Synthesis of 13-substituted derivatives of berberine: Aggregation-induced emission enhancement and pH sensitive property

Abstract Five kinds of berberine derivatives with hydroxy, methyl, ethyl, benzyl and (4-methyl)benzyl group at 13-position have been synthesized and characterized. 13-hydroxyberberine chloride exhibits aggregation-induced emission enhancement (AIEE) property because of hydroxy group, which is beneficial to increase the rigidity of molecule by enlarging conjugated system. Optical properties in pure solution, CH3OH/H2O mixed solution, amorphous and crystalline state were comparatively investigated. Polymeric morphology and particle size of 13-hydroxyberberine chloride with different water fractions (0–90 vol%) were obtained by scanning electron microscope (SEM) and dynamic light scattering (DLS) method respectively, which provided reasonable explanation that the formation of small globular nanoparticles in mixed solution is conducive to the fluorescence emission. The single crystal structure of 13-hydroxyberberine chloride was determined by single-crystal X-ray diffraction. Crystallographic data indicated that the main mechanism of the AIEE phenomenon is the existences of J-aggregation (head to tail dipole stacking) combined with molecular planarization. The calculation done by DFT showed that the HOMO–LUMO bandgap is in accordance with experimental data. To further explore the biomedical application of 13-hydroxyberberine, its cell viability and cell imaging performance were examined, which demonstrate that 13-hydroxyberberine shows definite fluorescent intensity. In all, 13-hydroxyberberine should be a promising candidate for different biomedical application such as pH fluorescence probe because of its response to the stimuli of pH value.