Acid–base and spectral properties of β-carbolines. Part 2. Dehydro and fully aromatic β-carbolines

Abstract

The absorption and fluorescence spectra of a set of dehydro and fully aromatic β-carboline (9H-pyrido[3,4-b]indole) derivatives have been investigated in aqueous solutions of varying acidity. Within the pH-range, cationic (nitrogen protonation) and neutral species are evidenced from both absorption and fluorescence spectra. Additional zwitterionic species can also be detected from the fluorescence spectra. In highly basic media, outside the pH-range, anionic species (pyrrolic nitrogen deprotonation) are formed. Ground state ionization constants for the different acid–base equilibria involving these species have been determined spectrophotometrically at 25 °C. Lowest excited singlet state pKas have been estimated from the Forster–Weller cycle. The influence of structural variations on the spectra and on the acid–base properties of these compounds is discussed.