Solvent viscosity effects on photochromic reactions of a diarylethene derivative as revealed by picosecond laser spectroscopy

Article abstract of DOI:10.1021/jp0206626

The solvent viscosity effect on photochromic reaction processes such as cyclization and cycloreversion of a diarylethene derivative, 2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA), was investigated by means of steady-state spectroscopy, picosecond transient absorption, and fluorescence measurements, The cyclization reaction yields from the excited open-ring isomer decreased with an increase in solvent viscosity. Picosecond transient absorption spectroscopy revealed that the cyclization reaction took place via two pathways; very rapid reaction immediately after the excitation in competition with the relaxation to the fluorescent state and the rather slow reaction from the relaxed fluorescent state with the time constant of several hundred picoseconds. The latter slow process was strongly dependent on the solvent viscosity, whereas the former rapid process was almost independent of the viscosity. This viscosity dependence of the reaction from the fluorescent state was attributed to the solvent molecular friction to the rotation of the thienyl moieties leading to the favorable geometry for the cyclization. For the cycloreversion reaction, much smaller solvent viscosity dependence was observed and reaction time constants less than 1-2 ps were estimated by transient absorption spectroscopy. By integrating these experimental results with the potential energy surfaces predicted from theoretical investigation, the mechanism of the photochromic reaction of TMTMA was discussed.

Full text of DOI:10.1021/jp0206626

8096
J. Phys. Chem. A 2002, 106, 8096-8102
Solvent Viscosity Effects on Photochromic Reactions of a Diarylethene Derivative As
Revealed by Picosecond Laser Spectroscopy
Hiroshi Miyasaka,*,† Takahiro Nobuto,‡ Masataka Murakami,† Akira Itaya,‡
Naoto Tamai,§ and Masahiro Irie*,|
Department of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, and CREST, Toyonaka,
Osaka 560-8531, Japan, Department of Polymer Science and Engineering, Kyoto Institute of Technology, and
CREST, Matsugasaki, Sakyo, Kyoto 606-8585, Japan, Department of Chemistry, Kwansei Gakuin UniVersity,
Gakuen, Sanda 669-1337, Japan, and Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu UniVersity, and CREST, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8531, Japan
ReceiVed: March 7, 2002; In Final Form: June 4, 2002
The solvent viscosity effect on photochromic reaction processes such as cyclization and cycloreversion of a
diarylethene derivative, 2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA), was investigated by means
of steady-state spectroscopy, picosecond transient absorption, and fluorescence measurements. The cyclization
reaction yields from the excited open-ring isomer decreased with an increase in solvent viscosity. Picosecond
transient absorption spectroscopy revealed that the cyclization reaction took place via two pathways; very
rapid reaction immediately after the excitation in competition with the relaxation to the fluorescent state and
the rather slow reaction from the relaxed fluorescent state with the time constant of several hundred picoseconds.
The latter slow process was strongly dependent on the solvent viscosity, whereas the former rapid process
was almost independent of the viscosity. This viscosity dependence of the reaction from the fluorescent state
was attributed to the solvent molecular friction to the rotation of the thienyl moieties leading to the favorable
geometry for the cyclization. For the cycloreversion reaction, much smaller solvent viscosity dependence
was observed and reaction time constants less than 1-2 ps were estimated by transient absorption spectroscopy.
By integrating these experimental results with the potential energy surfaces predicted from theoretical
investigation, the mechanism of the photochromic reaction of TMTMA was discussed.
Introduction
1,2-Bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA)
is one of the diarylethene derivatives fulfilling several of these
excellent properties. This compound undergoes the following
cyclization and cycloreversion photochromic reactions (Scheme
1). The Stokes shift between the absorption and fluorescence
of the open-ring isomer of TMTMA is rather large and solvent
dependent.19 These fluorescence properties indicated some
conformational rearrangement for the relaxation process in the
S1 potential surface to produce the fluorescent state with a large
dipole moment. Because the photocyclization yield was found
to decrease with an increase in the solvent polarity,19 it was
suggested that the twisted conformation with the intramolecular
CT character was responsible for the fluorescence state, which
led to the decreased chance for the cyclization. Our previous
investigation by means of picosecond transient measurement,11
however, revealed that the photocyclization reaction of TMTMA
in solutions had a very rapid reaction channel with the time
constant of e10 ps, which was much shorter than the fluores-
cence lifetime. To elucidate the contribution of cyclization
reaction in the fluorescent state, we have investigated the solvent
viscosity effect on the photochromic reaction processes. On the
basis of the reaction yields of photochromic processes, pico-
second transient absorption, and time-resolved fluorescent
measurements, we will discuss the reaction profiles of TMTMA
in the following.
