Charge Injection and Transport in Metal-Containing Conducting

Article pubs.acs.org/cm

Charge Injection and Transport in Metal-Containing Conducting Polymers: Spectroelectrochemical Mapping of Redox Activities Wenjun Liu,† Weijie Huang,‡ Chun-Hsing Chen,† Maren Pink,† and Dongwhan Lee*,† †

Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States Carl Zeiss SMT, Inc., One Corporation Way, Peabody, Massachusetts 01960, United States

‡

S Supporting Information *

ABSTRACT: Electropolymerization of tris(dioximate) cage complexes furnished metal-containing conducting polymers (MCPs) that deposit directly onto the electrode surface as uniform films. The injection of electrons into, or removal of electrons from, these electroactive materials proceeds via different pathways with different rates, the underlying molecular mechanisms of which were investigated by a combination of electrochemical, spectroscopic, and focused-ion-beam−scanning electron microscopy (FIB-SEM) cross-section analysis studies. For cobalt-containing polymers, both the metal centers and πconjugated organic backbone work cooperatively as hopping stations for migrating holes, whereas the reduced polymer utilizes less-efficient self-exchange between cobalt(II) and cobalt(I) centers for electron transport. A small molecule model of such reductively doped polymer was prepared independently, which provided compelling electrochemical and spectroelectrochemical evidence to support the structural integrity of the metal centers upon redox switching. A well-defined metal-to-ligand charge transfer (MLCT) band of the n-doped polymer was exploited further as a straightforward spectroscopic tool to quantify the number of redox-active metal centers directly and to estimate the lower distance limit of diffusional charge transport across the bulk material. KEYWORDS: conducting polymer, transition metal, redox-active, spectroelectrochemistry, charge transport

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(Figure 1A).14,21,22 Alternatively, the π-conjugated polymer backbone can actively participate in the ET reaction, either by mediating direct electronic coupling between two adjacent metal centers through superexchange (Figure 1B),23−26 or by functioning as “hopping stations” for the migrating charge carriers (Figure 1C). Most of the MCPs prepared by anodic polymerization have oligo/polypyrrole- or oligo/polythiophene-based conjugation units.20,27−29 As anticipated, such electron-rich organic segments can effectively facilitate hole injection to the metal centers (analogous to p-doping). On the other hand, the delivery of electrons (analogous to n-doping) into such a system must rely primarily on direct outer-sphere ET between metal centers; the organic CP backbone usually remains electronically inactive under reductive conditions.5,8,18−20 How ef f iciently can electrons hop to the metal centers embedded within such hybrid constructs? A quantitative answer to this important scientific question should help formulate rational design principles for MCP-based electrocatalysts that can deliver reducing equivalents to exogenous substrates.4−17 We have recently communicated that tris(dioximate) cage complexes30 of redox-active transition metals can be function-

INTRODUCTION Conducting polymers (CPs) having structurally well-defined metal centers offer opportunities to develop chemical sensors and catalysts with molecular-level understanding of metal− polymer and metal−substrate interactions.1−10 Among different synthetic strategies to access such hybrid materials, electropolymerization of preassembled transition metal complexes allows facile access to polymer-supported and surface-bound electrocatalysts that can drive chemical transformations of relevance to energy conversion and storage, including hydrogen production,11−13 CO2 reduction,14−16 and O2 activation.17 The use of metal-containing CPs (MCPs) in such multielectron redox processes requires that the structural integrity of the metal−ligand module should be maintained during and after electropolymerization. In addition, metal centers embedded within the polymer matrix should also retain the redox activities that are inherent to their molecular precursors. For such idealized systems, charge carriers delivered from the electrode can be transported through repetitive self-exchange (eq 1) via one of the mechanisms schematically shown in Figure 1.5,8,18−20 Mn + 1 + Mn + → Mn + + Mn + 1

(1)

For systems in which the polymer backbone serves mainly as insulated structural support, electron transfer (ET) occurs through space between the metal centers in close proximity © 2012 American Chemical Society

Received: July 26, 2012 Revised: August 27, 2012 Published: August 28, 2012 3650

dx.doi.org/10.1021/cm302361p | Chem. Mater. 2012, 24, 3650−3658

Chemistry of Materials

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distances over which electrons can migrate across the polymer matrix. Electrochemical measurements on MCP often produce convoluted current−voltage patterns reflecting the redox chemistry of the metal centers superimposed on features from organic π-conjugation as well as from double-layer charging.32,33 As such, significant background signals must be taken into account in data analysis. In our complementary spectroelectrochemical approach reported in this work, the use of optical properties that are intrinsic to the metal center ensures that the measured signals arise exclusively from the redox activities of the metallic component of the MCP.

