A fast photochromic molecule that colors only under UV light

Article abstract of DOI:10.1021/ja810032t

We demonstrate that photochromism based on light-driven reversible bond cleavage can enable rapid coloration upon UV light irradiation and successive fast thermal bleaching within tens of milliseconds at room temperature. We have succeeded in developing a

Full text of DOI:10.1021/ja810032t

Published on Web 03/10/2009
A Fast Photochromic Molecule That Colors Only under UV Light
Yuta Kishimoto and Jiro Abe*
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin UniVersity, 5-10-1 Fuchinobe,
Sagamihara, Kanagawa 229-8558, Japan
Received December 24, 2008; E-mail: jiro_abe@chem.aoyama.ac.jp
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This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jacs.
Scheme 1. Photochromism of (a) HABI, (b)
1-NDPI-8-TPI-naphthalene and (c) pseudogem-BisDPI[2.2]PC
Photochromic materials are a well-known class of molecules that
change color upon irradiation with light; the photogenerated species
can be converted back to the initial species either thermally or by
subsequent irradiation with a specific wavelength of light. The main
interest in photochromic materials has been in their applications,
including optical waveguides and shutters, optical data storage, and
ophthalmic plastic lenses.1 There is also increasing interest in the
use of photochromic materials to modulate conductivity, fluores-
cence, magnetism, and shape at the bulk level.2 Increasing switching
rates, particularly the thermal bleaching rate, is indispensable for
certain applications, such as optical data processing and light
modulators. For application to real-time image processing at video
frame rates,3 the return to the initial state should be accomplished
within tens of milliseconds; however, the thermal back-reaction of
a colored species to its colorless form is generally on the time scale
of tens of seconds to hours, which precludes their practical use in
certain applications. In this work, we demonstrate that photo-
chromism based on light-driven reversible bond cleavage can enable
rapid coloration with UV light irradiation and successive fast
thermal bleaching within tens of milliseconds at room temperature.
Photochromic materials showing such intense photocoloration and
fast thermal bleaching performance could be promising materials
for possible fast light modulator applications.
Hexaarylbiimidazole (HABI), shown in Scheme 1a, was discov-
ered in the early 1960s by Hayashi and Maeda, and it has
subsequently attracted significant interest because of its unusual
physical properties.4 Various stimuli, such as heat, light, and
pressure, readily cleave HABI into a pair of 2,4,5-triphenylimida-
zolyl radicals (TPIRs), which thermally recombine to reproduce
their original imidazole dimer. The solution of HABI changes from
colorless to purple under UV irradiation. Indeed, this photochromic
behavior can be attributed to the photoinduced homolytic bond
cleavage of the C-N bond between the imidazole rings. The
dissociation of HABI into the geminate radical pair occurs with a
time constant of 80 fs along the repulsive potential energy surface
of the first excited singlet state.4i The thermal bleaching process,
however, is relatively slow. In organic solvents, the thermal
transformation of the TPIRs to HABI requires several minutes at
room temperature.
cannot diffuse into the medium to yield free radicals. A kinetic
study of the thermal back-reaction of the colored species showed
that the thermal bleaching process obeys first-order kinetics with a
half-life of 179 ms in benzene at 25 °C. This thermal bleaching
rate is fast enough to be utilized in ophthalmic lenses. Although
this development was noteworthy in that the thermal back-reaction
was drastically accelerated while the optical density in the colored
state was maintained, it was not satisfactory for application to real-
time image processing at video frame rates. We were able to make
a green spot move quickly and follow the movement of UV light
irradiation in solution, but an afterimage could be recognized for
1 s or less with the naked eye.4f
In order to increase the thermal bleaching rate, the photogenerated
radical pairs should be more closely spaced. Thus, we have designed
and synthesized
a
novel HABI derivative, pseudogem-
We recently developed a unique photochromic molecule, 1-NDPI-
8-TPI-naphthalene (Scheme 1b), that exhibits improved photochro-
mic performance in coloration and thermal bleaching rates as well
as a greater optical density in the colored state.4f 1-NDPI-8-TPI-
naphthalene is a HABI derivative containing a naphthalene moiety
that tethers two triarylimidazole units. 1-NDPI-8-TPI-naphthalene
cleaves photochemically into 1-NDPIR-8-TPIR-naphthalene in a
manner similar to HABI derivatives, and the solution changes from
colorless to green. However, unlike conventional HABI derivatives,
the photogenerated radical pair in 1-NDPIR-8-TPIR-naphthalene
bisDPI[2.2]PC, which has a paracyclophane (PC) moiety that tightly
couples two TPIR units (Scheme 1c). The molecular structure of
pseudogem-bisDPI[2.2]PC was determined by X-ray crystallo-
graphic analysis and is shown in Figure 1a, along with that of
1-NDPI-8-TPI-naphthalene in Figure 1b. The C-N bond length
connecting the two imidazole rings in pseudogem-bisDPI[2.2]PC
[1.4876(15) Å] is approximately equal to that in 1-NDPI-8-TPI-
naphthalene [1.488(2) Å]. pseudogem-BisDPI[2.2]PC undergoes a
photochromic reaction involving a color change from colorless to
blue upon UV irradiation both in the solid form and in solution at
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C O M M U N I C A T I O N S
Figure 1. Crystal structures of (a) pseudogem-bisDPI[2.2]PC and (b)
1-NDPI-8-TPI-naphthalene4f with thermal ellipsoids at 50% probability.
