Stepwise Solid‐Phase Synthesis and Solid‐State Electrochemistry of Redox‐Active Viologen Core/Shell‐Structured Modified Silica Materials

Abstract

Core/shell‐structured viologen‐modified Stöber silica particles with diameters of approximately 235 nm are prepared in a stepwise, solid‐phase synthesis. The nonporous and spherical silica base particles are obtained through a sol–gel process from tetraethoxysilane followed by a post‐synthetic calcination step and characterized by means of scanning electron microscopy, nitrogen adsorption/desorption experiments, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Gay‐Lussac pycnometry in water, and dynamic light scattering. Coating with triethoxysilane yields SiH‐terminated particles, which are further reacted with the linker, 11‐bromo‐1‐undecene, in a thermally induced hydrosilylation. The resulting kinetically inert and thermodynamically stable SiC bond ensures strong attachment to the silica surface. The viologen units are generated by alkylation with 1‐methyl[4,4′]bipyridinium iodide directly on the particle surface. The successful attachment is confirmed by diffuse reflectance UV/Vis, DRIFT, and 13C cross‐polarization/magic‐angle spinning nuclear magnetic resonance spectroscopy. The viologen shell of the particles is electrochemically addressable. In cyclic voltammetry experiments, under three‐phase junction conditions with modified gold and paraffin‐impregnated graphite electrodes, the immobilized viologen units are reduced to the neutral state in two consecutive electron‐transfer reactions via a mono radical cation species. The in situ electrolysis of suspended particles leads to a material with a diamagnetic core (silica) and a paramagnetic shell (viologen mono radical cations). Electron paramagnetic resonance experiments demonstrate that the radical cations are strongly attached to the particle surface. The material and electrochemical properties of the particles are compared to amorphous silica and to a viologen‐modified Stöber material based on a well‐established immobilization strategy (condensation of alkoxysilane to surface silanol groups).

Redox-Active Silica Nanoparticles. 9.1 Synthesis, Electrochemistry, and Diffusion Properties of Caged Octakis(N-ferrocenoyl-3- aminopropyl)silsesquioxane

“…Our present work explores nonporous silica nanoparticulate materials which are surface modified by redox-active molecules (see ref and earlier papers of this series). Spherical particles with sizes down to ∼50 nm have been achieved with the Stöber process, , and covalent surface attachment by both condensation (resulting in Si–O–Si attachment) and alkene hydrosilylation (resulting in Si–C attachment) ,− was successfully used to immobilize ferrocene, ,, organometallic catalysts, , and organic molecules. ,, Electrochemical data support a structural model of particles immobilized on the electrode and redox processes that include electron-hopping transport on the particle surface.…”

“…Our present work explores nonporous silica nanoparticulate materials which are surface modified by redox-active molecules (see ref and earlier papers of this series). Spherical particles with sizes down to ∼50 nm have been achieved with the Stöber process, , and covalent surface attachment by both condensation (resulting in Si–O–Si attachment) and alkene hydrosilylation (resulting in Si–C attachment) ,− was successfully used to immobilize ferrocene, ,, organometallic catalysts, , and organic molecules. ,, Electrochemical data support a structural model of particles immobilized on the electrode and redox processes that include electron-hopping transport on the particle surface. Silica particles with a redox-active shell and comparable or even smaller sizes have been characterized by the Murray and Reinhoudt groups with similar results.…”

Electrochromism of Methylviologen (Paraquat)

“…Some examples of electrochromic materials are tungsten oxide (WO 3 ) [3,4,5,6,7], polyaniline (PANI) [8,9,10,11], and viologen (Vio) [12][13][14][15][16][17].…”