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RESULTS AND DISCUSSION Synthesis and Characterization. Metal-templated [3 + 2]-type condensation reactions between diphenylglyoxime (dpg) and bithiopheneboronic acid produced the cobalt(II) and iron(II) cage complexes 1 and 2, respectively (see Scheme 2). While their structural analogues 3 and 4 (Scheme 2) could

Figure 1. Mechanisms of self-exchange between Mn+ and Mn+1 in MCPs (metal is shown in blue; ligand is light gray; polymer backbone is dark gray). The situation in panel A or B is described by eq 1, whereas the bridge-mediated noncoherent hopping shown in panel C involves the Mn+−π•+−Mn+ intermediate en route to the product Mn+1−π−Mn+ from the reactant Mn+−π−Mn+1.

Scheme 2. Chemical Structures of Cage Monomers and Polymers

alized with π-extended arylboronic acids as capping groups (see Scheme 1).31 A sequential electropolymerization of the Scheme 1. Metal-Templated Assembly of Inorganic Cage Monomers and Their Electropolymerization to MCPs

be isolated after simple stirring in MeOH at room temperature,31 the assembly of 1 and 2 required extended heating in boiling THF, which presumably reflects the conformational flexibility of the dpg ligand (used for 1 and 2) relative to the ring-fused nioxime ligand (used for 3 and 4). Introduction of the electron-withdrawing phenyl groups in the ligand backbone might also lower the nucleophilicity of the oxime OH groups in the condensation reaction with the arylboronic acid. This inductive effect, on the other hand, anodically shifts the reduction potential of the metal center, thereby allowing the isolation and characterization of the corresponding reduced metal complex (vide infra). In CD2Cl2 at T = 298 K, the iron(II) complex 2 shows a well-resolved 1H NMR pattern, suggesting a low-spin d6 electronic configuration. On the other hand, the cobalt(II) complex 1 displays broad paramagnetic resonances. The solution magnetic moment of μeff = 1.78 μB at T = 298 K determined for 1 by the Evans method is consistent with the S = 1/2 ground spin-state of a low-spin d7 system. This interpretation is supported further by X-band EPR spectroscopy (T = 77 K) on a frozen CH2Cl2 solution sample of 1. As

monomers that are essentially isostructural but differ in the identity of the metal ions embedded inside the “cage” furnished electroactive bilayer materials on the electrode surface. Comparative electrochemical studies have subsequently established efficient hole-injection capabilities of these systems, which allow charge carriers to migrate readily across the interface between different MCP layers.31 In this paper, we report our detailed investigation on the electron-injection properties of MCP systems constructed from inorganic cage complexes. In addition to providing well-defined cyclic voltammetric (CV) signals associated with the n-doping process, the system produces intense and characteristic optical bands that arise from metal-to-ligand charge-transfer (MLCT) transitions at the reduced metal center. A combination of UV− vis spectroelectrochemical studies and focused-ion-beam− scanning electron microscopy (FIB-SEM) cross-section analysis have allowed us to experimentally (i) confirm the structural integrity of the material upon n-doping, (ii) quantify the number of redox-active metal centers, and (iii) assess effective 3651



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shown in Figure S1 in the Supporting Information, the anisotropic g values and hyperfine splitting patterns of 1 are characteristic of a cobalt-centered unpaired spin.34−36 X-ray crystallographic analysis on a single crystal of 1 unambiguously confirmed the chemical connectivity, along with the spatial arrangement of the N6-donor sets adopting a pseudotrigonal prismatic geometry (Figure 2a). Two chemically

Figure 3. Cyclic voltammograms of 1−4 (1.0 mM) on glassy carbon electrodes in CH2Cl2 with nBu4NPF6 (0.1 M) as supporting electrolyte. Scan rate = 100 mV/s.

extended from the cage core remain neutral under reducing conditions. Therefore, no polymerization occurred. Preparation, Spectroscopic, and Structural Characterization of Reduced Cobalt(I) Cage Complex. The reversible reduction wave of the cobalt(II) complex 1 with a modestly negative E1/2 value (Figure 3) suggested the feasibility of using chemical reducing agents to access the corresponding cobalt(I) complex. Upon treating with cobaltocene (1.5 equiv), a dark red solution of 1 in either THF or CH2Cl2 immediately furnished green precipitates. This material, however, could not be dissolved in common organic solvents for further characterization. The reaction was thus carried out in the presence of n Bu4NBr as a salt metathesis reagent (Scheme 3). Crystallization from a mixture of toluene and hexanes furnished the cobalt(I) complex 5 as a dark green solid.

