Ultrafast laser spectroscopic study on photochromic cycloreversion dynamics in fulgide derivatives: One-photon and multiphoton-gated reactions

Article abstract of DOI:10.1039/b900999j

Cycloreversion processes of three photochromic fulgide derivatives in toluene solution were investigated by means of picosecond and femtosecond laser photolysis methods. Femtosecond laser irradiation revealed that the cycloreversion processes upon visible one-photon excitation took place within a few ps in these three derivatives. On the other hand, drastic enhancements of the reaction yield were observed under picosecond laser exposure. Excitation intensity effect of the reaction yield and dynamic behaviors revealed that the successive two-photon absorption process leading to higher excited states opened the efficient cycloreversion channel in the three derivatives. The reaction yields in higher excited states were found to be quite large in these three systems, 0.35-0.55, while those in the S₁ state were 0.048-0.21. The correlation of the reaction yield in the S₁ state with that in S _n states suggest the character of the electronic states connected by the optical absorption plays an important role in the control of the cycloreversion reaction. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009.

Full text of DOI:10.1039/b900999j

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Ultrafast laser spectroscopic study on photochromic cycloreversion
dynamics in fulgide derivatives: one-photon and multiphoton-gated
reactions
Yukihide Ishibashi,ab Tetsuro Katayama,a Chikashi Ota,a Seiya Kobatake,c
Masahiro Irie,d Yasushi Yokoyamae and Hiroshi Miyasaka*ab
Received (in Montpellier, France) 16th January 2009, Accepted 24th March 2009
First published as an Advance Article on the web 16th April 2009
DOI: 10.1039/b900999j
Cycloreversion processes of three photochromic fulgide derivatives in toluene solution were
investigated by means of picosecond and femtosecond laser photolysis methods. Femtosecond
laser irradiation revealed that the cycloreversion processes upon visible one-photon excitation
took place within a few ps in these three derivatives. On the other hand, drastic enhancements of
the reaction yield were observed under picosecond laser exposure. Excitation intensity effect of the
reaction yield and dynamic behaviors revealed that the successive two-photon absorption process
leading to higher excited states opened the efficient cycloreversion channel in the three derivatives.
The reaction yields in higher excited states were found to be quite large in these three systems,
0.35–0.55, while those in the S1 state were 0.048–0.21. The correlation of the reaction yield in the
S1 state with that in Sn states suggest the character of the electronic states connected by the
optical absorption plays an important role in the control of the cycloreversion reaction.
From the viewpoint of the application, the photochromic
systems for actual utilization require several conditions such as
(a) thermal stability, (b) high durability, (c) rapid response,
Introduction
Photochromism is a photoinduced reversible isomerization in
a chemical species between two forms with different colors.
(d) high sensitivity, and (e) non-destructive readout capability.
The quick change of molecular properties via the photo-
Since the reaction in the excited state occurs in competition
with other processes in a finite lifetime, a fast rate of the
much attention from the viewpoints of the fundamental
photochromic reaction (rapid response) is of crucial impor-
induced chemical-bond reconstruction has been attracting
chemical reaction processes and the application to opto-
electronic devices.1–17
tance for an increase in the reaction yield (high sensitivity) and
a decrease in undesirable side reactions, resulting in low
photodegradation (high durability). On the other hand, the
Among various photochromic molecules, diarylethene3,4
and fulgide5,6 derivatives undergo cyclization and cyclo-
non-destructive read-out capability is in conflict with the
reversion reactions in 6p-electrocyclic systems in ultrafast time
above properties fulfilling conditions (b)–(d). Hence, some
external gated-functions are required for the photochromic
scale.18–33 Both isomers in the photochromic reactions are
thermally stable at the room temperature and different from
systems to attain the non-destructive capability while reading
each other not only in their absorption spectra, but also in
the data by the absorption of the light.
various physical and chemical properties such as fluorescence
Over the years, we reported laser-induced enhancement
spectra,11 refractive indices,12 oxidation/reduction potentials,13
(450ꢀ) of the cycloreversion reaction in diarylethene deriva-
and chiral properties.14 In addition, the photochromic
tives in solution,24 polymer films25 and crystalline phases.26 The
fluence and pulse duration effects of the reaction profiles
reaction can take place even in the crystalline phase.15–17
These features in fulgide and diarylethene derivatives have
revealed that higher excited states attained by multiphoton
been attracting much attention as most promising for applica-
tions such as rewritable optical memory and switches.
absorption could open effective photochromic reaction
channels. In addition, the efficient cycloreversion reaction was
also observed in a fulgide derivative28 in a higher excited state
a Division of Frontier Materials Science, Graduate School of
pumped by successive two-photon absorption. This multiphoton-
Engineering Science, Center for Quantum Science and Technology
gated reaction may provide an approach to erasable memory
under Extreme Conditions, Osaka University, Toyonaka, Osaka,
560-8531, Japan. E-mail: miyasaka@chem.es.osaka-u.ac.jp;
Fax: +81-(0)6-6850-6244; Tel: +81-(0)6-6850-6241
b CREST, JST, Toyonaka, Osaka, 560-8531, Japan
c Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University, Sumiyoshi, Osaka, 558-8585, Japan
d Department of Chemistry, School of Science, Rikkyo University,
Nishi-Ikebukuro, Toshima, Tokyo, 171-8501, Japan
media with non-destructive read-out capability in fulgide and
diarylethene derivatives. Not only from the viewpoint of the
application but also from the basic viewpoints of photo-
chemistry, the selective excitation to a specific electronic state
leading to the target reaction seems to provide an important
approach for the control of photochemical reactions.
