Hückel's rule
The lowest triplet state of an annulene is, according to Baird's rule, aromatic when it has 4"n" π-electrons and antiaromatic when the π-electron count is 4"n" + 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.