Langmuir 1992,8, 959-964
959
Electrochemical Investigation of Novel Polymerizable Thiophene/Ferrocene Conjugates Roberta Back and R. Bruce Lennox* Chemistry Department, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received November 17, 1991. In Final Form: December 10, 1991
A series of 3-alkyl-substituted thiophenes has been synthesized, where the 3-position is substituted by
-(CH2)2-O-(CH2).-OC(0)Fc( n = 6,8,10,12,16;Fc = ferrocene). The electrochemical properties of both
the ferrocene and the thiophene moieties have been investigated in detail. The ferrocenelferricinium couple is quasi-reversible at Pt, and oxidation of the thiophene at =+2 V leads to the deposition of a monolayer of a poly(thiophene) species. Pulsed potential, fixed potential, and cycled potential techniques all yield polymer surface films whose ferrocene pendant group exhibits quasi-ideal surface wave behavior. Monolayer and not multilayer films were uniquely accessible with these materials. Electrochemical deposition of these 3-alkylthiophene derivatives onto a Pt electrode precoated with a conducting 3-methylthiophenefilm was successful and was also limited to monolayer equivalent coverage. Copolymerization of the thiophene/ferroceneconjugates with 3-methylthiopheneresults in a film with a greater than monolayer equivalent coverage of ferrocene derivative. A comparison of the electropolymerization conditions provides insight into the deposition mechanism of these poly(thiophene) conjugates onto an electrode surface and establishes a methodology for deliberately producing monolayer coverages of conducting polymers.
Introduction The modification of electrode surfaces has greatly expanded opportunities in electrochemistry.'s2 There are two extreme approaches in this field of research. The underlying purpose of one approach involves the generation of novel electrochemical phenomena, such as electrocatalysis or selective electrochemical detection. The other approach uses electrochemistry as a tool to probe the behavior, on a molecular level, of given materials at the electrode surface. Conducting polymers are interesting materials with which to modify electrode surfaces. Electropolymerization of a monomer directly onto an electrode surface offers a unique view of the resulting polymer's characteristics, including ita conductivity,permeability, redox properties, selectivity toward other species, and solubility. Conducting polymers may also act as a convenient "organicanchor" with which to attach various other groups to an electrode surface.3 This may be achieved chemically, through modification of the polymer structure, or physically, wherein a molecule is adsorbed by the polymer on the electrode surface. Polypyrroles and polythiophenes are particularly adaptable to such studies. Each is polymerized through an oxidative polymerization mechanism which proceeds through an electrochemically accessible radical cation intermediate.* Electropolymerizationof these monomers results in the deposition of the resulting polymers onto the electrode surface. Numerous derivatives are readily obtained through substitution at either the 3-position of the ring or on the nitrogen of pyrrole prior to polymerization. B y judicious derivatization and choice of polymerization conditions, these polymers may be synthesized to yield both metallic properties and enhanced solubility in a variety of processing solvent^.^ (1)Murray, R. W. Acc. Chem. Res. 1980, 13, 135. (2) Abrufia, H.D. Coord. Chem. Reu. 1988,86, 135. (3) Roncali, J.; Garreau, R.; Delabouglise, D.; Garnier, F.; Lemaire, M. Synth. Met. 1989,28, C341. (4) Genies, E. M.; Bidan, G. J. Electroanal. Chem. 1983, 149, 101.
