Langmuir 1992,8, 2455-2460
2455
Surfactant-Intercalated Clay Films Containing Metal Phthalocyanines James F. Rusling,’ Maryam F. Ahmadi, and Naifei Hut Department of Chemistry, Box U-60,University of Connecticut, Storrs, Connecticut 06269-3060 Received February 24,1992.I n Final Form: June 24, 1992 We previously showed that clay-surfactant films containing metal phthalocyanines catalyze electrochemical reductive dechlorinations. Cobalt(I1) phthalocyanine (CoIIPc)was a much better catalyst than the corresponding iron complex. This paper reports studies of these catalytic films by spectroscopic, X-ray, and electron microscopic methods. Scanning electron microscopy cross sectional images of films of didodecyldimethylmmonium bromide, clay, and CoIIPc were considerably different from the stacked layers observed for pure composites. Previously observed phase transitions are characteristic of surfactant bilayers. The general morphology of these films appears to feature a heterogeneous mixture of CoIIPc crystals and surfactant bilayers intercalated between clay layers. Electronic spectra and X-ray diffraction patterns suggest that iron phthalocyanine (FeIIPc)is present in oxidized forms in these films. CoIIPc films are better dechlorination catalysts partly because CoIIPc remains intact in the films, while FeIIPc is decomposed.
Introduction Ordered composites of clay and surfactants can be made by reacting cation exchanging clay colloids with insoluble amphiphilic cations.’ Films can be cast onto solid surfaces from suspensions of these composites in organic solvents. Surfactant films intercalated between linear ionic polyme13293 have been prepared by similar methods. Gel-toliquid crystal phase transitions for surfactant composite films occur at temperatures close to those of bilayer suspensions of the same surfactants. These results combined with X-ray diffraction and electron microscopy studies suggest that the films are arranged in stacked surfactant bilayers intercalated between clay or polymer layers. 1-3 Preparation and casting of composites as described above are a convenient way to prepare ordered multiple bilayer films of Surfactants with structures and properties related to biomembranes.‘ Films with thicknesses in the micrometer range containing thousands of bilayers are easily prepared. Permeability is controlled by the phase of the Neutral, water soluble solutes pass through the films in the liquid crystal state but are blocked when the films are brought to the solidlike gel phase. In addition to controlling permeability, other possible applications include coatings for piezoelectric5 or amperometric senS O ~ S , ~supports J for ordering biological macromoleculesld + On leave from Beijing Normal University, Beijing, China. (1) Okahata, Y.; Shimizu, A. Langmuir 1989,5,954-959. (2) (a) Shimomura,M.; Kunitake, T. Polym. J.1984,16,187-190. (b) Kunitake,T.;Tsuge,A.; Nakashima,N. Chem. Lett. 1984,1783-1786. (c) Nakmhima, N.; Kunitake, M; Kunitake, T.; Tone, S.; Kajiyama, T.
and inorganic complexes,1cand membranes for controlling vectorial electron transport.8 We recently evaluated clay-surfactant films containing redox mediators for electrochemical catalysis. Composite films of didodecyl- and dioctadecyldimethylammonium bromide (DDAB and DODAB) and clay cast on pyrolytic graphite electrodes acted as barriers toward hydrophilic multivalent ions6but incorporated hydrophobic ions and neutral molecules from aqueous solutions. Clay-surfactant films containing neutral metal phthalocyanines catalyzed reductive dechlorination of trichloroacetic acid? Cobalt(I1) phthalocyanine was a much better catalyst in the composites than the corresponding iron complex. Charge transport rates were excellent when the films were in liquid crystal phases but poor in solidlike gel states. Gel-to-liquid crystal phase transitions were detected by voltammetry6 and differential scanning calorimetry.’ Clay-surfactant films containing metal phthalocyanines showed excellent stability in catalytic applications and were usable for a month or more. In this paper, we report results of several types of experiments to characterize composite films containing metal phthalocyanines: (i) square wave voltammetry to establish redox properties of the films; (ii) electronic absorption spectroscopy, which provides insight into microenvironment, and oxidation and aggregationstatesof the catalyst; (iii) scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses, which provide insight into film morphology, structure, and catalyst distribution; (iv) X-ray powder diffraction providing the d-spacing of the composites.
