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Title: Understanding electronic energy transport in biologically relevant systems : the photochemistry of sunscreens and the photophysics of photosynthesis
Author: Baker, Lewis A.
ISNI:       0000 0004 6496 5119
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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This thesis focusses on two areas. The first is elucidating the ultrafast photoprotective mechanisms exhibited by a number of commercial and natural sunscreening agents, through the use of femtosecond pump-probe transient absorption spectroscopy, coupled with ab initio electronic structure calculations. The second is understanding the electronic energy transport properties of pigment-protein complexes found in photosynthetic organisms, through the use of quantum dynamics simulations. Oxybenzone, titanium dioxide, octocrylene and ethylhexyl triazone are all studied given their prevalence in commercial sunscreen products. We deduce that oxybenzone relaxes through an enol-keto isomerism (~400 fs) followed by back-isomerisation commensurate with vibration energy transfer to the surrounding solvent (~5-8 ps). Titanium dioxide is then considered in multicomponent suspensions with oxybenzone, where we find that the photodynamics exhibited by each component can be considered independent from one another. Octocrylene is shown to undergo the majority of its photodynamics within ~2 ps, displaying remarkable efficiency as an ultraviolet light chromophore, relaxing through nonradiative internal conversion pathways. Studies of ethylhexyl triazone are presented, where results suggest this molecule relaxes through a number of ultrafast processes, ranging from ~400 fs, ~20 ps and ~200 ps, involving a large change in nuclear geometry, which couples excited states to the ground state through a conical intersection. Sinapoyl malate is the predominant sunscreening agent synthesised in arabidopsis thaliana which is deposited into the upper epidermis of its leaves. This molecule, along with its biological precursor sinapic acid, is shown to relax through ultrafast pathways (~10-30 ps), which we suggest is mediated by a trans-cis isomerism, in stark contrast to the recent time-resolved gas-phase measurements indicating the solvent environment alters the photodynamics significantly. Considering the second half of this thesis, we study the Fenna-Matthews-Olson pigment-protein complex found in green sulphur bacteria, and the light-harvesting complex II found in the spinach plant. Employing a simple quantum master equation, the Haken-Strobl model, we highlight a computationally tractable approach for describing these large, complicated systems. To this end, we perform an enormous array of simulations which include a simple description of environmental perturbations and find, for the first time, the full extent of the robustness of these pigment-protein complexes. Most strikingly, for the Fenna-Matthews-Olson complex, we find that up to 50% of the available pigments may be removed, with a small drop of 20% in electronic energy transport, displaying an incredible robustness to network disruption.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Q Science (General) ; QC Physics ; QD Chemistry