The Fenna−Matthews−Olson (FMO) photosynthetic complex found in green sulfur bacteria has been one of the favorite “model” systems for biological energy transfer over the last decades. However, even after 40 years of studies, quantitative knowledge about its energy-transfer properties is limited. 

We applied two-dimensional electronic spectroscopy with full polarization control to provide an accurate description of the electronic structure and population dynamics in the complex. The sensitivity of the technique has further allowed us to spectroscopically identify the eighth bacterio-chlorophyll molecules recently discovered in the crystal structure. The time evolution of the spectral structure, covering time scales from tens of femtoseconds up to a nanosecond, reflected the energy flow in FMO and enabled us to extract an unambiguous energy-transfer scheme.

Another significant result obtained from the measurements was proving that the long-lived quantum beats (QBs), visible in 2DES of the FMO, were exclusively vibrational in origin, rather then originating from superpositions of excitonic states. We further found that specific vibrational coherences were produced via vibronically coupled excited states. The presence of such states suggested that vibronic coupling is relevant for photosynthetic energy transfer.