Hybrid Synthetic Biological-Polaritonic Light Harvesting Systems

It has been observed that when plasmon modes are coupled to light-harvesting complexes from plants and bacteria, they are split to yield hybrid light-matter states called plexcitons. Modelling of the spectra suggests that the protein complex serves as a scaffold on which the transition dipole moments are organised within the plasmon mode volume. The objective of this thesis is to determine whether these effects can be replicated using synthetic scaffolds for this organisation. The simplest such system consists of a monolayer of bacteriochlorins adsorbed at a gold surface, a system which is structurally close to the arrangement found in chlorosomes, the antenna complexes of green sulfur bacteria. A synthetic bacteriochlorin was prepared with thioacetate anchoring groups to form SAMs on gold films and nanostructures. Splitting of the plasmon mode was observed, but the coupling energy obtained was ~ 0.12 eV, short of the threshold for strong coupling (~ 0.24 eV). Analysis of the system by X-ray photoelectron spectroscopy suggests a surface density of ~ 3.7 x 10-17 m^-2, which may be below the limit for strong coupling of these molecules. Poly(cysteine methacrylate) brushes were also used as scaffolds. Chlorophyll a, purified from spinach, was derivatised by modification of the phytyl ester to give a succinimidyl active ester, which was then attached to the poly(CysMA) brushes. Strong coupling, leading to a pronounced splitting in the plasmon mode approach a coupling energy of ~ 0.5 eV, was observed as a function of decreasing density of the polymer brush scaffold, allowing more dipoles to derivatise the brush layer. Polymer brush-pigment hybrid materials therefore may be attractive candidates for further development of hybrid biological polaritonic light-harvesting systems.