In the last twenty years, much effort has been directed into finding methods to ensure the efficient transmission of energy across long distances in molecular photonic materials. Exciton diffusion lengths in molecular systems are typically of the order of 10 nm, thus constraining the design of various devices. The photosynthetic light-harvesting complexes (LHCs) found in plants and bacteria have been the subject of considerable interest due to their potential role for efficient exciton transfer in molecular systems. Peptide units in these pigment-protein complexes organise chlorophyll and carotenoid molecules into specific and unique arrangements. However, the high cost of large-scale manufacturing and the natural susceptibility of the protein to damage during processing makes light-harvesting complexes unsuitable for utilisation as photonic materials. A recent study demonstrated that surface-grafted poly (amino acid methacrylate) brushes can be used to design biomimetic pigment-polymer complexes, enabling strong coupling of excitons to confine optical modes with an efficiency exceeding that found for pigment-protein complexes in photosynthesis.
This thesis explores the possibility of designing synthetic photonic materials inspired by natural light-harvesting complexes to facilitate the efficient transport of excitons across extended distances. In particular, the utilisation of poly (2-dimethylamino ethyl methacrylate) (PDMA) scaffolds formed by atom transfer radical polymerisation (ATRP) to support chlorophyll a (Chl a) molecules is examined. The hypothesis that tertiary amine pendant groups on the PDMA could coordinate with the metal ion at the core of the Chl a tetrapyrrole ring was examined. Chl a was isolated from spinach and incubated with PDMA grown to a thickness of ~30 nm from BIBB-APTES initiator surfaces. The binding of chlorophyll to PDMA scaffolds was investigated using X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopy (UV-vis). Binding curves were measured for the attachment of Chl a to PDMA scaffolds. Chl a concentrations in PDMA scaffolds were found to be as high as ~2M, ca. 3 times the concentration found in biological light-harvesting complexes. By partially quaternising the PDMA brush, it is possible to control the density of chlorophyll, which provides potential for future utilisation in biologically inspired energy transfer systems.
The findings provide strong evidence in support of the hypothesis that Chl a forms a bond with PDMA brushes by coordinating the tertiary amines in the pendant groups with the Chl a metal centre. It was hypothesised that by incorporating strong plasmon-exciton coupling, it would be possible to achieve long-range energy transfer in these films. Through the plasmon mode, coherent energy exchange can in principle occur between spatially separated pigments in the strong coupling regime. Gold surfaces were functionalised by the adsorption of thiols with bromine initiators at their tail ends. PDMA was grown from the initiator functionalised planar gold surfaces and characterised using XPS, atomic force microscopy (AFM), and spectroscopic ellipsometry (SE) techniques. When grown under the same conditions as used to grow PDMA brushes from BIBB-APTES, including a temperature of 90 °C, PDMA layers grown on gold were found to be very thin, attributed to the poor stability of the thiol initiator layer at elevated temperatures. However, on reduction of the temperature to 50 ºC, stable polymer layers were formed, albeit with slightly reduced thickness because of slower kinetics at the lower temperature. For fully dense brush layers, chlorophyll binding was found to be sterically inhibited. This, mixed self-assembled monolayers consisting of DTBU and MUL were formed to reduce the grafting density. Using this approach, it was found that larger concentrations of chlorophyll could be achieved in the brush layer. XPS data indicated that the best results were obtained for an initiator mole fraction in the thiol film, ꭓBr (Au), of 0.24, which facilitated a fourfold increase in the amount of bound Chl a.
The incorporation of plasmonic materials into PDMA brushes was investigated as an alternative approach to creating pigment-polymer antennas on gold surfaces. However, the incorporation of gold nanoparticles of different sizes did not yield strong plasmon-exciton coupling. This was mainly due to the difference in energy between the localised surface plasmon resonance (LSPR) of the nanoparticles and excitons in Chl a. Further exploration involving hollow gold nanoparticles (HGN) and silver nanoparticles (AgNPs) also did not achieve the desired coupling.