Experimental Section Chemicals and Solutions. Didodecyldimethylammonium
kano, I(.J. Chem. Soc., Chem. Common. 1990,443-444. (3) (a) Okahata, Y.; Enna, G.; Taguchi, K.; Seki, T. J. Am. Chem. SOC. 1986,107,5300-5301. (b) Okahata, Y.; Enna, G. J.Phys. Chem. 1988, 92,4646-4551. (c) Okahata,Y.; Enna, G.; Takenouchi,K. J.Chem. SOC., Perkin Tram. 2 1989,835-843.
bromide (DDAB, 99+ % ), dioctadecyldimethylammonium bromide (DODAB, 99+%), and iron and cobalt phthalocyanine (97+ % ) were from Eastman Kodak. Cetyltrimethylammonium bromide (CTAB, hexadecyltrimethylammonium bromide) was Fisher certified grade, 99.8%. Solvents were spectroscopic grade. All other chemicals were reagent grade. Bentonite clay (Bentolite H)was from Southern Clay Producb and had a cation exchange capacity of 80 mequivI100 g. Apparatus and Procedures. A Bioanalytical Systems BAS100electrochemistrysystem was used for Osteryoung-typesquare
Texter, J., Eds.; Plenum: New York, in press.
(8) Gratzel, M.HeterogeneousPhotochemicalElectronTransfer;CRC Press: Boca Raton, FL, 1989.
Macromolecules1986,18,1515-1516. (d)Higashi,N.; Kajiyama,T.;Kunitake, T.; Praee, W.; Ringadorf, H.; Takahara, A. Macromolecules 1987, 20,29-33. (e) Nakashima, N.; Eda, H.; Kunitake, M.; Manabe, 0.;Na-
(4) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (5) Okahata, Y.; Ebato, H. Anal. Chem. 1991,63, 203-207. (6) Hu, N.; Rueling, J. F. Anal. Chem. 1991,63, 2163-2168. (7) Rusling, J. F.; Hu, N.; Zhang,H.; Howe, D.; Miaw, C.-L.; Couture, E. In Electrochemistry in MicroheterogeneousFluids; Mackay, R. A.,
0743-7463/92/2408-2455$03.00/00 1992 American Chemical Society
Ruling et al.
2456 Langmuir, Vol. 8,No. 10, 1992 wave voltammetry (SWV). The working electrode was a basal plane pyrolyticgraphite (HPG-99,Unioncarbide) disk (geometric A = 0.2 cm2). Electrodes were prepared by sealing a disk into polypropylene pipet tips as described previouslf or by sealing to glass tubes with heat shrinkable tubing. Pyrolytic graphite (PG) electrodes were rough polished with 600-grit S i c paper on a metallographic polishing wheel prior to coating. Surfactant-clay composites were prepared by reacting clay colloids and surfactant in aqueous suspension as described previously.6 Purified, freeze-dried composite was suspended in chloroform (2 mg mL-1) for preparing films. Metal phthalocyanine (MPc) solutions in chloroform (10 mM) were mixed 1:l with the composite suspension. For voltammetry and SEMI EDX, 120 pL of the composite MPc suspension was deposited with a micropipet onto a 0.2 cm2 pyrolytic graphite disk. Chloroformwas evaporated overnight in air. Dry film thicknesses by SEM were 20-30 pm? The three-electrode cell for SWV included the pyrolytic graphite (PG) working electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) as reference. Ohmic drop of the cell was about 90 % compensated by the BAS100. Experimenta were thermostated at 25.0 f 0.1 OC. All solutions were purged with purified nitrogen to remove oxygen before SWV. Absorption spectroscopy was done using Perkin-Elmer Model h3B or Milton Roy Spectronic 3000 Array UV-Vis spectrophotometers. The reference for composite films was usually a plain glass slide. For visible spectroscopy,MPc-compositesuspensions were prepared as above, but with 1r M MPc to obtain measurable absorbance spectra. An 80-pL portion of the suspension was cast onto a masked 0.5-cmZarea of a glass microscope slide and chloroform was evaporated in air overnight. Dry thickness was estimated at ca. 10 pm by SEM. Scanning electron micrscopy (SEM) and energy dispersive X-ray analysis (EDX) were done with an Amray 