frequencies can fluctuate under the envelope of the vibrational bands. In the case of
non-resonant energy transfer, the existing energy and momentum difference
between donor and acceptor modes is compensated by environmental degrees of
freedom.
Energy transfer is an important feature in 2D IR spectroscopy since clear physical
interpretations about the underlying mechanisms have been developed for different
cases [1, 10, 43, 52]. From a comparison between experimental results and theoretical
predictions detailed conclusions can then be drawn with respect to the molecular
structure. As an example, energy transfer rates strongly depend on the distance
between donor and acceptor modes, as well as the relative orientations between the
transition dipole moments [10, 145–147]. Energy transfer has consequently been used
as a ‘‘molecular ruler’’ to determine distances of molecules in condensed phase
samples [43]. Typically, intermolecular energy transfer is relevant for distances of
functional groups of less than a nanometer [148, 149]. In contrast, intramolecular
energy transfer has been observed up to distances of 6 nm [150]. If one considers
condensed phase systems, the concentration of a sample molecule in bulk solution is in
many cases fairly low (* mM). If aggregation of the molecules can be neglected, the
average distance between two particles of the same type is then often too large for
intermolecular energy transfer to be observed experimentally and intermolecular
energy transfer predominately takes place between the solute and the solvent.
However, once the molecules are brought closer together, there exist a couple of cases
for which energy transfer can play a significant role for the ultrafast dynamics.
Intermolecular energy transfer has been observed in concentrated (*M) solutions of
ions in liquids [151]. A special case is encountered for bond-mediated energy transfer
in cases where two molecules exhibit chemical interactions such as hydrogen bonds
[152–154]. Besides bulk environments, other examples have been reported, where thin
films of molecules exhibited vibrational energy transfer between chemically identical
molecules [155, 156].
3.1.5.1 Intermolecular Energy Transfer To indicate how detailed molecular
information can be obtained by the observation of energy transfer from 2D IR data,
Fig. 10 shows a recently reported example of in-depth analyzed energy transfer in
thin films (* 1 lm thickness) of Pentaerythritol tetranitrate (PETN), a model
system to investigate intermolecular interactions in explosives [157]. PETN exhibits
four nitrate ester functional groups (Fig. 10a), which, in 2D IR spectra, give rise to
an intense asymmetric stretch transition at about 1660 cm
-1 (A 1 ) with a weak
shoulder at 1685 cm
-1 (A 2 ), when the molecule is dissolved in bulk solution
(Fig. 10b and c). The second band stems from distinct conformational heterogeneity
of the molecule. These two peaks become considerably broadened for molecules in
PETN films (B 1 , B 2 , Fig. 10e and f). Looking at the time-dependence of the 2D IR
signals moreover reveals distinct differences between the bulk solution and thin-film
samples, i.e. the appearance of cross peaks between the two bands (B 12 ), only in the
case of the thin film. This growing-in of cross peaks, which was determined to
happen with a time constant of about 2 ps for the PETN film, is a typical signature
of energy transfer in 2D IR measurements [10, 75, 148].
Top Curr Chem (Z) (2017) 375:86
123
140
Reprinted from the journal
non-resonant energy transfer, the existing energy and momentum difference
between donor and acceptor modes is compensated by environmental degrees of
freedom.
Energy transfer is an important feature in 2D IR spectroscopy since clear physical
interpretations about the underlying mechanisms have been developed for different
cases [1, 10, 43, 52]. From a comparison between experimental results and theoretical
predictions detailed conclusions can then be drawn with respect to the molecular
structure. As an example, energy transfer rates strongly depend on the distance
between donor and acceptor modes, as well as the relative orientations between the
transition dipole moments [10, 145–147]. Energy transfer has consequently been used
as a ‘‘molecular ruler’’ to determine distances of molecules in condensed phase
samples [43]. Typically, intermolecular energy transfer is relevant for distances of
functional groups of less than a nanometer [148, 149]. In contrast, intramolecular
energy transfer has been observed up to distances of 6 nm [150]. If one considers
condensed phase systems, the concentration of a sample molecule in bulk solution is in
many cases fairly low (* mM). If aggregation of the molecules can be neglected, the
average distance between two particles of the same type is then often too large for
intermolecular energy transfer to be observed experimentally and intermolecular
energy transfer predominately takes place between the solute and the solvent.
However, once the molecules are brought closer together, there exist a couple of cases
for which energy transfer can play a significant role for the ultrafast dynamics.
Intermolecular energy transfer has been observed in concentrated (*M) solutions of
ions in liquids [151]. A special case is encountered for bond-mediated energy transfer
in cases where two molecules exhibit chemical interactions such as hydrogen bonds
[152–154]. Besides bulk environments, other examples have been reported, where thin
films of molecules exhibited vibrational energy transfer between chemically identical
molecules [155, 156].
3.1.5.1 Intermolecular Energy Transfer To indicate how detailed molecular
information can be obtained by the observation of energy transfer from 2D IR data,
Fig. 10 shows a recently reported example of in-depth analyzed energy transfer in
thin films (* 1 lm thickness) of Pentaerythritol tetranitrate (PETN), a model
system to investigate intermolecular interactions in explosives [157]. PETN exhibits
four nitrate ester functional groups (Fig. 10a), which, in 2D IR spectra, give rise to
an intense asymmetric stretch transition at about 1660 cm
-1 (A 1 ) with a weak
shoulder at 1685 cm
-1 (A 2 ), when the molecule is dissolved in bulk solution
(Fig. 10b and c). The second band stems from distinct conformational heterogeneity
of the molecule. These two peaks become considerably broadened for molecules in
PETN films (B 1 , B 2 , Fig. 10e and f). Looking at the time-dependence of the 2D IR
signals moreover reveals distinct differences between the bulk solution and thin-film
samples, i.e. the appearance of cross peaks between the two bands (B 12 ), only in the
case of the thin film. This growing-in of cross peaks, which was determined to
happen with a time constant of about 2 ps for the PETN film, is a typical signature
of energy transfer in 2D IR measurements [10, 75, 148].
Top Curr Chem (Z) (2017) 375:86
123
140
Reprinted from the journal