Non-uniform excited state electronic-vibrational coupling leads to enhanced exciton transport in pigment protein complexes

Photosynthetic organisms exploit interacting quantum degrees-of-freedom, namely intra-pigment electron-vibrational (vibronic) and inter-pigment dipolar couplings (J-coupling), to rapidly and efficiently convert light into chemical energy. These interactions result in wavefunction configurations that delocalize excitation between pigments and pigment vibrations. Our study uses multi-dimensional spectroscopy to compare two model photosynthetic proteins, the Fenna-Matthews Olson (FMO) complex and Light Harvesting 2 (LH2) and confirm that long-lived excited state coherences originate from the vibrational modes of the pigment. Within this framework, theJ-coupling of vibronic pigments should have a cascading effect in modifying the structured spectral density of excitonic states. We show that FMO effectively couples all its excitations to a uniform set of vibrations while in LH2, its two chromophore rings each couple to a unique vibrational environment. We simulate energy transfer in a simple model system with non-uniform vibrational coupling to demonstrate how modification of vibronic coupling strength can modulate energy transfer. Since increasing vibronic coupling increases internal relaxation, strongly coupled vibronic states can act as an energy funnel, which can potentially benefit energy transport.