1810microscope using a tungsten filament. DDAB films for SEM/EDX analysis were coated on PG electrodesusing the same preparation methods as for electroanalysis. The entire electrode assembly was fiied on the mounting stage of the SEM with electrical connection through the connecting wire. For cross-sectionalviews, composite coatings were prepared on very thin disks of pyrolytic graphite and freeze fractured after immersion in liquid nitrogen. Prior to analysis by SEM, 5 nm of gold was coated onto samples with a Model SC 500 sputter coater (Bio-Rad). EDX was done using a Phillips North America PV-9800 EDAX system. Beam diameter was 2 pm. X-ray diffraction studies were done with a Scintag XDS 2000 powder diffractometer using a Cu Ka source at 45 kV and 40 mA. Scan rate was 0.5 deg/min. Films for X-ray diffraction were prepared on glass microscope slides from chloroform dispersions with compositions described above. Before analyses, the dry films were soaked in 0.1 M KBr for 2-3 h, washed with water, and stored in a closed desiccator with a small amount of water in the bottom to maintain hydration.
+
Results Electrochemistry. Squarewave voltammetrywas used to measure reduction and oxidationpotentials of the metal phthalocyanines in the films. For the film containing cobalt phthalocyanine (CoPc),the first reduction peak at about -420 mV (all vs SCE)corresponds to the reversible CdIPclCoIPc-redox couple as reported previously6 (Figure la). Ita shape reflects overlapped peaks due to adsorption of CoPc on the electrode. By comparison with peak potentials reported for CoIIPc in organic solventa,9JO the second reduction peak at about -1350 mV most likely corresponds to the Co1Pc-/Co1Pc2-couple. No oxidation peak was observed for CoIIPc. The current increase at 700 mV (Figure lb) is probably caused by oxidation of bromide ions. (9)Lever, A.B.P.; Licoccia, S.;Magnell, K.; Minor, P. C. Adu. Chem. Ser. 1982,No. 201, 231-251. (10) Ruling, J. F.;Owlia, A. J. Electroanab Chem. Interfacial Electrochem. 1987,234,297.
0.001 -100
'
'
"
'
'
"
'
'
.
700
300
"
1100
'
" 1500
-E, mV v8 SCE
1
-0.25 1
-0.05 0.00 700
500
100
300
E, mV v8 SCE
Figure 1. Square wave voltammograms at 250 Hz,25 mV pulse height for CoPcclay-DDAB films on PG disks in 0.1 M KBr: (a) cathodic scan; (b) anodic scan. 0.30
A
0
-0.16
-0.10
400
800 -E, mV VI
1200 SCE
I
'
700
1600
I
I
500
3 00
100
-100
E, mV v8 SCE
Figure 2. Squere wave voltammograms at 250 Hz,25 mV pulse height for FePcclay-DDAB films on PG disks in 0.1 M KBr: (a) cathodic scan; (b) anodic scan.
Two reduction peaks were found for films to which iron phthalocyanine (FePc) had been added (Figure 2). The peak at -580 mV is in the potential range for an FeuPc reduction. The peak at -1260 mV, by comparison with literature?JO is in the same range as a Fe1Pc-/Fe*Pc2couple. An oxidation peak for FePc films was observed at 100 mV (Figure 2b).
Langmuir, Vol. 8, No. 10, 1992 2467
Clay-Surfactant Films Containing CdIPc
Table I. Electronic Absorbance Peakr for Metal Phthalocyanines in Different Media A, values, nm cobalt phthalocyanine iron phthalocyanine medium4 (A*)* DMSO (1.0) 658 596 653 630 590 CHzClz (0.82) 665 706 657 552 benzene (0.59) 667 599 708 674 hexane (4.08) 671 746 709 680 tetradecane (-0.08) 670 596 706 660
0.10
0.054
0.1 M CTAB
668 670 668
clay/DDAB clay/DODAB WAVELENGTH (nm) 0.mo
609 639 638
707 146 748
606 603
616 681 682
594 592
4 Enough metal phthalocyanine was added to make each solution 1 pM. In the most hydrophobic solvents, some particles remained which were removed by filteringbeforethe spectra weretaken. Taft r* parameters from ref 12.
