2696
J. Phys. Chem. 1986, 90, 2696-2702
after 35 min of 4He bombardment, the analysis is performed at a maximum depth of 2.0 nm. This extreme surface sensitivity causes shielding phenomena to be an important consideration, especially in the case of air-exposed samples.35 In spite of the large scatter of the data in Figure 12, a change in the slope of the curve seems to occur at approximately 1% Mo. This value corresponds to a change from tetrahedral to octahedral dominating species A decreased slope as detected by UV reflectance ~pectroscopy.~ in I S S data is opposite to the change in slope observed for Mo/A1203 as a result of a tetrahedral-to-octahedral coordination change.lg Perhaps, the presence of heteropoly Mo on the surface of the Si02causes the observed decrease.
distribution of the silica support. One may consider that during the pore-filling impregnation technique, the smaller pores of the support are not filled with the ammonium paramolybdate solution. This leaves some droplets on the outer surface of the Si02particle, which yields an increased local Mo concentration on drying and calcination. Eventually, this will favor the formation of MOO, microcrystals at Mo loadings of 22%. This interpretation explains the difference between impregnated silica- and alumina-supported catalysts: the broader pores of the alumina support allow for a more complete introduction of Mo solution during impregnation. It is also obvious from our data that the technique of fixation of Mo using the q3-allyl complex is not presenting the same drawbacks, as no Mo surface segregation and no Moo3 crystallization is found on either AF or S F catalysts. The Raman results indicate the presence of a silicomolybdic anion species at the surface of Mo/Si02 catalysts at loadings as low as 1% Mo. The finding suggests a possible explanation for the observed ISS results on these catalysts. Moreover, at low loadings more of the silicomolybdic species is present in the S I than in the SF catalysts. This is consistent with the surface segregation of Mo observed on S I catalysts.
Conclusion Our extensive study of O2 chemisorption has led to the conclusion that this technique is not liable to provide a quantitative determination of the Mo dispersion. Notwithstanding the effect of the degree of reduction, it is clear that the O2chemisorption is very site-specific as evidenced by the large variations observed for N [ 0 2 ] / N [ M o ]with Mo content, on S I and AI catalysts. The ESCA and Raman data of S I and SF catalysts can be simultaneously interpreted by taking into account the pore size
Registry No. Mo, 7439-98-7; 02,7782-44-7.
Selective Incorporation of Pendant Redox Sites into Preformed Polymers Lawrence D. Margerum, Thomas J. Meyer,* and Royce W. Murray* Kenan Laboratories, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514 (Received: July 22, 1985; In Final Form: January 29, 1986)
A series of pendant donor and acceptor redox polymers has been prepared by reactions between poly(chloromethy1styrene-co-styrene) (I, PS-CH2Cl) and [PTZI- and/or MQ'CI- (PTZ- is phenothiazine radical anion, MQ'Cl- is Nmethyl-4,4'-bipyridinium chloride). The degree of loading of redox sites was controllable in a systematic and reproducible way. Preparation of a polymer with both PTZ and pyridinium sites leads to an intramolecular donoracceptor (D-A) interaction. The loadings of pendant redox sites on the polymers were determined by 'HNMR and confirmed by elemental analysis. Electrochemical studies on the polymers both in homogeneous solutions and as films show that multiple oxidizing or reducing equivalents can be transferred from the polymers within a narrow potential window.
This paper describes an approach to preparing soluble redox polymers containing controllable amounts of pendant donors and acceptors as a first step toward using such polymers in photochemical quenching schemes. The polymers are based on copolymers of styrene and chloromethylstyrene (I, PS-CH2C1), with
Soluble electroactive polymers have interesting and unusual properties, such as the ability to transfer many electrons per polymer molecule within a narrow potential window,' and regions of high local concentrations of redox sites in a solution relatively dilute in polymer molecules. We have been interested in the development of synthetically versatile pathways to polymeric materials containing redox sites, and the investigation of their redox properties as photochemical electron-transfer reagents. Previously, we have shown that vinyl-containing polypyridyl complexes of ruthenium and osmium can be electropolymerized to produce insoluble, electroactive films on electrode surfaces.2 In addition, polypyridyl complexes of Ru have been attached to soluble poly(viny1pyridine) polymers and their chemical and photochemical properties inve~tigated.~
+CH,-$H*CH,-$Ht
CH2CI
I
styrene acting as spacer both to enhance reactive site availability and to increase flexibility within the chains.4 The redox sites were bound chemically by nucleophilic displacement of C1- by phenothiazine radical anion (PTZ-) and/or monomethylated 4,4'bipyridine (MQ') to form the copolymers: poly-[ 10-(benzylvinyl-co-styry1)phenothiazineco- (chloromethylstyrene-co-styrene)] (11, abbreviated as PS-CH2-[PTZlX,where x = % incorporation of redox sites per chloromethyl site available) and poly[Nmethyl-N'- (benzylvin yl) -co-st yryl) -4,4'- bipyridine-co- (chloromethylstyrene-co-styrene)] (111, PS-CH2-[PQ2+],). The synthetic
(1) (a) Smith, T. W.; Kuder, J. E.; Wychich, D. J . Polym. Sci., Polym. Chem. Ed. 1976, 14, 2433. (b) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J . Am. Chem. SOC.1978, 102,4248. (c) Saji, T.; Pasch, N. F.; Weber, S. E.; Bard, A. J. J . Phys. Chem. 1978,82, 1101. (d) Funt, B. L.;
Hsu, L. C.; Hoang, P. M.; Martenot, J. P. J . Polym. Sci., Polym. Chem. Ed. 1983,21,953. (e) Morishima, Y.; Itoh, Y.; Koyagi, A. J . Polym. Sci., Polym. Chem. Ed. 1983, 21, 953. (2) (a) Murray, R. W. In Electroanalytical Chemistry, Bard, A. J., Ed.; Dekker: New York, 1984, and references therein. (b) Murray, R. W. Annu. Rev. Mater. Sci. 1984, 14, 145. (3) (a) Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1981.20, 27. (b) Calvert, J. M.; Casper, J. V.; Binsted, R. A.; Westmoreland, T.D.; Meyer, T.J. J . Am. Chem. SOC.1982, 104, 6620. (c) Westmoreland, T. D.; Calvert, J. M.; Murray, R. W.; Meyer, T. J. J . Chem. SOC.,Chem. Commun. 1983, 65.
