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 (ΦF). We found that fluoride and hydroxide ions are promising promoters that enhance their values of ΦF in aprotic and aqueous solutions. For example, an increase of ΦF by a factor of 102 was observed when palmatine incorporated 40 equiv TBAF in dimethyl sulfoxide (DMSO). The cause of the enhancement in ΦF 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−, PF 6 - , and ClO 4 - 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 ΦF of alkaloids.