Synthetic Metals, 15 ( 1 9 8 6 ) 219 - 227 219

POLYPYRROLE-PHTHALOCYANINE

M. V E L A Z Q U E Z R O S E N T H A L , T. A. SKOTHEIM and C. A. L INKOUS

Department of Applied Science, Brookhaven National Laboratory, Upton, N Y 11973 (U.S.A.)

Abstract

Polypyrrole films incorporating sulfonated phthalocyanines have been synthesized on Pt and indium-tin-oxide electrodes from aqueous solution. We report on the spectroelectrochemical and stability properties of the films incorporating tetra- and mono-sulfonated cobalt-phthalocyanine and tetra- sulfonated metal-free phthalocyanine. The films are strongly electrochromic and have four distinctly different color stages in the potential range 1.2 V to --1.6 V versus SCE. The films are more stable as electrodes than polypyrrole complexed with inorganic counterions and show a higher degree of crystaUinity.

Introduction

Polypyrrole film electrodes have been grown on various substrates in- corporating a number of different counterions to form highly conducting complexes between the polycation and the counterion [1- 6]. Standard inorganic counterions have been used in most of the work reported. Only a limited amount of work has been done to date to search for different types of counterions, e.g., larger organic molecules, which could lead to different con- figurations and ordering of the polymer. Counterions that are themselves electroactive and electro-optic species could also lead to materials with novel electrical and optical properties.

In this context we have investigated polypyrrole (PP) and poly (N- methyl pyrrole) (PMP) films incorporating large organic macrocycles as counterions, specifically complexes formed between polypyrrole and various sulfonated phthalocyanines. We have polymerized polypyrrole from aqueous solution in the presence of a number of sulfonated phthalocyanines, which are thereby incorporated as anions in the polymer matrix. The work reported here concerns films made with tetra- and mono-sulfonated cobalt- phthalocyanine (TSCoPC and MSCoPC, respectively) and tetra-sulfonated metal-free phthalocyanine (TSH:PC). The films obtained are strongly electrochromic, and, in the case of PP-CoPC, have four distinctly different color stages in the voltage range 1.2 V to --1.6 V (versus SCE), correspon- ding to different states of oxidation of the CoPC as well as that of the poly-

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pyrrole itself. The electrochromic properties of thin PC films alone have been investigated previously. [ 7 ].

PC films incorporating phthalocyanines and porphyrins as electro- catalysts have previously been studied as electrodes for oxygen reduction [2 - 5]. The PP-PC films also appear to be more environmentally stable forms of polypyrrole than PP films complexed with inorganic anions, as measured by monitoring the electronic conductivi ty as a function of time for films stored in ambient atmosphere [6]. In some cases, the conductivi ty has been observed to increase substantially over a period of about two to three months upon exposure to the atmosphere [6].

This paper discusses the electrochemical and spectroelectrochemical characteristics of sulfonated cobalt and metal-free phthalocyanines incorpo- rated as anions in polypyrrole and poly(N-methyl pyrrole).

Experimental

Pyrrole (Aldrich) was purified by fractional distillation and stored under nitrogen. Acetonitrile (Aldrich) was refluxed and distilled from Call:. Tetraethylammonium tetraf luoroborate (TEABF4) (Aldrich) was purified by t reatment with activated charcoal, recrystallized from methanol, dried and stored under vacuum. All acetonitrile electrolyte solutions were stored over activated alumina. Water was triply distilled, millipore-filtered quality.

The electrochemical measurements were performed with a PAR 173 potent ios ta t and a PAR 175 universal programmer. The u.v.-vis spectra were taken with a Perkin-Elmer spectrophotometer , and thickness measurements were done with a Dektak surface profilometer.

