photochemistry of macromolecular metal complexes. iii. synthesis, spectral and electrochemical...

Photochemistry of Macromolecular Metal Complexes. Ill.* Synthesis, Spectral and Electrochemical Properties of Macromolecular Bound Protoporphyrin in Aqueous Solution

V. NARAYANAN and P. NATARAJAN'

Department of Inorganic Chemistry, School of Chemistry, University of Madras, Cuindy Campus, Madras 600 025, India

SYNOPSIS

The macromolecular bound protoporphyrin I X and its metal complexes, poly- ( protoporphyrin-co-acrylamide) , cobalt ( I1 ) [ poly ( protoporphyrin-co-acrylamide) 1, zinc- ( 11) [ poly (protoporphyrin-co-acrylamide) ], and manganese (111) [ poly (protoporphyrin-co- acrylamide ) ] chloride were synthesized. The absorption and emission spectra have been obtained for the macromolecular porphyrins. The lifetime of the excited singlet state of the protoporphyrin I X was found to decrease from 13.7 to 6.2 ns after polymerization. The cyclic voltammograms of polymeric protoporphyrin coated electrodes have been obtained. 0 1992 John Wiley & Sons, Inc. Keywords: photochemistry porphyrin macromolecules metal complexes

I NTRO DU CTI 0 N

Porphyrins and their meta. complexes have been identified to play a significant role in many biological reactions.2 Consequently there have been extensive investigations to understand the structure3 and reactivity4 of free base porphyrins and their metal complexes. Some of the naturally occurring porphyrins implicated in oxygenation5 and photosynthesis6 are insoluble in water and hence the redox properties of such porphyrins and their metal complexes could not be studied in aqueous solutions. However, water- soluble derivatives of the protoporphyrin IX have been synthesized and their thermal and photochemical properties have been inve~tigated.~ These derivatives do not completely represent the naturally occurring systems and hence we have attempted to synthesize water-soluble porphyrins by binding covalently the protoporphyrin IX to water-soluble macromolecule and investigated the spectroscopic and electrochemical properties. Macromolecular

* For Part 11, see Ref. 1. To whom all correspondence should be addressed.

Journal of Polymer Science: Part A Polymer Chemistry, Vol. 30,247&24&3 (1992) 0 1992 John Wiley & Sons, Inc. CCC 0&37-624X/92/122475-14

metal complexes and organic dyes show unusual properties.*

EXPERIMENTAL

Synthesis of Macromolecular Bound Protoporphyrin IX

Iron ( 111) protoporphyrin IX chloride Fe( PPIX) CI was prepared following the procedure of Fisher' and purified by recrystallization in a mixture of acetic acid and water. Free base protoporphyrin IX ( H2PPIX) was obtained by the demetallation of Fe(PP1X)Cl according to the procedure of Ramsey."

Copolymerization of protoporphyrin IX with acrylamide was carried out by the following procedure." Protoporphyrin IX ( 150 mg) and acrylamide (30 g) were dissolved in 300 mL of water-pyridine ( 6 : 1 v / v ) mixture and the solution was thoroughly deaerated. To the solution potassium peroxodisul- phate (300 mg) dissolved in a minimum amount of water was added. The contents were deaerated and sealed in a glass tube and the polymerization reaction was allowed to proceed at 60°C with stirring.

2476

2476 NARAYANAN AND NATARAJAN

After 24 h the contents of the tube were slowly poured into a large excess of methanol and the precipitate was collected by filtration and dried in vacuum. The precipitate was repeatedly dissolved in water and reprecipitated using methanol as nonsol- vent. The pale-brown amorphous solid was dried in a vacuum oven at 50°C and stored in a vacuum des- iccator. Silica gel thin layer chromatogram indicated the absence of unreacted monomeric protoporphyrin IX. The poly ( protoporphyrin IX-co-acrylamide ) [ poly ( PPIX-co-AM) ] was water soluble and the ratio of the protoporphyrin IX to acrylamide in the macromolecular chain was found to be 1 : 2500 f 20 as determined from the absorbance at 396 nm for a solution containing a known weight of poly ( PPIX- co-AM); from the known molar absorptivity for the PPIX, the ratio of PPIX to acrylamide in the mac- tomolecular chain was calculated. The macromolecular porphyrins are polydisperse and the molecular weight of the samples have not been determined. The presence of two vinyl groups in PPIX offers the possibility of forming some crosslinking. Since the polymers prepared are soluble it is presumed that extensive crosslinking has not occurred.

