Chemical Physics Letters 372 (2003) 216–223
www.elsevier.com/locate/cplett
Fluorescence spectra of trans- and cis-azobenzene –
emission from the Franck–Condon state
H. Satzger a, S. Sp€
orlein a, C. Root a, J. Wachtveitl b, W. Zinth a, P. Gilch
b
a,*
a
Sektion Physik, Ludwig-Maximilians-Universit€at, Oettingenstr. 67, DE-80538 M€unchen, Germany
Institut f€ur Physikalische und Theoretische Chemie, Universit€at Frankfurt am Main, Marie-Curie-Str. 11,
D-60439 Frankfurt am Main, Germany
Received 27 November 2002
Abstract
The photodynamics of the two isomers of azobenzene in DMSO after np excitation are monitored via transient
absorption and steady state emission spectroscopy. The excited state decay of the cis-form is close to single-exponential
with a time constant of 0.1 ps. The trans-isomer decays in a biphasic fashion with characteristic times of 0.34 and 3.0 ps.
This biphasic behaviour has no impact on its fluorescence intensity. It is concluded that the fluorescence probes the
dynamics in the vicinity of the Franck–Condon region.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
Since the discovery of the two isomers (trans
and cis) of azobenzene in 1937 [1,2] their interconversion has been subject to numerous experimental and theoretical studies (for reviews see
[3,4]). To a great deal these studies were motivated
by the fact that this isomerization can be induced
photochemically. This photo-isomerization opens
the route to a variety of (potential) applications
ranging from its use as a chemical actinometer [5]
over its use as an optical storage device (see e.g.
*
Corresponding author. Fax: +49-89-2180-9202.
E-mail address: [email protected] (P.
Gilch).
[6]) to very recent work on protein folding where
an azobenzene moiety served as an ultrafast trigger
[7]. The mechanism of this molecular trigger and
the timescale of its operation has been studied by
optical [8–10] and IR [11] femtosecond absorption,
emission spectroscopy [12], and picosecond resonance Raman experiments [13].
Since both isomers of azobenzene exhibit two
distinct transitions in the visible (np ) and the near
UV range (pp ) four different experiments are
possible. Most studies have delt with the (thermally
stable) trans isomer [9,10,12,13] and focused on
differences between the dynamics and the mechanism when exciting either the np transition or the
pp (for a very recent discussion see [14]). Comparative studies on the trans and cis-isomer when
exciting the same electronic state (np ) have only
been performed by one group [8,11]. The results for
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00364-6
H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
the trans-isomer are summarized as follows: After
excitation at 435 nm close to the maximum of the
np transition a positive transient absorption is
detected that decays bi-exponentially with time
constants of 0.32 and 2.1 ps in ethanol [8]. For an
excitation wavelength of 420 nm Lednev et al. [10]
obtained values of 0.6 and 2.6 ps in n-hexane. The
behaviour of the cis-form is distinctly different [8].
Here, the decay is dominated by one time constant
of 0.18 ps. Tuning the excitation wavelength to the
red wing of the np transition (480 nm) results in
qualitatively identical signatures with slightly
modified time constants (see below). The different
dynamics of the two isomers has been attributed to
the different slopes of the S1 ðnp ) surface in the
vicinity of the Franck–Condon point [8]. Within
this interpretation the slope for the cis form is
steeper leading to a ballistic movement of the initial
wavepacket towards the conical intersection with
the ground state. The trans Franck–Condon region
is regarded as relatively flat which could initiate a
crossover from a ballistic to a diffusive motion on
the way to the conical intersection. In this Letter
informations obtained by absorption techniques
are supplemented by steady state fluorescence
measurements since fluorescence is particularly
sensitive to motion in the vicinity of the Franck–
Condon region.
Despite the extensive spectroscopic material no
comparative experiments on the fluorescence of
the two isomers at room temperature have been
reported. Fujino et al. [12] measured the emission
spectrum of trans-azobenzene after excitation to
its S2 (pp ) state. The spectrum contains two
bands peaking at 390 nm and 665 nm. The two
bands were assigned to direct emission from
the S2 state and to emission from the S1 after
internal conversion. The intensity of the latter
amounts to only a few percent as compared to
the former. The fluorescence quantum yield of
the 665 nm emission was estimated to be of the
order of 106 explaining why emission data on
azobenzene are so scarce. In this study results
from stationary fluorescence spectra and timeresolved absorption measurements for trans- and
cis-azobenzene are combined to obtain a unified
picture of the initial photodynamics after np
excitation.
