Photochromism of diarylethene single molecules in polymer matrices

Article abstract of DOI:10.1021/ja069131b

Robust fluorescent photoswitching molecules, having perylene bisimide as the fluorescent unit and diarylethene as the switching unit, were prepared, and their photochromic reactions were measured at the single-molecule level in various polymer matrices. T

Full text of DOI:10.1021/ja069131b

Published on Web 04/14/2007
Photochromism of Diarylethene Single Molecules in Polymer
Matrices
Tuyoshi Fukaminato,† Tohru Umemoto,† Yasuhide Iwata,† Satoshi Yokojima,‡
Mitsuru Yoneyama,‡ Shinichiro Nakamura,‡ 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
Mitsubishi Chemical Group, Science and Technology Research Center, Inc. and CREST-JST,
1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
Received December 20, 2006; E-mail: iriem@rikkyo.ac.jp
Abstract: Robust fluorescent photoswitching molecules, having perylene bisimide as the fluorescent unit
and diarylethene as the switching unit, were prepared, and their photochromic reactions were measured
at the single-molecule level in various polymer matrices. The histograms of the fluorescent on and off
times were found to deviate from normal exponential distribution and showed a peak when the molecules
are embedded in rigid polymer matrices, such as Zeonex or poly(methyl methacrylate) (PMMA). In soft
polymer matrices, such as poly(n-buthyl methacrylate) (PnBMA), exponential distribution was observed
for the on and off times. The abnormal distribution suggests that the quantum yields of the photoreactions
are not constant and the molecules undergo the reactions after absorbing a certain number of photons. A
multilocal minima model was proposed to explain the environmental effect.
is useful not only to probe microenvironments15,20-26 but also
to characterize the photochemical reactivity of individual
molecules.27 The detailed analysis of the reactivity of each
molecule is indispensable to develop ultrahigh-density single-
Introduction
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† Kyushu University.
‡ Mitsubishi Chemical Group, Science and Technology Research Center,
Inc. and CREST-JST.
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J. AM. CHEM. SOC. 2007, 129, 5932-5938
10.1021/ja069131b CCC: $37.00 © 2007 American Chemical Society

Photochromism of Diarylethene Single Molecules
A R T I C L E S
Scheme 1 . Fluorescent Photochromic Molecule 1 and 2
molecule optical memory17,18,28,29 (each molecule stores 1 bit
of optical information) as well as photoswitching devices.
Although we recently reported on the on/off digital fluorescence
switching of single photochromic molecules embedded in a
polymer film by irradiation with UV and visible light,29 the
analysis of the reaction mechanism was incomplete because of
lack of photostability of the chromophores used. The number
of switching cycles was limited to a few cycles. The histograms
of the photochemical events shown in the papers29 were
constructed by collecting many molecules in different microen-
vironments. It is required to synthesize highly stable photo-
chromic molecules, which enable us to follow the photo-
chromism of the same molecule many times (>100 times) in
the same microenvironment.
Recently, we have succeeded to synthesize robust fluorescent
photochromic diarylethene derivatives 1 and 2 (Scheme 1),
which undergo efficient and stable photochromic reactions.30
The molecules have large absorption coefficients, high fluo-
rescence quantum yields, and photostability. The excellent
characteristic properties opened detailed discussions of photo-
chromic reactions at the single-molecule level. In this study,
we measured the photochromic reactions in various polymer
matrices and analyzed the reactivity at the single-molecule level.
We found a novel environmental effect in the photochromic
reactions of the single molecules.
Figure 1. (a) Absorption and (b) fluorescence spectra in dichloromethane
solution of 2 (C ) 6.6 × 10-6 M) upon 313 nm light irradiation: (s)
open-ring isomer, (‚ ‚ ‚ ‚ ‚) photostationary state, (- - - -) closed-ring
isomer.
photochromic molecule, which enables us to follow the pho-
tochromic reactions many times without decomposition. As
candidates of such molecules, we designed and synthesized
fluorescent diarylethene derivatives having perylene bisimide
as shown in Scheme 1.
The perylene bisimide unit was chosen as the fluorescent unit
because of its high fluorescence quantum yield (∼1.0), large
molecular extinction coefficient (88 000 M-1 cm-1 at λmax), and
photostability.31 The diarylethenes contain one (1a) or two (2a)
methoxy groups at the reactive carbons. The methoxy group
controls the cycloreversion quantum yield.32
Figure 1 shows absorption and fluorescence spectral changes
of 2 upon irradiation with UV light in dichloromethane.
