Digital photoswitching of fluorescence based on the photochromism of diarylethene derivatives at a single-molecule level

Article abstract of DOI:10.1021/ja047169n

Photochromic reactions of diarylethene derivatives were detected at a single-molecule level by using a fluorescence technique. Fluorescent photoswitching molecules in which photochromic diarylethene and fluorescent bis(phenylethynyl)anthracene units are l

Full text of DOI:10.1021/ja047169n

Published on Web 10/21/2004
Digital Photoswitching of Fluorescence Based on the
Photochromism of Diarylethene Derivatives at a
Single-Molecule Level
Tuyoshi Fukaminato,† Takatoshi Sasaki,† Tsuyoshi Kawai,† Naoto Tamai,‡ and
Masahiro Irie*,†
Contribution from the Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu UniVersity, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan, and
Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity,
Gakuen 2-1, Sanda 669-1337, Japan
Received May 14, 2004; E-mail: irie@cstf.kyushu-u.ac.jp
Abstract: Photochromic reactions of diarylethene derivatives were detected at a single-molecule level by
using a fluorescence technique. Fluorescent photoswitching molecules in which photochromic diarylethene
and fluorescent bis(phenylethynyl)anthracene units are linked through an adamantyl spacer were
synthesized, and switching of fluorescence upon irradiation with UV and visible light was followed in solution
as well as on polymer films at the single-molecule level. Although in solution the fluorescence intensity
gradually changed upon irradiation with UV and visible light, digital on/off switching between two discrete
states was observed at the single-molecule level. The “on”- and “off”-times were dependent on the power
of UV and visible light. When the power of UV and visible light was increased, the average on- and off-
times became short in proportion to the reciprocal power of the light. The response-times were found to
show distribution. The distribution of the on- and off-times is considered to reflect the difference in the
micro-environment as well as conformation of the molecules.
Introduction
the A/B ratio changes upon photoirradiation. Although such
analogue behavior is so far named “switching”, the performance
is not “switching” in the literal sense of the word.
If we can detect the photochromic reaction at a single-
molecule level, real digital photoswitching is anticipated. Recent
advances in fluorescence microscopy have allowed the detection,
imaging, and spectroscopy of single molecules at room
temperature.3-21 Although the single-molecule detection tech-
Photochromism is defined as a reversible transformation of
a chemical species induced in one or both directions by
absorption of electromagnetic radiation between two forms, A
and B, having different absorption spectra.1 Therefore, the
reaction is represented by the following scheme.
hν
A {hν′ or ∆} B
(3) Examples of reviews and special issues for single-molecule detection: (a)
Ambrose, W. P.; Goodwin, P. M.; Jett, J. H.; Orden, A. V.; Werner, J. H.;
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E.; Orrit, M.; Wild, U. P. Single-Molecule Optical Detection, Imaging and
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Keller, R. A. Single Molecule Detection in Solution: Methods and
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According to this reaction scheme, bulk molecular properties
are expected to switch between two discrete states upon
photoirradiation.2 In other words, the observed properties or data,
such as absorption intensity or refractive index, digitally switch
on and off between the two states. Yet, in bulk systems, where
average behavior is observed, this effect is masked by the
inherent averaging process. The experimental data gradually
change upon photoirradiation, because under normal conditions
there exists a vast number of molecules in the system and only
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† Kyushu University.
‡ Kwansei Gakuin University.
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J. AM. CHEM. SOC. 2004, 126, 14843-14849
14843

A R T I C L E S
Fukaminato et al.
nique is successfully applied to characterize the properties of
individual molecules in various micro-environments,22-31 the
direct observation of the photochromic reaction at the single-
molecule level has not yet been accomplished and single-
molecule photocontrol is limited.32-35 For the observation, it
is indispensable to prepare a photochromic molecule, which
changes the fluorescence intensity along with the photochromic
reaction and has high photostability. Green fluorescence protein
is the only example that exhibits the optically induced switching
of fluorescence at room temperature.19 There are no artificial
fluorescent photochromic molecules that can be applied to the
single-molecule experiment. It is required to newly design and
synthesize robust fluorescent photochromic or photoswitching
molecules. As the photoswitching unit, diarylethene derivatives
with heterocyclic aryl groups are the most promising candidates
because of their fatigue resistant photochromic performance.36
The derivatives can repeat photoinduced coloration/decoloration
cycles more than 104 times, and the photocolored isomers are
stable for more than a thousand years at 30 °C.37 When an
appropriate fluorescent unit is connected to the chromophores,
the photochromic reaction can be detected by the fluorescence
intensity change even at the single-molecule level.38
In this study, a new robust fluorescent diarylethene derivative
with one methoxy group at the reactive carbon has been
synthesized in addition to the previous derivative38a with two
methoxy groups, and their photoswitching performance was
studied in detail in solution (an ensemble system) as well as on
polymer films at the single-molecule level. At the single-
molecule level, digital switching between two distinct states was
observed, and the different behavior of the two derivatives
indicated that the switching of fluorescence is unambiguously
based on the photochromic reaction.
