Illuminating antiaromaticity

Discovered at the dawn of the modern chemical age, aromaticity is a concept deeply familiar to multiple generations of chemists. Benzene was first isolated in 1825 by Michael Faraday, from an oily residue of the compressed gas used to light the lamps of his Royal Institution laboratory, and its stable and unreactive nature relative to its peers was soon recognised. Our understanding of benzene’s bonding, structure and stability has matured in step with advances in chemical theory. Over many decades, the reliable stabilising influence of the benzene ring has been usefully incorporated into countless molecular structures.

But put benzene under the spotlight and less familiar behaviour emerges. As benzene becomes photoexcited its character shifts radically. The molecule synonymous with stability becomes highly unstable, rearranging near-instantaneously to form fulvene and benzvalene, an explosive white powder. In the photoexcited state, benzene is antiaromatic.