Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations

Aromaticity is one of the most widely applied concepts within chemistry. It plays a major role in the rationalization of a range of chemical properties in the electronic ground state, despite the fact that it cannot be uniquely defined and only indirectly be measured and quantified. Although the concepts of aromaticity and antiaromaticity traditionally are considered only for the singlet ground state of molecules, these concepts can also be shown to influence electronically excited states of both singlet and triplet multiplicities. They allow for rationalization of a number of different photophysical and photochemical observations, observations that are usually not set in relation to an excited state (anti)aromatic influence. The field then progressed into a more dormant phase which lasted from the 1980s until the late 1990s, despite several highly important experimental findings being made during those decades. However, it has seen a revival in the past few years, primarily within the theoretical and computational chemistry community.