Direct observation of the nuclear motion during ultrafast intramolecular proton transfer

Article abstract of DOI:10.1016/j.molstruc.2004.01.038

The skeletal motions contributing to the reaction path of the ultrafast excited state intramolecular proton transfer (ESIPT) are determined directly from time resolved measurements. We investigate the ESIPT in the compounds 2-(2′-hydroxyphenyl)benzothiazole, 2-(2′-hydroxyphenyl)benzoxazole and ortho-hydroxybenzaldehyde by UV-visible pump-probe spectroscopy with 30 fs resolution. The proton transfer is observed in real time and a characteristic 'ringing' of the molecule in a small number of vibrational modes is found after the reaction. The results show that a bending motion of the molecular skeleton reduces the proton donor-acceptor distance and an electronic configuration change occurs at a sufficient contraction leading to the bonds of the product conformer. The process evolves as a ballistic wavepacket propagation on an adiabatic potential energy surface. The proton is shifted by the skeletal motions from the donor to the acceptor site and tunneling has not to be considered.

Full text of DOI:10.1016/j.molstruc.2004.01.038

Journal of Molecular Structure 700 (2004) 13–18
www.elsevier.com/locate/molstruc
Direct observation of the nuclear motion during
ultrafast intramolecular proton transfer
*
S. Lochbrunner , K. Stock, E. Riedle
¨ ¨ ¨
Lehrstuhl fur BioMolekulare Optik, Sektion Physik, Ludwig-Maximilians-Universitat, Oettingenstrasse 67, 80538 Munchen, Germany
Received 24 November 2003; revised 27 January 2004; accepted 29 January 2004
Abstract
The skeletal motions contributing to the reaction path of the ultrafast excited state intramolecular proton transfer (ESIPT) are
determined directly from time resolved measurements. We investigate the ESIPT in the compounds 2-(20-hydroxyphenyl)benzothiazole,
2-(20-hydroxyphenyl)benzoxazole and ortho-hydroxybenzaldehyde by UV–visible pump-probe spectroscopy with 30 fs resolution.
The proton transfer is observed in real time and a characteristic ‘ringing’ of the molecule in a small number of vibrational modes is found
after the reaction. The results show that a bending motion of the molecular skeleton reduces the proton donor–acceptor distance and an
electronic configuration change occurs at a sufficient contraction leading to the bonds of the product conformer. The process evolves as a
ballistic wavepacket propagation on an adiabatic potential energy surface. The proton is shifted by the skeletal motions from the donor to the
acceptor site and tunneling has not to be considered.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Excited state intramolecular proton transfer; Hydrogen dynamics; Ultrafast spectroscopy; Wavepacket motion; Vibrations
1. Introduction
proton transfer (ESIPT). Molecules exhibiting ESIPT
typically contain an H-chelate ring with an intramolecular
hydrogen bond. In the ground state, they adopt the enol
form in which the hydrogen is bound to an oxygen atom.
In the first electronically excited state, the keto form with
the hydrogen transferred to a nitrogen or second oxygen
atom at the other side of the ring is the more stable form
[6]. After optical excitation of the enol form ESIPT takes
place and the keto form is generated. Scheme 1 shows the
process for the compounds 2-(20-hydroxyphenyl)ben-
zothiazole (HBT), 2-(20-hydroxyphenyl)benzoxazole
(HBO) and ortho-hydroxybenzaldehyde (OHBA) that are
investigated in this study. The ESIPT leads to a large
Stokes shift between the fluorescence and the absorption
spectrum [6] that has been reported for the compounds
under investigation (HBT: Refs. [7,8]; HBO: Ref. [9];
OHBA: Ref. [10]). In our experiments, the ESIPT is
initiated by an ultrashort light pulse that promotes all the
molecules investigated simultaneously from the ground
state to the electronically excited state. Due to the
intramolecular character and the ring stabilizing hydrogen
bond, the starting geometry is fixed and the dynamics and
mechanism of PT are studied under well defined
conditions.
The dynamics of hydrogen bonds and the reactive proton
transfer (PT) are intimately connected with the stability,
functionality and conformational dynamics of biomolecules
as well as the acidic or basic behavior of many compounds
[1]. Much work has been undertaken to characterize these
processes and the associated potential energy surfaces
(PESs) and to unravel the underlying mechanisms. Steady
state spectroscopy has been very successfully applied to find
and characterize the vibronic eigenstates involved in these
processes [2–4]. Because these states sample large areas of
the PESs, the obtained results are very challenging to
analysis especially with respect to the dynamics.
Experiments with extremely high time resolution
represent a direct and complementary access to the
dynamics [5]. They are able to observe the changes and
motions on the molecular level in real time. For this
purpose, it is necessary to start the process at an exactly
defined time. This is possible in excited state intramolecular
*
Corresponding author. Tel.: þ49-8921809254; fax: þ49-8921809202.
E-mail address: stefan.lochbrunner@physik.uni-muenchen.de
(S. Lochbrunner).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.01.038

