Conducting Polymers Containing In-Chain Metal Centers

Electron transport in electrochemically polymerized films of a bithienyl-substituted {Os(tpy)2} complex has been studied. The voltammetric response as...
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J. Phys. Chem. B 2003, 107, 10431-10439

10431

Conducting Polymers Containing In-Chain Metal Centers: Homogeneous Charge Transport through a Quaterthienyl-Bridged {Os(tpy)2} Polymer Johan Hjelm,† Robyn W. Handel,‡ Anders Hagfeldt,† Edwin C. Constable,‡ Catherine E. Housecroft,‡ and Robert J. Forster*,§ Department of Physical Chemistry, Uppsala UniVersity, Box 579, SE-751 23 Uppsala, Sweden, National Centre for Sensor Research, Dublin City UniVersity, GlasneVin, Dublin 9, Ireland, and Department of Chemistry, UniVersity of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland ReceiVed: April 28, 2003; In Final Form: June 24, 2003

Electron transport in electrochemically polymerized films of a bithienyl-substituted {Os(tpy)2} complex has been studied. The voltammetric response associated with the Os2+/3+ process is unusually ideal under both semi-infinite linear and finite, diffusion conditions. Significantly, ac impedance results indicate that the ratelimiting step for charge transport is electron hopping at overpotentials more negative than -50 mV or more positive than +100 mV. In contrast, at smaller overpotentials the rates of electron hopping and counterion diffusion are similar. The electron hopping diffusion coefficient determined using dual electrode voltammetry is (2.6 ( 0.5) × 10-6 cm2 s-1. Significantly, this rate of charge transport is more than 2 orders of magnitude larger than those found for comparable metallopolymers in which the osmium centers are linked by nonconjugated bridges. This result is discussed in terms of the effect of bridge conjugation and the relatively close proximity in energy of the redox center and bridge states.

Introduction

SCHEME 1

Conjugated polymers that exhibit high electronic conductivities play key roles in the emerging field of low-cost molecular electronics. In particular, linking metal centers with useful electrochemical, photophysical, or catalytic properties to the backbone, or within the chain itself, is a topic of intense interest.1-3 Ruthenium and osmium oligopyridine complexes have been incorporated in such polymeric assemblies by synthesis of polymers in solution and their subsequent deposition onto electrode surfaces or by electropolymerization procedures.4-10 From a geometrical point of view, suitable building blocks for the construction of topographically linear polynuclear molecules or polymers are the {Ru(tpy)2} and {Os(tpy)2} motifs (tpy is 2,2′:6′,2′′-terpyridine).11 Adopting the strategy of Pickup,12,13 Swager,4,8,9,14 Zotti,6,7 Wolf,5 and others,1-3 for the electrochemical polymerization of preformed thiophene-substituted transition metal complexes, we have previously demonstrated the electrochemical synthesis of a rodlike polymer from [Ru(ttpy)2]2+ (ttpy ) 4′-(2-thienyl)-2,2′:6′,2′′-terpyridine).15 The resulting conjugated metallopolymer produced in this way consists of 2,2′-bithienyl-bridged {Ru(tpy)2} moieties. In this contribution, we report on bithiophene-substituted complex [Os(bttpy)2]2+ (bttpy ) 4′-(5-(2,2′-bithienyl))-2,2′: 6′,2′′-terpyridine) that produces a rodlike 2,2′:5′,2′′:5′′,2′′′quaterthienyl-bridged redox polymer (Scheme 1) that exhibits unusually ideal redox properties. This system allows the effects of extensive charge delocalization and the difference in energy of the metal center and bridge states on the charge transport process to be investigated in an unusually detailed way. Specifically, the formal potential of the Os2+/3+ redox process is within 250 mV of accessible bridge states. By addition of a †

Uppsala University. University of Basel. § Dublin City University. ‡

nucleophile scavenger, BF3, to the supporting electrolyte solution, the tendency of the oxidized bridges to undergo a following chemical reaction can be effectively eliminated and a welldefined and reversible response for the thienyl bridge is observed. The rate of homogeneous charge transport through the film has been determined using a range of transient and pseudo-steady-state techniques. These measurements reveal that the oxidation state of the osmium centers can be switched at a high rate, approximately 2 orders of magnitude larger than previously nonconjugated reported systems. These data allow

10.1021/jp035139l CCC: $25.00 © 2003 American Chemical Society Published on Web 09/04/2003