We have chosen to study the system of polythiophene with pendant ferrocene groups, with particular regard to the modification of platinum electrode surfaces by this system. Similar systems to date include viologen-functionalized poly(3-alkylthienylene) Ppbferrocene-functionalized polypyrrole,6c*dbipyridine-functionalizedpolypyrrole6eand polythiophene,6fanthraquinone-, phenothiazine-, and anthracene-functionalizedpolypyrrole,6g and polypyridinyl-ruthenium complexespolymerizedthrough pyrrole units.6h We have synthesized a series of monomers, designated Mn, consisting of a ferrocene ester unit attached to a 2-(3-thienylethoxy) group by an alkyl chain; chain lengths, n, range from 6 to 16 carbons. Our choice of the thiophene monomer stems from many inherent and useful properties of thiophene. The anodic electropolymerization of thiophene is accessible 'in acetonitrile. Furthermore, long-chain substituents at the 3position of thiophenes have been shown to form a polymer soluble in common organic solvents5 when the alkyl chain is of sufficient length, making large scale characterization and manipulation feasible. The synthesis of derivatives with substituents at the 3-position is generally easier to achieve with thiophene than with pyrrole, whose ring nitrogen must be protected.' The use of 2-(34hienyl)ethanol as a starting material will give monomers with an ether linkage in the substituent at the 3-position. Such monomers, once polymerized, have exhibited the ability to complex cations such (5)(a) Chang, A X . ; Blankespoor, R. L.; Miller, L. L. J. Electroanal. Chem. 1987,236, 239. (b) Elsenbaumer, R. L.; Jen, K. Y.; Oboodi, R. Synth. Met. 1986, 15, 169. ( c ) Blankespoor, R. L.; Miller, L. J. Chem. Soc., Chem. Commun. 1985,SO. (6) (a) Chu, C. F.; Wrighton, M. S. Am. Chem. SOC.Symp. Ser. 1988, No. 378,408. (b) Bauerle, P.; Gaudl, K.-U. Adu. Mater. 1990,2 (4), 185. ( c ) Inagaki, T.; Hunter, M.; Yang, X. Q.; Skotheim, T. A,; Okamoto, Y. J. Chem. Soc., Chem. Commun. 1988, 126. (d) Eaves, J. G.; Mirrazaei, R.; Parker, D.; Munro, H. S. J. Chem. SOC.,Perkin Trans. 2 1989,373. (e) Deronzier, A.; Moutet, J . 4 . Acc. Chem. Res. 1989,22, 249. (0 Mirrazaei, R.; Parker, D.; Munro, H. S. Synth. Met. 1989,30, 265. (g) Andrieux, C. P.; Audebert, P. J. ElectroanaL Chem. 1990, 285, 163. (h) J.ElectroanaL Chem. 1990,285, Cosnier, S.; Deronzier, A.; Roland, J.-F. 133.
(7) Delabouglise, D.; Garnier, F. Adu. Mater. 1990, 2 ( 3 , 91.
0743-7463/92/2408-0959$03.00~0 0 1992 American Chemical Society
960 Langmuir, Vol. 8, No. 3,1992
S
O
H
Back and Lennox
, DMAP
Fe
e n=6,8,10,12,16 69-94%
75%
CH2C12
e
Mn
CH2C12
17-70%
Figure 1. Synthesis route of monomer series, M6 to M16.
as Li+ in a loose crown ether type of structure.8 This in turn leads to enhanced conductivitiesof the polymer when such cations are part of the supporting electrolyte. A polymeric backbone is expected to provide a reliable and stable anchor for the ferrocene moiety. An added benefit of polythiophene originates from the fact that sulfur has a tendency to physisorb to metals such as gold and ~ l a t i n u m .This ~ property may enhance the adsorption of polymer to the electrode and thus improve the physical stability of our system, as well as the extent of polymer/ electrode communication. The ferrocene group is a well-characterizedredox group and provides an internal electrochemical probe for the materials discussed below. In addition, it has been successfully applied to electrocatalysissituations1° and is synthetically accessible as a variety of derivatives. By using different ferrocene derivatives, one also has the possibility of 'tuning" the Eo of the probe. The variety of chain lengths available also provides a parameter, on the molecular level, with which to explore the effect of distance on the rate of electron transfer. The goals of this work are 2-fold. Firstly, we wish to use our series of thiophenelferrocene derivatives to study the electron transfer processes occurring a t the electrode surface as a function of a variety of experimental conditions. These electron transfer processes include redox activity of both the polymer backbone and the ferrocene ester attached to the polymer. Secondly,we wish to explore methods for producing ultrathin films of conducting polymers in a controlled fashion. Given that the thinner the film, the higher its conductivity," routes to produce ultrathin films in a reproducible fashion are highly desirable. (8) Roncali, J.; Garreau, R.; Delabouglise, D.; Garnier, F.; Lemaire, M. J. Chem. SOC.,Chem. Commun. 1989,679. (9) Chambers, J. Q. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, Vol XII, pp 329-
502. (IO) Pettersson, M. Anal. Chim. Acta 1983, 147, 359. (11) Yassar, A.; Roncali, J.; Garnier, F. Macromolecules 1989,22,804.