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S. Lochbrunner et al. / Journal of Molecular Structure 700 (2004) 13–18
and crosses the pump beam with a small angle of 38.
The sample transmission is measured by detecting the
energy of the probe pulses with a photodiode behind the
sample. To account for small variations of the probe beam, a
reference beam is split off before the sample and measured
with a second photodiode. The sample is a 70 mm thick free
flowing liquid jet or a flow cell with a channel thickness of
120 mm. In both cases, the excited volume of the sample
solution is replaced by a fresh one between successive laser
shots. The ESIPT molecules are dissolved in cyclohexane
with a concentration of 1022–1023 M resulting in an
absorption of about 50% of the pump light.
Scheme 1.
Previous pump-probe experiments demonstrated that
3. Results and discussion
the ESIPT is very fast [7,11] and proceeds even on a
sub-100 fs time-scale [9,12]. Measurements with an
improved time resolution on 2-(20-hydroxy-50-methylphe-
nyl)benzotriazole (TINUVIN P) showed for the first time
that skeletal vibrational modes are coherently excited
after the process [13].
3.1. Wavepacket dynamics of ESIPT
Fig. 1 shows the transient transmission after photoexcita-
tion of HBT (347 nm excitation wavelength) and HBO
(340 nm) at probe wavelengths in the blue and red wing of
the keto emission spectra. All traces show an increase of the
transmission with a delay of several 10 fs and characteristic
oscillations. The Fourier transforms of the oscillatory
contributions are depicted in the insets of Fig. 1. The
transmission increase is due to the onset of the keto
emission. Bleaching of the enol form does not contribute
because the enol absorption is in the UV and well separated
from the probed spectral region. We can therefore be
absolutely sure that the observed kinetics are due to the
dynamics of the molecule in the excited electronic state. The
signal oscillations are caused by the coherent excitation of
specific vibrational modes in the keto form as a result of the
ESIPT. The coherently excited modes lead to oscillatory
shifts of the emission spectrum that are detected as
transmission changes at the fixed probe wavelength [8,17].
The transmission decrease at time zero is caused by the
excited state absorption (ESA) from the S1 state to higher
electronic states. The ESA leads to an increased absorption
immediately when the pump pulse has promoted the
molecule to the S1 state.
We investigated the ESIPT of several compounds by
transient absorption spectroscopy with a time resolution of
30 fs and observed the nuclear motions during this process
[10,14,15]. The transient absorption signals exhibit the
characteristic features of vibrational wavepacket dynamics.
The vibrational modes of the molecular skeleton
contributing to the reaction path are identified from the
oscillatory signal components. We performed this analysis
for HBT [8,15] and OHBA [10] (see Scheme 1). Here, we
give a comparison of the results and include in the analysis
new data on HBO. The common features of the nuclear
motions during the PT are elaborated giving insight in the
course of the reaction path. We discuss the interplay
between the electronic and nuclear degrees of freedom and
demonstrate that the proton itself plays a rather passive role.
2. Experimental
The transient absorption measurements were performed
with a pump-probe spectrometer based on two non-
collinearly phase matched optical parametric amplifiers
(NOPAs) [14,16]. The NOPAs are pumped by a home built
Ti:sapphire laser delivering approximately 100 fs long NIR
pulses at around 800 nm with a repetition rate of 1 kHz. The
NOPAs provide pulses tunable throughout the visible
spectral region that are compressed by a fused silica prism
sequence to below 20 fs. The output of one NOPA is
frequency doubled in a nominally 100 mm thick BBO
crystal cut for type I phase matching and the dispersion of
the resulting UV pulses is compensated with a second fused
silica prism sequence. The UV pulses are guided to a
motorized delay stage and then focused onto the sample to a
spot size of 200 mm. The output of the second NOPA is used
as probe beam. It is focused to a size of 80 mm at the sample
In the case of OHBA, the transient absorption behaves in
a similar way (see Fig. 2). A transmission increase due to the
rise of the product emission is observed shortly after an
initial decrease due to the onset of ESA at delay time zero.
Subsequent oscillatory contributions with characteristic
frequencies are found that are damped on a picosecond
time-scale.
For a quantitative analysis, a model function is fitted to
the time resolved data (solid lines in Fig. 1 and Fig. 2) [8].
The transmission increase is modeled with a delayed step-
like emission rise. The values of the fitted delays are given
for the presented measurements in Table 1. To account for
the ESA, the function contains a step-like transmission
decrease at time zero. Both step-like contributions are
convoluted with the cross-correlation. The oscillatory signal

