The idea that excitonic (electronic) coherences are of fundamental importance to natural photosynthesis gained popularity when slowly dephasing quantum beats (QBs) were observed in the two-dimensional electronic spectra of the Fenna–Matthews–Olson (FMO) complex at 77 K. These were assigned to superpositions of excitonic states, a controversial interpretation, as the strong chromophore–environment interactions in the complex suggest fast dephasing. Although it has been pointed out that vibrational motion produces similar spectral signatures, a concrete assignment of these oscillatory signals to distinct physical processes is still lacking. Here we revisit the coherence dynamics of the FMO complex using polarization-controlled two-dimensional electronic spectroscopy, supported by theoretical modelling. We show that the long-lived QBs are exclusively vibrational in origin, whereas the dephasing of the electronic coherences is completed within 240 fs even at 77 K. We further find that specific vibrational coherences are produced via vibronically coupled excited states. The presence of such states suggests that vibronic coupling is relevant for photosynthetic energy transfer.

当在77 K的Fenna–Matthews–Olson（FMO）复合体的二维电子光谱中观察到缓慢移相量子拍（QB）时，激子（电子）相干性对于自然光合作用至关重要。由于复杂的强发色团与环境相互作用提示快速移相，因此将其分配给激子态的叠加，这是一个有争议的解释。尽管已经指出振动运动会产生相似的频谱特征，但是仍然缺少将这些振荡信号具体分配给不同的物理过程的方法。在这里，我们在理论建模的支持下，使用偏振控制的二维电子光谱技术，重新研究了FMO复合物的相干动力学。我们表明，寿命长的QB的起源仅是振动，而电子相干的移相即使在77 K时也可在240 fs内完成。我们进一步发现，特定的振动相干是通过振动耦合激发态产生的。这种状态的存在表明，振动耦合与光合作用的能量转移有关。