10432 J. Phys. Chem. B, Vol. 107, No. 38, 2003 new strategies for optimizing redox switching in metallopolymers to be identified, especially in terms of tuning redox center and bridge states. Experimental Section Materials and Reagents. Tetra-n-butylammonium tetrafluoroborate, [n-Bu4N][BF4] (Aldrich), was dried for at least 12 h in a vacuum oven at 80 °C before use. Boron trifluoride diethyl ethereate, BF3‚OEt2, (Aldrich, purified, redistilled) and acetonitrile (Aldrich, anhydrous) used in the electrochemical experiments were used as received. N-Ethylmorpholine, [NH4][PF6], K2[OsCl6], ethane-1,2-diol, and 2-acetylpyridine were purchased from Aldrich and used as received; KtBuO was dried in a desiccator under vacuum before use. Dry THF was freshly distilled from sodium/benzophenone before use. Synthesis of 2,2′-Bithiophene-5-carbaldehyde. A twonecked 50 mL round-bottomed flask containing dry DMF (2 mL) was cooled in an ice bath to 0 °C and POCl3 (400 µL, 4.28 mmol) was added slowly and the solution stirred for 30 min. A solution of 2,2′-bithiophene (500 mg, 3.01 mmol) in DMF (0.5 mL) was added to give a clear yellow solution. The reaction solution was allowed to warm to room temperature and then heated to 100 °C for 2 h. The solution was left to cool and poured onto ice. After the ice had melted, it was neutralized with saturated aqueous NaOAc solution to give a light colored precipitate. The mixture was left in a refrigerator overnight and the solid filtered off and washed with water. The filtrate was extracted with chloroform (2 × 20 mL), and the extract was dried and concentrated in vacuo to give a brown oil that was purified by column chromatography (Al2O3/toluene) to give additional aldehyde as a light yellow solid (163 mg, 35%). 1H NMR (CDCl3, 300 MHz): δ 9.86 (1H, s, CHO), 7.67 (1H, d, J 4.04 Hz, C4,), 7.36 (2H, d, J 4.41 Hz, D3 + D5), 7.25 (1H, d, J 3.67 Hz, C3), 7.07 (1H, d, J 4.41 Hz, D4). FAB-MS: m/z 195 ([M + H]+). IR (solid, cm-1): 1651 (s), 1542 (m), 1508 (m), 1446 (m), 1423 (m), 1377 (m), 1323 (w), 1299 (m), 1222 (m), 1161 (m), 1083 (w), 1053 (m), 983 (w), 891 (w), 844 (m), 798 (s), 752 (m), 713 (s), 659 (s), 563 (m), 520 (s). Synthesis of 4′-(2,2′-Bithien-5-yl)-2,2′:6′,2′′-terpyridine (bttpy). 2-Acetylpyridine (115 µL, 1.0 mmol) was added dropwise to a solution of K[tBuO] (173 mg, 1.5 mmol) in dry THF (20 mL). This was stirred at room temperature for 30 min to give a creamy yellow suspension, to which a solution of 2,2′bithiophene-5-carbaldehyde (100 mg, 0.5 mmol) in dry THF (2 mL) was added. The solution turned clear brown and was left stirring for 20 h at room temperature, after which a solution of [NH4][Oac] (2.5 g, 0.039 mol) in 2:1 EtOH/AcOH (30 mL) was added and the reaction mixture heated to reflux for 5 h. The reaction mixture was cooled to room temperature, and solids were removed by filtration. The filtrate was poured onto ice and left at room temperature for 3 h to give a precipitate that was collected by filtration to give bttpy as a yellow solid (136.9 mg, 67%). 1H NMR (CDCl3, 300 MHz): δ 8.75 (2H, dm, J 4.78 Hz, A6), 8.66 (2H, s, B3), 8.64 (2H, d, J 8.08 Hz, A3), 7.88 (2H, dt, J 1.83, 7.35 Hz, A4), 7.70 (1H, d, J 4.04 Hz, C4,), 7.36 (2H, ddd, J 1.10, 4.78, 7.35 Hz, A5), 7.27 (1H, d, J 4.04 Hz, C3), 7.27 (1H, d, J 5.14 Hz, D3,), 7.24 (1H, d, J 3.67 Hz, D5), 7.06 (1H, dd, J 3.67, 5.14 Hz, D4). 13C NMR (CDCl3, 100 MHz): δ 156.0 (B2), 155.9 (A2), 149.1 (A6), 143.0 (B4 + C2, C5 or D2), 140.2 (C2, C5 or D2), 137.0 (C2, C5 or D2), 136.9 (A4), 127.9 (D4), 126.5 (C3), 125.0 (D5), 124.6 (C3), 124.2 (D3), 123.9 (A5), 121.3 (A3), 116.6 (B3). ES-MS: m/z 420.059 (calc for C23H15N3NaS2, [M + Na+] 420.060). FABMS: m/z 398 ([M + H]+). IR (solid, cm-1): 3062 (w), 2923

Hjelm et al. (w), 2854 (w), 1735 (w), 1581 (m), 1562 (m), 1542 (m), 1458 (m), 1396 (m), 1226 (m), 1145 (w), 1091 (w), 1045 (w), 1006 (w), 987 (w), 879 (w), 837 (w), 783 (s), 740 (m), 713 (m), 655 (m), 617 (m), 563 (m), 516 (s). Synthesis of [Os(bttpy)2][PF6]2. A suspension of bttpy (62.3 mg, 0.157 mmol) and K2OsCl6 (26.3 mg, 0.086 mmol) in ethane-1,2-diol (10 mL) containing 10 drops of N-ethylmorpholine was heated to reflux in a modified domestic microwave oven at (600 W) for 6 min, after which the solution was allowed to cool to room temperature. A saturated aqueous solution of [NH4][PF6] was added and the resultant precipitate was filtered off over Celite and dissolved in acetonitrile. The solvent was then removed in vacuo and the crude product purified by chromatography (silica, MeCN-saturated aqueous KNO3-H2O, 7:1:0.5 then MeCN-saturated aqueous KNO3-H2O, 7:2:2). The major red-brown fraction was collected and further purified by preparative TLC to give [Os(bttpy)2][PF6]2 as a brown solid (43.2 mg, 43%). 1H NMR (CD3CN, 300 MHz): δ 8.92 (4H, s, B3), 8.63 (4H, d, J 7.72 Hz, A3), 8.04 (2H, d, J 4.04 Hz, C4), 7.80 (4H, dt, J 1.47, 7.72 Hz, A4), 7.54 (2H, d, J 4.04 Hz, C3), 7.48 (2H, dd, J 1.10, 5.14 Hz, D3), 7.47 (2H, dd, J 1.10, 3.67 Hz, D5), 7.31 (4H, d, J 5.51 Hz, A6), 7.19 (2H, dd, J 3.67, 5.14 Hz, D4), 7.11 (4H, dt, J 1.47, 6.98 Hz, A5). ES-MS: m/z: 1131 ([M - PF6]+), 493 ([M - 2PF6]2+). IR (solid, cm-1): 1608 (m), 1465 (m), 1427 (m), 1400 (m), 1353 (m), 1284 (m), 1245 (m), 1161 (w), 1083 (w), 1026 (m), 964 (w), 817s [PF6], 779 (s), 748 (s), 725 (m), 678 (m), 651 (m), 621 (m), 551s. Electrodes. Platinum interdigitated microelectrode arrays with 25 electrode pairs (3 mm length, 5 µm electrode width, and 5 µm interelectrode spacing purchased from Abtech Scientific) were used for the dual-electrode voltammetry and in situ conductivity measurements. Platinum disk microelectrodes of 25 µm diameter were used for transient charge transport measurements and were fabricated from platinum microwires (Goodfellow Metals Ltd.) according to a previously described procedure.16 For the impedance spectroscopy measurements, 2 mm diameter Pt disk electrodes sealed in Kel-F (CH Instruments) were used as working electrodes. A coiled Pt-wire was used as the counter electrode and a double-junction Ag/Ag+ (10mM AgNO3, 0.1 M [n-Bu4N][BF4], MeCN) half-cell was used as the reference electrode. The outer reference electrode compartment was filled with acetonitrile containing 0.1 M [n-Bu4N][BF4]. The solutions were degassed with argon before, and were maintained under a blanket of argon during, the experiments. Glassware was dried in a vacuum oven at 80 °C overnight or flamed using a Bunsen burner before preparing solutions and running experiments. The electrodes were polished on a soft polishing pad (Struers, OP-NAP) with an aqueous suspension of 0.3 µm alumina (Buehler) and sonicated for 5 min in MQ-water to remove any remaining polishing material from the electrode surface. The working electrodes were rinsed thoroughly with acetone and dried in air before insertion into the cell previous to electrodeposition of polymer films. The microelectrode arrays were cleaned by placing a drop of freshly mixed H2SO4:30% H2O2 (50:50) on the array for about 10 s followed by rinsing in a stream of deionized water (∼100 mL) to quench the reaction followed by drying in a stream of nitrogen. Polymer-coated electrode arrays were cleaned by repeating the same procedure until the array was clean, as monitored by optical microscopy, scanning electron microscopy (SEM), and voltammetry in a ferrocene containing solution. The reference electrode was calibrated externally by carrying out cyclic voltammetry (at 100 mV/s) in acetonitrile containing 0.5-1 mM ferrocene and 0.1 M [n-Bu4N][BF4]. All potentials