Photochromism is a photoinduced reversible isomerization
in a chemical species between two forms. The instant property
change arising from the chemical-bond reconstruction via
photoexcitation has been attracting much attention1-10 not only
from the viewpoint of the fundamental chemical reaction
processes but also from the viewpoint of the application to
optoelectroinic devices such as rewritable optical memory and
switches.
Among various photochromic molecules, diarylethenes with
heterocyclic rings have been developed as a new type of
thermally stable and fatigue-resistant photochromic compound.
Some of these molecules have no thermochromicity even at 150
°C, the colored forms are estimated to be stable for more than
1900 years at 30 °C, and the cyclization and cycloreversion
cycles can be repeated more than 104 times while adequate
photochromic performance is maintained.3-10 In addition, the
photochromic reaction can take place even in the crystalline
phase.5,10 Direct detection of the reaction dynamics11-17 revealed
that most of these photochromic reactions take place in the time
region e10 ps and the multiphoton-gated reaction pathway was
recently found, which may lead to the realization of the
nondestructive readout capability.18
* Corresponding authors. E-mail: miyasaka@chem.es.osaka-u.ac.jp,
irie@cstf.kyushu-u.ac.jp. Fax: +81-(0)6-6850-6244, +81-(0)92-642-3568.
† Osaka University.
Experimental Section
‡ Kyoto Institute of Technology.
A picosecond laser photolysis system with a repetitive mode-
locked Nd3+:YAG laser was used for transient absorption
§ Kwansei Gakuin University.
| Kyushu University.
10.1021/jp0206626 CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/09/2002

Photochromic Reactions of a Diarylethene Derivative
J. Phys. Chem. A, Vol. 106, No. 35, 2002 8097
SCHEME 1
spectral measurements.20 The third harmonic pulse (355 nm)
or with 15 ps fwhm and 0.5 mJ output power or the second
harmonic (532 nm) with 15 ps fwhm and 0.5-1 mJ were used
for excitation. The excitation pulse is focused into a spot with
a diameter of ca. 0.15 cm. Picosecond white continuum
generated by focusing a fundamental pulse into a 10 cm quartz
cell containing D2O and H2O mixture (3:1) was employed as a
monitoring light. A sample cell with 1 cm length was used in
the measurement with the excitation at 355 nm, whereas a 0.2
cm cell was employed for the 532 nm excitation. The repetition
rate of the laser excitation was kept low (<0.2 Hz), and the
solution sample was circulated. Steady-state fluorescence and
absorption spectra were respectively recorded by a Hitachi
F-4500 fluorometer and a Shimadzu MPS-2000 spectropho-
tometer.
Fluorescence decay curves were measured by single photon
timing spectroscopy.21 A second harmonics (∼360 nm, pulse
width ∼200 fs) of a hybridly mode-locked, dispersion-
compensated femtosecond dye laser (Coherent Satori 774)
pumped by a continuous-wave (cw) mode-locked Nd3+: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, Model 360-80, 25D,
and 305). A microchannel-plate photomultiplier (Hamamatsu,
MCP R2809U) was used as a detector, which gave an instrument
response function of ∼30 ps (fwhm). The fluorescence decay
curves were analyzed by a nonlinear least-squares iterative
convolution method based on a Marquardt algorithm.
1,2-Bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA)
was synthesized and purified as reported previously.19 Isooctane,
cyclohexane, and n-hexane (Dotite, Spectrosol) were used as
received. cis- and trans-decalins (Wako Special Guarantee) were
purified by passing through a column of silica gel (Wako
Wakogel 200). Samples (ca. 10-4 M) were deaerated by
irrigating with N2 gas. All the measurements were performed
at 22 ( 2 °C.
