Aromaticity switches

Switching between qualitatively different types of p-electron delocalization may be effected by a number of factors. The most typical case of switching occurs when the formal oxidation level of a p-electron system is changed, transforming an aromatic system (4n+2-electrons) into an antiaromatic one (4n electrons) or vice versa.Other switching mechanisms include tautomeric equilibria and conformational dynamics.

22-Hydroxy-m-benziporphyrin is an example of tautomerically controlled aromaticity switch. It forms an equlibrium mixture of two tautomers: a phenol form, dominated by the benzenoid aormaticity of the m-phenylene ring, and a keto ("semiquinone") form, characterized by 20-electron macrocyclic antiaromaticity.

A,D-Di-p-benzihexaphyrin may adopt three conformations differing in the twist level of the p-conjugated system (T0 - planar form, T1 - Möbius band, T2 - figure eight form with two half-twists). The macrocycle possesses a 28-electron conjugation pathway, which corresponds to macrocyclic antiaromaticity in forms T0 and T2. In the case of the T1 species, the rules of aromatic stabilization are reversed (Heilbronner-Hückel rules), imparting to the T1 form features characteristic of aromaticity (e.g. a diatropic ring current). Switching between the three forms with different topologies may be controlled in several different ways, e.g by changing solvent and temperature, or by acid-base chemistry.