0022-3654186 , ,12090-2696SO1.5010 I
(4) Arshady, R.; Reddy, B. S. R.; George, M. H. Polymer 1984, 25,716.
0 1986 American Chemical Societv -
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2697
Incorporation of Redox Sites into Polymers
I
CH2
I
I1
E+~H-CH~*CH-C~+~~CH-CH~-H-CH-CH~+ I X I I I y 2
I-.#
I
CH2CI
Q
I +pF6-
CH,
I11
approach offers both synthetic flexibility and the possibility of sequential additions leading to mixed donor-acceptor polymers. Several reports of soluble polymers containing electroactive donors or acceptors have appeared, most of which involve polymerization of vinyl monomers such as 3-vinyl-10-methylphenothiazine,le vinylferrocene,laSb 2-vinylnaphthalene, and 9vinylanthracene.lc A report has also appeared on the modification of PS-CH2Cl with (MQ+),5*6but the solution electrochemical properties were not described.
Experimental Section Materials. Tetraethylammonium perchlorate (Et4NC104) and tetrabutylammonium perchlorate (Bu4NC104)were prepared by standard literature procedure^.^ Electrochemical solvents acetonitrile (CH3CN), dimethylformamide (DMF), propylene carbonate (PC), and methylene chloride (CH2C12,all Burdick and Jackson) were stored over activated 4-A sieves. Tetrahydrofuran (Aldrich Gold Label) was refluxed over sodium metal and distilled. All other solvents were reagent grade. Phenothiazine (PTZ, Aldrich) and azobis(isobutyronitri1e) (AIBN) were recrystallized from ethanol, 10-methylphenothiazine ( 10-CH3-PTZ) was sublimed under reduced pressure, l,l'-dimethyl-4,4'-bipyridinium (PF6- salt, paraquat) was metathasized from the c1- salt, and l-methyl-4,4'-bipyridinium iodide8 ([MQ'II-) was converted to the chloride salt by ion exchange using CGA-3 16 anion-exchange resin (J. T. Baker). Styrene (Aldrich) was washed with 5% (wt/vol) sodium hydroxide; it and chloromethylstyrene (Polysciences, meta:para = 6040) were distilled under reduced pressure. Preparation of Polymers. The method of Arshady et al.? was used to copolymerize the monomers styrene and chloromethylstyrene with AIBN in chlorobenzene to obtain the 1:l copolymer poly(chloromethy1styrenestyrene) (PS-CH2Cl). The copolymer had an average molecular weight Mw = 7600 as determined by (5) Soto, H.; Tamamura, T. J . Appl. Polym. Sci. 1979, 24, 2075. (6) (a) Mortimer, R. J.; Anson, F. C. J . Electroanal. Chem. 1984, 138, 325. (b) Lee,P. C.; Matheson, M. S.;Meisel, D. Isr. J . Chem. 1982,22, 133. (7) Sawyer, D. T.; Roberts, J. L. Experimental Electrochemistry for Chemists; Wiley-Interscience: New York, 1974; p 212. (8) Figard, J. E.; Paukstelis, J. V.; Byrne, E. F.; Petersen, J. D. J . Am. Chem. Soc. 1977, 99, 8417.