Fluorescence intensity of both compounds (1 and 2) decreases
when the diarylethene unit converts from the open- to the closed-
ring isomers.30 In order to know the fluorescence quenching
mechanism,33 the fluorescence quantum yield of 1 was measured
in hexane, toluene, and ethyl acetate solutions. The fluorescence
quantum yields were determined to be 0.98, 0.96, and 0.97,
respectively, by utilizing N,N′-bis(1-hexylheptyl)perylene-3,4,9,-
10-tetracarboxyl bisimide in toluene as a reference (Φf ≈ 1.0).
The constant fluorescence quantum yield irrespective of solvent
polarity indicates that the contribution of electron transfer is
negligible in the quenching process.
The reversible fluorescence intensity change is ascribed to
the intramolecular energy transfer switching as shown in Figure
2. When the diarylethene unit is in the open-ring form, the
perylene bisimide unit gives strong fluorescence because the
energy state of the diarylethene unit is higher than the fluorescent
state. When the diarylethene unit converts to the closed-ring
Results and Discussion
Molecular Design of Fluorescent Photochromic Molecules.
In a previous paper,29 we prepared fluorescent photochromic
diarylethenes having a bis(phenylethynyl)anthracene unit. Al-
though the molecules were successfully used to demonstrate
photoswitching of fluorescence at the single-molecule level, the
fluorescent anthracene unit decomposed after a few cycles before
the decomposition of the diarylthene switching unit. For the
detailed analysis of the photochromic reactions at the single-
molecule level, it is required to develop a robust fluorescent
(31) Langhals, H. HelV. Chem. Acta 2005, 88, 1309-1343.
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4574-4578.
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Hampel, C.; Mu¨llen, K.; Verhoeven, J. W.; Van der Auweraer, M.; De
Schryver, F. C. J. Am. Chem. Soc. 2002, 124, 9918-9925. (b) Holman,
M. W.; Liu, R.; Adams, D. M. J. Am. Chem. Soc. 2003, 125, 12649-
12654.
(28) (a) Irie, M. Photo-ReactiVe Materials for Ultrahigh-Density Optical
Memory; Elsevier: Amsterdam, The Netherlands, 1994. (b) Irie, M. Chem.
ReV. 2000, 100, 1685.
(29) (a) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature 2002,
420, 759-760. (b) Fukaminato, T.; Sasaki, T.; Kawai, T.; Tamai, N.; Irie,
M. J. Am. Chem. Soc. 2004, 126, 14843-14849.
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J. AM. CHEM. SOC. VOL. 129, NO. 18, 2007 5933

A R T I C L E S
Fukaminato et al.
Figure 2. Fluorescence energy transfer switching based on photochromic reaction of diarylethene derivatives.
Table 1. Photocyclization (Φofc), Photocycloreversion (Φcfo) Quantum Yields of 1 and 2, and Fluorescence Properties of Compounds 1, 2,
and N,N′-Bis(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl Bisimide in Dichloromethane
a
compound
Φof
Φcf
λf,max/nm
Φf
τ/ns
c
o
1a
1b
2a
2b
0.26
-
-
489, 525
-
0.98
<0.001
0.97
<0.001
1.0
4.1
1.4 × 10-3 b
(4.5 ( 1.5) × 10-3
0.13
-
-
489, 526
-
3.9
d
4.0
4.1 × 10-5 c
N,N′-bis(1-hexylheptyl)-
-
-
490, 526
perylene-3,4,9,10-
tetracarboxyl bisimide
b
c
d
a Measured by irradiation with 343 nm light. Measured with 592 nm light. Measured with 622 nm light. Not determined.
form, the state becomes lower than the fluorescent state and
the fluorescence of the perylene bisimide unit is efficiently
quenched. The fluorescence lifetime measurement confirmed
this mechanism. The fluorescence lifetime of the open-ring form
was measured to be 4.1 ns. The lifetime decreased to less than
5 ps when the diarylethene unit converts to the closed-ring form,
as shown in Table 1. Very efficient intramolecular energy
transfer took place in the molecule. Similar fluorescence
switching was also observed for compound 2.