Results and discussion

Polymer synthesis The tetra- and mono-sulfonated phthalocyanines were synthesized

according to published procedures [8, 9]. The PP-MSCoPC, PP-TSCoPC, PP-TSH2PC and PMP-TSCoPC films were grown by electrochemical oxidation of pyrrole in an aqueous solution consisting of 0.2 M of a sodium salt of the phthalocyanine as the only anion present in the electrolyte, resulting in a complex formation between polypyrrole and the sulfonated phthalocyanine. The films were grown on 1 cm 2 platinum or indium-tin- oxide (ITO) substrates at a potential of 0.65 V (versus SCE) with current densities of 200 - 300 pA/cm:, or by cycling the potential between 0.0 V and 0.55 V with a sweep rate of 50 mV/s. The counterelectrode was a 5 cm 2 platinum mesh. The polymerization at constant potential required 25 - 30 mC/cm: for 0.1 pm film thickness compared with 20 mC/cm 2 for 0.1 #m for polypyrrole complexed with inorganic anions (e.g., BF4-).

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The deposit ion of PP-TSCoPC starts at 0.4 V, versus 0.6 V for PP-BF 4 in aqueous solution. This could be due to complex formation in solution between pyrrole and phthalocyanine, resulting in partial reduction of the pyrrole.

Once grown, the films were thoroughly rinsed with water and dried with an N2 stream. Thin films (1000 A - 2000 A) were dark blue with a smooth surface as seen under an optical microscope.

The electrochemical behavior of the films was studied both in aceto- nitrile (TEABF4) and aqueous (NaC104) electrolytes. All electrolyte solutions were degassed with N2 and kept under inert atmosphere through- out the studies. The films used for cyclic vol tammetry studies were 500- 1000/~ thick.

Phthalocyanine concentration Figure 1 shows the volume concentration of sulfonated CoPC in the

PP-CoPC films as a function of sulfonation and method of preparation in aqueous solution. The PC concentrat ion was calculated from its optical absorption at 660 nm. As can be seen from the Figure, the PC concentration in the film decreases with increasing thickness both for mono- and tetra- sulfonated species when the films are formed at a constant potential. Constant potential oxidation results in the formation of a depletion layer near the electrode from the parasitic oxidation of the CoPC, which occurs at 0.3 V, as compared to 0.6 V for pyrrole wi thout CoPC in the solution. This

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Fig. i. Concentration of CoPC in polypyrrole films calculated from the absorbance at 660 nm for tetra-sulfonated and mono-sulfonated CoPC incorporated in polypyrrole. Films were synthesized either potentiostatically at 0.65 V (vs. SCE) or by cycling between 0.0 V and 0.55 V with a sweep rate of 50 mV/s.

2 2 2

eventually lowers the concentrat ion of the CoPC in the film. Cycling the potential negative of the CoPC reduction potential regenerates the CoPC in the region next to the electrode and prevents the formation of a depletion region of unoxidized CoPC, which is apparently necessary for the incorpo- ration of PC in the film. The concentrat ion of CoPC in the PP film is then maintained for increasing PP film thickness.

The films used in the present study were deposited potentiostatically and with a thickness <0.1 pm and had a constant PC concentrat ion of ~15%.

Cyclic voltammetry When TSCoPC is incorporated in a PP film, all the redox processes of

the CoPC are evident and superimposed on the reduction/oxidation reactions of the PP itself. This is apparent in the cyclic voltammogram of Fig. 2, per- formed in acetonitrile electrolyte. The large background currents in the anodic regime are interpreted as capacitive charging currents associated with the large effective area of the porous oxidized PP electrode [ 10]. However, the PP-CoPC film is never fully reduced to an insulating state, as is apparent from the remaining charging current even at cathodic potentials. This is in contrast to the behavior of PP-BF4 and PP-C104, which in the reduced state exhibit small charging currents corresponding to a fraction of the geometric area. The polymer itself is insulating and the capacitive charging is due to the exposed substrate because of the porous nature of the film.

