A Spectroscopic Investigation of Luminescent Cr(III) Complexes

Photochemistry, in which visible or ultraviolet light is used as a catalyst to promote the formation of new chemical species with distinct chemical properties, is of growing importance for molecular synthesis, therapeutic diagnostics and new technological applications such as light emitting diodes. Absorption of light energy frequently involves the creation of paramagnetic excited states or charge carriers that play vital roles in electron transfer events, displaying significantly different chemistry to ground state processes. However, these species often evade detection due to their incredibly short lifetimes.

In this project we will apply TR-EPR methods to yield accurate resolution of kinetics, molecular profiles, and structural models which cannot be accessed under steady-state conditions due to rapid relaxation processes. TR-EPR will provide fundamental knowledge of charge transport mechanisms required for the optimization of efficient photosynthetic systems and devices. This research is directly relevant to EPSRC Strategic Priority for Photonic Materials and the related Materials for Energy Applications themes.

This project will provide a Ph.D. training regimen in advanced spectroscopy and chemical synthesis, in a collaborative nature between an experienced project team. Two areas of initial research activity will focus on (i) Photon Upconversion by Ir(III) Complexes, and (ii) Sustainable sensitizers for dye-sensitized semiconductor solar cells. Both research areas will yield significant advances within the project timeframe.

Photon upconversion has received significant attention as a means of increasing the efficacy of light harvesting processes, photoredox catalysis and for bioimaging agents. The Co-I has recently demonstrated leading conversion efficiencies using novel Ir(III)-based donor:acceptor systems through triplet-triplet upconversion (Chem. Eur. J. (2018), DOI:10.1002/chem.201801007). TR-EPR is ideally suited to fully disentangle the spin interactions of the photoexcited system (e.g. transient organic triplets, spin-orbit coupling parameters), results guiding rational design of donor:acceptor pairs for maximum upconversion and selective emission wavelengths.

Photoexcitation of metal oxide semiconductors creates charge carriers that can be utilised in surface redox processes and in electrical thin-film devices. The PI has recently provided unique insights into this area by utilising EPR to identify intrinsic/dopant oxidation state changes during photoirradiation. The development of copper-based sensitizers (by the Co-I) will be explored to maximise photoexcitation profiles and optimise charge-transfer processes providing a sustainable, low-cost route towards solar cells of the future.