The crucial step in the conversion of solar to chemical energy in Photosynthesis takes place in the reaction center where the absorbed excitation energy is converted into a stable charge separated state by ultrafast electron transfer events. However, the fundamental mechanism responsible for the near unity quantum efficiency of this process is unknown. Here we elucidate the role of coherence in determining the efficiency of charge separation in the plant photosystem II reaction centre (PSII RC) by comprehensively combining experiment (two-dimensional electronic spectroscopy) and theory (Redfield theory). We reveal the presence of electronic coherence between excitons as well as between exciton and charge transfer states which we argue to be maintained by vibrational modes. Furthermore, we present evidence for the strong correlation between the degree of electronic coherence and efficient and ultrafast charge separation. We propose that this coherent mechanism will inspire the development of new energy technologies.

The spatial structure of the J-aggregates of meso-tetra(4-sulfonatophenyl)porphine (TPPS4) formed in acidic aqueous solutions and coated on silicon substrates was analyzed by means of atomic force microscopy (AFM). The AFM experiments indicate the presence of the stripelike J-aggregate structures on the surface. The size of the individual stripes ranged from 4.5 × 40 × 200 to 4.5 × 40 × 1000 nm3 (height × width × length). The length of the stripelike structures varied, whereas stripe width and thickness remained unchanged. The stripes stacked into larger domains, “fibers”, containing typically 2−20 stripes aligned parallel with a longitudinal shift with respect to each other. The size of individual stripes remained the same when interacting side-by-side in fibers. At lower magnification it is clearly seen that fibers form a network-like structure. The J-aggregates assemble into large (up to several millimeters) bushlike structures. It takes several weeks to form such structures in acidic aqueous solutions. On the basis of our experimental findings, it is also suggested that the stripes consist of rings that form nanotube-like TPPS4 J-aggregates, which are flattened as a result of attractive interactions with the substrate.

Chlorosomes are light-harvesting antennae that enable exceptionally efficient light energy capture and excitation transfer. They are found in certain photosynthetic bacteria, some of which live in extremely low-light environments. In this work, chlorosomes from the green sulfur bacterium Chlorobaculum tepidum were studied by coherent electronic two-dimensional (2D) spectroscopy. Previously uncharacterized ultrafast energy transfer dynamics were followed, appearing as evolution of the 2D spectral line-shape during the first 200 fs after excitation. Observed initial energy flow through the chlorosome is well explained by effective exciton diffusion on a sub-100 fs time scale, which assures efficiency and robustness of the process. The ultrafast incoherent diffusion-like behavior of the excitons points to a disordered energy landscape in the chlorosome, which leads to a rapid loss of excitonic coherences between its structural subunits. This disorder prevents observation of excitonic coherences in the experimental data and implies that the chlorosome as a whole does not function as a coherent light-harvester.