immobilized carbon monoxide (CO) at the interface between aqueous electrolytes
and indium-tin-oxide (ITO)/platinum (Pt) electrodes has been investigated dependent on the applied electrochemical potential to the ATR surface [218]. The CO
molecule sensitively responds to the applied potential through the vibrational Stark
effect [219–222], which influences the CO stretching frequency by two distinct
mechanisms. In a first mechanism, application of the potential modifies the electron
density in the CO bond, thus strengthening or weakening it with varying potential.
In a second mechanism, the relative energetic positions of ground- and excited
vibrational states in the anharmonic electronic ground state potential of the
molecule are modified by the interfacial electric field [221, 223]. This originates
from slightly larger average bond lengths in the vibrationally excited state as
compared to the ground state, which results in different values for the dipole
moment, depending on the degree of vibrational excitation. Experimentally, the CO
frequency was thus modulated (Fig. 16b and c) in a potential range between about
-1.0 to 0.4 V (against an Ag/AgCl reference electrode). Interestingly, despite a
more than 30 cm
-1 shift in the vibrational frequency in this potential range, the
vibrational dynamics of the adsorbed CO are only slightly influenced by the
application of the interfacial electric field. Vibrational relaxation (Fig. 16 (a), left
scale) becomes only slightly slower (ca. 20%) for positive potentials (0.4 V, open
circles) as compared to negative potentials. In a similar manner, spectral diffusion,
as determined from the CLS method (Fig. 16a, right scale), becomes only slightly
faster (ca. 10%) for positive potentials (0.4 V, open circles). Such comparatively
mild variations in the ultrafast dynamics indicate that the interfacial electric field is
not strong enough to result in structural changes in the hydrogen bond network of
water molecules at the interfaces, which interacts with the adsorbed CO [204, 218].
Although 2D ATR IR has been so far mainly used to investigate ultrafast
dynamics of immobilized molecules at solid-liquid/gas interfaces, the technique is
intrinsically sensitive also to molecules from an isotropic environment above the
interface, e.g. the bulk electrolyte solution. This is due to the generation of the
evanescent wave at the ATR interface, which penetrates up to several micrometers
into the medium of lower refractive index [224]. Therefore spectro-electrochemistry
can also be performed for isotropic samples such as redox-active metal complexes.
This has been demonstrated by electrochemical switching between stable redox
species of ferro- and ferricyanide in aqueous solution (Fe
III/II (CN) 6
3-/4- , Fig. 16d
and e). Starting from the pure Fe
III species (2113 cm
-1 ) at positive potentials
(? 0.5 V against Ag/AgCl), a decrease of the electrochemical potential to 0 V starts
reducing the metal center, thus partially creating Fe
II species at lower vibrational
frequencies (2040 cm
-1 ). With these two demonstrations spectro-electrochemistry,
i.e. vibrational Stark-shift spectroscopy and redox-chemistry, 2D ATR IR has set the
stage for ultrafast investigations directly at electrode-electrolyte interfaces. This
rapidly growing field of research is expected to reveal important insight into
structural dynamics under electrochemical conditions, which are relevant in the
fields of catalysis or energy research. It is noteworthy that alternative approaches for
measuring ultrafast dynamics under electrochemical conditions have also been
presented, which can partly be applied for analogous investigations. External
reflection 2D IR spectroscopy has been introduced by Bredenbeck et al. to measure
Top Curr Chem (Z) (2017) 375:86
123
156
Reprinted from the journal
and indium-tin-oxide (ITO)/platinum (Pt) electrodes has been investigated dependent on the applied electrochemical potential to the ATR surface [218]. The CO
molecule sensitively responds to the applied potential through the vibrational Stark
effect [219–222], which influences the CO stretching frequency by two distinct
mechanisms. In a first mechanism, application of the potential modifies the electron
density in the CO bond, thus strengthening or weakening it with varying potential.
In a second mechanism, the relative energetic positions of ground- and excited
vibrational states in the anharmonic electronic ground state potential of the
molecule are modified by the interfacial electric field [221, 223]. This originates
from slightly larger average bond lengths in the vibrationally excited state as
compared to the ground state, which results in different values for the dipole
moment, depending on the degree of vibrational excitation. Experimentally, the CO
frequency was thus modulated (Fig. 16b and c) in a potential range between about
-1.0 to 0.4 V (against an Ag/AgCl reference electrode). Interestingly, despite a
more than 30 cm
-1 shift in the vibrational frequency in this potential range, the
vibrational dynamics of the adsorbed CO are only slightly influenced by the
application of the interfacial electric field. Vibrational relaxation (Fig. 16 (a), left
scale) becomes only slightly slower (ca. 20%) for positive potentials (0.4 V, open
circles) as compared to negative potentials. In a similar manner, spectral diffusion,
as determined from the CLS method (Fig. 16a, right scale), becomes only slightly
faster (ca. 10%) for positive potentials (0.4 V, open circles). Such comparatively
mild variations in the ultrafast dynamics indicate that the interfacial electric field is
not strong enough to result in structural changes in the hydrogen bond network of
water molecules at the interfaces, which interacts with the adsorbed CO [204, 218].
Although 2D ATR IR has been so far mainly used to investigate ultrafast
dynamics of immobilized molecules at solid-liquid/gas interfaces, the technique is
intrinsically sensitive also to molecules from an isotropic environment above the
interface, e.g. the bulk electrolyte solution. This is due to the generation of the
evanescent wave at the ATR interface, which penetrates up to several micrometers
into the medium of lower refractive index [224]. Therefore spectro-electrochemistry
can also be performed for isotropic samples such as redox-active metal complexes.
This has been demonstrated by electrochemical switching between stable redox
species of ferro- and ferricyanide in aqueous solution (Fe
III/II (CN) 6
3-/4- , Fig. 16d
and e). Starting from the pure Fe
III species (2113 cm
-1 ) at positive potentials
(? 0.5 V against Ag/AgCl), a decrease of the electrochemical potential to 0 V starts
reducing the metal center, thus partially creating Fe
II species at lower vibrational
frequencies (2040 cm
-1 ). With these two demonstrations spectro-electrochemistry,
i.e. vibrational Stark-shift spectroscopy and redox-chemistry, 2D ATR IR has set the
stage for ultrafast investigations directly at electrode-electrolyte interfaces. This
rapidly growing field of research is expected to reveal important insight into
structural dynamics under electrochemical conditions, which are relevant in the
fields of catalysis or energy research. It is noteworthy that alternative approaches for
measuring ultrafast dynamics under electrochemical conditions have also been
presented, which can partly be applied for analogous investigations. External
reflection 2D IR spectroscopy has been introduced by Bredenbeck et al. to measure
Top Curr Chem (Z) (2017) 375:86
123
156
Reprinted from the journal