The metal complexes of macromolecular PPIX were synthesized by the following method. The co- polymer poly ( PPIX-co-AM) obtained as detailed above was dissolved in water and stoichiometric amounts of metal halides or acetates were added to the solution and heated to 50-60°C for 30-45 min with stirring. The completion of the reaction was ascertained by monitoring the absorption spectra which show characteristic features for the metal porphyrin complex. In the case of Mn ( 111) complex we could not completely coordinate all the porphyrin sites in the polymers.

The monomeric metalloporphyrins Co"PPIX, Zn"PPIX, and Mn"'( PPIX) C1 were prepared by the following method.12 Protoporphyrin IX was dissolved in boiling reagent grade DMF and stoichiometric amounts of the metal halides or other salts were added to the boiling solution and the reaction was allowed to proceed for 30-45 min. The completion of the reaction was indicated by the absence of fluorescence maximum at 620 nm for the free base protoporphyrin IX on excitation at 420 nm. In those cases where the metalloporphyrins show emission, the completion of the reaction was determined by monitoring the disappearance of the absorption bands of free base protoporphyrin IX at 630 nm. After the completion of the reaction the reaction mixture was cooled in ice water for 15 min. Cold water (100 mL) was added to this when the metal-

loprotoporphyrin IX separates out. The precipitate was filtered off, washed with small quantities of water, and dried in a vacuum oven at room temperature. These metallorprotoporphyrins were obtained in pure form by silica gel column chromatography using chloroform as the eluent.

Acrylamide obtained commercially was recrys- tallized from chloroform several times, and pyridine was purified by treating with KOH for several hours and vacuum distilling it before use. Dimethylform- amide obtained commercially was distilled under P205 in vacuum before use. Other reagents used in the investigation were of analytical grade.

Absorption spectra were measured using a Beck- man model 25 UV-VIS spectrophotometer. Emission spectra were obtained in a Perkin-Elmer model MPF-44B fluorescence spectrophotometer. Infrared spectral measurements were carried out using a model IR-408 Shimadzu infrared spectrophotometer. Cyclic voltammograms were obtained using a PAR set-up containing model 173 potentionstat / galvanostat, 176 current follower and 175 sweep generator modules with a saturated calomel electrode as the reference electrode. The sample solutions were deaerated by bubbling with oxygen-free nitrogen at 25°C. The cyclicvoltammograms of the porphyrins have been obtained by coating the porphyrins on to the pyrolytic graphite electrodes as films or with the porphyrins dissolved in homogeneous solution. Macromolecular porphyrin coated electrodes were prepared using pyrolytic graphite disks.13 Pyrolytic graphite disks were cut from the cylindrical stock (Union Carbide) with the basal planes of the graphite perpendicular to the axis of the cylinder. The disks were sealed at the end of a glass tube by means of heat-shrinkable polyolefin tubing. Electrical contact was made to the rear face of the disks using a few drops of mercury. The mounted graphite disks were freshly prepared before each set of experiments by cutting a thin section through the polyolefin sheath and the graphite disk with a scalpel. The exposed area of each disk was 0.16 cm2. The polymeric metalloporphyrins and free base porphyrin were coated on to the electrode by placing a small aliquot of the polymer solution of known concentration on the electrode surface and evaporating the s01vent.l~ Metalloporphyrins were also coated on to the electrodes using the same procedure.

Emission lifetime studies were carried out in a Applied Photophysics (U.K.) model SP-70 Nano- second Fluorescence spectrometer by time-corre- lated single photon counting technique.

