Acid–base and spectral properties of β-carbolines. Part 1. Tetrahydro-β-carbolines

Abstract

The absorption and fluorescence spectra of a set of tetrahydro-β-carboline(9H-1,2,3,4-tetrahydropyrido[3,4-b]indole) derivatives in the H_I/pH/H_– range of –11 to +18 have shown the presence of four different molecular species, namely: dication, cation, neutral and anion. Ionization data for the prototropic equilibria involving these species have been obtained spectrophotometrically at 25 °C and comparatively analysed by the Hammett acidity function and the excess acidity methods. The changes of acidity or basicity experienced by those species upon excitation to their lowest singlet excited states have been estimated from the Förster–Weller cycle. The influence of structural variations on the spectral and acid–base properties of these compounds is discussed.