Photoreactivity Driven by Excited State Aromaticity Gain or Antiaromaticity Relief.

Gain of aromaticity or relief of antiaromaticity along a reaction path are important factors to consider in mechanism studies. Analysis of such changes along potential energy surfaces has historically focused on reactions in the electronic ground state (S0) but can also be utilized for excited states. In the lowest ππ* states the electron counts for aromaticity and antiaromaticity follow Baird's rule where 4n π-electrons indicate aromaticity and 4n+2 π-electrons antiaromaticity. Yet, there are also cases where Hückel's rule plays a role in the excited state. The electron count reversals of Baird's rule compared to Hückel's rule explain many altered physicochemical properties upon excitation of (hetero)annulene derivatives. Here we illustrate how gain of excited state aromaticity (ESA) and relief of excited state antiaromaticity (ESAA) impact on photoreactivity and photostability. Emphasis is placed on recent findings supported by results of quantum chemical calculations, and photoreactions in a wide variety of areas are covered.