PHOTOCHEMISTRY OF MACROMOLECULAR METAL COMPLEXES 2477

0.8,

l A

350 450 550 650

Wo~lenqlh ,nm

PPlX / DMF

0 I ,

c"

0.8 1

350 450 550 650 750

Wavelength , nm

Co(lll PPlX / DMF

0.1 I D

3 0.6

350 450 550 650 750

Wovelenpth , nm

Z n ( II 1 PPlX / DMF

350 450 550 650 75 0

Wavelenqfh, nm

Mn(l l l ) (PPIX) CI/DMF

Figure 1. DMF, (C) Zn(I1)PPIX in DMF, ( D ) Mn(III)(PPIX)Cl in DMF.

Electronic absorption spectrum of: ( A ) PPIX in DMF, ( B ) Co (I1 ) PPIX in


00--- 350 450 550 650

Wovelength, nm

Poty ( PPlX - CO-AM 1 / H2O

2.0,

0.01 1 I I I I I I

350 450 550 650

l B

400 500 600 700

Wovelanqth , nm

Co(ll) [poly (PPIX-co-AM )]/H20

Figure 2. Electronic absorption spectrum of: (A) Poly(PP1X-co-AM) in water, ( B ) Co(I1) [poly(PPIX-co-AM)] in water, (C ) Zn(II)[poly(PPIX-co-AM)] in water, (D) Mn( 111) [poly( PPIX-co-AM)] C1 in water.


RESULTS AND DISCUSSION

Macromolecules are known to affect the photo- physical and photochemical properties of dyes l5 and metal complexes l6 covalently bound to the chain. .The spectroscopic properties of porphyrins bound to synthetic macromolecules are not well known. In order to understand the photochemical and redox properties of the polymer bound porphyrins it is es- sential to have details on the nature of the excited states and extent of aggregation 7*17 between the porphyrins present in the macromolecular chain. The electronic absorption spectra of the monomeric and macromolecular protoporphyrin IX and the metal complexes with Mn, Zn, and Co have been recorded by dissolving the monomers in DMF and the polymers in water and are presented in Figures 1 and 2 and the spectral details are given in Table I. The absorption spectrum of protoporphyrin IX in pyridine is characteristic of an intense band with maximum at 409 nm ( E = 1.25 X lo5 M-' cm-') (Soret band) with the other bands occurring at 505, 540, 576, 605, and 630 nm. Although PPIX is insoluble in water, poly ( protoporphyrin IX-co-acrylamide ) is readily soluble in water. The visible absorption spectrum of poly ( protoporphyrin IX-co-acrylamide ) in aqueous solution is characterized by an intense and sharp peak for the Soret band at 396 nm and other bands at 503,527,613, and 630 nm as shown in Figure 2 (A). The shift of 13 nm of the Soret band for polymer porphyrin is due to the saturation of the vinyl groups in the monomeric porphyrins after polymerization. Broadening of the absorption spectral bands of the polymeric porphyrin species in wa-

ter indicates that there is some degree of aggregation between the porphyrin units in the macromolecule. Spectra of polymer-bound metalloporphyrins also exhibit such behavior indicating aggregation of the porphyrins in aqueous solution. Since the ratio of PPIX to acrylamide is rather low (1 : 2500) the broadening could also be due to site inhomogeneity in the polymer and also due to the fact that the spectra for the monomer is obtained in DMF while that of the polymer was obtained in aqueous medium.

The emission spectra of PPIX and poly (PPIX- co-AM) and their metal complexes were measured using pyridine and water, respectively, and are presented in Figure 3 and in Table 11. The emission spectra of PPIX and poly ( PPIX-co-AM) show maximum at 618 and 580 nm. The emission spectra of polymeric metalloprotoporphyrins and monomeric metalloprotoporphyrins differ from the polymeric protoporphyrins and monomeric protoporphyrin." For metalloporphyrins there appears a sharp and intense peak around 580 nm and a peak around 620 nm with low intensity.