Results and Discussion
Molecular Design of Fluorescent Photochromic Molecules.
The molecules that we synthesized are shown in Scheme 1.
Photochromic diarylethene and fluorescent bis(phenylethynyl)-
anthracene units are covalently linked through a rigid adamantyl
spacer group. Methoxy substituents are introduced at the reactive
carbons to decrease the cycloreversion quantum yield.39 Two
derivatives having one or two methoxy groups were prepared.
Their photochromic performances are similar to each other
except the cycloreversion quantum yield. The effectiveness of
the methoxy substituents for the observation of the single-
molecule photoswitching will be discussed in detail in the last
part. The bis(phenylethynyl)anthracene unit with methoxy
substituents was employed as the fluorescent unit, because of
its high fluorescence quantum yield, appropriate fluorescence
spectral region, and fairly good fatigue resistant property.
1a converts to 1b upon irradiation with UV (300-350 nm)
light, and 1b returns to 1a upon irradiation with visible (>450
nm) light.39,40 The relative energy levels of the component
chromophores are also shown in Scheme 1. The bis(phenyl-
ethynyl)anthracene unit has characteristic absorption and fluo-
rescent bands at 488 and 503 nm, respectively.41 The fluores-
cence spectrum well overlaps the absorption spectrum of the
closed-ring form of the diarylethene unit. Therefore, the
fluorescence is efficiently quenched when the diarylethene unit
converts from the open- to the closed-ring forms. On the other
hand, when the diarylethene is in the open-ring form, its energy
level is higher than the level of the bis(phenylethynyl)anthracene
unit and the fluorescence quenching does not take place. The
detailed quantitative data are shown in the following section.
Fluorescence Change in Ensemble Solution upon Pho-
toirradiation. Figure 1a shows the absorption spectral change
of 1a in a toluene solution (an ensemble system) upon irradiation
with 313 nm light. A visible absorption band at around 630 nm
gradually increased and reached a photostationary state. The
absorption band is due to the closed-ring isomer 1b, and the
gradual increase is due to the photocyclization of the diaryl-
ethene unit.39,40 Figure 1b shows the fluorescence spectral
change along with the photocyclization. The initial fluorescence
quantum yield of 1a was 73%. The fluorescence intensity
gradually decreased in proportion to the conversion from 1a to
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Digital Photoswitching of Fluorescence
A R T I C L E S
Scheme 1
1b. At the conversion of 66%, the fluorescence decreased to
34% of the initial intensity. 1b was almost nonfluorescent. Upon
irradiation with >450 nm light, the fluorescence again gradually
returned to the initial intensity.
The fluorescence lifetimes and the quantum yields of 1a and
1b were measured and are shown in Table 1. The lifetime of
1a was 4.9 ns, while it decreased to 4.5 ps for 1b. The lifetime
as well as the fluorescence quantum yield of 1a are similar to
those of the bis(phenylethynyl)anthracene molecule.41 This result
confirms that the fluorescence is scarcely perturbed by the open-
ring form of the diarylethene unit. On the other hand, the very
short lifetime of 1b indicates that the excited-state energy of
the bis(phenylethynyl)anthracene unit is effectively intramo-
lecularly transferred to the closed-ring form of the diarylethene
unit. From the fluorescence lifetimes, the energy-transfer rate
was estimated to be 2.2 × 1011 s-1
.
