In total transient 2D IR spectroscopy in its many different variants has opened up
a completely new way of measuring chemical structure and its evolution in sample
systems as complex as proteins from picoseconds to milliseconds. These studies are
expected to have a profound impact on the understanding of both biological
systems, as well as, e.g., functional devices such as solar cell materials. The total
potential of transient 2D IR spectroscopy has up to now not been evaluated in all
aspects. It is, therefore, assumed that several additional experimental extensions will
evolve in future works, some of which are already under way. These additional
aspects will be evaluated further in Sect. 5.4.
4.5 Mixed Vibrational-Electronic 2D Spectroscopies
The preceding sections have covered experimental developments that were realized
by adding new pulses to a standard 2D IR pulse sequence. Very recently, other
variants of multi-dimensional vibrational spectroscopy have been presented that
make use of a substitution of selected pulses in the 2D pulse sequence by pulses
from other frequency ranges, in particular from the UV/VIS region
([ 10000 cm
-1 ). This way, so-called mixed 2D vibrational-electronic (VE) and
electronic-vibrational (EV) signals have been obtained, depending on which of the
pulses is substituted. Such methods are able to cover large parts of exclusively the
cross peak regions in Fig. 1 and reveal information that is very different from
standard 2D IR spectroscopy. In this section, as well as the succeding Sect. 4.5.2
these developments are presented, their potential is discussed and selected
applications are commented.
4.5.1 2D Vibrational-Electronic (VE) Spectroscopy
To start with, consider the case where the probe pulse in a standard 2D IR sequence
is replaced by a near-infrared (NIR) or VIS pulse. This results in the pulse sequence
for VE spectroscopy (Fig. 25a). This method has initially been designed to
investigate dynamics of intramolecular couplings between vibrational and electronic
degrees of freedom and vibronic couplings in metal complexes [272], but the
principle is more general and can be applied to other systems as well. Intramolecular
properties of that sort are expected to play key-roles in photo-biology and functional
materials where molecular struture is often architectured to exhibit a particular
function. 2D VE spectroscopy also reports on mode-specific vibrational-electronic
frequency-frequency correlations via the observable lineshapes of the signals as
well as on intra and intermolecular vibrational relaxation.
The general principle of 2D VE is similar to that of standard 2D IR spectrocopy
and involves two phase-stable IR pump pulses that initially excite a coherence and
subsequently a population in the sample along with a delayed probe pulse, which is
now spectrally located in the NIR/VIS range and a local oscillator for heterodynedetection. To generate a 2D VE signal the coherence delay between the pump pulses
is scanned for a fixed population delay between the second pump and the probe
pulse. The temporal evolution along the population time axis then reports on the
ultrafast dynamcis of the sample. In addition, the dynamics of the signal may also
Top Curr Chem (Z) (2017) 375:86
123
172
Reprinted from the journal
a completely new way of measuring chemical structure and its evolution in sample
systems as complex as proteins from picoseconds to milliseconds. These studies are
expected to have a profound impact on the understanding of both biological
systems, as well as, e.g., functional devices such as solar cell materials. The total
potential of transient 2D IR spectroscopy has up to now not been evaluated in all
aspects. It is, therefore, assumed that several additional experimental extensions will
evolve in future works, some of which are already under way. These additional
aspects will be evaluated further in Sect. 5.4.
4.5 Mixed Vibrational-Electronic 2D Spectroscopies
The preceding sections have covered experimental developments that were realized
by adding new pulses to a standard 2D IR pulse sequence. Very recently, other
variants of multi-dimensional vibrational spectroscopy have been presented that
make use of a substitution of selected pulses in the 2D pulse sequence by pulses
from other frequency ranges, in particular from the UV/VIS region
([ 10000 cm
-1 ). This way, so-called mixed 2D vibrational-electronic (VE) and
electronic-vibrational (EV) signals have been obtained, depending on which of the
pulses is substituted. Such methods are able to cover large parts of exclusively the
cross peak regions in Fig. 1 and reveal information that is very different from
standard 2D IR spectroscopy. In this section, as well as the succeding Sect. 4.5.2
these developments are presented, their potential is discussed and selected
applications are commented.
4.5.1 2D Vibrational-Electronic (VE) Spectroscopy
To start with, consider the case where the probe pulse in a standard 2D IR sequence
is replaced by a near-infrared (NIR) or VIS pulse. This results in the pulse sequence
for VE spectroscopy (Fig. 25a). This method has initially been designed to
investigate dynamics of intramolecular couplings between vibrational and electronic
degrees of freedom and vibronic couplings in metal complexes [272], but the
principle is more general and can be applied to other systems as well. Intramolecular
properties of that sort are expected to play key-roles in photo-biology and functional
materials where molecular struture is often architectured to exhibit a particular
function. 2D VE spectroscopy also reports on mode-specific vibrational-electronic
frequency-frequency correlations via the observable lineshapes of the signals as
well as on intra and intermolecular vibrational relaxation.
The general principle of 2D VE is similar to that of standard 2D IR spectrocopy
and involves two phase-stable IR pump pulses that initially excite a coherence and
subsequently a population in the sample along with a delayed probe pulse, which is
now spectrally located in the NIR/VIS range and a local oscillator for heterodynedetection. To generate a 2D VE signal the coherence delay between the pump pulses
is scanned for a fixed population delay between the second pump and the probe
pulse. The temporal evolution along the population time axis then reports on the
ultrafast dynamcis of the sample. In addition, the dynamics of the signal may also
Top Curr Chem (Z) (2017) 375:86
123
172
Reprinted from the journal