the mentioned fields are based on the structural and dynamical properties of
molecules, 2D IR can be considered the perfect method for analytical purposes.
Spectroscopy of molecules at two-dimensional interfaces is particularly challenging
because there is only a low number of contributing molecules in the focal region of
the laser spot. Moreover, in the IR spectral range ultrafast molecular spectroscopy is
often considered difficult due to often very low absorption coefficients of functional
groups (\ 1000 cm
-1 ). However, to date a series of methods, which are able to
resolve ultrafast, multi-dimensional IR spectra from two-dimensional interfaces, has
been developed by different groups. All of these methods exhibit distinct strengths
and weaknesses that have been thoroughly compared before, both from an
experimental and theoretical signal point of view [15]. In this section, an overview
is presented on the demonstration of recent insights from the sample perspective.
The first demonstration that ultrafast 2D IR spectroscopy can be sensitive enough
to obtain signals from only monolayer thin samples of organic molecules at twodimensional interfaces was reported by Bredenbeck et al. by use of homodynedetected 2D sum-frequency generation (SFG) spectroscopy [172]. Although limited
structural and dynamic information was revealed in that study, the report triggered
subsequent ground-breaking developments of surface 2D IR spectroscopy. Detailed
investigations employed the high sensitivity of 2D IR in transmission and external
reflection BOXCARs geometry and focused on strongly absorbing metal-carbonyl
headgroups in self-assembled organic monolayers at solid-liquid/gas interfaces.
These experiments looked at mainly spectral diffusion dynamics in dependence of
different experimental parameters such as solvent properties, surface morphology or
monolayer structural defects [173–178]. The high absorption coefficients of metalcarbonyl complexes ([ 1000 M
-1 cm
-1 ) [173] strongly facilitated the generation
of the nonlinear signals from the monolayers. The application of plasmonic
substrates together with surface-enhanced 2D attenuated total reflectance (ATR) IR
spectroscopy [179] recently then allowed pushing the sensitivity to molecular
monolayers equipped with azide and nitrile functional groups that exhibit absorption
coefficients as low as 550 to \ 200 M
-1 cm
-1 [180, 181]. Figure 12 shows 2D
ATR IR spectra of the asymmetric stretch vibration from azide functional groups,
attached to linear alkyl chains of short [2-azidoethanthiol (2-N3), (a) and (b)] and
long [11-azidoundecanethiol (11-N3), (c) and (d)] on ultrathin gold (Au) layers
(average thickness 1 nm). Spectral diffusion was characterized for these systems by
the help of the CLS method (white lines), and the values were found to drastically
depend on the distance between the Au surface and the functional group. The
interpretation is that a close proximity to the rough surface results in a high degree
of structural heterogeneity and slower spectral diffusion for the short-chain
monolayers. This difference in the dynamics was rationalized with a lower quality
of packing between the monolayer constituents of the short-chain versus the longchain sample molecules (2-N3 vs. 11-N3) based on the varying degrees of interchain hydrophobic interaction.
In the context of the formation of self-assembled monolayers, important
questions often exist regarding the interaction between the monolayer constituents.
If molecules are tightly packed in well-defined aggregates, the intermolecular
distance is generally less than 1 nm, which is sufficiently close proximity for the
Top Curr Chem (Z) (2017) 375:86
123
146
Reprinted from the journal
molecules, 2D IR can be considered the perfect method for analytical purposes.
Spectroscopy of molecules at two-dimensional interfaces is particularly challenging
because there is only a low number of contributing molecules in the focal region of
the laser spot. Moreover, in the IR spectral range ultrafast molecular spectroscopy is
often considered difficult due to often very low absorption coefficients of functional
groups (\ 1000 cm
-1 ). However, to date a series of methods, which are able to
resolve ultrafast, multi-dimensional IR spectra from two-dimensional interfaces, has
been developed by different groups. All of these methods exhibit distinct strengths
and weaknesses that have been thoroughly compared before, both from an
experimental and theoretical signal point of view [15]. In this section, an overview
is presented on the demonstration of recent insights from the sample perspective.
The first demonstration that ultrafast 2D IR spectroscopy can be sensitive enough
to obtain signals from only monolayer thin samples of organic molecules at twodimensional interfaces was reported by Bredenbeck et al. by use of homodynedetected 2D sum-frequency generation (SFG) spectroscopy [172]. Although limited
structural and dynamic information was revealed in that study, the report triggered
subsequent ground-breaking developments of surface 2D IR spectroscopy. Detailed
investigations employed the high sensitivity of 2D IR in transmission and external
reflection BOXCARs geometry and focused on strongly absorbing metal-carbonyl
headgroups in self-assembled organic monolayers at solid-liquid/gas interfaces.
These experiments looked at mainly spectral diffusion dynamics in dependence of
different experimental parameters such as solvent properties, surface morphology or
monolayer structural defects [173–178]. The high absorption coefficients of metalcarbonyl complexes ([ 1000 M
-1 cm
-1 ) [173] strongly facilitated the generation
of the nonlinear signals from the monolayers. The application of plasmonic
substrates together with surface-enhanced 2D attenuated total reflectance (ATR) IR
spectroscopy [179] recently then allowed pushing the sensitivity to molecular
monolayers equipped with azide and nitrile functional groups that exhibit absorption
coefficients as low as 550 to \ 200 M
-1 cm
-1 [180, 181]. Figure 12 shows 2D
ATR IR spectra of the asymmetric stretch vibration from azide functional groups,
attached to linear alkyl chains of short [2-azidoethanthiol (2-N3), (a) and (b)] and
long [11-azidoundecanethiol (11-N3), (c) and (d)] on ultrathin gold (Au) layers
(average thickness 1 nm). Spectral diffusion was characterized for these systems by
the help of the CLS method (white lines), and the values were found to drastically
depend on the distance between the Au surface and the functional group. The
interpretation is that a close proximity to the rough surface results in a high degree
of structural heterogeneity and slower spectral diffusion for the short-chain
monolayers. This difference in the dynamics was rationalized with a lower quality
of packing between the monolayer constituents of the short-chain versus the longchain sample molecules (2-N3 vs. 11-N3) based on the varying degrees of interchain hydrophobic interaction.
In the context of the formation of self-assembled monolayers, important
questions often exist regarding the interaction between the monolayer constituents.
If molecules are tightly packed in well-defined aggregates, the intermolecular
distance is generally less than 1 nm, which is sufficiently close proximity for the
Top Curr Chem (Z) (2017) 375:86
123
146
Reprinted from the journal