Hydrogen atoms and solvent molecules have been omitted, and nitrogen
atoms are highlighted in blue.
room temperature. Under continuous irradiation, the solution of
pseudogem-bisDPI[2.2]PC reaches photostationary equilibrium very
quickly, and after irradiation ceases, the absorption decreases very
rapidly according to monoexponential thermal bleaching kinetics.
It is worth noting that complete bleaching is achieved within 200
ms in benzene at 25 °C. Figure 2a shows transient vis-NIR
absorption spectra of pseudogem-bisDPI[2.2]PC 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 900 nm can be ascribed to the colored species,
pseudogem-bisDPIR[2.2]PC. All of the absorption bands decay with
the same time constant, indicating the presence of a single
conformation for the colored species. The half-life of the colored
species is 33 ms at 25 °C. Figure 2b shows the time profile of the
transient absorbance at 400 nm measured over the temperature range
from 5 to 40 °C. The thermal bleaching process obeys first-order
kinetics, and the half-life of the colored species varies from 198
ms at 5 °C to 10 ms at 40 °C, as shown in Figure 2c. The enthalpies
and entropies of activation (∆Hq and ∆Sq, respectively) for the
thermal back-reaction were estimated from an Eyring plot over the
temperature range from 5 to 40 °C. The Eyring plot produced an
excellent straight line (Figure S5 in the Supporting Information),
and the ∆Hq and ∆Sq values estimated from standard least-squares
analysis of the Eyring plot are 59.8 kJ mol-1 and -19.1 J K-1
mol-1, respectively. The free energy barrier (∆Gq ) ∆Hq - T∆Sq)
is 65.5 kJ mol-1 at 25 °C.
Fast thermal bleaching kinetics enables a solution to change color
only where the solution is irradiated with UV light, because the
thermal bleaching rate is much faster than the diffusion rate of the
colored species at room temperature (a movie showing this is
available). It should be noted that the afterimage seen in the
photochromic reaction of 1-NDPI-8-TPI-naphthalene is not visible
to the naked eye for pseudogem-bisDPI[2.2]PC system. In view of
the thermal bleaching rate, the fast photochromism of pseudogem-
bisDPI[2.2]PC is applicable to real-time image processing at video
frame rates. In addition, the photogeneration of paramagnetic radical
species attributable to the homolytic bond cleavage is one of the
important features of the photochromism of HABI derivatives. From
the viewpoint of spin state, the photochromic reaction of pseudo-
gem-bisDPI[2.2]PC can be regarded as a transformation from the
diamagnetic colorless state to a paramagnetic colored state. Thus,
in addition to the optical properties, the magnetic properties can
also be switched reversibly between the two states simply by turning
the optical stimulation on and off. The fast optical switching in
both color and spin states cannot be realized by any other currently
available photochromic system.
Figure 2. (a) Transient vis-NIR absorption spectra of pseudogem-
bisDPI[2.2]PC in degassed benzene at 25 °C (2.1 × 10-4 M, 10 mm light-
path length). 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). (b) Decay profiles of the colored
species generated from pseudogem-bisDPI[2.2]PC, monitored at 400 nm
in degassed benzene (1.5 × 10-4 M). The measurements were performed
over the temperature range from 5 to 40 °C. (c) Temperature dependence
of the half-life of the colored species in degassed benzene determined from
the measurements of the nanosecond laser flash photolysis experiment.