Charge Transport in a Quaterthienyl-Bridged Polymer

J. Phys. Chem. B, Vol. 107, No. 38, 2003 10433

SCHEME 2

are quoted with respect to the redox potential of the ferrocene/ ferrocenium couple unless otherwise stated. Procedures. Electrochemistry was performed using a CH Instruments model 660A electrochemical workstation (chronoamperometry, cyclic voltammetry, impedance spectroscopy) and the bipotentiostat from a CH Instruments model 900A (scanning electrochemical microscope) was used for four-electrode measurements (in situ conductivity, dual-electrode voltammetry). The in situ conductivity measurements were carried out using a modified version of the software for the CH Instruments model 900A bipotentiostat. SEM imaging was carried out using a Hitachi S-3000N variable pressure scanning electron microscope. Electropolymerization of the complex was carried out by scanning the potential of the electrodes (potentiodynamic deposition) between +0.2 and +1.35 V in acetonitrile solutions (0.1 M [n-Bu4N][BF4]) containing 0.5-0.7 mM of the complex and approximately 5% BF3‚OEt2. All electrochemical measurements were carried out in 0.1 M [n-Bu4N][BF4] in acetonitrile + 5% BF3‚OEt2 added, except the measurements carried out to determine the ionic resistance of the polymer films which were carried out in 0.1 M [n-Bu4N][BF4] in acetonitrile. RC constants of the microelectrodes were determined using small amplitude potential step experiments carried out with a custom-built highspeed potentiostat with a rise-time of less than 10 ns. Results Synthesis. A one-pot synthesis17 of 4′-(2-thienyl)-2,2′:6′,2′′terpyridne from thiophene-2-carbaldehyde and 2-acetylpyridine18 has been previously reported and has been extended here to produce the more highly conjugated derivative 4′-(5-(2,2′bithienyl))-2,2′:6′,2"-terpyridine (bttpy). The key intermediate, 2,2′-bithiophene-5-carbaldehyde, was prepared as a pale yellow solid in 35% yield by the Vilsmeir formulation of 2,2′bithiophene with POCl3 and DMF.19,20 The reaction of 2,2′bithiophene-5-carbaldehyde with 2-acetylpyridine gave the desired 4′-(5-(2,2′-bithienyl))-2,2′:6′,2′′-terpyridine (bttpy) as a yellow solid in 67% yield (Scheme 2). This compound was fully characterized by conventional methods. The complex [Os(bttpy)2][PF6]2 was prepared by a standard synthetic method involving the reaction of bttpy with K2OsCl6 under reducing conditions. These coordination reactions proceed very smoothly when performed in ethane-1,2-diol in a modified