217
2. Experimental section
The femtosecond transient absorption set-up
consists of a home-built titanium sapphire based
laser amplifier system running at a repetition rate
of 1 kHz. It delivers pulses with durations of 90 fs
and an energy of 1 mJ at a central wavelength of
800 nm. A non-collinear optical parametric amplifier (NOPA) [15] was used to tune the excitation
wavelength, a CaF2 based continuum generator
[16] and a multichannel detector [17] allow broad
band probing of the absorption changes. In the
experiments presented here 480 nm excitation
pulses with an energy of 700 nJ and a duration of
50 fs were crossed with the white light probe
pulses at the sample location (spot diameters of
200 lm for the pump pulses and 50–80 lm for the
probe pulses, 80 fs cross correlation width). The
sample solutions (10 mM) were pumped through a
fused silica cuvette with 500 lm optical pathlength.
The low emission quantum yield of azobenzene
requires special procedures to record its emission.
With a standard fluorimeter (we tested for instance
the Fluorolog 1680 0.22m Double Spectrometer
from Specs) no emission from azobenzene can be
detected. Therefore, we used a highly sensitive
Raman spectrometer to record the fluorescence
emission. This spectrometer consists of an argon
ion laser (Coherent, Innova 300) as excitation
source, reflective optics to collect the fluorescence
emission and an imaging spectrometer (ISA, HR
460) coupled to a liquid nitrogen cooled CCD
camera (Princeton Instruments) as a detector. The
excitation wavelength was tuned to the 488 nm line
of the ion laser. Other laser lines as well as plasma
lines were suppressed by a prism set-up. In order
to avoid significant photoisomerization by the laser light, the excitation power was kept low at
30 lW. The spot diameter at the sample was
130 lm. Emitted light was collected in a 180 °
back-scattering geometry by a Cassegrainian microscope objective (Ealing, NA ¼ 0.5, f ¼ 13.4
mm). The emitted light passed a notch filter
(Kaiser, HNSF 488.0) to suppress scattered laser
light. The spectrometer was equipped with a 150
lines/mm grating which allows the simultaneous
recording of the whole emission spectrum. The
218
H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
spectral sensitivity of the set-up has been corrected
for using a blackbody radiator as a reference.
Sample solutions (43 mM azobenzene in
DMSO, large volumes of P100 ml) for the fluorescence measurements were pumped rapidly
through fused silica cuvettes (1 mm optical pathlength). Care was taken to ensure that photoisomerization due to the excitation laser light did not
influence the recorded spectra. The trans-sample
was kept in the dark to suppress any cis-formation.
Cis-azobenzene was prepared by illumination of a
solution (200 ml) of trans-azobenzene with a
mercury lamp (350 mW at the excitation wavelength 316 nm selected with suitable filters, exposure time 50 h). The progress of the photoreaction
was monitored by absorption spectroscopy and
the illumination was stopped when the photostationary spectrum was reached.
3. Results
3.1. Transient absorption measurements
In order to ensure comparability of transient
absorption and steady state emission data (excitation wavelength 488 nm), femtosecond absorption
experiments were performed with the excitation
wavelength tuned to 480 nm (20 nm FWHM). The
plot of the induced absorption change DA versus
delay time and detection wavelength is shown in
Fig. 1a for trans- and in Fig. 2a for cis-azobenzene.
Excitation of trans-azobenzene results in a multiphasic absorption change. A global analysis of the
data with a multi-exponential trial function required three components and an offset to account
for the formation of a stable photoproduct. The
data analysis produced the decay associated spectra depicted in Fig. 1b. The three time constants
determined for trans-azobenzene are 0.34, 3.0, and
12 ps. The spectrum of the 0.34 ps component
represents the decay of a broad absorption band in
the visible (450–600 nm) and the decay of a sharper
feature in the near UV (<400 nm). The 3.0 ps
component has a decay associated spectrum that
resembles the spectrum of the 0.34 ps component:
The sharp peak in the UV is present – though redshifted by at least 30 nm – and the broad features in
Fig. 1. (a) Femtosecond transient absorption of trans-azobenzene in DMSO after 480 nm excitation. Note that the time axis
is linear from )1 to 1 ps and logarithmic from there on. (b)
Decay associated spectra extracted by a global tri-exponential
fit from the data given in (a).
the visible nearly coincide in shape and in amplitude. The spectrum associated with the 12 ps time
constant is considerably weaker in amplitude than
the two other spectra and becomes pronounced
only in the red wing of the trans pp absorption
band (<420 nm). Apart from the spectrum of the
offset it is the only spectrum with a negative contribution (around 440 nm).
