Preparing Multimetallic Architectures for the Control of Multiple Excited States

Lead Research Organisation: University of Oxford
Department Name: OxICFM CDT

Abstract

This project will investigate multimetallic complexes of trivalent lanthanide ions. A trivalent lanthanide ion is a 3+ ion of any of the lanthanide elements (from cerium to lutetium). A multimetallic complex is a molecular entity which contains two or more different metal atoms and a ligand framework holding the complex in place. To make well-defined multimetallic lanthanide complexes, where the molecular distribution of the different ions is known and consistent this project will use kinetically inert complexes. Kinetically inert complexes do not 'scramble' in solution i.e. once the metal is in one binding site it remains in that binding site.
The photophysical properties of the complexes will be studied; to examine how they behave when they have light shone on them. The photophysical properties of the lanthanide ions are generally dominated by their f-electrons, and f-f transitions. An f-f transition is where the ion goes from one energy state to another, which occurs solely within the f-orbitals of an ion. When the lanthanide ions have light shone on them (are illuminated) they can absorb light, undergoing a transition and moving to an excited state (a state higher in energy than the ground state, the lowest energy state, it is in this state that the complex usually exists). According to the selection rules which govern how molecules absorb and emit light these f-f transitions have a very low probability of occurring, thus, the ions do not absorb well. It is common to sensitise a complex, by appending a group which does absorb light well. Sensitisation is the name given to building in another part of the molecule which can absorb light well. Once this part has absorbed the light it goes from its ground state (where it exists normally) to an electronically excited state. An excited state is a name given to any state of a molecule which is higher in energy than the ground state. The excited state can transfer its energy to the lanthanide ion, which is then moved to its excited state, whilst the other part is returned to its ground state.
Whilst energy transfer from a sensitising molecule is fairly well understood, what is not well understood is energy transfer between lanthanide ions in a molecular system. This project wants to look at how the lanthanide ions transfer energy between each other in multimetallic complexes.
This project will also look at what happens when a complex contains two lanthanides in their excited state. The lifetime of a molecule that emits light is the average amount of time the molecule exists in the excited state. For organic species, such as the ligands used to sensitise lanthanide emission, this is commonly on the order of 10s of nanoseconds (ns), one nanosecond is one billionth of a second. For the lanthanide ions this can be thousands to millions of times longer. The lifetimes are, comparatively, long for the same reason that the ions do not absorb light well, the f-f transitions have a very low probability of occurring.
By exploiting the longer lifetimes of lanthanide ions, and that the sensitising ligand returns to the ground state after it has transferred its energy, we envision that we can go through the process a second time, exciting a second lanthanide ion, before the first has emitted light (decayed). The processes required for sensitisation are typically faster than the average lifetimes of several lanthanide ions, thus, this is feasible. To the best of our knowledge this has not been attempted before.
This project falls within the EPSRC synthetic coordination chemistry research area.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/S023828/1 01/04/2019 30/09/2027
2329371 Studentship EP/S023828/1 01/10/2019 30/09/2023 Cameron Gray