domestic microwave reaction with the reaction being complete within 10 min. Purification by exhaustive chromatography gave the desired complex as a brown solid in 45% yield. The 1H NMR chemical shifts of this complex are very similar to those of other {Os(Xtpy)2} systems and closely resemble those of [Os(ttpy)2]2+. The electrospray mass spectrum of [Os(bttpy)2][PF6]2 in acetonitrile exhibited peaks corresponding to {[Os(bttpy)2][PF6]}+ and {[Os(bttpy)2]}2+ showing the expected isotopomer distributions. The complex is colored with an intense MLCT band at 513 nm typical of an [Os(Xtpy)2]2+ center. The spin-forbidden singlet-triplet transition is also observed at 683 nm, again a typical observation for an [Os(Xtpy)2]2+ system with high spin-orbit coupling. A ligand centered absorption associated with the bithienyl substituent is observed at 375 nm. General Electrochemical Properties. The cyclic voltammogram of an acetonitrile solution of [Os(bttpy)2][PF6]2 exhibits a reversible metal-centered oxidation process at +0.48 V. Consistent with the behavior reported previously15 for [Ru(ttpy)2][PF6]2, repetitive cycling of an acetonitrile solution (0.1 M [n-Bu4N][BF4], 5% BF3‚OEt2) between +0.20 and +1.35 V results in the formation of a redox active film of a conducting polymer. After the electrode is rinsed with acetone and acetonitrile, a smooth, transparent, deep red (thin films) to black (thicker films) film can be observed on the electrode surface. Figure 1 illustrates typical slow sweep cyclic voltammograms for an electrogenerated poly-{Os(bttpy)2} film in monomer free electrolyte. The polymer film exhibits a well-defined metalcentered redox wave centered at +0.52 V corresponding to the Os2+/3+ redox process. The peak shape of thinner films (Γ < 6 × 10-8 mol cm-2) is independent of scan rate, υ, up to at least 100 mV s-1, and as shown in the inset of Figure 1, the peak height scales linearly with scan rate. Repetitive cycling between +0.2 and +0.75 V over a 1 h period does not change the shape of the voltammograms, and the peak current decreases by less than 5%. These results demonstrate that under these conditions the responses are electrochemically reversible and the polymer films adhere strongly to the electrode surface. Moreover, for υ < 100 mV s-1, the peak-to-peak separation, ∆Ep, is less than 20 mV and independent of scan rate, and the full width at halfmaximum (fwhm) is 110-120 mV. Where there are no lateral interactions between surface-confined redox centers, and a rapid equilibrium is established with the electrode, a zero peak-to-

10434 J. Phys. Chem. B, Vol. 107, No. 38, 2003

Figure 1. Cyclic voltammograms of a poly-{Os(bttpy)2}-coated platinum electrode (A ) 7.85 × 10-5 cm2, Γ ) 3.3 × 10-8 mol cm-2) at 20, 40, 60, 80, and 100 mV s-1. The inset shows a plot of anodic peak current vs scan rate.

peak separation and a fwhm of 90.6 mV are expected for a oneelectron transfer.21 The observation that the peaks are somewhat broader than theoretically predicted for an ideal Nernstian system may indicate that there are weak destabilizing interactions between the metal centers. After correcting for the background charging current, we can determine the surface coverage Γ, i.e., the number of moles of {Os(tpy)2} centers per unit area, from the charge passed, Q, under exhaustive electrolysis conditions. The surface coverage obtained from the CV illustrated in Figure 1 is 3.3 × 10-8 mol cm-2. By varying the number of voltammetric cycles used in the electropolymerization step, we can vary the surface coverage from a coverage of about 1 × 10-9 to 1 × 10-6 mol cm-2 while maintaining close to ideal voltammetric responses. Assuming that one molecular layer is a hexagonally close-packed layer of hard spheres of the same effective radius as either an {Os(tpy)2} complex (7 Å), or as an {Os(bttpy)2} complex (13 Å), the highest surface coverage corresponds to either approximately 10 000 or 40 000 molecular layers. The {Os(tpy)2} centers are linked by a quaterthienyl spacer (Scheme 2) that is itself electroactive and exhibit redox waves at approximately +0.73 and +1.03 V. The fact that metal center and bridge states are separated by less than 0.25 eV is expected to lead to enhanced coupling between the redox centers perhaps via an electron hopping mechanism. This issue will be addressed in detail in future publications by systematically varying the identity of the metal center and the number of thiophene groups in the bridge so as to control the energy difference of redox and bridge states. However, it is important to note that these ligand-based redox reactions are not fully electrochemically reversible when the potential interval is extended in the positive direction and the bridge states are irreversibly oxidized at potentials more positive than +0.75 V. Overoxidation is a common phenomenon in the electrochemistry of organic conducting polymers such as polythiophene and polypyrrole and is often accompanied by a loss of conjugation.22 For the materials presented here, the irreversibility observed for the bridge states is possibly caused by nucleophilic attack, e.g., by trace water, on the oxidized bridging ligands. Holding the potential at high positive values in acetonitrile based electrolyte without added BF3 causes the ligand-based redox waves to collapse and triggers a dramatic reduction in the dc conductivity of the film. This observation demonstrates the importance of the conjugated bridges in achieving rapid homogeneous charge transport. To overcome

Hjelm et al.

Figure 2. Cyclic voltammograms of a poly-{Os(bttpy)2}-coated platinum microelectrode (A ) 4.91 × 10-6 cm2, Γ ) 5.8 × 10-7 mol cm-2) at 100, 200, 300, 400, and 500 mV s-1. The inset is a plot of the logarithm of the anodic peak current vs the logarithm of the scan rate.

the problem of bridge degradation, about 5% BF3‚OEt2 was added to the acetonitrile solutions used for both electropolymerization and for charge transport studies. This concentration allowed thick films to be produced by electropolymerization and stabilized the bridge redox activity indefinitely at moderate potentials. Therefore, it appears that adding this Lewis acid deactivates trace water and other nucleophiles. Homogeneous Charge Transport. Figure 2 illustrates cyclic voltammograms obtained for a poly-{Os(bttpy)2}-coated platinum microelectrode at 100, 200, 300, 400, and 500 mV s-1 where the surface coverage is 5.8 × 10-7 mol cm-2. The inset is a plot of the logarithm of the anodic peak current vs the logarithm of the scan rate. In contrast to the Gaussian-like peaks observed at slow scan rates that are illustrated in Figure 1, at higher scan rates the cyclic voltammograms become similar to those expected for an electrochemically reversible couple in solution. For example, for surface coverages in excess of approximately 5 × 10-7 mol cm-2, the peak-to peak separation is less than 75 mV for 50 e υ < 5000 mV s-1, which compares favorably with the 57 mV separation expected for a reversible electrochemical reaction involving the transfer of a single electron under semi-infinite linear diffusion conditions. The inset of Figure 2 shows that for 50 e υ < 5000 mV s-1 plots of log ip vs log υ are linear with a slope of 0.52 ( 0.04, which is entirely consistent with the theoretical slope of 0.5 expected for a diffusion-controlled response. The charge transport dynamics through relatively thick polymer films (Γ ) (6-8) × 10-7 mol cm-2) deposited on 25 µm diameter Pt-microdisk electrodes have been examined using cyclic voltammetry at scan rates from 0.005 up to 5 V s-1. Microelectrodes allow the ideality of the voltammetric response to be probed over a wide experimental time scale without complications from the electrode response time or ohmic effects. The Randles-Sevcik equation allows the product DCT1/2C to be determined from the slope of a plot of ip vs υ1/2 using data collected where the voltammetric response is under semi-infinite linear diffusion control and the depletion layer thickness is significantly less than the film thickness:

