Hexameric Macroring of Gable-Porphyrins as a Light-Harvesting Antenna Mimic

Abstract

Construction of a self-assembled supramolecular macroring that has distances and orientations of porphyrin dimer units in close analogy to those of the natural light-harvesting complexes was achieved. In natural light-harvesting complexes, bacteriochlorophyll-a's are arranged in macroring structures by coordination from imidazolyl side chains. A structural determination of a light-harvesting antenna complex (LH2) elucidated the arrangement of 18 bacteriochlorophyll-a's in a slipped-cofacial way with C₉ symmetry in B850 in 1995. To obtain such an elegant macroring architecture as an artificial light-harvesting complex, we connected slipped-cofacial dimers of imidazolylporphyrins in a gable-porphyrin orientation. The introduction of zinc assembled by coordination porphyrins with originally a broad molecular weight distribution (MWD). When coordination bonds were cleaved and reorganized under high dilution conditions using chloroform/methanol solution, the MWD was perfectly converged. This crop gave particle images of a uniform height by atomic force microscopy measurements. Further purification was successfully achieved by gel permeation chromatography, and the first eluting component gave a diameter corresponding to the cyclic hexamer of gable-porphyrins from a small-angle X-ray scattering measurement with synchrotron radiation. In summary, porphyrin assemblies in a macroring arrangement were constructed using the gable-porphyrin motif, and their photophysical properties are highly interesting.