Tunable multiple light emissions of core–shell structures based on rare earth ions doped on the surfaces of organic cocrystals

Abstract

The charge transfer (CT) interactions play a vital role in tuning the luminescence of organic crystals. The enhanced energy transfer (ET) effect in rare earth (RE) ions is a significant method to achieve long-lifetime fluorescence. These studies are of great significance in the fields of photoelectric functional materials. However, the effect of CT interactions on the process of ET from the cocrystal ligand group to RE ions is unknown. In this work, we have doped Eu³⁺ ions, Tb³⁺ ions and Eu³⁺/Tb³⁺ mixed ions on the surfaces of Phen–TCNB (Phen = 1,10-phenanthroline, and TCNB = 1,2,4,5-tetracyanobenzene) to construct organic cocrystal-type core–shell structures by the epitaxial growth method. The core–shell structures exhibited multiple photoluminescence depending on the types and proportions of RE³⁺ ions that are doped on the surfaces of the cocrystals. Experimental and theoretical investigations prove that the ET enhancements from ligand groups to Eu³⁺ ions originate from appropriate energy differences between the lowest triplet states of Phen–TCNB and the lowest excited state of RE³⁺ ions. In contrast, the reduced Tb^{3+ 5}D₄ lifetime is caused by the energy back transfer process since the energy difference becomes small. These results reveal that the multiple luminescences of the cocrystal-type core–shell structures can be adjusted by the CT and ET, and this study provides a new strategy for developing novel optoelectronic materials.