3-Aminoquinolinc fluoresces intensely in concentrated acid, dilute acid, neutral and dilute basic aqueous solutions as well as in a variety of organic solvents. However, in moderately concentrated alkaline solutions its fluorescence is quenched. The electronic absorption spectra of the various prototropic forms of 3-aminoquinoline have been studied'*' and the pK, values for the various ground-state prototropic reactions have been determinedj. In the range pH 14 to Ho -10, three ground-state prototropic spccics, the neutral molecule, a monocation protonated at the ring nitrogen, and a doubly protonated cation have been observed (Fig. 1). It is of spectroscopic and photochemical interest to identify the fluorescing species derived from 3-aminoquinoline and to evaluate the nature of the excited state processes by which they are converted from one prototropic form to another. Since solvent shifts of electronic spectra, cspccially those related to hydrogen-bonding, differ from protonation shifts more in degree than in kind, comparison of protonation-induced shifts of absorption and fluorescence spectra with solvent shifts should yield clues about the nature of the interactions of the solute, in ground and electronically excited states, with the solvent.
-NH &) (t) F== QQ""' e wNH2 c I'+' mNH3 2'+' I It m 1p FIN. 1. Prototroplc cquilibrla of3-ammoqumolinc. (I) Immo anlon ; (Il)nCUtrd SPCCiCS ;(III)ring-Protonatcd monocatlon ; (IV) dication. EXPERIMENTAL 3-Aminoquinoline (K&K Laboratories, Inc., Plainview, N.Y.) was purified by three recrystallizations from 50% ethanol-water. The purity was established by the agreement of the corrected excitation spectra with the electronic absorption spectra. Mallinckrodt reagent-grade sulfuric acid and Matheson, Coleman and Bell spectroquality chloroform, p-dioxane, n-heptane and acetonitrile were employed as solvents. Ethanol was distilled under low pressure to remove traces of benzene before use. Sodium hydroxide solutions were prepared from the carbonate-free solution by dilution with distilled deionized water. In the intermediate pH range, phosphate, acetate and borate buffers were employed.