Figure 2. (a) X-ray structure of 1 (in two different views) as ORTEP diagrams with thermal ellipsoids at 50% probability. Two chemically equivalent but crystallographically independent molecules were identified in the asymmetric unit, for which only one is shown. Hydrogen atoms have been omitted for clarity. (b) A qualitative dorbital splitting diagram illustrating the origin of Jahn−Teller distortion observed in the solid state. The z-axis lies along the B···Co···B vector in this coordinate notation.

Scheme 3. Chemical Synthesis of 5 by One-Electron Chemical Reduction of the Cobalt(II) Precursor 1

equivalent, but crystallographically independent molecules 1A and 1B were identified in the lattice (Figure S2 in the Supporting Information). In both 1A and 1B, the metal−ligand distances of the two Co−N bonds that are syn to each other are substantially different from those of the rest (Table S1 in the Supporting Information). Such geometric properties reflect the Jahn−Teller distortion of the low-spin d7 system subjected to a trigonal prismatic ligand environment, as qualitatively described in Figure 2b. Metal-Centered and Ligand-Dependent Redox Activities. In order to probe the electronic consequences of ligand modification, the redox behavior of 1−4 was studied by cyclic voltammetry. Our previous studies on 3 and 4 focused exclusively on the oxidation chemistry of the MII/MIII redox couple,31 which overlaps with irreversible oxidation (and subsequent C−C bond coupling) of the bithiophene capping groups. In this investigation, we wished to investigate the delivery of electrons to the cage-embedded metal centers by voltage sweep in the cathodic direction. As shown in Figure 3, the cobalt(II) complexes 1 and 3 displayed clean one-electron reduction waves at E1/2 = −1.03 and −1.34 V (vs Fc/Fc+), respectively. On the other hand, the corresponding iron(II) complexes 2 and 4 showed quasi-reversible reduction processes at E1/2 = −1.53 and −1.76 V, respectively. The systematic shifts in the reduction potential across the series 1−4 thus establish that (i) for a given metal, electronwithdrawing substituents on the ligand backbone make the complex easier to reduce; and (ii) for a given ligand, the cobalt(II) complex is easier to reduce than the corresponding iron(II) analogue. As anticipated, the bithiophene units

The 1H NMR spectrum of 5 has broad features spanning a large spectral window of 0−22 ppm, which, along with the solution magnetic moment of μeff = 4.00 μB determined by the Evans method at T = 298 K, suggests the presence of a highspin (S = 1) d8 metal center.34,36,37 Single crystals of 5 suitable for X-ray crystallography were obtained by layering of hexamethyldisiloxane over a toluene solution of this material. As shown in Figure 4, the tris(dioximate) cage ligand of 5 adopts a trigonal prismatic geometry as in the precursor 1, but with narrowly distributed metal−ligand distances of 1.981(5)− 2.007(4) Å (Table S2 in the Supporting Information). Upon one-electron reduction, the average Co−N bond distance increases to 1.994(5) Å, compared with the value of 1.962(4) Å determined for the cobalt(II) precursor 1, which is 3652



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Polymer Synthesis. Electropolymerization is a straightforward synthetic method to prepare MCPs from a wide range of redox-active bifunctional monomers.28,29 Under ambient conditions, repeated potential sweeps between −0.4 V and +1.4 V (vs Ag/Ag+) on 1 in CH2Cl2 furnished amorphous thin films of poly-1 (Scheme 2 and Figure S3 in the Supporting Information), which uniformly covered the surface (Figure 6b).

Figure 4. ORTEP diagrams of (a) 5 and (b) the tris(dioximate) metal core with thermal ellipsoids at 50% probability. The bithienyl groups on the ligand are disordered over two positions, for which only one model is shown. Hydrogen atoms have been omitted for clarity.

consistent with an increasing electronic population of the antibonding MO. On the other hand, metric parameters associated with the cage ligand framework remain largely unchanged upon conversion of 1 to 5. The average CN and C−C bond distances of 5 (1.307(8) and 1.478(9) Å, respectively) are comparable to the corresponding parameters (1.297(6) and 1.466(7) Å, respectively) determined for 1, and characteristic of a closed-shell diimine functionality. Taken together, the one-electron reduction of 1 primarily involves metal-centered, rather than ligand-centered, redox activities.34,36,37 The essentially superimposable structures of 1 and 5 confirm little structural reorganization in the ligand sphere upon redox change, which is an important requirement for efficient ET (vide infra). As shown in Figure 5, the cobalt(II) precursor 1 has an intense MLCT band at λmax = 500 nm, in addition to the

Figure 6. (a) Cyclic voltammogram of poly-1 deposited on an ITOcoated glass electrode (in CH3CN with nBu4NPF6 (0.1 M); scan rate = 25 mV/s). (b) A cross-sectional FESEM image of poly-1 created by FIB milling. A layer of Pt was deposited on top to obtain a precise edge profile prior to milling.

