Metal Complexes of Radical Ligands – Electron Hopping Leading to Novel Optical Properties

Summary

This project involves the synthesis, electrochemistry and host: guest chemistry of metal complexes of redox-active (non-innocent) ligands. An example is the dioxolene ligands, which can readily cycle between the catechol, semiquinone and quinone oxidation states. Such processes allow electrons to shuttle between the metal and organic parts of a dioxolene complex molecule, giving them intensely dark colours that extend into the infra-red. Alternatively, with appropriate metal/dioxolene combinations similar electron rearrangements can take place abruptly under the action of a physical stimulus, leading to thermochromic or photochromic crystalline materials.

We have been studying novel mixed-valent radical compounds supported by ligands containing multiple dioxolene rings. These are often stable under ambient conditions, strongly coloured and often exhibit intense near-infra-red absorptions. The intense colours arise from hopping of unpaired electrons around the poly-dioxolene ligand scaffold, between the part-oxidised aromatic rings. Analysis of these near-IR spectra defines the mechanism of this electron hopping, which we can turn on or off depending on the ligand design. An example of this is in Chemical Communications 2019, 55, 2281.

We now want to apply the same principles to make electrochromic framework crystals, and molecular cage compounds, from similar components. The radical oxidation products have intense colours, with strong absorptions in the visible and near-IR regions. Near-IR absorbers like these, whose absorptions can be switched on and off electronically, can be very useful in fibre-optic communications devices.

The electron-hopping process means the frameworks should also show semiconducting properties or even metallic conduction. If the frameworks can be made sufficiently porous, they could also serve as green oxidation catalysts for alcohol or aldehyde vapours, or as components in sensor devices for some small molecule gases like the nitrogen oxides. All these applications hinge on the unique properties of electron mobility around the metal/dioxolene scaffold used to construct these materials.

This project involves organic and inorganic synthesis, crystallography, electrochemistry and other techniques for studying radical products (EPR, UV/vis/NIR and IR spectroscopies, magnetic measurements and DF calculations). Some of the dioxolene ligands we use require multi-step syntheses, so a PhD on this topic will involve a significant amount of organic ligand synthesis.

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