e Department of Materials Chemistry, Graduate School of
Engineering, Yokohama National University, Tokiwadai, Hodogaya,
Yokohama, 240-8501, Japan
In order to explore the applicability and generality of the
multiphoton-gated reaction in 6p-electrocyclic photochromic
ꢁc
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compounds, we have investigated the photochromic reaction
profiles of three fulgide derivatives under ultrafast laser pulsed
excitation in the present work. In the following, we will present
the reaction dynamics depending on the pulse duration of the
pump laser light sources and the excitation intensity effects of
the photochromic reactions.
3-isopropylidene-4-[1-(2-methyl-3-benzo[b]furyl)butylidene]-
dihydrofuran-2,5-dione (F2) and 3-adamantylidene-4-[1-(2,5-
dimethyl-3-furyl)ethylidene]dihydrofuran-2,5-dione (F3) were
synthesized and purified.34 Quantum yields of cyclization and
cycloreversion were, respectively 0.18 and 0.048 for F1,
0.39 and 0.11 for F2, and 0.12 and 0.21 for F3.34 Toluene
(Wako, infinity pure grade) was used without further purifica-
tion. All the measurements were performed under O2-free
condition at 22 ꢃ 2 1C.
Experimental
A picosecond laser photolysis system with a repetitive mode-
locked Nd3+:YAG laser was used for transient absorption
spectral measurements in the ps–ns time region.33 The second
harmonic (532 nm) with 15 ps excitation pulse was focused
into a spot with a diameter of ca. 1.5 mm. Picosecond white
light continuum generated by focusing a fundamental pulse
into a 10 cm quartz cell containing D2O–H2O (3 : 1) was
employed as a monitoring light. A sample cell with 2 mm
optical length cell was used. The sample solution was
circulated during the measurement under a repetition rate
o0.1 Hz and the transient spectrum was obtained with only
a one-shot laser exposure. For the measurement of the excita-
tion intensity dependence with the picosecond laser pulse, a
pinhole with a diameter of ca. 1 mm was placed before the
sample solution. The intensity of the picosecond laser light was
measured by a laser power meter (Gentec, ED-200).
Results and discussion
Steady-state absorption spectra
Three fulgide derivatives, F1, F2 and F3, undergo photo-
chromic reactions between the open and the closed isomer
(C-form) as shown in Scheme 1. In general, open forms of the
fulgide derivative have two isomers, E- and Z-forms. Colorless
E-form can turn to the colored C-form in competition with the
cis–trans isomerization to the colorless Z-form upon UV light
irradiation, while the Z-form does not undergo the cyclization
reaction to yield the C-form. On the other hand, visible light
irradiation of the C-form induces the cycloreversion reaction
to the E-form
Fig. 1 shows the steady-state absorption spectra of the three
fulgide derivatives in toluene solution. The dotted line in each
of the figures shows the spectrum of the open forms (E and Z
isomers), while the solid line in each of the figures is the
spectrum under the photostationary state obtained with
330 nm light irradiation. The closed-forms (C-forms) of F1,
F2 and F3, respectively, have absorption maxima at 494, 475
and 519 nm in the visible region. On the other hand, absorp-
tion bands of the open forms of F1, F2 and F3 are located only
in the UV region.
To investigate the dynamic behaviors under femtosecond
laser light excitation, a dual NOPA/OPA laser system was
used for transient absorption spectroscopy. The details of the
system have been described elsewhere.24,26–28 Briefly, the
output of
a femtosecond Ti:Sapphire laser (Tsunami,
Spectra-Physics) pumped by the SHG of a cw Nd3+:YVO4
laser (Millennia V, Spectra-Physics) was regeneratively
amplified with 1 kHz repetition rate (Spitfire, Spectra-Physics).
The amplified pulse (1 mJ pulseꢂ1 energy, 85 fs fwhm, 1 kHz)
was divided into two pulses with the same energy (50%). These
fundamental pulses (802 nm) were guided into a non-collinear
optical parametric amplifier (NOPA) system (TOPAS-white,
Light-Conversion) or an optical parametric amplifier (OPA)
system (OPA-800, Spectra-Physics). The NOPA output can
cover the wavelength region between 500 and 1000 nm with
1–40 mW output energy with ca. 20 fs fwhm. On the other
hand, the OPA output pulses are converted to the SHG, THG,
FHG, or sum frequency mixing pulse. These pulses can cover
the wavelength region between 300 and 1200 nm with
1–10 mW output energy with ca. 120 fs fwhm. White light
continuum was generated by focusing the fundamental light at
802 nm into a 1 mm quartz window. Polarization of the two
pulses was set at the magic angle for all the measurements. The
signal and the reference pulses were detected with multichannel
diode array systems (PMA-10, Hamamatsu) and sent to a
personal computer for further analysis. Spectra were calibrated
for group velocity dispersion. From the cross correlation
trace at the sample position, the pulse duration of the cross
correlation between the NOPA output and the super-
continuum was ca. 80 fs for spectra. A rotating sample cell
with an optical length of 2 mm was utilized and the absorbance
of the sample at the excitation wavelength was set to B1.0.
