Quantum-optical brushes: active control of strong plasmon-exciton coupling in programmable biomimetic pigment-polymer antenna complexes grown by surface-initiated polymerisation from gold nanostructures

Theory suggests that exciton diffusion lengths in molecular materials could be enhanced by several orders of magnitude in the strong light-matter coupling regime. Photosynthetic light-harvesting complexes (LHCs) from plants and bacteria are strongly coupled to localised surface plasmon resonances (LSPRs) in gold nanostructure arrays, yielding delocalised states (plexcitons) that mix the properties of light and matter. However, proteins are not suitable for putative applications of molecular photonic materials. Here we demonstrate the fabrication of programmable, biomimetic plexcitonic antenna complexes, in which poly(amino acid methacrylate) scaffolds grown by atom-transfer radical polymerisation organise chlorophylls within LSPRs to achieve strong light-matter coupling. Modelling the system as coupled oscillators yields Rabi energies up to twice those achieved with LHCs. The energies of the plexcitons are programmed by varying the degree of polymerisation, scaffold packing density and chlorophyll loading. Moreover, synthetic plexcitonic antenna complexes display pH and temperature responsiveness, facilitating active control of strong plasmon-exciton coupling. Programmable plexcitonic antenna complexes offer promise as a new kind of biologically-inspired metamaterial.