Bending a Photonic Wire into a Ring

Natural light harvesting systems absorb sunlight and transfer its energy to the reaction centre, where it is used for photosynthesis. Synthetic chromophore arrays provide useful models for understanding energy migration in these systems. Research has focussed on mimicking rings of chlorophyll molecules found in purple bacteria, known as ‘light-harvesting system 2’. Linear meso-meso linked porphyrin chains mediate rapid energy migration, but until now it has not been possible to bend them into rings. Here we show that oligo-pyridyl templates can be used to bend these rod-like photonic wires, to create covalent nanorings. The macrocycles consist of 24 porphyrin units, with a single butadiyne link. Their elliptical conformations have been probed by scanning tunnelling microscopy (STM). This system exhibits two types of excited state energy transfer processes: (a) from the central template to the peripheral porphyrins and (b) from the 24 light-absorbing porphyrin units to the π-conjugated butadiyne-linked porphyrin dimer segment.