Fluorescent properties of hydrogen-bonded ellipticine: A special effect of fluoride anion

Fluorescence properties of ellipticine, a plant alkaloid, was studied in the presence of hydrogen-bond acceptors, such as F−, CH3COO−, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and N-methylimidazole in different solvents. Global analysis of the spectrophotometric titration data proved that ellipticine deprotonation in the ground state occurred only in the case of the interaction with two F− anions in acetonitrile. The fluorescence band of the 1:1 complex of anions showed large Stokes shift implying significant proton displacement upon excitation along the hydrogen-bond. The lack of ellipticine-DBU complex emission was rationalized by quick proton transfer in the excited state. The kinetics and mechanism of the excited ellipticine quenching were also revealed.