*
7
Films COPC
0
CoPc
FePc
0.060
6901
I
u' I _
0.040
I
670
'
5 0 0 5 5 0 ~ " 7 5 0 8 0 0 8 5 0
WAVELENGTH (nm)
650 -0.30
0
0.10
0.50
0 0
0.90
I
1.30
-* II Figure 4. Influence of Taft's A* solvent parameter on maxima of longest wavelength peaks of metal phthalocyanines in various solvents. A* values are given in Table I. Maxima for CoPc in DDAB- and DODAB-clay films are arbitrarily located on the linear regression line for the CoPc solution data.
WAVELENGTH (nm)
Figure 3. Visible absorbance spectra of (A) 1 pM ConPc in DMSO (B) 1 pM ConPc in 0.1 M aqueous CTAB, (C) ConPc in clay-DDAB film.
UV Spectra. Electronic spectra of metal phthalocyanines in the visible region reflect their state of aggregation.11 In DMSO, the peak for CoPc at 658 nm and a small peak at 596 nm are attributed to the monomer (Figure 3A).1°J1 Peaks for association dimers of metal phthalocyanines are generally found as shoulders on the short wavelength side of the main monomer peak." When CoPc is dissolved in aqueous micellar 0.1 M cetyltrimethylammonium bromide (CTAB), a relatively large peak attributed to dimer is found at 609 nm along with the monomer peak at 668 nm (Figure 3B). This is presumably because the ~~
(11) (a) Schelly, 2.A.; Farina, R. D.; Eyring, E. M.J. Phya. Chem. 1970,74,617-620.(b)Schelly,2.A.; Huward, D. J.; Hemes, P.; Eyring, (c) Gruen, L.C.; Blagrove, E. M.J. Phya. C h m . 1970,74,3*3042. R. J. A u t . J. Chem. 1972,25,2553-2558;1973,26,319-323. (d) Boyd, P.D. W.; Smith, T. D. J. Chem. SOC.,Dalton Tr0n.s. 1972,839-843.(e) Farina, R. D.; Halko, D. J.; Swinehard, J. H. J. Phya. Chem. 1972,76, 2343-2348. (0Abel, E. W.; Pratt, J. M.;Whelan, R. J. Chem. SOC.,Dalton Trans. 1976,509-514. (g) Yang, Y.;Ward, J. R.; Seiders, R. P. Inorg. Chem. l986,24,1765-1769.
water-insoluble CouPc is solubilized in restricted hydrophobic regions of the micelles at concentrations which drive the formation of dimers. Composite clay surfactant filmscontaining CoPc had prominent monomer peaks near 669 nm with a relatively small dimer peak at about 639 nm (Figure 3C). Spectra of CouPc were recorded in dimethyl sulfoxide (DMSO), methylene chloride, tetradecane, hexane, and benzene, as well as in composite clay films of DDAB and DODAB (Table I). The main monomer peak showed a definite shift toward longer wavelengths as solvent polarizability decreased. A linear plot was obtained when ,A of the monomer peak was plotted against Taft's polarizability/dipolarityl2parameter x* (Figure 4). When ,A of CoPc in DDAB and DODAB composites are placed on this regression line, a value of T* = 0.1 is found. This is about midway between values for tetradecane and diethyl ether. In DMSO, spectra of FePc were similar to those of CoPc, with a monomer peak close to 653 nm and a dimer shoulder at about 630 nm (Figure 5A, Table I). However, in less polar solvents, in 0.1 M CTAB, and in the composite films (Figure 5B), broad peaks were observed at wavelength about 700 nm (Table I). After a day or more, spectra obtained from methylene chloride and DMSO solvents showed the increasing development of these peaks. The positions of the longest wavelength peaks for freshly (12)(a)Taftsolvent polarity indiceswere derivedbased on wavelength shifta in absorption spectra for indicator solutes in a wide range of solventa.'*b The A* parameter is an indicator of solvent polarity and Abraham, M.H.; polarizability. (b) Kamlet, M.J.; Abboud, J. L.-M.; Taft, R. W. J. Org. Chem. 1983,48,2877.