Figure 1. (a) Ground-state absorption spectra of the open-ring isomer
(solid line) and the photostationary state under the exposure at 396 nm
(dotted line) of TMTMA in isooctane. The closed-ring isomer of
TMTMA has absorption maxima at 375 and 552 nm. (b) Fluorescence
spectrum of the open-ring isomer of TMTMA in isooctane excited at
355 nm (solid line) and the fluorescence excitation spectrum (dotted
line) monitored at 490 nm fluorescence.
TABLE 1: Steady-State Absorption and Fluorescence
Maxima in Alkane Solutions
absorption
λ
max/nm
fluorescence
solvent
open-isomer
closed-isomer
λ
max/nm
n-hexane
isooctane
cyclohexane
trans-decalin
cis-decalin
331
332
333
334
334
552
552
552
554
552
490
490
491
494
494
1 was also observed in other alkane solutions. The spectral band
shape and their absorption maxima in these alkane solutions
were almost identical with those in n-hexane solution, as listed
in Table 1.
Figure 1b shows the fluorescence spectrum of the open-ring
isomer in isooctane solution (solid line) excited at 355 nm. The
fluorescence spectral band shape and its maximum was also
independent of alkane solvents, as listed in Table 1, indicating
that the structure of the relaxed fluorescent state was similar
among these solvents. The excitation spectrum for 490 nm
fluorescence (dotted line) was confirmed to be almost the same
with the absorption spectrum. No fluorescence from the closed-
ring isomer was detected.
Results and Discussion
Steady-State Absorption and Fluorescence Spectra. Figure
1a shows ground state absorption spectra of TMTMA in
isooctane solution. The solid line is the spectrum of the open-
ring isomer of TMTMA. The light exposure at UV light changed
the color of the solution from yellow to red. The dotted line
corresponds to the spectrum at the photostationary state under
the exposure at 396 nm. The broad absorption in the visible
region with the absorption maximum at 552 nm was safely
ascribed to the closed-ring isomer because the absorption
maxima and the spectral band shape were almost the same as
those in n-hexane solution.11,19 In addition, the cycloreversion
reaction leading to the open-ring isomer took place under the
light exposure in the visible region. An absorption change due
to the photochromic reactions similar to that shown in Scheme
Photochromic Reaction Yields. The reaction yields of the
cyclization (O to C) and cycloreversion (C to O) were obtained
from the time dependence of the spectral change under the
steady-state light irradiation. The light at 546 nm was employed

8098 J. Phys. Chem. A, Vol. 106, No. 35, 2002
Miyasaka et al.
TABLE 2: Photochromic Reaction Yields of TMTMA in
Alkane Solution at 22 °C
reaction yield
cyclization
(O f C)
cycloreversion
solvent
viscosity/cPa
(C f O)
n-hexane
isooctane
cyclohexane
trans-decalin
cis-decalin
0.313
0.504
0.980
2.128
3.381
0.13
0.10
0.086
0.076
0.072
0.160
0.153
0.140
0.136
0.131
Figure 3. Difference spectrum between the transient absorption at 40
ps and that at 0 ps after the excitation of the open-ring isomer TMTMA
in trans-decalin solution (see text).
a Solvent viscosity at 20 °C.22
Figure 2. Transient absorption spectra of open-ring isomer TMTMA
in trans-decalin solution, excited with a picosecond 355 nm laser pulse.
for the cycloreversion process, whereas the light at ca. 396 nm
(isosbestic point) was used for the cyclization reaction. In both
the measurements, the hexane solution of TMTMA was irradi-
ated under the same condition and used as a reference.19 The
reaction yields were listed together with the solvent viscosity22
in Table 2. Although both the cyclization and the cycloreversion
yields decreased with an increase in the solvent viscosity, the
cyclization reaction was more sensitive to the viscosity.
Dynamic Detection of Cyclization Reactions. Figure 2
shows time-resolved transient absorption spectra of the open-
ring isomer of TMTMA in trans-decalin solution, excited with
a picosecond 355 nm laser pulse. The time evolution of the
spectra can be divided into three stages. At the first stage
(immediately after the excitation), a broad absorption spectrum
whose intensity increases toward the blue region appears within
the time resolution of the apparatus, followed by the growth of
a shoulder around 540-570 nm in the several tens of picosec-
onds time region. In the second stage in several hundreds of
picoseconds after the excitation, the absorbance shorter than
ca. 500 nm gradually decreases, together with the appearance
of the two absorption peaks at 460 and 560 nm as well as the
broad spectrum in the wavelength region longer than 650 nm.
