Möbius Aromatic [28]Hexaphyrin Germanium(IV) and Tin(IV) Complexes: Efficient Formation of Triplet Excited States

[28]Hexaphyrin Ge ^IV and Sn ^IV complexes were synthesized in high yields by reactions of [28]hexaphyrin with GeCl ₄ or SnCl ₄ in the presence of triethylamine. Both complexes display distinct 28π Möbius aromatic character and possess a trigonal bipyramidal geometry at the central Ge ^IV or Sn ^IV atom. The equatorial hydroxy group of the Ge ^IV complex was smoothly exchanged with neutral nucleophiles, such as phenol derivatives and thiophenol, with retention of configuration. In the Sn ^IV complex, intersystem crossing to the T ₁ state is remarkably enhanced owing to the effective heavy-atom effect, thus allowing the formation of the T ₁ state in high yield. The T ₁ states of the Ge ^IV and Sn ^VI complexes were found to be antiaromatic on the basis of the transient absorption features in line with the Baird rule.