Infrared spectra of the protoporphyrin IX, poly (protoporphyrin IX-co-acrylamide ) and its derivatives were obtained using KBr pellets. The IR spectra show peaks at 1610 cm-' due to uc,c(vinyl), 980 cm-' due to 8CH out-of-plane (vinyl ) , and 900 cm-' due to b C H Z out-of-plane (vinyl) which corresponds to the protoporphyrin." In the case of poly (protoporphyrin IX-co-acrylamide) the peaks at 1610, 980, and 900 cm-' are missing, indicating that the vinyl groups are saturated due to polymerization of protoporphyrin IX with acrylamide.

Table I. Macromolecular Bound Protoporphyrin IX and Its Metal Complexes

Absorption Spectral Properties of the Monomeric and

Complex Absorption Maxima

(nm)

PPIX

PPIX-DME

Poly( PPIX-co-AM) Co(I1)PPIX Co(I1) [poly(PPIX-co-AM)] Zn(I1)PPIX Zn(I1) [poly (PPIX-co-AM)] Mn(II1) (PP1X)Cl Mn( 111) [poly( PPIX-co-AM)]Cl Fe( 111) (PP1X)Cl

409, 505, 540, 576, 605, 631;

409, 505,540, 576,605,631;

396, 503,537, 565,613,630 418, 532,557 413, 527,560 418, 545,583 407, 536,575 367, 418,465, 550,585 367, 408,458, 540,574 410,532,556

c =1,25,00OM-' cm-' at 409 nm

c = 1,25,OOOM-' cm-' at 409 nm


In the case of metalloporphyrins the ~ c H ~ , out-of- plane (vinyl) 900 cm-' peak is shifted to higher frequency, i.e., in CoPPIX to 950 cm-l, in Mn(PP1X)Cl to 940 cm-l, and in ZnPPIX to 925 cm-'. However, there is no appreciable change in the VC=C frequency (1610 cm-') peak, the change in 1610 cm-' peak is within _+5 cm-'. In the case of polymeric metalloporphyrins the peaks due to vinyl groups are not seen in the IR spectra.

PMR spectra of protoporphyrin IX was obtained in DMSO-cl,. Three peaks are obtained due to the vinyl groups a t 6 = 8.10,6.25, and 6.10. These peaks are due to three protons present in the vinyl groups which are highly shielded." PMR spectra of poly ( protoporphyrin IX-co-acrylamide ) were taken in DzO which show no peaks beyond 6 = 5.0. This clearly indicates that the vinyl groups are saturated due to polymerization of protoporphyrin with acrylamide.

Porphyrin excited states (singlet) are short lived and the singlet state lifetimes are of the order of

i 0 0 7 -

59 0 670 750

Wovelength, nm

PPIX

nanoseconds. The decay profile of the emission from the excited state of the polymer sample in a typical case is shown in Figure 4. The excited singlet state lifetime for protoporphyrin IX was found to be 13.7 f 0.1 ns. In the case of poly ( protoporphyrin IX-co- acrylamide) the lifetime was found to be 6.2 f 0.1 ns. The singlet state lifetime of Zn"PP1X was found to be 2.8 f 0.1 ns. The singlet state lifetime of polymeric protoporphyrin is much lower than the monomer. It should be mentioned that the lifetime of the monomer is measured in DMF, whereas the lifetimes of the polymer is determined in aqueous medium. It is possible that in aqueous solution there is consid- erable amount of porphyrin aggregation. Due to aggregation there may be effective self-quenching which will decrease the lifetime of the singlet states. On the other hand, the emitting states may also relax rapidly due to the presence of the polymer chain, which decreases the lifetime of the singlet state of the macromolecular porphyrin.

Cyclic voltammograms of PPIX, Co"PPIX,

B

600 Hbvclenpth , nm

ZnPPlX

Figure 3. Room temperature emission spectrum oE (A) PPIX in chloroform, ( B ) Zn( 11) PPIX in chloroform, ( C ) Poly( PPIX-co-AM) in water, (D) Zn (11) [poly( PPIX- co-AM)] in water.


