Topics in Current Chemistry (2018) 376:28
1 3
a chemist is possibly NMR spectroscopy. Fortunately, this is also the method with
which chemists are most intimately familiar in terms of signal interpretation. There
exist many analogies between multidimensional optical spectroscopy and nuclear
magnetic resonances [90–92]. For instance, quantized molecular states (either vibrational, electronic, or other) are the analogues of the spin configurations, albeit without a need to make them spectroscopically visible by the application of electric or
magnetic external fields. Also in optical spectroscopy, the properties and dynamics
of the molecular states are obtained by following the (laser-induced) FID in time.
As will be repeatedly demonstrated in the following contributions, spreading of the
optical signals in two dimensions allows for example the precise determination of
spectral linewidths and their dissection into homogeneous and heterogeneous contributions. The disentanglement of heterogeneity in the molecular spectra will be
one of the central topics in three contributions of this collection, namely the 2D
electronic, 2D infrared, and 2D Raman spectroscopies. Heterogeneous contribution
to the signal is widely known to be associated with the microscopic dephasing (fluctuations of energy levels). Such information cannot easily be determined from linear
or one-dimensional spectroscopy methods like transient absorption.
Also in close analogy to, e.g., widely known multidimensional COSY and
NOESY experiments in NMR spectroscopy, there exists tremendous interest in
optical multidimensional spectroscopy regarding the observation of so-called cross
peaks between different resonances. Such cross peaks may be observed for all types
of different resonances and can have manifold origin that need to be, however, evaluated on a case by case basis. For example, static 2D spectra can report on the existence of electrostatic (through space), mechanic (through bond) or electronic coupling in a sample system [93]. In case of optical measurements, the relative intensity
between diagonal and cross peak signals is very sensitive to structural properties
of the investigated sample, i.e., to relative distances and angles, which determine
the coupling strengths. Careful determination of relative intensities under different
experimental conditions can then be used in conjunction with theoretical models
of different levels to determine precisely molecular structure [33, 93, 94], or even
the structure of molecular aggregates [95]. The contribution on 2D IR spectroscopy
[96] depicts how multidimensional techniques using infrared light can be applied
to recover structural information from vibrational coupling. Cross peaks and couplings are also the topic of other contributions. Intra- and intermolecular coupling
between electronic states is reviewed in the 2D electronic spectroscopy contribution.
The contribution on 2D THz spectroscopy deals with low-frequency couplings, e.g.,
between intramolecular vibrational modes in the liquid phase. Similarly important is
the calculation of such couplings. Recent development of numerical methods to calculate the complete and accurate 2D electronic response of chromophores including
inter-chromophoric coupling is presented in the last contribution of this collection.
Thinking moreover about molecular dynamic effects in NMR spectroscopy, one
needs to consider the time dependence of both diagonal as well as cross peaks. In
multidimensional NMR spectroscopy, the diagonal peaks are generally stationary, since the possibility of a “population time axis” is often not necessary and not
exploited. This is clearly different in the optical counterparts, where extensive use
is made out of the fact that pump-induced difference signals are detected and the
12
Reprinted from the journal
1 3
a chemist is possibly NMR spectroscopy. Fortunately, this is also the method with
which chemists are most intimately familiar in terms of signal interpretation. There
exist many analogies between multidimensional optical spectroscopy and nuclear
magnetic resonances [90–92]. For instance, quantized molecular states (either vibrational, electronic, or other) are the analogues of the spin configurations, albeit without a need to make them spectroscopically visible by the application of electric or
magnetic external fields. Also in optical spectroscopy, the properties and dynamics
of the molecular states are obtained by following the (laser-induced) FID in time.
As will be repeatedly demonstrated in the following contributions, spreading of the
optical signals in two dimensions allows for example the precise determination of
spectral linewidths and their dissection into homogeneous and heterogeneous contributions. The disentanglement of heterogeneity in the molecular spectra will be
one of the central topics in three contributions of this collection, namely the 2D
electronic, 2D infrared, and 2D Raman spectroscopies. Heterogeneous contribution
to the signal is widely known to be associated with the microscopic dephasing (fluctuations of energy levels). Such information cannot easily be determined from linear
or one-dimensional spectroscopy methods like transient absorption.
Also in close analogy to, e.g., widely known multidimensional COSY and
NOESY experiments in NMR spectroscopy, there exists tremendous interest in
optical multidimensional spectroscopy regarding the observation of so-called cross
peaks between different resonances. Such cross peaks may be observed for all types
of different resonances and can have manifold origin that need to be, however, evaluated on a case by case basis. For example, static 2D spectra can report on the existence of electrostatic (through space), mechanic (through bond) or electronic coupling in a sample system [93]. In case of optical measurements, the relative intensity
between diagonal and cross peak signals is very sensitive to structural properties
of the investigated sample, i.e., to relative distances and angles, which determine
the coupling strengths. Careful determination of relative intensities under different
experimental conditions can then be used in conjunction with theoretical models
of different levels to determine precisely molecular structure [33, 93, 94], or even
the structure of molecular aggregates [95]. The contribution on 2D IR spectroscopy
[96] depicts how multidimensional techniques using infrared light can be applied
to recover structural information from vibrational coupling. Cross peaks and couplings are also the topic of other contributions. Intra- and intermolecular coupling
between electronic states is reviewed in the 2D electronic spectroscopy contribution.
The contribution on 2D THz spectroscopy deals with low-frequency couplings, e.g.,
between intramolecular vibrational modes in the liquid phase. Similarly important is
the calculation of such couplings. Recent development of numerical methods to calculate the complete and accurate 2D electronic response of chromophores including
inter-chromophoric coupling is presented in the last contribution of this collection.
Thinking moreover about molecular dynamic effects in NMR spectroscopy, one
needs to consider the time dependence of both diagonal as well as cross peaks. In
multidimensional NMR spectroscopy, the diagonal peaks are generally stationary, since the possibility of a “population time axis” is often not necessary and not
exploited. This is clearly different in the optical counterparts, where extensive use
is made out of the fact that pump-induced difference signals are detected and the
12
Reprinted from the journal