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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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propagation through the film and counter ion diffusion to maintain film
neutrality. At a high frequency rate of the ac impedance-perturbation
signal, the rate of electron transfer may be the controlling factor for
the charge propagation through the film. In this case the complex plane
plot is a semicircle and resistance Rct to the charge transfer can be
determined from the diameter of the semicircle. The resistance of the
electrode-film-electrolyte system Rs is shown in the complex plane as the
difference between the zero origin of the plot and intercept of the
semicircle with the axis Zreal at high frequencies. At low frequencies
the charge propagation may become diffusion controlled. At this point the
complex plane plot becomes linear with a slope of n/4.
Film thickness and the charge-transfer process are crucial factors in
the design of porphyrinic-film amperometric sensors. Both of these
processes influence the range of

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Figure 1. A continuous scan cyclic voltammogram of nickel(ll) tetrakis(3-
methoxy-4-hydroxyphenyl)porphyrin [(TMHPP)Ni] indicating film formation
(a), reaction pathway for the electrochemical oxidation of (TMHPP)Ni in
basic aqueous media (l-ll) and the reaction leading to polymerization of
fully oxidized (TMHPP)Ni (III-V)
(b), UV-Visible spectrum of polymeric film of (TMHPP)Ni deposited on
transparent indium oxide electrode
(c). In lla, b, through V, only one of the oxidized meso substituents
in the a or position is shown.
44/Porphyrin-Based Electrochemical Sensors
Figure 2. Scanning electron micrograph of polymeric (TMHPP)Ni (a), An
impedance spectrum for thin-layer polymeric film on a solid electrode
(b), An impedance spectrum for polymeric-(TMHPP)Ni film on a glassy
carbon electrode (potential 0.52 V, film thickness 0.4 ) ().
frequencies over which electron-transfer kinetics, diffusion or charge
saturation determine charge propagation. For thick films, charge
saturation may be observed only at very low frequencies because diffusion
is the parameter that dominates over a large frequency range. The complex
impedance plot obtained for polymerized (TMHPP)Ni exhibited a kinetic-
control region and a diffusion-controlled region (Figure 2c). The
diffusion coefficient for charge transfer, Dct is 4 x 10 - 7cm2s 1, which
is relatively large in comparison to other polymeric materials (10 ~9~
10" l2cm2s ') (12). Furthermore, the resistance to charge
Figure 3. Possible spatial arrangements of free bases and metalated 1,10-
phen(TMHPP)2 in polymeric films, (a), In the case of 1,10-phen(TMHPP)2
metalated at the phenantroline N (1) N(10) centers in both porphyrin
rings; (b), As in (a) with different spatial arrangement.
transfer, Rct =91 ohms, is low compared to other polymeric materials such
as polymerized vinylferrocene film, 300-2600 ohms, and polymerized
diethylaminoporphyrin, about 4000 ohms.27 The electrical properties of
polymerized (TMHPP)Ni make this material very suitable for an
amperometric sensor. These properties can be additionally improved by
modification of the porphyrin structure.
The 1,10-phenanthroline can be attached to the porphyrin by
carboxamide-/)-phenylene bridges at the 4- and 7-positions of the
phenanthroline.17 This linkage forms a diporphine species in which the
phenanthroline acts as a "spacer" between the two porphyrin rings (Figure
3). The oxidation process of (l,10-phen)(TMHPP)2Ni observed during
continuous scan cyclic voltammetry is very similar to that observed for
(TMHPP)Ni. Figure 3 shows types of stacking promoted by the
phenanthroline linkage in (1,10-phen)(TMHPP)2Ni at the phenanthroline N-
(l) and N-(10) centers. Also, interactions between two porphyrin rings
located fairly close together in the same molecule would be expected.
Because the 3-methoxy-4-hydroxyphenyl substituents are both polar and
could be involved in hydrogen bonding within the stacked films,
intermoleeular attractions probably also occur. A scanning electron
micrograph of a polymer of (l,10-phen)(TMHPP)2Ni shows morphology of the
film to be relatively smooth with no microspheroid features visible with
a magnification of 1890:1 (Figure 4). An electrochemically active area
(measured based on Fe2 4 / Fe3 + redox reaction) which reflects a
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