In the third stage (on and after ca. 1.5 ns following the
excitation), almost no evolution of the spectra was observed.
The absorption maximum at 560 nm observed in the time region
longer than ca. 1.5 ns can be safely assigned to that of the
closed-ring isomer of TMTMA, because the peak position and
the spectral band shape are the same as those exhibited in Figure
1a.
Figure 4. Time profiles of transient absorbance of open-ring isomer
TMTMA in trans-decalin in the initial 200 ps following the excitation
with a picosecond 355 nm laser pulse, monitored (a) at 675 nm and
(b) at 560 nm. Solid lines are convolution curves calculated on the
basis of laser pulse durations and the time constants (see text).
evident from Figure 3 that the peak of the difference absorption
located around 560 nm and a slight decrease of the absorbance
was observed at ca. 650 nm. Because the absorption maximum
and the band shape of the difference spectrum in Figure 3 are
very close to those of the closed-ring isomer, the rapid spectral
evolution in a few tens of picoseconds time region is attributable
to the photocyclization to result in the formation of the closed-
ring isomer of TMTMA. The rapid appearance of the closed-
ring isomer was also observed in other alkane solutions.
To estimate the time constant of the rapid photocyclization,
we analyzed the time profiles of transient absorbance at 675
and 560 nm (Figure 4). In this figure, solid lines are results
calculated by taking into account the durations of the excitation
and monitoring laser pulses and time constants of absorbance
changes. In this calculation, the time dependence of the transient
absorbance was assumed to be biphasic. In the time profile at
675 nm, the instrument-limited rise of the absorbance was
followed by the rapid decay, whose time constant was set to be
10 ps in the calculation. After this rapid decay, the slow increase
of the absorbance was observed. This slow increase of the
transient absorbance will be discussed later. On the other hand,
the time profile at 560 nm shows a slower rise than the response
function of the apparatus. The time constant of 10 ps was used
also for the calculation of this rise behavior. The laser
spectroscopy with 15 ps pulse width may make it difficult to
discriminate the some relaxation processes such as vibrational
cooling from the intrinsic reaction of the closed-ring isomer
First, we concentrate our discussion on the spectral evolution
immediately after the excitation. For elucidating the origin of
this rapid change, the difference between the spectra at 0 and
40 ps was calculated, as shown in Figure 3. Both of the spectra
used for the calculation were normalized at ca. 450 nm. It is

Photochromic Reactions of a Diarylethene Derivative
J. Phys. Chem. A, Vol. 106, No. 35, 2002 8099
value at ca. 2 ns after the excitation. As was shown in Figure
1a, the closed-ring isomer of TMTMA has little absorption at
675 nm. In addition, such an increase in the absorbance with
the same time constant was also observed in longer wavelength
region where no absorption due to the closed-ring isomer exists.
By integrating the above results with the fact that the time
constant of the rise in the absorbance was identical with that of
the decay of the fluorescent state, the increase of the absorbance
is ascribed to the formation of other species such as the triplet
state. Similar temporal behaviors were obtained also in other
alkane solutions.