gel permeation chromatography using polystyrene standards. Anal. Calcd. for 53% chloromethylation: C, 78.92; H, 5.58; C1, 14.51. Found: C, 78.05; H, 6.47; C1, 14.24. Poly- [ IO-(benzylvinyl-co-styry1)phenothiazine-co- (chloromethylstyrene-co-styrene)](ZZ,PS-CH2-[PTZ],). The extent of PTZ incorporation depends on the amount of [PTZI- anion generated. In a typical reaction leading to 27% loading, phenothiazine (0.166 g, 0.80 mmol) dissolved in 3 mL of T H F was added to a T H F solution containing 1 equiv of lithium diisopropylamine (LDA, Aldrich) to give an immediate color change from pale green to the bright yellow of [PTZI-. After the [PTZI- solution was stirred for 20 min it was added dropwise to a stirring solution of the polymer PS-CH2C1(I, 0.638 g, 2.5 mmol, in 10 mL of THF). The solution was gently heated at reflux until most of the yellow color had dissappeared (48 h) and then reduced in volume to ca. 5 mL. The product was precipitated by pouring into methanol, collected by suction filtration, washed, redissolved in methylene chloride, reprecipitated into methanol, and dried under vacuum. Yield: 0.60 g. Anal: Calcd. for PS-CH2-[PTZ]27-PS-[CH2C1]73: C, 80.99; H, 6.39; N , 1.26; C1, 8.64. Found: C, 80.32; H, 6.11; N, 1.53; CI, 8.63. Poly- [N-methyl-"-( benzylvinyl-co-styry1)-4,4'-bipyridiniumco-(chloromethylstyrene-co-styrene)](ZZZ,PS-CH2-[P@+],). For the pyridinium ion the number of redox sites bound was also controlled by the amount of [MQ+]Cl- added and the reaction time. To a solution of PS-CH2C1 (0.184 g, 0.72 mmol) in 10 mL of DMF was added 0.180 g of [MQ+]Cl- (0.8 mmol) in 10 mL of DMF. The solution was deareated, heated in an oil bath at 90 "C for 24 h, and concentrated to 1-2 mL on a rotary evaporator. Methanol was added to solubilize the precipitate; the resulting solution was added dropwise to acetone and the precipitated quaternized paraquat polymer was collected as the dichloride salt, washed with acetone, and dried under vacuum. Yield: 0.100 g. The PF6- salt of the polymer was isolated by dissolving the chloride salt in methanol and adding two volumes of water and then NH4PF6in excess to obtain a swollen precipitate. The solid was isolated by centrifuge, washed with water, collected on a glass frit, and dried under vacuum. The light brown solid was redissolved in methylene chloride and reprecipitated by adding to methanol. Anal: Calcd. for PS-CH2-[PQ(PF6)2]36-~~-PS[CH2C1]@:C, 61.49; H, 5.12; N, 2.45. Found: C, 62.14; H, 5.37; N , 2.55. PS-CH2Cl to Which Phenothiazine and Paraquat Are Both Bound, ZV, PS-CH2-[PTZ].-co-PS-CH,-[PQ2+],. To a solution of PS-CH2-[PTZ]38in DMF (0.200 g, 0.63 mmol, containing 2.4 mequiv of Cl-/g of polymer) was added [MQ+]Cl- (0.206 g, 0.91 mmol). The solution was degassed and heated under nitrogen in an oil bath at 90-100 O C for 4 days, cooled, and taken to dryness on a rotary evaporator, and the resulting solid was washed with water several times to remove unreacted [MQ'ICl-. The armygreen solid was redissolved in a minimum amount of DMF and H 2 0 was added, followed by excess aqueous NH4PF6to precipitate the PF6- salt. The solid was collected on a glass frit and washed with H 2 0 , methanol, and then diethyl ether. Yield: 235 mg. The pyridinium content of the material was determined by electrochemistry and NMR, and the results of both techniques were consistent with the elemental analysis. Anal: Calcd for PSCH2- [PTZ] ~ ~ - c o - P S - C H [PQ(PF6)J ~~O-CO-PS[CHZCl]42: C, 71.50; H, 5.56; N , 2.78; C1, 3.53. Found: C, 69.44; H, 5.53; N, 2.78; C1, 2.61. Electrochemistry. Electrochemical experiments on solutions of the polymers were performed in either dry methylene chloride or dimethylformamide with 0.1 M tetra-n-butylammonium perchlorate (Bu4NC104)or hexafluorophosphate (Bu4NPF6) supporting electrolyte. Electrochemical potentials were recorded vs. the NaC1-saturated calomel electrode (SSCE). Cyclic voltammetry was carried out with a locally constructed potentiostat and signal g e n e r a t ~ r and , ~ coulometry with a PAR 173 potentiostat and PAR 179 digital coulometer. Teflon-shrouded Pt and glassy (9) Woodward, W. S.; Rocklin, R. D.;Murray, R. W. Chem. Biomed. Enuiron. Instrum. 1979, 9, 95.
2698 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Margerum et al.
TABLE I: Reaction Conditions and Product Analyses of Donor and Acceptor Polymers reaction H' NMR peak area,c starting nucleophile molar feed ratio, bound nucleophile: polymer used nucleophile:-CH2C1 unreacted -CH2C1 3:2 1:0.03 PS-CH&I' [PTZ] -b PS-CH2CI [PTZI1.1:l 1:0.33 PS-CH2CI [PTZI1:2 1:1.63 1:3 1:2.7 PS-CH2CI [PTZI[MQ+] 1.1:l 1:1.8 PS-CHIC1 PS-CH,-[PTZ] 27 [MQ'] CI1.1:1 1:0.3:2.4 PTZ:PQ2':CH2C1 PS-CHI- [PTZ]38 [MQ'ICI2: 1 1:0.53:1.1 PTZ:PQ2':CH2CI
abbrev used in text
%
incorporationC 97 75 38 27 36 8
PS-CH2-[PTZ]97 PS-CHI- [PTZ] 75 PS-CH,-[PTZ] 38 PS-CH2-[PTZ] 27 PS-CH2-[PQ2']3, PS-CH2-[PTZ]2 7 - ~ 0 PS-CH2- [PQ2'] 8 PS-CH2- [PTZI~B-COPS-CHI- [PQ2+]20
20
a PS-CH2CI = poly(m,p-chloromethylstyrene-co-styrene); 53% chloromethyl. [PTZI- = phenothiazine radical anion (see Experimental Section). e l H NMR peak areas from benzyl region (4-6 ppm) at 250 MHz in Me2SO-d6. *[MQ']CI- = N-methyl-4,4'-bipyridinium chloride. "ercentage of original -CH2CI sites which have reacted; directly from NMR data (elemental analysis confirmed all the results f2% incorporation).
