Topics in Current Chemistry (2018) 376:35
1 3
1 Introduction
The dynamics and function of complex molecular systems can be understood
as resulting from the interaction between three different interacting sub-units
composed of the electronic, vibrational, and environmental degrees of freedom.
Focusing particularly on photoreactions—i.e., excited state reactions—they may
be described as (non radiative) transitions between distinct electronic states of
one or several molecules. Depending on the nature of the coupling responsible for
such a transition in a given case, the photoreaction may correspond to a charge
or energy transfer, or an internal conversion, an intersystem crossing, etc. Within
the Born–Oppenheimer (BO) approximation, electronic states are characterized
by multidimensional electronic potential energy surfaces (PESs) representing the
energy of the electronic subsystem as a function of all internal nuclear coordinates (3 N-6 for N nuclei) treated as fixed, external parameters. The vibrational
degrees of freedom, which drive the molecular system along the photoreactive
path from the Franck–Condon state (i.e., initially produced by the photon absorption) to the photoproduct, contribute to the reaction coordinate. All other molecular (and solvent, in condensed phase) vibrational degrees of freedom constitute
a large thermal bath—the environment—responsible for very fast energy relaxation and dissipation. There is not only fundamental interest in understanding how
these three sub-units interact to perform a given photoreaction and govern its
quantum yield, but also in understanding how such interactions shape the molecular functionality.
The mechanism of photoreactions may be elucidated by identifying the conformations and vibrational dynamics of transient electronic states successively populated, from the Franck–Condon state to the vibrationally and thermally relaxed
photoproduct. Vibrational spectroscopies (i.e., Raman and IR spectroscopies)
have long been exploited to reveal vibrational activity and conformations of stationary or transient molecular states. In this contribution, we will review recent
experimental developments, which implement in various ways stimulated Raman
scattering to monitor vibrational dynamics along the course of a photoreaction,
with a time resolution typically below 100 fs, therefore allowing to resolve the
vibrational activity accompanying ultrafast photoreactions. In condensed phases,
the vibrational energy relaxation and dissipation to the environment occurs on the
0.1–10 ps time scale, which is in many cases faster than the photoreaction itself.
We shall consider as “ultrafast” the photoreactions occurring on a similar time
scale or faster. One difficulty inherent to their investigations comes from the fact
that there is no time scale separation between the various relevant processes.
To investigate the dynamics and functions of molecular systems on ultrashort
time scales, time-resolved UV–VIS spectroscopy has been used since the advent
of femtosecond laser light sources. Time-resolved transient absorption—or socalled pump-probe spectroscopy—exploits the non-linear response of the complex system described above upon interaction with coherent laser light pulses.
More generally, in the regime of weak-field light–matter interaction, the use
of short, coherent laser pulses enables the preparation of controlled, coherent
208
Reprinted from the journal
1 3
1 Introduction
The dynamics and function of complex molecular systems can be understood
as resulting from the interaction between three different interacting sub-units
composed of the electronic, vibrational, and environmental degrees of freedom.
Focusing particularly on photoreactions—i.e., excited state reactions—they may
be described as (non radiative) transitions between distinct electronic states of
one or several molecules. Depending on the nature of the coupling responsible for
such a transition in a given case, the photoreaction may correspond to a charge
or energy transfer, or an internal conversion, an intersystem crossing, etc. Within
the Born–Oppenheimer (BO) approximation, electronic states are characterized
by multidimensional electronic potential energy surfaces (PESs) representing the
energy of the electronic subsystem as a function of all internal nuclear coordinates (3 N-6 for N nuclei) treated as fixed, external parameters. The vibrational
degrees of freedom, which drive the molecular system along the photoreactive
path from the Franck–Condon state (i.e., initially produced by the photon absorption) to the photoproduct, contribute to the reaction coordinate. All other molecular (and solvent, in condensed phase) vibrational degrees of freedom constitute
a large thermal bath—the environment—responsible for very fast energy relaxation and dissipation. There is not only fundamental interest in understanding how
these three sub-units interact to perform a given photoreaction and govern its
quantum yield, but also in understanding how such interactions shape the molecular functionality.
The mechanism of photoreactions may be elucidated by identifying the conformations and vibrational dynamics of transient electronic states successively populated, from the Franck–Condon state to the vibrationally and thermally relaxed
photoproduct. Vibrational spectroscopies (i.e., Raman and IR spectroscopies)
have long been exploited to reveal vibrational activity and conformations of stationary or transient molecular states. In this contribution, we will review recent
experimental developments, which implement in various ways stimulated Raman
scattering to monitor vibrational dynamics along the course of a photoreaction,
with a time resolution typically below 100 fs, therefore allowing to resolve the
vibrational activity accompanying ultrafast photoreactions. In condensed phases,
the vibrational energy relaxation and dissipation to the environment occurs on the
0.1–10 ps time scale, which is in many cases faster than the photoreaction itself.
We shall consider as “ultrafast” the photoreactions occurring on a similar time
scale or faster. One difficulty inherent to their investigations comes from the fact
that there is no time scale separation between the various relevant processes.
To investigate the dynamics and functions of molecular systems on ultrashort
time scales, time-resolved UV–VIS spectroscopy has been used since the advent
of femtosecond laser light sources. Time-resolved transient absorption—or socalled pump-probe spectroscopy—exploits the non-linear response of the complex system described above upon interaction with coherent laser light pulses.
More generally, in the regime of weak-field light–matter interaction, the use
of short, coherent laser pulses enables the preparation of controlled, coherent
208
Reprinted from the journal