The presence of cobalt in this material was confirmed by energy-dispersive X-ray (EDX) analysis (Figure S4 in the Supporting Information). The iron-containing polymer poly-2 was prepared in a similar fashion. As shown in Figure 6a, the cyclic voltammogram of poly-1 has (i) oxidation waves of the CoII/CoIII redox couple overlaid with the oxidative doping of the oligothiophene polymer backbone, and (ii) reduction wave at E1/2 = −0.91 V arising exclusively from the CoII/CoI redox couple. Both poly-3 and poly-4 also showed well-resolved metal-based reduction II I processes at Ered 1/2(M /M ) = −1.24 and −1.69 V, respectively (Figure S5 in the Supporting Information). While the CV II III signals around the Eox 1/2(M /M ) of the polymer-embedded metal centers are obscured by the overlapping features from the oligothiophene-based oxidation and, therefore, are less II I informative,31 the similar Ered 1/2(Co /Co ) parameters obtained for the well-defined one-electron reduction process of the monomeric (Figure 3) and the corresponding polymeric system (Figure 6a and Figure S5 in the Supporting Information) unambiguously confirmed that the cage metal complex 1 retains its structure during the electropolymerization process.

Figure 5. Electronic absorption spectra of 1 (red) and 5 (blue) in CH2Cl2 at T = 298 K.

absorption at λmax = 305 nm and a shoulder peak at λ ≈ 380 nm arising from the π−π* transitions of the bithiophene and dpg groups. As anticipated, the essentially isostructural cobalt(I) complex 5 exhibits similar ligand π−π* transitions at λmax = 320 and 400 nm. In addition, a broad (λ = 600−800 nm) and intense (ε = 8700 M−1 cm−1) red-shifted band was observed, which is characteristic of the MLCT transition from the cobalt(I) center to the imine ligand π* orbitals.34−37 3653



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Scheme 4. Charge Diffusion in n-Doped poly-1 via SelfExchange between Cobalt(II) and Cobalt(I) Centers within the Polymer

The ability to carry out electropolymerization on preassembled metal complexes without compromising their structural integrity has significant practical implications. While post-modification of preassembled polymers to introduce metal centers as part of the side chains could result in incomplete metallation, direct electropolymerization allows access to MCPs with every repeating unit carrying metal centers. Charge Injection and Transport. The CV studies shown in Figure 6a have established that poly-1 can readily be oxidized or reduced. A careful comparison between the CVs of poly-1 in cathodic (Figure 7a) and anodic (Figure 7b) scans, however,

separated by insulating organic spacers. The concentration− distance profile of the reduced (denoted as P) and neutral (denoted as R) portions of n-doped poly-1 can thus be represented by the charge dif f usion model shown in Figure 8a, which qualitatively describes the charge distribution established at a short time after applying a voltage beyond the peak potential (Ep).21 Figure 7. Scan rate-dependent cyclic voltammograms of poly-1 in CH3CN with nBu4NPF6 (0.1 M) as supporting electrolyte in (a) reductive, and (b) oxidative scans. Scan rates were varied as (i) 100 mV/s, (ii) 75 mV/s, (iii) 50 mV/s, and (iv) 25 mV/s.

revealed markedly different peak shapes. As shown in Figure 7a, poly-1 displays a broad reduction wave with a diffusion tail and large ΔEp (defined as the peak-to-peak separation; ΔEp = |Ep,a − Ep,c|) values at scan rates of v = 25−100 mV/s, with ΔEp = 120 mV determined at v = 25 mV/s. This current−voltage pattern is similar to that obtained from freely diffusing redoxactive species, and characteristic of slow and diffusion-limited charge-transport processes.32,33 This interpretation is further supported by the linear dependence of the cathodic peak current (ip,c) on the square-root of the scan rate v1/2 (Figure S6a in the Supporting Information). With the oligothiophenebased polymer backbone remaining charge neutral and serving as redox-innocent structural scaffold, the metal centers are the only redox-active components in n-doped poly-1. Considering that the physical diffusion of the metal centers that are immobilized as part of the polymer matrix should be significantly restricted, a question arises as to the origin of this electrochemical signature of an apparent dif fusion process. For poly-1 subjected to cathodic bias (Figure 7a), repetitive and directional self-exchange reactions (eq 1) occur between neighboring cobalt(II) and cobalt(I) centers to drive a “net migration” of the cobalt(I) centers away f rom the electrode, and the cobalt(II) centers toward the electrode (see Scheme 4). Accordingly, the time-dependent changes in the spatial distribution of the cobalt(I) and cobalt(II) species within thin film poly-1 is equivalent to the physical diffusion of discrete cobalt(II) and cobalt(I) species within the diffusion layer in solution-based cyclic voltammetry. For such diffusional charge transport,21,22,38,39 (i) exchange of electrons between metal centers or (ii) migration of counterions to balance the moving charge becomes the rate-limiting step. We suspect that the rate of electron injection into poly-1 is limited by the slow charge diffusion, which originates from the slow self-exchange ET reactions between metal centers that are

