Mechanism of Aggregation-Induced Emission of Berberine and Palmatine in Aqueous Solution

Highly concentrated berberine and palmatine in aqueous solutions exhibit relatively strong fluorescence. The fluorescence decay was analyzed using biexponential analysis, and the shorter and longer lifetime components were assigned to the fluorescence of the monomeric forms of berberine and palmatine and the aggregation-induced emission (AIE) of these molecules, respectively. The obtained thermodynamic parameters suggest that these molecules aggregate through electrostatic interactions mediated by counterions. This electrostatic interaction-mediated aggregation inhibits the quenching of the photoexcited state of berberine and palmatine via intramolecular electron transfer by increasing the charge transfer state energy level. Consequently, the AIE mechanism of berberine and palmatine in aqueous solution can be explained by the recovery of their fluorescence quantum yields. The maximum wavelength and bandwidth of these AIE spectra were shorter and narrower than those of the fluorescence spectra of the monomeric forms, indicating a decrease in the Stokes shift and stabilization of the photoexcited state of berberine and palmatine through aggregation. Aggregation also restored the ability of berberine and palmatine to generate singlet oxygen. This AIE mechanism can be applied to control the photosensitizer activity of alkaloid molecules for phototherapy.