The cyclization/cycloreversion quantum yields of 1 and
2 were also measured and summarized in Table 1. The
cyclization quantum yield was determined by comparing the
photocyclization rates of samples and furyl-flugide with
the standard procedure.34 1a and 2a have similar cyclization
quantum yields, but the cycloreversion quantum yields are
markedly different. The cycloreversion quantum yield of 2b
is 1% of that of 1b. The decrease in the quantum yield is
attributed to the methoxy substituent effect at the reactive
carbons.32
Photochromism at the Single-Molecule Level. Typical
Figure 3. Fluorescence intensity trajectories of (a) 2 and (b) 1 embedded
on Zeonex thin films (∼100 nm) irradiated with both 488 nm light
(fixed power, 100 W/cm2) and weak 325 nm light (270 µW/cm2)
simultaneously. The long lasting on-state from 90 s is ascribed to the
fluorescence of perylene bisimide alone after the decomposition of the
diarylethene unit.
fluorescence intensity trajectories for 2a and 1a are shown in
Figure 3a and 3b, respectively. The sample for the single-
molecule measurement was prepared by spin-coating a toluene
solution of the dye (compound 1 or 2; around 10-11 M) and
polymer (Zeonex, PMMA, PnBMA, and Arton; around 2 wt
%) on a quartz cover glass at 4000 rpm. The closed-ring isomers
of the diarylethene derivatives (1b and 2b) were isolated by
HPLC and used as the dye. When the closed-ring isomers are
(34) (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.
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Photochromism of Diarylethene Single Molecules
A R T I C L E S
100 molecules, the average possible switching cycles of 1
in the same microenvironment were estimated to be
around 100.
In the above experiment amorphous polyolefine (Zeonex, Tg
≈ 130 °C) was used as the host polymer matrix. To know the
effect of environment on the photochromic reaction, two more
polymers, which have different Tg values, were examined. One
is poly(methyl methacrylate) (PMMA) having a Tg of
105 °C, and the other is poly(n-buthylmethacrylate) (PnBMA)
having a Tg of 21 °C. To reveal the detailed photochemical
reactivity of individual molecules in these polymer matrices,
the time traces of single molecule 1b were measured under
irradiation with both visible (488 nm, 100 W/cm2) and weak
UV (325 nm, 270 µW/cm2) light.
The fluorescence intensity switching between two discrete
states was observed in all polymer matrices. Figure 5 shows an
example of the on and off histograms observed for a single
molecule in Zeonex. The on and off states are clearly discrimi-
nated as shown in Figure 5b. Figure 5c and 5d show the
histograms of on and off times, respectively. The on as well as
off histogram showed a peak at a certain on or off time.
Although exponential distribution is anticipated from the simple
photochemical events, this was not the case. The abnormal
distribution of the histograms was observed in most of the
molecules (>80%), and the shapes were very similar among
the molecules. Any remarkable difference in the distribution
between the early events (0-100 s) and later events (100-200
s) was not observed.
The shapes of the histograms are dependent on the polymer
matrices as shown in Figure 6. In Zeonex and PMMA matrices,
the histograms show a peak at a certain on or off time. On the
other hand, in PnBMA, the on- and off-time histograms are
normal exponential shapes as shown in Figure 6b and 6e. In
PnBMA, with Tg being below or nearly equal to the measuring
temperature, the embedded molecules are considered not to be
strictly fixed and freely rotate in the polymer matrix. The result
suggests that the abnormal histogram is ascribed to the rigidity
of the polymer matrices. When the polymer matrices have high
Tg values and are rigid enough, the molecules showed the
abnormal histograms. This was further confirmed by using a
polymer (modified Arton-amorphous polyolefin) having ex-
tremely high Tg values (Tg ) 300 °C) as shown in Figure 6c
and 6f. The difference in the histograms will be discussed in
the following section.
Figure 4. Time traces of fluorescence intensity of 1 embedded in a Zeonex
thin film irradiated with both 488 nm light (fixed power, 100 W/cm2) and
325 nm light (variable power): (a) 270 µW/cm2, (b) 54 µW/cm2, (c) 27
µW/cm2.
dispersed in the polymer matrices, the conformation of the
molecules is fixed in photoactive antiparallel one, even after
conversion to the open-ring isomers. The antiparallel conforma-
tion is a necessary condition for the molecules to undergo the
photocyclization reaction.35 Both compounds showed one-step
photobleaching as is expected for a single molecule. The average
detected photon numbers of fluorescence before decomposition
were around 5 × 105, which indicates excellent photostability
of these molecules.
