Blue-light-emitting cationic water-soluble polyfluorene derivatives with tunable quaternization degree

The design, synthesis and characterization of novel blue emission cationic water-soluble conjugated polymers based on the polyfluorene (PF) derivatives with amino-terminal groups are described. Water solubility was realized through quaternization of the amino group, which permits a control of the cationic degree, which in turn determines the solubility of the polymers in organic solvents and water. Better solubility in polar solvents was accompanied by a spectral blue shift for polymers with a higher quaternization degree. In the presence of a trace amount of some weak organic acids, the neutral polymer also showed a high solubility in water. Instead of forming a quaternized salt, the ¹H NMR spectra indicated that only electrostatic interaction existed between the acid and the amino-terminal groups, which increased the affinity of the polymers with those polar solvents. In addition to the special solubility, good thermal stability as well as the intense fluorescence both in solutions and as films endows this series of materials with the status of most attractive candidates as the transporting/emitting layer in multilayer device fabrication.