Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S1 state

Article abstract of DOI:10.1039/b419236b

Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the photoinduced isomerization reactions of a rotation-restricted trans-azobenzene (trans-AB) derivative capped by a crown ether (1), a chemically similar open d

Full text of DOI:10.1039/b419236b

View Article Online / Journal Homepage / Table of Contents for this issue
R E S E A R C H P A P E R
Femtosecond fluorescence up-conversion spectroscopy of a
rotation-restricted azobenzene after excitation to the S1 statew
T. Pancur,a F. Renth,*a F. Temps,*a B. Harbaum,b A. Kruger,b R. Hergesb and
¨
c
¨
Chr. Nather
a Institut fur Physikalische Chemie, Christian-Albrechts-Universitat zu Kiel, Ludewig-Meyn-Str.
8, D-24098 Kiel, Germany. E-mail: renth@phc.uni-kiel.de, temps@phc.uni-kiel.de
¨
¨
b Institut fur Organische Chemie, Christian-Albrechts-Universitat zu Kiel, Otto-Hahn-Platz 3,
D-24098 Kiel, Germany
¨
¨
c Institut fur Anorganische Chemie, Christian-Albrechts-Universitat zu Kiel,
Otto-Hahn-Platz 6–7, D-24098 Kiel, Germany
¨
¨
Received 23rd December 2004, Accepted 24th March 2005
First published as an Advance Article on the web 11th April 2005
Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the photo-
induced isomerization reactions of a rotation-restricted trans-azobenzene (trans-AB) derivative capped by a crown
ether (1), a chemically similar open derivative (2), and unsubstituted trans-AB (3) after excitation to the S1 (np*)
state at l ¼ 475 nm in dioxane solution. The observed biexponential temporal fluorescence profiles for 1 and 2
were almost indistinguishable within experimental error. The fitted fast fluorescence decay times (ꢀ2s) for the
two compounds were t1 (1) ¼ (0.79 ꢀ 0.20) and t1 (2) ¼ (1.05 ꢀ 0.20) ps, compared to t1 (3) ¼ (0.37 ꢀ 0.06) ps.
The second decay components could be described with t2 (1) ¼ (20.3 ꢀ 9.5) resp. t2 (2) ¼ (19.0 ꢀ 6.0) ps, vs. t2
(3) ¼ (3.26 ꢀ 0.85) ps. The very similar lifetimes strongly suggest that trans–cis isomerization of 1 and 2 after
S1 excitation is governed by the same mechanism. Since 1 cannot isomerize by a simple large-amplitude rotation
of one of the phenyl rings about the central NN bond, the isomerization dynamics of both ABs should be better
described as ‘‘inversion’’ at the N atom(s) rather than large-amplitude ‘‘rotation’’.
via a semi-linear planar CNNC transition state.
1. Introduction
The photo-induced cis–trans isomerization of azobenzene (AB)
is of substantial interest for optical data-storage devices1,2 or
laser-triggered molecular switches.3–8 However, the detailed
mechanisms and dynamics of this prototypical reaction are
still a matter of controversy. A longstanding question is
whether the isomerization takes place as ‘‘inversion’’ on one
of the N atoms or as large-amplitude ‘‘rotation’’ about the
central NN bond or whether it requires more complex nuclear
motions.9,10
Important experimental evidence pertaining to the ensuing
Transient absorption and fluorescence decay measurements
carried out in the past few years showed that the isomerization
of trans-AB after S1 excitation takes place on time scales
between a few hundred femtoseconds and several pico-
seconds.11–16 The corresponding reaction of cis-AB has been
found to be even faster.15,16 Most workers assumed the above-
mentioned semi-linear ‘‘inversion’’ pathway.11–13,15 However,
the ‘‘rotational’’ isomerization route has recently received
substantial support from a number of quantum chemical
calculations. In particular, calculations on the semi-linear
inversion mechanism in the S1 state predicted a potential
energy barrier for that pathway.14,17,18 In contrast, torsional
motion has been suggested to lead to an easily accessible
conical intersection (CI) between the S1 and S0 states that
can mediate an ultrafast radiationless relaxation to the ground
state near the 901 point of the CNNC out-of-plane rotation
pathway.17–20 In addition, Diau recently proposed a ‘‘con-
certed inversion’’ channel.17 Chang et al. reported experimental
evidence supporting the rotational isomerization route in low
viscosity solvents (hexane) and the concerted inversion route in
high viscosity solvents (ethylene glycol).21 A time-resolved
investigation of the photochemical dynamics of the capped
reaction pathways has been reported by Rau and co-workers,
who carried out quantum yield measurements for capped AB
derivatives, in which a large-amplitude rotational motion of
the aromatic rings during isomerization should not be feasible.
