title: Ultrafast Transient Absorption Spectroscopy of Light Harvesting Materials
Creators: name: Thwaites, Owen
Publisher: name: University of Liverpool Repository
Year: 2024

description: The need to reduce the reliance on highly polluting fossil fuels is well recognised. One
method of enabling a more sustainable energy mix is by harvesting the sun’s energy. Many
different methods of harvesting solar energy have been investigated, however further work
is required to optimise these methods. Understanding the photodynamics of light harvesting
materials is the first step in developing new, and improved, materials. In this
thesis, transient absorption spectroscopy is used to probe two well studied light harvesting
systems; the biological reaction centre-light harvester 1 supercomplex of the Rhodobacter
sphaeroides purple bacteria and the inorganic metal chalcogenide antimony selenide,
which is a promising light harvesting material for photovoltaic and photo-electrochemical
devices.
Rhodobacter sphaeroides has naturally evolved to: i) collect solar energy, ii) funnel energy
to catalyst centres and iii) use harvested energy to generate chemical energy, a key
contributor to steps i) and ii) is the reaction centre-light harvester 1 supercomplex. My
work provides a greater understanding of the importance of protein structure and excitonic
states on energy transfer within natural light harvesting systems. Enhancing the
understanding of natural diversity and evolution within photosynthetic systems will help
enable the construction of biohybrid devices and bio-inspired light harvesting devices. The
first study in this thesis examines the influence of transmembrane polypeptides on the
excitation energy transfer within reaction centre-light harvester 1 supercomplexes. The
structure of these supercomplexes is well understood but prior to the studies in this thesis,
there was a limited understanding of the effect of structural changes on the ultrafast
photodynamics. By performing transient absorption spectroscopy of modified reaction
centre-light harvester 1 supercomplexes in which the transmembrane polypeptides PufX
and PufY have been removed, I find that PufX plays a crucial role in excitation energy
transfer kinetics. While despite PufY removal having a significant effect on structure,
there is minimal effect on light harvester 1 → reaction centre excitation energy transfer. 
A further study is performed to investigate the effect of pump excitation energy on the excitation
energy transfer between the light harvester 1 and the reaction centre. In this study
an order of magnitude faster light harvester 1 → reaction centre excitation energy transfer
is found at higher pump excitation energies. To elucidate information about this accelerated
excitation energy transfer, the photodynamic process that occur within the first
10 ps are assigned to annihilation within a single multiexcited reaction centre-light harvester
1 supercomplex that define as intra(light harvester 1 exciton-exciton annihilation)
and annihilation after excitation energy transfer between neighbouring light harvester 1
rings within aggregations of reaction centre-light harvester 1 supercomplexes inter(light
harvester 1 exciton-exciton annihilation). The excess energy following annihilation leads
to an accelerated excitation energy transfer from light harvester 1 to reaction centre.
This work unravels the complex photophysics of this complicated biological system and
highlights how excitation energy transfer can be accelerated by perturbing the donor or
acceptor excitonic state.
A different strand to this thesis is an investigation of the ultrafast charge carrier processes
within antimony selenide. A material of interest to the photovoltaic community for
use as a light absorber in multilayered thin film devices. The studies performed in this
thesis highlight that the reflectance of antimony selenide thin films can not be treated
as negligible and has to be taken into account when attempting to understand the ultrafast
charge carrier properties. This is done by combining the changes in transmission
and reflection to calculate the change in absorbance enabling a greater understanding of
the charge carrier properties upon excitation. Additionally, the oscillations in the early
time transient absorption spectra of antimony selenide, caused by coherent phonons, are
investigated to enable the analysis of frequency modes which are non-trivial to observe
with even advanced Raman spectroscopy apparatus.