different chain-length (n) and tags, as well as reporter groups attached to the ends of
the chain. Of particular use proved the azide asymmetric stretch vibration and CO
stretch vibrations of several types of carbonyl functional groups [75, 170].
Figure 11c represents a typical magnitude RA 2D IR spectrum of a sample from the
lower left class of molecules with n = 2, where the azide group has been excited
and several lower frequency groups of the succinimide are probed. Note that there
exists no diagonal line since the signals are located exclusively in the cross peak
region. Following the evolution of the single cross peak signals over time for each
peak yields kinetics as schematically drawn in (d), which exhibit clear maxima of
the intensity that report on the arrival time of the excitation energy at the specific
mode, followed by an exponential decay due to cooling of the sample. When
varying the chain length of the sample molecule, the intensity maxima of the cross
peaks are found to be continuously delayed. This observation represents the
propagation speed of the energy in the molecule via the slopes of the plots, (e).
Several interesting aspects are revealed by the experimental data: First, the speed
of propagation within the sample is constant over large distances, which already
implies a ballistic energy propagation mechanism. Second, different propagation
speeds are observed, depending on the chemical nature of the tag and the reporter
units. Similar results can also be observed by varying the chemical nature of the
chain [170]. Third, the speed in extremely high, reaching values of about 1.5 nm/ps,
which exceeds even the speed of sound in some metals [171]. The reason for the
different speeds was identified to originate from a wave packet formation after
initial excitation: For the varying samples, different types modes from the chain of
repeating units contribute to the wave packet propagation. Depending on the actual
folding structure of the molecules in solution phase, the propagation of the wave
packet can be significantly affected, thereby allowing obtaining information of the
molecular conformations. Overall, the clear linear behavior shows that chain as well
as tag/reporter properties control the energy propagation in the sample and excludes
the involvement of ‘‘through space’’ energy transfer between the functional groups,
as this would result in a strongly nonlinear distance dependence (Eq. 5). The
possibility of controlling the energy transport in materials through rational design of
the molecular structure is an interesting aspect for material science and technological applications. Next to furthering the fundamental understanding of the materials,
the optimization of these effects may allow designing energy dissipation materials
in electronic devices.
3.2 Structural Dynamics of Molecules in Confined Environments
3.2.1 2D IR from Molecules Confined in Two Dimensions: Surfaces and Interfaces
In recent years, ultrafast molecular spectroscopy has undergone a significant
development towards surface-related investigations. This evolution was mainly
triggered by progress in fields of energy science, heterogeneous catalysis and
chemical sensing [15]. 2D IR spectroscopy allows obtaining direct information of
how molecules under different forms of confinement behave and how confinement
influences molecular structure and properties. As many of the important aspects in
Top Curr Chem (Z) (2017) 375:86
123
145
Reprinted from the journal
the chain. Of particular use proved the azide asymmetric stretch vibration and CO
stretch vibrations of several types of carbonyl functional groups [75, 170].
Figure 11c represents a typical magnitude RA 2D IR spectrum of a sample from the
lower left class of molecules with n = 2, where the azide group has been excited
and several lower frequency groups of the succinimide are probed. Note that there
exists no diagonal line since the signals are located exclusively in the cross peak
region. Following the evolution of the single cross peak signals over time for each
peak yields kinetics as schematically drawn in (d), which exhibit clear maxima of
the intensity that report on the arrival time of the excitation energy at the specific
mode, followed by an exponential decay due to cooling of the sample. When
varying the chain length of the sample molecule, the intensity maxima of the cross
peaks are found to be continuously delayed. This observation represents the
propagation speed of the energy in the molecule via the slopes of the plots, (e).
Several interesting aspects are revealed by the experimental data: First, the speed
of propagation within the sample is constant over large distances, which already
implies a ballistic energy propagation mechanism. Second, different propagation
speeds are observed, depending on the chemical nature of the tag and the reporter
units. Similar results can also be observed by varying the chemical nature of the
chain [170]. Third, the speed in extremely high, reaching values of about 1.5 nm/ps,
which exceeds even the speed of sound in some metals [171]. The reason for the
different speeds was identified to originate from a wave packet formation after
initial excitation: For the varying samples, different types modes from the chain of
repeating units contribute to the wave packet propagation. Depending on the actual
folding structure of the molecules in solution phase, the propagation of the wave
packet can be significantly affected, thereby allowing obtaining information of the
molecular conformations. Overall, the clear linear behavior shows that chain as well
as tag/reporter properties control the energy propagation in the sample and excludes
the involvement of ‘‘through space’’ energy transfer between the functional groups,
as this would result in a strongly nonlinear distance dependence (Eq. 5). The
possibility of controlling the energy transport in materials through rational design of
the molecular structure is an interesting aspect for material science and technological applications. Next to furthering the fundamental understanding of the materials,
the optimization of these effects may allow designing energy dissipation materials
in electronic devices.
3.2 Structural Dynamics of Molecules in Confined Environments
3.2.1 2D IR from Molecules Confined in Two Dimensions: Surfaces and Interfaces
In recent years, ultrafast molecular spectroscopy has undergone a significant
development towards surface-related investigations. This evolution was mainly
triggered by progress in fields of energy science, heterogeneous catalysis and
chemical sensing [15]. 2D IR spectroscopy allows obtaining direct information of
how molecules under different forms of confinement behave and how confinement
influences molecular structure and properties. As many of the important aspects in
Top Curr Chem (Z) (2017) 375:86
123
145
Reprinted from the journal