Final Report Summary - COSUN (Cooperative Phenomena in Supramolecular Nanostructures)
This project developed new template-directed strategies for the synthesis of large pi-conjugated macrocycles; molecular nanorings and nanoballs, mostly built from metalloporphyrin units. We also investigated cooperative phenomena in these nanostructures. Electronic delocalization was probed using time-resolved fluorescence spectroscopy, electron paramagnetic resonance spectroscopy (EPR) and nuclear magnetic resonance spectroscopy (NMR). The singlet excited states are highly delocalized and mimic the light-harvesting chlorophyll arrays responsible for gathering energy from sunlight during photosynthesis. The triplet excited states are less delocalised than the singlets, but they are still more delocalized than expected from previous work. The singly charged polarons are localized over two-three porphyrin units, but highly charged multi-cations and multi-anions have totally delocalized electronic structures, resulting in aromatic ring currents. This extreme delocalization suggests that they could be useful in electronic devices, such a single-molecule transistors. The global aromaticity in these systems matches perfectly with the predictions of Hückel's 4n+2 rule, even up to rings with 162 pi-electrons. This is remarkable because the conventional wisdom is that aromatic ring currents do not arise in macrocycles with more than about 22 pi-electrons. Rings made entirely from carbon atoms were also investigated. Molecules of cyclo[18]carbon were imaged by atomic force microscopy, at 5 kelvin on a sodium chloride surface, demonstrating that they have a polyynic structure, with alternating single and triple bonds. This was a significant achievement; it is the first time that molecules of a carbon allotrope based on linear two-coordinate sp atoms has been structurally characterized or studied in a condensed phase.