Light-Harvesting Heptadecameric Porphyrin Assemblies

New porphyrin assemblies containing 17 porphyrin molecules are constructed by using free base TPP-type porphyrins having eight pyrazine moieties 1. Spectroscopic titration of dimeric [meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]zinc(II) 2 with these porphyrins shows that the processes of the formation of the heptadecameric porphyrin assemblies may be analyzed as eight independent equilibrium processes with an identical binding constant. All binding constants are larger than 5 × 107 M-1 which is the determinable upper limit of the present titration method. In all cases, the fluorescence spectrum of the 1:8 mixture of 1 and 2 consists of the major fluorescence of 1 and the minor one of 2·pyrazine complex even in the presence of the large excess of the antenna pigments. The observed spectra are well reconstructed by the form of f a F 1 + f b F 2, where F 1 and F 2 are the fluorescence of 1 and the 2·pyrazine complex measured separately at the corresponding concentrations. Interestingly, the general trend that values of f a are nearly equal to those of r 564 × (1 − f b) in all cases is found, where r 564 is the absorption ratios of the 2·pyrazine moiety and the central free base porphyrin in the assemblies at 564 nm. The observation indicates the excitation of the central porphyrin is directly enhanced by the absorption of the antenna pigments even in such large scale assemblies. Thus, the antenna effect for 1 having largest r 564 results in 77 times fluorescence enhancement of the central free base porphyrin. The systems also show interesting dependency of energy-transfer efficiencies on the topological arrangement of the antenna elements.