Excitonic-Vibronic Coupling in Natural and Artificial Light-Harvesting Systems

Abstract

The origin of long-lasting quantum coherent oscillations is deemed to be present in highly ordered systems of small scale, but the fact of being present in natural photosynthetic light-harvesting complexes (LHC) has puzzled researchers for years now. The initial incident of photons excites the chromophoric pigments inside the protein, the exciton formed is transported efficiently to other pigments to reaction center for charge separation process. The question of whether one can replicate this quantum coherent mechanism in artificial light-harvesting systems remains open. Exciton energy transfer observed in all the natural systems, due to the extended quantum superpositions of energy levels interplay between the excitonic-vibronic coupling can enhance the energy transfer process. Herein I have overviewed the theoretical background with particular excitonic-vibronic energy transfer mechanism that occurs in natural LHC (phycobilins, LH2, and Fenna–Matthews–Olson (FMO) complexes). The total Hamiltonian excitonic-vibronic description followed in major works is explained in detail, and computing individual parameters like chromophore site energies and coupling parameters are presented. Artificial light-harvesting systems of H- and J-aggregates use donor and acceptor concept to understand the absorption and photoluminescence properties. Frenkel exciton theory explains the excitonic energy transfer mechanism using the Coulomb coupling among aggregates and the extended Frenkel-Holstein theory explains the excitonic-vibronic coupling in aggregates. A brief overview of excitonic-vibronic coupling from simple oligoacenes till polymeric materials is given in detail.