Femtosecond dynamics and stimulated emission from the S2 state of a liquid crystalline trans-azobenzene

Article abstract of DOI:10.1016/S0009-2614(98)00251-6

The excited-state S₂ dynamics of an azobenzene derivative, trans-BMAB, was investigated in solution by femtosecond transient absorption and picosecond single-photon timing fluorescence spectroscopies. The fluorescence was observed at 400 nm with a lifetime ~250 fs. The S₁ state with an absorption maximum at 410-415 nm was formed with a time constant of ~250 fs from the S₂ state followed by relaxation with a lifetime of ~2.3 ps in n-hexane. The stimulated emission from the S₂ state overlapped with the femtosecond transient absorption spectra just after excitation, which was confirmed by polarization absorption spectroscopy.

Full text of DOI:10.1016/S0009-2614(98)00251-6

15 May 1998
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Chemical Physics Letters 288 1998 77–82
Femtosecond dynamics and stimulated emission from the S2 state
of a liquid crystalline trans-azobenzene
a
a
b
b
Jun Azuma , Naoto Tamai , Atushi Shishido , Tomiki Ikeda
a
Department of Chemistry, School of Science, Kwansei Gakuin UniÕersity, 1-1-155 Uegahara, Nishinomiya 662, Japan
b
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226, Japan
Received 1 December 1997; in final form 9 February 1998
Abstract
The excited-state S2 dynamics of an azobenzene derivative, trans-BMAB, was investigated in solution by femtosecond
transient absorption and picosecond single-photon timing fluorescence spectroscopies. The fluorescence was observed at 400
nm with a lifetime ;250 fs. The S1 state with an absorption maximum at 410–415 nm was formed with a time constant of
;250 fs from the S2 state followed by relaxation with a lifetime of ;2.3 ps in n-hexane. The stimulated emission from the
S2 state overlapped with the femtosecond transient absorption spectra just after excitation, which was confirmed by
polarization absorption spectroscopy. q 1998 Elsevier Science B.V. All rights reserved.
on the isomerization reaction. Thin polymer films,
Langmuir–Blodgett films, and liquid crystals of
azobenzene derivatives have been investigated as
1. Introduction
Photochromic reactions of some organic molecules
are of considerable interest because of their wide
applications, including optical information process-
ing, data storage and nonlinear optics 1,2 . The
photochromism is based on simple photochemical
reactions such as bond cleavage, pericyclic, proton
w
x
promising systems for various applications 7–9 . In
these systems, the physicochemical properties such
as absorption spectrum, dipole moment, refractive
index, and molecular conformation can be reversibly
changed through the trans–cis isomerization of
azobenzene. In contrast to stilbene, in which the
dynamics of isomerization seems to be well under-
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x
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transfer and isomerization reactions 1 . To under-
stand the primary processes of photochromic reac-
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tions, we have recently examined spirooxazine 3 ,
stood 10,11 , the excited-state properties and the
isomerization mechanism of azobenzene are still un-
clear. Furthermore, it appears that the isomerization
mechanism is different from that of stilbene, as it
depends on the excitation wavelength. For the p–p)
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x
diarylethene derivatives 4,5 and salicylideneaniline
w x
6 by femtosecond to picosecond transient absorp-
tion and picosecond fluorescence spectroscopies.
These molecules undergo C–O bond cleavage, peri-
cyclic, and proton transfer reactions, respectively,
with time constants of a few hundred fs to 1 ps
depending on the type of reactions.
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.
S2 excitation, a rotational mechanism has been
proposed for the isomerization process, whereas the
isomerization proceeds through an inversion pathway
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.
w x
On the other hand, azobenzene and its derivatives
are representative photochromic compounds based
under the n–p) S1 excitation 12 .
A recent study of trans-azobenzene has indicated
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 98 00251-6