500 580 660 r40 800

Wovelength, nm

POIY (PPIX-cO-AM 1

I I I 1 I I 500 560 660 740

Wovelength , nm

Z n ( I 1 ) [poly (PPIX-cO- AM)]

Figure 3 (continued from the previous page)

Zn "PPIX, and Mn 'I1 (PPIX ) C1 coated as a film by evaporating a solution placed onto a pyrolytic graphite electrode (amounts of porphyrin or metalloporphyrins in the films around 1 X lo-' M ) obtained by immersing the electrode in a thoroughly degassed solution containing 0.1M perchloric acid

or trifluoroacetic acid as supporting electrolyte are shown in Figure 5 and the redox potentials are given in Table 111. The separation between cathodic and anodic peaks ( E , - EPa) is ca. 60-150 mV. The peak currents increase linearly with the scan rate of the applied potential (Fig. 6 ) as expected for the

Table 11. Macromolecular Bound Protoporphyrin IX and Its Metal Complexes

Luminescence Properties of Monomeric and

Compound Emission Maxima

Solvent (nm)

PPIX Chloroform 580,618,676 PPIX-DME Chloroform 580,618,676 POly( PPIX-co- AM) Water 580,618,675 Zn(I1)PPIX Chloroform 575,616 Zn(I1) [poly( PPIX-co-AM)] Water 575,616


, B -

.. . . . .* .,. , ...-/ :.... ............. .!. :. . ..... ...... .................... .... . ........... . ............. . . . . . . . . ........ . . . . : -. .: ... .:...:- ...... *-:.:.. .;.*3"

;. .. ............ ... . .......... .... ........... . . - (I) 0.0 .: .....r - a . c ~:.:.'. : .: ::... -.;.:.. a

v (I) Q) - s - a .-

-26.0 I I I 1 I I I I I I ,

5

t 4 ( I ) 3 c C 3 0 0

r 0 2 0 z 0

3 1

0

..

A

" I - 'r- ' ' I - I

I I I I I I I I I L 100 200 300 400 5c

Channel number -

Channel number -

0 100 200 250

Correlation channel - Figure 4. M poly( PPIX-co-AM) excited at 396 nm in water ( - - * * * ) together with a respective lamp profile ( - - - - - - ) . Solid line (in the decay profile) represent fits to the fluorescence decay profile for poly ( PPIX- co-AM) in water. (B) Residuals of the decay profile. (C) The autocorrelation function.

( A ) Fluorescence decay profile obtained from


Q


Table 111. Protoporphyrin IX and Its Metal Complexes

Redox Properties of Monomeric and Macromolecular Bound

Redox Potential

Complex Coated Electrode Homogeneous Solution

(V vs. SCE) (V vs. SCE)

PPIX PPIX-DME Poly( PPIX-co-AM) Co(I1)PPIX Co(I1) [poly(PPIX-co-AM)] Zn(I1)PPIX Zn( 11) [poly( PPIX-co-AM)] Mn(III)(PPIX)Cl Mn(II1) [poly(PPIX-co-AM)]Cl

0.46 k 0.01 0.46 f 0.01 0.46 k 0.01 0.67 k 0.01 0.38 k 0.01 0.57 -t 0.01 0.59 k 0.01 0.71 f 0.01 0.62 k 0.01

-

- 0.40 f 0.02

0.10 f 0.02

0.34 f 0.02

0.45 k 0.02

-

-

-

reactants attached to electrode surfaces.21 The peak currents of the voltammograms with the amount of porphyrin in the film remaining the same, decreased at lower pH. This does not result from stripping of the porphyrin from the surface by the acidic elec- trolytes because an electrode that exhibits low response a t pH 0.3 yields a large response almost in- stantly for pH 14.0 when the electrode is transferred from 0.1 M trifluoroacetic acid to 0.1 M sodium hy- droxide.

