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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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Also, porphyrins with /V-methyl-4-pyridinium substituents, can be
polymerized with a formation of conductive electrocatalytic films.17 The
most convenient method of polymerization and deposition of porphyrin
films on the surface of solid support is continuous scan voltammetry.
When detailing the electrochemical polymerization of the most useful
metalloporphyrin tetrakis (3-methoxy-4-hydroxyphenyl) porphyrin Ni(II),
(TMHPP)Nin, a continuous scan cyclic voltammogram shows the growth
pattern indicating film formation (Figure la). Thin-film formation occurs
only after oxidation of the porphyrin ring (process la and Ha). Peaks
(Ilia, c) are due to the redox reaction of the Ni(II)/Ni(III) couple in
the polymeric film (Figure lb). They are not observed after demetalation
of the film in acidic media. Ni(II) or Ni(III) can be reincorporated into
the film from neutral or basic solution. The process of reincorporation
of nickel into the polymeric porphyrin film was applied to the
development of an amperometric sensor for nickel.23 The oxidation of
Ni(II) to Ni(III) occurs only in polymeric (TMHPP)Ni and is not
observable in the monomeric porphyrin. Generally, the oxidation of Ni(II)
to Ni(III) in metalloporphyrins is difficult. However, the oxidation has
been observed in nonaqueous media in the potential range of 0.99-1.23 V
vs SCE.26 Different oxidation mechanisms of Ni(II) to Ni(III) in
porphyrins were proposed to occur depending upon the nature of the
porphyrin ring system. One of the most common mechanisms entails an
intramolecular electron transfer from the Ni(II) to the porphyrin ring
following the two-electron oxidation of the porphyrin ring to the
dication. In aqueous media the oxidation of dissolved Ni(II) to Ni(III)
has been known to be very difficult due to the strong hydration of nickel
ions. However, this unfavorable condition does not exist when nickel is
coordinated in the highly hydrophobic polymeric film. Therefore, it is
not surprising that a Ni(II) / Ni(III) redox couple is observed in
polymeric (TMHPP)Ni. The formation of a yellow-green film is clearly
visible during the electrochemical oxidation of (TMHPP)Ni in 0.1 M NaOH
on semiconducting indium-oxide-coated transparent electrodes.
The film is characterized by a Soret band at 434.0 nm and small band
at 538.4 nm (Figure lc). When (TMHPP)Ni is dissolved in basic solution, a
partial oxidation occurs which leads to the formation of a quinoid-type
system in the substituents on the 5 and 15 positions of the porphyrin
core
(species Ha). The portion of (TMHPP)Ni that is not oxidized chemically
could be oxidized electrochemically (wave I). The products of both the
chemical and the electrochemical oxidation are the same. During the
second process, which is solely electrochemical (wave II), another pair
of electrons is lost finally forming highly reactive orthoquinone species
(Ha, b) absorbed on the electrode surface. Two 3-methoxy-
4-hydroxy substituents in the 5 and 15 positions on the porphyrin ring
are oxidized according to this mechanism. It appears that two types of
processes take place during formation of the polymeric film. A
"horizontal" process, due to the absorption of fully oxidized (TMHPP)Ni,
results in the formation of monolayers of porphyrin material on the
surface of the electrode. Then, the absorbed, fully oxidized (TMHPP)Ni,
with its two orthoquinone-like substituents can oxidize the parent
molecule to species II being itself reduced to a semiquinone species V.
Interaction between species (II and V) can result in formation of
polymeric bond. This "vertical" process is responsible for the build up
in the multilayer system. The process of electrochemical oxidation of
molecules in the subsequent layer, loss of methoxy group, formation of
the orthoquinone-like substituents and further reaction with the parent
molecule would be repeated resulting in further polymerization.
Therefore, the growth of the polymeric film results from the chemical
oxidation of the parent porphyrin molecule at the film-solution interface
following the electrochemical oxidation. (TMHPP)Ni during its
electrochemical oxidation rapidly forms thick, mechanically stable,
compact films.
1. Morphology and ac Impedance of Polymeric Films
In micrographs of (TMHPP)Ni films, numerous craters are observed within a
relatively smooth surface (Figure 2a). The charge propagation through the
deposited porphyrinic film can be determined by ac impedance
spectroscopy. One way of depicting the data obtained from the impedance
measurements is with a plot of capacitive impedance (Zim) vs faradaic
impedance (Zreal) in complex plane diagram (Figure 2b). The overall
polymeric-film-redox process, which is a fundamental step in any
amperometric application of the porphyrinic sensors, consists of an
electron transfer between the porphyrin and the carbon support, charge
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