In particular, they studied the photoisomerization of an azo-
benzophane9 and the AB derivative 1, in which the two
aromatic rings are connected by a crown ether.10 The experi-
mental results were found to depend on whether the molecules
were excited to the S1(np*) or to the S2(pp*) electronic state. In
particular, the quantum yields for isomerization of 1 and
unsubstituted trans-AB (3) following excitation to the S1 state
were found to be practically the same within experimental error
(F E 0.25 and 0.29), while substantial differences (F E 0.11
and 0.29, respectively) were encountered after excitation to the
S2 state.10 Rau et al. thus concluded that S1 excitation leads to
isomerization by inversion, which they assumed to take place
w Electronic supplementary information (ESI) available: Experimental
details and crystal structure. See http://www.rsc.org/suppdata/cp/b4/
b419236b/
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 8 5 – 1 9 8 9
1985

View Article Online
AB 1, in which the rotational isomerization pathway does not
appear possible, thus seems to be of great interest.
In this publication, we report on first results of a femto-
second time-resolved experimental study of the dynamics of
trans-1 (3,30-bis(1,10-diaza-4,7,13,16-tetraoxa-18-crown-6-car-
bonyl)-trans-AB) and the chemically closely related open deri-
vative trans-2 (3,30-bis(diisopropylaminocarbonyl)-trans-AB)
in dioxane solution following excitation to the S1 state. 1 has
previously been studied with femtosecond time resolution only
after excitation to the S2 state at l ¼ 303 nm,13 while 2 has not
been investigated at all to our knowledge. The molecules were
excited to the S1 state at l ¼ 475 nm and the ensuing dynamics
were monitored by femtosecond fluorescence up-conversion
spectroscopy. A comparison of the results for the two AB
derivatives with each other and with data for the parent
compound trans-AB (3) provides some interesting new insight
into the ensuing isomerization mechanisms.
Fig. 1 X-Ray structure of 1.
10ꢁ4 M with E40 000 to 50 000 excitation pulses per time step.
Five scans taken on different days were averaged to improve
the signal-to-noise ratio. Measurements of 2 were carried out at
a concentration of 1 ꢂ 10ꢁ3 M, resulting in a correspondingly
higher signal-to-noise ratio, and two scans were averaged.
trans-AB (3), purchased from Merck (98%), was measured at
a concentration of 7.5 ꢂ 10ꢁ3 M.
2. Experimental section
Experiments were carried out at room temperature employing
a flow sample cell with 0.2 mm sapphire windows and 1 mm
optical path length.16 Excitation light pulses at l ¼ 475 nm
with E50 fs (Gaussian fwhm) duration were provided by a
home-built non-collinear optical parametric amplifier
(NOPA)22 pumped by E200 mJ pulses from a regeneratively
amplified Ti:sapphire femtosecond laser system (Clark MXR
CPA 2001, l ¼ 775 nm, E150 fs fwhm, 1 kHz repetition rate).
Excitation pulses of approximately 0.5 mJ were focused into the
sample cell to a spot size of E400 mm. Fluorescence was
collected and refocused into a 0.2 mm BBO crystal (GWU)
for up-conversion by type II sum frequency generation with the
775 nm gate pulses (E120 mJ) from the Ti:Sa laser using a pair
of off-axis parabolic mirrors (Melles-Griot, f ¼ 119 mm). A
Schott OG550 filter between the mirrors removed scattered
pump light. The upconverted light was bandpass filtered
(Schott UG11), focused onto the entrance slit of an f ¼ 0.1 m
double monochromator (Jobin-Yvon HR 10), and detected
with a photomultiplier (Hamamatsu R1527P) connected to a
preamplifier (Stanford Research SR 445) and a gated photon
counter (Stanford Research SR 400). Temporal fluorescence
profiles were recorded by stepping the time delay of the gate
pulses using a computer-controlled linear translation stage
(Physik Instrumente M-126CG). The instrument response
function (IRF), determined by upconverting scattered Raman
light due to the CH stretching vibrations of the solvent or by
cross correlation of the pump and gate pulses, was represented
by a width parameter of tirf E 0.21 ps (standard deviation of a
normalized Gaussian, corresponding to a fwhm of E0.50 ps).