ip ) (2.69 × 105)n3/2ADCT1/2Cυ1/2

(1)

where DCT is the homogeneous charge transport diffusion coefficient, C is the concentration of osmium centers within the polymer film, and A is the geometric electrode area.

Charge Transport in a Quaterthienyl-Bridged Polymer

Figure 3. Cottrell plots of the anodic and cathodic potential step experiments carried out using the same poly-{Os(bttpy)2}-coated platinum microelectrode as in Figure 2.

The values of DCT1/2C observed for the oxidation and reduction processes associated with the Os2+/3+ reaction were experimentally indistinguishable at (3.6 ( 0.6) × 10-7 mol cm-2 s-1/2. In cyclic voltammetry, the data are obtained at discrete experimental time scales dictated by the scan rate, and the concentration profile varies continuously throughout the experiment. In contrast, chronoamperometry provides a more complete picture of the temporal evolution of the diffusion field from semi-infinite linear diffusion to finite diffusion as the depletion layer approaches the polymer film/solution interface. In contrast, chronoamperometry can provide temporally continuous data. In chronoamperometry experiments the potential was held at the initial value so as to fully oxidize or reduce the polymer film, it was then stepped to a value at least 180 mV more positive or negative that E°Os2+/3+. Using a large amplitude potential step ensures that the rate of heterogeneous electron transfer is sufficiently large to ensure that the current response is controlled by the rate of homogeneous charge transport. The electrode response times given by the product of the resistance, R, and double layer capacitance, Cdl, for the 25 µm diameter Pt disk microelectrodes used were less than 2 µs, and the resistance was sufficiently small to ensure that the ohmic drop was less than 5 mV. Figure 3 shows examples of Cottrell plots of i(t) vs t-1/2 where 30 e t e 150 ms. Consistent with semi-infinite linear diffusion control, the responses from both the anodic and cathodic processes are completely linear over this time scale. For 0.005 < t < 4 ms, i.e., at short time scales, but where double layer charging is complete, the current response does not follow the Cottrell equation, suggesting that semi-infinite linear diffusion does not provide an adequate description of homogeneous charge transport over this period. Similar short-time-scale phenomena have been reported previously for nonconjugated systems, but this issue is not addressed in detail here.23 The slope of the Cottrell plot illustrated in Figure 3 yields DCT1/2C values of (4.5 ( 1.0) × 10-7 mol cm-2 s-1/2 for both oxidation and reduction of the polymer. The observation that DCT1/2C values for film oxidation and reduction are similar despite the fact that that charge compensating counterions ingress during oxidation to maintain electroneutrality suggests that the materials are rapid ion conductors. Moreover, the values are consistent with that obtained using cyclic voltammetry, (3.6 ( 0.6) × 10-7 mol cm-2 s-1/2. It is perhaps important to note that although the bridge connecting the metal center is expected to be electronically conducting, the conjugation is broken at each metal center. Moreover, neighboring bridging ligands will be approximately orthogonal with respect to one another due to the coordination geometry of the octahedral bis-terpyridine

J. Phys. Chem. B, Vol. 107, No. 38, 2003 10435

Figure 4. Steady-state voltammogram obtained by operating a poly{Os(bttpy)2}-coated interdigitated platinum microelectrode array in generator-collector mode. The scan rate was 5 mV s-1. Forward and backward scans are shown. i1 ) generated current, i2 ) collected current.

complex. Under these conditions, charge transfer between metal centers is expected to be diffusional in character even if electron rather than ion movement represents the rate determining step. Dual-Electrode Voltammetry. Both cyclic voltammetry and potential step chronoamperometry are transient techniques that rapidly change the redox composition of the film and require significant ion transport across the polymer/solution interface. In contrast, dual-electrode voltammetry at interdigitated microelectrode arrays, IDAs, operated in generator-collector mode can provide powerful insight into homogeneous charge transport without macroscopically changing the redox composition of the polymer film.24 Moreover, with this approach, it is possible to determine the homogeneous charge transport diffusion coefficient independently of the fixed site concentration. Platinum interdigitated microelectrode arrays were coated by potentiodynamic electropolymerization in which both electrodes were scanned simultaneously between +0.20 and +1.35 V at 100 mV s-1 with a 20 mV potential offset between the two electrode fingers. When the polymer film bridges the gap, a drain current flows and the resistance between the electrodes decreases dramatically. Typically, sufficient polymer was deposited during the initial two voltammetric scans but a further one or two scans was applied to ensure that a continuous film is formed across the gap. SEM and optical microscopy imaging of the coated arrays reveal that thinner films, where Γ is less than 2 × 10-8 mol cm-2, are smooth and exhibit very few surface features. In contrast, films that are used for transient charge transport measurements and ac impedance measurements are substantially thicker and show significant surface roughness and granular irregularities. Figure 4 shows the steady-state voltammogram obtained using a poly-{Os(bttpy)2}-coated platinum interdigitated microelectrode array. The potential of one electrode was held at 0.20 V whereas the other was cycled at 5 mV s-1 from +0.20 to +0.75 V. The collection efficiency, defined as the ratio of “generated”, i1, to “collected”, i2, is close to unity, which is consistent with the reversible cyclic voltammetry response observed. The halfwave potential is identical to the formal potential found using slow scan cyclic voltammetry to within 20 mV. The charge transport diffusion coefficient can be determined using

