How does excited-state antiaromaticity affect the acidity strengths of photoacids?

Abstract

Photoacids like substituted naphthalenes (X = OH, NH₃⁺, COOH) are aromatic in the S₀ state and antiaromatic in the S₁ state. Nucleus independent chemical shifts analyses reveal that deprotonation relieves antiaromaticity in the excited conjugate base, and that the degree of “antiaromaticity relief” explains why some photoacids are stronger than others.