An ensemble of Frenkel exciton model Hamiltonians for an aqueous solution of the peridinin-chlorophyll-a protein (PCP) system is parameterized from first principles calculations. Geometry optimized MD configurations are used to sample the influence of local structural fluctuations on chromophore excitation energies, electronic couplings and electronic-vibrational couplings. The model Hamiltonians are used in dissipative quantum dynamics calculations to explore the influence of initial excitation on early time energy transfer and relaxation dynamics. It is found that an initial Franck-Condon excitation high into the peridinin (Per) exciton manifold results in rapid resonant energy transfer on a ~50 fs timescale between the Per and chlorophyll-a (Chl-a) excited state manifolds across an excitation energy gap of ~6000 cm-1 in excellent agreement with recent experimental two-dimensional electronic spectroscopy (2DES) results. Analysis of other 2DES experiments that used a lower energy, broadband excitation pulse centered between the bottom of the Per S2 manifold and the Chl-a Qy band, yield a significantly slower energy transfer timescale of ~220 fs. Approximate quantum dynamical simulations using our model Hamiltonian ensemble exploring different quantum initial excitations of the system suggest that the different timescales observed in these experiments arise from exciting different initial populations and coherences, for example, the slower timescale results from the lower energy excitation of a coherent superposition of Per S2 and Chl-a Qx states for which the vibronic dynamics does not satisfy the resonant energy transfer condition resulting in slower non-resonant relaxation process. A simple quasi-classical analytical model provides a mechanistic understanding of how the excitation energy transfer timescale depends on initial preparation of electronic excited state populations or coherences.

First-principles study of peridinin chlorophyll-a protein: Influence of initial excitation on excitation energy transfer dynamics

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