In the time profile at 560 nm, the decrease of the transient
absorbance was observed in the subnanosecond time region as
shown in Figure 5c. The time profile at 560 nm mainly involves
dynamic behaviors of the fluorescent state and the closed-ring
isomer. The time profile of the fluorescent state at 560 nm,
AF560, is represented as
-t
τf
AF560 ) ꢀF560cF exp
(1)
( )
Here, ꢀF560, cF, and τF are respectively the extinction coefficient
of the fluorescent state at 560 nm, the initial concentration of
the fluorescent state, and the fluorescent lifetime. In this
equation, the concentration of the transient species is integrated
over the cell length. To analyze the dynamic behaviors of the
closed-ring isomer, the contribution of the transient absorbance
due to the fluorescent state (broken line) was subtracted on the
basis of the initial value of the transient absorbance and the
fluorescent lifetime. The initial value of the transient absorbance
of the fluorescent state, ꢀF560cF, was estimated in the following
way. As was shown in Figures 2 and 3, the closed-ring isomer
was produced immediately after the excitation and the transient
absorption spectrum even at 40 ps comprised the contributions
from the close-ring isomer and the fluorescent state. From Figure
3, the absorbance due to the closed-ring isomer at 40 ps was
estimated to be 0.05. Hence, the rest of the transient absorbance
at 560 nm (ca. 0.085) at 40 ps was attributed to the fluorescent
state. The lifetime of the fluorescent state (475 ps) was used
for the calculation. The dynamic behaviors of the transient
absorbance from which the contribution from the fluorescent
state was subtracted were shown as open circles in Figure 5c,
where the rise of the transient absorbance was observed. In the
case that the closed-ring isomer is produced also from the
fluorescent state, the time profile of the closed-ring isomer at
560 nm in the subnanosecond to nanosecond time region is given
by
Figure 5. Time profiles of transient absorbance of open-ring isomer
TMTMA in trans-decalin, excited with a picosecond 355 nm laser pulse,
monitored (a) at 475 nm, (b) at 675 nm, and (c) at 560 nm.
formation. This result, however, indicates that there exists a rapid
process with the time constant of e10 ps in the photocyclization
pathways, as was observed in n-hexane solution.11 In other
alkane solutions, a similar rapid appearance of the closed-ring
isomer was observed with the time constant of e10 ps.
Second, we discuss the evolution of the spectra in the
nanosecond time region. Figure 5 shows time profiles of the
transient absorbance at 475, 675, and 560 nm, respectively. The
time profile at 475 nm shows the decrease in the subnanosecond
time region with a time constant of 480 ps, followed by a much
longer decay. The fluorescence decay profile of the open-ring
isomer TMTMA in trans-decalin was described by more than
one phasic process. On the assumption that the decay profile
was biphasic, the time constants and preexponential factors were
respectively obtained to be 475 ps (99%) and 3.91 ns (1%).
The complex behaviors of the fluorescence decay may be due
to the distribution of the conformation of TMTMA in the ground
state.19 Namely, diarylethene derivatives with five-membered
heterocyclic rings have at least two conformations with the two
rings in mirror symmetry (parallel conformation, P) and in C2
symmetry (antiparallel conformation, AP). It is known that the
photochemical cyclization can proceed only from the AP
conformer. As will be shown later, the cyclization reaction takes
place also in the fluorescent state. Hence, the longer component
might be due to the fluorescence of the P conformer, although
further investigation is necessary to clearly assign this compo-
nent. In any event, it is worth noting here that the main
component of the fluorescence lifetime agrees with the decay
time constant of the transient absorbance at 475 nm within the
experimental error. Hence, the absorption spectrum with its
maximum around 475 nm was attributed to the fluorescent state
of the open-ring isomer TMTMA.
-t
AC560 ) ꢀC560cFΦFfC 1 - exp
+ ꢀC560cI
(2)
( )
[
]
τf
Here, ꢀC560 and ΦFfC are respectively the extinction coefficient
of the closed-ring isomer at 560 nm and the quantum yield of
the cyclization reaction in the fluorescent state. The concentra-
tion of the closed-ring isomer formed immediately after the
excitation is given as cI in eq 2. Because the time profile of
transient absorbance at 560 nm is represented by the summation
of eqs 1 and 2 in the presence of the slow cyclization reaction
channel from the fluorescent state, the decay profile of the
transient absorbance can be observed in the case where the ΦFfC
value was small. On this point, it is worth noting here that
neither the rise nor the decay in the subnanosecond to
nanosecond time region was observed for the dynamic behaviors
of TMTMA in n-hexane after the excitation at 355 nm.11 In
addition, the decay behavior at 560 nm was pronounced in
On the other hand, the absorbance at 675 nm shows an
increase in intensity with the time constant of 490 ps following
the rapid decay around the time origin and reaches the plateau

8100 J. Phys. Chem. A, Vol. 106, No. 35, 2002
Miyasaka et al.