Three photochromic fulgide derivatives, 3-isopropylidene-4-
[1-(2,5-dimethyl-3-furyl)ethylidene]dihydrofuran-2,5-dione (F1),
Scheme 1
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Fig. 2 Time-resolved transient absorption spectra of the closed form
of F1(c), in toluene solution, excited with a ca. 20 fs FWHM 510 nm
laser pulse with 2.0 mJ pulseꢂ1 output power. Scattering of excitation
light influences the monitoring wavelength region from 490 to 520 nm.
Fig. 1 Steady-state absorption spectrum of photostationary state of
the three fulgide derivatives, (a) F1, (b) F2 and (c) F3 in toluene
solution (solid lines), irradiated at 330 nm. The dotted lines show the
open forms (E- and Z-forms). The main component in the photo-
stationary state is due to the C-form.
Femtosecond laser photolysis
Fig. 2 shows time-resolved transient absorption spectra of
F1(c) in toluene solution, excited with a fs laser pulse centered
at 510 nm. A negative signal in the wavelength region of
450–525 nm appears within the response function of the
apparatus, together with a positive signal at ca. 570 nm and
a negative one in the wavelength region 4620 nm. With
increasing delay time, the negative absorbance in the
450–525 nm regions becomes reduced and is followed by a
constant negative absorbance. This negative absorbance could
be safely assigned to the bleaching of the ground state, because
the spectral band shape is similar to the steady-state absorption
spectrum of F1(c). On the other hand, the positive signal
at ca. 570 nm and negative one 4620 nm decrease to the
baseline with an increase in the delay time after the excitation.
The negative signal in the wavelength region 4620 nm could
be ascribed to the stimulated emission of the excited state of
F1(c) because the ground state of F1(c) has no absorption in
this region. In addition, the time constant of the decrease of
this negative signal was in agreement with the recovery of the
transient bleaching of the ground state, as will be shown in
Fig. 3. On the other hand, the positive signal at ca. 570 nm
could be mainly attributed to the Sn ’ S1 absorption of F1(c)
because the decay time constant of this band was almost the
same with those of the recovery of the bleaching signal and
the stimulated emission. At and after 50 ps following the
excitation, no temporal evolution was observed in the transient
spectra. In addition, it was confirmed after the fs laser
Fig. 3 Time profiles of transient absorbance of F1(c) in toluene
solution, excited with
a femtosecond 510 nm laser pulse with
ca. 20 fs FWHM and 2.0 mJ pulseꢂ1 output power; (a) monitored at
470 nm, (b) 532 nm, (c) 580 nm and (d) 680 nm. Solid black lines are
the best calculated curves by taking into account the pulse durations,
the time constants, and the reaction yield.
irradiation that the steady-state absorption covering the UV
region consisted of only those of the open- and closed-isomers.
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UV light irradiation led to perfect recovery of the closed form.
From these results, it can be concluded that the cycloreversion
reaction was completed within 50 ps following the excitation.
Fig. 3 shows time profiles of F1(c) in toluene solution, of
which transient absorption spectra were shown in Fig. 2. As
was observed in Fig. 2, the time profile at 470 nm shows a
quick appearance of the negative absorption due to the
transient bleaching of the ground state, followed by the
recovery in a few tens of picoseconds. At and after ca. 40 ps
following the excitation, constant negative absorbance
remained. This remaining negative absorbance was attributed
to the permanent bleaching due to the cycloreversion reaction,
as was explained in the previous section. The time profile was
analyzed with the biphasic recovery function with faster and
the slower time constants of (1.1 ꢃ 0.1) ps and (9.5 ꢃ 1.0) ps,
respectively. The amplitude factors for the faster, the slower,
and the constant negative components are, respectively, 0.62,
0.30 and 0.08. Because the faster time constant is the major
component of the ground-state recovery and was in good
agreement with the time constant of the stimulated emission
at 680 nm, it could be concluded that the time constant of
1.1 ps was due to the lifetime of the excited state. On the other
hand, the slower time constant may be attributed to the
vibrational cooling in the ground state of the closed-form
F1(c). The ground state of F1(c) produced immediately after
the nonradiative deactivation from the excited state is not
thermally relaxed. This hot molecule generally releases its
excess energy in a few tens of ps.35–38
transient absorption spectra and time profiles of Fig. 2 and 3
were independent of the excitation intensity of the femto-
second laser pulse with an output of 1–3 mJ pulseꢂ1
.