Rwling et al.
2468 Langmuir, Vol. 8, No. 10, 1992
BCINTA,ci/US&
88.21 14.11 29.43 22.01 800.0
560.0
480.0 400.0 320.0
240.0 WAVELENQTM (nm)
17.66 14.72 12.62 11.04
i
I
160.0 80.0
0.0
IN: mjn205n5.ni I D : COPS DATE: 2 / 5 / 9 2 TIME: 9:15
1
88.21
em
700
750
41.14 29.43 2 2 . 0 7
600.0j1
480 .O -
ow
8%
420.0-
WAVELENGTH (nm)
Figure 5. Visible absorbance spectra of (A) 1 WM FeIIPc in DMSO; (B) FePc-clay-DDAB film.
b
S T L P : 0.030
17.66 14.12 12.62 11.01
9’ 1.51060
Wt:
9.82
8.84
5‘
60
pol
so 40
30
basal macine.A clay alone 10.2 f 0.4 composite alone 25.5 f 0.5 composite + CoPc 23.8 f 0.8 composite + FePc 29.1 f 0.6 0 See Experimental Section for preparation and compositions. 28,dea 8.66 3.46 3.14 3.03
100
t70
Table 11. X-ray Diffraction Results for DDABClay Composite Films
film additivea
8.84
BCINTAO/USA
PT: 3.600
540.0BW
9.82
20
~
prepared solutions of FePc gave no correlation with the solvent parameter a* (Figure 4). X-ray Diffraction. The lowest angle 28 reflection for clay colloid films intercalated with surfactants can be used to obtain the interlayer basal spacing through the Bragg relation.12J5 These small angle peaks were observed for clay-DDAB composites with and without MPc’s (Table 11). These peaks were rather broad, suggesting a degree of disorder in the films. Values for hydrated films were roughly within experimentalerror of each other. The basal spacings for unhydrated films were much smaller. Although only one peak at 28 3.46O was found for the pure clay-DDAB composite, a series of additional peaks occurred in the 4-10’ region when MPc’s were present in the films. For CoPc films, the first peak marked 23.8 A gives the clay basal plane spacing of the film (Figure 6a). Peak between 28 4O and loo in the CoPc film pattern are at nearly identical positions to those found for CoPc powder (Figure 6b). We conclude that there are crystals of ConPc in the film. The FePc composite film clear1 shows the basal plane reflection peak marked 29.1 (Figure 7a). However, peaks between 28 4O and loo are different from those in the X-ray spectrum of Fe*IPc powder. We conclude that these additional peaks in the FePc films are not caused by crystals of FenPc. ScanningElectron Microscopy/EnergyDispersive X-ray Analyris. SEM images of the films before use in aqueous solution appeared distinctly different from those of films that had been soaked in aqueous solutions. Thus,
AT
(13) Shi, C.; Ruling, J. F.;Wang, Z.;Willis, W. S.; Winiecki, A. M.; Suib, 5. L. Langmuar 1989,5, 650.
