Supramolecular organization of light-harvesting porphyrin macrorings

Porphyrin‐based supramolecular nanostructures have been produced by the self‐assembly of porphyrin macrorings with three benzoic acid groups (Acid‐R) on each side of the rings through cooperative carboxyl–carboxyl hydrogen bonds. Structures of the organized Acid‐R were analyzed by AFM, and two clear distribution peaks were observed at 3 and 27 nm in the height‐distribution histogram. From the overall assessment, the higher objects are considered to be one‐dimensional structures standing vertically on the mica substrate. The height corresponds to an 11‐mer of a unit Acid‐R. Light‐harvesting functions were examined by using fluorescence titration, whereby an energy‐acceptor molecule (Tripod 2 ) was employed that strongly interacted with Acid‐R units (association constant: 2.0×10⁸ M ^?1), specifically from the inner pore. The titration results showed that the apparent stoichiometry Tripod 2 ]/Acid‐R] was <0.5, and that the value was concentration dependent. Titration results reasonably account for the scheme in which Tripod 2 only interacts with each terminal in the organized Acid‐R. The number of organization was fitted to a 10‐mer of Acid‐R in a 6.8×10^?7 M solution, and was consistent with that estimated from the AFM results. In the composites of organized Acid‐R/Tripod 2 , a singlet excitation energy transfer occurred among the Acid‐R units, and to Tripod 2 . The energy‐transfer rate constants were estimated by using the decamer model, which employed kinetic parameters obtained from steady‐state and time‐resolved fluorescence experiments.