Figure 8. Idealized concentration−distance profiles of reactant (denoted as R; solid line) and product (denoted as P; dotted line) for (a) “very slow” and (b) “very fast” charge transport, respectively, at a short time after application of a potential beyond the peak potential in cyclic voltammetry.21 The x-axis represents the distance from the electrode surface, where x = 0 and x = d correspond to the electrode surface and the top of the polymer film, respectively.

In stark contrast to the diffusional charge transport properties observed upon n-doping, poly-1 under oxidative conditions shows sharp and symmetric oxidation waves with small ΔEp values (∼10 mV at 25 mV/s) at scan rates up to v = 100 mV/s (Figure 7b). In addition, the peak current (ip,a) is directly proportional to the scan rate v (see Figure S6b in the Supporting Information). These behaviors are consistent with surface-confined redox activities for which charge transport rates are faster than the experimental time scale.21,22 Unlike the reduction of poly-1, in which only the cobalt centers become redox active, oxidative doping of poly-1 involves both the metal centers and oligothiophene polymer backbone having similar oxidation potentials (Figure 6a). An increased number of redox-active sites that participate in electron hopping over shorter distances (Figure 1) could thus be responsible for the rapid removal of electrons from (e.g., hole injection into) poly-1. The concentration−distance profile of the oxidized (P) and neutral (R) portions of the p-doped poly-1 can be approximated by the model shown in Figure 8b. Here, an equilibrium is rapidly established with a uniform ratio between P and R across the bulk material following efficient hole injection.21 Access to both CoIII and CoI oxidation states 3654



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from the common CoII cage has thus allowed us to unveil different pathways available for the charge carriers in oxidized and reduced polymers. Spectroscopic Mapping of Electroactivities. With a better understanding of the charge transport properties obtained from the CV studies described in the previous section, we decided to investigate voltage-dependent optical properties of poly-1 using UV−vis−NIR spectroelectrochemistry. As shown in Figure 9a, the optical bands of p-doped poly-

reduction of the metal centers in the polymer. Using eq 2, the maximum absorbance of the MLCT band in this n-doped poly1 was converted to a surface coverage of ΓCo = 3.8 × 10−8 mol/ cm2 (A = absorbance of fully reduced poly-1 at λ = 700 nm; ε = 8700 M−1 cm−1 = molar absorptivity of 5 at λ = 700 nm): ΓCo =

A 1000ε

(2)

This spectroscopically determined surface coverage ΓCo is comparable to that determined by using conventional electrochemical techniques. Specifically, a total charge of 6.35 mC (after subtraction of the background signal measured using otherwise identical but unmodified ITO electrode) was required to fully reduce the same sample of poly-1 on an ITO electrode of known area (0.7 cm × 1.5 cm), which corresponds to a surface coverage of ΓCo = 6.3 × 10−8 mol/cm2. We suspect that a partial reoxidation of the electrochemically generated cobalt(I) species under ambient conditions might be responsible for the slightly large value of the electrochemically determined ΓCo value compared with the spectroscopically determined ΓCo. Electrochemical reduction of such “regenerated” cobalt(II) species should consume additional electrons to result in an overestimation of the number of redox-active metal centers, whereas spectroscopic determination of surface coverage should suffer less from such experimental artifacts under ambient conditions. Efficiency of Charge Injection and Transport. Our spectroelectrochemical studies described in the previous section have convincingly demonstrated that a well-defined MLCT transition from the reduced metal center could be used as a straightforward analytical tool to quantify the amount of electrons injected into MCP. For systems comprising metal centers homogenously dispersed within the solid matrix, such as poly-1, the number of redox-active sites (e.g., cobalt cage) should increase linearly with increasing thickness of the material. On the other hand, the efficiency of ET to the remotely located metal centers would diminish with increasing distance from the electrode surface. As such, there should exist a critical thickness for an effective delivery of charges into electrode-supported materials. A further increase in the film thickness beyond this optimal value would only result in the buildup of functionally irrelevant layers that are inherently redox-active but cannot actually take electrons from the electrode. In order to address this point, we prepared poly-1 of different film thickness by varying the number of voltage sweep cycles in electropolymerization. As shown in Figure 10a, changing the number of scan numbers from 5 to 20 resulted in a systematic increase in the film thickness of poly-1 from 182 nm to 396 nm, as determined by FIB milling and FESEM crosssectional analysis. The amount of cobalt(II) sites within the polymer that can take up electrons were quantified by measuring the intensity of the MLCT band of the n-doped material (Figure 10b), in a manner similar to that described in Figure 9b. The intensity of the MLCT band indeed increases proportionally with increasing film thickness (Figure 10c), which confirms that electrons can be injected and transported at least up to ∼400 nm through continuous self-exchange (see Scheme 4).

