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 x 10(8)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 x 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 de-camer model, which employed kinetic parameters obtained from steady-state and time-resolved fluorescence experiments.