The decay of the S1 state of cis-azobenzene is
dominated by a 0.1 ps component as determined
by a global analysis. This analysis yields two additional time constants of 0.9 and 5.6 ps. The
dominant spectrum associated with a time constant
of 0.1 ps again has a broad feature in the visible and
a sharper peak in the UV. The spectra of the slower
H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
219
spectrum exhibits a pronounced wing in the red
part extending to the NIR. The emission spectrum
of cis-azobenzene was calculated from the spectrum of a photostationary sample. The fraction of
trans-azobenzene in the sample (30 %) was determined via absorption spectroscopy using the
extinction coefficients [3] of both isomers. The
emission spectrum of the trans form was then
scaled by a factor of 0.3, subtracted from the
photostationary spectrum and the resulting difference spectrum was normalized to the portion of
excitation light absorbed by the cis form. The
Fig. 2. (a) Femtosecond transient absorption of cis-azobenzene
in DMSO after 480 nm excitation. The contribution of the residual trans-isomer has been corrected for. (b) Decay associated
spectra extracted by a global tri-exponential fit from the data
given in (a).
components are very similar to each other and to
the 12 ps spectrum of trans-azobenzene. Apart
from minor changes in the time constants the results reported here are in good agreement with
those reported earlier [8,10] although the solvents
and the excitation wavelengths differ.
3.2. Steady state fluorescence
Excitation of trans-azobenzene in DMSO at
488 nm generates a very weak and broad emission
that is comparable in amplitude to the Raman
signal of the DMSO solvent. After subtraction of
the solvent background and spectral correction a
spectrum is obtained that peaks at 640 nm and
has a width of 250 nm (FWHM) (Fig. 3a). The
Fig. 3. Absorption and emission spectra of azobenzene (excitation wavelength 488 nm). (a) Trans-azobenzene. (b) Cis-azobenzene (corrected for a residual contribution of the trans-form).
The emission spectra are corrected for the spectral sensitivity of
the spectrometer. A properly scaled solvent background has
been subtracted. To determine the scaling factor, an increasing
percentage of the solvent background is subtracted until the
Raman line around 580 nm of the DMSO solvent completely
vanishes (the sharp structure around 580 nm in the emission
spectrum shows the small artefacts that still remain after
subtraction).
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H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
emission spectrum of cis-azobenzene obtained by
this procedure (Fig. 3b) peaks at 600 nm and has a
width of 160 nm (FWHM). It is narrower than
that of the trans-isomer and lacks the wing in the
red spectral region. The integrated emission intensity of cis-azobenzene is very close to that of
trans-azobenzene, it amounts to 80%. This relative
emission quantum yield of the two samples opens
a route to information on the relative lifetimes of
the emissive states of the two isomers. For snr sf
the emission quantum yield /f is given by
/f ¼
snr
:
sf
ð1Þ
Here sf is the radiative and snr is the non-radiative
time constant of the emissive state. The radiative
lifetime can be estimated from the absorption
spectrum via the Strickler–Berg relation [18]
Z
1
9 2 3 1
sf ¼ 2:880 10 n h~
d ln m~:
ð2Þ
mf ihm
The Strickler–Berg relation was originally derived
for molecules with non-reactive excited states. The
application to azobenzene might, therefore, be
debatable. However, since here only the ratio of
lifetimes is of interest, and the two molecules
(trans- and cis-azobenzene) are spectroscopically
very similar (absorption and emission), the use of
this relation seems to be justified. Furthermore, the
Strickler–Berg relation was successfully used to
estimate radiative lifetimes of other rapidly isom1
erizing molecules [19]. The expression h~
m3
f ihm is the
3
reciprocal of the mean value of m~ and was determined by integrating the emission spectra. The
refractive index n of the solvent DMSO was set
equal to 1.478 [20]. The integral in Eq. (2) covers
the lowest energy (np ) transition in the absorption
spectra, being the extinction coefficient. The radiative lifetimes sf obtained are 530 ns 1 and 200 ns
for trans- and cis-azobenzene, respectively. Comparing the ratio of these two lifetimes (2.65) with
the ratio of the emission quantum yields affords a
ratio of the non-radiative lifetimes equal to
snr;cis =snr;trans ¼ 0:3.