Figure 9. Changes in the UV−vis spectra of poly-1 as a function of applied voltage (vs Ag/Ag+; denoted in the plot) in (a) oxidative and (b) reductive scans.

1 is dominated by strong polaronic/bipolaronic transitions20,40 at longer wavelengths (λ = 600−1100 nm) from the oxidized oligothiophene unit. A voltage scan to the cathodic direction, however, produced a strong MLCT band at λ = 600−800 nm from the reduced metal center (Figure 9b). The essentially superimposable MLCT transitions observed for the chemically synthesized and structurally characterized cobalt(I) complex 5 (Figure 5) and electrochemically reduced poly-1 (Figure 9b) establish that (i) the cage repeating unit in the polymer retains its structural integrity upon reduction of the metal center, and (ii) the n-doping of MCP proceeds via outer-sphere ET process with minimal structural reorganization in the metal−ligand sphere. While the MLCT band of the cobalt(II) center (Figure 5) is obscured by the strong π−π* transitions of the organic component of the polymer (Figure 9) and thus becomes less useful as a diagnostic tool, a significantly red-shifted MLCT of the cobalt(I) center does not suffer from such interference. This fortuitous situation allowed us to directly quantify the surface coverage of the metal center (= ΓCo) in poly-1 by using the molar absorptivity (ε) at λ = 700 nm of the independently synthesized cobalt(I) complex 5 (Figure 5) as the metal site model of n-doped poly-1. A negative potential of E = −1.1 V (vs Ag/Ag+) was thus applied to poly-1 on an ITO electrode until no further increase in the intensity of the MLCT band was observed (Figure S7 in the Supporting Information), which indicated a complete

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SUMMARY AND OUTLOOK An interplay between electroactive metals and π-conjugated organic polymers gives rise to redox properties that cannot be 3655