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J. Azuma et al.rChemical Physics Letters 288 1998 77–82
Scheme 1.
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.
that the isomerization pathway passes through an
inversion mechanism based on an analysis of reso-
nance Raman excitation profiles and theoretical cal-
a magnetic gyre pump Micropump, 040-332 during
the measurements to avoid the excitation of the
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.
photoproduct cis-form . The concentration of trans-
BMAB was ;2.0=10y4 M for both transient ab-
sorption and fluorescence decay measurements.
The laser system for transient absorption spec-
troscopy consisted of a hybridly mode-locked, dis-
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culations 13 . A transient absorption spectroscopic
study has been carried out for trans-azobenzene in
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various solvents 14 . It was shown that under excita-
tion to the S2 state at 303 nm, the lifetime of the S2
state is ;1 ps, being independent of the solvent
polarity andror dielectric constant. A long-lived
component existing for 13–16 ps is also observed,
which can be explained by the relaxation from the
intermediate to the ground state of trans-azobenzene.
The isomerization dynamics of cis-azobenzene ex-
cited to the S1 state has also been examined by
Ž
persion-compensated femtosecond dye laser Coher-
ent Satori 774 which was pumped by a cw mode-
locked Nd:YAG laser Coherent Antares 76S 3 .
The output of the dye laser was amplified to an
energy of ;400 mJ at a center wavelength of 720
.
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. w x
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nm by a regenerative amplifier system Continuum
RGA60 and PTA60 with a repetition rate of 10 Hz.
.
w
x
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.
femtosecond transient absorption spectroscopy 15 .
However, most of these experiments are based on the
rise and decay dynamics observed at selected wave-
lengths, and time-dependent spectral data are still
unclear.
The second harmonics ;360 nm obtained by a 1
mm BBO crystal was used as an excitation pulse. A
remaining fundamental pulse was focused into a 1
cm water cell to generate a white-light continuum as
probe and reference pulses. The system response
function of the pump–probe method was estimated
to be ;200 fs fwhm. The signal was analyzed by a
In the present study, we have investigated the
excited-state dynamics of a liquid crystalline trans-
X
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azobenzene 4-butyl-4 -methoxyazobenzene, abbrevi-
microcomputer-controlled ICCD detector Princeton
. Ž
.
.
Instruments, ICCD-576-G at each optical delay us-
ated here as BMAB Scheme 1 in solution through
the excitation to the S2 state by femtosecond tran-
sient absorption and picosecond fluorescence spec-
troscopies. S2 fluorescence and the stimulated emis-
sion from the S2 state of trans-BMAB were observed
in single-photon timing and transient absorption
spectra, respectively. The femtosecond polarization
dynamics of transient absorption was also examined.
From these results, the relaxation dynamics of S2
and S1 states of trans-BMAB has been discussed.
Ž
.
ing a translation stage Sigma Koki, STM-500X . A
temporal dispersion of the white-light continuum
was corrected for the transient absorption spectra.
The fluorescence spectrum and its decay curves
were measured by picosecond single-photon timing
spectroscopy by using the same dye laser without
amplification. The repetition rate of the excitation
pulse was reduced to 3.8 MHz with an external pulse
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.
picker Conoptics, Model 360-80, 25D, and 305 .
The detection system was a combination of a mi-
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crochannel-plate photomultiplier Hamamatsu, MCP
.
Ž
2. Experimental
R2809U , a monochromator Japan Spectroscopic,
CT-10 , a constant fraction discriminator Tennelec,
TC454 , and a time-to-amplitude converter Tenne-
lec, TC864 , which gave an instrument response
function of ;30 ps fwhm. The spectral sensitivity
of the system was not corrected. The rise and decay
.
.
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BMAB was synthesized and purified according to
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.
the literature 16,17 . The sample was dissolved in
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.
n-hexane spectroscopic grade, Kishida Chemicals
and allowed to flow through a 2 mm flow cell using

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J. Azuma et al.rChemical Physics Letters 288 1998 77–82
79
curves of transient absorption and fluorescence were
analyzed by a non-linear least-squares iterative con-
volution method based on a Marquardt algorithm
w
x
18 .
3. Results and discussion
3.1. Absorption spectra
Fig. 1 shows the absorption spectra of trans- and
cis-BMAB in n-hexane. Cis-BMAB was obtained by
the irradiation of trans-form at 350 nm with a 150 W
Xe-lamp for 16 h. The intensity of the p–p) ab-
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.
sorption S2 §S0 at 345 nm in the trans-form is
dramatically reduced and its peak wavelength is
shifted to the shorter wavelength at 305 nm by the
trans-to-cis isomerization. In addition, the intensity
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.
of the n–p) absorption S1 §S0 with a peak at
;450 nm is enhanced by the formation of cis-
BMAB. This result can be interpreted in terms of
selection rules based on the symmetry differences of
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the cis- and trans-forms 1 . The current excitation
wavelength at ;360 nm corresponds to the excita-
tion to the S2 state.
Ž . Ž .
a Fluorescence decay curve of trans-BMAB dot in
n-hexane excited at 360 nm and monitored at 410 nm along with
Fig. 2.
3.2. Fluorescence properties
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.
the instrument response function —— scat and the simulation
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.
curve smooth line . The decay curve was analyzed by the sum of
a two-exponential function. The weighted residual is shown in the
upper part, and the channel width of detection is 1.11 psrchannel.
The S1 state of trans-BMAB is generally non-
emitting because of the forbidden transition and iso-
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merization reaction in the S1 manifold 1 . Fig. 2a
Ž .
b
Fluorescence spectrum of trans-BMAB obtained from the
illustrates a fluorescence decay curve of trans-BMAB
product of the total fluorescence intensity and the amplitude factor
of the fast-decay component at its respective wavelength.
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.
excited to the S2 state lex ;360 nm and observed
at 410 nm along with the instrument response func-
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.
tion 30 ps fwhm . It is clear that the fwhm of the
fluorescence decay curve is almost the same as the
system response function, indicating the extremely
short fluorescence lifetime. The decay curve was
analyzed as a first approximation by a sum of two
exponentials. The fast decay component has a life-
time shorter than 2 ps and its amplitude factor is
larger than 0.997, originating from the S2 state of
trans-BMAB as shown later. The long decay compo-
nent with a lifetime of 1.0 ns is probably due to an
impurity.
Fig. 1. Absorption spectra of trans- and cis-BMAB in n-hexane.
The concentration is 6.5=10y5 M.

