Mass spectral studies were carried out using HP1100 LC–MS mass spectrometer bypassing the LC step. 1H NMR and 13C NMR were measured on an AVANCE III 400 MHz Digital NMR spectrometer with chemical shifts reported in ppm (in DMSO-d6; TMS as internal standard). Elemental analysis of C, H, O and N was taken in a Leco CHNS-932. UV–vis spectra were recorded on a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). Fluorescence emission spectra were recorded on a RF5301PC instrument (Shimadzu). All the pH values were measured with a model PHS-3C meter (Shanghai Leici Equipment Factory, China).