The photocyclization quantum yield of 1a (0.12) is smaller
than that of the diarylethene molecule (0.52).42 This decrease
is ascribed to two factors. One factor is the overlapping of the
absorption band of the bis(phenylethynyl)anthracene unit at the
UV region (313 nm). Comparison of the absorption coefficients
of the two chromophores, bis(phenylethynyl)anthracene and
diarylethene units, indicates that 47% of the absorption of 1a
at 313 nm is due to the bis(phenylethynyl)anthracene unit. Only
a half of the absorbed photons by 1a can be used for the
photocyclization reaction. Another factor is the excited-state
energy transfer from the diarylethene unit to the bis(phenyl-
ethynyl)anthracene unit through the adamantyl spacer, because
the energy level of the bis(phenylethynyl)anthracene unit is
lower than that of the open-ring form of the diarylethene unit
(Scheme 1). The energy transfer is, however, much lower than
the efficiency from the excited bis(phenylethynyl)anthracene unit
to the closed-ring form of the diarylethene unit. This is ascribed
to the difference in the lifetimes of the two excited states. The
lifetime of the open-ring form of the diarylethene is around 1
ps,43 while the lifetime of the bis(phenylethynyl)anthracene is
Figure 1. (a) Absorption and (b) fluorescence spectral changes of 1 in
toluene upon irradiation with 313 nm light. The concentration of 1 in toluene
was 1.2 × 10-5 M.
(42) Kobatake, S.; Matsumoto, Y.; Irie, M., in preparation.
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Table 1. Photocyclization (Φofc), Photocycloreversion (Φcfo) Quantum Yields of 1 and 2, and Fluorescence Properties of Compounds 1, 2,
and 1,5-Dimethoxy-9,10-bis(phenylethynyl)anthracene in Toluene
a
compound
Φof
Φcf
λf,max/nm
Φf
τ/ns
c
o
1a
1b
2a
2b
0.12
0.12
503, 539
503, 537
501, 537
0.73
<0.001
0.73
4.9
<8 × 10-5 b
1.5 × 10-3 c
0.0045 ( 0.0015
4.9
1,5-dimethoxy-9,10-
0.72
5.0
bis(phenylethynyl)anthracene41
a Measured by irradiation with 313 nm light. b Measured with 630 nm light. c Measured with 600 nm light.
both the bis(phenylethynyl)anthracene unit and the closed-ring
form of the diarylethene unit. Interestingly, some of the spots
(2 and 3) appeared after irradiation with visible light for 5 s,
but others (1 and 4) appeared only after 10 s of irradiation.
Different molecules showed different response-times.
Among 11 bright spots, 7 spots did not show any response
to UV and visible light. These stable spots are considered to be
in a nonphotoactive state. Diarylethenes are well known to take
two kinds of conformations, parallel (mirror symmetry) and
antiparallel (C2 symmetry), and only the antiparallel conformer
can undergo the photocyclization reaction.44 The parallel
conformer is photochemically inactive. In the molecules that
give stable bright spots, the diarylethene unit is in a parallel
conformation or less probably in a photoinactive nonflexible
antiparallel conformation. The photochemical response can give
us information concerning the conformation of the diarylethene
unit.
Figure 2. Fluorescence intensity changes of 1 upon alternate irradiation
with 313 and >450 nm light in toluene. The concentration of 1 was 5 ×
10-7 M.
4.9 ns. The short lifetime scarcely suppresses the photocycliza-
tion reaction, and the long lifetime results in efficient fluores-
cence quenching. The photocycloreversion quantum yield of 1b
was very low, less than 10-4. The very low quantum yield is a
necessary condition to clearly detect the photoswitching per-
formance at the single-molecule level.
Figure 2 shows the typical analogue-type photoresponse of
1 in a toluene solution (an ensemble system). Upon irradiation
with 313 nm light, the fluorescence intensity gradually decreased
in 10 min, while it increased upon irradiation with >450 nm
light. It took about 5 h for 1b to return to 1a under the present
experimental conditions.
Photoswitching of Fluorescence Based on Photochromism
at the Single-Molecule Level. The photoresponse behavior of
1 dramatically changed at the single-molecule level. Compound
1 was dispersed on a Zeonex polymer film (thickness: ∼100
nm) by spin-coating a toluene solution containing 2 × 10-11
M of 1a, and the fluorescence change upon photoirradiation
was followed at the single-molecule level. The polymer film
was placed on the sample stage of a confocal microscope with
an oil immersion lens (100×, N.A. ) 1.4) equipped with an
avalanche photodiode detector (APD).
Figure 3 shows the images of the single molecules. All bright
spots are due to fluorescence from single 1a molecules. Upon
UV (325 nm, 0.27 mW) light irradiation for 5 s, some of the
spots, indicated by circles 1-4, disappeared, as shown in image
(b). The lost spots again appeared, as shown in images (c) and
(d), upon continuous irradiation with 488 nm light, which excite
This interpretation was confirmed by dispersing nonfluores-
cent closed-ring isomers 1b on the Zeonex film. Images (e-h)
of Figure 3 show the photoresponse of the polymer film.