S. Lochbrunner et al. / Journal of Molecular Structure 700 (2004) 13–18
15
Fig. 2. Time resolved transmission change (open circles) for OHBA excited
at 340 nm probed at 500, 530, and 630 nm together with the fitted model
functions (solid lines). The Fourier transforms of the oscillatory
contributions are depicted in the insets.
Fig. 1. (a) Time resolved transmission change due to excitation of HBT at
347 nm probed at 505 and 597 nm. (b) Transient transmission of HBO
excited at 340 nm and probed at 500 and 560 nm. The data (open circles)
are depicted together with the results of the fitting procedure (solid lines).
The insets show the Fourier transforms of the oscillatory signal
components.
follows a path on the S1 PES that is governed by the steepest
descend. It stays quite confined and no splitting or strong
spreading is observed. According to the Ehrenfest theorem
the center of gravity of the wavepacket moves in an almost
classical fashion [19]. This allows us to describe the process
by a model with classical features. Furthermore, these
contributions are fitted by exponentially damped cosine
functions. The frequencies of the dominant contributions are
listed in Table 1. The damping times are found to be on the
order of 500 fs [8,10]. This is in excellent agreement with
vibrational redistribution times of about 700 fs found by
time resolved IR spectroscopy [18].
Table 1
Fit results for the emission delay and the two dominant frequencies
Compound Wavelength Delay First frequency Second frequency
(cm21
(nm)
(fs)
)
(cm21
)
The rise of emission indicates the delay time when the
molecule reaches the area of the PES belonging to the
product state. Since the emission rise can be modeled by a
delayed step instead of an exponential change and because
the molecules show coherently excited vibrational motions
after the ESIPT, we conclude that the whole process evolves
as a ballistic wavepacket motion [8,10,15]. The wavepacket
launched by the pump pulse at the Franck–Condon point
HBT
505
597
39
52
113
106
254
256
HBO
OHBA
500
560
90
80
114
110
273
271
500
530
630
41
44
46
285
275
296
427
419
420