1 and 2 were synthesized as described in the electronic
supplementary information (ESI)w following a modification
of the procedure of Shinkai et al.23 Both compounds (E95 and
97% purity, respectively) were checked by NMR spectroscopy.
Compound 1 was additionally characterized by single crystal
X-ray diffraction.w The structure of 1 which is depicted in
Fig. 1 confirmed the trans configuration of the azo moiety. In
addition, it showed that the two aromatic rings are slightly
non-coplanar (71) and the aminocarbonyl groups are twisted
by 581 with respect to the aromatic planes. The main impurity
of 1 which could not be removed by simple means was the
corresponding azoxybenzene. However, this molecule does not
show any absorption at the excitation wavelength used24 so
that it does not affect the fluorescence measurements. Further
details can be found in the ESI.w
3. Results
3.1. UV absorption spectra
The investigations were initiated by measuring stationary UV/
Vis spectra of the three molecules of interest. All three spectra
are broad and unstructured. The weak first absorption max-
imum of trans-AB (3) is located at l ¼ 445–450 nm (S1 ’ S0),
the strong second transition peaks at l ¼ 319 nm (S2 ’ S0).24
A closer inspection suggests that the S1 ’ S0 absorption
maximum of trans-1 may be slightly blue-shifted compared
to trans-AB (Dl E 5 nm), while the S2 ’ S0 maximum appears
red-shifted by about the same amount. The position of the
S1 ’ S0 band maximum of trans-2 is virtually identical with
that of trans-AB, while the S2 ’ S0 maximum is slightly red
shifted (Dl r 3 nm). However, these differences are very
minor. Thus, the meta-substitutions on the aromatic rings of
the trans-AB nucleus by the aminocarbonyl groups and the
crown ether appear to have little influence on the np* and pp*
excited electronic states of the AB moieties of the molecules.
Significant differences between the UV spectra of 1 or 2 and the
parent 3 were only observed at wavelengths of l r 280 nm,
where the derivatives exhibit additional absorption bands
presumably due to the carbonyl groups, but this difference is
not relevant in the present study of the S1 state.
3.2. Time-resolved measurements
The l ¼ 475 nm pump pulses in the time-resolved experiments
excited the molecules in the red wings of their S1 ’ S0
absorption bands. Fluorescence was monitored at two detec-
tion wavelengths, l ¼ 622 and 655 nm, close to the peak of the
emission spectrum of trans-AB.14–16 Since the data taken at the
two wavelengths did not show detectable differences they were
averaged. The resulting temporal fluorescence profiles for the
three molecules are displayed in Fig. 2. Although the data for 1
and 2 are somewhat noisy due to the poor solubilities and
the limited number of experimental runs, it can be seen that the
time profiles of the rotation-restricted capped AB 1 and
the open derivative 2 are very similar. Both decay curves are
biexponential, with two distinctive time constants. As can be
clearly seen, the fluorescence from both derivatives decays
more slowly than from the parent compound 3.
Sample solutions were prepared using dry dioxane (Merck,
Uvasol) and pumped through the cell using a peristaltic pump
(Ismatec, MS-CA 2/860, 6.8 ml minꢁ1 flow rate). The number
of experimental runs that could be made was limited by the
small amounts of substance available. Measurements of the
poorly soluble 1 were carried out at a concentration of 5 ꢂ
For a quantitative analysis, the measured time profiles were
fitted with a sum of two decaying exponentials convoluted with
the IRF using a non-linear least-squares routine based on the
Levenberg–Marquardt algorithm.25 The two time constants
1986
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 8 5 – 1 9 8 9
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

View Article Online
scales (t1 and t2) are well separated, indicating that they should
belong to two genuine dynamical processes.
In the following, we compare the fluorescence decay times of
the three ABs of interest, consider work on related stilbenes,
and discuss the dynamical processes which may be responsible
for the observed two decay time scales.
4.1. Observed fluorescence decay times
The striking result of the present work that sheds interesting
light on the photoisomerization dynamics of ABs is that the
temporal fluorescence decay profiles of the rotation-restricted
capped AB 1 and the chemically similar open AB 2 are almost
indistinguishable (cf. Fig. 2).
