8.07 (d, J =
10.0 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.74À7.73 (m, 2H),
7.54À7.52 (m, 1H), 7.46À7.40 (m, 3H). FABÀMS: m/z 302
[M + H]+.
1-(1-Pyrenyl)-2-phenylethane-1,2-dione (2). To a solution of
1 (4.03 g, 13.3 mmol), sodium hydrogen carbonate (1.30 g, 15.4
mmol), and tetraethylammonium bromide (2.80 g, 13.3 mmol)
in CH2Cl2 (100 mL) was added the potassium permanganate
(6.40 g, 40.5 mmol) in water, and the reaction mixture was
vigorously stirred for 40 h. Saturated solution of sodium
hydrogen sulfite in water was added to quench excess oxidant.
The reaction mixture was filtered, and the residue was washed
with CH2Cl2 and water. The organic layer was separated, dried
over Na2SO4, and concentrated in vacuo. The residue was
purified with silica gel column chromatography hexane/CHCl3
= 5/1 as eluent to give a orange powder (2), 2.48 g (7.43 mmol,
56%). 1H NMR (500 MHz, DMSO-d6), δ: 9.49 (d, J = 9.8 Hz,
1H), 8.61À8.24 (m, 8H), 8.07 (d, J = 7.9 Hz, 2H), 7.82 (t, J =
7.0 Hz, 1H), 7.66 (t, J = 7.0 Hz, 2H). FABÀMS: m/z 335
[M + H]+.
pseudogem-(4,5-Diphenyl-1H-imidazol-2-yl-4-pyrenyl-5-phe
nyl-1H-imidazol-2-yl)[2.2]paracyclophane (4). 3 (513 mg, 1.13
mmol), 2 (584 mg, 1.75 mmol), and ammonium acetate (957
mg, 12.4 mmol) were refluxed in acetic acid (5 mL) for 24 h.
After cooling to room temperature, the reaction mixture was
neutralized with aqueous NH3. The resulting precipitate was
dissolved CH2Cl2 and washed with water. The organic layer
was dried over Na2SO4, filtered, and concentrated in vacuo.
The crude mixture was purified with silica gel column chro-
matography using CH2Cl2 as eluent to give a yellow powder
780 mg (the mixture of structural isomers). This was used in
the next step without further purification. FABÀMS: m/z 769
[M+H]+.
photosensitivity was proved to be quite effective even for the
polymer systems carrying [2.2]PC-bridged imidazole dimer.2c
In this study, we propose another approach to enhance the
photosensitivity to UVA light based on the introduction of larger
aromatic groups as a sensitizing unit. As described in our previous
paper, the introduction of a pyrenyl group on the 2-position of
the imidazole ring of hexaarylbiimidazole was found to induce
the CT-like transition from the frontier molecular orbitals
delocalized over the pyrenyl moieties and Im1 to the LUMO
delocalized mainly in Im2 on the basis of the TDDFT
calculations.5 Moreover, the intense absorption band attributable
to the localized πÀπ* transition band of the pyrenyl moieties was
revealed to enhance the photosensitivity. Here we developed a
new type of [2.2]PC-bridged imidazole dimer introduced a
pyrenyl group on the 4-position of imidazole ring and investi-
gated the photochromic properties of the molecule.
pseudogem-PPI-DPI[2.2]PC. All manipulations were carried
out with the exclusion of light. Under nitrogen, to a solution of 4
(765 mg, 0.995 mmol) in benzene (40 mL) was added the
solution of potassium ferricyanide (8.24 g, 25.0 mmol) and KOH
(4.85 mg, 86.4 mmol) in water (50 mL), and the reaction mixture
was vigorously stirred for 30 min at room temperature. The
organic layer was separated, exhaustively washed with water, and
concentrated in vacuo. Then the crude mixture of the structural
isomers was purified with silica gel column chromatography
hexane/AcOEt = 3/2 as eluent. The separated pseudogem-PPI-
DPI[2.2]PC was recrystallized from CH2Cl2/hexane to give
yellow crystal, 90 mg (0.117 mmol, 12%). 1H NMR (500 MHz,
CDCl3), δ: 8.63 (d, J = 9.2 Hz, 1H) 8.12À8.08 (m, 2H), 8.02 (d,
J = 9.2 Hz, 1H), 8.00À7.90 (m, 3H), 7.86 (d, J = 7.9 Hz, 1H),
7.70 (d, J = 7.9 Hz, 1H), 7.47À7.40 (m, 3H), 7.36À7.31 (m, 3H),
7.27À7.26 (m, 2H), 7.20À7.17 (m, 3H), 7.11À7.07 (m, 3H),
6.91À6.82 (m, 3H), 6.71À6.82 (m, 3H), 6.71À6.64 (m, 2H),
6.56 (d, J = 7.3, 1H), 6.50À6.48 (m, 1H) 4.68À4.63 (m, 1H),
3.58À3.53 (m, 1H), 3.35À3.19 (m, 4H), 3.11À3.06 (m, 2H).
