Fluorescence enhancement for alkaloids by anions: Spectroscopic and electrochemical characterizations

Alkaloids such as palmatine and berberine are potential phototoxins, but they exhibit low quantum yields for fluorescence (Φ_F). We found that fluoride and hydroxide ions are promising promoters that enhance their values of Φ_F in aprotic and aqueous solutions. For example, an increase of Φ_F by a factor of 10² was observed when palmatine incorporated 40 equiv TBAF in dimethyl sulfoxide (DMSO). The cause of the enhancement in Φ_F was shown to be the interaction of fluoride and hydroxide ions with the quaternary nitrogen atoms contained in the target alkaloid. The effect of substitution of fluoride with other anions such as Cl^-, Br^-, I^-, PF₆^-, and ClO₄^- is insignificant. In aqueous media, the efficacy of fluoride and hydroxide ions was greatly reduced. Nevertheless, introducing DNA helped restore the effectiveness of fluoride in enhancing the value of Φ_F of alkaloids.