In particular, the fluorescence profiles of all three ABs
studied exhibit distinctive biexponential decays. The ‘‘fast’’
fluorescence components (E75% rel. amplitude) disappear
on the sub-picosecond to picosecond time scale. The respective
lifetimes of the derivatives 1 and 2 (t1 ¼ 0.79 and 1.05 ps) are
only about 2.5 times longer than for the parent molecule 3
(t1 ¼ 0.37 ps). A larger difference (factor of six) between 1 and
2 on the one hand and 3 on the other was found for the ‘‘slow’’
fluorescence components (E25% rel. amplitude); the observed
lifetimes are t2 E 20 ps for 1 and 2 vs. t2 E 3.3 ps for 3.
Furthermore, the t1 value for 1 appears to be even slightly
shorter than that for 2, despite of the extra hindrance of the
isomerization due to the crown ether, while the t2 values are
virtually identical. The ratio of the fast and slow fluorescence
amplitudes for all three ABs is a1/a2 E 3/1.
Fig. 2 Measured fluorescence decay curves of the trans-ABs 1, 2, and
3 in dioxane. The experimental data are given by the open symbols, the
fitted profiles by the solid lines.
and two amplitudes were used as adjustable parameters. In
addition, it was checked by varying tirf that the time resolution
of the fluorescence measurements agreed with the IRF derived
from the Raman signal and the pump-gate cross correlation.
The fit results are collected in Table 1. The decay times of the
first fluorescence components were found to be
t1(1) ¼ 0.79 ps, t1(2) ¼ 1.05 ps, t1(3) ¼ 0.37 ps;
the second components were described with
We note that it is by no means obvious whether the ultrafast
kinetics of a barrierless or near barrierless reaction like the
isomerization of AB (S1) should follow a mono- or bi- or
multi-exponential decay law. Dynamical models developed
for the similar near barrierless excited state isomerization of
cis-stilbene (trans-stilbene has a significant potential energy
barrier for isomerization and is described by unimolecular rate
theory29) indeed predicted non-exponential decays not attrib-
uted to two distinctive dynamical processes.30 However, the
experimental data on cis-stilbene available at the time showed
mono-exponential behaviour.30–32 The reported exponential
decay time (E1 ps) for cis-stilbene is significantly longer than
that for trans- and cis-AB. Further, recent work showed that
there is a shallow well on the excited cis-stilbene PES, support-
ing some low-frequency vibrations.33,34 The trans–cis and cis–
trans isomerization reactions of AB (S1) thus appear to be
much better prototypical examples for barrierless reactions
than the cis-stilbene reaction.
In agreement with the literature on AB, we take the very
clear separation between the observed two decay time scales
(t1,t2) for AB and especially for the AB derivatives as evidence
for (at least) two distinctive dynamical processes.14,15,21,26,27
The striking similarities then strongly suggest that photoisome-
rization of the AB derivatives 1 and 2 is governed by the same
molecular mechanisms. Whether or to which extent this also
applies for the unsubstituted parent AB 3 remains a question
which we defer to the end of this discussion. The comparison of
the experimental decay times for the parent AB on the one
hand and the derivatives on the other hand suggests that there
are at least subtle differences.
t2(1) ¼ 20.3 ps, t2(2) ¼ 19.0 ps, t2(3) ¼ 3.26 ps.