FABÀMS: m/z 767 [M + H]+.
2. EXPERIMENTAL SECTION
2.1. Synthesis. All reactions were monitored by thin-layer
chromatography using 0.2 mm E. Merck silica gel plates (60F-
254). Column chromatography was performed on silica gel
(Wakogel C-300). All reagents were purchased from TCI, Wako
Co. Ltd., Aldrich Chemical Co., Inc., and Acros Oraganics, and were
used without further purification. 1H NMR spectra were recorded at
500 MHz on a JEOL JMN-ECP500A spectrometer. Chemical shifts
are reported in parts per million (ppm) relative to tetramethylsilane,
and the coupling constants (J) are reported in hertz (Hz). FAB mass
spectra were measured with an MStation MS-700 (JEOL) spectro-
meter using 3-nitrobenzyl alcohol as a matrix.
1-Phenylethynylpyrene (1) and 1-(1-pyrenyl)-2-phenylethane-
1,2-dione (2) weresynthesized according to Scheme 2. pseudogem-
PPI-DPI[2.2] (5) was synthesized according to Scheme 2 using
pseudogem-[4-formyl-13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]-
paracyclophane (3) as a starting material. pseudogem-[4-Formyl-
13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]paracyclophane2b (3) was
prepared according to a literature procedure.
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Scheme 2. Synthetic Procedure of pseudogem-PPI-DPI[2.2]PC
2.2. X-ray Crystallographic Analysis. The diffraction data of
the single crystal were collected on the Bruker APEX II CCD area
detector (Mo Kα, 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 carried out by the
Bruker APEXII software package with SHELXT program.6 All
non-hydrogen atoms were anisotropically refined.
2.3. DFT Calculation. All calculations were carried out using the
Gaussian 03 program (Revision E.01).7 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 performed at the MPW1PW91/6-31
+G(d) level of the theory for the optimized geometries.
crystal X-ray crystallographic analysis (Figure 1). The CÀN
bond length connecting two imidazole rings (1.487(3) Å) is
approximately equal to that of pseudogem-bisDPI[2.2]PC
(1.4876(15) Å). The imidazole ring attached with the pyrenyl
moiety acts as an electron-donating unit. The pyrenyl moiety is
linked to the 4-position of the electron-donating Im1. The bond
distances of the pyrenyl moiety are almost equal to those of un-
substituted pyrene. In addition, we could not observe a signifi-
cant difference in the bond lengths in the [2.2]PC moiety, Im1,
Im2, and the phenyl rings of pseudogem-PPI-DPI[2.2]PC com-
pared with those of pseudogem-bisDPI[2.2]PC. Thus, it is
expected that the pyrenyl moiety is relatively weakly coupled
with other parts in the molecule.
3.2. UVÀVis Absorption Spectra. Figure 2 shows the
UVÀvis absorption spectra of pseudogem-PPI-DPI[2.2]PC and
pseudogem-bisDPI[2.2]PC in benzene at room temperature,
along with the calculated oscillator strength indicated by the
red perpendicular lines for pseudogem-PPI-DPI[2.2]PC. The
experimental absorption spectrum is in good agreement with
the theoretical one. Pseudogem-bisDPI[2.2]PC does not show a
distinct absorption band in the UVA light region, whereas
pseudogem-PPI-DPI[2.2]PC shows an intense absorption band
in the UVA light region. The S0 f S1 transition at 501 nm of
pseudogem-PPI-DPI[2.2]PC demonstrated by the TDDFT cal-
culation is attributable to the HOMO f LUMO transition. The
oscillator strength for this transition is almost zero (f = 0.0002)
due to the small orbital overlap between HOMO localized on
Im1 and LUMO localized on Im2. The S0 f S5 transition at
356 nm of pseudogem-PPI-DPI[2.2]PC can be described by the
HOMO f LUMO+1 transition, which is characterized by the
2.4. Measurement. Time-resolved visÀNIR absorption spec-
tra were recorded on a Unisoku TSP-1000 time-resolved spectro-
photometer. A Continuum Minilite II Nd:YAG (Q-switched)
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 MD2000). The steady state UVÀvis absorption
spectra were carried out with a Shimadzu UV3150 spectrometer.