carbon electrodes were polished with 1-pm diamond paste (Buehler) . When concentrations of the polymers are cited they refer to the concentrations of redox sites in M . Polymer films on electrodes were made by slowly evaporating 5-10 pL of a dilute solution of polymer placed onto inverted Pt or carbon disk electrodes in a nearly solvent saturated atmosphere. The coated electrodes are placed in solutions of electrolytes (in solvents that do not redissolve the polymer) for electrochemical analysis. Apparent surface coverages (r,mol/cm2) were calculated from the equation r = QJnFA, where n is the number of electrons per redox site reduced or oxidized, F is Faraday's constant, A is electrode area in cm2, and Q, is the charge (area) under the cyclic voltammetric reductive or oxidative wave. Spectroelectrochemistry. Polymer solutions were evaporated onto one end of glass slides coated with highly doped SnOz films, and electrical contact was made by clamping a Pt foil onto the other end. The electrode and electrolyte solution were placed in a two-compartment cell consisting of a quartz cuvette (1 X 1 cm) and a side arm for the reference electrode separated by a fine glass frit. UV-visible transmission spectra were recorded with a Hewlett Packard Model 8450A UV-visible diode array spectrophotometer. In a typical experiment, the cell was placed in the sample beam, a spectrum recorded and stored, the electrode potentiostated to a potential where the film was oxidized or reduced, and another spectrum taken and stored. Difference spectra were then generated by the HP 8450A which included only contributions to the absorbance from the polymeric film. Other Measurements. 'H N M R measurements were obtained on a Brueker 250-MHz spectrometer in 99% Me2SO-d6 and referenced internally to Me2S0. Size exclusion chromatography was performed using a Waters lo3A Styragel column and a ISCO V4 detector (Adat = 280 nm) with THF and a flow rate of 1.0 mL/min. Elemental analyses were performed by Galbraith Laboratories, Knoxville, T N .
Results and Discussion Addition of known amounts of either [PTZI- or [MQ+]Cl- to PS-CH2Cl leads to controllable loadings of the redox sites onto the polymeric backbone.
I
A
I
4
1
10
.
1
e
1
i,
.
pdm
1
1
i
.
6
Figure 1. ' H NMR at 250 MHz in Me2SO-d6for the donor and/or acceptor polymers prepared in this study: A, starting polymer, PS-CH2CI; B, donor polymer, PS-CH2-[PTZI2,; C, acceptor polymer, PSCH2-[PQ2'I2,; D, donor-acceptor polymer, PS-CH2-[PTZ]38-co-PSCH2-[PQ2']20. Resonances for Me2S0 and H 2 0 are not shown for clarity. (See text for peak assignments.)
materials in photochemical quenching schemes, which entails detailed knowledge of polymer composition. As an example, for the case of PS-CH,-PTZ, attempts were made to modify from PS-CH2Cl [PTZIPS-CH2-[PTZlX-~o-PS-[CH2C1], 4% to 97%of the chloromethyl sites, based on added [PTZI-, and I I1 it was necessary to develop a routine technique for polymer (1) analysis. Polymer Characterization. The reaction stoichiometries used PS-CHzCl + MQ'C1for [PTZI- and [MQ+]Cl- substitution reactions are summarized PS-CH2- [PQ(C~)~],-CO-PS[CHZCI], (2) in Table I as are the analyses for the redox polymer products. I11 Table I compares the molar ratio of nucleophile added to available Controlled loadings in these reactions leave a residue of chloro-CH2Cl sites compared to the actual amount incorporated. The methylated sites so that a second reaction can be carried out to actual amount incorporated is reported as percent of original obtain new polymeric materials containing mixtures of redox sites. -CH2C1 sites substituted. 'H N M R was an effective way to analyze the loadings and also gave information about polymer I1 + MQ'ClPS-CH~-[PTZ],-CO-PS-CH~-[PQ(C~)~]~ (3) macrostructure. Shown in Figure 1 are representative 250-MHz IV spectra of the polymers I, 11, 111, and IV. As is typical for polymers, relaxation times are long and structural detail is lost,1° As noted in the Introduction, one of our goals is to use these
+
-+
-+
+
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2699
Incorporation of Redox Sites into Polymers
TABLE II: Eo' Values and Number of Electrons Transferred (n,) in Donor/Acceptor Polymers Using Cyclic Voltammetry"
sample 10-CHq-PTZ PS-CHi- [PTZ]7S PS-CH2-[PTZ]27 PQ(PF6)2 PS-CH2-[PQ2+]27 PS-CHI- [PTZ] ~ * - c o PS-CH2-[PQ2+]20
Eo', V vs. SSCE" +0.7g +0.788 +0.788 -0.38* -0.3 1 * +0.788
mol wtb 213 11227 8907 458 10751
[i,,dcI? N m M 19.32 5.63 5.96 8.57 3.03
N,'
1 22.3 8.02 1 8.02
100(n,/Nc), % 100
nb
1 19.3 6.91 1 6.75
86.7 86.1 100
84.2
-0.33 'All 100 mV/s at glassy carbon electrodes (area = 0.0755 cm2). The differences in EO'values of polymer-bound and monomeric redox sites are due mainly to the more electron-withdrawing abilityI4 of the benzyl group on the polymer backbone. bBased on PS-CH2C1 = 7600 g/mol ( D p = 29.7) determined by GPC in THF. e N c = average number of redox centers per chain. dFrom slope of ipk vs. concentration of redox sites per plot. CFrom eq 5 in text. fCH,CN/O.l M Bu4NC104. 8CH2Cl2/0.1M Bu4C104. *DMF/O.l M Bu4NC104.