TABLE 3: Fluorescence Lifetimes of the Open-Form
TMTMA and Time Constants Obtained by Transient
Absorption Spectroscopy
transient absorbance
fluorescence at
475 nm
(decay)
675 nm
(rise)
560 nm
(decay)
490 nma τf (Ai)
n-hexane
355 ps (0.993)
5.77 ns (0.007)
390 ps (0.997)
1.09 ns (0.003)
425 ps (0.997)
8.75 ns (0.003)
475 ps (0.990)
3.91 ns (0.01)
510 ps (0.996)
1.12 ns (0.004)
330 ps
400 ps
440 ps
480 ps
550 ps
360 ps
380 ps
400 ps
450 ps
530 ps
b
isooctane
390 ps
440 ps
490 ps
500 ps
cyclohexane
trans-decalin
cis-decalin
a Double-exponential function, A1exp(-t/τ1)+A2exp(-t/τ2), was as-
sumed for the fluorescence decay. The longer time constant was less
reliable because the time window of the measurement was not
sufficiently wide to resolve it. b Neither decay nor rise of the absorbance
was detected in n-hexane solution (see text).
Figure 6. (a) Solvent viscosity dependence of the fluorescent lifetime
of the open-ring isomer of TMTMA. (b) Solvent viscosity dependence
of the photoinduced cyclization reaction yield of the open-ring isomer
of TMTMA.
solution with large viscosity such as trans-decalin and cis-
decalin. These results indicated that the cyclization reaction took
place also in the fluorescent state of the open-ring isomer in
addition to the rapid process prior to the relaxation to the
fluorescent state and the ΦFfC value decreased with an increase
in the solvent viscosity.
value. For these systems, it was reported that the isomerization
reaction rate constants actually increased with decreasing solvent
viscosity but the deviation from the complete linear relation
was experimentally observed between the rate constant and the
1/η, which was mainly attributed to the difference between the
microscopic molecular friction and the macroscopic viscos-
ity.23,24
The solvent viscosity effect on reaction profiles of the excited
open-ring isomer of TMTMA was summarized in Table 3 where
fluorescent lifetimes and the time constants obtained by the
transient absorption measurements in alkane solutions were
listed. In each of solutions, time constants obtained from the
transient absorption spectroscopy were in agreement with the
fluorescence. The time constants at 475 and 675 nm were
respectively ascribed to the decay of the fluorescent state and
the rise of the long-living species such as triplet state. Except
for the temporal behavior in n-hexane, the decay behavior was
observed for the time profile at 560 nm, as mentioned in the
above section. Table 3 also shows that the fluorescent lifetimes
as well as the time constants in transient absorbance increase
with an increase in the solvent viscosity, whereas the cyclization
reaction yield decreases with an increasing viscosity of the
solvent as was shown in Table 2.
To more precisely elucidate this viscosity dependence, the
reciprocal value of the fluorescence lifetime, 1/τf, was plotted
as a function of 1/η in Figure 6, where the dependence of the
cyclization reaction yield on 1/η is also shown for the
comparison. Here, 1/η is the reciprocal value of the viscosity.
Figure 6a shows that 1/τf increases almost linearly with an
increase in 1/η value. Such viscosity dependence of the
fluorescence lifetime was reported for the excited stilbene,23
its derivatives,24 and cyanine dyes,25 in which systems the
isomerization accompanied with large conformational change
takes place in the fluorescent state. In relation to the viscosity
effect of the molecular diffusive motion and the reaction time
constant, it is known that the time constant of the rotational
relaxation time of the solute in solution phase scales with the
solvent viscosity.26 The simple Debye-Stokes-Einstein model
predicts that the reciprocal value of the rotational relaxation
time is in proportion with kBT/Vη. Here, kB, T, V, and η are
respectively the Boltzmann constant, temperature, molecular
volume, and viscosity of the solvent. In analogy with this
Debye-Stokes-Einstein model, it has been expected that the
rate constant of these isomerization processes with large
geometrical change linearly increases with an increase in 1/η
As briefly mentioned in the introductory section, the open-
ring isomer of TMTMA has a rather large Stokes shift and is
dependent on the solvent polarity. These fluorescence properties
suggested some conformational rearrangement for the relaxation
process in the S1 potential surface to produce the fluorescent
state with a large dipole moment. Because the photocyclization
yield was found to decrease with an increase in the solvent
polarity,11,19 it was also suggested that the twisted conformation
with the intramolecular CT character was responsible for the
fluorescence state, which led to the decreased chance for the
cyclization. Even in nonpolar alkane solutions, the Stokes shift
was estimated to be more than 4800 cm-1 from Figure 1b,
indicating that large conformational rearrangement also occurs
after the excitation, even in nonpolar solution. Hence, the
conformational change leading to the rather flat structure of two
thienyl moieties in a plane seems a key process for the
cyclization reaction in the fluorescent state. The almost linear
viscosity dependences of the lifetime of the fluorescent state
and the reaction yield indicate that the conformational rear-
rangement process takes place against the molecular friction,
as observed for other photochemical processes such as isomer-
ization with large geometrical change. To summarize the above
results and discussion, it is concluded that, not only the rapid
reaction channel prior to the fluorescent state formation but also
the cyclization reaction takes place in the fluorescent state and
the slower cyclization process in the fluorescent state was
suppressed with an increase in the solvent viscosity.