DCT )

iL dp N ‚ Q (N - 1)

(2)

where iL is the limiting current (taken as the average value of i1 and i2 at 0.65 V, i.e., 130 mV past the formal potential of the Os(III/II) couple), d is the inter-electrode distance (5 µm), p is

10436 J. Phys. Chem. B, Vol. 107, No. 38, 2003 the center-to-center distance between neighboring electrodes (10 µm), Q is the Faradaic charge collected in a slow potential scan where exhaustive electrolysis of the film takes place, and N is the number of fingers (digits) in the array. There are two significant advantages to this approach. First, unlike cyclic voltammetry and chronoamperometry, the diffusion coefficient can be determined without having prior knowledge of the concentration of osmium centers within the film. Second, the potential of the electrode is changed sufficiently slowly so that the current response is not influenced by macroscopic ion transport in and out of the film. Thus, the DCT values determined should be true electron hopping diffusion coefficients. The diffusion coefficient determined using this approach is (2.6 ( 0.5) × 10-6 cm2 s-1. Significantly, this value is at least 2 orders of magnitude larger than values previously reported by Murray and Chidsey for pendant {Os(bpy)2X2} moieties on nonconjugated backbones.24 Once the DCT value has been determined using the dual electrode technique, it is theoretically possible to use this value in conjunction with the DCT1/2C values obtained from cyclic voltammetry or chronoamperometry to extract a concentration for the osmium centers. However, for this concentration to be meaningful, the nature of the rate determining step has to be the same in the two experiments. Thus, although the cyclic voltammetry and chronoamperometry data indicate that a diffusion-like process controls the overall switching rate, these data do not exclude the possibility of coupled electron and ion movement and the calculation of a fixed site concentration is avoided. The dual-electrode measurements give a DCT value 2 orders of magnitude larger than that reported for the electropolymerized nonconjugated polymer poly-[Os(bpy)2(4-vinylpyridine)22+]. This result is significant because lower charge transport rates might be expected for the terpy system for two distinct reasons. First, in the poly-{Os(bttpy)2} system discussed here, molecular modeling suggests that the in-chain intersite separation is of the order of 2.65 nm, which is significantly larger than the 1.4 nm expected for the poly-[Os(bpy)2(4-vinylpyridine)22+] system. Second, the terpy-quaterthienyl bridging ligands lead to a very rigid, linear structure with relatively few intra-strand linkages compared to the reticulated structure of the vinylpyridine-based systems. The fast charge transport properties of the poly-{Os(bttpy)2} system suggest that either hole-type superexchange involving the bridge-based HOMO level or sequential electron hopping between the metal centers and the bridge-based HOMO level, i.e., resonant superexchange, provides efficient electronic coupling between adjacent osmium centers.1 Some recent work carried out by Pickup and co-workers on conjugated polymers with Os and Ru oligopyridyl centers coordinated directly to the polymer backbone has demonstrated increased charge transport rates as the energy gap between the backbone oxidation redox process of the polymer and the M(III/II) decreases.13 The energy gap reported for the polymer poly-[Ru(5,5′-bis(2-thienyl)-2,2′bithiazole)(bpy)22+] is approximately 300 mV. For poly-{Os(bttpy)2} this energy gap is approximately 200 mV and the DCT1/2C values reported in this work correspond well with those reported for the former polymer. Structurally, there are some significant differences between these two systems. The distance between adjacent redox centers on the same chain is larger in poly-{Os(bttpy)2}, and the polymer backbone is interrupted by the metal centers in this polymer, whereas it is continuous in poly-[Ru(5,5′-bis(2-thienyl)-2,2′-bithiazole)(bpy)22+]. It appears an important factor governing the charge transport rate enhancement in these two systems is the matching of the energies of

Hjelm et al.

Figure 5. In situ conductivity measurement using a poly-{Os(bttpy)2}coated interdigitated platinum microelectrode array. The scan rate was 5 mV s-1, and the offset potential was 20 mV.

the metal redox couple and the polymer redox couple (polymer HOMO). In Situ Conductivity. dc conductivity measurements were carried out by the method developed by Wrighton and coworkers.25 A small potential difference, Eoffset, was applied between the two electrodes (typically 20 mV), after which the electrodes were scanned at 5 mV s-1 while the potential difference was maintained. The drain current, iD, that flows between the two electrodes can be related to the film conductivity, σfilm, by applying Ohm’s law and dividing the device, or Zaretsky, cell constant, 0.13 cm-1, by the measured resistance.26 The film conductivities obtained were between 0.5 and 1 × 10-4 S cm-1 (seven independent films of varying film thickness). As illustrated in Figure 5, the maximum conductivity occurs at the formal potential, indicating that redox conduction rather than electronic conductivity is the dominant charge transport mechanism. Consistent with the behavior of an ideal reversible system, the absolute currents measured at the two working electrodes were identical to within 5%. Significantly, there is little hysteresis between the forward (scanning from lower to higher potentials) and backward scans, indicating that the system is electrochemically reversible over this restricted potential range. The residual conductivity observed at potentials above +0.65 V is most likely due to redox conduction by the first bridge-based redox-couple (centered at approximately +0.73 V). It is important to note that the film conductivities measured here are most likely underestimates of the absolute conductivity of the material. To obtain the absolute film conductivity, the film thickness needs to be known, and it was not possible to reliably measure the film thickness with any of the available methods, such as step profilometry/AFM, or ellipsometry. For the surface coverages typically used for the dual electrode electron diffusion and in situ conductivity measurements, optical microscopy, scanning electron microscopy, and tapping-mode AFM indicate that in their dry state, the films are significantly thinner than the digit height of the IDA electrodes. However, the films exhibit significant roughness that prevents an accurate thickness from being obtained. Since the films are significantly thinner than the digit height, the conductivity measurements performed here probably underestimate the absolute conductivity of the polymer. ac Impedance. Electrochemical impedance has proven to be a useful tool to separate the ionic, Ri and electronic resistances, Re, in redox polymers and conjugated polymers.27-31 In this way, an insight into the nature of the rate determining step can be obtained. The impedance response was measured at potentials