10 0
9
10
Figure 6. X-ray diffraction powder pattern for (a) CoPcclayDDAB film and (b) CoPc powder. all SEM and EDX experimenta were done on samples that had been previously soaked in aqueous 0.1 M KBr, then air-dried, to closer approximate the condition of the films previously used for electrochemical catalysis. SEM images of the top of the films were very different for composites with and without MPc’s (Figure 8). Films containing MPc’s appeared to include crystalline structures. These structures were different for CoPc films, which appeared as a collection of needlelike crystals, than for FePc films. The FePc composite filma appeared more amorphous, but a few needlelike crystals could be men. The above morphologies were confirmed by crosssectional SEM images of the films after freeze fracture. The pure clay surfactant composite films showed (Figure 9a) a layered structure similar to that reported for other surfactant However, cross sections of the CoPc f i i appeared as crystals (Figure9b). Cross sections of the FePc compositefilms revealed a rather thick layered structure with a few needlelike structures (Figure 9c). EDX of composite films detected silicon, aluminum, and the metal from MPc’s. Traces of potassium and chlorine were also found, but no bromine was detected. Analyses at different apota on the sample surface revealed a relatively constant Al/Si ratio consistent with the aluminosilicate clay as the source for these two elementa. For CoPc films, this ratio had a 13% relative standard deviation. In contrast, the Co/Si ratio varied considerably from spot to spot and had a relative standard deviation of 63 7%. This suggesta a heterogeneous distribution of CoPc in the film. The Al/Si ratio in FePc films measured by EDX had a reproducibility of about 10%. The Fe/Si ratio had a
hngmuir, Vol. 8, No. 10, 1992 2459
Clay-Surfactant F i l m Containing CdIPc
-1
88.27 44.14 2gk43 22.,07 17.,66 14:72 12.62 2000.0
n
1800.0
\
9.82
8.84
loa 90
?
1600.0
11.04
80
O
1400.0
70
540.0.
b
480.0420.0 360.0-,~~ 300 .O
{
Figure 7. X-ray diffraction powder pattern for (a) FePc-clayDDAB film and (b) FePc powder.
.
standard deviation of 26% Iron may be distributed more homogeneously in its composites than is CoPc.
Discussion Cobalt PhthalocyanineCompositeFilms. Electronic spectra and electrochemistry of the films to which CoIIPc was added are consistent with the presence of CoHPc.In organic solventa, the main monomer peak of CoIIPc shows a linear correlation with the Taft polarizability/dipolarity parameter A*. In the films, the monomer peak appears at values close to those in the nonpolar solvents (cf. Figure 4). Results suggest that the molecules responsible for the spectrum are present in a relatively nonpolar environment, which, however, is somewhat more polar than a tetradecane environment. The fact that no oxidation peak is observed for CoIIPc in the film is consistent with the presence of weak axial ligands which stabilize CorlPc toward oxidation.9 The UV spectrumin 0.1 M CTAB micelles reflects heavy dimerization of CoIIPc. The spectra of CorlPcin the films have a large monomer peak and only a small dimer peak. Thus, the degree of dimerization of CorlPc in the films seems relatively small compared to CTAB micelles. X-ray diffraction results are consistent with a structure featuring surfactant intercalated between the clay layers. The interlayer spacing is increased greatly in the surfactant clay films compared to pure clay films (Table 11). This spacing, however, is somewhat smaller than the 30 A found previously for DDAB-montmorillonite clay composites.1 The observed spacing in our films, made with a different clay with a 40 % smaller cation exchange
Figure 8. SEM top views of (a, top) clay-DDAB film and (b, bottom) CoPc-clay-DDAB film.
capacity, was on the order of 15-20A after subtracting 9.8
A for the thickness of the clay layers from the values in
Table 11. The X-ray diffraction patterns suggest the presence of crystallized CorlPcin the films. This is confirmed by the SEM images which clearly show crystals. EDX spot analyses suggesting heterogeneous distribution of Co in the films is also consistent with the presence of crystals. SEM cross-sectionalviews of the pure clay-DDAB films clearly show the layered structure observed previously for polymerized surfactant bilayer composites.2dBk The layered structure is obscured when CoPc is present in the films. Iron PhthalocyanineComposite Films. The voltammetric oxidation peak at 100 mV vs SCE for these films showed that the iron present is quite easily oxidized. Electronic spectra in DMSO reflect the presence of monomer and dimer of Fer1Pc.l0J1However, in less polar solvents, much broader peaks at longer wavelengths are observed which showed no correlation with the Taft A* solvent parameter. Iron(I1)phthalocyaninetetrasuEonatedissolved in water is irreversibly oxidized by air.14 FeIIPc suspended in organic solvent is oxidized by air to a dimeric p-oxo species with an FerlLO-Ferrllinkage.15 Voltammetry of this dimer in pyridine revealed a single-electron oxidation peak at 0.47 V vs SCE and two one-electron reduction peaks at (14) McLendon, G.; Martell, A. E. Inorg. Chem. 1977,16, 1812. (15) Ercolani, C.; Gardini, M.; Monacelli, F.; Pennesi, G.; h i , G. Inorg. Chem. 1983,22,2584.
