Siamese-twin porphyrin: a pyrazole-based expanded porphyrin providing a bimetallic cavity.

Expanded porphyrins are synthetic analogues of porphyrins consisting of at least 17 atoms in a cyclic conjugated framework that contains at least three pyrrole or pyrrole-like heterocyclic subunits. These macrocyclic systems have aroused great interest as near-IR-absorbing or -emitting chromophores and as two-photon dyes; their cavities are large enough to function as anion-recognition systems, and they may exhibit intriguing conformations. Hexaphyrin 1 is a representative expanded porphyrin that exists in a number of conformations and oxidation states, capable of forming a variety of monoand bimetallic complexes, including the formation of M bius aromatic systems.