Charge Transport in a Quaterthienyl-Bridged Polymer

Figure 6. Complex plane plot of the impedance response of a poly{Os(bttpy)2}-coated platinum electrode (A ) 0.033 cm2) at the formal potential (a) and at an overpotential of -180 mV (b). Γ ) 1.3 × 10-7 mol cm-2.

within 200 mV of the formal potential using a small scale ac perturbation, typically 5 mV. At a potential where no Faradaic reactions occur, the polymer film on the electrode surface acts simply as an additional resistance that is in series with the resistance of the solution and through which the electrode/ electrolyte double-layer charges. The ionic resistance can therefore be measured from the high-frequency real axis intercept that is equal to the sum of Rs and Ri, in a complex plane plot by subtracting the uncompensated solution resistance, Rs, obtained from the real-axis intercept of a bare electrode. The ionic resistance and double-layer capacitance were measured on three independent polymer films at +0.05 V, which ensures that the film is in the fully reduced form. The short-time-scale or high-frequency response can provide a useful insight into the local microenvironment within the film and the ionic content through measurements of the interfacial capacitance and film resistance. These data reveal that the polymer film ionic resistance is unusually low, being less than 10 Ω even for the thickest films (1.5 × 10-7 mol cm-2). The interfacial (doublelayer) capacitance was extracted from the high-frequency data by plotting the imaginary impedance (-Z′′) vs 1/ω, where ω is the angular frequency, rad s-1. Bare and coated electrodes yielded identical capacitances at this potential (13 µF cm-2), indicating a highly solvated interior of the polymer films. As illustrated in Figure 6, the impedance response at greater overpotentials (outside the interval -50 mV < η < +100 mV, where η is overpotential) consists of two distinct regions, first, a region of nearly 45° slope, which at lower frequencies changes to an almost vertical line. The 45° region is diagnostic of a diffusion or migration process. The contribution from this diffusion-like process is minimized close to the formal potential where the redox conductivity is at a maximum. For overpotentials outside the interval -50 < η < +100 mV the length of the diffusion-migration region depends strongly on the overpotential. Within this interval the electronic and ionic resistances are of similar magnitude. At overpotentials more negative than -50 mV or more positive than 100 mV the potential dependent resistance associated with the 45° diffusion-migration region can be attributed solely to electron transport. All DCT1/2C values reported here were determined from data recorded at potentials where the electronic resistance is at least 8 times larger than the ionic resistance. Two different approaches have been used to extract electron diffusion coefficients from the potential dependent impedance data. He and Chen used the critical frequency where semiinfinite diffusion behavior changes to finite diffusion, taken as the point where the high-frequency 45° region changes to the

J. Phys. Chem. B, Vol. 107, No. 38, 2003 10437

Figure 7. Low-frequency (faradaic) capacitance as a function of potential for a poly-{Os(bttpy)2}-coated platinum electrode (A ) 0.033 cm2). Γ ) 9.1 × 10-8 mol cm-2.

TABLE 1 107DCT1/2C/ mol cm-2 s-1/2

technique: cyclic voltammetry chronoamperometry ac impedance (dual-transmission rail model) ac impedance (critical frequency approach) dual-electrode voltammetry (polymer-coated IDAs)

106DCT/ cm2 s-1

3.6 ( 0.6 4.5 ( 1.0 2.6-5.3 1-4 2.6 ( 0.5

almost vertical line (low-frequency region) to determine the diffusion coefficient.32 This transition should occur at f ) 6DCT/ l2, where f is the frequency in Hz and l ()Γ/C) is the thickness of the film and is described by

DCT1/2C )

( ) f‚Γ2 6

1/2

(3)

As shown in Table 1, this method of analysis gives DCT1/2C values in the range (1-4) × 10-7 mol cm-2 s-1/2. Another way to determine the diffusion coefficient from the impedance data is to use the theoretical framework for a finite dual-transmission rail.27 In this model the film is viewed as consisting of uniform polymer penetrated by solution-filled pores. The polymer and the collection of pores are described by separate resistance rails. At overpotentials where the ionic resistance is negligible compared to the electronic resistance, the electronic resistance can be determined from the lowfrequency real axis intercept, Rlow, using

Re ) 3(Rlow - Rhigh)

(4)

where Rlow is the low-frequency real axis intercept and Rhigh is equal to Rs + Ri. The electronic resistance is related to the charge transport diffusion coefficient, DCT, through

Re )

l2 Γ2 ) ClowDCT C D C2 low CT

(5)

where Clow is the low-frequency capacitance, i.e., the Faradaic capacitance of the film. The low-frequency capacitance can be obtained from the slope of a plot of the imaginary impedance (-Z′′) vs 1/ω, where ω is the angular frequency, rad s-1. As illustrated in Figure 7, the Faradaic capacitance goes through a maximum at the formal potential of the polymer. The dualtransmission rail method of analysis yields DCT1/2C values in the range (2.6-5.3) × 10-7 mol cm-2 s-1/2. Figure 8 shows the DCT1/2C values as a function of potential determined using

10438 J. Phys. Chem. B, Vol. 107, No. 38, 2003

Hjelm et al. characteristics of the quaterthienyl-terpyridine bridge, DPhys, is assumed to be zero. The metal center to metal center distance obtained using molecular modeling is 2.65 nm giving a selfexchange rate constant of approximately 2.4 × 109 M-1 s-1. Despite the redox centers within the same chain being separated by 2.65 nm, this kSE is approximately 2 orders of magnitude larger than those typically found for solution phase reactants41 and at least an order of magnitude larger than for fully compressed monolayers where the reactants are in intimate physical contact.42-44 Conclusions