Fig. 4 shows the transient absorption spectra of F2(c) in
toluene solution, excited with a fs laser pulse centered at
510 nm with ca. 20 fs pulse duration. A broad positive band
with a maximum at 610 nm appears within the response of the
apparatus and decreases to the baseline with an increase in the
delay time. This band could be ascribed to the Sn ’ S1
absorption by the analysis of the time constant in Fig. 5. After
the decrease of this band, the negative absorption spectrum
with a minimum around 475 nm remains. Since the absorption
minimum and spectral band shape of this negative absorption
spectrum were identical with those of the ground-state
absorption spectrum of F2(c), we can safely attribute this negative
transient spectrum to the bleaching of the ground state. At and
after 50 ps following the excitation, no temporal evolution was
observed in the transient spectra at least up to the time region of a
few ms. In addition, UV-light irradiation to this negative constant
absorbance led to perfect recovery to the base line. From these
results, it can be concluded that the cycloreversion reaction was
completed within 50 ps following the excitation.
Fig. 5 shows time profiles of transient absorbance of F2(c) in
toluene solution, monitored at several wavelength points. The
time profile at 610 nm in Fig. 5(a) shows a quick appearance of
the positive signal, which is followed by the decay to the
baseline in a few tens of picoseconds. The solid line in this
figure is the result calculated on the basis of a biphasic decay
with time constants of 1.8 ps and 13.3 ps. Pre-exponential
factors for the faster and the slower components were, respec-
tively, 0.58 and 0.42. The faster time constant of 1.8 ps could
be ascribed to the lifetime of the excited state of F2(c), because
similar time constants were obtained as a major component in
other wavelength regions. On the other hand, the slower time
constant may be due to the vibrational cooling in the ground
state of the closed-form F2(c) as mentioned above.
In the time profile monitored at 532 nm in Fig. 3(b), the
positive absorbance appearing immediately after the excitation
is followed by the decay in a few tens of picoseconds. The
positive absorbance immediately after the excitation indicates
that the molar absorption coefficient of the excited state at
532 nm is larger than that of the ground state. At and after
ca. 40 ps following the excitation, a negative constant
absorbance was observed. As was explained for Fig. 3(a),
this negative absorbance was safely attributed to the cyclo-
reversion reaction. The time profile was analyzed with a
biphasic decay function with faster and the slower decay time
constants of 1.1 ps and 10.5 ps. The time profile at 580 nm in
Fig. 3(c) shows the quick appearance of the positive
absorbance, followed by the decay to the baseline in a few
tens of picoseconds. The solid line in this figure is the result
calculated with the biphasic decay function. The faster and the
slower time constants and their amplitude factors were,
respectively 1.20 ps, 0.83, and 9.6 ps, 0.17. The faster time
constant corresponds to the lifetime of the excited state,
indicating that this signal is attributable to the Sn ’ S1
absorption. On the other hand, the longer decay time constant
may be ascribed to the vibrational cooling of the hot S0 state,
because the wavelength at 580 nm is close to the ground-state
absorption and the time constant of the decay was similar to
those observed in Fig. 3(a) and (b).
The time profile at 680 nm, which could be assigned to the
stimulated emission, is exhibited in Fig. 3(d). The decay was
reproduced by a single exponential function with a time
constant of 1.2 ps. This result supports that the faster time
constant observed in Fig. 3(a)–(c) can be assigned to the
excited-state lifetime of F1(c). It was confirmed that the
Fig. 4 Time-resolved transient absorption spectra of the closed-form
of F2(c), in toluene solution, excited with ca. 20 fs FWHM 510 nm
laser pulse with 2.0 mJ pulseꢂ1 output power. Scattering of excitation
light influences the monitoring wavelength region from 490 to 520 nm.
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Table 1 Cycloreversion reaction yields of the three fulgide derivatives
in toluene solution and time constants (ps) obtained by transient
absorption spectroscopy
Transient absorbance
Reaction
yield
Positive
signal
Bleaching
signal
Stimulated
emission
F1
F2
F3
0.048
0.11
0.21
1.2 ꢃ 0.2
9.6 ꢃ 1.0
1.1 ꢃ 0.1
9.5 ꢃ 1.0
1.2 ꢃ 0.2
—
1.8 ꢃ 0.2
1.8 ꢃ 0.2
9.5 ꢃ 1.5
13.3 ꢃ 0.6
0.7 ꢃ 0.2
9.3 ꢃ 0.9
0.61 ꢃ 0.1
7.0 ꢃ 1.0
0.7 ꢃ 0.1
0.6 mJ mmꢂ2 output power. As stated in the Experimental
section, the sample solution was circulated during the
measurement under the repetition rate o0.1 Hz and the data
were obtained only with one-shot laser exposure for each
spectrum.
Time resolved transient absorption spectra of F1(c) in
Fig. 6(a) show a negative absorption band around 500 nm
and a positive one around 600 nm appearing in the early stage
after excitation. The former band is assigned to the bleaching
signal of F1(c) and the latter is ascribed to the excited state
and/or the hot band in the ground state of F1(c), as was
explained for Fig. 2. This positive absorption signal decreases
to the baseline within 50 ps after the appearance of the pulsed
excitation. On the other hand, the bleaching signal slightly
recovered also within 50 ps after the excitation. In addition,
this negative absorption spectrum was observed even at
several seconds after the excitation without circulation of the
sample. The UV light irradiation after several hundreds shots
of the picosecond laser pulse perfectly recovered the absor-
bance of F1(c). Hence, the negative absorption spectrum at
and after 50 ps was due to the cycloreversion reaction process.
