1 Bio-inspired polymers with polaritonic properties from visible to infrared a material play ground to mimic purple bacteria light -harvest ing resonators.

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Bio-inspired polymers with polaritonic properties from visible to infrared:

a material playground to mimic purple bacteria light-harvesting resonators.

Samuel Thomas Holder1, Carla Estévez-Varela2, Isabel Pastoriza-Santos2, Martin Lopez-

Garcia3, Ruth Oulton1 and Sara Núñez-Sánchez2

1 Quantum Engineering Technology Labs, University of Bristol, Bristol, UK

2 CINBIO, Universidade de Vigo, 36310 Vigo, Spain

3 International Iberian Nanotechnology Laboratory, Braga, Portugal

ABSTRACT

Light-harvesting complexes in natural photosynthetic systems, such as those in purple

bacteria, consist of photo-reactive chromophores embedded in densely packed “antenna”

systems organized in well-defined nanostructures. In the case of purple bacteria, the

chromophore antennas are composed of natural J-aggregates such as bacteriochlorophylls

and carotenoids. Inspired by the molecular composition of such biological systems, we create

a library of organic materials composed of densely packed J-aggregates in a polymeric matrix,

in which the matrix mimics a protein scaffold. This library of organic materials shows polaritonic

properties which can be tuned from the visible to the infrared by choice of the model molecule.

Inspired by the molecular architecture of the light-harvesting complexes of Rhodospirillum

molischianum bacteria, we study the light-matter interactions of J-aggregate-based nanorings

with similar dimensions to the analogous natural nanoscale architectures. Electromagnetic

simulations show that these nanorings of J-aggregates can act as resonators, with

subwavelength confinement of light while concentrating the electric field in specific regions.

These results open the door to bio-inspired building blocks for all-organic metamaterials while

offering a new perspective on light-matter interactions at the nanoscale in densely packed

organic matter in biological organisms including photosynthetic organelles.

Keywords: J-aggregates, organic polaritonics, photosynthesis, biomimetic, excitonic

metamaterials, purple bacteria.

Introduction

Natural biological systems have evolved complex structures at the molecular level which have

been carefully optimised for specific functions. This complexity can be observed, for example,

in membrane proteins where chemical selectivity is achieved through specific molecular

key/site pairs1 or, in the arrangement of chromophore molecules in photosynthetic complexes

(PCs)2. These PCs are composed of supramolecular chromophores contained in well-defined

nanostructures where proteins play the role of responsive scaffolds.3–7 These molecular

arrangements have evolved to optimise light capture and exciton transport between

chromophores, with proteins modulating energy pathways between them depending on sun

irradiation conditions.8 A significant amount of prior work has focussed on the investigation of

PCs as absorbers and emitters by ultrafast spectroscopy and quantum modelling.9,10 However,

these studies neglect the optical properties of the photosynthetic matter and hence, the

potential photonic modes supported by the molecular nanostructures. In this work, inspired by

the composition of the natural photosynthetic nanostructures of light-harvesting complexes

(LHCs) of purple bacteria, we build up a library of bio-inspired organic matter which we use to

model the light-matter interaction of photosynthetic nanostructures.

LHCs of purple bacteria are composed of organic matter with a high concentration of dye

molecules without presenting quenching (0.5-0.6 M)11,12 They show annular structures

composed of densely packed J-aggregates of π- conjugated organic molecules (carotenoids,

bacteriochlorophylls, etc)13–15 These supramolecular assemblies form a natural J-aggregate

with an exciton delocalised across the monomers. Previous research on light-trapping and

exciton transport strategies in natural systems typically analyses photosynthetic matter as an

effective dielectric medium with a dispersive value of the refractive index. This definition of a

macroscopic effective index of the membranes has furthermore been used in advanced

models that describe proteins within photosynthetic complexes.4,16 While this may be an

appropriate approximation for materials mainly composed of collagen or cellulose,

photosynthetic LHCs nanostructures of purple bacteria are composed of densely packed J-

aggregates with strong absorptions and delocalized excitons which can promote a significant

modification of the local refractive index, with important consequences for how light interacts

with these organic nanostructures.11,12,17,18 Our previous work demonstrates that thin films

composed of densely packed J-aggregates within a polymer have strongly modified optical

properties, achieving negative values of the real part of the permittivity and being able to

support surface exciton polaritons (SEPs).19,20 Here we demonstrate that these polaritonic

properties are not unique to a single specific molecular compound. On the contrary, the

polaritonic character can be achieved with a whole catalogue of molecules across the VIS and

