SnIV Porphyrin Scaffolds for Axially Bonded Multiporphyrin Arrays: Synthesis and Structure Elucidation by NMR Studies

A simple, one-step, supramolecular strategy was adopted to synthesize Sn-IV-porphyrin-based axially bonded triads and higher oligomers by using meso-pyridyl Sn-IV porphyrin, meso-hydroxyphenyl-21,23-dithiaporphyrin, and Ru-II porphyrin as building blocks and employing complementary and non-interfering (SnO)-O-IV and (RuN)-N-II interactions. The multiporphyrin arrays are stable and robust and were purified by column chromatography. H-1, H-1-H-1 COSY and NOESY NMR spectroscopic studies were used to unequivocally deduce the molecular structures of Sn-IV-porphyrin-based triads and higher oligomers. Absorption and electrochemical studies indicated weak interaction among the different porphyrin units in triads and higher oligomers, in support of the supramolecular nature of the arrays. Steady-state fluorescence studies on triads indicated the possibility of energy transfer in the singlet state from the basal Sn-IV porphyrin to the axial 21,23-dithiaporphyrin. However, the higher oligomers were weakly fluorescent due to the presence of heavy Ru-II porphyrin unit(s), which quench the fluorescence of the Sn-IV porphyrin and 21,23-dithiaporphyrin units.