3. RESULTS AND DISCUSSION
3.1. X-ray Crystallographic Analysis. The molecular struc-
ture of pseudogem-PPI-DPI[2.2]PC was determined by single
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Figure 2. UVÀvis absorption spectra for pseudogem-PPI-DPI[2.2]PC
and pseudogem-bisDPI[2.2]PC and TDDFT calculations for pseudogem-
PPI-DPI[2.2]PC. The calculated spectra (MPW1PW91/6-31+G(d)//
MPW1PW91/6-31G(d)) are shown by the red perpendicular lines.
Figure 1. ORTEP representation of the molecular structure of pseudo-
gem-PPI-DPI[2.2]PC with thermal ellipsoids (50% probability), where
nitrogen atoms are highlighted in blue. The hydrogen atoms are omitted
for clarity.
localized πÀπ* transition of the pyrenyl moiety (Figure 3). The
large orbital overlap between HOMO and LUMO+1 results in
the large oscillator strength (f = 0.6019). Indeed, the photo-
sensitivity to UVA radiation can be improved by displacing the
phenyl ring by the pyrenyl moiety comparing with that of the
parent compound, lacking the sensitizing group. Our previous
work demonstrated that the introduction of larger aromatic
groups, instead of phenyl rings, is an effective method to improve
the photochromic properties of HABI derivatives that have
higher sensitivity and an absorption band in the longer wave-
length region.5 The absorption tail in the UVÀvis absorption
spectrum of the HABI derivative with a pyrenyl unit on the
2-position of the imidazole rings (Py-HABI) is fairly long and it
extends even beyond 450 nm. This long absorption tail can be
assigned to the CT-like transition from the frontier molecular
orbital delocalized over the pyrenyl moiety and Im1 to the
LUMO delocalized mainly in Im2 on the basis of the TDDFT
calculations. In fact, Py-HABI shows coloration by the visible
light irradiation. The time-resolved transient absorption spec-
troscopy and fluorescence measurements showed that the pyr-
enyl unit influences the dissociation reaction rate by modifying
the surface crossing between the attractive potential surface of
the locally excited state of pyrenyl moiety and the repulsive
potential surface along the CÀN bond elongation axis. Though
the pyrenyl moiety is introduced on the 4-position of the
imidazole ring for pseudogem-PPI-DPI[2.2]PC, we consider that
the difference in the location of the pyrenyl group would not be
an essential matter. Thus, we considered that the photosensitivity
of pseudogem-PPI-DPI[2.2]PC would also be enhanced in a
similar manner as Py-HABI. Consequently, we concluded that
the introduction of a pyrenyl group is an effective method for
enhancing the photosensitivity to the UVA radiation region for
the photochromic [2.2]PC-bridged imidazole dimer.
Figure 3. Relevant molecular orbitals of pseudogem-PPI-DPI[2.2]PC
obtained at the MPW1PW91/6-31+G(d) level.
UV light irradiation and the photogenerated species immediately
turns back to the initial colorless state according to monoexpo-
nential kinetics when UV irradiation ceases. The transient visÀ
NIR absorption spectra of the colored species, pseudogem-PPIR-
DPIR[2.2]PC, of the parent imidazole dimer, pseudogem-PPI-
DPI[2.2]PC, are shown in Figure 4. A sharp absorption band
at 400 nm and a broad absorption band ranging from 500 to
3.3. Laser Flash Photolysis Measurement. The photochro-
mic property of pseudogem-PPI-DPI[2.2]PC was investigated by
laser flash photolysis measurement. The solution of pseudogem-
PPI-DPI[2.2]PC rapidly reaches the photostationary state by the
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Figure 5. Transient visÀNIR spectra for pseudogem-PPIR-DPIR[2.2]PC
and pseudogem-bisDPIR[2.2]PC and the theoretical spectrum by the
TDDFT calculations for pseudogem-PPIR-DPIR[2.2]PC. The calculated
spectra (UMPW1PW91/6-31+G(d)//UMPW1PW91/6-31G(d)) are
shown by the red perpendicular lines.
