Fluorene-based novel highly emissive fluorescent molecules with aggregate fluorescence change or aggregation-induced emission enhancement characteristics

Abstract

A series of luminogens containing formamide or fluorenone units, based on fluorene, have been synthesized and characterized by NMR spectroscopy, mass spectrometry and elemental analysis. Their aggregate fluorescence change and aggregation-induced emission enhancement (AIEE) characteristics were investigated by luminescence and UV/Vis spectroscopies. The results indicate that luminogens 1d and 2d, containing formamide units, exhibit significant fluorescence color changing upon aggregate formation, which is a newly observed uncommon and interesting aggregation-induced emission phenomenon. Although the luminogen 3d, with one fluorenone unit and one fluorene unit, was weakly emissive in solution, it was highly fluorescent in the aggregated state and demonstrates typical AIEE characteristics. In comparison with 3d, the luminogen 4d with one fluorenone unit and two fluorene units showed weaker AIEE behavior. In thin-films and in the solid state, luminogen 1d shows intense yellow-green emission, and other luminogens show strong yellow emitting. Among these luminogens, 4d exhibits the highest solid-state emission quantum yield.