Six-Coordinate Zinc Porphyrins for Template-Directed Synthesis of Spiro-Fused Nanorings

Five-coordinate geometry is the standard binding mode of zinc porphyrins with pyridine ligands. Here we show that pseudo-octahedral six-coordinate zinc porphyrin complexes can also be formed in solution, by taking advantage of the chelate effect. UVvisNIR titrations indicate that the strength of this second coordination is ca. 68 kJ mol(-1). We have used the formation of six-coordinate zinc porphyrin complexes to achieve the template-directed synthesis of a 3D pi-conjugated spiro-fused array of 11 porphyrin units, covalently connected in a nontrivial topology. Time-resolved fluorescence anisotropy experiments show that electronic excitation delocalizes between the two perpendicular nanorings of this spiro-system within the experimental time-resolution of 270 fs.