Multi-stimuli Photo and Redox-active Nanostructured Mesoporous Silica Films on Transparent Electrodes

Abstract

Silica matrices hosting transition metal guest complexes may offer remarkable platforms for the development of advanced functional devices. We report here the elaboration of ordered and vertically oriented mesoporous silica thin films containing covalently attached tris(bipyridine)iron derivatives using a combination of electrochemically assisted self-assembly (EASA) method and Huisgen cycloaddition reaction. Such a versatile approach is primarily used to bind nitrogen-based chelating ligands such as (4-[(2-propyn-1-yloxy)]4'-methyl-2,2'-bypiridine, bpy') inside the nanochannels. Further derivatization of the bpy'-functionalized silica thin films is then achieved via a subsequent in-situ complexation step to generate [Fe(bpy)₂ (bpy')]²⁺ inside the mesopore channels. After giving spectroscopic evidences for the presence of such complexes in the functionalized film, electrochemistry is used to transform the confined diamagnetic (S=0) ${\left({Fe}_{LS},{\left(b,p,y\right)}_2,\left(b,p,y^{\text{'}}\right)\right)}^{2+}$ species to paramagnetic (S=1/2) oxidized ${\left({Fe}_{LS},{\left(b,p,y\right)}_2,\left(b,p,y^{\text{'}}\right)\right)}^{3+}$ species in a reversible way, while blue light irradiation (λ=470 nm) enables populating the short-lived paramagnetic (S=2) ${\left({Fe}_{HS},{\left(b,p,y\right)}_2,\left(b,p,y^{\text{'}}\right)\right)}^{2+}$ excited state. [Fe(bpy)₂ (bpy')]²⁺ -functionalized ordered films are therefore both electro- and photo-active through the manipulation of the oxidation state and spin state of the confined complexes, paving the way for their integration in optoelectronic devices.

Keywords: electrochemistry; mesoporous silica; photo-switching; redox-switching; thin films.