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J. Azuma et al.rChemical Physics Letters 288 1998 77–82
The product of the total intensity and an ampli-
tude factor of the fast component at respective wave-
length is plotted as a function of wavelength. The
fluorescence spectrum has a peak at ;400 nm, as is
clearly shown in Fig. 2b. This can probably be
assigned to the fluorescence from the S2 state of
trans-BMAB for the following reasons. For the S2
fluorescence of azobenzene, Morgante and Struve
have reported a lifetime of ;5 ps by using a
picosecond Nd:glass laser and the optical Kerr gate
w
x
method 19 , although they have not reported the
fluorescence spectrum. The fluorescence spectrum
from the S2 state of trans-azobenzene and its quan-
tum yield were examined by Hamai and Hirayama,
who reported that the fluorescence spectrum of
trans-azobenzene has a maximum at 385 nm with Ff
;1.7=10y5 and maximum at ;410 nm with Ff
;0.9=10y5 for 1-methoxy azobenzene 20 . They
have also estimated the S2 lifetime of trans-azoben-
zene to be ;60 fs by using the Strickler–Berg
equation. Our experimental data showing a fluores-
cence maximum of ;400 nm and a lifetime shorter
than 2 ps are in good agreement with the reported
values of the S2 fluorescence of trans-azobenzene
and its derivative.
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x
3.3. Transient absorption spectra
To analyze the ultrafast dynamics of trans-BMAB
in the excited S2 state, the transient absorption spec-
tra in n-hexane were measured with the excitation at
360 nm as illustrated in Fig. 3. The spectrum just
after the excitation has a peak at ;490 nm and very
broad absorption in the longer wavelength region. As
clearly indicated in the figure, the very rapid decay
with a time constant shorter than 0.5 ps was ob-
served at wavelength longer than ;600 nm and at
;490 nm. No rise component was detected at these
bands, indicating that the spectrum just after the
excitation is probably due to the Sn §S2 absorption
of trans-BMAB. In addition, a broad flat part at the
onset of the transient absorption ranging from 390 to
470 nm region was clearly observed just after the
excitation. As discussed later, this is originated from
the stimulated emission of the S2 ™S0 fluorescence.
After the disappearance of the Sn §S2 absorption
bands, a new and intense absorption band appears at
shorter wavelength region. The spectrum at a delay
Ž .
Fig. 3. a A bird’s-eye view of the transient absorption spectra of
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.
trans-BMAB in n-hexane excited at 360 nm S2 excitation and
probed with the white-light continuum at the magic angle condi-
Ž .
tion. The spectra are illustrated up to 3 ps after excitation. b The
transient absorption spectra of trans-BMAB in n-hexane at se-
lected time windows after the excitation.
time longer than 0.3 ps has a peak at ;415 nm and
a shoulder at ;550 nm. It has been reported by
Lednev et al. that the transient absorption spectrum
of trans-azobenzene in n-hexane appears in the range

