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Topics in Current Chemistry (2018) 376:28
sample systems undergoes different forms of relaxation. Next to the type of trivial
information on energy relaxation for which no multidimensional methods would be
needed, significant information is revealed by, e.g., the temporal evolution of infrared and Raman vibrational frequencies and of the multidimensional line shapes of
the signals [97–99]. Frequency shifts during the evolution of a photochemical reaction can be traced back to how chemical bondings are forming and breaking in the
femtosecond and picosecond time scale. The multidimensional vibrational coherence spectroscopy, but also the 2D infrared and 2D Raman contributions, show for
photoreactions involving small (e.g., photodissociation of triiodide, hydrogen bonding) as well as large molecular system (e.g., isomerization of stilbene) how reaction mechanisms can be developed from such experimental frequency shifts. The
second significant information provided by multidimensional spectroscopy, i.e., the
evolution of lineshapes, is originated due to the changes in the environmental interactions through structural fluctuations on the timescale of femtoseconds to picoseconds and longer after excitation. Such effects are known to cause “spectral diffusion”, which is highly relevant for vibrational spectroscopy methods, but to a lesser
extent than electronic counterparts [100]. Detailed insight into the origin of this type
of evolution is then generally obtained from a comparison of experimental signals
and simulations based on molecular dynamics program packages. Even more important, however, are the temporal dynamics of the cross peaks, which may exhibit
kinetic (incoherent) as well as oscillatory (coherent) contributions. In this regard,
incoherent dynamics can generally originate from energy transfer dynamics in the
weak coupling limit between a donor and acceptor state, just like the effect often
observed in Förster resonant energy transfer spectroscopy [101, 102]. The dynamics
of such energy transfer is a good source for determination of structural properties of
the sample, since the transfer rates are highly sensitive to molecular orientation and
distances [103, 104]. Another source of incoherent evolution of cross peaks is represented by chemical exchange, i.e., the inter-conversion between different molecular
states that are spectroscopically distinguishable. Good examples in this context are
the breaking and new formation of chemical bonds like those encountered during
the isomerization of double bonds in organic molecules, or the loss/rearrangement
of ligands in metal complexes [105–107]. A crucial requirement for these incoherent effects to be observed in multidimensional optical signals is the matching of the
timescales for the relevant processes with the energy relaxation dynamics of the
pump-induced perturbations. If energy relaxation is too fast compared to energy
transfer or chemical exchange, there is simply not enough time for an appreciable
transfer or a reaction to occur before the difference signals have ceased. Different
strategies have been devised (e.g., “triggered exchange spectroscopy”) [90] to overcome that limitation of ultrafast spectroscopy, some of which are discussed in detail
in the contribution on infrared multidimensional spectroscopy [96]. Regarding the
second point of coherent contributions, the strong coupling regime, for example, can
result in oscillatory cross peak intensities, depending on the modulus of the coupling strength relative to the spectral separation of interacting resonances [41]. Next
to this, coherent oscillatory contributions in electronic multidimensional spectra
may originate from both coherent coupling of electronic states as well as from vibrational wave packet evolution in ground- or excited electronic states. Much effort
13
Reprinted from the journal
Topics in Current Chemistry (2018) 376:28
sample systems undergoes different forms of relaxation. Next to the type of trivial
information on energy relaxation for which no multidimensional methods would be
needed, significant information is revealed by, e.g., the temporal evolution of infrared and Raman vibrational frequencies and of the multidimensional line shapes of
the signals [97–99]. Frequency shifts during the evolution of a photochemical reaction can be traced back to how chemical bondings are forming and breaking in the
femtosecond and picosecond time scale. The multidimensional vibrational coherence spectroscopy, but also the 2D infrared and 2D Raman contributions, show for
photoreactions involving small (e.g., photodissociation of triiodide, hydrogen bonding) as well as large molecular system (e.g., isomerization of stilbene) how reaction mechanisms can be developed from such experimental frequency shifts. The
second significant information provided by multidimensional spectroscopy, i.e., the
evolution of lineshapes, is originated due to the changes in the environmental interactions through structural fluctuations on the timescale of femtoseconds to picoseconds and longer after excitation. Such effects are known to cause “spectral diffusion”, which is highly relevant for vibrational spectroscopy methods, but to a lesser
extent than electronic counterparts [100]. Detailed insight into the origin of this type
of evolution is then generally obtained from a comparison of experimental signals
and simulations based on molecular dynamics program packages. Even more important, however, are the temporal dynamics of the cross peaks, which may exhibit
kinetic (incoherent) as well as oscillatory (coherent) contributions. In this regard,
incoherent dynamics can generally originate from energy transfer dynamics in the
weak coupling limit between a donor and acceptor state, just like the effect often
observed in Förster resonant energy transfer spectroscopy [101, 102]. The dynamics
of such energy transfer is a good source for determination of structural properties of
the sample, since the transfer rates are highly sensitive to molecular orientation and
distances [103, 104]. Another source of incoherent evolution of cross peaks is represented by chemical exchange, i.e., the inter-conversion between different molecular
states that are spectroscopically distinguishable. Good examples in this context are
the breaking and new formation of chemical bonds like those encountered during
the isomerization of double bonds in organic molecules, or the loss/rearrangement
of ligands in metal complexes [105–107]. A crucial requirement for these incoherent effects to be observed in multidimensional optical signals is the matching of the
timescales for the relevant processes with the energy relaxation dynamics of the
pump-induced perturbations. If energy relaxation is too fast compared to energy
transfer or chemical exchange, there is simply not enough time for an appreciable
transfer or a reaction to occur before the difference signals have ceased. Different
strategies have been devised (e.g., “triggered exchange spectroscopy”) [90] to overcome that limitation of ultrafast spectroscopy, some of which are discussed in detail
in the contribution on infrared multidimensional spectroscopy [96]. Regarding the
second point of coherent contributions, the strong coupling regime, for example, can
result in oscillatory cross peak intensities, depending on the modulus of the coupling strength relative to the spectral separation of interacting resonances [41]. Next
to this, coherent oscillatory contributions in electronic multidimensional spectra
may originate from both coherent coupling of electronic states as well as from vibrational wave packet evolution in ground- or excited electronic states. Much effort
13
Reprinted from the journal