4. Discussion
Considering the detailed molecular mechanisms responsible for
the observed dynamics, we start with the following premises: (i)
The observed ultrashort fluorescence lifetimes of trans-AB 3
and its derivatives 1 and 2 studied in the present work are
related to ultrafast nonradiative electronic relaxation and trans–
cis isomerization of the molecules. Both processes are assumed
to take place through CIs between the S1 and S0 potential energy
surfaces (PES) of the molecules near the mid-points of the trans–
cis isomerization reaction coordinates.11,14–21,26,27 As has been
shown, S1 fluorescence and excited state (Sn ’ S1) absorption
decay on the same time scale and this decay is accompanied by
the appearance of ‘‘hot’’ ground state absorption as signature of
the S1 - S0 internal conversion.11,12,15,28 Thus, nonradiative
electronic relaxation and isomerization of AB are closely con-
nected. (ii) The measured S1 - S0 absorption spectra do not
indicate significant differences between the respective excited
electronic state structures of the three molecules studied. This
suggests that the initial states of the molecules after excitation to
the S1 PES should be comparable. (iii) The observed fast sub-
picosecond to picosecond fluorescence decay times show that the
trans - cis isomerization of AB is a barrierless or nearly
barrierless reaction. (iv) The observed two fluorescence time
Table 1 Fluorescence decay times and relative amplitudes for the
rotation-restricted capped AB derivative 1, the open derivative 2, and
the unsubstituted parent compound 3 in dioxane solution
4.2. Comparison with related stilbenes
As mentioned, the longstanding question is whether the reac-
tion coordinate for the photoisomerization of AB is better
characterized as ‘‘rotation’’ about the central NN bond or
‘‘inversion’’ at one (or both) of the N atoms. While the actual
pathways may be more complex, a fundamental classification
of the dynamics along these two lines is justified by comparison
of the photoisomerization quantum yields of the cited capped
ABs9,10 (e.g., 1) and similar capped stilbenes. In particular,
trans-AB
a1 (%)a
t1/fsa
a2 (%)a
t2/fsa
1
2
3
a
75.3(95)
72.2(61)
79.7(74)
0.79(20)
1.05(20)
0.37(6)
24.7(56)
27.8(51)
20.3(39)
20.3(95)
19.0(60)
3.26(85)
2s standard deviations in parentheses.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 8 5 – 1 9 8 9
1987

View Article Online
Rau and Waldner recently reported that the photoisomeriza-
tion quantum yield for a trans-stilbenophane is F ¼ 0.35 This
highly intriguing result is rationalized by the blocking of the
‘‘rotational’’ reaction coordinate in the stilbenophane.
At least initially, the wavepacket should move mainly along the
NNC bending coordinate. This conclusion has been supported
by a recent semiclassical dynamics calculation.20 The longer t1
values of 1 and 2 compared to the parent molecule 3
are attributed to increased solvent friction due to the extended
side chains. In addition, the direct wavepacket motion in
the direction of steepest descent on the S1 PES may be
accompanied by some rapid partial intramolecular vibra-
tional redistribution (IVR) among vibrational modes
which are strongly coupled with the NNC bending mode.
The molecules were prepared with about 2000–3000 cmꢁ1 of
internal energy above the S1 origin, corresponding to
several vibrational quanta which may be partially redistri-
buted.
The slow fluorescence decay times of t2 E 20 ps for the AB
derivatives are attributed to their transition to the S0 state
through the CI along the inversion coordinate. This dynamics
can be pictured as indirect, ‘‘diffusive’’ motion on the S1 PES.
The molecules may execute a sort of ‘‘meandering’’ motion,
miss the CI on their first pass, and survive on the S1 PES until
they find the CI from different directions at a later time. The
diffusive motion is partially a consequence of the fact that the
CI is not located exactly on the initial direct path of the
wavepacket and partially a consequence (and manifestation)
of fast partial ‘‘IVR’’. Following Diau,17 the CI between the S1
and S0 states is located along the concerted inversion coordi-
nate at an almost linear CNNC configuration. The funneling of
the wavepacket through the CI and the subsequent wavepacket
evolution on the S0 PES determine whether the product
molecules end up in the cis or the trans configurations. The
higher number of vibrational degrees of freedom and the
increased solvent friction of the AB derivatives lead to a much
slower ‘‘diffusive’’ arrival of the excited wavepacket at the
CI, hence the increase of t2 from E3.3 ps to E20 ps
with increasing molecular size. Additionally, on the time
scale of E20 ps the molecules in the S1 state experience
substantial vibrational relaxation by the solvent. Thus, the t2
values for the derivatives are essentially attributed to
vibrationally relaxed S1 molecules. For the unsubstituted AB,
t2 is too short for a large degree of vibrational relaxation
within the S1 state. Eventually, since the current experiments
had to be optimized for parallel polarization detection to
observe the extremely weak fluorescence from 1 and 2, it
cannot be excluded that rotational depolarization may play a
small role.