Figure 4. Transient visÀNIR absorption spectra of pseudogem-PPIR-
DPIR[2.2]PC in degassed benzene at 25 °C (2.1 Â 10À5 M, light path
length 10 mm). Each of the spectra was recorded at 2 ms intervals after
excitation with a nanosecond laser pulse (excitation wavelength,
355 nm; pulse width, 5 ns; power, 8 mJ/pulse).
1000 nm can be ascribed to the colored species. The broad
absorption band covers whole range of visible light region and its
absorbance is approximately equal throughout the visible region.
Thus, pseudogem-PPI-DPI[2.2]PC shows instantaneous colora-
tion from colorless to black upon UV light irradiation. Previously
reported [2.2]PC-bridged imidazole dimers show the color
change from colorless to blue or green due to the presence of a
valley around 500 nm in the transient visÀNIR absorption
spectra, whereas pseudogem-PPIR-DPIR[2.2]PC colors black in
benzene. It is worth noting that pseudogem-PPI-DPI[2.2]PC
colors black in a single component system. In our previous work,
we reported that the absorption band of pseudogem-bisDPIR-
[2.2]PC at longer wavelength regions is attributable to the
intramolecular radicalÀradical interaction.2f The introduction
of the pyrenyl moiety would affect the spin density distribution
and intramolecular radicalÀradical interaction. Indeed, pseudo-
gem-PPIR-DPIR[2.2]PC absorbs the longer-wavelength light
than pseudogem-bisDPIR[2.2]PC. The TDDFT calculation for
pseudogem-PPIR-DPIR[2.2]PC was carried out to explain this
red-shift in absorption spectrum as shown in Figure 5. The blue
and red curves indicate the experimental visÀNIR absorption
spectrum of pseudogem-bisDPIR[2.2]PC and pseudogem-PPIR-
DPIR[2.2]PC, respectively. The red perpendicular lines are the
theoretical oscillator strength for pseudogem-PPIR-DPIR-
[2.2]PC. The S0 f S1 transition at 1047 nm (f = 0.0071) and
the S0 f S2 transition at 936 nm (f = 0.0834) are described by the
αHOMO f αLUMO and βHOMO f βLUMO transitions of
pseudogem-PPIR-DPIR[2.2]PC. These transitions are character-
ized by intramolecular CT transition from the electron-donating
pyrenyl moiety to the 5π-electron imidazole ring.
Figure 6 shows decay profiles of the colored species pseudogem-
PPI-DPI[2.2]PC. The thermal bleaching process obeys the first-
order kinetics, and the half-life of the colored species is 12 ms in
benzene at 25 °C. The thermal bleaching rate of pseudogem-PPIR-
DPIR[2.2]PC is larger than that of pseudogem-bisTPIR[2.2]PC
(33 ms in benzene at 298 K), indicating that the introduction of
pyrenyl moiety can preserve the rapid thermal bleaching char-
acteristic to the [2.2]PC-bridged imidazole dimer. The decay
profiles were measured over the temperature range from 5 to
40 °C. The activation parameters for the thermal back-reaction
were estimated from the Eyring plots. The Eyring plots gave an
excellent straight line, and ΔH‡ and ΔS‡ values were estimated
from the standard least-squares analysis of the Eyring plots. The
Figure 6. Decay profiles of the colored species pseudogem-PPIR-DPIR-
[2.2]PC monitored at 400 nm in degassed benzene (2.1 Â 10À5 M, light
path length 10 mm). The measurements were performed in the temp-
erature range from 5 to 40 °C.
enthalpy of activation ΔH‡ is 55.0 kJ molÀ1, and the entropy of
activation ΔS‡ is À26.6 J molÀ1
K
À1, respectively. The free
energy barrier (ΔG‡ = ΔH‡ À T ΔS‡) at 25 °C is 63.0 kJ molÀ1
.