A similar temporal evolution of transient spectra was
observed for F2(c) and F3(c) in Fig. 6(b) and (c). In the early
stage after the excitation, the positive bands around 600 nm
due to the excited state and thermally unrelaxed ground state
of F2(c) and F3(c) and the bleaching signal around 500 nm
appeared. The former positive absorption decreased to the
baseline or negative value, while no apparent recovery was
observed for the latter bleaching signal and the negative
constant value remained at and after 50 ps following the
excitation. In both systems, the negative transient spectrum
was attributed to the cycloreversion reaction process as for
F1(c).
Fig. 5 Time profiles of transient absorbance of F2(c) in toluene
solution, excited with femtosecond 510 nm laser pulse with
ca. 20 fs FWHM and 2.0 mJ pulseꢂ1 output power; (a) monitored at
610 nm, (b) 475 nm, and (c) 532 nm. Solid black lines are the best
calculated curves by taking into account the pulse durations, the time
constants, and the reaction yield.
a
Fig. 5(b) shows the time profile of the transient absorbance
monitored at 475 nm. The positive absorbance in the early
stage after the excitation indicates that the molar absorption
coefficient of the excited state is larger than that of F2(c) in the
ground state. The positive absorbance decreased in the initial
10 ps following the excitation and was followed by a constant
negative absorbance. This negative constant signal is safely
ascribed to the cycloreversion reaction, as was observed in
Fig. 4. The time profile was reproduced by a double exponential
function with time constants of (1.8
ꢃ
0.2) ps and
(9.5 ꢃ 1.5) ps. Fig. 5(c) shows the time profile monitored at
532 nm, showing the similar time profile as that at 475 nm. The
positive absorption appears immediately after the excitation,
followed by faster and slower decays with time constants of
(1.7 ꢃ 0.15) ps and (14.0 ꢃ 1.0) ps. Weak permanent negative
absorbance remained at and after ca. 50 ps, which was due to
the cycloreversion process as was observed in Fig. 4. No
excitation intensity dependence was observed for the time
constants or the ratio of three pre-exponential factors in
these time profiles or spectra for excitation intensity of
1–3 mJ pulseꢂ1. These results indicate that the excited state
deactivates into the ground state with a time constant of
ca. 1.8 ps in competition with the cycloreversion reaction.
For F3(c), the transient absorption spectra and time profiles
were also observed by means of the same femtosecond laser
system. The time constants of these three systems are listed in
Table 1.
Fig. 7 exhibits the time profiles of closed-forms of F1, F2
and F3 in toluene solution following picosecond 532 nm laser
excitation with 0.6 mJ mmꢂ2 output power. The time profile at
610 nm of F1(c) in Fig. 7(a) shows the positive absorbance
appearing within the response of the apparatus, followed by
the decay to the baseline in several tens of picoseconds. The
solid line in Fig. 7(a) is the curve calculated on the basis of the
pulse widths of the exciting and monitoring pulses (15 ps),
decay time constants, and amplitude factors obtained by
measurements under femtosecond laser exposure. The curve
thus calculated reproduces the experimental results well,
Picosecond laser photolysis
Fig. 6 exhibits the time-resolved transient spectra of the three
fulgide derivatives, F1(c), F2(c) and F3(c), in toluene solution
following the picosecond 532 nm laser excitation with
ꢁc
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Fig. 6 Time-resolved transient absorption spectra of three fulgide derivatives in toluene solution, excited with a 15 ps FWHM 532 nm laser pulse
with 0.6 mJ mmꢂ2 output power: (a) F1(c), (b) F2(c) and (c) F3(c).
indicating that the time profile obtained by the picosecond
laser pulsed excitation is almost the same with that obtained
by the femtosecond laser excitation. On the other hand,
Fig. 7(b) shows the time profile at 495 nm. The negative
absorption appearing within the response of apparatus slightly
recovered and attained a negative constant value. This
negative absorption signal is due to the cycloreversion process
from F1(c) to F1(o).
the result calculated on the basis of Scheme 2 with the
cycloreversion reaction yield, Fo of 0.50. The curve thus
calculated reproduced the experimental result fairly well,
although the quantum yield under steady-state light
irradiation was 0.11. This result indicates that the cyclo-
reversion of F2(c) was also enhanced by the picosecond
532 nm laser excitation.
Also for F3(c) in toluene solution, the same analysis was
applied. The time profile at 500 nm in Fig. 7(e) shows the
appearance of the negative absorbance within the response
function of the apparatus, followed by no remarkable
temporal evolution. To reproduce the experimental result
within the framework of Scheme 2, an apparent cycloreversion
reaction yield Z 0.40 was required, although the quantum
yield under steady-state light irradiation was 0.21. Summarizing
the above results and discussion, it can be concluded that
these three fulgide derivatives undergo effective cycloreversion
reaction under the picosecond 532-nm laser excitation.