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S. Lochbrunner et al. / Journal of Molecular Structure 700 (2004) 13–18
observations indicate that no significant energy barrier is
encountered along the reaction path.
The emission rise indicates the arrival of the wavepacket
in the keto region of the PES. Its delay is a measure for the
duration of the ESIPT. In the case of HBT and OHBA, the
observed delays are on the order of 40–50 fs (see Table 1)
[8,10]. The delays observed for HBO are 80–90 fs and quite
similar to the exponential time constants found in previous
studies [9,20,21]. They are significantly longer than those of
HBT and OHBA. We think that in the case of HBO the
initial acceleration in the Franck–Condon region is weaker
than in the other two molecules. However, in all cases, the
delay is on the sub-100 fs time-scale. It is, however, still too
long for a barrierless motion of only the proton [12].
This indicates that skeletal deformations determine the
duration of the process and the inertia of the contributing
nuclei is responsible for the surprisingly large delay.
The similarities in the observed dynamics of HBT, HBO
and OHBA are striking. Measurements on the significantly
larger ESIPT compound N,N0-bis(salicylidene)-p-phenyle-
nediamine reveal again a transfer time of about 50 fs [22]
and the coherent wavepacket dynamics observed in the case
of TINUVIN P [13] has very similar features compared to
the molecules investigated in this study. This similarity
exists even so the excited state lifetime of TINUVIN P is
only 120 fs and thereby 100–1000 times shorter than for the
other compounds [17]. Recently, time resolved ionization
measurements also found a coherent wavepacket motion
associated with the ESIPT of o-hydroxyacetophenone [23].
From these similarities, we conclude that fine variations in
the electronic structure are not important for the dynamics.
This is especially supported by the results on OHBA where
an oxygen atom is the proton acceptor whereas most of the
other compounds have a nitrogen atom at this position.
Fig. 3. Calculated vibrational eigenmodes and frequencies of HBT that
correspond to the dominant oscillatory signal contributions.
normal modes are found to be very similar in both states.
This is indicated by the good agreement between the
calculated and observed frequencies and was confirmed by
high level ab initio calculations on HBT [24].
In Fig. 3, the two vibrational eigenmodes are depicted
that dominate the oscillatory signal contributions in HBT
and HBO. The calculated vibrational frequencies of HBT
are given in Fig. 3 and coincide almost perfectly with the
observed frequencies listed in Table 1. In the case of HBT,
we demonstrated that in total four modes are coherently
excited, but the contribution of the two modes not shown
here is much smaller [8,15]. In HBT and HBO, the same
modes are coherently excited showing that their reaction
paths are very similar. Both modes lead to in-plane
deformations of the H-chelate ring and especially to the
variation of the distance between the hydrogen donating
oxygen atom and the accepting nitrogen atom. Several other
vibrational modes exist with frequencies in the observable
range up to 600 cm21. But they are not or only very weakly
coherently excited indicating that they represent defor-
mations that are of minor relevance for the ESIPT process.
Fig. 4 shows the normal modes that give rise to the
oscillatory signal contributions in the case of OHBA. Both
modes lead again to in-plane deformations of the H-chelate
ring. The mode calculated at 274 cm21 and observed around
285 cm21 (compare Fig. 4 and Table 1) is a bending motion
of the carbonyl group and a counter rotation of the rest of the
molecule to conserve angular momentum. The 420 cm21
mode changes the in-plane angles of the entire molecular
skeleton. In the case of HBT and HBO the two
dominant modes lead to a bending or a contraction of the
whole molecular skeleton whereas the 285 cm21 mode of
OHBA is an almost local bending of the carbonyl group.
3.2. Skeletal motions during the transfer
The coherent excitation of vibrational modes has its
origin in the molecular motion along specific coordinates
during the ESIPT [8,24]. In the process, the molecule
follows a curved reaction path in the multi-dimensional
space spanned by the nuclear degrees of freedom. A small
number of vibrational coordinates contributes significantly
to the reaction and only for these modes the distortion of the
molecule along the reaction path has large projections.
When the molecule arrives at the product minimum, it has
momentum along the same coordinates. This causes
coherent vibrational motions in modes that correspond to
these coordinates. The identification of the coherently
excited modes reveals the coordinates contributing to the
reaction path. This can be accomplished by comparing the
oscillation frequencies observed in the experiments with
calculated vibrational normal modes. We restrict ourselves
in most cases to density functional theory calculations in the
electronic ground state because ab initio calculations in the
excited state are very challenging and the low frequency

