Multi-Stimuli Responsive Luminescent β-Diketones and Difluoroboron Complexes with Heterocyclic Substituents.

Emissive β-diketones (bdks) and difluoroboron complexes (BF2bdks) exhibit multi-stimuli responsive luminescence, including solvatochromism, viscochromism, aggregation induced emission, thermal and mechanochromic luminescence, halochromism and pH sensing. In this study, a series of six-membered heterocycle-substituted (piperidine, morpholine, 1-methyl piperazine) bdk ligands and boron complexes were synthesized, and their luminescent properties were investigated. All the compounds exhibited red-shifted emission in more polar solvents due to intramolecular charge transfer as well as higher emission intensity in more viscous environments. In response to solubility changes in water/tetrahydrofuran mixtures, while the piperazine bdk ligand showed aggregation caused quenching, the piperidine and morpholine bdks displayed enhanced emission upon aggregation. In the solid state, all ligands exhibited mechanochromism. More dramatic halochromism was observed for the piperidine boron dye spin cast film. In solution, for the boron dyes under varying pH values (1-13), different protonated and deprotonated forms were analyzed according to the measured emission spectra. Graphical abstract Multi-stimuli responsive luminescent properties were investigated for the six-membered heterocycle-substituted β-diketone ligands and difluoroboron complexes.