Langmuir 1999, 15, 1491-1497
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Coordination Polymers Based on Bis(bipyridyl)alkane Ligands: Film Preparation and Charge-Transport Characteristics Stanton Ching* Department of Chemistry, Connecticut College, New London, Connecticut 06320
C. Michael Elliott Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received August 18, 1998. In Final Form: November 10, 1998 Electroactive coordination polymers have been prepared by reacting Fe2+ and Co2+ with bis(bipyridyl)alkane (bba) ligand films that have been spin coated onto glassy carbon electrodes. The resulting films exhibit redox characteristics similar to those of electropolymerized vinyl(bipyridyl) metal complexes. The stability of the electrode-bound films increases as the length of the linking alkyl chain decreases. Better stability is also obtained with Fe2+ films than Co2+ films and with the alkyl-linking group attached at the 4-position rather than the 5-position. Charge-transport measurements using chronoamperometry have been conducted on poly-FeII/bba films in which the alkyl linker consists of 2, 4, and 6 methylene groups. Values of D1/2C (D ) charge-transfer diffusion coefficient; C ) concentration of redox sites) range from 9.5 × 10-8 to 2.3 × 10-8 mol/cm2 s1/2 at 298 K and follow a trend of decreasing D1/2C with increasing alkyl chain length for the 4-position-linked FeII/bba polymers. No such trend is observed for the analogous 5-position-linked systems. Activation parameters Ea and ∆Hq range from 22 to 31 kJ/mol, whereas ∆Sq ranges from -30 to -75 J/mol K. The results are consistent with a mechanism in which the increase in methylene chain length causes a decrease in the rate of charge transport due to the lower frequency of electron-transfer collisions between neighboring redox sites. However, polymer segmental motion is also believed to play a secondary role in this mechanism.
Introduction Bis(bipyridyl)alkane molecules (bba, Figure 1) are versatile ligands in the synthesis of dimeric and oligomeric metal complexes. The coordination chemistry of these ligands has produced numerous complexes with interesting structures and novel electrochemical and photochemical properties.1-9 In the laboratory of one of the authors, bba ligands have been used to synthesize dimeric complexes with approximate D3-symmetry, [M2(bba)3]4+ (M ) Fe2+, Ru2+, Co2+, Ni2+, Mn2+), in which each of the three ligands bridge the two metal centers. The ligands consist of two bipyridyl moieties each linked at the 4-position by an alkyl chain containing 2 or more methylene groups.5-9 Alkyl linkers with inserted oxygen and sulfur atoms have also been prepared.8,9 In addition, the dimers can be generated in mixed-metal and mixed-valent forms.8,9 In an effort to broaden the scope of the bba chemistry, we used these ligands as cross-linking agents in the formation of redox polymers. The systems are similar to those of poly-[M(vbpy)3]2+ (M ) Fe, Ru, Os; vbpy ) 4-vinyl4′-methyl-2,2′-bipyridine), which are produced by elec(1) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, Germany, 1995; Chapter 9. (2) Youinou, M.-T.; Ziessel, R.; Lehn, J.-M. Inorg. Chem. 1991, 30, 2144. (3) Garber, T.; Van Wallendael, S.; Rillema, D. P.; Kirk, M.; Hatfield, W. E.; Welch, J. H.; Singh, P. Inorg. Chem. 1990, 29, 2863. (4) Furue, M.; Kuroda, N.; Nozakura, S. Chem. Lett. 1986, 1209. (5) Elliott, C. M.; Freitag, R. A.; Blaney, D. D. J. Am. Chem. Soc. 1985, 107, 4647. (6) Serr, B. R.; Andersen, K. A.; Elliott, C. M.; Anderson, O. P. Inorg. Chem. 1988, 27, 4499. (7) Ferrere, S.; Elliott, C. M. Inorg. Chem. 1995, 34, 5818. (8) Larson, S. L.; Hendrickson, S. M.; Ferrere, S.; Derr, D. L.; Elliott, C. M. J. Am. Chem. Soc. 1995, 117, 5881. (9) Elliott, C. M.; Derr, D. L.; Ferrere, S.; Newton, M. D.; Liu, Y. P. J. Am. Chem. Soc. 1996, 118, 5221.
