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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>-polyÂ(2-(acrylÂoyloxy)Âethyl
ferroceneÂcarboxylate) (PS-<i>b</i>-PAEFc). PS-<i>b</i>-PAEFc with different PAEFc volume fractions (PS<sub>154</sub>-<i>b</i>-PAEFc<sub>51</sub>, PS<sub>154</sub>-<i>b</i>-PAEFc<sub>26</sub>, and PS<sub>154</sub>-<i>b</i>-PAEFc<sub>12</sub>, where the subscripts represent the polymerization
degree of each block; <i>f</i><sub>PAEFc</sub> = 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 PS<sub>154</sub>-<i>b</i>-PAEFc<sub>51</sub> and PS<sub>154</sub>-<i>b</i>-PAEFc<sub>26</sub> thin films but not on the surfaces
of PS<sub>154</sub>-<i>b</i>-PAEFc<sub>12</sub> 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<sup>+</sup>/Fc)
regardless of <i>f</i><sub>PAEFc</sub> and film thickness.
For PS<sub>154</sub>-<i>b</i>-PAEFc<sub>51</sub> and PS<sub>154</sub>-<i>b</i>-PAEFc<sub>26</sub>, thicker films afforded
larger faradaic peak currents and exhibited diffusion-controlled voltammograms
at faster sweep rates. PS<sub>154</sub>-<i>b</i>-PAEFc<sub>26</sub> produced voltammograms less influenced by solvent-induced
swelling than PS<sub>154</sub>-<i>b</i>-PAEFc<sub>51</sub>, reflecting the improved morphological stability of PAEFc microdomains
by redox-inert PS frameworks. In contrast, PS<sub>154</sub>-<i>b</i>-PAEFc<sub>12</sub> films yielded similar faradaic peak
currents regardless of film thickness and exhibited voltammograms
indicative of surface-confined species. These observations suggest
that PS<sub>154</sub>-<i>b</i>-PAEFc<sub>51</sub> and PS<sub>154</sub>-<i>b</i>-PAEFc<sub>26</sub> films contain continuous
PAEFc microdomains extending from the electrode to the surface, in
contrast to the PS<sub>154</sub>-<i>b</i>-PAEFc<sub>12</sub> 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<sup>–11</sup> cm<sup>2</sup>/s) for PS<sub>154</sub>-<i>b</i>-PAEFc<sub>51</sub> and PS<sub>154</sub>-<i>b</i>-PAEFc<sub>26</sub>. 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