NIR spectral range, mirroring the broad range of chromophores appearing in photosynthetic

organisms. These polaritonic properties arise from the presence of strong electric dipoles from

delocalised Frenkel excitons in the constituent J-aggregates which are embedded within the

polymer matrix. As LHCs are composed of densely packed J-aggregates within protein

scaffolds we propose that the unusual optical properties required for SEPs may exist in the

matter forming LHC nanostructures. Therefore exciton-polariton modes could play a role in

the well-defined nanostructures of LHCs21–23 of purple bacteria, with structural similarities

between these natural nanoscale systems and nanoscale metamaterial building blocks.24,25

To investigate the physics of polaritons in LHCs of purple bacteria, we study the light-matter

interactions of LHC architectures at the nanoscale by electromagnetic simulations, using as

models the architecture of the LHC-2 of Rhodospirillum molischianum bacteria, together with

the optical properties of a library of organic materials composed by artificial J-

aggregates.12,26,27 These simulations revealed that LHC-2 composed of densely packed J-

aggregates act as nano-resonators confining the light at the nanoscale. The optical response

obtained depends strongly on the polarization of the incident light, revealing that when the

incident polarization is contained in the photosynthetic membrane, the electric field is

concentrated in the centre and edges of the nanorings, enhancing interaction with the reaction

centre and promoting the coupling between closed LHCs. This offers a new perspective on

the understanding of light-matter interactions in natural photosynthetic complexes of purple

bacteria, demonstrating how supramolecular nanostructures of densely packed J-aggregates

can shape and optimise the interaction of light and transport of energy within photosynthetic

organelles. Understanding this mechanism could boost light-harvesting efficiency in

metamaterial devices through a new family of molecular materials that mimic the carefully

optimised molecular arrangement, concentration, and architectural design of natural

photosynthetic systems, with J-aggregate and polymer materials forming the building blocks

of such a platform.

Bio-inspired polaritonic library

In analogy with proteins in the biological systems, we mixed J-aggregates with a water-based

polymer which physically separates the J-aggregates giving robustness to the final bulk

photosynthetic-mimetic material. We controlled the conformal arrangement of the molecular

aggregates to promote self-assembly into J-aggregates by increasing the dye concentration

in final dye-polymer water solutions.28 As a library of J-aggregates, we used commercial

water-soluble closed-chain cyanine dyes with optical responses from the visible to the infrared.

The structure of the monomers of the four J-aggregates selected for this work is shown in

Figure 1a-d: J562, J587, J619 and J798 (see methods for complete name). The Figure 1.a-

d shows the extinction of the monomer obtained from the dye solution in ethanol for the four

cyanine dyes with peaks at 504 nm (J562), 519 nm (J587), 554 nm (J619) and 664 nm (J562).

For a water-based polymer we selected poly (vinyl alcohol) (PVA) with a molecular weight of

85,000-124,000. The four cyanine water solutions (25 mM) were mixed with 6 wt% aqueous

PVA solution (3:1 volume ratio) as previously described.19 As shown in Figure 1.a-d, all four

cyanine-PVA solutions show a narrow absorption peak red-shifted with respect to the peak of

the monomeric dye that confirms the formation of J-aggregates (measurement protocol in

section 1, supplementary). The J-aggregate:PVA films were prepared by spin-coating (10000

rpm) of dye-PVA solutions on glass substrates obtaining a final mass ratio of J-aggregate to

PVA in the bulk material of around 1:1.

