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 pKa values in acetonitrile were determined by spectrophotometric methods. The pKa values in the first excited singlet state (pKa*) 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 pKa of the ground and the electronic excited singlet states in different organic media (MeOH, EtOH, PriOH, ButOH, MeCN and CH2Cl2) were also calculated.

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