Initially, the film is nonfluorescent, because all molecules are
in the nonfluorescent closed-ring form.45 Upon irradiation with
visible (488 nm, 200 W/cm2) light for 10 s, 12 molecules
became bright, as shown in image (f). These bright spots except
one spot (shown by a circle) disappeared upon irradiation with
UV light (325 nm, 0.27 mW/cm2) for 5 s as shown in image
(g). In this system, 11 molecules are in the photoactive
antiparallel conformation. Therefore, the molecules can undergo
reversible photocyclization reactions. The bright spots again
appeared upon irradiation with visible light. The dark/bright
cycles could be repeated upon alternate irradiation with visible
and UV light. The bright spots in image (g) disappeared possibly
due to photodecomposition.
To reveal the detailed switching characteristics of the
molecules, the time trace of single-molecule 1b was measured
under irradiation with both visible (488 nm, 100 W/cm2) and
UV (0.027 mW/cm2) light, as shown in Figure 4a. Initially, the
molecule was in the nonfluorescent off-state. At 5.6 and 21.0
s, the nonfluorescent molecule was abruptly switched to the
fluorescent on-state. These digital responses are due to photo-
isomerization from 1b to 1a by the visible light irradiation. At
15.7 and 30.1 s, the fluorescence molecule was abruptly
switched to the nonfluorescent off-state. These digital responses
are due to photoisomerization from 1a to 1b by UV light
irradiation. The fluorescence intensity switched on and off
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Digital Photoswitching of Fluorescence
A R T I C L E S
Figure 3. (a-d) Single-molecule fluorescence imaging of 1a embedded on a Zeonex thin polymer film. (a) Before irradiation. (b) After irradiation with UV
light (325 nm, 0.27 mW/cm2) for 5 s. After irradiation with visible light (488 nm, 200 W/cm2) for (c) 5 s, and (d) an additional 5 s. (e-h) Single-molecule
fluorescence imaging of 1b embedded on a Zeonex thin polymer film. (e) Before irradiation. (f) After irradiation with visible light (488 nm, 200 W/cm2)
for 10 s. (g) After irradiation with UV light (325 nm, 0.27 mW/cm2) for 5 s. (h) After irradiation with visible light (488 nm, 200 W/cm2) for 10 s: imaging
size, 10 µm × 10 µm (64 pixels × 64 pixels); integration time, 100 ms/pixel, N2 atmosphere. The distorted images are due to the low resolution of the
measuring system. The number of pixels is limited to 64 × 64.
Figure 4. (a-c) Time traces of fluorescence intensity of 1 embedded on Zeonex thin films irradiated with both strong 488 nm light (fixed power,
100 W/cm2) and weak 325 nm light (variable power). (d-f) Histograms of on-time. The histograms were constructed by collecting on-times of 1-3 events
for each molecule. Power of 325 nm light: (a,d) 0.027 mW/cm2; (b,e) 0.054 mW/cm2; (c,f) 0.27 mW/cm2. Average on-time: (d) 11 s (73 molecules);
(e) 5.7 s (94 molecules); (f) 1.2 s (80 molecules).
between two discrete states based on the photochromic reaction
of the diarylethene unit.
photocycloreversion reaction is mainly controlled by the visible
light. On the other hand, when the unit is in the open-ring form
or the molecule is in the on-state, the weak UV light is effective
at inducing the photocyclization reaction. The photocyclization
quantum yield is around 10%, which means that only 10 photons
are enough to induce the photocyclization reaction or switch
from the on-state to the off-state. The single-molecule switch
is very sensitive to UV light.
The on-time was dependent on the power of the UV light.
When the power was increased, the duration of the on-time
became short. As can be seen in Figure 4a-c, the on-time
decreased in proportion to the power of the UV light. In this
experiment, UV light is irradiated from the beginning. When
the diarylethene unit is in the closed-ring form or the molecule
is in the off-state, UV light scarcely affects the photocyclo-
reversion reaction, because the UV power is very weak in
comparison with the power of visible light (around 1/106). The
To obtain the details of the digital photoresponsive behavior
of individual molecules, we measured the on-time for 50-100
molecules by changing the power of the UV light, as shown in
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A R T I C L E S
Fukaminato et al.