For the case of stilbene and for several polyenes, it is well
known that the cis–trans isomerization coordinate is much
more complex than a simple large-amplitude rotation.33,34 The
actual isomerization coordinate for stilbene is assumed to be a
mixture of several internal coordinates showing torsional mo-
tion around the ethylenic CC bond, including out-of-plane CH
wag and other oscillatory motions on the multidimensional
PES, which minimize solvent friction.30,31 The term ‘‘hula-
twist’’ motion has been coined to describe the combined
effect.33,34 However, the photoisomerization quantum yield
of F ¼ 0 for the trans-stilbenophane shows that under the
experimental conditions used neither rotation nor hula-twist
nor inversion nor another type of motion appears to be a
feasible reaction coordinate for that molecule.35
With these premises, the contrast between the photoiso-
merization quantum yield of F ¼ 0 for the stilbenophane35
compared with the ‘‘normal’’ quantum yields found for the
similar rotation-restricted azobenzophanes and the crown
ether capped AB 1 (F E 0.24 and 0.29)10 can be rationalized
only if the capped ABs undergo photoisomerization by ‘‘in-
version’’. The same route is thus suggested for the open AB
derivative 2. The inversion may not take place via the originally
proposed semi-linear CNNC configuration, for which there
appears to be a potential energy barrier.17,18 Instead, Diau
proposed a concerted inversion pathway, via a CI between the
S1 and S0 states at an almost linear CNNC configuration.17 It is
of interest in this context that according to a picosecond
Raman study the NN bond of trans-AB in the S1 state keeps
a double bond character.36 In addition, we would like to
mention the possibility of a ‘‘hula-twist’’ reaction coordinate
similar to the stilbene case, involving some out-of-plane mo-
tions of the N atoms as a ‘‘non-planar inversion’’ variant. The
hula-twist motion, which has not yet been theoretically ex-
plored in sufficient detail, would lead to the same orientation of
the phenyl ring as a planar inversion, whereas rotation of the
phenyl group about the NN bond exchanges the C atoms on
the two sides of the ring.
4.3. Mechanistic explanation of the bi-exponential decay
profiles
Having identified inversion as the reaction pathway for the AB
derivatives 1 and 2, the origin of the biexponential decay curves
moves into focus. Here, possible differences have to be taken
into account between the dynamics of unsubstituted AB15,21,27
and the AB derivatives. We shall discuss common features first
and specific differences afterwards.
Considering the unsubstituted parent trans- or cis-AB, the
‘‘inversion’’ mechanism has received support from Nagele et al.
and Satzger et al.,11,15,27 who compared time-resolved absorp-
tion results on trans- and cis-AB after S1 (np*) and after S2
(pp*) excitation. Transient signals of trans-AB in ethanol or
DMSO in the case of S1 excitation were found to exhibit decay
times of t1 E 0.3 and t2 E 2–3 ps; those of cis-AB could be
described with decay times of t1 E 0.1–0.17 and t2 E 0.9–1.0
ps, in excellent agreement with the fluorescence up-conversion
data on the S1 state obtained in our laboratory16 and by Lu
et al.14 Apart from an additional very fast decay time (t0.1 ps)
attributed to internal conversion from the S2 to the S1 state,
virtually the same decay times were found following S2 excita-
tion, in agreement in this case with fluorescence up-conversion
data by Fujino et al.37 These results were interpreted by Satzger
et al. in terms of a two step model, whereby an ultrafast
radiationless transition (t0.1 ps) takes the molecules first
from S2 to S1. The actual isomerization then follows from
the subsequent large-amplitude motions on the S1 PES towards
the CI with the S0 state close to the midpoint of the cis–trans
isomerization coordinate. Further slow processes appearing on
the 5–20 ps time scale were attributed to vibrational cooling of
the produced hot ground state molecules. Likewise, Lednev
et al.,13 who reported transient absorption measurements of 1
after excitation to the S2 state, found an exponential decay with
The starting point for the dynamics is defined by the location
of the initial wavepacket prepared on the S1 PES. Considering
the np* nature of the S1 ’ S0 transition, where a non-bonding
electron is removed from one of the N atom lone pairs, the
Franck–Condon region is determined by a high degree of
excitation of the S1 NNC bending vibration. Incidentally, this
vibration coincides basically with the inversion reaction co-
ordinate. In addition, there is a shortening of the CN bond in
the S1 state compared to the S0 state. The Franck–Condon
determined initial location of the excited wavepackets on the
S1 PES at t ¼ 0 should be similar for all three ABs.
