contribute to the charge-injection process, whereas the injection for the third
configuration is negligible. It can be envisioned that future applications of transient
2D IR spectroscopy to similar systems will lead to valuable insights with regard to
the performance of heterogeneous photo-catalysts, or several forms of dyesensitized solar cells [159].
Kubarych et al. have established transient 2D IR spectroscopy to also allow
studying differences in solvation dynamics between ground and excited electronic
states in metal complexes [254]. The authors investigated solvation dynamics of the
thermally equilibrated triplet state of Re-carbonyl complexes, which is populated
after UV excitation. It could be determined that the spectral diffusion dynamics in
the triplet state are three times slower as compared to the electronic ground state.
These differences were attributed to the markedly different charge distributions in
the molecules in the different electronic states. Future studies can be envisioned,
which do not consider thermally equilibrated excited state systems, but which look
at differences in spectral diffusion dynamics of vibrationally excited species as
compared to thermally relaxed molecules. Such experiments could, therefore, be
used to shed additional light on relaxation dynamics in excited electronic states.
Other versions of transient 2D IR spectroscopy investigated electronic relaxation
dynamics, as well as excited state vibrational features in biological molecules such
as carbonyl carotenoids [255, 256]. Excitation wavelength, as well as solventdependent spectral signatures and distinct cross peaks could be resolved in these
studies, which originated from coupled stretching modes of the conjugated chain in
electronic states of different symmetry. Besides revealing detailed insight into the
special dynamics of these samples, it should be noted that these studies are of the
few, which demonstrate that transient 2D IR spectroscopy can be applied to other
sample systems besides strongly IR-absorbing metal-carbonyl complexes. As such,
the method is likely to find in future much broader application than demonstrated so
far.
4.4.1 Extending the Observation Window for 2D IR Spectroscopy
One particular issue in ultrafast vibrational spectroscopy is the comparatively short
lifetime of the excited vibrational levels. Strong coupling of the sample to the
environment, as well as rapid IVR often leads to signal decay on the timescale of a
few picoseconds. This limits the temporal observation window and makes in
intrinsically difficult to observe processes that occur on longer timescales. Recently,
Bredenbeck et al. have proposed a method that can circumvent that lifetimelimitation by combining VIS and IR excitation, similar to triggered exchange
spectroscopy. In their method of vibrationally promoted electronic resonance
(VIPER) 2D IR spectroscopy a vibrationally resonant 2D IR sequence is combined
with an additional UV/VIS excitation pulse that is for itself non-resonant with any
electronic transition of the sample (Fig. 23b) [270]. The UV/VIS excitation pulse is
arranged to arrive after the IR excitation pulse at the sample position. Only the
combination of IR and UV/VIS excitation can promote the system to an excited
electronic state. By this method the applicability of 2D IR spectroscopy can be
extended to timescales that are much longer compared to the vibrational relaxation
Top Curr Chem (Z) (2017) 375:86
123
168
Reprinted from the journal
configuration is negligible. It can be envisioned that future applications of transient
2D IR spectroscopy to similar systems will lead to valuable insights with regard to
the performance of heterogeneous photo-catalysts, or several forms of dyesensitized solar cells [159].
Kubarych et al. have established transient 2D IR spectroscopy to also allow
studying differences in solvation dynamics between ground and excited electronic
states in metal complexes [254]. The authors investigated solvation dynamics of the
thermally equilibrated triplet state of Re-carbonyl complexes, which is populated
after UV excitation. It could be determined that the spectral diffusion dynamics in
the triplet state are three times slower as compared to the electronic ground state.
These differences were attributed to the markedly different charge distributions in
the molecules in the different electronic states. Future studies can be envisioned,
which do not consider thermally equilibrated excited state systems, but which look
at differences in spectral diffusion dynamics of vibrationally excited species as
compared to thermally relaxed molecules. Such experiments could, therefore, be
used to shed additional light on relaxation dynamics in excited electronic states.
Other versions of transient 2D IR spectroscopy investigated electronic relaxation
dynamics, as well as excited state vibrational features in biological molecules such
as carbonyl carotenoids [255, 256]. Excitation wavelength, as well as solventdependent spectral signatures and distinct cross peaks could be resolved in these
studies, which originated from coupled stretching modes of the conjugated chain in
electronic states of different symmetry. Besides revealing detailed insight into the
special dynamics of these samples, it should be noted that these studies are of the
few, which demonstrate that transient 2D IR spectroscopy can be applied to other
sample systems besides strongly IR-absorbing metal-carbonyl complexes. As such,
the method is likely to find in future much broader application than demonstrated so
far.
4.4.1 Extending the Observation Window for 2D IR Spectroscopy
One particular issue in ultrafast vibrational spectroscopy is the comparatively short
lifetime of the excited vibrational levels. Strong coupling of the sample to the
environment, as well as rapid IVR often leads to signal decay on the timescale of a
few picoseconds. This limits the temporal observation window and makes in
intrinsically difficult to observe processes that occur on longer timescales. Recently,
Bredenbeck et al. have proposed a method that can circumvent that lifetimelimitation by combining VIS and IR excitation, similar to triggered exchange
spectroscopy. In their method of vibrationally promoted electronic resonance
(VIPER) 2D IR spectroscopy a vibrationally resonant 2D IR sequence is combined
with an additional UV/VIS excitation pulse that is for itself non-resonant with any
electronic transition of the sample (Fig. 23b) [270]. The UV/VIS excitation pulse is
arranged to arrive after the IR excitation pulse at the sample position. Only the
combination of IR and UV/VIS excitation can promote the system to an excited
electronic state. By this method the applicability of 2D IR spectroscopy can be
extended to timescales that are much longer compared to the vibrational relaxation
Top Curr Chem (Z) (2017) 375:86
123
168
Reprinted from the journal