the desorbed state (insets to Fig. 3.3b). In the re-adsorbed Y-type n-octadecanol
bilayer the hydrocarbon chains have an average tilt of 20
and 30
in the outer and
inner leaflets, respectively.
This example illustrates that in situ PM IRRAS is an excellent analytical technique for the studies of the conformation, packing and orientation of the hydrocarbon chain in amphiphilic molecules in thin films adsorbed in electrode surfaces.
Moreover, the advantage of the isotopic substitution allows a layer-by-layer analysis
of structural changes taking place in the entire supramolecular assembly. In situ PM
IRRAS examination of the polar head group region of amphiphilic molecules is
more demanding. The OH stretching and bending modes in the hydroxyl group of
n-octadecagon contribute to the IR spectrum. However, these modes are usually
wide and become indistinguishable from the non-linear background of the PM IRRA
spectrum.
In situ PM IRRAS was used to study potential-dependent changes in the structure
and orientation of both the hydrophobic (hydrocarbon chain) and hydrophilic (pyridine moiety) parts in 4-pendadecylpyridine molecules adsorbed on the Au(111)
surface [22]. The PM IRRA spectra of 4-pendadecylpyridine in the monolayer and
X-type bilayer assemblies in the 1650–1350 cm
À1 spectral region are shown in
Fig. 3.4. In this spectral region the pyridine ring gives four absorption modes. They
arise from the in-plane ring stretching modes. In the solution spectrum of
4-pendadecylpyridine, representing the PM IRRA spectrum of randomly distributed
molecules, their maxima are located at 1610 (a 1 ), 1559 (a 1 ), 1499 (b 1 ) and 1414
(b 1 ) cm
À1 [28, 29]. The IR absorption modes at 1468 and 1455 cm
À1 arise from the
deformation modes of the methylene and methyl groups in the hydrocarbon chain of
4-pendadecylpyridine (Fig. 3.4). In the mono- and bilayer-assemblies the position of
the maximum of the in-plane ring stretching modes in the pyridine moiety depend on
the potential applied the Au (111) electrode, thus on the environment around the
polar head group (Fig. 3.4).
The IR absorption modes centered at 1610 and 1421 cm
À1 are observed in the
potential range in which the films are adsorbed on the Au surface and are assigned to
the pyridine moiety being in a direct contact with the metal surface [22]. The in plane
ring stretching modes at 1603 and 1414 cm
À1 are visible only in the bilayer assembly
at negative, desorption potentials (Fig. 3.4a). They are assigned to the pyridine
moieties which are in contact with the electrolyte solution. In the monolayer film
these modes appear as a shoulder. The mode at 1610 cm
À1 is strong. These results
indicate that even at the most negative potential applied to the Au(111) electrode
(E ¼ À0.89 V vs SCE) the pyridine ring interacts with the gold surface. Figure 3.4
shows also that except for the shape, the intensities of these modes change with
potential, indicating potential-dependent changes in the orientation of the pyridine
moiety in the films. The transition dipole vector of the in-plane ring stretching mode
with a 1 symmetry (around 1610 cm
À1 ) is parallel to the C 2v symmetry axis of the
4-pendadecylpyridine molecule. In the monolayer assembly the calculated θ a1 angle
changes from 63
in the adsorbed to 68
in the desorbed state. In the bilayer
assembly, at the adsorption potentials, the pyridine rings in 4-pendadecylpyridine
in one leaflet make a direct contact with the Au surface while the other they are
52
3 In Situ PM IRRAS Studies of Redox-Inactive Molecular Films Adsorbed on. . .
bilayer the hydrocarbon chains have an average tilt of 20
and 30
in the outer and
inner leaflets, respectively.
This example illustrates that in situ PM IRRAS is an excellent analytical technique for the studies of the conformation, packing and orientation of the hydrocarbon chain in amphiphilic molecules in thin films adsorbed in electrode surfaces.
Moreover, the advantage of the isotopic substitution allows a layer-by-layer analysis
of structural changes taking place in the entire supramolecular assembly. In situ PM
IRRAS examination of the polar head group region of amphiphilic molecules is
more demanding. The OH stretching and bending modes in the hydroxyl group of
n-octadecagon contribute to the IR spectrum. However, these modes are usually
wide and become indistinguishable from the non-linear background of the PM IRRA
spectrum.
In situ PM IRRAS was used to study potential-dependent changes in the structure
and orientation of both the hydrophobic (hydrocarbon chain) and hydrophilic (pyridine moiety) parts in 4-pendadecylpyridine molecules adsorbed on the Au(111)
surface [22]. The PM IRRA spectra of 4-pendadecylpyridine in the monolayer and
X-type bilayer assemblies in the 1650–1350 cm
À1 spectral region are shown in
Fig. 3.4. In this spectral region the pyridine ring gives four absorption modes. They
arise from the in-plane ring stretching modes. In the solution spectrum of
4-pendadecylpyridine, representing the PM IRRA spectrum of randomly distributed
molecules, their maxima are located at 1610 (a 1 ), 1559 (a 1 ), 1499 (b 1 ) and 1414
(b 1 ) cm
À1 [28, 29]. The IR absorption modes at 1468 and 1455 cm
À1 arise from the
deformation modes of the methylene and methyl groups in the hydrocarbon chain of
4-pendadecylpyridine (Fig. 3.4). In the mono- and bilayer-assemblies the position of
the maximum of the in-plane ring stretching modes in the pyridine moiety depend on
the potential applied the Au (111) electrode, thus on the environment around the
polar head group (Fig. 3.4).
The IR absorption modes centered at 1610 and 1421 cm
À1 are observed in the
potential range in which the films are adsorbed on the Au surface and are assigned to
the pyridine moiety being in a direct contact with the metal surface [22]. The in plane
ring stretching modes at 1603 and 1414 cm
À1 are visible only in the bilayer assembly
at negative, desorption potentials (Fig. 3.4a). They are assigned to the pyridine
moieties which are in contact with the electrolyte solution. In the monolayer film
these modes appear as a shoulder. The mode at 1610 cm
À1 is strong. These results
indicate that even at the most negative potential applied to the Au(111) electrode
(E ¼ À0.89 V vs SCE) the pyridine ring interacts with the gold surface. Figure 3.4
shows also that except for the shape, the intensities of these modes change with
potential, indicating potential-dependent changes in the orientation of the pyridine
moiety in the films. The transition dipole vector of the in-plane ring stretching mode
with a 1 symmetry (around 1610 cm
À1 ) is parallel to the C 2v symmetry axis of the
4-pendadecylpyridine molecule. In the monolayer assembly the calculated θ a1 angle
changes from 63
in the adsorbed to 68
in the desorbed state. In the bilayer
assembly, at the adsorption potentials, the pyridine rings in 4-pendadecylpyridine
in one leaflet make a direct contact with the Au surface while the other they are
52
3 In Situ PM IRRAS Studies of Redox-Inactive Molecular Films Adsorbed on. . .