Four district oxidation states of the CoPC are observed in the range 1.2 V to --1.6 V, as reported in the literature [7, 11, 12]. The blue form

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represents the neutral form. On oxidation at 0.8 V, a violet color is observed. This oxidation is assigned to a ring orbital. On reduction beyond --0.4 V, a blue to yellow/green transition takes place. This is associated with a one- electron reduction where the additional electron is essentially confined to an orbital located on the central cobalt atom and represented as CoIpC - (yellow/green) and the blue form as CouPC [11]. The second reduction stage, at --1.5 V, is assigned to a ring orbital and represented as CoIPC 2- [11]. The color transition is from yellow/green to red/brown. The CoIPC 2- form is silghtly soluble in MeCN-TEABF4. Similar results were obtained with mono- sulfonated CoPC.

The PP-CoPC films required a certain break-in period of several cycles in which currents increased up to a stable value, where they remained for several hours of continuous cycling between --1.0 V and 0.4 V at rates varying from 0.1 to 5 V/s. The break-in effect could be due to morphological changes in the film allowing improved electrolyte penetration. The peak currents increased linearly with scan rate, as expected for a surface-bound species.

Stability PP films incorporating inorganic counterions (e.g., BF4) can only be

cycled reversibly to about 0.6 V before irreversible oxidation of the PP film itself occurs. The PP-CoPC films could be cycled to 1.2 V repeatedly without any significant decay. When the potential was scanned to --1.6 V repeatedly, an irreversible decay of the film electroactivity occurred. This effect, where the decomposition of PP at anodic potentials is prevented, has also been observed with other organic electroactive groups incorporated into PP as counterions [ 13].

A significant difference between PP-BF4 or PP-C104 and PP-TSCoPC is that the CoPC appears to be irreversibly bound to the PP matrix, whereas the inorganic anions do not. BF4 and C104 leave the PP matrix upon reduction of the polymer and are at least partially reinserted upon reoxidation when the switching is performed in an electrolyte containing the original anion. When PP-PC is cycled between oxidized (0.4 V) and reduced (--1.0 V) states in an electrolyte consisting of aqueous 0.1 M NaC104, where the PC anion itself is soluble, the PC itself is oxidized and reduced, but does not leave the PP matrix. With PP-TSH2PC there is no detectable loss of PC in the film even after 104 cycles, as shown in Fig. 3. The PC concentration was calculated from the PC absorbance at 660 nm.

With PP-TSCoPC a slow decay of the PC absorbance takes place initially, followed by more rapid decay at about 104 cycles. No CoPC could be detected in the solution, however. We attribute the loss in absorbance at 660 nm to a loss in the switching capability of the PP matrix itself, as seen in Fig. 4(a) in the region below 500 nm. At 0.4 V the film should be fully re- oxidized but resembles a film that is partially reduced. The absorbance of a fully reduced film without PC anions is shown in Fig. 4(b).

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Fig. 3. Stability of PP-PC and PMP-PC electrodes to electrochemical switching between the reduced (--0.8 V) and oxidized (0.4 V) states as measured by monitoring the absorbance at 660 nm in the oxidized state as a function of the number of switching cycles.

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Fig. 4. (a) Spectroelectrochemistry of PP-TSCoPC in aqueous 0.1 M NaCIO4 for a film in its as-made oxidized state poised at 0.4 V and reduced once to --0.8 V. Also shown is the spectrum of the same film at 0.4 V after 7 X 10 3 cycles. (b) Spectra of PP-BF4 in oxidized (0.5 V) and reduced (--0.8 V) states in MeCN-TEABF4.

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The spectroelectrochemistry of PP-TSH2PC and PP-MSCoPC is similar to that of PP-TSCoPC.

