In this perspective, different expert opinions are gathered on the definition and concept of aromaticity.

Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4n + 2] π-aromatic in the ground state, become [4n + 2] π-antiaromatic in the first ¹ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o-Salicylic acid undergoes ESPT only in the “antiaromatic” S₁ (¹ππ*) state, but not in the “aromatic” S₂ (¹ππ*) state. Stokes’ shifts of structurally related compounds [e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates] vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation. Significance Excited-state proton transfer (ESPT) is universally recognized as a reaction that relaxes the energy of a photoexcited organic compound. It is commonly found in many light-driven processes. Here we identify decisive principles underlying why and when ESPT happens. Our computational investigation of prototypical ESPT reactions finds that the occurrence of ESPT can be explained by an electron-counting rule—Baird’s rule, which remains largely ignored despite having a near–50-y-old history. We emphasize that this surprising connection not only explains the mechanistic principle of ESPT reactions, but it also predicts whether hydrogen bonding interactions that form within and between organic compounds might strengthen or weaken when irradiated by light. Recognizing this relationship has tremendous interpretive merit for organic photochemistry.