Topics in Current Chemistry (2018) 376:35
1 3
systems in condensed phase. In particular, the ability to directly follow Raman shifts
of only a few wavenumbers as well as Raman amplitudes during a photoreaction is a
central feature of time- as well frequency-domain Multi-VCS methods.
The recent success of multi-VCS in clarifying the dynamics of a myriad of
molecular system is mainly due to advances in key optical technologies of ultrashort
pulse generation and the ability of tuning the excitation and probing spectra. On the
one hand, multi-VCS in the time domain requires short pulses, i.e., below 15 fs to
effectively induce high-frequency vibrational coherence. On the other hand, spectral tuneability has been shown to be a central piece when disentangling e.g. ground
from excited-states vibrational dynamics as well as addressing different chromophores. These two aspects have become experimentally more accessible in recent
years by the combination of commercial non-collinear optical parametric amplifiers
(nc-OPA) and broadband chirped mirrors. As also shown in this contribution, this
has enabled the application of multi-VCS to a wide range of different chromophores
absorbing from the UV, over the visible and up to the near-infrared spectral region.
Present experimental developments, e.g., towards microscopy, are the promise for
turning multi-VCS into a so-called “high-content” analytical technique [167–168].
In particular, the combination of e.g., multi-VCS with super-resolution microscopy
exemplifies the ability to identify chemical compounds with spatial resolutions
beyond the diffraction limit without any chemical labeling [169].
In spite of the success of multi-VCS, there are still several ongoing research topics. One of these topics is the extraction of the pure excited state vibrational dynamics. The complexity of molecular signal extraction is present in all multi-VCS
methods and can be due to different causes (spectral overlap between ground- and
excited-state absorptions, signal distortion due to other optical signals contributions,
etc.). The development of cautious data post-processing, for example, to isolate reliably the specific stimulated Raman spectra of interest is still an intense research
topic.
Another ongoing research topic in multi-VCS is the calculation of the evolution
of Raman spectra in excited molecular states. Compared to other multidimensional
techniques like 2D electronic or 2D infrared spectroscopies, the application of theoretical methods to calculate the evolution of Raman spectra in excited molecular
states still is in its infancy, and in the overwhelming majority of the experimental
cases, analysis of multi-VCS spectra is still done in a very qualitative way. From
the theoretical point-of-view, models (like sum-over-states (SOS) and multimode
Brownian oscillator) and nonlinear signal calculation techniques are well known,
but there have been very few examples where the experimental optical signal in
multi-VCS has been completely numerically simulated [102]. A major challenge for
accurate modeling of third-order non-linear spectroscopy signals (VCS or UV–Vis
2DES) obtained in complex molecular systems still resides in the accurate quantum
chemical simulation of excited molecular states and of their photoreaction dynamics. The contribution by Segarra-Martin et al. in this collection reviews the present
state-of-the-art theoretical developments for the simulation of 2DES spectroscopy
signal in complex molecular systems. Rapid progress in theoretical development is
opening new horizons towards quantitative modeling of experimental signal based
on realistic models of complex systems.
236
Reprinted from the journal
1 3
systems in condensed phase. In particular, the ability to directly follow Raman shifts
of only a few wavenumbers as well as Raman amplitudes during a photoreaction is a
central feature of time- as well frequency-domain Multi-VCS methods.
The recent success of multi-VCS in clarifying the dynamics of a myriad of
molecular system is mainly due to advances in key optical technologies of ultrashort
pulse generation and the ability of tuning the excitation and probing spectra. On the
one hand, multi-VCS in the time domain requires short pulses, i.e., below 15 fs to
effectively induce high-frequency vibrational coherence. On the other hand, spectral tuneability has been shown to be a central piece when disentangling e.g. ground
from excited-states vibrational dynamics as well as addressing different chromophores. These two aspects have become experimentally more accessible in recent
years by the combination of commercial non-collinear optical parametric amplifiers
(nc-OPA) and broadband chirped mirrors. As also shown in this contribution, this
has enabled the application of multi-VCS to a wide range of different chromophores
absorbing from the UV, over the visible and up to the near-infrared spectral region.
Present experimental developments, e.g., towards microscopy, are the promise for
turning multi-VCS into a so-called “high-content” analytical technique [167–168].
In particular, the combination of e.g., multi-VCS with super-resolution microscopy
exemplifies the ability to identify chemical compounds with spatial resolutions
beyond the diffraction limit without any chemical labeling [169].
In spite of the success of multi-VCS, there are still several ongoing research topics. One of these topics is the extraction of the pure excited state vibrational dynamics. The complexity of molecular signal extraction is present in all multi-VCS
methods and can be due to different causes (spectral overlap between ground- and
excited-state absorptions, signal distortion due to other optical signals contributions,
etc.). The development of cautious data post-processing, for example, to isolate reliably the specific stimulated Raman spectra of interest is still an intense research
topic.
Another ongoing research topic in multi-VCS is the calculation of the evolution
of Raman spectra in excited molecular states. Compared to other multidimensional
techniques like 2D electronic or 2D infrared spectroscopies, the application of theoretical methods to calculate the evolution of Raman spectra in excited molecular
states still is in its infancy, and in the overwhelming majority of the experimental
cases, analysis of multi-VCS spectra is still done in a very qualitative way. From
the theoretical point-of-view, models (like sum-over-states (SOS) and multimode
Brownian oscillator) and nonlinear signal calculation techniques are well known,
but there have been very few examples where the experimental optical signal in
multi-VCS has been completely numerically simulated [102]. A major challenge for
accurate modeling of third-order non-linear spectroscopy signals (VCS or UV–Vis
2DES) obtained in complex molecular systems still resides in the accurate quantum
chemical simulation of excited molecular states and of their photoreaction dynamics. The contribution by Segarra-Martin et al. in this collection reviews the present
state-of-the-art theoretical developments for the simulation of 2DES spectroscopy
signal in complex molecular systems. Rapid progress in theoretical development is
opening new horizons towards quantitative modeling of experimental signal based
on realistic models of complex systems.
236
Reprinted from the journal