To elucidate the solvent effect on this fast cyclization reaction
process, transient absorption spectra at 40 ps after the excitation
with an 355 nm laser pulse were plotted in Figure 7, where the
absorption intensity was normalized at 465 nm. As is clear in
this figure, no remarkable differences among these spectra were
observed. This result indicates that the fast reaction channel was
scarcely dependent on the solvent viscosity. By integrating the
above spectra with the results shown in Figure 6, the intercept

Photochromic Reactions of a Diarylethene Derivative
J. Phys. Chem. A, Vol. 106, No. 35, 2002 8101
Figure 7. Transient absorption spectra of the open-ring isomer
TMTMA in n-hexane, isooctane, cyclohexane, trans-decalin, and cis-
decalin, observed at 40 ps following the excitation with a picosecond
355 nm laser pulse. These spectra were normalized at 450 nm.
Figure 9. (a) Transient absorption spectra of the closed-ring isomer
of TMTMA in cis-decalin solution, excited with a picosecond 532 nm
laser pulse. (b) Time profile at 550 nm. Solid lines are convolution
curves on the basis of the pulse widths of pump and probe pulses and
time constants.
Figure 8. Schematic representation of potential profiles of TMTMA.
The reaction coordinate is given as the distance between two carbon
atoms that is to be connected via the cyclization reaction.
TMTMA in the fluorescent state is rather high and, conse-
quently, the energy barrier is smaller than that in polar solution,
which opens the cyclization reaction in the fluorescent state.
Cycloreversion Reaction. Figure 9a shows the time-resolved
transient absorption spectra of the closed-ring isomer of
TMTMA in cis-decalin solution, excited with a picosecond 532
nm laser pulse. Immediately after the excitation, the depletion
around 560 nm and positive absorption around 600-750 and
400-450 nm are observed. The former negative absorption can
be safely assigned to the bleaching of the closed-ring isomer.
The time profile of the absorption signal at 550 nm is exhibited
in Figure 9b, indicating that the slight recovery of the bleaching
of the closed-ring isomer immediately after the excitation is
followed by the constant value. Moreover, the positive signal
around 700 nm disappeared within a few tens of picoseconds.
Because the decay time constant of this positive signal was
estimated to be almost identical with that of the recovery of
the negative absorption around 560 nm, this signal might be
assigned to the Sn r S1 absorption of the closed form. In
addition, the contribution from the hot band of the closed-ring
isomer in the ground state, which was produced via the internal
conversion from the S1 to S0 state, might be involved in the
positive absorption around 700 nm. By means of the picosecond
transient absorption spectroscopy with the pulse duration of 15
ps, we could not quantitatively discriminate these two contribu-
tions.
value in Figure 6b may be attributed to the contribution from
the fast reaction channel in the total cyclization reaction.