but the peaks and peak areas nontheless can be assigned to determine composition. In the case of PS-CH2Cl, I, Figure lA, the polymeric backbone protons occur as a broad peak centered at 1.8 ppm, the benzyllic protons are centered at 4.5 ppm, and in the aromatic region resonances appear due to styryl and meta,para-substituted chloromethylstyryl protons. Direct integration of the benzyl region (4-6 ppm) relative to the aromatic region (6-8 ppm) shows a 1:l ratio of styryl:chloromethylstyryl, which agrees with the elemental analysis data. Size exclusion chromatography of this polymer gave an average Mw of 7600. Shown in Figure 1B is the result of a reaction between equimolar amounts of -CH2Cl groups on the polymer and [PTZI-. Binding of PTZ shifts the benzyl peak to lower field vs. -CH2CI due to shielding effects. Direct integration of the benzyl peak areas (-CH2PTZ:-CH,-Cl) gave the relative loading of PTZ onto the polymeric backbone (see Table I). In this case, 75% of the available -CH2CI sites were converted into PTZ sites. Similarly, reaction of [MQ+]Cl- with PS-CH2C1 in D M F leads to a paraquat-type polymer as shown previously by several other worker^.^.^ Figure 1C shows the 250-MHz spectrum of a reaction mixture (24 h) in which a 1:l molar ratio of [MQ+]Cl- to PSCH2Cl was used. Comparison to the 'H N M R spectrum of the monomer PQ(Cl), leads to the following assignments: 4.3 ppm (N+-CH,), 8.8, 9.1 ppm (pyridine-H), and 5.3 ppm (styreneCH,-N+). Integration of peak areas showed that 26% of the available -CH2Cl sites had reacted. By comparison, Anson and co-workers reported that a 1-week reaction time resulted in 100% loading, although molar ratios were not cited.6a Intramolecular Charge Transfer. In contrast to the colorless single-site redox polymers I1 and 111, addition of [MQ+]Cl- to a solution of PS-CH2-[PTZ]38with subsequent heating leads to an increasingly colored solution. Isolation of the product results in an army-green powder. The color appears to arise from intramolecular, donor-acceptor, charge-transfer transitions.
Figure 1D shows the 'H N M R spectrum of the product, in which peak assignments can be readily made by comparison to the spectra of the separate single site polymers in Figure 1 B and C. Direct integration of the three peaks for different benzyl-X chemical sites (4-6 ppm) leads to a PTZ:-PQ2+:-CH2C1 ratio of approximately 1:OS:l.l. In other words, the product contains the original 38% of PTZ polymeric sites, another 20% of the -CH2Cl sites that now bind -PQ2+, with the remainder as unreacted -CHZCl. Donor-acceptor ( E A ) complexes are known for PTZ and PQ2+ monomers in solution,lla where a donor-acceptor charge-transfer band occurs at 480 nm.*lb There are previous examples of intramolecular polymeric D-A complexes,I2 one of the first being (10) Klopffer, W. Introduction to Polymer Spectroscopy; Springer-Verlag: New York, 1984; pp 139, 159. ( 1 1 ) (a) Foster, R. Organic Charge-Transfer Complexes; Academic: London, 1969; p 292, 358. (b) Sullivan, B. P.,unpublished results.
a0
R
8
8
R
U
€
i
~
8
11
WAVELENGTH (NM)
Figure 2. Visible difference spectrum between a DMF solution of the copolymer PS-CH2-[PTZ]j8-co-PS-CH2-[PQ(PF6),]2, and a DMF solution containing the same average concentration of PTZ and PQ2+sites, except as PS-CH2-[PTZIj8 and PS-CH2-[PQ(PF6)2]26. Both samples were adjusted to be 1.1 mM in PTZ sites and 0.5 mM in PQ2+sites.
that of Turner and StalkalZain which 1:l polymerization of 1(2-anthry1)ethyl methacrylate and 2,4,7-trinitro-9-fluorenylmethacrylate led to an orange polymer with a charge-transfer band centered at 490 nm. More recently, Iwatsuki and co-workers13 have reported D-A copolymers based on copolymers of cyanomethphenyl methacrylates and m-(N,N-dimethy1amino)phenyl acrylates where difference spectra between the D-A copolymer and equal concentrations of the two homopolymers reveals an intramolecular charge-transfer absorption band in the region 400-450 nm. Figure 2 shows a difference spectrum between a D M F solution [PQ(PF6)2]20,IV, of the copolymer PS-CH2-[PTZ]38-~o-PS-CH2and a DMF solution which contains a mixture of the same average concentrations of PTZ and PQ2+sites but present in the single-site redox polymers PS-CHI- [PTZ]75 and PS-CH2-[PQ2+(PF6)2]26. The shoulder at 480 nm in the difference spectrum provides clear evidence for a charge-transfer complex between PTZ and PQ2+ in the copolymer. (No intermolecular charge-transfer transitions are seen in the mixture of single site redox polymers.) In this experiment there is necessarily a much larger local concentration of D-A complexes (>7-8 times) in the mixed copolymer IV than when the two single-site redox polymers are simply mixed, which promotes the charge-transfer interaction. Solution Electrochemistry. As shown by Bard and Anson,lbSc voltammetry is a valuable tool for evaluating the ability of redox polymer chains to transfer multiple oxidizing or reducing equiv(12) (a) Turner, S . R.; Stolka, M. S. Macromolecules 1978, 1 1 , 835. (b) Schulz, R. C.; Fleischer, D.; Henglein, A,; Bossler, H. M.; Trisnadi, J.; Tanka, H. Pure Appl. Chem. 1974,38,227. (c) Tomono, T.; Hasegawa, E.; Tsuchida, E. J . Polym. Sci., Polym. Chem. Ed. 1977, 15, 571. (d) Simionescu, C. 1.; Grigoras, M.; Barboiu, V.Polym. Bull. 1983, 9, 537, 577. (e) Simionescu, C. I.; Onofrei, G.;Grigoras, M. Makromol. Chem., Rapid. Commun. 1984, 5, 229. (13) Iwatsuki, S.; Itoh, T.; Shimizu, Y.; Iwata, Y.; Matsuhiro, S . Macromolecules 1985, 18, 1.