Figure 5. (a-c) Time traces of fluorescence intensity of 1 embedded in Zeonex thin films irradiated with both strong 488 nm light (variable power) and
weak 325 nm light (fixed power, 0.054 mW/cm2). (d-f) Histograms of off-time. The histograms were constructed by collecting off-times of 1-3 events for
each molecule. Power of 488 nm light: (a,d) 50 W/cm2; (b,e) 100 W/cm2; (c,f) 300 W/cm2. Average off-time: (d) 23 s (81 molecules); (e) 11 s
(57 molecules); (f) 4.0 s (107 molecules).
Figure 4d-f. The average on-time correlated to the reciprocal
power of the UV light. When the power was increased from
0.027 to 0.27 mW/cm2, the average on-time decreased from 11
to 1.2 s. The distribution of the on-time is considered to reflect
the difference in micro-environments as well as the conformation
of the molecules.
The off-time was also examined by changing the power of
visible (488 nm) light, as shown in Figure 5. The off-time, or
the interval between adjacent on-states, decreased with an
increase in the power. It decreased from 23 to 4.0 s by increasing
the power from 50 to 300 W/cm2.
Photoswitching of Fluorescence of a Molecule with One
Methoxy Substituent at the Reaction Site of the Diarylethene
Unit. Figure 6 shows the fluorescence switching of compound
2 dispersed on the Zeonex film, just as the previous experiment
used for compound 1, at the single-molecule level. The triplet
blinking-like decrease of fluorescence10,17,46 is due to the
photochromic reaction from fluorescent 2a to nonfluorescent
2b. This was confirmed by changing the power of the UV light.
When the power was decreased from 0.27 to 0.027 mW/cm2,
the frequency of the fluorescence switching dramatically
decreased. The decrease in the frequency indicates that the
switching is not due to the triplet blinking but due to the
photochromic reaction.
The molecule 2b has a photocycloreversion quantum yield
of 1.5 × 10-3, which is more than 20-50 times larger than
that of 1b. The average off-time of the fluorescence for
compound 2 is 220 ms under irradiation with visible light (488
nm, 100 W/cm2), while it is around 11 s for compound 1 under
irradiation with the same light intensity. The decrease of the
off-time is ascribable to the change of the cycloreversion
Figure 6. Time traces of fluorescence intensity of 2 embedded on a Zeonex
(46) Vosch, T.; Hofkens, J.; Cotlet, M.; Ko¨hn, F.; Fujiwara, H.; Gronheid, R.;
thin film irradiated with both strong 488 nm light (fixed power, 100 W/cm2)
Van Der Biest, K.; Weil, T.; Herrmann, A.; Mu¨llen, K.; Mukamel, S.; Van
and weak 325 nm light (variable power): (a) 0.27, (b) 0.054, (c) 0.027
mW/cm2.
der Auweraer, M.; De Schryver, F. C. Angew. Chem. Int. Ed. 2001, 40,
4643-4648.
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Digital Photoswitching of Fluorescence
A R T I C L E S
model 360-80, 25D, and 305). A microchannel-plate photomultiplier
(Hamamatsu, MCP R2809U) was used as a detector, which gave an
instrument response-time constant of ∼30 ps (fwhm). The emission
wavelength of 510 nm was selected by a monochromator. The
fluorescence decay curves were analyzed by the nonlinear least-squares
iterative convolution method based on a Marquardt algorithm. For the
nanosecond resolution measurements, a pulsed N2 laser (Hamamatsu
Photonics, LN-203, 337.1 nm, 600 ps) was used as a light source, and
the emission was detected by a gated streak-scope system (Hamamatsu
Photonics, C4334-1, C5094, and C-4792) in the single-photon counting
mode.
quantum yield. In other words, the fluorescence switching is
unambiguously based on the photochromic reaction. When the
bleaching quantum yield is larger than 10-2, as observed in
normal photochromic compounds, the switching phenomena are
hardly detected. The very low photocycloreversion quantum
yield is a necessary condition to clearly detect the digital
fluorescence switching between two discrete states due to the
photochromic reaction.
Conclusions
Fluorescent photoswitching molecules in which photochromic
diarylethene and fluorescent bis(phenylethynyl)anthracene units
are covalently linked through an adamantyl spacer were
synthesized, and the fluorescence switching of the diarylethene
derivatives was observed by irradiation with UV and visible
light at the single-molecule level. Digital on/off switching
between two discrete states was observed on polymer films at
the single-molecule level. This provides unequivocal evidence
that the photochromic reaction takes place between two discrete
states. The fluorescent diarylethene derivatives can be potentially
utilized for ultrahigh-density erasable optical data storage
(1 bit per molecule, ∼peta bit/in2), when novel techniques to
address each molecule by photon are developed.