S. Lochbrunner et al. / Journal of Molecular Structure 700 (2004) 13–18
17
not stop its evolution but propagates further according to the
nuclear momenta. This ringing of the molecule corresponds
to the damped oscillatory motions around the keto minimum
that are observed in the experimental signal.
3.3. Electronic configuration change and adiabatic
evolution
Time resolved IR experiments show for HBT that after
UV excitation the CyO stretch vibration appears on a time-
scale of 30–50 fs [25]. This demonstrates that in the course
of the ESIPT the H-donor bond is indeed broken and new
bonds are formed. The occupation of the electronic orbitals
changes during the process and the electronically excited
enol form has a different electronic configuration as the
resulting S1 keto form. However, the observation of a
well-defined wavepacket after the transfer indicates that the
change of the electronic configuration is not associated with a
jump between two energetically separated electronic
states. Rather we describe the PT as an evolution on a single
adiabatic PES. During the evolution, the wave function
changes its electronic character. To discuss the properties of
the adiabatic PES we start out with the two diabatic surfaces
belonging to the enol and the keto configuration that would
be the relevant surfaces if the electronicconfigurations would
not mix. In the case of strong coupling two adiabatic surfaces
emerge from the mixing of the two diabatic PESs and the
reaction path follows the energetically lower one [26].
The path involves several molecular coordinates that
contribute with varying magnitude resulting in changes of
the path direction in coordinate space. The initial accelera-
tion in the Franck–Condon region points toward the
minimum of the diabatic PES of the S1-enol configuration
as was demonstrated for HBT by ab initio calculations [24].
However, the enol and the keto configuration couple very
strongly with each other. This leads to an efficient mixing of
both configurations and to a strong deformation of the
resulting adiabatic PES with respect to the original diabatic
PESs. In the region of the diabatic enol minimum, the
adiabatic PES has a slope toward the region with increasing
configuration mixing. The mixing is most efficient where the
two diabatic PESs intersect. The change of the electronic
configuration takes place when the wavepacket propagates
through this part of the adiabatic PES. Thereafter, the slope of
the PES is directed toward the keto minimum. Because of the
strong coupling between both configurations a barrierless or
almost barrierless reaction path results. From the coupling of
the two diabatic PESs a second adiabatic PES at higher
energies is expected. However, it will be difficult to locate
this state in the dense manifold of higher electronically
excited states.
Fig. 4. Calculated vibrational eigenmodes and frequencies of OHBA that
correspond to the dominant oscillatory signal contributions.
However, all these motions have in common that they vary
very efficiently the donor–acceptor distance. Even the
OHBA mode at 420 cm21 influences significantly this
distance (see Fig. 4).
Because the donor–acceptor distance is changed by each
coherently excited vibrational mode in all three molecules
we conclude that during the PT it adapts values that differ
strongly from the equilibrium geometry of both the enol and
the keto form. It has to be drastically reduced before the
proton can be bonded to the acceptor atom. In the case of
HBT, we found indications that the bending of the
molecular skeleton is the dominant initial motion and
leads indeed directly to a shortening of the donor–acceptor
distance [8]. Because the coherently excited vibrations
likewise affect in-plane angles and bond lengths of the
H-chelate ring these parameters seem to experience also a
readjustment during the ESIPT. The ab initio calculations
totally confirm this picture derived from the experimental
observations and find a decrease of the donor–acceptor
˚
distance of up to 0.3 A along the minimum energy path from
the Franck–Condon point to the keto minimum [24].
From these findings, the following picture of the nuclear
motions during the transfer emerges [8,10,24]. After the
molecule is promoted by the optical excitation to the
Franck–Condon point, it performs a bending motion that
reduces the donor–acceptor distance. When the hydrogen
atom is shifted together with the oxygen atom sufficiently
near to the acceptor atom a change of the electronic wave
function is possible without the need of extra energy or a
tunneling process through a barrier. The new electronic
wave function is associated with the formation of a new
bond between the hydrogen and the acceptor atom and the
change of the strength of other bonds. A new equilibrium
geometry is associated with the new bonds and the
molecular skeleton performs motions in order to adjust to
this geometry. Thereby the wavepacket evolves also
along other coordinates that are strongly coupled to the
donor–acceptor distance. When the molecule geometry
arrives at the keto minimum of the excited state PES, it does
Reliable ab initio calculations dealing with the ESIPT
mechanism are very complex and challenging [24,27]. The
reason is that the shape of the PES is determined by two
electronically excited configurations and the coupling
between them. It is necessary to use an accurate description

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S. Lochbrunner et al. / Journal of Molecular Structure 700 (2004) 13–18
of the electronic configurations and to account for electron
correlation effects [27]. However, such calculations were
recently performed for HBT and find in nice agreement with
our experimental results that a contraction of the donor–
acceptor distance is needed to transfer the proton and that
this contraction seems to result from skeletal motions [24].
The propagation of the wavepacket along the reaction
path is first of all associated with skeletal motions of the
molecule (see above). The proton is initially shifted together
with the oxygen atom and after the configuration change
together with the acceptor atom. It stays all the time in or
nearby the minimum of its local potential well that is moved
by the skeletal motions. Several findings support this point
of view. If the ESIPT would mainly be the direct motion of
the proton from the donor site to the acceptor site it should
be much faster because of the small proton mass [12] or it
must be hindered by a barrier. Tunneling through the barrier
as well as thermally activated crossing over the barrier
should not lead to a ballistic wavepacket motion of the
whole system as observed. Furthermore, if the proton
would not stay at its local minimum an excitation of its local
vibrational mode should occur. However, the excitation of
such high frequency modes seems to be highly
unlikely according to IR experiments [18,25] and energy
considerations [8].
Acknowledgements
We are indebted to Alexander J. Wurzer, Tanja Bizjak,
Regina de Vivie-Riedle, and Vincent De Waele for valuable
contributions and discussions and gratefully acknowledge
funding by the Deutsche Forschungsgemeinschaft.
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Transient absorption measurements with 30 fs time
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