Electrochemical Characterization and Catalytic Application of Gold-Supported Ferrocene-Containing Diblock Copolymer Thin Films in Ethanol Solution

This paper reports the electrochemical behavior and catalytic property of electrode-supported thin films of polystyrene-<i>block</i>-poly­(2-(acryloyloxy)­ethyl ferrocenecarboxylate) (PS-<i>b</i>-PAEFc) in an ethanol (EtOH) solution. The electrochemical properties of PS-<i>b</i>-PAEFc films with different PAEFc volume fractions (<i>f</i>_PAEFc = 0.47, 0.30, and 0.17) in 0.1 M ethanolic sodium hexafluorophosphate (NaPF₆) were compared with those in an acetonitrile (MeCN) solution of 0.1 M tetrabutylammonium hexafluorophosphate. Pristine PS-<i>b</i>-PAEFc films did not afford significant faradaic currents in the EtOH solution because EtOH is a nonsolvent for both PS and PAEFc. However, the films could be rendered redox-active in the EtOH solution by applying potentials in the MeCN solution to induce the redox-associated incorporation of the supporting electrolytes into the films. Atomic force microscopy images verified the stability of PAEFc microdomains upon electrochemical measurements in these solutions. Cyclic voltammograms measured in the EtOH solution for PS-<i>b</i>-PAEFc with the larger <i>f</i>_PAEFc were diffusion-controlled regardless of ellipsometric film thickness (23–152 nm) at relatively slow scan rates, in contrast to those in the MeCN solution that were controlled by surface-confined redox species. The electron propagation efficiency in the EtOH solution was significantly lower than that in the MeCN solution because of the poorer swelling of the films, which limited the migration of counterions and the collisional motions of the ferrocene moieties. PS-<i>b</i>-PAEFc films were applied as electrochemically responsive heterogeneous catalysts based on the ferrocenium moieties for Michael addition reaction between methyl vinyl ketone and ethyl 2-oxocyclopentanecarboxylate (E2OC) in 0.1 M NaPF₆/EtOH. The catalytic activities of thin films were similar regardless of <i>f</i>_PAEFc, suggesting that the catalytic reaction took place for the reactants that could penetrate through the film and reach PAEFc microdomains communicable with the underlying electrode. Interestingly, the permeability of PS-<i>b</i>-PAEFc films provided a means to control the reaction selectivity, as suggested by negligible reaction of E2OC with <i>trans</i>-4-phenyl-3-buten-2-one

Electron Propagation within Redox-Active Microdomains in Thin Films of Ferrocene-Containing Diblock Copolymers

This paper reports the electrochemical behavior of redox-active microdomains in thin films of ferrocene-containing diblock copolymers, polystyrene-<i>block</i>-poly­(2-(acryl­oyloxy)­ethyl ferrocene­carboxylate) (PS-<i>b</i>-PAEFc). PS-<i>b</i>-PAEFc with different PAEFc volume fractions (PS₁₅₄-<i>b</i>-PAEFc₅₁, PS₁₅₄-<i>b</i>-PAEFc₂₆, and PS₁₅₄-<i>b</i>-PAEFc₁₂, where the subscripts represent the polymerization degree of each block; <i>f</i>_PAEFc = 0.47, 0.30, and 0.17, respectively) was synthesized by sequential atom transfer radical polymerization. PS-<i>b</i>-PAEFc films of controlled thicknesses (20–160 nm) were prepared on gold substrates via spin-coating and characterized by ellipsometry. Microdomains were observed via atomic force microscopy on the surfaces of PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆ thin films but not on the surfaces of PS₁₅₄-<i>b</i>-PAEFc₁₂ thin films. Electrochemical behavior of films was assessed by cyclic voltammetry and chronocoulometry in acetonitrile solution. The redox potential of ferrocene moieties was similar (ca. + 0.29 V vs Fc⁺/Fc) regardless of <i>f</i>_PAEFc and film thickness. For PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆, thicker films afforded larger faradaic peak currents and exhibited diffusion-controlled voltammograms at faster sweep rates. PS₁₅₄-<i>b</i>-PAEFc₂₆ produced voltammograms less influenced by solvent-induced swelling than PS₁₅₄-<i>b</i>-PAEFc₅₁, reflecting the improved morphological stability of PAEFc microdomains by redox-inert PS frameworks. In contrast, PS₁₅₄-<i>b</i>-PAEFc₁₂ films yielded similar faradaic peak currents regardless of film thickness and exhibited voltammograms indicative of surface-confined species. These observations suggest that PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆ films contain continuous PAEFc microdomains extending from the electrode to the surface, in contrast to the PS₁₅₄-<i>b</i>-PAEFc₁₂ films which contain isolated PAEFc microdomains buried within the PS matrix. Electron propagation took place only through PAEFc microdomains that could electrically communicate with the underlying electrode. Apparent diffusion coefficients within PAEFc microdomains were similar (≈ 2 × 10^–11 cm²/s) for PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆. The relatively low efficiency in electron propagation was attributable to ineffective electron self-exchange reaction within the PAEFc microdomains and/or limited counterion migration through the acetonitrile-swollen microdomains. These results provide guidance in design of redox-active metalloblock copolymers for various applications, which include electrocatalysis, electrochemical mediation in enzyme sensors, and redox-controlled molecular deposition