Changes in weak-base pKBH+ values upon excitation to the first excited singlet state

Abstract

Attention is drawn to the possible uses of excited state pK_BH⁺*(hereafter pK_β*) studies on compounds which only protonate in the ground state in concentrated acid solutions. Fixing the ground state pK_BH⁺(hereafter pK_β) value with the primary nitroaniline H₀^′ scale and calculating pK_β* by the Förster cycle method gives a moderately good indication of the region of acidity in which the fluorescence spectrum will change from the base to the conjugate acid form for some compounds (1- and 2-naphthoic acids, 2-naphthamide, phthalide, and perhaps benzoic acid), but for several others complications have been encountered. For example, with chalcone, 4′-methoxychalcone, and 1,1-diphenylethylene, fluorescence emission by the protonated or unprotonated forms could only be obtained from solutions in which the corresponding ground-state species predominated, despite the large shifts in the absorption and fluorescence spectra on protonation.