Linker-Based Control of Electron Propagation through Ferrocene Moieties Covalently Anchored onto Insulator-Based Nanopores Derived from a Polystyrene–Poly(methylmethacrylate) Diblock Copolymer

Abstract

This paper reports the effects of linker length on electron propagation through ferrocene moieties covalently anchored onto insulator-based cylindrical nanopores derived from a cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymer. These nanopores (24 nm in diameter, 30 nm long) aligned perpendicular to an underlying gold electrode were modified via esterification of their surface COOH groups with OH-terminated ferrocene derivatives having different alkyl linkers (FcCO(CH(2))(n)OH; n = 2, 5, 15). Cyclic voltammograms were measured in 0.1 M NaBF(4) at different scan rates to assess the efficiency of electron propagation through the ferrocene moieties. The redox peaks of the anchored ferrocenes were observed at nanoporous films decorated with FcCO(CH(2))(15)OH and FcCO(CH(2))(5)OH, but not at those with FcCO(CH(2))(2)OH. Importantly, the higher electron propagation efficiency was observed in the use of the longer linker, as shown by the apparent diffusion coefficients (ca. 10(-12) cm(2)/s for n = 15; ca. 10(-13) cm(2)/s for n = 5; no electron propagation for n = 2). The observed electron propagation resulted from electron hopping across relatively large spacing that was controlled by the motion of anchored redox sites (bounded diffusion). The longer linker led to the larger physical displacement range of anchored ferrocene moieties, facilitating the approach of the adjacent ferrocene moieties within a distance required for electron self-exchange reaction. The linker-based control of redox-involved electron propagation on nanostructured, insulating surfaces will provide a means for designing novel molecular electronics and electrochemical sensors.

Block Copolymer‐Derived Monolithic Polymer Films and Membranes Comprising Self‐Organized Cylindrical Nanopores for Chemical Sensing and Separations

“…[83] These studies showed the significant influence of microdomain morphologies on electron propagation efficiency [82] and their applicability as a mediator for glucose oxidase. [83] PS-b-PMMA-derived cylindrical nanopores covalently decorated with ferrocene moieties were used to investigate electron propagation based on bounded diffusion, [84] which was an electron self-exchange reaction between adjacent redox-active moieties covalently tethered to an immobile framework. [85] OH-terminated ferrocene derivatives with different alkyl linker lengths (Figure 3 b) were tethered to the nanopores through esterification with the surface À COOH groups.…”

Electron Hopping through Redox Moieties Anchored to Well‐Defined Nanostructures

“…As expected from the theoretical study, [21] we observed more efficient electron hopping through ferrocenes anchored with longer linkers. [4] This observation implies that the electron hopping is controlled by the dynamic properties of the redox moieties (Figure 2, left) that reflect the linker properties such as length and flexibility. Currently, we are further exploring the fundamental characteristics of electron hopping through nanopore-anchored redox moieties.…”

“…[4,5] These studies aim to quantitatively understand parameters that govern the efficiency of charge transport based on electron hopping through redox moieties confined within nanoscale structures. Fundamental knowledge obtained will directly aid in engineering redox-active mediator layers for enzyme-based electrochemical sensors and for electrocatalysis, and may also provide valuable insight into long-range electron transfer in biological systems and organic electronics.…”

“…[19] More importantly, his suggestion led to electrochemical studies of redox moieties covalently anchored to the nanopore surface (Figure 1). [4,5] These moieties cannot physically diffuse to the underlying electrode, and thus can propagate electrons based only on electron hopping, an electron self-exchange reaction between adjacent redox moieties in sufficiently close proximity. Electron hopping was extensively studied in the 1970s-1990s to understand the charge-transport characteristics of redox polymers.…”

“…[4] First, the effective spacing between adjacent ferrocene moieties, which was defined by the density of the surface -COOH groups, was larger (ca. 1.2 nm) than the size of ferrocene (ca.…”