To obtain information on the mechanism of this enhance-
ment in the apparent cycloreversion reaction yield, we
explored the excitation intensity effect of the cycloreversion
reaction. Fig. 8 shows the relation between the excitation
intensity and the reaction amount monitored as negative
transient absorption signals at 100 ps after the excitation, at
which all of the cycloreversion reaction was completed. The
ordinate, the conversion efficiency, is the normalized negative
absorbance, ꢂDAmax(100 ps)/Amax. Here, DAmax(100 ps) and
Amax are, respectively the transient absorbance at 100 ps and
the ground state absorbance at the wavelength of the absorp-
tion maximum of the S1 ’ S0 transition in the visible region;
494 nm for F1(c), 475 nm for F2(c) and 519 nm for F3(c). Both
abscissa and ordinate in Fig. 8 are logarithmic. Each slope in
Fig. 8 was 1.5–2.0 in the rather low excitation intensity region,
indicating that the cycloreversion reaction is quadratic in
proportion with the incident laser intensity. With further
increase in the excitation intensity, saturation tendency was
observed for all fulgide derivatives. This saturation is partly
due to a decreased concentration of the molecules in the
ground state in high excitation region, because the maximum
value of the ordinate, –DAmax(100 ps)/Amax is unity.
For the analysis of the time profile at 495 nm, we tentatively
assumed the simple reaction scheme that the excited state of
F1(c) undergoes deactivation into the ground state in competi-
tion with the cycloreversion reaction leading to the production
of F1(o), as shown in Scheme 2. Here, the reaction yield of the
cycloreversion, Fo is represented as ko/(kn + ko). Here, kn and
ko are, respectively the rate constant of the deactivation into
the ground state and that of the cycloreversion reaction. In the
analysis, 1/(kn + ko), was set to the decay time constant of the
positive absorption signals of 1.2 ps, and the relative
amplitude factors of the fast and the slow components were
set to be the same with those obtained by the femtosecond
laser excitation.
Solid and dotted lines in Fig. 7(b) are the curves calculated
with two Fo values. The dotted line with Fo value obtained by
the steady-state light irradiation, 0.048, does not reproduce the
experimental result. On the other hand, the solid line is the
result calculated with the Fo value of 0.60, which reproduces
the experimental result. To reproduce the experimental result
with no remarkable decay of the negative absorbance, it
was revealed that a Fo value Z 0.6 was necessary. That is,
Fo = 0.60 is the minimum reaction yield estimated by the
above procedure. The result in Fig. 7(b) indicates that
the cycloreversion reaction is drastically enhanced under the
picosecond laser pulse excitation in F1(c), as observed in the
diarylethene derivatives24–27 and a fulgide derivative.28
Fig. 7(c) and (d), respectively show time profiles at 610 and
475 nm of F2(c) in toluene solution, excited with a picosecond
532 nm laser pulse. As for F1(c) in Fig. 7(a), the time profile at
610 nm was reproduced by the curve calculated with the pulse
durations and the time constants obtained by the fs laser
photolysis. On the other hand, the solid line in Fig. 7(d) is
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molecule has no ground-state absorption. The number of the
molecules excited by the simultaneous two-photon absorption
is represented by Ne = dNgI2. Here, d and Ng are the two-
photon absorption cross-section and the number of the
ground state molecules, respectively. I is the peak intensity
of the excitation pulse with the dimension of the number of
photons divided by the unit area size and the time. On the
other hand, the latter stepwise two-photon absorption process
takes place in such a manner that the transient species
produced via the first one-photon absorption of the ground-
state molecule absorbs a second photon, resulting in the
production of higher excited states. Since the second photon
absorption by the transient species takes place in competition
with the first photon absorption by the ground-state molecule,
the number of the photons in a laser pulse is an important
factor for this stepwise two-photon absorption process.
In the present case, the pulse width and the excitation
intensity of the femtosecond laser pulse were ca. 1/1000 and
1/100 smaller than those of the picosecond pulse. Hence, the
peak intensity of the femtosecond laser pulse, I, is almost ten
times larger than that of the picosecond laser.40 However, the
excitation intensity effect was not observed in the femtosecond
laser excitation. These results therefore may lead to the
conclusion that the stepwise two-photon absorption is
responsible for the drastic enhancement of the cycloreversion
reaction in the present systems as observed in diarylethene
derivatives24–27 and a fulgide derivative.28 However, the
lifetime of S1 state in each fulgide derivative was ca. 1/10
shorter than that of the hot molecules and picosecond pulse
width. The vibrationally hot molecules in the electronic
ground state immediately after the deactivation from the S1
state may contribute to the drastic enhancement of the cyclo-
reversion reaction via second-photon absorption.
Fig. 7 Time profiles of the transient absorbance of three fulgide
derivatives in toluene solution, excited with a 15 ps FWHM, 532 nm
laser pulse with 0.6 mJ mmꢂ2 output power, and observed at two
wavelengths where the positive absorption and the bleaching signal
occur: (a) at 610 nm and (b) 495 nm for F1(c), (c) at 610 nm and
(d) 475 nm for F2(c) and (e) at 500 nm for F3(c). Solid and dotted lines
are convolution curves calculated with pump and probe pulse widths
of 15 ps and double exponential decay-time constants obtained under
femtosecond pulse excitation. In this calculation, the reaction yield
was set as a parameter. The dotted lines were set to the reaction yields
under steady-state light irradiation, while the grey bold lines, with
reaction yields of 0.60, 0.50 and 0.40, best reproduce the experimental
results in (b), (d) and (e), respectively (see text).