Figure 1. Methylene-linked bba ligands: (a) 4,4′-bba (4n0); (b) 5,5′-bba (5n0).
trochemical polymerization.10 The chemically generated metal/bba polymers, however, differ from the electropolymerized systems in significant ways. For instance, bba ligands can be used to generate polymer films with metal cations such as Co2+ which are not amenable to electropolymerization in the form of (vinyl)bipyridyl or related complexes. The Co(vbpy)32+ complex, for example, electropolymerizes poorly ostensibly because the first reduction process is metal-based rather than ligand-based,11 whereas Co/bba polymers form readily. Additionally, the length and structure of the linking methylene chain in M/bba polymers is determined by the choice of the bba ligand used. In contrast, the polymer matrix of an electrogenerated poly-[M(vbpy)3]2+ film can be constructed by joining the vinyl moieties either in head-to-head or head-to-tail configurations (or some mixture thereof); consequently, such films contain linking methylene chains that vary randomly in both length and structure. (10) Denisevich, P.; Abruna, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1982, 21, 2153. (11) Potts, K. T.; Usifer, D. A.; Guadalupe, A.; Abruna, H. D. J. Am. Chem. Soc. 1987, 109, 3961.
10.1021/la9810509 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/23/1998
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We have begun to explore the synthesis and properties of electrode-bound redox polymers based on bba ligands. In an earlier study some general synthetic procedures for forming metal/bba films, as well as related coordination polymers of bis(terpyridyl)alkanes and mixed (bipyridylterpyridyl)alkanes were reported.12 The electrocatalytic properties of these films toward CO2 and O2 reduction were also described. In this article, we report on the synthesis and properties of a series of Fe2+ and Co2+ redox polymer films formed from bba ligands linked in either the 4- or 5-position. We also describe the charge-transport characteristics of the poly-Fe/bba films and suggest a mechanism for the apparent charge diffusion. The M/bba polymers offer an excellent opportunity to study charge transport as a function of the polymethylene chain structure. The distance between immobilized redox sites can be varied without other major modifications in the film structure, which is a situation often difficult to achieve with other polymers having immobilized redox centers.1-9 The bba ligands considered here are given a three-digit shorthand notation, “mn0”.13 The metalated polymers are abbreviated as poly-M/mn0. Experimental Section Chemicals. 4,4′-Dimethyl-2,2′-bipyridyl (4,4′-DMB) was obtained from Reilly Tar Chemicals, Indianapolis, and 5,5′dimethyl-2,2′-bipyridyl (5,5′-DMB) was prepared by a modified version of the method reported by Sasse and Whittle.14 Both were recrystallized from ethyl acetate before use. Procedures reported previously were used to prepare 4-vinyl-4′-methyl-2,2′bipyridyl (vbpy)15 and [Fe(vbpy)3](PF6)2.16 All other chemicals were used as received. Solutions of Fe(ClO4)2 and Co(ClO4)2 were prepared from Fe(ClO4)2‚6H2O and Co(ClO4)2‚6H2O obtained from Alfa Chemicals. High-purity solvents from Burdick and Jackson were used for electrochemical measurements. Tetrabutylammonium (TBA) hexafluorophosphate (TBAPF6) and tetrabutylammonium perchlorate (TBAClO4) for supporting electrolyte were obtained from Aldrich Chemicals. Ligand Synthesis. The 4n0 and 5n0 ligands were prepared from 4,4′-DMB and 5,5′-DMB using previously reported procedures.7 In a typical synthesis, 5-10 g of bipyridyl was dissolved in dry tetrahydrofuran (THF) and treated with 1 mol equiv of lithium diisopropylamide under N2 at -78 °C. One-half equivalent of the appropriate coupling reagent was then added to produce the dimeric bipyridyls: 1,2-dibromoethane was used to prepare 420 (40% yield) and 520 (70% yield); ethylene glycol di-p-tosylate was used to prepare 440 (30% yield) and 540 (45% yield); 1,4dibromobutane was used to prepare 460 (20% yield) and 560 (50% yield). The products were purified by flash chromatography using silica gel and 10% v/v acetone/dichloromethane followed by recrystallization from ethyl acetate. Polymer Films. Metalated films of 4n0 and 5n0 were prepared by spin-coating techniques. Typically, 8 µL of 100 mM bba in chloroform was applied to a 3-mm-diameter glassy carbon disk electrode and spin coated at 2000 rpm. After drying, a 10-µL drop of 40 mM Fe(ClO4)2 or Co(ClO4)2 in acetonitrile was applied, resulting in formation of dark red coordination polymers for Fe2+ (12) Feldheim, D. L.; Baldy, C. J.; Sebring, P.; Hendrickson, S. M.; Elliott, C. M. J. Electrochem. Soc. 1995, 142, 3366. (13) Methylene-linked, 2,2′-bipyridyl ligands are given the abbreviation “mn0”, in which “m” represents the position of the methylene link on the bipyridyl rings, “n” gives the number of methylene groups in the link, and “0” indicates that there are no substituents on the bipyridyl nitrogen atoms. Thus, bis(bipyridyl)alkanes linked at the 4-position by 2, 4, and 6 methylene groups are designated 420, 440, and 460, respectively. Likewise, analogous methylene links at the 5-position give 520, 540, and 560. Metal-containing redox polymers bear the name of the metal followed by the ligand notation separated by a slash, for example, poly-Fe/420 or poly-Co/520. (14) Sasse, W. H. F.; Whittle, C. P. J. Am. Chem. Soc. 1961, 83, 1347. (15) Spiro, T.; Ghosh, P. J. Am. Chem. Soc. 1980, 102, 5543. (16) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 3334.
