In photosynthesis in Nature, light-harvesting systems absorb sunlight and then transfer the energy to the photosynthetic reaction center. For this, they use light-harvesting pigments such as chlorophylls. The light-harvesting system 2 (LH2) in purple bacteria, for example, is composed of two concentric rings of bacteriochlorophyll units, one with nine chlorophylls and one with 18 chlorophylls. The latter shows ultrafast energy transfer in the ring.

Synthetic models of such light-harvesting systems could help to understand how they transfer energy. Rings composed of porphyrin units can serve as models for LH2. However, the energy transfer in the models is usually much slower than in Nature, because the coupling between the porphyrin units is challenging to replicate.

Alex Saywell, University of Nottingham, UK, Laura M. Herz, Harry L. Anderson, University of Oxford, UK, and colleagues have synthesized nanorings that consist of 24 porphyrin units and a single butadiyne link (simplified structure pictured). The butadiyne breaks the otherwise high symmetry, which makes the energy transfer in the system easier to study. The team used a linear oligo-porphyrin precursor, which was “bent” into the desired shape. For this, the researchers used two oligo-pyridyl templates, which stack in a staggered manner, forming a spoke-like structure with 24 binding sites in total at the outer ends of the “spokes”. These sites then coordinate the 24 zinc atoms in the linear oligo-porphyrin precursor, “bending” it into a ring.

This templating step is followed by a palladium-catalyzed oxidative coupling to close the ring. The template is removed by replacing it with pyridine ligands. The desired nanoring was obtained in a yield of 26 %. The team then studied energy transfer processes in the ring and found that an energy transfer from the porphyrin units to the butadiyne-linked segment can take place.

• Bending a photonic wire into a ring,
  Henrik Gotfredsen, Jie-Ren Deng, Jeff M. Van Raden, Marcello Righetto, Janko Hergenhahn, Michael Clarke, Abigail Bellamy-Carter, Jack Hart, James O’Shea, Timothy D. W. Claridge, Fernanda Duarte, Alex Saywell, Laura M. Herz, Harry L. Anderson,
  Nat. Chem. 2022.
  https://doi.org/10.1038/s41557-022-01032-w