Cyclic voltammograms of the macromolecular bound porphyrins, poly ( PPIX-co-AM), Co"- [ poly ( PPIX-co-AM) 1 , Zn"[ poly ( PPIX-co-AM) 1, and Mn"'[ poly( PPIX-co-AM)] C1 coated as films by evaporating a solution of the polymer placed onto a pyrolytic graphite electrode (polymer bound porphyrin derivatives 2-3 X 10-"M) obtained by immersing the electrode in a thoroughly degassed solution containing 0.1M perchloric acid or 0.1M trifluoroacetic acid as supporting electrolyte are shown in Figure 7 and the redox potentials are given in Table 111. The voltammograms are well-defined with the peak separation between anodic and cathodic waves at ca. 60-120 mV. The peak currents increase linearly with the scan rate of the applied potential (Fig. 6 ) and the anodic and cathodic peak potentials are very nearly equal as expected for the reactants attached to electrode surfaces.'l There is slight shift in the Ell2 values, i.e., 40-80 mV. This deviation from the expected behavior for the reactants attached to the electrode surface may be due to the resistance of the polymer chain for electron transfer at the electrode. The saturation of the two vinyl groups may also have some effect on the redox potentials.

In homogeneous solution the cyclic voltammograms of poly ( PPIX-co-AM) , Co"[ poly (PPIX-co-

AM ) ] , Zn *I[ poly ( PPIX-co-AM ) ] , and Mn"'- [ poly ( PPIX-co-AM ) ] C1 were obtained. The voltammograms are not well-defined and also the peak separation a t ca. 200-300 mV is very high. There is a shift in value, i.e., 100-200 mV as compared for thin films on electrodes which is presumably due to the slower diffussion coefficient of the polymer.

In all the above cases the peak separation between anodic and cathodic waves is ca. 60-120 mV instead of 0 (for thin films on electrodes), and the wave shapes are asymmetric, as can be seen from almost negligible decline in the current after the peaks. This behavior could be a consequence of a slower charge transfer kinetics at the e l e c t r ~ d e . ~ ~ , ~ ~ It is also possible that the porphyrin species are present in dif- ferent environmental sites on the electrode surfaces.

Catalysis of the electroreduction of dioxygen by transition metal porphyrins is an active area. Por- phyrin catalyzed reduction of dioxygen at glassy carbon electrodes coated with Co"PP1X at pH 14.0 is shown in Figure 7 (D) . The Epc of oxygen reduction with no catalyst is ca. -0.60 V. This potential is shifted to ca. +0.06 V in the presence of Co"PP1X (adsorbed) as catalyst. The data using other catalysts are given in Table IV. At all pH values the catalyzed reduction of O2 proceeds at potentials well- separated from the surface waves for the absorbed catalyst so that these waves could be inspected (at higher amount of sensitivities) to determine if they are affected by exposure to dioxygen. In no case did the addition of dioxygen produces significant change in the position of the surface waves. At pH 14 the cyclic voltammograms for the reduction of dioxygen at the coated electrode corresponds to a totally reversible couple. Essentially equal anodic and cathodic peak currents are obtained and the peak potentials are separated by 60-80 mV. A peak splitting


I 1 I I I I 1 0 0 20 30 40 50--11i

SCAN RATE (3 1, mv/rat. -

t

SCAN RATE ( 3 1 ,mv/uc.-

I . CO(II [ ~O!~(PPIX-CO-AM) ] II. 2n (I1 1 [poly ( PPlX -co-AM ] 111. Mn(l!l )[poly ( PPIX-co-AM) 1 CI

Figure 6. Co(I1) [poly(PPIX-co-AM)], (11) Zn(I1) [poly(PPIX-co-AM)], (111) Mn(II1) [poly(PPIX-co-AM)].