Materials. Detailed synthetic procedures of compounds 1a and 2a
are described in the Supporting Information. The molecules were
carefully purified by GPC and HPLC. The molecular structure was
1
confirmed via H NMR, elemental analysis, and mass spectroscopy,
and the purity was evaluated by HPLC. The closed-ring isomers 1b
and 2b were prepared by irradiating the hexane or toluene solutions
with UV light and were isolated by HPLC.
Single-Molecule Detection. Samples for single-molecule measure-
ments were prepared by spin-coating a 2 × 10-11 M toluene solution
of open-ring isomers (1a and 2a) and closed-ring isomers (1b and 2b)
isolated by HPLC on a quartz cover glass coating with a Zeonex film
(50-100 nm) at 4000 rpm. Zeonex is a kind of amorphous polyolefin
of Nippon Zeon Co. Ltd.
The single-molecule fluorescence was detected using a confocal
microscope (TCS-NT, Leica) with an oil immersion lens (Leica PL
APO CS, NA 1.4) equipped with an avalanche photodiode (APD) in
single-photon counting mode (EG&G, SPCM AQR-14) as the detector.
Appropriate notch (Kaiser Optical) and long-pass (Chroma) filters were
placed in the detection path to suppress remaining excitation light and
detect the fluorescence longer than 500 nm. The fluorescence intensity
transients were measured with a bin time of 20 ms or 10 ms. The
excitation source was an argon ion laser at 488 nm (Spectra Physics
Stabilite 2017). A He-Cd laser at 325 nm (KINMON ELCTRIC,
IK3151R-E) was used for the photocyclization. Circular polarized light
was used as the excitation and reaction light for equally exciting
molecules. All single-molecule experiments were carried out under
nitrogen atmosphere.
Experimental Section
General. Solvents used in the photochemical measurements were
spectroscopic grade and were purified by distillation before use.
Steady-state absorption and fluorescence spectra in solution were
measured with an absorption spectrophotometer (Hitachi U-3500) and
a fluorescence spectrophotometer (Hitachi F-3010), respectively. Pho-
toirradiation was carried out using a 1000 W high-pressure mercury
lamp or a 500 W xenon lamp as the light source. Monochromic light
was obtained by passing the light through a monochromator (Ritsu
MV-10N) or a band-pass filter (∆λ1/2 ) 15 nm).
The cyclization quantum yield was determined by comparing the
photocyclization rates of samples and furyl-fulgide with the standard
procedure.47 The cycloreversion quantum yield was also measured using
furyl-fulgide in toluene as a reference.47 The fluorescence quantum yield
(Φf) was evaluated utilizing 9,10-bis(phenylethynyl)anthracene as the
standard of Φf ) 1.0.48
Fluorescence decay curves with the picosecond time resolution were
measured by the time-correlated single-photon counting spectroscopy.
A second harmonic (420 nm, pulse width ∼200 fs) of a hybrid mode-
locked dispersion compensated femtosecond dye laser (Coherent Satori
774) pumped by a CW mode-locked Nd:YAG laser (Coherent Antares
76S) was used as an excitation source. A repetition rate of the excitation
pulse was reduced to 3.8 MHz with an external pulse picker (Conoptics,
Acknowledgment. This work was partly supported by a
Grant-In-Aid for Fundamental Research Program (S) (No.
15105006) and also by a Grant-In-Aid for The 21st Century
COE Program, “Functional Innovation of Molecular Informat-
ics”, from the Ministry of Education, Culture, Science, Sports,
and Technology of Japan. We thank Nippon Zeon Co., Ltd. for
the supply of octafluorocyclopentene. T.F. is thankful for the
Research Fellowships of the Japan Society for the Promotion
of Science for Young Scientists.
(47) (a) Yokoyama, Y.; Kurita, Y. J. Synth. Org. Chem. Jpn. 1991, 49, 364-
372. (b) Hellar, H. G.; Langan, J. R. J. Chem. Soc., Perkin Trans. 2 1981,
341-343.
(48) (a) Berlman, B. Handbook of Fluorescence Spectra of Aromatic Molecules;
Academic: New York and London, 1971. (b) Haugland, R. P. Handbook
of Fluorescence Probes and Research Chemicals, 6th ed.; Molecular
Probes: Eugene, 1996.
Supporting Information Available: Detailed synthetic pro-
cedures. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA047169N
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