extended towards UV/VIS or THz regions as well to address electronic and very
low-frequency, intermolecular modes, respectively.
5.4 Extensions of Transient 2D IR Spectroscopy
Other future developments of 2D vibrational spectroscopy concern the broadening
of the range of applications of transient 2D IR methods. In principle, one would like
to combine 2D IR spectroscopy with any type of possible perturbation of the sample
systems to be able to address a maximum range of scientific questions in chemistry,
physics, biology and material science. Depending on the involved mechanism of
perturbation and the response of samples, however, different timescales to be
studied will become relevant, which may range from picoseconds all the way up to
milliseconds, seconds or even minutes and longer. In that way, this approach will
involve a likely transition between transient 2D IR spectroscopy to what may be
referred to as ‘‘real-time 2D IR spectroscopy’’ [86].
Among the most important additional external stimuli to be implemented is the
triggered change of proton concentrations in liquid phases to yield so-called pHjumps [375–378]. Such triggered variations in the pH are particularly relevant in
biological systems such as folding and denaturation of proteins, but also acidcatalyzed chemical reactions and material chemistry. A general way to achieve the
pH-jump is to employ photo-acids, which, after the interaction with an ultrashort
laser pulse, release protons due to a change of the pK a value in the excited electronic
state [379–381]. Accessible timescales for the proton release are in the nanosecond
regime, but depend on the actual mechanism behind the dissociation [382–385]. pHjumps as large as two or three units have been demonstrated experimentally [384].
In that way pH-jump induced transient 2D IR spectroscopy is expected to enable
measuring a large range of structural changes in molecules.
Further methods to perturb the sample are the so-called pressure-jump techniques
(p-jump) [386]. Also this variant has previously been developed to study mainly
biological samples such as proteins [387] and investigate folding dynamics. But also
other applications can be envisioned such as dynamics of adsorbed molecules under
two- and three-dimensionally confined conditions [388, 389] or dynamics in
polymer samples [390]. Contrary to T- and pH-jumps, the p-jump is not induced by
an ultrashort laser pulse, but rather with the help of either mechanical stimuli, or
electrical burst diaphragms. This limits the achievable temporal resolution, but still
the sub-microsecond range has been achieved experimentally with pressure
differences as large as 2500 MPa [386]. With these developments, there exist
currently a powerful toolbox of fast sample perturbations, which cover most of the
important thermodynamic and kinetic variables.
Making use of the already existing combination of 2D IR spectroscopy and
electrochemistry (Sect. 4.1), other variants of transient 2D IR spectroscopy to be
developed might involve the laser-induced change of electrode potentials. In that
way, molecular dynamics near electrodes might be investigated that are based on a
so-called potential-jump (V-jump) of a pre-charged electrode [391, 392]. Also, here,
a laser pulse is used initially to act on the electrode material to induce the V-jump.
Effectively, it is assumed that the laser pulse heats up the electrode, thereby first
Top Curr Chem (Z) (2017) 375:86
123
186
Reprinted from the journal
low-frequency, intermolecular modes, respectively.
5.4 Extensions of Transient 2D IR Spectroscopy
Other future developments of 2D vibrational spectroscopy concern the broadening
of the range of applications of transient 2D IR methods. In principle, one would like
to combine 2D IR spectroscopy with any type of possible perturbation of the sample
systems to be able to address a maximum range of scientific questions in chemistry,
physics, biology and material science. Depending on the involved mechanism of
perturbation and the response of samples, however, different timescales to be
studied will become relevant, which may range from picoseconds all the way up to
milliseconds, seconds or even minutes and longer. In that way, this approach will
involve a likely transition between transient 2D IR spectroscopy to what may be
referred to as ‘‘real-time 2D IR spectroscopy’’ [86].
Among the most important additional external stimuli to be implemented is the
triggered change of proton concentrations in liquid phases to yield so-called pHjumps [375–378]. Such triggered variations in the pH are particularly relevant in
biological systems such as folding and denaturation of proteins, but also acidcatalyzed chemical reactions and material chemistry. A general way to achieve the
pH-jump is to employ photo-acids, which, after the interaction with an ultrashort
laser pulse, release protons due to a change of the pK a value in the excited electronic
state [379–381]. Accessible timescales for the proton release are in the nanosecond
regime, but depend on the actual mechanism behind the dissociation [382–385]. pHjumps as large as two or three units have been demonstrated experimentally [384].
In that way pH-jump induced transient 2D IR spectroscopy is expected to enable
measuring a large range of structural changes in molecules.
Further methods to perturb the sample are the so-called pressure-jump techniques
(p-jump) [386]. Also this variant has previously been developed to study mainly
biological samples such as proteins [387] and investigate folding dynamics. But also
other applications can be envisioned such as dynamics of adsorbed molecules under
two- and three-dimensionally confined conditions [388, 389] or dynamics in
polymer samples [390]. Contrary to T- and pH-jumps, the p-jump is not induced by
an ultrashort laser pulse, but rather with the help of either mechanical stimuli, or
electrical burst diaphragms. This limits the achievable temporal resolution, but still
the sub-microsecond range has been achieved experimentally with pressure
differences as large as 2500 MPa [386]. With these developments, there exist
currently a powerful toolbox of fast sample perturbations, which cover most of the
important thermodynamic and kinetic variables.
Making use of the already existing combination of 2D IR spectroscopy and
electrochemistry (Sect. 4.1), other variants of transient 2D IR spectroscopy to be
developed might involve the laser-induced change of electrode potentials. In that
way, molecular dynamics near electrodes might be investigated that are based on a
so-called potential-jump (V-jump) of a pre-charged electrode [391, 392]. Also, here,
a laser pulse is used initially to act on the electrode material to induce the V-jump.
Effectively, it is assumed that the laser pulse heats up the electrode, thereby first
Top Curr Chem (Z) (2017) 375:86
123
186
Reprinted from the journal