With poly(N-methyl pyrrole) (PMP) the sulfonated CoPC is no t as tightly bound to the polymer matrix, as exhibited by the decrease in con- centration with cycling time (Fig. 3), which is accompanied by coloring of the solution. In addition, the film does no t switch color to the yellow/green state, but to a lighter blue color when PMP is reduced and darker blue when PMP is reoxidized. This means that the PC is not reduced and reoxidized. Its blue color is superimposed on the light and dark color of the PP itself in its reduced and oxidized states, respectively. This color contrast fades within about 30 min of switching. This is accompanied by the reduction and eventual disappearance of the peaks of the cyclic voltammogram, leaving only a broad feature characteristic of the capacitive charging currents of a porous, large surface area, electrode.

It appears, therefore, that CoPC is not electronically coupled to the PMP matrix because of steric hindrance. Infrared spectra of PP-TSH2PC show no evidence of chemical bonding of the phthalocyanine to PP. The tight binding of H2PC and CoPC to PP could therefore be a ~-bonding of the flat PC molecules to the aromatic pyrrole moiety, a bonding configuration that is sterically hindered by the methyl group of PMP. This could also be an explanation for the increased ordering of PP-CoPC as observed with X-ray diffraction {vide infra ).

PP is known to be fairly stable to air exposure when it is in the oxidized state, and highly sensitive to irreversible air-oxidation in its reduced state. When a PP-BF4 film is exposed to air in its reduced, yellow/green, state (pp0), it will slowly oxidize irreversibly and darken. A subsequent cyclic voltammetric scan shows considerable loss of redox properties, leaving only a featureless cyclic voltammogram after only a few cycles.

When PP-CoPC is exposed to air in the reduced, yellow/green state, it immediately turns dark blue, undergoing spontaneous oxidation orders of magnitude faster than reduced PP-BF4. When the film is reconnected in the electrochemical cell in a deaerated solution, the color switching propert ies of the film remain, with only a slow degradation with repeated cycling. The spontaneous oxidation of reduced PP-CoPC is probably related to the central Co atom accepting an oxygen molecule as an extra ligand and therefore becoming oxidized [ 14].

The increased electrochemical stability of PP-PC films is accompanied by an increased stability of the electronic conduct ion properties of dried films. In some cases the electronic conductivity increases substantially over a period of 2 - 3 months, resulting in conductivities in the 40 - 50 (S cm -1) range [6]. The conductivi ty of PP-BF4, on the other hand, slowly decreases over a similar period.

The increase in conductivi ty is too slow to be a result of the influence of oxygen or moisture on the thin (~1 pm) porous films. Initial studies of the crystallinity of the films may provide an explanation. X-ray diffraction data of a fresh PP-CoPC film showed a much higher degree of crystallinity

226

than an equivalent film of PP-BF4 [15]. After the film had aged for 11 days, the degree of crystallinity and the size of the microcrystals appeared to have increased.

The degree of crystallinity is sensitive to the electrochemical growth conditions. Rapid growth at higher potentials (0.8 V and >0.5 mA/cm 2) produced films with no detectable crystallinity.

The more ordered structure imposed on polypyrrole with the incorpo- ration of large organic macrocycles may account for the increased stability of the films compared with BF4-doped polypyrrole. It is conceivable that the more localized charging of the film expected with smaller anions results in degenerative conditions associated with higher electric fields at the localized charges.

The increase of the conductivity with time could be due to a more ordered structure resulting from annealing of the PP-PC matrix and imposed by the large and fiat macrocycles.

Conclusion

The increased stability and the richness of the electro-optical properties of the PP-PC films clearly show the importance of systematic studies of new classes of counterions for synthesizing improved conducting polymeric materials. Fully delocalized counterionic charges may lead to materials with requisite environmental stability for practical applications. Furthermore, anions with delocalized charges are likely to be electro-optic molecules within the switching range of the host polymer matrix, and the complexation can therefore produce new materials with additional degrees of freedom compared with polymers incorporating non-electroactive inorganic counterions.

Acknowledgements

The authors would like to thank O. Inganiis and S. W. Feldberg for useful discussions and G. Senum for technical assistance. This work was supported by the Division of Chemical Sciences, U. S. Department of Energy.

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