Figure 1: a-d) Molecular structure of the monomers of the selected cyanine library for this work. They

are named here according to the absorption peak of the J-aggregate conformation (complete name of

molecules in methods section). e-h) Normalized absorbance of monomeric dye solutions in ethanol

(doted lines, 100 µM) and J-aggregate:PVA mixtures (solid line) for the J-aggregates e) J652, f) J587,

g) J619 and h) J789. i) Picture of the J-aggregate:PVA films on top of black cardboard. From left to the

right: J562, J587, J619 and J789 films. Optical microscope images of a 250 µm square area for dye (j)

J562 (k) J587 and (l) J619 measured in reflectance microscopes under Köhler Illumination

configuration. The scale bar is 50 µm.

Figure 1.i shows a picture of the four samples obtained. All the samples show a metallic lustre,

each in a different spectral range, observed by the naked eye and through a microscope

(Figure 1.j-l). This high reflectance has a well-defined, vivid colour, because it is associated

with a narrow region of negative real electric permittivity created by the intense absorption of

the densely packed J-aggregates making up each film19. While metals generally have negative

real electric permittivity in a broad spectral range, and therefore are reflective in a broad

spectral range, these J-aggregate based materials exhibit this behaviour in a narrow spectral

range, located at shorter wavelengths than the J-aggregate absorption, giving them a vividly

coloured reflectance. Reflectance close to 60% is achieved for all four samples (see Figure

2.a). Besides, it is independent of the angle of incidence, demonstrating that it is inherent to

the optical properties of the material and is not due to the creation of a Fabry-Perot cavity. The

J-aggregate:PVA films were analysed by Atomic Force Microscopy (supplementary section

2). Table 1 shows the thickness and roughness of the different J-aggregate:PVA films. In

general, the films are homogeneous on a 100nm length scale. Regardless the dyes, the J-

aggregate:PVA films present a roughness comparable to thatof a metal film obtained by

thermal evaporation (1-2 nm)29 being the films with the largest roughness those obtained with

J562 and probably due to randomly orientated bundles of J-aggregates at the film surface. For

example, the J562 film with the largest roughness shows grain-like features (Figure S2.a).

Further work is required to investigate potential domain structure within J-aggregate:PVA

materials: in this work we have assumed that the films are optically homogenous.

Figure 2: (a) Measured (solid lines) and fitted data (dashed lines) of un-polarised reflectance of J-

aggregate:PVA films at an angle of incidence of 45, 50, 55, 60, 65 and 70 degrees. b) Real (solid line)

and imaginary (dashed line) parts of the permittivity for the different J-aggregate:PVA films as indicated

obtained by fitting the unpolarised reflectance data only. Real part of the permittivity of PVA from

published data (grey dotted line)30. c) Experimental and modelled transmission through the polymer

films using the optical properties obtained from reflectance and the thickness from AFM. Panels from

top to bottom: J562, J587, J619 and J789 films.

The optical properties of the films were estimated by fitting the unpolarised reflectance as a

function of the incidence angle. The samples were modelled as a stack of two smooth layers:

a thin J-aggregate:PVA film on top of a semi-infinite glass substrate. The thickness of the J-

aggregate:PVA film was fixed to the measured value for each sample (Table 1).The resulting

real and imaginary parts of the relative electric permittivity of each film are shown in Figure

2.b. To confirm the measured optical properties of each film, we measured the transmission

spectrum through each sample and compared the experimental values with the modelled

transmission spectrum (see Figure 3.c). The good agreement between measured and

modelled data demonstrates the accuracy of the obtained optical properties of each material.

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1Bio-inspiredpolymerswithpolaritonicpropertiesfromvisibletoinfrared:amaterialplaygroundtomimicpurplebacterialight-harvestingresonators.SamuelThomasHolder1,CarlaEstévez-Varela2,IsabelPastoriza-Santos2,MartinLopez-Garcia3,RuthOulton1andSaraNúñez-Sánchez21QuantumEngineeringTechnologyLabs,UniversityofBr...

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1 Bio-inspired polymers with polaritonic properties from visible to infrared a material play ground to mimic purple bacteria light -harvest ing resonators.

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