Rusling et al.
2460 hngmuir, Vol. 8, No. 10, 1992
Table 111. Results of EDX Spot Analyses of Clay-DDAB films atomic ratios mot no.
1 2 3 4 5 6 mean i 8
1 2 3 4 5
6 mean i s
Figure 9. SEM cross-sectional views of (a, top) clay-DDAB film, (b, middle) CoPc-clay-DDAB film, and (c, bottom) FePcclay-DDAB film.
-0.59 V and -0.95 V.I6 In 96 % sulfuric acid or as powders, the two crystalline forms of the p-oxo dimers gave absorbance maxima at 695 nm, a wavelength considerably longer than that of the FerlPc monomer. These p-oxo dimers are irreversibly oxidized by oxygen.15 The electrochemical behavior of the p-oxo dimer is qualitatively similar to what we observe for the composite FePc film. Quantitative comparison is difficult since the (16)Bottomley,L.A.; Ercolani, C.; Gorce, J.-N.;Penneai, G.;h i , G. Inorg. Chem. 1986,25,2338.
CoPc Film AlISi 0.238 0.160 0.186 0.195 0.183 0.198 0.193 f 0.026 (*13%) FePc Film AIISi 0.215 0.231 0.234 0.193 0.230 0.186 0.215 f 0.021 (&lo%)
CoISi 1.48 0.088 1.9 42 5 2 2.86 3.58 2J6f 1-29(163%) FeISi 0.91 1.56 1.02 0.84 0.92 1.27 1.09 f 0.28 (i26%)
experiments on the dimer were done in pyridine,lGa strong axial ligand which would have considerable influence on the redox potentials. Thus, the observed oxidation and reduction peaks in the films could come from FePcspecies, p-oxo dimer, or other products of FePc oxidation. However, the spectra in nonpolar organic solvents and in the composite films clearly show large, broad electronic absorbance bands at wavelengths longer than that of the FerlPcmonomer. This peak forms rapidly when FeIIPc is dissolved in nonpolar solvents and its slow growth with time was observed in DMSO and methylene chloride. This behavior is consistent with the observed destabilization of the FeIIPc oxidation state by weak axial ligand^.^ We conclude that the FerrPcoriginally placed in the composite films is a t least partly oxidized, probably through formation of the p-oxo dimer and perhaps further oxidation of this species. This conclusion is supported by the X-ray diffraction data, which show that the peaks in the 4-10' 28 region are not due to FeIIPc crystals. Conclusions The morphology of clay-DDAB composite films is clearly changed by the presence of CoIIPc. The gross structure appears as a collection of CoIIPc crystals, rather than the stacked layers observed in the pure composite. However, gel-to-liquid crystal phase transitions were clearly observed in electrochemical catalysis at transition temperatures close to those found for DDAB bilayer suspension^.^*^ This suggests the presence of surfactant bilayers. However, the basal plane spacing of the clay at about 15-20 A, is considerably smaller than the 33.4 A required to accommodate two extended DDAB chains normal to the aluminosilicate layers. Thus, considerable tilting' and self-intercalation of the hydrocarbon chains must be considered. The general picture that suggests itself is a rather heterogeneous mixture of CoIIPc crystals and DDAB bilayers. Our results suggestthat composite filmscontaining ConPc are better catalysts than those containing FeIIPc partly because CorlPcremains intact in the films while FeIrPcis present in irreversibly oxidized forms.
Acknowledgment. This work was supported by U.S. PHS Grant No. ES03154awarded by the National Institute of Environmental Health Sciences. We thank E. J. Neth, Yanfei Shen, and S. Suib for help with X-ray and SEM analyses and David Howe for helpful discussions. Registry No. DDAB, 3282-73-3;CoIIPc, 3317-67-7;FeIIPc, 132-16-1.
