Photochemical behaviour of β-carbolines. Part 4.1 Acid–base equilibria in the ground and excited states in organic media

Abstract

The protonation equilibrium of six β-carbolines has been studied in organic media. The pK_a values in acetonitrile were determined by spectrophotometric methods. The pK_a values in the first excited singlet state (pK_a*) were calculated from the Förster cycle and the values compared with those obtained by fluorimetric titration. It is concluded that the acid–base equilibrium is not established during the lifetime of the excited state. The differences between the pK_a of the ground and the electronic excited singlet states in different organic media (MeOH, EtOH, PrⁱOH, Bu^tOH, MeCN and CH₂Cl₂) were also calculated.