have defined the delay T with respect to the third light-matter interaction of the IR
fields (lower long arrow) [250]. In fact, most of the reported transient 2D IR
experiments have utilized 2D IR spectra generation using a Fabry–Perot filter
[252, 265], where the initial two IR interactions both take place under the envelope
of the same excitation pulse. This avoids complications in the data analysis based on
the necessity to scan additional delays between E IR1,2 , which may partially overlap
with the T delay to some extent (e.g. at very short T delays). If the delay between the
initial excitation and the IR excitation is sufficiently long, i.e. when excited state
vibrational equilibration and solvation has completed, then transient 2D IR spectra
can be interpreted as a third-order 2D IR spectrum from an excited electronic state.
Experiments of that sort have been used to study structural dynamics in
biomolecules or bio-molecular sub-units [38, 258, 259, 266], solvation dynamics
in excited electronic states [254, 267], or electronic relaxation dynamics and
associated vibrational signals of excited electronic states. Other examples exist
regarding structural dynamics in large organic molecules [255, 256], chargeinjection dynamics between adsorbates and mesoporous semiconductors [250], or
spectral shifts in charge-transfer states of inorganic complexes [251].
Other implementations of transient 2D IR spectroscopy exist, which utilize the
UV excitation pulse after the initial interaction with the two IR fields (Fig. 20b). In
that particular case, the UV/VIS pulse intercepts the relaxation during the IR
population delay and promotes the system to an additional non-equilibrium state,
e.g. again an excited electronic state or a photochemical reaction channel. Following
this interception, the probe pulse interrogates the total perturbation of the system,
which consists of IR, as well as UV/VIS excitation. That variant is commonly
referred to as ‘‘triggered exchange transient 2D IR spectroscopy’’
[51, 252, 253, 268]. A possible interpretation of this type of transient 2D IR is
that the ground state IR spectrum is correlated with the excited state IR spectrum, or
even a starting material to a photo-product [250, 252]. Such experiments have
revealed spectral signatures of metal-to-ligand charge transfer in carbonyl
complexes [252], photo-induced ligand migration in proteins [269], or different
electron-transfer efficiencies of adsorbates at semiconductor surfaces [250].
Selected highlight examples of both of the transient 2D IR implementations are
discussed further below in this section.
A final note is due on the interpretation of the spectra. As the IR sequence is
resonant with IR transitions from the sample molecules that have undergone preexcitation, as well as those that did not, a subtraction of the two types of 2D IR
spectra is often needed to derive a transient 2D IR spectrum, i.e. with and without
the pre-excitation interaction.
The first example to be discussed here is the observation of transient hydrogen
bond dynamics in a beta-turn of a cyclic peptide, cyclo(Boc–Cys–Pro–Aib–Cys–
OMe) (Fig. 21a). Hamm et al. demonstrated that transient 2D IR can be used to
follow in real time the weakening of an intramolecular hydrogen bond and the
subsequent opening of a beta-turn upon UV-photolysis of a disulfide bridge [258].
The considered sample exhibits several distinct carbonyl functional groups (colorcoded in (a)), which can be observed in the amide-I region of the IR spectrum (color
code in Fig. 21b). Following 266 nm photolysis of the disulfide-bridge, the carbonyl
Top Curr Chem (Z) (2017) 375:86
123
165
Reprinted from the journal
fields (lower long arrow) [250]. In fact, most of the reported transient 2D IR
experiments have utilized 2D IR spectra generation using a Fabry–Perot filter
[252, 265], where the initial two IR interactions both take place under the envelope
of the same excitation pulse. This avoids complications in the data analysis based on
the necessity to scan additional delays between E IR1,2 , which may partially overlap
with the T delay to some extent (e.g. at very short T delays). If the delay between the
initial excitation and the IR excitation is sufficiently long, i.e. when excited state
vibrational equilibration and solvation has completed, then transient 2D IR spectra
can be interpreted as a third-order 2D IR spectrum from an excited electronic state.
Experiments of that sort have been used to study structural dynamics in
biomolecules or bio-molecular sub-units [38, 258, 259, 266], solvation dynamics
in excited electronic states [254, 267], or electronic relaxation dynamics and
associated vibrational signals of excited electronic states. Other examples exist
regarding structural dynamics in large organic molecules [255, 256], chargeinjection dynamics between adsorbates and mesoporous semiconductors [250], or
spectral shifts in charge-transfer states of inorganic complexes [251].
Other implementations of transient 2D IR spectroscopy exist, which utilize the
UV excitation pulse after the initial interaction with the two IR fields (Fig. 20b). In
that particular case, the UV/VIS pulse intercepts the relaxation during the IR
population delay and promotes the system to an additional non-equilibrium state,
e.g. again an excited electronic state or a photochemical reaction channel. Following
this interception, the probe pulse interrogates the total perturbation of the system,
which consists of IR, as well as UV/VIS excitation. That variant is commonly
referred to as ‘‘triggered exchange transient 2D IR spectroscopy’’
[51, 252, 253, 268]. A possible interpretation of this type of transient 2D IR is
that the ground state IR spectrum is correlated with the excited state IR spectrum, or
even a starting material to a photo-product [250, 252]. Such experiments have
revealed spectral signatures of metal-to-ligand charge transfer in carbonyl
complexes [252], photo-induced ligand migration in proteins [269], or different
electron-transfer efficiencies of adsorbates at semiconductor surfaces [250].
Selected highlight examples of both of the transient 2D IR implementations are
discussed further below in this section.
A final note is due on the interpretation of the spectra. As the IR sequence is
resonant with IR transitions from the sample molecules that have undergone preexcitation, as well as those that did not, a subtraction of the two types of 2D IR
spectra is often needed to derive a transient 2D IR spectrum, i.e. with and without
the pre-excitation interaction.
The first example to be discussed here is the observation of transient hydrogen
bond dynamics in a beta-turn of a cyclic peptide, cyclo(Boc–Cys–Pro–Aib–Cys–
OMe) (Fig. 21a). Hamm et al. demonstrated that transient 2D IR can be used to
follow in real time the weakening of an intramolecular hydrogen bond and the
subsequent opening of a beta-turn upon UV-photolysis of a disulfide bridge [258].
The considered sample exhibits several distinct carbonyl functional groups (colorcoded in (a)), which can be observed in the amide-I region of the IR spectrum (color
code in Fig. 21b). Following 266 nm photolysis of the disulfide-bridge, the carbonyl
Top Curr Chem (Z) (2017) 375:86
123
165
Reprinted from the journal