Correspondence of Fluorescing States of Naphthols and Naphtholate Anions and its Effect on the Calculation of pKa∗ from Spectral Shifts

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Abstract
The validity of dissociation constants of electronically excited acids and bases, calculated from spectral shifts1,2 accompanying protolytic dissociation, especially where shifts of fluorescence spectra are employed, is dependent, among other factors, upon fluorescence orginating from excited states of the same electronic configuration in both acid and conjugate base.3 In some of the molecules whose excited state acid-base properties have been most extensively studied, notably some derivatives of naphthalene, energy level reversals have been shown to accompany dissociation in the lowest excited singlet state in aqueous solutions.4 The spectroscopic properties and excited state dissociation phenomena of the naphthols have been studied extensively from several points of view.5-9 While the calculation of the excited state dissociation properties of β-naphthol, in which fluorescence occurs from the 1Lb state in both anion and neutral molecule, presents no particular problem with regard to correspondence of the lowest excited singlet states of conjugate acid and base, it appeared that in α-naphthol, by analogy with α-naphthylamine4, fluoescence might occur from the 1La state in the conjugate base and from the 1Lb state in the conjugate acid. Thus it seemed that a careful consideration of the states from which the fluorescences of the conjugate species derived from α-naphthol in fluid aqueous solutions orginated, would be useful.

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