5 General Applicability and Limitations of 2DRR Spectroscopy
The 2DRR signatures of vibronic coherence transfer discussed in this chapter are
subject to several constraints. Firstly, the photoinduced reaction should generally be
faster than the vibrational period (i.e., impulsive regime). It may be possible for a
vibronic coherence transfer mechanism to promote phase coherence in an ensemble
of systems where the reaction time is greater than or equal to the vibrational period;
however, the prevalence of such mechanisms in chemical systems is not presently
clear. Secondly, it is desirable to have reactants and products with non-overlapping
electronic transitions. This constraint can be relaxed if the vibrational resonances of
the reactant and product are well-separated and known beforehand, thereby allowing
signal components to be assigned. Thirdly, the 2DRR method is sensitive to Franck–
Condon active modes regardless of anharmonicity. Fourthly, the 2DRR method is
limited to systems with modest optical densities because of the possibility of
cascades. Optical densities less than 1.0 will usually be acceptable based on our
calculations [25]. However, highly concentrated systems like molecular crystals will
be problematic.
The constraints listed above are specific to studies of vibronic coherence transfer.
In fact, 2DRR signals can be detected for any system with Franck–Condon active
modes with relatively few constraints. Of course, cascades should be ruled out with
control experiments as in any other fifth-order vibrational spectroscopy. Beyond
that, the challenge is simply a matter of generating adequate signal strength. 2DRR
experiments conducted on non-reactive systems yield information about linebroadening mechanisms. As in other 2D vibrational spectroscopy techniques, the
2DRR vibrational line shapes elongate with respect to the diagonal axis for
inhomogeneously broadened transitions. We have observed such elongated 2DRR
line shapes in the low-frequency vibrational modes of myoglobin [25, 26]. It should
be noted that such 2DRR experiments are relatively insensitive to anharmonic
couplings because Franck–Condon active modes contribute whether the modes are
harmonic or not. However, in recent work, it has been predicted that sensitivity to
anharmonic couplings can be achieved if the first pulse is pre-resonant with the
electronic transition for a related 2D Raman technique [35].
The signatures of vibronic coherence transfer identified in this work generalize to
any ultrafast process that can be photoinduced (e.g., energy or electron transfer). We
suggest that electron transfer may be easier to study with 2DRR than is energy
transfer. The reason is that one of the key requirements for fast energy transfer is
spectral overlap between the donor’s emission spectrum and the acceptor’s
absorption spectrum. It will not be possible to distinguish donor and acceptor
modes by tuning incident laser beams in this situation, particularly if extremely
broadband pulses are employed. Nonetheless, 2DRR studies of vibrational
coherence transfer in systems like light harvesting proteins may be possible if the
donor and acceptor possess readily distinguished vibrational mode frequencies. In
contrast, applications to electron transfer reactions will be straightforward if the
oxidized and/or reduced species have intense electronic transitions. For such
Top Curr Chem (Z) (2017) 375:87
123
266
Reprinted from the journal
The 2DRR signatures of vibronic coherence transfer discussed in this chapter are
subject to several constraints. Firstly, the photoinduced reaction should generally be
faster than the vibrational period (i.e., impulsive regime). It may be possible for a
vibronic coherence transfer mechanism to promote phase coherence in an ensemble
of systems where the reaction time is greater than or equal to the vibrational period;
however, the prevalence of such mechanisms in chemical systems is not presently
clear. Secondly, it is desirable to have reactants and products with non-overlapping
electronic transitions. This constraint can be relaxed if the vibrational resonances of
the reactant and product are well-separated and known beforehand, thereby allowing
signal components to be assigned. Thirdly, the 2DRR method is sensitive to Franck–
Condon active modes regardless of anharmonicity. Fourthly, the 2DRR method is
limited to systems with modest optical densities because of the possibility of
cascades. Optical densities less than 1.0 will usually be acceptable based on our
calculations [25]. However, highly concentrated systems like molecular crystals will
be problematic.
The constraints listed above are specific to studies of vibronic coherence transfer.
In fact, 2DRR signals can be detected for any system with Franck–Condon active
modes with relatively few constraints. Of course, cascades should be ruled out with
control experiments as in any other fifth-order vibrational spectroscopy. Beyond
that, the challenge is simply a matter of generating adequate signal strength. 2DRR
experiments conducted on non-reactive systems yield information about linebroadening mechanisms. As in other 2D vibrational spectroscopy techniques, the
2DRR vibrational line shapes elongate with respect to the diagonal axis for
inhomogeneously broadened transitions. We have observed such elongated 2DRR
line shapes in the low-frequency vibrational modes of myoglobin [25, 26]. It should
be noted that such 2DRR experiments are relatively insensitive to anharmonic
couplings because Franck–Condon active modes contribute whether the modes are
harmonic or not. However, in recent work, it has been predicted that sensitivity to
anharmonic couplings can be achieved if the first pulse is pre-resonant with the
electronic transition for a related 2D Raman technique [35].
The signatures of vibronic coherence transfer identified in this work generalize to
any ultrafast process that can be photoinduced (e.g., energy or electron transfer). We
suggest that electron transfer may be easier to study with 2DRR than is energy
transfer. The reason is that one of the key requirements for fast energy transfer is
spectral overlap between the donor’s emission spectrum and the acceptor’s
absorption spectrum. It will not be possible to distinguish donor and acceptor
modes by tuning incident laser beams in this situation, particularly if extremely
broadband pulses are employed. Nonetheless, 2DRR studies of vibrational
coherence transfer in systems like light harvesting proteins may be possible if the
donor and acceptor possess readily distinguished vibrational mode frequencies. In
contrast, applications to electron transfer reactions will be straightforward if the
oxidized and/or reduced species have intense electronic transitions. For such
Top Curr Chem (Z) (2017) 375:87
123
266
Reprinted from the journal