Vibronic Coherence in a Minimal Pigment-Protein Complex Probed by Two-Dimensional Electronic-Vibrational Spectroscopy.

Observations of coherence in light-harvesting pigment-protein complexes have sparked significant interest in their role in energy transfer processes. However, resolving and understanding the origin of coherence in photosynthetic complexes has been challenging due to their structural complexity and overlapping spectral signals. Water-soluble chlorophyll-binding protein (WSCP), which binds only four chlorophyll molecules, offers a minimal model to study coherence and exciton dynamics. Using two-dimensional electronic-vibrational (2DEV) spectroscopy, we identify distinct excited-state features that exhibit ultrafast downhill energy transfer. In addition, we observe oscillations in the excited-state absorption signals at 284 cm-1, which are attributed to vibronic coherence due to their extended coherence lifetime and dominant appearance in the excited electronic state. This observation in 2DEV spectra is potentially enabled by coherence-to-population transfer, where the population dynamics reflect the oscillation of coherence. Our first-principles calculations predict low-frequency vibrational modes, which may contribute to the observed oscillations. These findings provide insight into energy transfer in natural pigment-protein complexes and demonstrate 2DEV spectroscopy as a probe of coherence.