changes in the structure and orientation of redox-active molecules deposited in
organized films on the metal electrode surface.
Polymers, polyelectrolyte films, coordinated network compounds (e.g. metalorganic frameworks) belong to important redox-active films which modify electrode
surfaces. These films are significantly thicker than uniformly oriented monolayer
assemblies. In situ PM IRRAS was applied to investigate potential dependent
changes in the composition of redox-active polyelectrolyte multilayers [3, 11] as
well as cobalt and copper hexacyanoferrate coordinated network compounds [4]
deposited on a polycrystalline gold electrode surface. A pyridine modified poly
(allylamine) (PAH) was used to synthesize a PAH-Os
(II) (CN) complex, whose
structure is shown in Fig. 4.4a [3]. Mulibilayers of PAH-Os
(II) (CN) and PAA
(polyacrylic acid) polyelectrolytes were deposited on a polycrystalline Au electrode
surface. At E > 0.5 V versus Ag/AgCl, the Os
(II) ions in the polyelectrolyte undergo
oxidation. Change in the oxidation state of the metal ion coordinated to the cyanide
group influences the frequency of the ν(CN) mode. Figure 4.4b shows the PM
IRRA spectra in the ν(CN) mode region of a five bilayers thick polyelectrolyte film
at different potentials applied to the Au electrode. At E < 0.50 V versus Ag/AgCl
one ν(CN) mode centered at 2030 cm
À1 is present in the spectra of the polyelectrolyte film. At E > 0.5 V a decrease in the intensity of the ν(CN) mode at
2030 cm
À1 and appearance of a new mode at 2090 cm
À1 are observed (red line
Fig. 4.4b). The hypsochromic shift of the ν(CN) mode is due to the oxidation of the
Os
(II) ions, which are coordinated to the cyanide groups [3]. During the positive
going potential scan the intensity of the ν(CN) mode at 2030 cm
À1 does not
decrease to zero. This result indicates that a fraction of the redox-active complex
is electrically isolated and does not participate in the oxidation reaction.
The analysis of the changes in the intensity of the two ν(CN) mode allowed the
determination of the fraction of the redox-active Os
(II)/(III) metal ions. It is equal to
0.57. Change of the oxidation state of the redox couple in the polyelectrolyte film
causes a charge imbalance in the film, which is compensated by a flux of counterions
and solvent. Thus, after oxidation of the Os
(II) ions an enrichment of anions in the
polyelectrolyte film is expected. In situ PM IRRAS was used to monitor potentialdependent changes in the intensity of the IR absorption modes originating from
NO 3
À ions present the electrolyte solution [3]. Figure 4.4c shows the PM IRRA
spectra of the ν(NO) mode. Two absorption modes centered at 1348 and 1328 cm
À1
contribute to this mode. The high wavenumber mode corresponds to the doubly
degenerated ν 3 mode with D 3h point symmetry group in the anhydrous state
[12]. Hydration of the NO 3
À ion lowers the symmetry and leads to the splitting of
the ν(NO) mode as shown in Fig. 4.4c [3]. The oxidation reaction of the polyelectrolyte film leads to an increase in the intensity of the ν(NO) mode indicating that a
larger amount of the NO 3
À ions is present in the vicinity of the electrode.
A similar spectroelectrochemical characteristic was done for [Fe(CN) 6 ]
4À/3modified PHA/PAA polyelectrolyte films [11]. Described above studies indicate
that in situ PM IRRAS with electrochemical control allows monitoring of the
population of redox species entrapped in the polyelectrolyte film giving information
about the electrical connectivity in redox-active multilayer assembles. Moreover,
102
4 In Situ PM IRRAS Studies of Redox-spi1;Active Molecular Films...
organized films on the metal electrode surface.
Polymers, polyelectrolyte films, coordinated network compounds (e.g. metalorganic frameworks) belong to important redox-active films which modify electrode
surfaces. These films are significantly thicker than uniformly oriented monolayer
assemblies. In situ PM IRRAS was applied to investigate potential dependent
changes in the composition of redox-active polyelectrolyte multilayers [3, 11] as
well as cobalt and copper hexacyanoferrate coordinated network compounds [4]
deposited on a polycrystalline gold electrode surface. A pyridine modified poly
(allylamine) (PAH) was used to synthesize a PAH-Os
(II) (CN) complex, whose
structure is shown in Fig. 4.4a [3]. Mulibilayers of PAH-Os
(II) (CN) and PAA
(polyacrylic acid) polyelectrolytes were deposited on a polycrystalline Au electrode
surface. At E > 0.5 V versus Ag/AgCl, the Os
(II) ions in the polyelectrolyte undergo
oxidation. Change in the oxidation state of the metal ion coordinated to the cyanide
group influences the frequency of the ν(CN) mode. Figure 4.4b shows the PM
IRRA spectra in the ν(CN) mode region of a five bilayers thick polyelectrolyte film
at different potentials applied to the Au electrode. At E < 0.50 V versus Ag/AgCl
one ν(CN) mode centered at 2030 cm
À1 is present in the spectra of the polyelectrolyte film. At E > 0.5 V a decrease in the intensity of the ν(CN) mode at
2030 cm
À1 and appearance of a new mode at 2090 cm
À1 are observed (red line
Fig. 4.4b). The hypsochromic shift of the ν(CN) mode is due to the oxidation of the
Os
(II) ions, which are coordinated to the cyanide groups [3]. During the positive
going potential scan the intensity of the ν(CN) mode at 2030 cm
À1 does not
decrease to zero. This result indicates that a fraction of the redox-active complex
is electrically isolated and does not participate in the oxidation reaction.
The analysis of the changes in the intensity of the two ν(CN) mode allowed the
determination of the fraction of the redox-active Os
(II)/(III) metal ions. It is equal to
0.57. Change of the oxidation state of the redox couple in the polyelectrolyte film
causes a charge imbalance in the film, which is compensated by a flux of counterions
and solvent. Thus, after oxidation of the Os
(II) ions an enrichment of anions in the
polyelectrolyte film is expected. In situ PM IRRAS was used to monitor potentialdependent changes in the intensity of the IR absorption modes originating from
NO 3
À ions present the electrolyte solution [3]. Figure 4.4c shows the PM IRRA
spectra of the ν(NO) mode. Two absorption modes centered at 1348 and 1328 cm
À1
contribute to this mode. The high wavenumber mode corresponds to the doubly
degenerated ν 3 mode with D 3h point symmetry group in the anhydrous state
[12]. Hydration of the NO 3
À ion lowers the symmetry and leads to the splitting of
the ν(NO) mode as shown in Fig. 4.4c [3]. The oxidation reaction of the polyelectrolyte film leads to an increase in the intensity of the ν(NO) mode indicating that a
larger amount of the NO 3
À ions is present in the vicinity of the electrode.
A similar spectroelectrochemical characteristic was done for [Fe(CN) 6 ]
4À/3modified PHA/PAA polyelectrolyte films [11]. Described above studies indicate
that in situ PM IRRAS with electrochemical control allows monitoring of the
population of redox species entrapped in the polyelectrolyte film giving information
about the electrical connectivity in redox-active multilayer assembles. Moreover,
102
4 In Situ PM IRRAS Studies of Redox-spi1;Active Molecular Films...