Photosynthetic Excitation Energy Transfer

Abstract

We investigate photosynthetic excitation energy transfer (EET) in the presence of realistic electron-phonon couplings and system Hamiltonians. The chlorophyll-based Fenna-Matthews-Olsen (FMO) complex represents the system with a fixed electronic Hamiltonian while the bilin-based phycobiliprotein PC645 light-harvesting complex served as the system with both configuration-dependent electronic Hamiltonians and intramolecular spectral densities. For both systems, the spectral densities are site-dependent. We solve for the dynamics of the reduced density matrix using the Modified Redfield Theory (MRT) and Coherent Modified Redfield Theory (CMRT) and obtain physical insight via statistical analysis.

First, we show that the site-varying intermolecular electron-phonon coupling of the FMO complex is optimized for EET. By considering two possible target sites, we identify two transport pathways of contradicting nature where one is dominated by coherent dynamics and the other by incoherent energy dissipation. Interestingly, it appears the realistic electron-phonon coupling configuration is such that both pathways are reasonably accommodated.

Next, we show that EET optimization also applies, at least for the 168 cm-1 vibrational mode, to the site-dependent intramolecular electron-phonon coupling of FMO. Additionally, we find further optimization via interplay with the site-dependent intermolecular parameters. The vibronic enhancement at the target site of BChl 3 (i.e. the chromophore commonly believed to be coupled to the reaction centre) is found to mainly originate from BChl 2. Surprisingly, BChl 4 does not contribute to the vibronic enhancement despite the resonant excitonic energy gap and strong coupling to BChl 3.

Lastly, we demonstrate that EET robustness in PC645 can be attributed to the presence of multiple strongly-coupled intramolecular vibrational modes spanning a range of frequencies and more than one target site. We also confirm that vibronic transport in PC645 is predominantly in the incoherent regime for the majority of the configurations.