Constructing triplet trap in Eu(III) complexes and corresponding application for halogen fluorescent optical sensing
In this paper, a series of Eu(III) complexes was designed using 2-thenoyltrifluoroacetone (TTA) as the first ligand, and phenanthroline derivatives as the second ligand. Their molecular structure was analyzed via their single crystals. Detailed photophysical analysis on these Eu(III) complexes suggested that ligand TTA was the major energy transfer antenna for emissive Eu(III). The phenanthroline-derived ligands showed low triplet levels (T1), leading to a reverse energy transfer (triplet trap) from Eu(III) 5D0 to T1 of diamine ligands. This procedure actually compromised emission performance of these Eu(III) complexes. It was found that halogen anions, especially fluorine ion, interacted with these diamine ligands, which heightened triplet level of these diamine ligands. With the elimination of such triplet trap in these Eu(III) complexes, their Eu(III) emission was greatly improved, showing emission turn-on effect. Detailed sensing performance was discussed carefully. Eu(TTA)3Phen showed a down-bending working curve following a two-site model Stern-Volmer equation. Eu(TTA)3DPPZ and Eu(TTA)3DPOX showed linear working curves with good selectivity. In addition to its emission turn-on sensing, Eu(TTA)3DPOX still showed ratiometric sensing. Detailed sensing mechanism was discussed and confirmed.