In order to more clearly explore the state absorbing the
second photon and elucidate the mechanism of nonlinear
cycloreversion reaction, we employed two picosecond
532 nm laser pulses for the excitation and investigated the
apparent cycloreversion yield as a function of the time interval
of these two pulses. Fig. 9(a) shows a schematic illustration of
the experimental setup. A 532 nm laser pulse was divided into
two pulses with same intensity and one of the two pulses,
Pump 2, was guided into the optical delay line to give the time
interval, DT, from the first pulse (Pump 1). The amount of
the cycloreversion reaction was monitored as the transient
absorption spectrum at 300 ps after the excitation of the
second or the first pulse. At a delay time of 300 ps the
cycloreversion reaction was completed.
Fig. 9(b) shows the correlation between the transient
absorbance of F1(c) at 500 nm observed at 300 ps and the
time interval between the two pulses, DT. This correlation
curve indicates that the apparent cycloreversion reaction yield
has a maximum value at DT = 0 and decreases with an
increase in the time interval between the two pulses. The solid
line in this figure is the result calculated with the reaction
scheme that the intermediate species produced by the first
photon absorption is excited again by the second photon
absorption leading to the cycloreversion reaction. In this
calculation, the lifetime of the intermediate species was varied
as a parameter. As clearly shown in this figure, the curve with
Scheme 2
The above results indicate that a two-photon process is
responsible for the enhancement of the cycloreversion process
under picosecond laser excitation. Usually, two-photon
absorption processes can be clarified into two cases such as
simultaneous absorption and stepwise absorption.39 In the
former simultaneous absorption process, the ground-state
molecule is excited even in the wavelength region where the
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Fig. 8 Excitation intensity dependence of the conversion efficiency of the three fulgide derivatives in toluene solution, observed at 100 ps after the
excitation with a 15 ps FWHM, 532 nm laser pulse (closed circles): (a) F1(c), (b) F2(c) and (c) F3(c). Thin and bold black lines in figures are,
respectively, the results calculated on the basis of Scheme 3(a) and (b) (see text).
open-ring form, is responsible also for the enhancement of the
cycloreversion reaction of the three fulgide derivatives in
toluene solution.
In order to quantitatively obtain the reaction yield in the
higher excited state, we applied the numerical simulation24–28
to the analysis of the results in Fig. 8. Parameters for this
simulation are indicated in Scheme 3(a) as italic styles.
Here, e532(S1), Fn(Sn) and Fo(Sn) are, respectively the molar
absorption coefficient of the S1 state at 532 nm, the yield
leading to the S1 state from the higher excited state (Sn)
attained by the second photon absorption, and the cyclo-
reversion reaction yield at the Sn state.
Fig. 9 (a) Schematic illustration of the double-pulse excitation of ps
532 nm laser light and time monitoring of the cycloreversion reaction.
(b) Correlation between the transient absorbance of F1(c) monitored
at 500 nm (at 300 ps) and the time interval between the two excitation
pulses, DT.
t = 1.0 ps reproduces the experimental result fairly well but
the time constant of 10 ps, which is the time constant of the
vibrational cooling, did not reproduce the experimental
results. This result clearly indicates that the S1 state, whose
time constant is 1.0 ps, is the main species absorbing the
second photon leading to the efficient cycloreversion reaction,
and that the hot ground state with a lifetime of 10 ps
(vibrational cooling) does not contribute to the efficient
cycloreversion. Summarizing the above results and discussion,
it can be safely concluded that the stepwise multiphoton
absorption via the S1 state prepared by the first photon
absorption, such as S0 + hn - S1 and S1 + hn - Sn
-
Scheme 3
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In this simulation, the molar absorption coefficient of the
S1 state at 532 nm was experimentally obtained by the analysis
of the time profile of transient absorbance monitored at
532 nm after the femtosecond 510 nm laser excitation in the
following manner. The time profile of the transient absorbance
at 532 nm, DA(t), is represented by eqn (1):
ꢀ
ꢁ
kn
ko
DAðtÞ ¼ eex
ꢂ
eg CexðtÞ ꢂ eg
Cexð0Þ ð1Þ
kn þ ko
kn þ ko
Here, eex, eg, kn, ko and Cex(t) are, respectively, the molar
absorption coefficient of the excited state, that of the ground
state, the rate constant of the deactivation into the ground
state from the excited state, that of the cycloreversion reaction,
and the concentration of the excited state. In this expression,
the time profile due to the vibrational cooling was ignored.
Since the constant negative absorbance at longer delay times
corresponds to the second term of eqn (1), we can acquire
Cex(0) with the cycloreversion reaction yield obtained by the
steady-state irradiation and eg of the S1 ’ S0 absorption at
532 nm. In addition, DA(0) corresponds to the term,
(eex ꢂ eg)Cex(0). Hence, we can estimate the molar absorption
coefficient of the excited state at 532 nm, as 7200 Mꢂ1 cmꢂ1 for
F1(c), 12 600 Mꢂ1 cmꢂ1 for F2(c), and 3600 Mꢂ1 cmꢂ1
for F3(c), respectively. Accordingly, the parameter required
for the numerical simulation is only the cycloreversion
reaction yield at higher excited state, Fo(Sn).