1
2100
The Journal of Physical Chemistry, Vol. 90, No. 12, 19616
Margerum et al.
0
1.0
VOLTS v s S S C E Figure 4. Cyclic voltammogram for a Pt disk coated with 2 X lo-*
mol/cm2of PTZ sites of PS-CH2-FTZ27, showing its response to repeated cycling between 0 and +1.1 V vs. SSCE in 0.1 M Et4NC104/propylene carbonate. Scan rate is 100 mV/s. S = 5 PA.
*10
0.0
-10
VOLTS v s S f C E
Figure 3. Solution cyclic voltammograms in Bu4NC1O4/CH2CI2 at a glassy carbon electrode for: curve A, PS-CH2-[PTZlZ7(2.7 mM in PTZ sites); curve B, PS-CH,-[PQ(PF,),],, (1.3 mM in PQ2+sites); curve C, co-polymer PS-CH2-[PTZ] 38-~~-PS-CH2[PTZ]p8-co-PS-CH2-[ PQ(PF6)2]20(2.8 mM in PTZ sites and 1.4 mM in PQ2+sites). The scan rates were all 100 mV/s. S = 1.5 PA.
alents. All of the polymers synthesized here are soluble in D M F and CH2C12,which are useful electrochemical solvents. Shown in Figure 3 and summarized in Table I1 are typical cyclic voltammetric responses for the PS-CH2-PTZ27,PS-CH2-[PQ(PF6)2]27,and PS-CH,-[PTZ] ss-co-CH2-[PQ(PF6)2]20polymers dissolved in CH2C12/0.1 M Bu4NC104. The redox waves are very well-formed, and in all cases AE, I90 mV, where AE, is the difference in peak potentials for the oxidative and reductive sweeps. All of the oxidation and reduction wave peak currents vary linearly with v1/2 (v is potential scan rate) and i , , = ip,a. All are characteristics of chemically reversible one-electron waves where the polymer redox sites are noninteracting or weakly interacting as discussed by Bard and Anson.Ib The cyclic voltammogram of the mixed copolymer in Figure 3C is simply an additive composite of the voltammograms of the two homopolymers in Figure 3, A and B. In the mixed copolymer, the oxidation and reduction processes must be relatively independent redox events, which is not surprising given the large difference in redox poentials between donor and acceptor. The in the copolymer reflects ratio of peak currents ip,,~prz):i,,,~pq~+) the 2:l PTZ/PQ2+ ratio derived from the N M R analysis. Since redox polymers are potential electrochemical electrontransfer reagents, it is of interest to know the number of electrons that are transferred per polymer chain. Bard, and Anson,lb and otherslc+ have shown that the number of electrons transferred electrochemically to or from a polymer containing np redox sites can be estimated from the relative values of diffusion-limited voltammetric currents and the relative molecular weights of the polymer (M,) and an appropriate monomer model compound ( M m ) ,as shown in
In eq 5, N , is the number of redox sites per polymeric chain (determined by the degree of polymerization and percent loading
from N M R measurements), and i,,,,,,,,/C, and ip,,/Cm are the slopes of plots of diffusion-controlled cyclic voltammetric peak currents vs. the concentration of redox sites. Equation 5 is based on the empirical relationship D,/Dln = (Mm/M,)055
(6) where D, and D, are the polymer and monomer diffusion coefficients, respectively, and the dissolved polymer molecule is assumed to be a spherical, random ~ 0 i l . l ~ Values oft+, calculated by using eq 5 are summarized in Table 11. The values of % and, therefore, the percentage of redox sites on the polymeric chain that are electroactive on the cyclic voltammetric time scale, 100(n,/N,), qualitatively match the loading obtained by the N M R assay. That is, in light of the approximations of eq 5 and 6, the values of 84-86% electroactivity of redox sites are not significantly distinguishable from the theoretical value of 100%. These data can be compared with previous n,/Nc = 72% for a phenothiazine 50%loaded copolymer (at 40 mV/s, Mw = 5 X 104),Ie78-90% for poly(viny1ferrocene) (at 2 mV/s, Mw = 4930),Ib and 50-100% for poly(vinylnaphtha1ene) and poly(vinylanthracene) (Mw = 2000-300 OOO).’” The polymers studied previously were all polymerized as vinyl monomers, and thus should have both higher and more homogeneous concentration of redox sites on the polymer backbone when compared to the polymers described here. The somewhat larger percent electroactivity in our polymers (even at faster potential sweep rates) suggests that the dynamics of polymeric chain flexibility may play as important a role in determining electron-transfer reactivity as the concentration of redox sites on the polymeric chain. Electrochemistry of Films on Electrodes. Slow evaporation of 10 ML of a T H F solution containing PS-CH2-[PTZ]27(0.25 mM in PTZ) onto a Pt disk electrode gives a smooth transparent film that in 0.1 M Et4NC104/propylene carbonate gave the cyclic voltammogram shown in Figure 4. There is a “break-in” period PTZ’ upon repetitive potential cycling in which two PTZ oxidative spikes coalesce into a single wave. There is also an initial rapid loss of electroactivity (to 60-70% of the original, in acetonitrile or propylene carbonate), which gradually stabilizes after a few cyclical scans. The N M R and solution electrochemical studies did not show two different oxidizable PTZ species and so the two waves in the film must have another origin. A more likely explanation is environmental changes at the PTZ sites which occur
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(14) Wilkens, R. G.; Tsukahara, K. J. Am. Chem. Soc. 1985, 107, 2632, and references therein. (15) Tanford, C. The Physical Chemistry of Macromolecules; Wiley: New York, 1961; p 362.