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atmosphere (Innovative Technology SPS 400) prior to use. Cobaltocene was purified by vacuum sublimation prior to use.45 The compound 2,2′-bithiophene-5-boronic acid was prepared according to literature procedures.46 The synthesis of 3 and 4, as well as the corresponding polymers, has been reported previously.31 All airsensitive manipulations were carried out under nitrogen atmosphere in a M. Braun drybox or by standard Schlenk line techniques. Physical Measurements. 1H NMR and 13C NMR spectra were recorded on a 400 MHz Varian Inova NMR spectrometer. Chemical shifts were reported versus tetramethylsilane (TMS) and referenced to the residual solvent peaks. X-band EPR spectra were recorded on a Bruker EMX-A spectrometer with an ER041X microwave bridge. Measurements were made at T = 77 K by loading solution samples into an EPR tube and placing it inside a finger dewar filled with liquid N2. MALDI-TOF mass spectra were obtained on a Bruker Biflex III MALDI-TOF mass spectrometer. UV−vis spectra were recorded on a Varian Cary 5000 UV−vis−NIR spectrophotometer (for solution samples) or an Agilent 8453 UV−vis spectrophotometer with ChemStation (for thin film samples). Elemental analysis was performed on a Thermo FlashEA 1112 Series CHNO-S elemental analyzer running Eager Xperience software under PC control. Solution magnetic moments were determined by Evans method47 using TMS or hexamethyldisiloxane as an internal reference. Electrochemical studies were typically carried out under ambient conditions (except for the reductive voltage scans on 1−4 (Figure 3), which were conducted under an argon atmosphere) with an Autolab model PGSTA30 potentiostat (Eco Chemie). A three-electrode configuration consisting of a working electrode (glassy carbon electrode or ITO-coated glass electrode), a Ag/AgNO3 (0.01 M in MeCN with 0.1 M nBu4NPF6) reference electrode, and a platinum coil counter electrode was used. All electrochemical potentials are reported to the Cp2Fe/Cp2Fe+ redox couple, unless otherwise noted. In situ spectroelectrochemistry studies were carried out using a polymer-modified ITO-coated glass electrode, a platinum mesh counter electrode, and a Ag/AgNO3 (0.01 M in MeCN with 0.1 M n Bu4NPF6) reference electrode. Films were grown on the surface of the working electrode under ambient conditions and washed with CH2Cl2. The polymer-modified electrode was transferred to a cuvette containing a MeCN solution of nBu4NPF6 (0.1 M). The UV−vis− NIR spectra were obtained as a function of applied potential using an Agilent 8453 UV−vis spectrophotometer. Electropolymerization and Characterization. Thin films of poly-1 were typically grown under ambient condition by repeated potential sweeps (between −0.4 and +1.4 V vs Ag/Ag+; scan rate = 100 mV/s) of a solution of 1 (1 mM) in CH2Cl2 with nBu4NPF6 (0.1 M) as a supporting electrolyte. The polymer-modified electrode was washed repeatedly with CH2Cl2 prior to further characterizations. Cross-Section and EDX Analysis. The cross-section of the electrode-deposited polymer was prepared with focused ion beam (FIB), using a Zeiss Auriga CrossBeam system (Peabody, MA). Electron-beam induced Pt deposition was used to protect the polymer thin film. The Pt deposition layer appears as the brightest layer at the top of the images. In a typical experiment, 2 keV electron energy was applied to perform the Pt deposition. All cross sections were generated with a 30 kV focused Ga ion beam. The cross sections were first created with a 2 nA beam, and then polished by applying 80 pA and 50 pA beams. The final imaging was measured at 2 kV, with the Zeiss incolumn energy-selective-backscattered detector. Backscattered imaging was chosen to minimize charging effect from the glass, and to maximize the material contrast between the layers. EDX spectra were acquired at 10 kV. Co Kα lines were detected from all the samples. Tris(diphenylglyoximato)bis(bithienylboron)cobalt(II) (1). A mixture of Co(OAc)2·4H2O (0.11 g, 0.42 mmol), diphenylglyoxime (0.30 g, 1.3 mmol), and 2,2′-bithiophene-5-boronic acid (0.18 g, 0.84 mmol) in THF (25 mL) was heated at reflux for 6 h under a nitrogen atmosphere. After cooling to room temperature, the reaction mixture was filtered through a short plug of Celite, and the filtrate was concentrated under reduced pressure. The residual material was extracted with CH2Cl2. Insoluble materials were removed by filtration through a short plug of Celite, and the filtrate was concentrated under

Figure 10. (a) Cross-sectional FESEM images of poly-1 grown on ITO-coated glass electrodes. Samples were prepared using different number of voltage scans in anodic polymerization: (i) 5 scans; (ii) 10 scans; and (iii) 20 scans. In order to obtain a precise edge profile in the cross-sectional analysis, a layer of Pt was deposited on top of each sample prior to FIB milling. (b) UV−vis spectra of poly-1 of the thickness (i) 182 nm, (ii) 316 nm, and (iii) 396 nm, obtained when no further spectral change was observed after applying a voltage of E = −1.1 V (vs Ag/Ag+) to the samples prepared according to the conditions described in panel a. (c) Absorbance of the MLCT band of poly-1, as a function of film thickness.

realized by individual components alone. One research avenue in this area focuses on designing conductive hybrid materials, in which the metals and organic components have similar oxidation potentials to facilitate charge hopping. In the work presented here, such “redox-matching”5,41,42 was demonstrated by a rapid injection of holes to the cobalt-containing poly-1. Upon injection of electrons, however, the organic component of poly-1 turns redox-silent and the electroactivity is localized exclusively at the metal centers. Access to both cobalt(III) and cobalt(I) oxidation states from the common cobalt(II) cage has thus helped elucidate different pathways of charge transport available upon oxidation and reduction of the polymer. We further exploited the reversible one-electron reduction chemistry of the cobalt centers to probe the structural and functional integrity of the polymer and their charge transport properties. For this purpose, a reduced cobalt(I) cage complex was prepared and characterized as a structural model for the repeating unit of n-doped poly-1. A rich set of electrochemical and spectroscopic data obtained from this molecular system was used to analyze the data obtained from electrode-supported polymeric systems, which are otherwise difficult to interpret. The structurally well-defined nature of poly-1 and the capability to deliver electrons to its metal centers lay a firm foundation for exploring the use of this and related materials as surface-bound electrocatalytic systems. Research elsewhere has recently demonstrated that structurally related cobalt cage complexes in solution can function as electrocatalysts for the conversion of protons to H2.35,43,44 Efforts are currently underway in our laboratory to explore the utility of MCPmodified electrodes in electrocatalytic applications.