The acceleration of the radical recombination reaction rate
was explained by the Marcus theory in our previous paper.2b,d
According to the standard Marcus theory, an increase in the
change in Gibbs free energy, ΔG0, between the reactant and the
product leads to a decrease in the free energy of activation, ΔG‡.
We have considered that the radical recombination reaction is
accelerated by the destabilization of the colored species. Indeed,
the radical recombination reaction rate of the colored species of
pseudogem-DPI-PI[2.2]PC with a [2.2]PC moiety that couples
dipheylimidazole and phenanthroimidazole groups, is accelerated
about 1000 times compared with that of pseudogem-bisDPIR-
[2.2]PC. The steric repulsion between the phenanthroimidazole
and the phenyl rigns facing each other should destabilize the
colored species. Consequently, the radical recombination reac-
tion rate is accelerated as explained by the Marcus theory.
Because the pyrenyl moiety is also sterically bulky, we considered
that the above discussion for the acceleration for the back-
reaction of the colored species of pseudogem-DPI-PI[2.2]PC
can be applied to that of pseudogem-PPI-DPI[2.2]PC. To explain
the acceleration of the radical recombination reaction of pseudo-
gem-PPIR-DPIR[2.2]PC, the Gibbs free energy of pseudogem-
PPIR-DPIR[2.2]PC and pseudogem-bisDPIR[2.2]PC were the-
oretically investigated by the DFT calculation. Because the
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absolute free energy could not be compared due to the diffe-
rence in the number of electrons and nuclei between them, the
relative free energies calculated from the difference between the
free energy of the colored species and that of the imidazole
dimer were compared. As mentioned in the Experimental
Section, the molecular structures were fully optimized at the
MPW1PW91/6-31G(d) level of the theory, and the zero-point
energy (ZPE) correction was obtained at the same level of
theory. The relative free energy of pseudogem-PPIR-DPIR-
[2.2]PC was 51.3 kJ molÀ1, whereas that of pseudogem-
bisDPIR[2.2]PC was 23.2 kJ molÀ1. Therefore, the relative
free energy of pseudogem-PPIR-DPIR[2.2]PC is higher than
that of pseudogem-bisDPIR[2.2]PC. Thus, the radical recombi-
nation reaction rate of pseudogem-PPI-DPI[2.2]PC can be
expected larger than that of pseudogem-bisDPI[2.2]PC by
following the Marcus theory.
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4. CONCLUSIONS
We have demonstrated that the introduction of a pyrenyl
moiety is effective method for enhancing the photosensitivity to
UVA light of the photochromic [2.2]PC-bridged imidazole
dimer. The localized excitation of the pyrenyl moiety with large
oscillator strength (f = 0.6019) leads to an increase in the
photosensitivity to UVA light. In addition, the introduction of
a pyrenyl moiety would affect the absorption spectrum of the
colored species. The broad absorption band in the UVA radiation
region results in black colored species fading in a time constant of
12 ms at room temperature. Moreover, it is revealed that the
absorption band at longer wavelength region reaches more than
1000 nm due to the intramolecular CT from the pyrenyl moiety
to 5π-electron imidazole ring. Thus, the present study demon-
strated the advantage of the introduction of pyrenyl moiety on
the 4-position of imidazole ring of [2.2]PC-bridged imidazole
dimer. We believe that the diversity of the molecular design
makes this class of photochromic molecules highly attractive for
the applications including smart windows, solar protection lenses
and decorative objects.
’ ASSOCIATED CONTENT
Supporting Information. 1H-NMR spectra of pseudo-
S
b
gem-PPI-DPI[2.2]PC, HPLC chromatograms, X-ray crystallo-
graphic analysis data, kinetics for the thermal back-reaction,
details of the DFT calculations, optimized structures, and atomic
coordinates. CIF file. 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 partially supported by a Grant-in-Aid for
Scientific Research (A) (No. 22245025) 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 matching fund subsidy from MEXT, and by
the NAIST Advanced Research Partnership Project.
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dx.doi.org/10.1021/jp204440s |J. Phys. Chem. A 2011, 115, 13332–13337