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J. Azuma et al.rChemical Physics Letters 288 1998 77–82
81
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x
from 370 to 450 nm and has a peak at 390 nm 14 .
This spectrum was obtained at 0.47 ps after the
excitation, and the main decay component was 0.9"
0.2 ps. They assigned this spectrum as the absorption
from the S2 state of trans-azobenzene. The spectral
shape of transient absorption with a maximum at
410–415 nm observed in our study is similar to that
observed by Lednev et al. The peak wavelength is
shifted to the red with an energy of ;1500 cmy1
,
which probably originates from the substitution of
azobenzene in the 4- and 4X-positions. The main
difference is the rise dynamics of this absorption
band. The transient absorption spectrum at 410–415
nm is observed after the disappearance of Sn §S2
absorption at ;490 nm, although the stimulated
emission from the S2 state is overlapped in this
wavelength region. In addition, the decay time of the
spectrum at 410–415 nm is strongly dependent on
the solvent viscosity, and has a lifetime of ;10 ps
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x
in ethylene glycol 21 . From these results, we as-
signed the absorption spectrum with a maximum at
410–415 nm to the Sn §S1 absorption of trans-
BMAB. The peak wavelength of this band shifts a
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.
little to the blue ;410 nm with increasing the
delay time. This may be attributed to the intramolec-
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x
ular vibrational redistribution in the S1 state 21 .
Furthermore, this absorption band disappears
completely within 10 ps, and a very weak absorption
spectrum with a maximum at ;450 and ;600 nm
can be observed within 20 ps, as indicated in Fig. 3b
although the signal to the noise ratio is rather bad.
This broad absorption spectrum remains up to 3 ns
Fig. 4. Polarization decay curves of transient absorption observed
Ž .
Ž .
at 410 nm a and 650 nm b . Polarization of the probe pulse was
set parallel and perpendicular with respect to the pump polariza-
Ž .
a
Ž .
b are the simulation curves for
tion. Smooth lines in
and
perpendicular and parallel polarization decays, respectively, by a
convolution of the pulse width and a two-exponential decay
function. The weighted residuals for the simulation of 410 nm
decay are shown in the upper part. The data were taken every 16.7
fsrchannel.
Ž
.
limit of our experiments . The absorption band at
;450 nm corresponds to the difference spectrum of
n–p) absorption in the ground state of cis- and
trans-BMAB. This result indicates that the trans–cis
isomerization of azobenzene is completed within 10
ps in n-hexane. The absorption spectrum at ;600
nm probably originates from the Tn §T1 absorption
that the rising parts of the parallel and perpendicular
components were clearly different from each other at
410 nm, but no difference was observed at 650 nm.
The parallel component at 410 nm is delayed for
200–300 fs, as compared to the perpendicular com-
ponent. This provides clear evidence that negative
absorption overlaps with the transient absorption of
the parallel component at 410 nm. One of the most
probable negative absorption is due to the stimulated
emission from the S2 state. Although the quantum
yield of the S2 fluorescence of trans-BMAB is on
w
x
of BMAB 21 .
3.4. Polarization absorption dynamics
To confirm the stimulated emission in the shorter
wavelength region and to analyze the excited-state
dynamics of trans-BMAB, the polarization dynamics
of transient absorption at 410 and 650 nm were
examined, as illustrated in Fig. 4. It should be noted

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J. Azuma et al.rChemical Physics Letters 288 1998 77–82
the order of 10y5, the probability of the stimulated
emission is determined by Einstein’s B factor and
therefore by the transition dipole moment of the
S2 §S0 absorption. We conclude, from the result of
transient absorption spectra as shown in Fig. 3 and
the polarization decay curves in Fig. 4a, that the
stimulated emission from the S2 state of trans-BMAB
is observed in the transient absorption spectra just
after the excitation.
The polarization decay curves of 410 and 650 nm
were analyzed by a two-exponential decay function.
The rise time of ;250 fs was observed at 410 nm
with a decay time of 2.3 ps, whereas no rise compo-
nent was detected at 650 nm. The decay time at 650
nm was ;250 fs, corresponding to the rise time at
410 nm. This time constant is not due to the rota-
tional relaxation but the intramolecular process of
trans-BMAB in n-hexane, since the rotational relax-
ation is much slower than this time constant. From
these results, it can be concluded that transient ab-
ization of trans-BMAB is completed within 10 ps.
Further investigation of the solvent dependence on
the isomerization dynamics of trans-BMAB is now
in progress.
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maximum at 410–415 nm is formed with a time
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;2.3 ps in n-hexane.
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12 H. Rau, in: H. Durr, H. Bouas-Lauran Eds. , Photochromism
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In conclusion, the femtosecond dynamics of
azobenzene derivative, trans-BMAB, was investi-
gated in n-hexane solution by femtosecond transient
absorption and picosecond fluorescence spectro-
scopies under excitation to the S2 state. It was found
that the fluorescence spectrum from the S2 state is
observed at 400 nm. The lifetime of the S2 state was
estimated to be ;250 fs by femtosecond transient
absorption spectroscopy. In addition, the S1 state of
trans-BMAB with an absorption maximum at 410–
415 nm is formed with a time constant of ;250 fs
from the S2 state. The stimulated emission from the
S2 state overlaps to the femtosecond transient ab-
sorption spectra just after the excitation. The isomer-
—
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