Ching and Elliott and dull yellow polymers for Co2+. Excess ligand and metal cation were removed by sequential rinsing with acetone, dichloromethane, and acetonitrile. This procedure consistently generated films with coverage of (2-4) × 10-8 mol/cm2 with 420, 440, 520, and 540 ligands. Comparable films with 460 and 560 required multiple spin coats of the ligand solution before treatment with metal cation. The film coverage (Γ) was determined by cyclic voltammetry using the charge obtained by integrating the area under the Fe2+/3+ oxidation wave or the Co2+/1+ reduction wave. Poly-[Fe(vbpy)3](PF6)2 was prepared by a standard procedure.10 Instrumental Measurements. Cyclic voltammetry experiments were carried out either with an EG&G Par 173 potentiostat/175 programmer or with a BAS 100B electrochemical analyzer. Amperometric measurements were recorded with the BAS 100B. Experiments were performed using standard threeelectrode cells using glassy carbon disk working electrodes (3mm diameter), a coiled platinum wire auxiliary electrode, and a silver wire quasireference electrode. Peak potentials are reported relative to saturated calomel electrode (SCE) using ferrocene as an internal reference. UV-Vis spectroelectrochemistry was performed using a Hewlett-Packard 8452A diode array spectrophotometer. The electrochemical cell consisted of two tin oxide-coated glass plates separated by an O-ring in a face-to-face arrangement. The polymer film was prepared on one plate while the other served as the auxiliary electrode. A silver wire inserted through the O-ring served as a quasireference electrode. A nitrogen-purged solvent/electrolyte solution was introduced through small syringe needles which were also inserted through the O-ring. Scanning electron microscopy was performed with a Phillips 505 SEM. Charge-Transport Measurements. Charge-transport rates through the Fe-containing redox polymer films were measured by chronoamperometry. The Cottrell equation (eq 1) was used to
i)
nFAD1/2C π1/2t1/2
(1)
extract the value of D1/2C, in which D is the apparent diffusion coefficient for charge transport and C is the concentration of the redox active sites in the film. Variable temperature measurements were performed in solutions of butyronitrile/0.1 M TBAPF6. Constant temperature was maintained using slush baths of ice water (273 K), carbon tetrachloride (250 K), acetonitrile (232 K), chloroform (210 K), and dry ice/acetone (196 K). The applied potential step was 0.0 to 1.4 V (vs Ag wire quasireference electrode). Cyclic voltammetric scans just before each experiment were used to establish that the final potential was at least of 300 mV beyond the Fe2+/3+ oxidation wave. (The Cottrell slopes were unchanged for potential steps ranging from 200 to 500 mV beyond the Fe2+/3+ oxidation wave.) Activation parameters were calculated from Arrhenius and Eyring plots as described in the literature.17
Results and Discussion Poly-M/bba-Modified Electrodes. Films of poly-M/ bba are not as smooth and well formed as those of redox polymers prepared by electrochemical polymerization of [M(vbpy)3]2+. Scanning electron microscopy images of polyFe/440 and poly-Fe(vbpy)32+ show the contrasting morphologies of these films (Figure 2). Despite their uneven appearance, poly-M/bba films could be prepared with very reproducible coverage by controlling the concentration of ligand in the spin-coating solution. For most of the ligands, films of 8 × 10-9 to 4 × 10-8 mol/cm2 were obtained from one-coat applications of 25-100 mM solutions. Thicker films were prepared with multiple coatings. Polymer films containing Fe2+ and Co2+ were prepared with the 4n0 and 5n0 ligands (n ) 2, 4, 6). Cyclic voltammograms of poly-Fe/420 and poly-Co/420 are shown (17) Daum, P.; Lenhard, J. R.; Rolison, D.; Murray, R. W. J. Am. Chem. Soc. 1980, 102, 4649.