In line with previous arguments,15,21,27 the fast initial decay
times (0.3 r t1 r 1 ps, respectively) are therefore attributed to
fast motion of the excited wavepackets on the S1 PES away
from the Franck–Condon region towards the CI (or CIs) with
the S0 PES. The direct motion leads the wavepacket along the
steepest descent on the S1 PES. The even faster (t1 r 0.1 ps)
decay of cis– compared to trans-AB is thus explained by a
steeper gradient of the S1 PES near the cis configuration.15,16
1988
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 8 5 – 1 9 8 9
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

View Article Online
a lifetime of t ¼ 2.6 ps, somewhat longer than the value of t1
found in the present work. In their case, the molecules were
concluded to experience a fast internal conversion to the S1
state within o0.5 ps. The differences between S2 vs. S1 excita-
tion thus reflect the different starting positions of the dynamics
of the wavepackets on the S1 PES.
References
1
2
T. Ikeda and O. Tsutsumi, Science, 1995, 268, 1873.
Z. F. Liu, K. Hashimoto and A. Fujishima, Nature, 1990, 347,
658.
3
4
I. Willner and S. Rubin, Angew. Chem., Int. Ed., 1996, 35, 367.
T. Hugel, N. B. Holland, A. Cattani, L. Moroder, M. Seitz and
H. E. Gaub, Science, 2002, 296, 1103.
However, recent femtosecond fluorescence depolarization
measurements of trans-AB excited to the S1 state in solvents
of different viscosity by Chang et al. showed that the dynamics
of the prototypical parent AB is considerably more complex.21
They observed that the fluorescence anisotropy of trans-AB in
hexane decays with the same time constant as the slow
fluorescence (t2). In contrast, the fluorescence anisotropy in
ethylene glycol did not show a discernable decay. They con-
cluded that both the rotation and the inversion routes are
important, but that their relative contributions depend on the
conditions. In low viscosity solvents (hexane) they assumed
isomerization predominantly via out-of-plane CNNC torsional
motion and the CI at the twisted configuration predicted by
theoretical work,17–20 while the concerted in-plane inversion
pathway17 was preferred in high-viscosity solvents (ethylene
glycol). Quoted differences between the decay times and photo-
isomerization quantum yields of trans- vs. cis-AB and S2 vs. S1
excitation thus reflect different starting points of the excited
wavepackets on the S1 PES, different relative contributions of
the rotation and inversion reaction pathways, and different
branchings of the wavepackets into the cis or trans directions at
the S1–S0 CIs.
5
N. B. Holland, T. Hugel, G. Neuert, A. Cattani-Scholz, C.
Renner, D. Oesterhelt, L. Moroder, M. Seitz and H. E. Gaub,
Macromolecules, 2003, 36, 2015.
L. Ulysse, J. Cubillos and J. Chmielewski, J. Am. Chem. Soc.,
1995, 117, 8466.
J. R. Kumita, O. S. Smart and G. A. Woolley, Proc. Natl. Acad.
Sci. USA, 2000, 97, 3803.
S. Sporlein, H. Carstens, H. Satzger, C. Renner, R. Behrendt, L.
Moroder, P. Tavan, W. Zinth and J. Wachtveitl, Proc. Natl. Acad.
Sci. USA, 2002, 99, 7998.
6
7
8
9
10 H. Rau, J. Photochem., 1984, 26, 221.
11 T. Nagele, R. Hoche, W. Zinth and J. Wachtveitl, Chem. Phys.
Lett., 1997, 272, 489.
12 I. K. Lednev, T.-Q. Ye, P. Matousek, M. Towrie, P. Foggi, F. V.
R. Neuwahl, S. Umapathy, R. E. Hester and J. N. Moore, Chem.
Phys. Lett., 1998, 290, 68.
13 I. K. Lednev, T.-Q. Ye, L. C. Abbott, R. E. Hester and J. N.
Moore, J. Phys. Chem. A, 1998, 102, 9161.
14 Y.-C. Lu, C.-W. Chang and E. W.-G. Diau, J. Chin. Chem. Soc.,
2002, 49, 693.
15 H. Satzger, S. Sporlein, C. Root, J. Wachtveitl, W. Zinth and
P. Gilch, Chem. Phys. Lett., 2003, 372, 216.
16 T. Pancur, PhD thesis, Christian-Albrechts-Universitat zu Kiel,
2004.
