Figure 3.

Absorption spectra of pHA (24, X = H; dashed, in red) in neutral water (λ_max = 278 nm), of pHA^– in 0.05 M aqueous NaOH (λ_max = 325 nm; solid, in yellow), and of HpHA⁺ in 70% aqueous HClO₄ (λ_max = 333 nm; dash–dot, in blue). The triplet excited state equilibria are shown in Scheme 18. The pK_a of ground state pHA is 7.9 ± 0.1 (the concentration quotient at ionic strength I = 0.1 M, 25 °C). Adapted with permission from ref (89). Copyright 2012 American Chemical Society.