Photochemical behaviour of beta-carbolines .4. Acid-base equilibria in the ground and excited states in organic media

The protonation equilibrium of six beta-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 Forster 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, Pr'OH, Bu'OH, MeCN and CH2Cl) were also calculated.