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Towards Hierarchical Structural Self-Assembly of Conjugated Polyelectrolyte Light Harvesting Complexes

  • Author(s): Segura, Carmen Julia
  • Advisor(s): Ayzner, Alexander L
  • et al.
Abstract

The dramatic decrease in the world’s finite reserve of fossil fuels has led to an increase

demand for alternative energy resources. Inspired by natural photosystems that were

fine-tuned by nature through billion year of evolution. The focus of this research aims

to mimic and improve upon nature's approach of capturing solar energy by constructing

an artificial photosystem. The method of approach was done by creating 'soft

supramolecular complexes' based on conjugated polyelectrolyte (CPE) complexes as

analogs to nature's light harvesting machinery. Conjugated polyelectrolytes (CPEs) are

a class of quasi-1D structures whose delocalization of pi-electrons through their

backbone enables the motion and transport of electrical charge and excitation energy;

In addition, they bear ionic sides chains per repeating monomer unit that enable these

non-polar macromolecules to be water soluble and facilitate their microstructural selfassembly.

In order to construct efficient 'light harvesting complexes' it is important to

understand the physics that drive the self-assembly and microstructure of the CPE

complexes and how this relates to their electronic energy transfer (EET) dynamics

between the donor and acceptor units of the CPE complexes. The initial phase of this

work focused on constructing 'light harvesting complexes' centered on pairing

oppositely charged and electronically coupled CPEs PFPI-donor and PTAK-acceptor.

Results from this initial investigation showed evidence of EET between the donorxi

acceptor complexes as well as the ability to tune the EET dynamics of the system by

varying the molar charge composition of the CPEs. The design of projects forward led

to a natural trajectory focused on building on the complexity by first incorporation of

ionic surfactants, mixed micelle systems with varying charge densities and finally the

templating of donor-acceptor CPEs as a layer-by-layer assembly onto a charged

liposome scaffold. The local micro and macrostructural morphology of the CPE

complexes was characterized using a combination of Dynamic light scattering (DLS),

small x-ray scattering (SAXS) and Confocal Laser Scanning microscopy (CLSM)

techniques. The use of scattering techniques and light microscopy are complementary

to one another since they access different length scales from nanometer details to the

visualization of CPE-liposome micron sized structures. These structural techniques

combined with optical techniques provided insight to the relation of morphology and

photo-physical properties of these CPEC-Surfactant/Lipid interactions. The results of

this dissertation demonstrate the capability to ironically assemble a stable

multicomponent modular light harvesting system. The influence of surface charge,

charge density of amphiphilic systems had on both the electronic structure and

microstructure of CPE was deeply explored. It was found that through careful choice,

ternary molecules and self-assembled structures such as surfactants and liposomes can

be used to manipulate the electronic structure and thus the energy transfer dynamics of

higher order CPE systems. This research forms a basis for the creation of a soft, lightweight

artificial photosystem with the potential of converting sunlight into chemical

fuels.

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