Cavity-Modified Exciton Dynamics in Photosynthetic Units

Abstract

Recently, exciton–photon strong coupling has been proposed as a means to control and enhance energy transfer in ensembles of organic molecules. Here, we demonstrate that the exciton dynamics in an archetypal purple bacterial photosynthetic unit, composed of six LH2 antennas surrounding a single LH1 complex, is greatly modified by its interaction with an optical cavity. We develop a Bloch–Redfield master equation approach that accounts for the interplay between the B800 and B850 bacteriochlorophyll molecules within each LH2 antenna, as well as their interactions with the central LH1 complex. Using a realistic parametrization of both the photosynthetic unit and optical cavity, we investigate the formation of polaritons in the system, revealing that these can be tuned to accelerate its exciton dynamics by 3 orders of magnitude. This yields a significant occupation of the LH1 complex, the stage immediately prior to the reaction center, with only a few-femtosecond delay after the initial excitation of the LH2 B800 pigments. Our theoretical findings unveil polaritonic phenomena as a promising route for the characterization, tailoring, and optimization of light-harvesting mechanisms in natural and artificial photosynthetic processes.

Barrier-free reverse-intersystem crossing in organic molecules by strong light-matter coupling

“…Strong light-matter coupling has emerged as a new tool for tailoring molecular properties without touching the chemical structure 8 , 9 . The formed hybrid states, referred to as polaritons, play key roles in modifying the photophysical and photochemical processes in the strong coupling regime 10 – 13 , such as the enhancement of Förster type and vibrational energy transfer 14 – 17 , tilting the ground-state reactivity landscape 18 , 19 , facilitating Bose–Einstein condensation and organic lasing 20 – 25 , reducing energy losses in photovoltaics 26 – 29 , manipulating triplet state dynamics 30 – 33 , maximizing superconducting current 34 and optimization of artificial photosynthesis 35 . In particular, we have previously demonstrated polariton-enhanced RISC in optical cavities by reducing the energy gap between singlet (polariton) and triplet states 36 , the method of which was then adopted to achieve an energy inversion of the two states 37 .…”

A theoretical perspective on molecular polaritonics

“…This problem can be resolved by applying the Markov approximation after taking into account the strong coupling, e.g., by using a Bloch-Redfield approach. 76,112,113 When exciton-phonon coupling is sufficiently strong that the above approach breaks down, it becomes necessary to explicitly include the vibrational modes of the molecules. The simplest approach is the so-called Holstein model, which treats only a single vibrational mode and describes each molecule as two displaced harmonic oscillators.…”

Theoretical Challenges in Polaritonic Chemistry

“…2,3 This new direction to modify and control the properties of molecular systems is nowadays known as polaritonic chemistry. [4][5][6] It has been shown to affect a wide range of processes, such as photochemical reactions both in single-molecule [7][8][9][10][11][12][13][14] and collective 2,3,[15][16][17][18][19][20] strong-coupling setups, as well as (possibly long-range) energy transfer, [21][22][23][24][25][26][27][28][29][30][31] and transitions between different spin multiplets, [32][33][34][35][36][37][38][39] among others. We emphasize that polaritonic chemistry is not a mere substitute of traditional chemistry techniques, as it can enable processes that are not possible in bare materials due to the long-range and collective nature of the polaritons.…”