Strong Coupling of Localized Surface Plasmons to Excitons in Light-Harvesting Complexes.

Anna Tsargorodska,Michaël L Cartron,Päivi Törmä,Graham J Leggett,Jeremy J Baumberg,C Neil Hunter,Cvetelin Vasilev,Olga A Mass,P Leslie Dutton,Goutham Kodali

doi:10.1021/acs.nanolett.6b02661

Abstract

Gold nanostructure arrays exhibit surface plasmon resonances that split after attaching light harvesting complexes 1 and 2 (LH1 and LH2) from purple bacteria. The splitting is attributed to strong coupling between the localized surface plasmon resonances and excitons in the light-harvesting complexes. Wild-type and mutant LH1 and LH2 from Rhodobacter sphaeroides containing different carotenoids yield different splitting energies, demonstrating that the coupling mechanism is sensitive to the electronic states in the light harvesting complexes. Plasmon–exciton coupling models reveal different coupling strengths depending on the molecular organization and the protein coverage, consistent with strong coupling. Strong coupling was also observed for self-assembling polypeptide maquettes that contain only chlorins. However, it is not observed for monolayers of bacteriochlorophyll, indicating that strong plasmon–exciton coupling is sensitive to the specific presentation of the pigment molecules.

Highlights

Photosynthetic organisms collect sunlight with extraordinarily high efficiencies, and the mechanisms that are responsible for this have been the subject of intense enquiry.[1]
Gold nanostructure arrays exhibit surface plasmon resonances that split after attaching light harvesting complexes 1 and 2 (LH1 and light-harvesting complex 2 (LH2)) from purple bacteria
It is not observed for monolayers of bacteriochlorophyll, indicating that strong plasmon−exciton coupling is sensitive to the specific presentation of the pigment molecules

Summary

Nano Letters

They have attracted growing interest for applications in biological sensing and analysis.[23,27−35] Recent studies have demonstrated plasmonic enhancement of fluorescence emission from light-harvesting complexes of bacteria[36,37] and for photosystem I from chloroplasts[38,39] via coupling between the plasmon band of a metal nanoparticle and a spectroscopic transition in the biomolecule. 4.2 × 1017 m−2 (see Supporting Information for justification) Based on these considerations there seems to be no reason to predict a reduced strength of plasmon−exciton coupling for BChl. If differences in the density of dipoles cannot explain the observation of splitting in the extinction spectra of arrays functionalized with light-harvesting complexes and maquettes, but not of arrays covered with BChl, the explanation must lie in the presentation of the pigment molecules within the plasmon mode volume: the observation of strong plasmon− exciton coupling requires not gold nanostructures and the presence of BChl and Crt within the LSPR mode volume. Surface plasmon resonances (LSPR) of gold nanostructures in macroscopically extended, periodic arrays are strongly coupled to excitons in the pigment molecules in lightharvesting complexes 1 and 2 from R. sphaeroides and chlorincontaining man-made 4-α-helical maquette proteins This coupling leads to a substantial splitting of the plasmon band and the observation of significant changes in the extinction spectrum.

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