Hybrid dansyl-triazine based mono and bidentate ligands for the selective sensing of fluoride anion through fluorescence enhancement

The design and synthetic strategy of hybrid mono and bidentate dansyl-triazine ligands (DTM and DTD) for selective sensing of fluoride anion are reported. The synthesised compounds were characterised using spectroscopic techniques such as ¹H NMR, LC-MS, IR, UV-vis absorption and emission and corroborated with theoretical methods. Studies showed that the bidendate ligand (DTD) exhibits intra-molecular charge transfer (ICT) from the donor dansyl fluorophore (HOMO) to the triazine acceptor (LUMO). On the other hand, electron density of the HOMO and LUMO of the mono ligand is localised on the dansyl group. Interestingly, these ligands showed selective detection of fluoride anions with a limit of detection (LOD) of 1.31 μM and 294 nM for the DTM and DTD ligands respectively. The electrostatic potential surface mapping suggests that the hydrogen bonding between NH of the ligand and fluoride ion as the underlying mechanism for sensing. Time-correlated single photon counting (TCSPC) analyses further validate this rationale, where DTM showed a lifetime of 3.56 ns and DTD 1.65 ns. Upon binding, the lifetime increases, this may be attributed to the restricted molecular rotation upon hydrogen bonding.