IPC vs. u plots. A ( I ) PPIX, (11) Co(II)PPIX, (111) Zn(II)PPIX, (IV) Mn(III)(PPIX)Cl; B: ( I )

of 30 mV would be expected if the electrode reaction adhered to the Nernst equation. The slightly larger splitting observed may be due to both uncompen-

Table IV. the Metalloprotoporphyrins and Macromolecular Bound Metalloprotoporphyrins

Catalytic Oxygen Reduction Potentials for

Catalytic Oxygen Reduction Potentials

Complex (V vs. SCE)

Co(I1)PPIX +0.16 Co(I1) [ ~ ~ ~ ~ ( P P I X - C O - A M ) ] -0.10 Zn(I1)PPIX 0.00 Zn(I1) [poly(PPIX-co-AM)] -0.20 Mn( 111) (PP1X)Cl +0.07 Mn(1II) [poly(PPIX-co-AM)]Cl -0.15

sated resistance in the cell and some small deviation from the Nernstian behavior. As the pH of the solution is decreased, the separation of the cathodic and anodic peak potentials for the 02/02H- couple increases. The increasing separation results pri- marily from a shift of the anodic peak potential to more positive value while the cathodic peak potential remains relatively constant up to pH 7. The mag- nitude of the anodic peak current also diminishes a t pH values below ca. 12.

One of the most striking features in the behavior of the metalloporphyrin and macromolecular bound metalloporphyrin catalysts is the significant separation between the potentials where the catalyst is oxidized and reduced and at the potentials where it exhibits its catalytic activity. The catalyst is reduced at potentials well ahead of those where dioxygen is reduced, suggesting that the formation of a M - O2

- 4 a.

c z W a a

a

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Ill II

50 0 5

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I50

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1.0

0.6

0.2

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v vl

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1.4

1.0

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of m

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por

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d py

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) Co(II)[poly(PPIX-co-AM)], (

B) Zn(II)[poly(PPIX-co-AM)], (C

) Mn(III)[poly(PPIX-co-AM)]C1.

(D) C

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for t

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adduct is a necessary but not a sufficient condition for the electroreduction of dioxygen to occur. The potential of the electrodes must also be sufficiently negative to reduce the M-O2 adduct. At pH 14, where the catalysts endow the electrode with full reversibility toward the 02/02H- couple, a possible reaction sequence for the catalyzed reduction is:

LCo(ll) + 0 2 - LCo(ll)O* (1)

LCo(1l)O~H- -L LCO(l1) + OzH- ( 4 )

All of the steps in this sequence are reversible but a t high rates of reaction, the binding of the substrate (dioxygen) to the catalyst, i.e., the forward direction of the step (1) or the reverse direction of the step ( 2 ) , becomes rate-limiting. The insensitivity of the peak potentials of the adsorbed catalyst to the addition of 0 2 or H202 is strong evidence that the equi- librium constants governing the formation of the Co(I1) adducts of the dioxygen and 02H- are not large.13 A similar result was obtained for Mn"'/ Mn" - tetra - (N,N',N" - trimethylani1inium)por- ~ h y r i n , ~ ~ for Fe"'/Fe"-protoporphyrin IX,25 for Coi"1/Co"-tetrasulfonatophthalocyanine,26 and for Co "'/Co"-tetrapyridylporphyrin 27 and the role of an oxygen adduct formed between O2 and the metal atom of the macrocycle complex is postulated.

The investigations are partly supported by UGC COSIST assistance and by a DST Thrust Area Programme. V.N. was supported by UGC Junior Research Fellowship.

REFERENCES A N D NOTES

1. K. Anbalagan and P. Natarajan, J. Polym. Sci. Part A Polym. Chem., 29,1739-1749 (1991).

2. ( a ) D. Dolphin, Ed., The Porphyrins, Academic, New York, 1979, Vol. VII; ( b ) P. O'Carra, in Porphyrins and Metalloporphyrins; K. M. Smith, Ed., Elsevier, Amsterdam, 1975, chapter 4, and references therein; (c ) K. Butze and K. Nakamoto, J. Inorg. Biochem.,

3. ( a ) M. Gouterman, in The Porphyrins; D. Dolphin, Ed., Academic, New York, 1978, Vol. 111, chapter 1, and references therein; (b) F. Adar, in The Porphyrins; D. Dolphin, Ed., Academic, New York, 1978, Vol. V, chapter 6, and references therein; ( c ) J. G. Radzisz-

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Received March 22, 1991 Accepted September 24, 1991

photochemistry of macromolecular metal complexes. iii. synthesis, spectral and electrochemical...

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