On the solvent viscosity independent reaction yield in the
fast reaction channel, it is worth noting here that the recent
theoretical calculation27 based on ab intio multiconfiguration
self-consistent calculation predicts the excited state potential
surface of diarylethene derivatives, as shown in Figure 8. In
this figure, the reaction coordinate is given as the distance
between two carbon atoms that is to be connected via the
cyclization reaction. According to this result, the excited state
potential surface has a local maximum around the position where
the open-ring isomer has the minimum energy in the ground
state. In addition, it was predicted that the cyclization reaction
yield is strongly dependent on the position of this maximum in
the excited state. By analyzing the present results with this
theoretical calculation, we propose that the branching toward
the cyclization and the twisted conformation on the excited
potential surface is determined at the excited Franck-Condon
state and this branching ratio does not depend on the solvents
examined in the present work. Actually, the solvent independent
absorption spectrum in the ground state supports the above idea
that the potential surfaces in the ground and excited states are
almost the same in these alkane solvents.
As mentioned in the introductory section, the cyclization yield
of TMTMA decreased with an increase in solvent polarity.19
Because the fluorescent state has the charge-transfer character,
the energy level at the minimum position of the fluorescent state
is stabilized with increasing solvent polarity, leading to a large
activation barrier for the cyclization reaction from the bottom
of the fluorescent state higher. On the other hand, in nonpolar
solution, the energy minimum of the open-ring isomer of
In the time region on and after ca. 40 ps following the
excitation, no evolution of the spectra was observed, as shown
in Figure 9a. The residual absorption spectra are almost identical
with the difference spectrum between the closed-ring and the
open-ring isomers, as shown in Figure 1a. Solid lines in Figure
9b are the calculated curves by taking into account the pulse

8102 J. Phys. Chem. A, Vol. 106, No. 35, 2002
Miyasaka et al.
durations of the exciting and the monitoring lights and the
quantum yield of the ring-opening reaction as listed in Table 2.
The time constants for the recovery of the negative absorption
are indicated in the figure. The calculated curves with time
constants of 1-2 ps reproduced the experimental results,
indicating that the cycloreversion reaction took place with the
time constant e1-2 ps. Similar time profiles and ultrafast
reaction with the time constant of e1-2 ps were also observed
other alkane solutions.
reaction. This was attributed to the very rapid reaction taking
place in the time region shorter than the diffusive motion of
the solvent.
Acknowledgment. We appreciate Dr. S. Nakamura in
Mitsubishi Chemical Corp. for his valuable discussion. The
present work was partly supported by the Grants-in Aid (No.
14340183 and No. 14050061) from the ministry of Education,
Science, Sports, and Culture of Japan.
As was shown in Table 2, the cycloreversion reaction was
also dependent on the solvent viscosity, indicating that the
motion of thienyl moieties takes an important role in the
cycloreversion reaction as in the cyclization reaction in the
fluorescent state of the open isomer. The viscosity dependence
of the cycloreversion reaction was, however, much smaller than
that of the cyclization in the fluorescent state. This weak
dependence of the cycloreversion process may be interpreted
along with the recent investigation on the deactivation process
of triphenylmethane (TPM) dyes in solution phase,28 which
undergoes ultrafast deactivation depending on the solvent
viscosity, and the rotation of phenyl rings take an important
role in this deactivation. From the dynamic laser spectroscopy
with 30 fs time resolution, it was revealed the solvent viscosity
became less effective for the faster deactivation process. This
result means that the diffusive friction is less effective for the
fast motion and, rather, inertial motion in the first solvation shell
becomes dominant. For the present cycloreversion processes,
the time constant of the excited state was estimated to be <1-2
ps, as shown in Figure 9. Because this time scale is comparable
with or slightly shorter than the diffusive motion of the solvent,
the cyclization yield was less dependent on the solvent viscosity,
as was observed for the deactivation processes of TPM dyes.
Concluding Remarks. The solvent viscosity effect on the
cyclization processes revealed the reaction mechanisms of the
open-ring isomer of TMTMA in the excited singlet state and
discriminated between the fast reaction channel immediately
after the excitation and the rather slow reaction pathway in the
fluorescent state. Recent theoretical calculation27 supports the
present experimental results. It was deduced that not only the
relative geometries of potential surfaces around the conical
intersection but also the local maximum position in the S1
potential surface play important roles in the cyclization process.
In the explanation of the fluorescence decay of the open-ring
form, we mentioned the parallel (P) conformer, which has been
reported to have no cyclization channel.5 As suggested by the
biphasic decay of the fluorescence, the P isomer might have a
longer fluorescence lifetime. On the dynamic behaviors of the
P isomer, we are now doing the investigation with a wider time
window.
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