Figure 8. DCT1/2C values determined at different overpotentials using a poly-{Os(bttpy)2}-coated platinum electrode. Γ ) 9.1 × 10-8 mol cm-2. The data analysis was carried out using the dual-transmission rail model (filled squares) and using the critical frequency approach used by He and Chen (open squares).

both types of analysis for the same electrode. The two sets of data differ by approximately 30%, and this systematic difference may arise from the somewhat arbitrary definition of the depletion layer thickness (6Dt). The obtained DCT1/2C values are in reasonable agreement with the values obtained using cyclic voltammetry and chronoamperometry. The ac impedance measurements show that the electronic and ionic resistance in the films are of the same order of magnitude where -50 mV e η e +100 mV. The fact that electron and counterion transport are coupled close to the formal potential suggests that the DCT1/2C values measured using cyclic voltammetry and chronoamperometry may contain an ion-transport contribution. However, it is important to note that the impedance and voltammetry DCT1/2C values agree to within a factor of 2, indicating that the ion transport contribution is relatively small. The observation that the diffusion coefficient drops significantly at the edges of the metal center redox wave is a phenomenon that has been observed previously by Chidsey and Murray.33 This behavior was attributed to the presence of more poorly conductive sites with E1/2 values both greater and less than the majority of the sites within the film. On the basis of their analysis, we expect the variation of DCT to be relatively small within (100 mV of the formal potential of the Os2+/3+ couple, and hence the potential dependence of DCT has a negligible impact on the DCT1/2C values determined using cyclic voltammetry. Self-Exchange Rate Constants. Given that electron hopping dominates the overall charge transport rate through these films, especially away from E°′Os2+/3+, it is possible to determine the rate of electron self-exchange between the immobilized osmium sites. By comparing this value with that observed for solution phase reactants, we can obtain insight into the strength of electronic coupling and the effect of the local microenvironment on the self-exchange dynamics. Electron hopping within polymeric matrixes has been considered by a number of workers including Dahms,34 Laviron,35 Save´ant,36 Ruff, Friedrich and co-workers,37,38 and Buck.39 The Dahms-Ruff40 expression allows the second-order rate constant, kSE, describing the dynamics of self-exchange between adjacent Os2+/Os3+ moieties, to be determined:

1 DCT ) DPhys + kSEδ2C 6

(6)

where DPhys describes physical diffusion in the absence of electron hopping, and δ is the inter-site separation between adjacent osmium centers moieties. Given that the rigid rod

A new 2,2′:6′,2′′-terpyridine ligand bttpy containing a pendant bithienyl unit has been synthesized and the homoleptic complex [Os(bttpy)2][PF6]2 prepared. Electropolymerization generates a metallopolymer containing {Os(tpy)2} moieties separated by quaterthienyl units that exhibit unusually ideal voltammetry for the Os2+/3+ redox processes. Moreover, the thienyl bridges undergo reversible redox switching when a nucleophile scavenger, BF3, is added to the electrolyte solution. The homogeneous charge transport diffusion coefficient associated with the Os2+/3+ centers was determined to be (2.6 ( 0.5) × 10-6 cm2 s-1 using polymer-coated interdigitated electrode arrays operated in generator-collector mode. This charge transport rate is approximately 2 orders of magnitude larger than those reported previously for structurally related systems. ac impedance measurements show that the electronic resistance is much higher than the ionic resistance at overpotentials larger than -50 and +100 mV and must be the rate-limiting step at those potentials. Closer to the formal potential of the redox couple, the ionic and electronic resistance are of the same order of magnitude and both ion diffusion and electron hopping contribute to the charge transport dynamics. Acknowledgment. J.H. and A.H. gratefully acknowledge financial support from the Swedish Natural Science Research Council (grant K 5104-20006267/2000). J.H. and R.F. thank Dr. Peixin He, CH Instruments, for providing a modified version of the software for the CHI900. We thank the referees for their insightful comments on the original manuscript. References and Notes (1) Pickup, P. G. J. Mater. Chem. 1999, 9, 1641. (2) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999, 48, 123. (3) Wolf, M. O. AdV. Mater. 2001, 13, 545. (4) Zhu, S. S.; Swager, T. M. AdV. Mater. 1996, 8, 497. (5) Zhu, Y.; Wolf, M. O. Chem. Mater. 1999, 11, 2995. (6) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Pagani, G.; Canavesi, A. Synth. Met. 1996, 76, 255. (7) Zotti, G.; Zecchin, S.; Schiavon, G.; Berlin, A. J. Electroanal. Chem. 2001, 506, 106. (8) Buey, J.; Swager, T. M. Angew. Chem., Int. Ed. 2000, 39, 608. (9) Kingsborough, R. P.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 8825. (10) Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992. (11) Constable, E. C. In Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998; Chapter 4. (12) Maclean, B. J.; Pickup, P. G. Chem. Commun. 1999, 2471. (13) Maclean, B. J.; Pickup, P. G. J. Phys. Chem. B 2002, 106, 4658. (14) Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 12568. (15) Hjelm, J.; Constable, E. C.; Figgemeier, E.; Hagfeldt, A.; Handel, R.; Housecroft, C. E.; Mukhtar, E.; Schofield, E. Chem. Commun. 2002, 284. (16) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (17) Spahni, W.; Calzaferri, G. HelV. Chim. Acta 1984, 67, 450. (18) Encinas, S.; Flamigni, L.; Barigelletti, F.; Constable, E. C.; Housecroft, C. E.; Schofield, E.; Figgemeier, E.; Fenske, D.; Neuburger, M.; Vos, J. G.; Zehnder, M. Chem. Eur. J. 2002, 8, 137.

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