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Figure 2. SEM images of FeII(bipyridyl) redox polymers: (a) chemically prepared poly-Fe/440; (b) electropolymerized poly[Fe(vbpy)3]2+.
in Figures 3 and 4. Redox potentials for the reversible waves of various Fe2+ and Co2+ polymer films are listed in Table 1. In addition to these reversible couples, polyFe/5n0 films exhibit an additional ill-defined third reduction wave just negative of -2.0 V. Likewise, each Co2+ film exhibits a broad second reduction wave centered in the range between -1.3 and -1.7 V. Qualitatively, the electrochemistry of each of the poly-M/bba films resembles the solution electrochemistry of the corresponding trisbipyridine metal complex. We infer from this observation that the vast majority, if not all, of the metal ions in the film are coordinated to three bipyridine moieties. Otherwise, different film redox chemistry would be expected. The potential data in Table 1 show that increasing the length of the methylene linkage causes the films to become slightly easier to oxidize and slightly harder to reduce. Qualitatively similar behavior is observed in the solution electrochemistry of analogous dinuclear [Fe2(4n0)3]2+ complexes.7 The potentials of the various redox processes of the dinuclear complexes are also shifted relative to those
of mononuclear analogue [Fe(4,4′-DMB)3]2+. As the alkyl chain length increases, the potentials of the dinuclear complexes approach those of [Fe(4,4′-DMB)3]. All of these potential shifts for [Fe2(bba)3] complexes (and by analogy poly-Fe/bba films) are explainable by site-site electrostatic interactions.7 The electrochemistry of poly-Fe/bba films is similar in most respects to that of electropolymerized Fe(vbpy)32+ complexes (again supporting the argument that each iron is coordinated to three bipyridine ligands). Cyclic voltammograms exhibit a reversible Fe2+/3+ oxidation wave and 2 or 3 reversible ligand-based reductions depending on the specific bba ligand (Figure 3). In the voltammetry of poly-Fe/bba films both the 2+/3+ and 1+/2+ redox processes exhibit prominent “charge-trapping” prewaves; that is, whenever the film is scanned through the 1+/2+ reduction prior to being scanned through the 2+/3+ oxidation, the oxidative prewave is present (and vice versa for the reductive prewave). If the potential is cycled only over the 2+/3+ or 1+/2+ waves, no trapping peaks are
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Ching and Elliott Table 1. Redox Potentials of Fe2+ and Co2+ bba Polymers Films E°(V) vs SCE
Figure 3. Cyclic voltammogram of poly-Fe/420 on a glassy carbon electrode. The electrolyte solution is 0.1 M TBAPF6/ CH3CN. The scan rate is 100 mV/s. The coverage is 8 × 10-9 mol/cm2.
Figure 4. Cyclic voltammograms of poly-Co/420 on a glassy carbon electrode. The first three scans are shown. The electrolyte solution is 0.1 M TBAPF6/CH3CN. The scan rate is 100 mV/s. The coverage is 2 × 10-9 mol/cm2.