Experimental Section 1. Synthesis. The monomer synthesis scheme is shown in Figure 1. Reacting ferrocene acid chloride with appropriate di-
ols in pyridine, with a catalytic amount of (dimethylamino)pyridine (DMAP) present, gave the corresponding ferrocene alcoholcompounds. Formation of bis(ferrocene)compoundswas minimized by working at high dilution with a %fold excess of diol. The ferrocene alcohols were then converted to the corresponding triflates, and the triflate moiety was displaced by 2(3-thieny1)ethanol to yield a series of ferrocene-derivatized thiophene molecules. In atypicalsynthesis, lOmmol(2.65g)offerroceneacidchloride was produced from 14 mmol(3.3 g) of ferrocene carboxylic acid (Aldrich) in a well-documentedprocedure.'% The acid chloride was dissolved in 30 mL of dry CHzCll and added dropwise to a solution of 30 mmol of diol (Aldrich) in 200 mL of dry pyridine (Aldrich), with a catalytic amount of (dimethy1amino)pyridine (DMAF') (Aldrich). This mixture was then allowed to stir at room temperature under Nz for up to 30 h. (Yieldsranged from 69 to 94%). This ferrocene-substituted alcohol was separated fromexceea dioland %(ferrocene) side product (usuallyminimal) by flash chromatography (60:40 petroleum ether:ether, Aldrich, silica gel Merck grade 60, Aldrich). The purified ferrocenesubstituted alcohol (5.3 mmol) and di-tert-butylpyridine (6.9 mmol) (Aldrich) were added in a 0.81.05 ratio to 30 mL of dry CBClz,at 0 OC. Trifluoromethanesulfonicanhydride (6.6 mmol) (Aldrich), dissolved in 15 mL of CHZC12, was added dropwise to the ferrocene solution.lZbAfter the addition was completed, the mixture was allowed to come to room temperature over l1/2 h. Without isolating the triflate, a further amount of base (4-fold excess) was added to the flask, followed by a 2-fold excess of 2-(34hienyl)ethanol (Aldrich). Product was isolated after up to 64 h and purified by flash chromatography (%lo petroleum ether:ether). 1H NMR (200 MHz, Varian XL200) spectra were consistent with the expected material. Yields were =70% for this procedure. Attempts to isolate the triflate lowered the yield of the last two steps drastically (to less than 20%). Compounds given the notation M were synthesized from linear diols with 6(M6), 8(M8), 10(M10),12(M12),and 16(M16)carbon units. 2. Electrochemistry. All electrochemicalexperiments were performed using a Bio-AnalyticalSystems (BAS) l00A electrochemistryworkstation. In a three-compartment cell,the working electrode was a BAS dsk Pt electrode (surface area of approx~~~~
~~~
(12) (a) Knobloch, F. W.; Rauscher, W. H. J. Polym. Sci. 1961,54,651. (b) Evans,P. A.; Sheppard, G. S. J . Org. Chem. 1990,55,5192.
Langmuir, Vol. 8, No. 3, 1992 961
Polymerizable ThiophenelFerrocene Conjugates Table I. Electrochemical Data for Monomers in 0.1 M TBAHP/CH&N and Surface Adsorbed Polymers in 0.1 M LiClO,/CH&N Em, mV AE: mV 90 650 M6 (8.0 mM) 100 650 M8 (9.8mM) 70 680 M8 (2.2mM) 100 670 M10 (9.9 mM) 70 710 M12 (2.0 mM) 70 M16 (2.2 mM) 660 40 600 PM6 610 10 PM8 610 20 PMlO 40 610 PM16 Values of AE are taken at a sweep rate of 100 mVIs. imately 0.027 cmz);the reference electrode (BAS)was a Ag/AgCl and the auxiliary electrode was a Pt gauze. All solutions were made up in acetonitrile (CH3CN) (Aldrich HPLC grade, used without further purification). Tetrabutylammonium hexafluorophosphate(TBAHP) (Aldrich)was recrystallizedoncefrom ethanol and dried at 78 OC under vacuum overnight. Lithium perchlorate (Aldrich)was dried at 78 OC under vacuum overnight. 3-Methylthiophene (Aldrich) used in copolymer and bilayer systems was used as purchased. 2.1. Electrochemistry of Monomers in Bulk Solution. The three shortest chain monomers, M6, M8, and M10, were studied in solutions of approximately 10 mM. The two longer chain monomers, M12 and M16, were studied in solutions of not more than 2 mM, due to their limited solubilities. A medium sized monomer (M8) was studied at both concentrations to investigate whether the bulk concentration affects the observed electrochemistry. Solutions of monomer were made only in 0.1 M TBAHP/CH,CN. Although the oxidation potential of thiophene is lower in 0.1 M LiCL04/CH3CN,such solutions were found to be unstable over the time period of the experiments. The electrochemistry of the ferrocene moiety was observed using cyclic voltammetry over a range of sweep rates. Carewas taken to avoid polymerizing thiophene in these experiments;this was achievedby maintaining Eapp < 1.00V. In a separate experiment, a cyclic voltammogram was run to +2.5 V to characterize the thiophene oxidation. 