H. Rau and E. Luddecke, J. Am. Chem. Soc., 1982, 104, 1616.
17 E. W.-G. Diau, J. Phys. Chem. A, 2004, 108, 950.
18 A. Cembran, F. Bernardi, M. Garavelli, L. Gagliardi and G.
Orlandi, J. Am. Chem. Soc., 2004, 126, 3234.
19 T. Ishikawa, T. Noro and T. Shoda, J. Chem. Phys., 2001, 115,
7503.
20 C. Ciminelli, G. Granucci and M. Perisco, Chem. Eur. J., 2004,
2327.
21 C.-W. Chang, Y.-C. Lu, T.-T. Wang and E. W.-G. Diau, J. Am.
Chem. Soc., 2004, 126, 10109.
22 T. Wilhelm, J. Piel and E. Riedle, Opt. Lett., 1997, 22, 1494.
23 S. Shinkai, T. Nakaji, Y. Nishida, T. Ogawa and O. Manabe,
J. Am. Chem. Soc., 1980, 102, 5860.
24 H.-H. Perkampus, UV-Vis Atlas of Organic Compounds, VCH,
Weinheim, 2nd edn., 1992, vol. 1.
25 W. H. Press, B. P. Flannery, S. A. Teukolsky and W. T. Vetterling,
Numerical Recipes in C, Cambridge University Press, Cambridge,
1988.
26 T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T. J. Martinez,
S. Lochbrunner, M. Schmitt, J. P. Shaffer, M. Z. Zgierski and
A. Stolow, J. Am. Chem. Soc., 2003, 125, 8098.
27 H. Satzger, C. Root and M. Braun, J. Phys. Chem. A, 2004, 108,
6265.
5. Conclusions
In conclusion, we have studied the temporal fluorescence decay
profiles of a rotation-restricted trans-AB derivative capped by
a crown ether (1), a chemically similar open derivative (2), and
the unsubstituted parent trans-AB (3) following excitation to
the S1 (np*) state. The observed biexponential fluorescence
profiles for 1 and 2 were almost indistinguishable within
experimental error, suggesting strongly that trans–cis isomeri-
zation of 1 and 2 after S1 excitation is governed by the same
mechanism. The results support the idea that the internal
conversion and isomerization of the molecules proceeds
through a conical intersection between the S1 and S0 states.
The very fast first fluorescence decay time is attributed to
the initial direct motion of the excited wavepacket on the
S1 PES away from the Franck–Condon region. The slower
second fluorescence decay time is assigned to the transi-
tion from to the S0 to the S1 state by a slower, ‘‘diffusive’’
motion. The isomerization of the capped AB 1 and the open
derivative 2 is concluded to proceed via a planar (and probably
concerted) ‘‘inversion’’ at the N atom(s) or a type of ‘‘hula-
twist’’ motion of the N atom(s) and phenyl ring(s). Further
theoretical studies are needed to fully characterize the possible
inversion routes.
28 P. Hamm, S. Ohline and W. Zinth, J. Chem. Phys., 1997, 106, 519.
29 J. Schroeder, T. Steinel and J. Troe, J. Phys. Chem. A, 2002, 106,
5510.
30 D. C. Todd, J. M. Jean, S. J. Rosenthal, A. J. Ruggiero, D. Yang
and G. R. Fleming, J. Chem. Phys., 1990, 93, 8658.
31 A. Abrash, S. Repinec and R. M. Hochstrasser, J. Chem. Phys.,
1990, 93, 1041.
32 L. Nikowa, D. Schwarzer, J. Troe and J. Schroeder, J. Chem.
Phys., 1992, 97, 4827.
33 K. Ishii, S. Takeuchi and T. Tahara, Chem. Phys. Lett., 2004, 398,
400.
Acknowledgements
34 W. Fuß, C. Kosmidis, W. E. Schmid and S. A. Trushin, Chem.
Phys. Lett., 2004, 385, 423.
35 H. Rau and I. Waldner, Phys. Chem. Chem. Phys., 2002, 4, 1776.
36 T. Fujino and T. Tahara, J. Phys. Chem. A, 2000, 104, 4203.
37 T. Fujino, S. Y. Arzhantsev and T. Tahara, J. Phys. Chem. A,
2001, 105, 8123.
The financial support of this work by the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie
is gratefully acknowledged. A.K. thanks the Fonds der Che-
mischen Industrie for a Liebig stipend, T.P. thanks the Fonds
der Chemischen Industrie for a PhD fellowship.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 8 5 – 1 9 8 9
1989