Environmental Effect of the Reactions. The appearance of
the peak in the histogram suggests that the quantum yield of
the molecule is not constant and increases with an increasing
number of absorbed photons. In other words, the molecule
remembers the number of absorbed photons and undergoes the
reaction after absorbing a certain number of photons. A possible
explanation for the abnormal histograms is given by the
multilocal minima model, as shown in Figure 7. The polymer
chains surrounding the diarylethene molecule work as the steric
hindrance. The effect of the steric hindrance will be reflected
on the potential energy surface as (i) deepening the potential
minima and creating the higher barriers and (ii) creating
additional local minima. These local minima may exist not only
for the ground state but also for the excited states. If the created
potential barriers are large enough, the thermally induced motion
to other local minima will be prevented and the optical excitation
The fluorescence switching was performed under irradiation
with both 488 nm (100 W/cm2) and 325 nm (270 µW/cm2)
light. Although the switching could be repeated more than a
few cycles for compound 2a, the cycle was limited to less
than 10 times because the cycloreversion quantum yield
of 2b is very small (Φcfo ) 4.1 × 10-5). On the other hand,
the cycles around 100 times were observed for compound 1a.30
The switching is not a physical blinking phenomenon. The
frequency was reduced upon decreasing the UV power from
270 µW/cm2 to 27 µW/cm2 as shown in Figure 4, which
indicates that the switching is due to the reversible photochromic
reactions. The switching cycles dramatically increased in
comparison with previous molecules. Based on the analysis of
(35) (a) Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63,
1311-1315. (b) Irie, M.; Miyatake, O.; Uchida, K. J. Am. Chem. Soc. 1992,
114, 8715-8716.
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J. AM. CHEM. SOC. VOL. 129, NO. 18, 2007 5935

A R T I C L E S
Fukaminato et al.
Figure 5. Analysis of single-molecule photochromism in a Zeonex thin film irradiated with both 488 nm light (fixed power, 100 W/cm2) and 325 nm light
(270 µW/cm2): (a) Time trace of fluorescence intensity of 1. (b) Histogram of fluorescence photoncounts during bin time (20 ms). The dashed line represented
the threshold value between on and off levels. (c) Histogram of on time. (d) Histogram of off time.
Figure 6. Histograms of (a-c) on time and (d-f) off time in PMMA (a, d), PnBMA (b, e), and Arton (c, f). The histograms were constructed from the time
trace of fluorescence intensity of 1 in polymer matrices irradiated with both 488 nm light (fixed power, 100 W/cm2) and 325 nm light (270 µW/cm2).
is required to go to other local minima. After the photoexcitation,
the open- or closed-ring isomer tries to switch to the closed- or
open-ring isomer, but the motion is prevented by a local
minimum in the excited state. Then, it may experience the
nonradiative decay and will be trapped by the local minimum
nearby. Thus, it requires many steps for the molecule to undergo
the reaction from open- (or closed-) to closed- (or open-)
isomers.
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Photochromism of Diarylethene Single Molecules
A R T I C L E S
Figure 8. Number of excitation dependence of the cyclization/cyclorever-
sion reactions. (a) The yield Yn(N) to induce the reaction for the N-th
excitation in the n-local minima system. (b) The probability Pn(N) to induce
the reaction for the first time after the N-th excitation in the n-local
minima system. The blue line with squares was for the two local minima,
which are open- and closed-ring isomers. The green line with triangles was
for the three local minima. The red line with circles was for the six local
minima.
Figure 7. A schematic diagram of the potential energy surface of
diarylethene (a) in the gas phase and (b) in the polymer matrix.
the exponential decay of Pn(N). This is normal behavior
observed in PnBMA. The yield for the N-th excitation is no
longer a constant for n > 2 (see Figure 8a). It starts from zero
and monotonically increases to approach pf as the number of
excitations increases. The larger the number of local minima n,
the slower the yield to approach pf. Figure 8b, which corresponds
to the experimental histograms for the on/off time, shows a peak
for n ) 3. The profiles for n ) 3-6 resemble the experimental
profiles of the on or off times. Another but similar explanation
is given by a model with a time-dependent potential energy
surface. For example, if the potential barrier in the excited-
state decreases whenever the diarylethene absorbs the photon,
we can obtain similar graphs as shown in Figure 8.
