Baird aromaticity in excited states and open-shell ground states

Book chapter, 2021

Baird aromaticity describes the stabilization of excited-state molecules due to cyclic conjugation in the same way as Hückel aromaticity is used for the electronic ground state. Baird’s rule is used to make a preliminary evaluation of a ring as aromatic or antiaromatic. For quantitative evaluation, a range of computational methods has been developed and validated. These methods need to be applied with caution and expertise, ensuring that they are used on the right excited state and that the excitation is localized to the ring in question. Baird aromaticity has found applications in, for example, fulvenes and other aromatic chameleons, expanded porphyrins and ground-state triplet molecules. Further research is needed to clarify how Baird’s rule applies to singlet excited states and macrocyclic and polycyclic systems. There is also a need for more knowledge on (anti)aromaticity changes along photoreaction pathways to get a detailed understanding of how it influences photoreactivity.