Highly Efficient Orange-Red/Red Excimer Fluorescence from Dimeric π-π Stacking of Perylene and Its Nanoparticle Applications

To achieve high-efficiency red excimer fluorescence, two novel perylene (PE) derivatives (mTPA-3PE and 2SF-3PE) are designed and synthesized with different monosubstituent triphenylamine (TPA) and spirofluorene (SF), respectively. The photophysical investigations reveal that mTPA-3PE and 2SF-3PE exhibit red/orange-red excimer fluorescence (637 and 610 nm) with long lifetimes (37.86 and 72.41 ns) in crystals but significantly different photoluminescence efficiencies (24 and 77%). Both crystal structure and excited-state property emphasize that the discreteness of the PE dimer with pi-pi stacking is essentially responsible for the high-efficiency excimer emission in the crystal. Using PE as a pi-pi emissive core in this work, highly efficient red excimer fluorescence is harvested in the crystal for the first time. Moreover, their nanoparticles with the same excimer fluorescence exhibit the advantage of easy processing and the promising application in cell imaging. Once again, our results validate the mechanism of excimer-induced enhanced emission (EIEE) by the discrete dimeric pi-pi stacking of PE in the solid state, which opens a new way to develop high-efficiency narrow band gap (e.g., near-infrared) light-emitting materials using the EIEE mechanism in the future.