observed. The explanation for these prewaves is that charge becomes kinetically trapped when the film is oxidized from 2+ to 3+ (or reduced from 2+ to 1+), and this charge is released when the film is reduced from 2+ to 1+ (or oxidized from 2+ to 3+). In general, the separation between the charge-trapping peak and the main reduction or oxidation wave decreases as the film coverage increases (again, similar to what is observed for Ru(vbpy)3 and Fe(vbpy)318). Cyclic voltammograms of poly-Co/bba films (Figure 4) exhibit a reversible Co2+/1+ reduction wave and a broad, barely perceptible Co2+/3+ oxidation wave (due to slow
polymer film
2+/3+
2+/1+
1+/0
0/1-
Fe/420 Fe/440 Fe/460 Fe/520 Fe/540 Fe/560 Co/420 Co/440 Co/460 Co/520 Co/540 Co/560
+0.879 +0.839 +0.829 +0.937 +0.935 +0.926 +0.112 +0.069 +0.065 +0.300 +0.227 +0.187
-1.471 -1.501 -1.558 -1.494 -1.512 -1.525 -1.127 -1.197 -1.193 -1.074 -1.091 -1.113
-1.651 -1.659 -1.714 -1.704 -1.691 -1.684
-2.060 -2.099 -2.163
electron-transfer kinetics for the 3+/2+ couple). Similar Co2+/3+ voltammetric waves are observed for monomeric [Co(4,4′-DMB)3]2+ in solution. The charge-trapping peak before the reduction wave gradually diminishes with repeated cycling, again, similar to the voltammetry of polyCo(vterpy)32+ (vterpy ) 4′-vinyl-2,2′:6′2′-terpyridyl19). Films of poly-Fe/bba are significantly more stable than their Co2+ counterparts. In particular, they show much more persistent electroactivity upon repeated voltammetric scans. The poly-Fe2+/3+ couple of Fe/4n0 polymers is completely stable even in multiple cyclic voltammetry experiments with numerous scans. The bipy reduction waves, on the other hand, degrade with prolonged scanning, losing about 25% of their initial peak height after 100 cycles. By contrast, the Co2+/1+ reduction wave of polyCo/420 decays more than twice as quickly. Not surprisingly, reduced forms of all of the polymers are extremely sensitive to oxygen. The reductive degradation of the film is greatly accelerated by exposure to even trace amounts of O2. In general, the Fe2+ films were noticeably less sensitive to oxygen than the Co2+ films. General film stability also improved with shorter methylene links. The patterns of general stability of poly-M/bba films mirror the electrochemical stability. The redox polymers with longer methylene chains tend to dissolve or delaminate more easily. For example, lower coverage was typically observed for films formed from 460 and 560 ligands. It was also generally found that polymers of 5n0 ligands were more soluble than those of 4n0 ligands. The greater solubility of poly-M/5n0 films parallels the greater solubility of the free 5n0 ligands relative to the 4n0 ligands. The coordinated metal also influences the physical properties of the film; Fe2+ films adhere more strongly to the electrode than the analogous Co2+ films. Attempts to prepare polymer films containing other transition metal ions thus far have been unsuccessful. Direct reactions of Ni2+ or Cu2+ with spin-coated 4n0 and 5n0 ligands generated films that dissolve or delaminate from the electrode. Attempts to generate Ru2+ films were likewise unsuccessful. Standard precursors for Ru(bpy)32+ complexes, such as Ru(DMSO)4Cl2, did not react at room temperature with spin-coated 4n0 and 5n0 films. Heating, on the other hand, resulted in loss of the ligand film. The dissolution of the film is attributed to the formation of soluble, dimeric [Ru2(bba)3]4+ complexes (and/or other small oligomers), which are known to be generated under such conditions.7 (18) Paulson, S. C. Spectroelectrochemical Characterization of a Unique Ruthenium Trisbipyridine-Linked Porphyrin Complex and Its Polymer Analogues. Ph.D. Thesis, Colorado State University, Fort Collins, CO. (19) Guadalupe, A. R.; Usifer, D. A.; Potts, K. T.; Hurrell, H. C.; Mogstad, A. E.; Abruna, H. D. J. Am. Chem. Soc. 1988, 110, 3462.
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Figure 5. Cottrell plots for the Fe2+/3+ oxidation of poly-Fe/ 420 at 250, 232, 210, and 196 K. The electrolyte solution is 0.1 M TBAPF6/butyronitrile. Inset: Cyclic voltammogram of the Fe2+/3+ couple for the poly-Fe/420 film with Γ ) 3 × 10-8 mol/ cm2.
