Abstract
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. UV–vis–NIR titrations indicate that the strength of this second coordination is ca. 6–8 kJ mol–1. We have used the formation of six-coordinate zinc porphyrin complexes to achieve the template-directed synthesis of a 3D π-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.
Highlights
We demonstrate that zinc porphyrins with two axial ligands can be formed in solution, when the whole system is suitably stabilized by the chelate effect
We show that sixcoordinate zinc centers can be used in the template-directed synthesis of spiro-linked porphyrin nanorings, which exhibit ultrafast energy migration between the two perpendicular fused macrocycles
To the best of our knowledge, six-coordinate zinc porphyrins with two axial pyridine ligands have never previously been detected in solution, they sometimes occur in crystal structures,[8] and zinc porphyrins with two axial DABCO or pyrazine ligands have been studied in supramolecular tweezer complexes.[9]
Summary
We demonstrate that zinc porphyrins with two axial ligands can be formed in solution, when the whole system is suitably stabilized by the chelate effect. We show that sixcoordinate zinc centers can be used in the template-directed synthesis of spiro-linked porphyrin nanorings, which exhibit ultrafast energy migration between the two perpendicular fused macrocycles.
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