The lowest triplet state of an annulene is, according to Baird's rule, aromatic when it has 4nπ-electrons and antiaromatic when the π-electron count is 4n + 2, where n is any positive integer. This trend is opposite to that predicted by Hückel's rule for the ground state, which is usually the lowest singlet state (S0). Baird's rule has thus become known as the photochemical analogue of Hückel's rule.[3]
Through various theoretical investigations, this rule has also been found to extend to the lowest lying singlet excited state (S1) of small annulenes.[3]
^Baird, N. Colin (1972), "Quantum organic photochemistry. II. Resonance and aromaticity in the lowest 3ππ* state of cyclic hydrocarbons", Journal of the American Chemical Society, 94 (7/12): 4941–4948, doi:10.1021/ja00769a025
^ abRosenberg, Martin; Dahlstrand, Christian; Kilså, Kristine; Ottosson, Henrik (2014-05-28). "Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations". Chemical Reviews. 114 (10): 5379–5425. doi:10.1021/cr300471v. ISSN0009-2665. PMID24712859.