It is possible to reproduce the environmental effect from this
model as follows. We assume that there are n local minima
and the probability to go to the next local minimum after the
excitation is pf. The probability to stay on the same local
minimum after the excitation is 1 - pf. Then, the probability
Pn(N) to induce the reaction for the first time after the N-th
excitation can be written as
N-1Cn-2 pfn-1(1 - pf)N-n+1
(N g n - 1)
Pn(N) )
(1)
{
(N < n - 1)
0
The corresponding quantum yield Yn(N) for the N-th excitation
can be written as
Conclusions.
Pn(N)
Single-molecule photochromic reactions in various polymer
matrices were measured and analyzed by utilizing robust
fluorescent diarylethene derivatives. We found a novel envi-
ronmental effect in the photochromic reactions of the single-
molecules. In order to explain the effect, we proposed a multi-
local minima model. The multi-local minima model successfully
reproduced the novel effect.
Yn(N) )
N-1
1 -
P (i)
n
∑
i)1
N-1Cn-2 pnf -1
N-1Ci pfi(1 - pf)n-2-i
(N g n - 1)
(N < n - 1)
n-2
∑
)
(2)
Experimental Section
i)0
Materials. Detailed synthetic procedures of compound 1a
and 2a were described in the Supporting Information. The
molecules were purified by GPC and HPLC carefully. The
molecular structure was confirmed by 1H NMR, elemental
analysis, and mass spectroscopy, and purity was evaluated by
HPLC. The closed-ring isomers 1b and 2b were prepared by
irradiating the cyclohexane or toluene solutions with UV light
and purified by HPLC.
{
0
From eq 2, it is easy to show that limNf∞ Yn(N) ) pf. For pf )
0.5, we plot Yn(N) and Pn(N) (Figure 8). Here, we further assume
that the number of excitation N is the normalized number of
absorbed photons, which is approximately corresponding to the
on or off time. In Figure 8, n ) 2 corresponds to the system
where there is no other local minima except the open- and
closed-ring isomers. Thus, the quantum yield in Figure 8a shows
the constant value pf for n ) 2. This is reflected in Figure 8b as
Single-Molecule Detection. Samples for single-molecule
measurements were prepared by spin-coating 2 × 10-11
M
toluene solution of the closed-ring isomers (1b and 2b) isolated
9
J. AM. CHEM. SOC. VOL. 129, NO. 18, 2007 5937

A R T I C L E S
Fukaminato et al.
by HPLC on a quartz cover glass coating with each polymer
film (50-100 nm) at 4000 rpm.
Acknowledgment. This work was partly supported by a
Grant-In-Aid for Fundamental Research Program (S) (No.
15105006), Grant-in-Aid for Young Scientist (B) (No. 18750119),
Grant-in-Aid for Scientific Research on Priority Areas (432)
(No. 16072214) from the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT) of Japanese Government, and
also by CREST, JST.
A single-molecule experiment was carried out at ambient
temperature using a confocal microscope (TCS-NT, Leica),
equipped with a 100×, 1.4 NA objective lens (Leica PL APO
CS, NA 1.4). The sample was excited with circularly polarized
488 nm light from an argon ion laser (Spectra Physics Stabilite
2017), and the fluorescence was collected and guided through
the appropriate notch (Kaiser Optical) and long-pass (Chroma)
filters. A pinhole was placed in the detection path to decrease
the focal depth. The fluorescence passed through a long-pass
filter (>500 nm) was detected by using single-photon-counting
avalanche photodiode (APD) detector (Perkin-Elmer/EG & G,
AQR-14). The fluorescence intensity transients were measured
with a bin time of 20 ms. Very weak UV light (325 nm, 270
µW/cm2) from a He-Cd laser (KINMON ELECTRIC, IK3151R-
E) was used for the photocyclization reaction. All measurements
were performed under a nitrogen atmosphere.
Note Added after ASAP Publication. After this paper was
published ASAP April 14, 2007, a typographical error was
corrected in the lower index of the summation in the second
part of eq 2. The corrected version was published ASAP April
19, 2007.
Supporting Information Available: Detailed synthetic pro-
cedures for all compounds and the derivation of the equation
for multilocal minima model. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA069131B
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5938 J. AM. CHEM. SOC. VOL. 129, NO. 18, 2007