Charge-Transport Measurements. Charge-transport measurements were carried out on poly-Fe/4n0 and poly-Fe/5n0 films using chronoamperometry. Because the Fe2+/3+ redox couple is stable to electrochemical cycling and is isolated from all other redox couples of the polymer, these films are well suited for these experiments. Spincoated films were prepared with coverage ranging from 2.4 to 3.4 × 10-8 mol/cm2 (determined coulometrically). At this coverage, the Fe2+/3+ voltammetric wave loses its characteristic surface-confined symmetry (Figure 3) and begins to take on a more diffusion-controlled appearance (inset in Figure 5). At lower temperatures, voltammetry becomes even more characteristic of a diffusion-limited process. The wave develops an increased tailing characteristic and ∆Ep grows considerably larger. The magnitude of the peak also become proportional to v1/2 instead of v. Typical Cottrell plots at various temperatures are shown in Figure 5. At higher temperature, the longer-time data (i.e., lower values of t-1/2) deviate from linear Cottrell behavior. This is as expected, as the limits of a finite diffusion are reached within the film on the experimental time scale.17 The results were fully reproducible for a given film and independent of whether the temperature was changed by cooling or warming. The data reported in Table 2 represent averages from experiments performed on three separately prepared films with similar coverage. The values of D1/2C in Table 2 are reliable to within at least (30%. Cyclic voltammograms of the films were unchanged before and after the variable temperature measurements. The Cottrell plots were used to extract charge-transport rates in the form of D1/2C at various temperatures (Table 2). Linear Arrhenius-type plots of ln(D1/2C) vs 1/T indicate that charge transport occurs by an activated process (Figure 6). The form of the relation with D1/2C expressed as a rate parameter is given by eq 2,17
D1/2C ) Do1/2C exp(-Ea/2RT)
(2)
which was used to determine activation energies, Ea,17 and to extrapolate values of D1/2C at room temperature. Activation enthalpies, ∆Hq, were estimated using the relationship Ea ) ∆Hq + RT.20,21 Values of D1/2C at room
Figure 6. Arrhenius plot of D1/2C vs 1/T for poly-Fe/420. Table 2. Temperature-Dependent Charge-Transport Data for Redox Polymer Films polymer film (Γ, mol/cm2) Fe/420 (3.1 × 10-8) Fe/440 (2.7 × 10-8) Fe/460 (3.4 × 10-8) Fe/520 (3.0 × 10-8) Fe/540 (2.4 × 10-8) poly[Fe(vbpy)3](PF6)2 (1.5 × 10-8) poly[Fe(vbpy)3](PF6)2 (5.7 × 10-9)
D1/2C × 109 (mol/cm2 s1/2) at various temperatures 273 K 250 K 232 K 210 K 196 K 54.7
27.8
29.3
16.4
20.0 17.5 14.0 4.60 22.0
6.92
3.84
8.16
3.92
2.52
13.2
6.43
4.02
2.00
11.9
6.18
3.83
2.36
9.86
4.86
2.75
1.76
2.75
2.26
1.49
1.03
7.45
4.29
2.87
12.6
14.8
temperature and activation parameters for the polymer films are listed in Table 3. Charge transport in redox polymers is a complicated phenomenon in which several mechanisms have been identified as possible rate-determining steps.22,23 For polymers with immobilized electroactive sites, these possibilities include 1) the intrinsic rate of electron hopping between neighboring redox sites, 2) the rate of polymer segmental motion needed to bring about the proper distance and orientation for electron exchange, and 3) the rate of counterion motion needed to maintain charge neutrality in the film. Moreover, each of these potential rate-determining steps is influenced differently by changes in polymer solvation. For poly-Fe/4n0 films, it is unlikely that the intrinsic rate of electron hopping (expressed by the quantity D in eq 2) differs significantly, because the basic Fe(4,4′DMB)32+-type redox site remains essentially constant in each of these polymers. It is concluded, therefore, that the gradual decrease in D1/2C which accompanies lengthening (20) Oyama, N.; Ohsaka, T.; Ushirogouchi, T. J. Phys. Chem. 1984, 88, 5274. (21) Ohsaka, T.; Yamamoto, H.; Oyama, N. J. Phys. Chem. 1987, 91, 3775. (22) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; p 191. (23) Oyama, N.; Ohsaka, T. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York 1992; Chapter 8.
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Table 3. Charge-Transport Data and Activation Parameters for Redox Polymer Films polymer film (Γ, mol/cm2) Fe/420 (3.1 × 10-8) Fe/440 (2.7 × 10-8) Fe/460 (3.4 × 10-8) Fe/520 (3.0 × 10-8) Fe/540 (2.4 × 10-8) poly[Fe(vbpy)3](PF6)2 (1.5 × 10-8) poly[Fe(vbpy)3](PF6)2 (5.7 × 10-9)
D1/2C at 298 Ka Ea ∆Hq ∆Sq (mol/cm2 s1/2) (kJ/mol) (kJ/mol) (J/mol K) 9.5 × 10-8
31
29
-30
4.9 × 10-8
29
27
-47
3.4 × 10-8
27
25
-62
2.8 × 10-8
24
22
-75
2.3 × 10-8
26
23
-73
5.4 × 10-8
17
14
-120
3.2 × 10-8
24
22
-72
a Obtained by extrapolation of the data in Table 2 to room temperature.
