A strategy for the assembly of multiple porphyrin arrays based on the coordination chemistry of Ru-centered porphyrin pentamers

An approach which employs pentameric porphyrin arrays as building blocks toward larger porphyrin arrays is described. Two flexible, and one relatively rigid, Ru-centered porphyrin pentamers (1-3) were synthesized and fully characterized. Their potential as building blocks toward larger porphyrin arrays has been studied via their coordination chemistry using bidentate and tetradentate ligands. DABCO (diazabicyclo[2.2.2]octane) can bind two monomeric porphyrins but was found to be too small to allow the complete formation of a 10-porphyrin array. On the other hand, titration of a larger bridging dipyridyl porphyrin ligand 17 (0.5 equiv) with 1 or 2 and tetrapyridyl ligand 18 (0.25 equiv) with 3 results in the formation of the 11-porphyrin and 21-porphyrin arrays, respectively, with the 21-porphyrin array containing porphyrins in three different metalation states. Changes in the chemical shift of the inner NH protons as well as the ortho- and meso-protons of the pyridyl groups of the porphyrin ligand clearly indicate the formation of large multiple porphyrin complexes. These studies demonstrate that by use of carefully designed building blocks and suitable bridging ligands, porphyrin arrays can be constructed with a dramatic increase in size in relatively few steps. Exploiting the fact that the strength of binding of pyridyl ligands is Ru > Zn > Ni, intra- vs intermolecular competition has been used to investigate aspects of the folding of the array. The photophysical properties of 3 are also described.