Over the last several decades, the light-harvesting protein complexes of purple bacteria have been among the most popular model systems for energy transport in excitonic systems in the weak and intermediate intermolecular coupling regime. In spite of this extensive body of scientific work, significant questions regarding the excitonic states and the photoinduced dynamics remain. Here, we address the low-temperature electronic structure and excitation dynamics in the light harvesting complex 2 (LH2) of Rhodopseudomonas (Rh.) acidophila by 2D electronic spectroscopy. We find that, although energy relaxation is very rapid, exciton mobility is limited over a significant range of excitation energies. This points to the presence of a sub-200 fs, spatially local energy relaxation mechanism, and suggests that local trapping might contribute substantially more in cryogenic experiments than under physiological conditions where the thermal energy is comparable to-or larger than-the static disorder.

Optical nonlinear spectroscopies carry a high amount of information about the systems under investigation; however, as they report polarization signals, the resulting spectra are often congested and difficult to interpret. To recover the landscape of energy states and physical processes such as energy and electron transfer, a clear interpretation of the nonlinear signals is prerequisite. Here, we focus on the interpretation of the electrochromic band-shift signal, which is generated when an internal electric field is established in the system following optical excitation. Whereas the derivative shape of the band-shift signal is well understood in transient absorption spectroscopy, its emergence in two-dimensional electronic spectroscopy (2DES) has not been discussed. In this work, we employed 2DES to follow the dynamic band-shift signal in reaction centers of purple bacteria Rhodobacter sphaeroides at 77 K. The prominent two-dimensional derivative-shape signal appears with the characteristic formation time of the charge separated state. To explain and characterize the band-shift signal, we use expanded double-sided Feynman diagram formalism. We propose to distinguish two types of Feynman diagrams that lead to signals with negative amplitude: excited state absorption and re-excitation. The presented signal decomposition and modeling analysis allows us to recover precise electrochromic shifts of accessory bacteriochlorophylls, identify additional signals in the B band range, and gain a further insight into the electron transfer mechanism. In a broader perspective, expanded Feynman diagram formalism will allow for interpretation of all 2D signals in a clearer and more intuitive way and therefore facilitate studying the underlying photophysics.

Vibronic coupling between molecules has been recently discussed to play an important role in photosynthetic functions. Furthermore, this type of coupling between electronic states has been suggested to define photophysical properties of chlorophylls, a family of photosynthetic molecules. However, experimental investigation of vibronic coupling presents a major challenge. One subtle way to study this coupling is by excitation and observation of the superpositions of vibrational states via transitions to vibronically mixed states. Such superpositions, called coherences, are then observed as quantum beats in non-linear spectroscopy experiments. Here we present polarization-controlled two-dimensional electronic spectroscopy study of the chlorophyll c1 molecule at cryogenic (77 K) temperature. By applying complex analysis to the oscillatory signals we are able to unravel vibronic coupling in this molecule. The vibronic mixing picture that we see is much more complex than was thought before.

The high efficiency of the special pair in initiating electron transfer in reaction centers is not well understood. By using 2DES, we identified the charge transfer state, which is responsible for the charge separation function.