the polymethylene chains from poly-Fe/420 to poly-Fe/ 460 is a result of decreasing concentration. More specifically, the frequency of successful electron-transfer collisions decreases as the average site-site separation increases. Solvent swelling also may contribute to the trend in D1/2C if the polymers with longer methylene chains are solvated more effectively. Given that the immobilized Fe(L)32+ units on average become farther apart, charge transport becomes progressively more dependent on polymer segmental motion as the alkyl chains are made progressively longer. Although segmental motion is expected to increase with the added flexibility of more methylene groups, the trend in Table 2 shows that this is not enough to offset the concomitant increase in electron hopping distance. Results from the Arrhenius calculations were applied to an Eyring analysis to obtain activation entropies, ∆Sq, through the relationship shown in eq 3.20,21
Do ) eλ2(kT/h) exp(∆Sq/R)
(3)
In this expression, Do comes from the preexponential term in eq 2, e is the base of the natural logarithm, and λ is the diffusion jump distance. The calculation was carried out assuming that the number of jumps is related to the electrode coverage, Γ, according to d/λ ) Γ/Γmonolayer, where d is the thickness of the film. This assumption then yields the relationship Do/λ2 ) (Do1/2C/Γmonolayer)2.17 If the monolayer coverage for poly[Ru(vbpy)3](ClO4)2, Γmonolayer ) 8 × 10-11 mol/cm2 is assumed as an approximation of Γmonolayer of the Fe/4n0 polymer,10 the parameters shown in Table 3 are obtained. These values are similar to those reported for immobilized poly(vinyl)ferrocene films.17 The relatively large negative activation entropy supports an increase for the electron-exchange transition state. This would be as expected if the rate-determining step involved collisions between isolated redox sites caused by polymer segmental motion. Thus, in the poly-Fe/4n0 series, the increasingly negative ∆Sq values are attributed to the greater entropic requirements of bringing redox sites together as the alkyl chain lengthens. This trend is accentuated by using a constant value of Γmonolayer in the calculation of ∆Sq, but this factor cannot be fully responsible for the trend. Indeed, to obtain comparable values of ∆Sq for poly-Fe/420, poly-Fe/440, and poly-Fe/460, a 50% decrease in monolayer coverage must be factored in for each additional pair of methylene groups in the chain. This should be considered an unreasonably large variation.
Figure 7. UV-Vis spectroelectrochemistry of poly-Fe/420 on a tin oxide electrode surface. The electrolyte solution is 0.1 M TBAPF6/CH3CN. The spectrum for the Fe2+ form of the polymer was obtained with the electrode potential at 0.0 V. The spectrum for the Fe3+ form was recorded with the electrode potential 300 mV beyond the Fe2+/3+ oxidation wave.
Rate limitations caused by counterion motion have been deemed insignificant for the poly-Fe/4n0 films. The trend in D1/2C is inconsistent with slow counterion migration, because the greater distance between redox sites and the enhanced polymer flexibility should increase rather than decrease the rate of ion movement through the film. Values of D1/2C also show very little dependence on coverage, which suggests facile permeation through the films. In addition, the values of D1/2C are not sensitive to changes in electrolyte anion for PF6-, ClO4-, and BF4-, nor electrolyte concentration (between 0.05 and 4.0 M). The freedom of ion movement through the Fe/bba polymers may be related to the high porosity of the films as indicated by the scanning electron microscopy (SEM) image in Figure 2. The small aggregated polymer particles are undoubtedly easier for counterions to penetrate than a uniformly well packed film, such as those of electropolymerized poly-[Fe(vbpy)3]2+. Permeation studies using cyclic voltammetry also show that poly-Fe/4n0 films do not block analyte molecules from access to the electrode surface in the same way as the electropolymerized polyFe(vbpy)3. For example, voltammograms of solutions of ferrocene recorded with electrodes having a 3 × 10-8 mol/cm2 film coverage of poly-Fe/4n0 exhibit less than a 50% reduction in peak current with almost no difference in wave shape compared with bare electrodes. By contrast, a 1.5 × 10-8 mol/cm2 film of electropolymerized polyFe(vbpy)32+ in the same ferrocene-containing solution results in an almost sigmoidal wave of greatly diminished current. A significant concern in this study was whether the loose aggregation of small polymer particles somehow limited electrochemical access to all of the redox sites in the film. Such a limitation would be expected to compromise data on film coverage and charge transport. The issue was addressed by spectroelectrochemistry performed on a poly-Fe/420 film prepared on optically transparent tin oxide electrodes. The UV-Vis spectrum, shown in Figure 7, indicates the complete transformation of Fe2+ to Fe3+ in the polymer film. Thus, despite the potentially poor interconnection of the polymer particles, these results indicate that all the redox sites in the film are accessible electrochemically.