Th!e Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2701
Incorporation of Redox Sites into Polymers
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. 4
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Figure 5. Spectroelectrochemistryof polymeric films on a Sn02 electrode. Visible difference spectra between: A, an oxidized (at +0.95 V vs. SSCE) and reduced film of PS-CH2-[F'"Z]75(2.6 X mol/cm2 in PTZ sites) in Et4NC104/propylenecarbonate, B;a reduced (at -0.7 eV vs. SSCE) and oxidized film of PS-CH2-[PQ2+]38 (9 X 10" mol/cm2 in PQ2+sites).
upon going from a relatively dry, electrically neutral, film to a solvent/electrolyte swollen film, with the change in state being partly induced by the oxidation. Break-in phenomena have been also observed in other films where the redox sites are initially electrically neutral.I6J7 After the break-in period, the film Eo' value of +0.76 V (AEp= 40 mV at 100 mV/s) is very close to the value of +0.78 V for the dissolved polymer in CH2C12. The overall stability of the PS-CH,-[PTZ], films on electrodes to continued potential cycling is limited. A complication is that solvents in which the neutral form is insoluble may solubilize the oxidized form. After extensive oxidative cycling of a coated electrode in propylene carbonate or in CH3CN, the amount of polymer on the surface is visibly smaller. N o electrochemical responses are seen in aqueous solutions, where electrolyte and solvent do not penetrate the film well, and it acts as an insulating barrier leading to poorly defined and unstable voltammograms in D M F / H 2 0 and C H 3 C N / H 2 0 solvent mixtures. PS-CH,-[PTZ], films are sufficiently stable in propylene carbonate/Et4NC104 that a visible spectrum of the oxidized film on an SnO, electrode can be obtained as shown in Figure 5A. The PS-CH2-[PTZ]7sfilm contained = 2.6 X mol/cm2 of PTZ sites and was oxidized at +0.95 V vs. SSCE for 30 min. The spectrum is the difference before and after oxidation. The peak at 510 f 2 nm is due to [PTZ]+ sites in the film as evidenced by the deep red color of the film. For comparison, a solution of the oxidized form of the polymer, PS-CHz-[PTZ]lS+, shows A, = 517 nm (CH,Cl,) and a solution of the monomer, 10-CH3= 514 nm in CH3CN.I8 [PTZ]', gives A,, PS-CH,-[PQ(PF,),]. Electrodes coated with the viologen polymer have been described by Anson;6a our results are similar. The electrochemistry is sensitive to the supporting electrolyte anion, and not all of the viologen sites react electrochemically. For instance, in PF6-or c10, electrolyte, both of which form insoluble viologen salts in water, the viologen reductions show large AEp values and are poorly resolved. However, Figure 6 shows the response of a relatively thick film of PS-CH,-[PQ(PF,),],, (rcald = 2 X mol/cm2 in PQ2+ sites) that was cast onto a Pt electrode, in tetra-n-butylammonium p-toluenesulfonate/H20 (16) (a) Daum, P.; Murray, R. W. J. Eleclroanal. Chem. 1979,103,289. (b) Daum, P.; Murray, R. W. J . Phys. Chem. 1981, 85, 389. (17) (a) Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S.R.; Chambers, J. Q.J. Am. Chem. Soc. 1980, 102,483. (b) Schroeder, A. H.; Kaufman, F. B. J . Electroanal. Chem. 1980, 113, 209. (18) Litt, M. H.; Radovic, J. J . Phys. Chem. 1974, 78, 1750.
0.0
- 0.7
V O L T S vs. S S C E
Figure 6. Steady-statecyclic voltammetry for a glassy carbon disk coated with 2 X 10" mol/cm2 of viologen sites of PS-CH,-[PQ(PF,),],, in 0.2 M [Bu4N](p-toluene-S03)/H20 at 50 mV/s. S = 5 pA.