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EXPERIMENTAL SECTION

General Considerations. All reagents were obtained from commercial suppliers and used as received unless otherwise noted. The solvents CH2Cl2, THF, and MeCN were saturated with nitrogen and passed through an activated Al2O3 column under nitrogen 3656



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reduced pressure. The residual material was triturate with MeOH (30 mL), and the solid material was isolate by filtration and washed repeatedly with MeOH. The crude product was dissolved into a small amount of CH2Cl2. A portion of MeOH was added slowly until precipitates began to form. The mixture was kept at −25 °C for 3 h. Compound 1 (0.13 g, yield = 27%) was isolated by filtration as a dark red solid. MS (MALDI) for C58H40B2CoN6O6S4 [M]+: calcd 1125.14, found 1125.13. Anal. Calcd for C58H40B2CoN6O6S4: C, 61.88; H, 3.58; N, 7.46. Found: C, 62.15; H, 3.65; N, 6.97. Tris(diphenylglyoximato)bis(bithienylboron)iron(II) (2). A mixture of Fe(OAc)2 (87 mg, 0.50 mmol), diphenylglyoxime (0.36 g, 1.5 mmol), and 2,2′-bithiophene-5-boronic acid (0.21 g, 1.0 mmol) in THF (25 mL) was heated at reflux for 6 h under nitrogen atmosphere. After cooling to room temperature, the reaction mixture was filtered through a short plug of Celite, and the filtrate was concentrated under reduced pressure. The residual material was extracted with CH2Cl2. Insoluble materials were removed by filtration through a short plug of Celite, and the filtrate was concentrated under reduced pressure. The residual material was triturate with MeOH (30 mL), and the solid material was isolate by filtration and washed repeatedly with MeOH. The crude product was dissolved into a small amount of CH2Cl2. A portion of MeOH was added slowly until precipitates began to form. The mixture was kept at −25 °C for 3 h. Compound 2 (0.11 g, 20%) was isolated by filtration as a dark red solid. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.49−7.47 (d, J = 7.6 Hz, 12H), 7.37−7.31 (m, 18H), 7.16−7.15 (d, J = 4.8 Hz, 2H), 7.063−7.056 (d, J = 2.8 Hz, 2H), 7.009−7.001 (d, J = 3.2 Hz, 2H), 6.98−6.95 (t, J = 4.2 Hz, 2H), 6.84−6.83 (d, J = 3.2 Hz, 2H); 13C NMR (100 MHz, CD2Cl2, 298 K): δ 156.8, 139.3, 139.0, 138.8, 131.45, 131.38, 130.4, 130.2, 128.4, 128.2, 124.8, 124.3, 123.7. MS (MALDI) for C58H40B2FeN6O6S4 [M]+: calcd 1122.14, found 1122.13. Tetrabutylammonium Tris(diphenylglyoximato)bis(bithienylboron)cobaltate(I) (5). To a mixture of 1 (94 mg, 0.083 mmol) and nBu4NBr (31 mg, 0.096 mmol) in THF (20 mL) was added dropwise a THF (5 mL) solution of cobaltocene (22 mg, 0.12 mmol). The reaction mixture was stirred at room temperature for 4 h, and filtered through a short plug of Celite. The dark green filtrate was concentrated under reduced pressure, and the residual material was extracted with toluene (15 mL). Insoluble fractions were removed by filtration. Vapor diffusion of hexane into the filtrate afforded 5 (16 mg, 14%) as green microcrystalline material, which was isolated by filtration and washed repeatedly with hexanes. Single crystals suitable for X-ray crystal analysis were grown by layering hexamethyldisiloxane over a toluene solution of this material. Anal. Calcd for C74H76B2CoN7O6S4·2C7H8: C, 68.08; H, 5.97; N, 6.32. Found: C, 68.71; H, 6.01; N, 5.91.

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ASSOCIATED CONTENT

S Supporting Information *

Spectroscopic and electrochemical data and crystallographic information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DTRA/ARO (W911NF-07-10533).

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REFERENCES

(1) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999, 48, 123−231. 3657



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Charge Injection and Transport in Metal-Containing Conducting

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