A homogeneous amorphous film of pseudogem-bisDPI[2.2]PC
can be readily prepared with concentrated solutions by spin-coating
or dip-coating (Figures S6-S8 in the Supporting Information). The
transparent amorphous film also shows fast thermal bleaching of
the photochromic reaction even at room temperature (Figure S9 in
the Supporting Information). Of course, the switching performance
of photochromic molecules within polymeric matrices has been an
important area of research in the development of optical solid-state
devices. Thus, we investigated the photochromic properties of a
poly(methyl methacrylate) (PMMA, MW ) 3.5 × 105) film doped
with 20 wt % pseudogem-bisDPI[2.2]PC. The dye-doped PMMA
film also undergoes the photochromic reaction, as does the
amorphous film. The transient absorption spectrum of the dye-doped
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C O M M U N I C A T I O N S
of the thermal bleaching rate is also important for recognition by
the human eye. It is difficult to detect a phenomenon that occurs
in less than 10 ms by the human eye, but the colored species derived
from pseudogem-bisDPI[2.2]PC has a half-life of tens of mil-
liseconds, which is favorable to detection by the human eye. Thus,
pseudogem-bisDPI[2.2]PC can potentially be applied to real-time
image processing at video frame rates. Our molecular design can
lead to the development of a new family of photochromic
compounds with unprecedented switching speeds and remarkable
stabilities, which could eventually evolve into solid-state photonic
materials with unique photoresponsive characteristics.
Acknowledgment. This work was supported by a Grant-in-Aid
for Science Research in a Priority Area “New Frontiers in
Photochromism (No. 471)” from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Figure 3. Time profiles of the transient absorbance at 400 nm of the PMMA
film doped with 20 wt % pseudogem-bisDPI[2.2]PC, measured at 25 °C:
(red O) freshly prepared sample; (blue 2) after irradiation with 10 000 laser
pulses (excitation wavelength, 355 nm; pulse width, 5 ns; power, 4 mJ/
pulse; repetition frequency, 1 Hz).
Note Added after Issue Publication. In the original version of this
article, which was first published on the Web on March 10, 2009, an
AVI video file was listed as part of the Supporting Information.
However, this video file is available as a Web-enhanced object in the
HTML version of the article and is not a part of the Supporting
Information. In the corrected version of the article, which was reposted
to the Web on January 26, 2010, the reference to this video file has
been removed from the Supporting Information Available paragraph.
PMMA film is almost identical to that of the solution (Figure S11
in the Supporting Information). While the decay of transient
absorbance at 400 nm does not obey simple first-order kinetics,
the half-life of the colored species is roughly estimated to be shorter
than 20 ms at 25 °C. This half-life is slightly shorter than that in
a benzene solution at the same temperature. Moreover, thousands
of switching cycles can be repeated consecutively without any sign
of degradation, even in the presence of molecular oxygen. The traces
in Figure 3 demonstrate the time profile of the transient absorbance
at 400 nm measured at 25 °C after irradiation with 10 000 laser
pulses, in order to evaluate the photochemical fatigue resistance.
The time profiles for the freshly prepared film and the film after
10 000 laser pulses are indistinguishable, indicating that pseudogem-
bisDPI[2.2]PC is indeed stable, making it attractive for practical
applications in optical solid-state devices. The excellent fatigue
resistance in PMMA matrices is particularly surprising in view of
the fact that the photogenerated radical of HABI is an excellent
hydrogen abstractor and has been used as an excellent initiator of
free-radical polymerization in combination with proper co-initiators,
such as hydrogen donors and efficient chain-transfer agents. The
remarkable stability of the colored biradical generated from the
photochromic reaction of pseudogem-bisDPI[2.2]PC can be at-
tributed to the inhibition of diffusion into the PMMA matrix and
the rapid geminate recombination of the nascent radical pair.
Actually, the photochemical reactions between photoactive sub-
stituents in the pseudogeminal positions in [2.2]paracyclophane were
reported by Hopf et al.5 in 1995.
Supporting Information Available: Synthesis of pseudogem-
bisDPI[2.2]PC, experimental details of the spectroscopic measurements,
Eyring plots, transient absorption spectra of the dye-doped PMMA film,
and crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
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In general, the faster the thermal bleaching rate is, the lighter is
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