medium. Apparently, the sulfonate anion can exchange with PF, within the film and, because of a higher ionic mobility, promote the electron-transfer reaction. [Et4N]Cl electrolyte behaves similarly. PS-CH2[PQ2+]36films are very stable in H 2 0and in repeated voltammetric cycling through the first viologen wave, but loss of electroactivity occurs when the potential is scanned into the second viologen reduction wave as noted for other viologens.I9 The charge under the voltammogram in Figure 5 corresponds to only 32%of the viologen sites known to be in the film. In 0.1 M [Et4N]Cl/propylene carbonate, the same film exhibited 80% electroactivity at slow sweep rates. Obviously, a combination of solvent and anions control ionic mobility and site accessibility. The spectrum obtained a t -0.7 V vs. SSCE for 30 min of electrolysis of a PS-CH2-[PQZ+],, polymer film evaporated onto a Sn02 electrode and reduced in rigorously degassed pH 9 phosphate buffer is shown in Figure 5B. The blue product film contains the PQ+ radical cation (shoulder at 605 nm) and the PQ+ dimer (A, = 552 f 4 nm).I9 A solution of PS-CH,-[PQ2+],, in 0.1 M Bu4NC104/CH2C12that had been electrochemically = 555 nm). Lee reduced gave essentially the same spectrum (A,et a1.6bhave noted that chemical reduction of aqueous PSCH2-[PQ2] solutions yields the dimer-polyviologen (A,, = 535, 365 nm) spectrum whereas Rabani et aLZ0have shown that pulse radiolytic reduction of other polyviologens produces only reduced cation radical (A = 600 nm), depending upon concentration and polyviologen composition. The latter spectra change over time, converting to the dimer radical at A = 545 nm. In most of the polyviologens, intermolecular processes were invoked to account for the spectral observations. Mixed Donot-Acceptor Copolymer. Figure 7 shows the cyclic voltammetry of a film of PS-CHz-[PTZ]38-co-PS-CH2-[PQ2+]20 on glassy carbon, in 0.1 M NaBF4/H20. This voltammogram has several points of interest. First, both PTZ"/+and PQ2+/+sites in the mixed copolymer are electrochemically reactive, in contrast to the fact that PS-CH2-[PTZ] film coatings are completely electroinactive in aqueous NaBF4. Secondly, the peak currents for PTZ oxidation and for PQ2+ reduction are approximately equal, yet the loading found by N M R was 2:l PTZ:PQ2+. This shows that a lower fraction of the PTZ sites are electroactive as compared to PQ2+sites. Thirdly, the fraction of PQ2+sites that are electroactive in the mixed copolymer is smaller than observed (19) Kosower, E. M.; Cotter, J. L. J . Am. Chem. SOC.1964, 86, 5524. (20) Sassoon, R. E.; Gershuni, S.; Rabani, J. J . Phys. Chem. 1985, 89, 1937.
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The Journal of Physical Chemistry, Vol. 90, No. 12, I986
+1.0
+ 0.5
0.
-0
VOLTS vs. S S C E Figure 7. Cyclic voltammetry for a glassy carbon disk coated with 2 X mol/cm2 of PTZ sites and 1.1 X mol/cm2 of PQ2+ sites of copolymer PS-CH2-[PTZ]38-co-PS-CH2-[PQ(PF6)2]20 in 0.1 M NaBF,/H20 at 100 mV/s. S = 1.2 @A.
for the homopolymer PS-CHz-[PQZ+]36 in [Et4N]Cl/Hz0. (A direct comparison with the mixed copolymer in [Et,N]Cl/H20 is not possible because C1- oxidation interferes with the PTZ pTZ+ step.) Finally, both FTZ and are electroactive in the mixed copolymer in CH3CN and propylene carbonate solvents, but the films appear to dissolve upon potential cycling. In the light of Figure 7, where both PTZ and PQ2+ are electroactive in the mixed copolymer film, it is interesting that a polymer blend prepared by evaporating a solution mixture of PS-CH,-[PTZ] and PS-CH2-[PQ2+] homopolymers produced, in aqueous NaBF4, gave a response from the viologen site only. The addition of PQz+ sites to the same polymer backbone as PTZ apparently provides in the mixed copolymer a sufficiently solvent swollen local environment to open up ionic channels to the PTZ sites within the film. That the blend shows no PTZ electrochemistry suggests that the polymeric chains do not randomly intertwine but are segregated, so that the swollen ionic regions with mobile anions are localized near the PQ2+sites.
w+
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Conclusions The above results establish general synthetic and characterization procedures for soluble redox polymers with controlled
Margerum et al. loading of redox sites. The appearance of light-catalyzed electron transfer between the electron donor and acceptor sites in solutions [PQ2+]2o (Figure of the copolymer PS-CH2-[PTZ]38-co-PS-CHZ2 and eq 4) gives evidence that intramolecular optical electron transfer is possible among redox sites along the polymer chain. At this point, we are unable to distinguish between optical charge-transfer processes between structurally nearest-neighbor redox sites, and chain-coiling processes which bring nonnearest neighbor redox sites into sufficiently close contact for optical charge transfer to be observed.z1 EPR evidence exists that intramolecular electron transfer can occur between redox sites in organic polymers,22 at rates depending on the spacing between sites. In the electrochemical context, both electron self-exchange between adjacent redox sites and chain-coiling processes could be important in multiple electron transfer of soluble redox polymers at electrodes. If the correlation times for intramolecular polymer motions are long compared to the residence time within the double layer, only a few of the redox sites on a chain need be involved in actual electron transfers with the electrode surface. Initial oxidation or reduction of these sites followed by rapid inter- and intrapolymer electron transfer (electron self-exchanges) could provide a basis for oxidation or reduction of most of the redox sites on the polymer chain, as observed by Table 11, during the residence time within the electrode diffusion layer. In this sense, these inter- and intrachain electron-transfer events of soluble polymers would bear analogy to the inter- and intrachain electron self-exchange processes which are known2 to provide electron conduction pathways in films of electroactive polymers on electrodes.
Acknowledgment. Acknowledgments are made to the Army Research OfficeDurham under Grant No. DAAG29-85-K-0121 and to the National Science Foundation under Grant No. CHE-8413450 for support of this research. We also thank Mr. Keith Wilbourn for GPC analysis. Registry NO. I, 54786-26-4; PTZ-, 68629-41-4; MQ'CI-, 4397-88-0; C, 7440-44-0; Pt, 7440-06-4; SnC12, 7772-99-8. (21) See Tazuke, S.; Nagahara, H. Mukromol. Chem. 1980, 181, 2199, 2207,2217 for a se.riRs of papers on structure effects of polymeric donors with monomeric acceptors. (22) (a) See ref 10, p 109. (b) Bloor, D. In Photon, Electron and Ion Probes of Polymer Structure and Properties, Dwight, D. W., Fabish, T. J., Thomas, H. R., Eds.; ACS Symposium Series 162; American Chemical Society: Washington, DC, 1981.