Coordination Polymers Based on bba Ligand Films
Charge-transport measurements for the poly-Fe/520 and poly-Fe/540 films reveal D1/2C values that are slightly less than those of their poly-Fe/4n0 analogues. Results for poly-Fe/560 were not obtained because of film loss during the course of the chronoamperometry experiments. Unlike the poly-Fe/4n0 polymer series, there is no trend that correlates D1/2C with methylene chain length in the poly-Fe/5n0 systems. Indeed, similar data are obtained for both poly-Fe/520 and poly-Fe/540. The observed charge-transport behavior in these coordination polymers can be rationalized from their solubility and segmental motion. It was previously noted that both the 5n0 ligands and metalated Fe2+ and Co2+ coordination polymers were noticeably more soluble in organic solvents than their 4n0 counterparts. This higher degree of solubility can reasonably be related to greater segmental motion in the redox polymer films (possibly due in part to a lower degree of cross-linking), which in turn increases the frequency of interactions between neighboring electroactive sites. In the poly-Fe/4n0 systems, the increase in methylene chain length results in decreasing concentration and therefore lower frequency of successful electron-transfer collisions. Thus, the chargetransport rate decreases. Significantly, this concentration effect is not offset by the increased segmental motion of the longer alkyl chains, which would tend to enhance charge transport. By contrast, the situation of the polyFe/5n0 films appears to be one in which enhanced polymer segmental motion negates the effects of lower redox site concentration. Thus there is essentially no difference in the charge-transport rates of poly-Fe/520 and poly-Fe/ 540. This explanation of the charge-transport rates is consistent with the relative solubilities of the redox polymers. Indeed, the greater solubility of the poly-Fe/ 5n0 films would suggest that these systems are probably less highly cross-linked and, thus, less rigid than their poly-Fe/4n0 counterparts. (Recall that charge-transport rates could not be measured for the Fe/560 film because of its high solubility.) The activation entropies of the redox polymers may also be indicative of how solubility and segmental motion contribute to charge transport in these systems. The data in Table 3 show a general correlation in which D1/2C decreases as ∆Sq becomes more negative. The ∆Sq values can be interpreted to indicate the segmental flexibility in the film which must be overcome in bringing redox sites together. Therefore, the rates of
Langmuir, Vol. 15, No. 4, 1999 1497
charge transport are similar for the poly-Fe/5n0 films because greater segmental flexibility (as indicated by ∆Sq) sufficiently offsets the effects of having a lower concentration of redox sites. By contrast, the poly-Fe/4n0 systems apparently have a smaller degree of segmental flexibility and so the concentration of redox sites is the dominant effect on the rate of charge transport. Conclusion Bis(bipyridyl)alkanes linked at the 4- and 5-positions are effective precursors for the transition metal redox polymers of Fe2+ and Co2+. Electrodes coated with polyM/bba films can be prepared by first spin coating the ligand onto an electrode, then reacting the ligand with a solution of the transition metal cation. These films exhibit electroactivity similar to their discrete molecular analogues, as well as to related electrochemically polymerized systems. Charge-transport measurements are consistent with a mechanism in which an increase in the linking methylene chain length causes a decrease in charge transport because of a lower frequency of electron-transfer collisions between neighboring redox sites. However, it appears that polymer segmental motion can offset this effect under circumstances of increased polymer solubility. The charge-transport rates in poly-Fe/4n0 and polyFe/5n0 systems are very similar to those of electrochemically polymerized vinyl(pyridine) and vinyl(bipyridine) complexes, for which D1/2C values generally occur in the range of (1-5) × 10-8.10,24-27 Thus, it appears that chargetransport rates for these two kinds of polymers are similar despite their very different physical morphologies (cf. Figure 2). The porous nature of the chemically generated poly-M/bba films suggest that these systems could be preferable in applications that favor high active surface area and facile mass transport, such as in electrocatalysis. Acknowledgment. The authors acknowledge support of this work by the National Science Foundation (CHE-971408). LA9810509 (24) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J. Am. Chem. Soc. 1982, 104, 2683. (25) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1991. (26) Ikeda, T.; Leidner, C. R.; Murray, R. W. J. Electroanal. Chem. 1982, 138, 343. (27) Leidner, C. R.; Denisevich, P.; Willman, K. W.; Murray, R. W. J. Electroanal. Chem. 1984, 164, 63.