1
Fujino et al. [12] derived a value of 670 ns based on a
simplified version of Eq. (2).
4. Discussion
The aim of this Letter is a deeper understanding
of the differences between the trans ! cis and
cis ! trans directions in the photo-isomerization of
azobenzene. We seek this understanding by comparison of time-resolved absorption spectroscopy
and steady state fluorescence measurements. For
the trans ! cis direction transient absorption afforded three time constants of 0.34, 3.0 and 12 ps.
The two shorter time constants carry nearly equal
amplitudes throughout the covered spectral range.
The dynamics of the cis ! trans direction can be
modelled by time constants of 0.1, 0.9 and 5.6 ps.
The dynamic behaviour is dominated by the 0.1 ps
component, the 0.9 ps component is much weaker
and bears no spectral resemblance with the 0.1 ps
contribution. For both directions, the dynamics on
the 10 ps time scale are due to molecular cooling
processes in the electronic ground state. This assignment is in accordance not only with earlier
transient absorption data [9], but also with results
from time resolved vibrational spectroscopy
[11,13]. For cis-azobenzene one time constant of
0.1 ps dominates the dynamics and can therefore
safely be assigned to the decay of the electronically
excited state. It is this time constant which enters
as snr Eq. (1) for the determination of the fluorescence quantum yield /f yielding /f ¼ 0:1ps=sf .
The situation is different for the trans-isomer. Here
the dynamics are bi-phasic with similar spectral
signature and amplitude, which rises the question
which time constant is a good candidate for snr in
Eq. (1). The quantum yield /f is proportional to
the temporal integral of the excited state decay.
For a bi-exponential decay with amplitudes A1;2
and time constants s1;2 this yields the following
relation
/f ¼
A 1 s1 þ A 2 s2 1
:
A 1 þ A 2 sf
ð3Þ
In our particular case (A1 A2 ) this would indicate
that /f ¼ ð0:34 ps þ 3:0 psÞ=2 ¼ 1:7 ps=sf . The
fluorescence spectrum of the trans-isomer would
then be much more intense (by a factor of 6) than
that of the cis form. However, the experimental
intensities are nearly equal. If one considers only
the first time constant of 0.34 ps for snr the
H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
Strickler–Berg analysis reproduces the experimentally observed ratio of fluorescence intensities. This
seems to indicate that only the short time constant
of the trans-azobenzene kinetics is associated with
the decay of the excited or more precisely the
emissive state. On the other hand, the close similarity of the decay associated spectra of the 0.34
and the 3.0 ps component have to be explained.
The similarity of the transient absorption spectra
suggest that they originate from the same electronic state. A possible resolution of this apparent
contradiction is that the 0.34 ps time constant
describes a movement of the wavepacket generated
by the optical excitation out of the Franck–Condon region and that the fluorescence emission
originates only from the Franck–Condon region.
This assignment is underscored by time resolved
fluorescence studies by Fujino et al. [12]. They
determined the fluorescence lifetime of the S1 state
of trans-azobenzene in hexane to be 0.50 ps and
found no longer living component.
To rationalize these different fluorescence and
absorption properties of the 0.34 and the 3.0 ps
components one has to consider which molecular
potential energy surfaces are responsible for the
respective spectra. The fluorescence spectrum is
governed by the difference between the electronic
ground state and the first excited state. The transient absorption spectrum probes the difference
potential between the excited state and all other
states accessible by the probe pulse. For the np
transition encountered here the contribution of the
stimulated emission is very weak and the transient
absorption spectrum is, therefore, dominated by
transitions to higher lying excited states. We assume that the potential energy surface of the
ground state is steeper and more structured than
those of the excited states (to be justified below).
Under this condition, the motion of a vibrational
wavepacket on the S1 surface will induce larger
changes in the fluorescence spectrum than in the
transient absorption signal (Fig. 4). This concerns
not only the frequencies of the spectra but via the
Franck–Condon factors and the transition moments also their intensities. In the following distinct points of these potential energy surfaces are
determined from our spectral data in order to
support this interpretation (Fig. 5).
221
Fig. 4. Schematic sketch of the movement of a wavepacket
along the potential energy surface of an excited state. Since here
the slope of the two excited states are similar this movement has
little influence on the transient absorption. The slopes of
ground and the first excited state differ significantly inducing
large changes in the emission frequency.
