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

Ghimire, Govinda, Yi, Yi, Derylo, Maksymilian A., Baker, Lane A., Ito, Takashi
Langmuir 2015 v.31 no.44 pp. 12307-12314
acetonitrile, atomic force microscopy, composite polymers, diffusivity, electrochemistry, electrodes, gold, moieties, polymerization, redox potential
This paper reports the electrochemical behavior of redox-active microdomains in thin films of ferrocene-containing diblock copolymers, polystyrene-block-poly(2-(acryloyloxy)ethyl ferrocenecarboxylate) (PS-b-PAEFc). PS-b-PAEFc with different PAEFc volume fractions (PS₁₅₄-b-PAEFc₅₁, PS₁₅₄-b-PAEFc₂₆, and PS₁₅₄-b-PAEFc₁₂, where the subscripts represent the polymerization degree of each block; fPAEFc = 0.47, 0.30, and 0.17, respectively) was synthesized by sequential atom transfer radical polymerization. PS-b-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-b-PAEFc₅₁ and PS₁₅₄-b-PAEFc₂₆ thin films but not on the surfaces of PS₁₅₄-b-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 fPAEFc and film thickness. For PS₁₅₄-b-PAEFc₅₁ and PS₁₅₄-b-PAEFc₂₆, thicker films afforded larger faradaic peak currents and exhibited diffusion-controlled voltammograms at faster sweep rates. PS₁₅₄-b-PAEFc₂₆ produced voltammograms less influenced by solvent-induced swelling than PS₁₅₄-b-PAEFc₅₁, reflecting the improved morphological stability of PAEFc microdomains by redox-inert PS frameworks. In contrast, PS₁₅₄-b-PAEFc₁₂ films yielded similar faradaic peak currents regardless of film thickness and exhibited voltammograms indicative of surface-confined species. These observations suggest that PS₁₅₄-b-PAEFc₅₁ and PS₁₅₄-b-PAEFc₂₆ films contain continuous PAEFc microdomains extending from the electrode to the surface, in contrast to the PS₁₅₄-b-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-b-PAEFc₅₁ and PS₁₅₄-b-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.