Fig. 10 Relation between the reaction yields of Fo(S1) and Fo(Sn) in
fulgide derivatives. The previous result of another fulgide system
(0.066 for Fo(S1) and 0.45 for Fo(Sn)) is also given.28 The solid line
is to guide the eye.
with high reaction yields plays an important role in the
enhancement of the cycloreversion reaction, as observed for
diarylethene derivatives24–26 and a fulgide derivative.28 As a
result, the generality of the multiphoton-gated process in the
system with the heterocyclic aryl groups was confirmed.
Finally, it is noteworthy that for F3(c), with the largest
Fo(S1) among the present three fulgide derivatives, has the
smallest cycloreversion reaction yield for Fo(Sn), as shown in
Fig. 10, where the previous results of a further fulgide system28
are also given. This tendency was also observed in diarylethene
derivatives. For example, while a diarylethene derivative with
a lower Fo(S1) of 1.3% has a higher reactivity of the higher
excited state,24 another derivative with Fo(S1) = 35% does
not.27a These results imply that the character of higher
electronic state attained by the second photon absorption
from the S1 state plays an important role for the efficient
cycloreversion. That is, the S1 molecular electronic wave
function feasible for the cycloreversion in the S1 state is not
feasible for the efficient reaction by the optical allowed
transition. At any rate, the present results suggest that the
two-photon gated cycloreversion is an effective method for
actual application, since it is effective for molecular systems
with low reaction yield in the S1 state. We are presently
undertaking the investigation of the relation between cyclo-
reversion reaction yield in higher excited states under stepwise
excitation and under one-photon excitation, the results of
which will be reported soon.
Thin black lines in Fig. 8 are results calculated with Fo(Sn)
of 0.50 for F1(c), 0.55 for F2(c) and 0.35 for F3(c), respectively.
Although the calculated curves reproduce the experi-
mental result in the region of the excitation intensity
o ca. 0.7 mJ mmꢂ2, the deviation between the experimental
results and the calculated curve was pronounced with a further
increase in the excitation intensity in all systems.
For this deviation, it is worth mentioning that the relaxation
from the Sn state of diarylethene derivative included the
internal conversion to the dark electronic state (denoted as
0
S1 in Scheme 3(b)) in addition to the cycloreversion reaction
and the relaxation to the S1 state.24–26 This S1 species was
0
assigned to the excited state which is located in the vicinity of
the S1 state but optically forbidden from the S0 state.24–26 In
addition, it was revealed that this species did not undergo
efficient cycloreversion even if it was pumped up into higher
excited state by the second photon absorption.
For the analysis of the present fulgide systems, we included
0
0
this S1 state in the analysis. The lifetime of this S1 state,
t(S01), was varied to reproduce the experimental results. Bold
black lines in Fig. 8 are the results calculated with the
cycloreversion reaction yield in the Sn state of 0.50 for F1(c),
0.55 for F2(c) and 0.35 for F3(c), and t(S01) of 11 ps, showing
that the experimental results are well reproduced by the
calculated results. The iterative simulation estimated the reac-
tion yield in the higher excited state was estimated at 0.50 ꢃ
0.05 for F1(c), 0.55 ꢃ 0.05 for F2(c) and 0.35 ꢃ 0.05 for F3(c).
Summarizing the above results and discussion, it can be
concluded that the stepwise two-photon process via the actual
intermediate S1 state, rather than the simultaneous two-
photon process, was responsible for the enhancement of the
cycloreversion process of the three fulgide derivatives under
the picosecond laser excitation. The production of the Sn state
Conclusion
Cycloreversion processes of three photochromic fulgide deri-
vatives in toluene solution were investigated by means of
picosecond and femtosecond laser spectroscopy. The bond
fission occurred within a few picoseconds. Picosecond pulsed
excitation of the closed-isomer of the diarylethene derivatives
led to the drastic enhancement of the cycloreversion reaction.
The dependence of the reaction profiles on the excitation
intensity, pulse duration, and the excitation wavelength
indicated that this enhancement is attributable to the produc-
tion of the higher excited state with a large reaction yield of the
cycloreversion attained via a successive two-photon process.
This process generally occurred in fulgide derivatives as well
as diarylethene derivatives and the generality of the stepwise
ꢁc
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two-photon process was confirmed for the 6p-electrocyclic
photochromic compounds. The correlation of the reaction
yield in the S1 state with that in Sn states suggest the character
of the electronic states connected by the optical absorption
plays an important role in the control of the cycloreversion
reaction.
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Acknowledgements
This work was partly supported by Grand-in-Aids for
Research in Priority Area (No. 471) from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT)
of the Japanese Government.
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M
The value of Ng is 6.02 ꢀ 1016 molecules cmꢂ3 for 1 ꢀ 10ꢂ4
concentration. By using the typical value of the two-photon
,
absorption cross-section, 10ꢂ50 cm4 s (photon molecule)ꢂ1 39
the
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ꢁc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1409–1419 | 1419