The difference of the ground state energies of
cis- and trans-azobenzene is obtained from their
heats of formations [21], the cis-form lying 0.6 eV
above the trans-isomer. The barrier for their interconversion is 1.6 eV as derived from the
temperature dependence of the isomerization rate
[22]. The steady state absorption spectra determine
the energies of the first two excited states (np and
pp ) of the trans- and the cis-form. The spectra of
the photo-excited states (np ) hold informations
on higher lying excited states. The decay associated spectra for the shortest time constant suggest
that for both isomers two higher states at 550 and
350 nm are optically accessible. After the decay
of the ultrafast component of the trans-form a
spectrum persists with peaks at 400 and 550 nm.
Of course this only fixes the difference between this
intermediate and the higher lying state. Concerning their absolute position the following boundaries are reasonable: Its energy has to be lower
than that of the primarily excited state (<2.8 eV)
and should be higher than the trans–cis-barrier
(>1.6 eV). This would place it as indicated in
the central part of Fig. 5. When comparing these
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H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
Fig. 5. Potential energy scheme of azobenzene. The energies in the center of the figure are derived from steady state and time-resolved
spectroscopy, chemical kinetics, and thermochemistry. The energy levels on the right- and left-hand side of the figure are results from
quantum chemical calculations [23].
experimental energies with quantum chemical
calculations [23] it is obvious that the calculations
can only reproduce the lowest transition energy
(S1
S0 , see Fig. 5). The energies of the higher
transitions tend to be overestimated [23,24].
Therefore, unfortunately, the interpretation of the
transient spectra which require these higher levels
cannot borrow from the calculations.
In Fig. 5, levels of the three forms (trans, cis and
intermediate) are tentatively correlated. These
correlations lead to the following interpretation of
the observed dynamics. The excited cis-isomer
experiences a steep potential close to the Franck–
Condon region resulting in a fast and directed
motion. This motion brings the wavepacket to the
conical intersection with the ground state and is
responsible for the efficient isomerization. The
excited state potential at the trans Franck–Condon
region is more shallow. Therefore, the motion out
of this region is slower. This motion is associated
with a slight decrease in energy and moves the
transition to the Ô350 nmÕ state out of the covered
spectral window and reduces the probability for
the emission to the ground state in the visible part
of the spectrum dramatically. This in turn explains
why the fluorescence measurement is only sensitive
to the fastest component. Apparently the initial
motion of the wavepacket brings the molecule to a
region of the S1 surface where a direct and fast
access to the conical intersection with the ground
state is not possible. This leads to observed slow
3.0 ps reaction.
In conclusion, we have demonstrated that absorption and emission techniques can probe different aspects of the excited state dynamics of
photo-reactive systems. Emission measurements
are mainly sensitive for changes in the vicinity of
the Franck–Condon region; transient absorption
H. Satzger et al. / Chemical Physics Letters 372 (2003) 216–223
spectra cover a broader area of the excited state.
This was demonstrated for the ultrafast photoisomerization of azobenzene. Here, differences
between the trans ! cis and cis ! trans direction
are detected. For the isomerization of cis-azobenzene is was demonstrated that depletion of the
Franck–Condon region and the excited state decay
are synchronous. The isomerization of trans-azobenzene is bi-phasic. A fast component of 0.34 ps
contributing to the emission and the absorption
signal is due to the initial motion out of the
Franck–Condon region. A much slower time
constant of 3.0 ps only observable via absorption
represents the excited state decay.
Note added in proof
After submission of this Letter, a paper on the
femtosecond fluorescence dynamics of azobenzene was published [25]. The authors observed a
dominant decay component of 200 fs for transazobenzene in hexane. This time constant is close
to the value of 340 fs (in DMSO) we determined
by transient absorption spectroscopy. Based on
the relative fluorescence quantum yields we predicted the 3 ps component of the absorption
measurement to be absent in the fluorescence
decay. The fluorescence decay traces in [25],
however, contain a long-lived component. This
long-lived component has a shorter lifetime
(1 ps) than the value obtained here and its
relative amplitude (with respect to the short
component) amounts only to 1:6 as compared
to 1:1 in the absorption experiments reported
here. Therefore, our finding that the dominant
part of the fluorescence emission decays within
some 100 fs is not in contrast to [25] and the
interpretation that this fast decay is associated
with the movement out of the Franck–Condon
region remains valid.
223
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