When excitons dance: mechanistic regimes of vibronic transport

Spectroscopic signatures of vibronic coherence, the delocalization between a donor and a vibrationally excited acceptor state, have been observed in both the phycobiliprotein PC645 antenna complex and the photosystem II reaction center. The extent to which vibronic coherence enhances photosynthetic light harvesting, however, remains controversial. The challenge in determining the importance of vibronic coherence has been the absence of a clear, quantitative assignment of vibronic mechanisms across different parameter regimes. Here, we use numerically exact hierarchical equations of motion (HEOM) calculations and detailed mechanistic analysis to establish the regimes of vibronic transport appropriate for prototypical photosynthetic heterodimers. Integrating the vibronic regimes with detailed atomistic simulations of PC645 we demonstrate that rapid transport from the highest-energy states to the lowest energy pigments in this pigment-protein complex occurs via an incoherent vibronic mechanism supported by a large reorganization energy and a broad collection of high-frequency vibrations. This work lays the foundation for further exploration of vibronic mechanisms in more complex systems such as the reaction center.