Effect of chlorine as substituent on the photochemistry and acid–base properties of β-carboline alkaloids

The UV-absorption, fluorescence excitation and emission spectra of the 6-chloro-, 8-chloro-, 6,8-dichloro-derivatives of nor-harmane, harmane and harmine and the 8-chloro-derivative of harmol were studied. These studies were performed in EtOH and in EtOH + 1% perchloric acid solutions (pa). Furthermore, fluorescence quantum yields (ϕf) in both media and in acetonitrile and acetonitrile + 1% perchloric acid solutions at 298 K were measured. The HOMO and LUMO energy, the positions (λmax) and oscillator strength (f) of the 1S1 ← 1S0 band for all the neutral and protonated β-carbolines studied were calculated and compared with the experimental data. The pKa values in aqueous solution for for 6-chloro-, 8-chloro- and 6,8-dichloro-nor-harmane, harmane and harmine and 8-chloro-harmol were spectrophotometrically measured (pKa(H2O))(pKa(H2O)). The change of the acid–base character of these compounds on going from the ground state (pKa) to the first electronic excited singlet state (pKa∗) as ΔpKa=pKa∗-pKa, in ethanol solution at 298 K were calculated (ΔpKa(EtOH)). Ground-state proton affinity (PA) for all the compounds studied defined as minus the enthalpy change of the reaction M + H+ → MH+ (gas state) were calculated. Basicity relative to pyridine (ΔHrPy) defined as the enthalpy change of the isodesmic reaction MH+ + Py → M + PyH+ in gas state and in water solution, were also calculated (ab initio calculations). The effect of chlorine as substituent on the photochemistry and acid–base properties of the β-carboline alkaloids is discussed.