Extracting the excitonic Hamiltonian of a chlorophyll dimer from broadband two-dimensional electronic spectroscopy

We apply Frenkel exciton theory to model the entire Q-band of a tightly bound chlorophyll dimer inspired by the photosynthetic reaction center of photosystem II. The potential of broadband two-dimensional electronic spectroscopy experiment spanning the Q(x) and Q(y) regions to extract the parameters of the model dimer Hamiltonian is examined through theoretical simulations of the experiment. We find that the local nature of Q(x) excitation enables identification of molecular properties of the delocalized Q(y) excitons. Specifically, we demonstrate that the cross-peak region, where excitation energy is resonant with Q(y) while detection is at Q(x), contains specific spectral signatures that can reveal the full real-space molecular Hamiltonian, a task that is impossible by considering the Q(y) transitions alone. System-bath coupling and site energy disorder in realistic systems may limit the resolution of these spectral signatures due to spectral congestion.

Automated summary

We used Frenkel exciton theory to simulate the entire Q-band of a chlorophyll dimer inspired by photosystem II. The potential of two-dimensional electronic spectroscopy experiment to extract the parameters of the model dimer Hamiltonian was examined through theoretical simulations. We found that by considering the local nature of Q(x) excitation, specific spectral signatures can reveal the full real-space molecular Hamiltonian, which is impossible by considering only Q(y) transitions. However, spectral congestion due to system-bath coupling and site energy disorder in realistic systems may limit the resolution of these spectral signatures.