Our current research interests and directions include:

I. Unravelling the primary photo-protection pathways of DNA

(a) Schematic DNA double helix with colour-coded bases. The shaded blue area represents the initial delocalisation of the excited state upon absorption of a UV photon. (b) The delocalisation collapses onto a single base pair (shaded in pink), where charge-transfer can be initiated. (c) The ensuing charge transfer induces bond scission on the backbone.

The ultraviolet component of the solar spectrum is capable of inducing damage in DNA, the genetic code of life essential for cell growth, development and function.(1) I will explore the primary photo-protection mechanisms of DNA by determining the extent of excited state delocalisation in model DNA systems, and how this prepares molecules for fast non-radiative relaxation. If excited states live for picoseconds, charge-transfer states can be accessed, generating electrons and holes that can destroy the genetic code via oxidative damage. If charge-transfer is initiated, how far do the ensuing electron and hole particles migrate? Mechanistic insights are essential understand whether an incoherent sequential hopping mechanism dominates charge-separation, or whether super-exchange plays a role. To address these questions, we must understand the how the intricately coupled electronic-nuclear landscape protects the genetic source-code of life. This will be explored using established multidimensional ultrafast optical spectroscopies (2, 3) and those I have pioneered. (4, 5)

II. Elucidating excitation transport mechanisms in photo-voltaic materials

Illustrative exciton transport / dissociation pathway inside a bulk heterojunction thin film

With increasing global energy demands, the efficient capture and storage of solar energy using low-cost, reliable photovoltaics (PVs) is of paramount importance to society. Two-dimensional electronic spectroscopy (2DES) (3, 6) will be used to gain mechanistic insights into the first step in PV photocurrent generation: exciton dissociation in thin film organic and hybrid inorganic-organic PVs. The role of coherent electronic or vibronic coupling between donor and acceptor organic semiconductors has only recently been considered, (7) but studies remain inconclusive and further investigation is required.

The question of whether the initially photo-excited states of solid-state inorganic-organic PVs such as lead halide perovskites and Cu₂ZnSnS₄ (CZTS) is excitonic in nature, or a semi-conductor, i.e. immediately free charge-carriers will be probed with temperature dependent 2DES. The reason for low carrier motilities will be characterised for CZTS, with the aim of identifying specific trap morphologies

III. Exploring Delocalised Energy Transport in Bacterial Reaction Centres

X-ray crystal structure of the bacterial reaction centre.

Reaction centres (RCs) play a pivotal role in photosynthesis, accepting energy from peripheral light harvesting antenna using it to induce charge-separation into electrons (and holes) with > 90% quantum efficiency. The electrons are used to drive essential downstream processes such as carbon fixation and carbohydrate synthesis. Despite extensive study by researchers from across the globe, the precise electronic structure of bacterial reactions centres remains unclear. 2D electronic-vibrational spectroscopy (4) will be used to elucidate the electronic coupling constants between the bacteriochlorophyll-a (B/P), bacteriophephytin-a (H), and carotenoid pigments. These parameters determine whether the RC electronic excited states can be described as localised transitions on individual chromophores (sites) or delocalised over multiple pigments (excitons).

IV. Nanoscale design principles of natural and artificial light harvesting systems

Purple bacteria grown in high and low light conditions reveal a difference in the ratio of LH1 (large rings): LH2 (small rings). From: F. Fassioli et al., Biophys. J. 97 2464 (2007).

The general macroscopic principles that underlie the near unity efficiency of natural light harvesting are becoming more apparent, (8) and provide inspiration for artificial solar devices. To date, these studies have limited spatial resolutions (typically > 20 μm), that limits our understanding of how the spatial arrangement of molecules on a nanometer length scales connects to their energetic and temporal landscapes. To counter this, the Oliver group will develop techniques that couple the advantages of multidimensional optical spectroscopies to confocal or super-resolution microscopies, revealing the influence of the intermolecular spatial heterogeneity on nanometre length scales. These experiments will allow for unprecedented insights on relevant molecular levels, and access information such as the morphologies associated with the most-efficient exciton-dissociation in photovoltaic materials, or microscopic inhomogeneity in natural light harvesting systems that likely underlies the very efficient macroscopic energy capture and transfer in higher plants. 

This research is funded by the Royal Society via a University Research Fellowship and by the Engineering and Physical Sciences Research Council.

References

1. Kohler B (2010) Nonradiative Decay Mechanisms in DNA Model Systems. J Phys Chem Lett 1(13):2047–2053.
2. Brixner T, Mancal T, Stiopkin IV, Fleming GR (2004) Phase-stabilized two-dimensional electronic spectroscopy. J Chem Phys 121(9):4221–4236.
3. Huxter VM, Oliver TAA, Budker D, Fleming GR (2013) Vibrational and electronic dynamics of nitrogen–vacancy centres in diamond revealed by two-dimensional ultrafast spectroscopy. Nat Phys 9(11):744–749.
4. Oliver TAA, Lewis NHC, Fleming GR (2014) Correlating the motion of electrons and nuclei with two-dimensional electronic-vibrational spectroscopy. Proc Natl Acad Sci USA 111(28):10061–10066.
5. Oliver TAA, Fleming GR (2015) Following Coupled Electronic-Nuclear Motion through Conical Intersections in the Ultrafast Relaxation of β-Apo-8′-carotenal. J Phys Chem B 119:11428–11441.
6. Cho M (2008) Coherent two-dimensional optical spectroscopy. Chem Rev 108(4):1331–1418.
7. Song Y, Clafton SN, Pensack RD, Kee TW, Scholes GD (2014) Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer–fullerene blends. Nat Commun 5:4933.
8. Blankenship RE (2014) Molecular Mechanisms of Photosynthesis (John Wiley and Sons, Oxford).