Exploring the Synthesis of Metalloporphyrin Equipped Pd12L24 Supramolecular Spheres.

The range of metalation states within porphyrins provide a facile approach to tunable optical, electrochemical and catalytic properties. Spatial organization of metalloporphyrin units can yield additional emergent properties, with precise organization in Pd12L24 (L = Ligand) supramolecular nanosphere systems made possible by metal-mediated self-assembly. Here, we explore synthetic approaches to form porphyrin-appended Pd12L24 nanospheres, featuring porphyrins with different metal centers. New synthetic strategies are devised and explored en route to porphyrin-equipped nanosphere building blocks. We determine that Pd2+, Cu2+, Fe3+, and Zn2+ metalated porphyrins can withstand the nanosphere formation process, while Mg2+ porphyrins experience transmetalation under nanosphere formation conditions. The free base porphyrins are easily metalated by the metal precursor [Pd(MeCN)4](BF4)2, required for Pd12L24 nanosphere formation, but free base porphyrin-containing nanospheres are readily formed under kinetic control when the precise stoichiometry is employed. With these newly developed synthetic methods, porphyrin-equipped building block compounds can form Pd12L24 nanosphere assemblies with porphyrins placed at both the exohedral and endohedral sites of the supramolecular construct. The cage-appended porphyrins retain metallation state dependent optical properties, although transient absorption measurements reveal much faster photophysics than molecular controls. Therefore, porphyrin-containing supramolecular nanospheres can now be explored as energy transduction systems with potential applications in artificial photosynthesis.