2.2. Electropolymerization. The electropolymerizations were performed from the aforementioned solutions. Constant potential experiments (+2.0 V) for a defined time were used to effect polymerization. Polymerization was also attempted using cyclicvoltammetry (repeatedlysweepingfrom 0.0 to +2.5 V) and double potential step (potential stepped from 0.0 to +2.0 V and back to 0). 2.3. Electrochemistry of Polymers. Once the polymer was formed on the electrode, the electrode surface was thoroughly rinsed with CH3CN and placed in a solution of 0.1 M LiClO4/ CH3CN. (The ferrocene couple shifts to less positive potentials in LiC10, than in 0.1 M TBAHP/CH3CN,l3making it easier to distinguishfrom the background current. At the higher potentials required in TBAHP, the peaks of the ferrocenecouplemay include the broad oxidation peak of the polymer backbone itself.) The polymer modified electrodes were then examined by cyclic voltammetry and chronocoulommetry. Results and Discussion 1. Electrochemistry of Monomers. The electro-
chemistry of the ferrocene-derivatized thiophene conjugates in bulk solution showed the ferrocene couple to be quasi-reversible. Cyclic voltammetry studies revealed a linear variation of peak currents with the square root of the sweep rate. Values of Ell2 and AE for the ferrocene couple in all monomers are shown in Table I. There is no obvious correlation between molecular weight and E l p . An irreversible oxidative peak corresponding to the oxidation of the thiophene unit appears at potentials =+2 V. This potential varies greatly depending on factors such (13)JBnsson, G.; Gorton, L.; Pettersson, L. Electroanalysis 1989,49.
100
I
i
1 60 40
a:
3
;
20 00
-2 0 -40
1
-60
U
-8 0
Figure 2. Cyclic voltammogram of 2 mM M12/0.1 M TBAHP/ CHsCN. Sweep rate = 100 mV/s; sweep begins at 200 mV and proceeds in anodic direction.
Y
0 U U 1
-0 3 -0 4
-05
, 10
08
06
Potential (V)
34
02
VS A g / A g C l
Figure 3. Cyclic voltammogram of Pt electrode coated with layer of PM8. Sweep rate = 100 mV/s; sweep begins at 200 mV and proceeds in anodic direction in 0.1 M LiC104/CH&N.
as the exact condition of the Pt electrode surface and the freshness of the monomer solution. A typical cyclic voltammogram of M12 from 0.2 to +2.5 V is shown in Figure 2. 2. Homopolymer. In the first electropolymerization attempt the three smaller ferrocene-derivatized thiophene monomers were dissolved in acetonitrile a t concentrations of =lO mM, along with 0.1 M TBAHP. This monomer concentration is the maximum allowable for these molecules; at higher concentrations ionic migration of the monomers is observed once the potential is sufficiently positive to oxidize ferrocene to the ferricinium form. All three electrochemical methods of polymerization successfully deposited a ferrocene species onto the electrode surface for each monomer. Chronocoulometry reveals that material oxidizable at +LO V adheres to the Pt electrode after each polymerization experiment. This is presumably the ferrocene oxidation but could include a component of the oxidation of the polythiophene backbone. Cyclic voltammetry indicates the presence of a surface wave centered at +600 mV, corresponding to the oxidation and reduction of the ferrocene ester. Peak currents of these cyclic voltammograms show linear dependence on potential sweep rate, with some deviation at high sweep rates. Peak separation is small but does increase at high sweep rates. A typical cyclic voltammogram, in this case of polyM8 (PM8), is shown in Figure 3. Table I lists Ell2 and AE values of the ferrocene couple for each polymer in the series. The Ell2 values of the polymers are all less positive than the Ell2 values of the monomers by 40-60 mV. We attribute this to the different
Back and Lennox
962 Langmuir, Vol. 8, No. 3, 1992 electrolytes. The effect of the perchlorate ion on the surface-bound ferrocene has been studied in detail.13 The authors speculate that the observed negative shift of Eo for the adsorbed ferrocene,which increases with increased perchlorate concentration, is caused by complex formation between the ferricinium ion and the perchlorate ions. In order to account for the behavior of peak currents and peak separations at high sweep rates, the resistance of the polymer was measured, and found to vary between 1and 2 kQ. The associated iR drop is
