Emissive β-diketones (bdks) and difluoroboron complexes (BF2bdks) show multi-stimuli-responsive luminescence in both solution and the solid state. A series of β-diketones (bdks) and boron coordinated dyes (BF2bdks) were synthesized with different cyclic amine ring substituents in the 4-position to explore ring size effects on optical properties. Multi-stimuli-responsive luminescence properties of bdks and BF2bdk dyes with four to seven membered rings (i.e. azetidine, pyrrolidine, piperidine and azepane) were studied, including solvatochromism, viscochromism, aggregation-induced emission (AIE), mechanochromic luminescence (ML) and halochromism. With increasing substituent ring size, red-shifted absorption and emission were observed in CH2Cl2 for both bdk ligands and boron dyes. Density functional theory (DFT) calculations revealed that intramolecular charge transfer (ICT) dominated π−π* transitions in all the ligands and dyes. In solution, the β-diketone ligands and boronated complexes displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. In addition, AIE properties of all the compounds were investigated in THF/H2O mixtures. While azetidine, pyrrolidine and piperidine substituted boron dyes showed aggregation-caused quenching, AIE was observed for the corresponding ligands. In contrast, the azepane ligand showed quenched emission when aggregated, but enhanced emission was observed in the azepane dye. In the solid state, ML and halochromism were investigated. For ML, smearing did not cause a dramatic red-shifted emission for any of the compounds as spin cast films, but a large emission wavelength change was observed for the piperidine ligand after melt quenching on weighing paper. Halochromism studies were performed utilizing trifluoroacetic acid and triethylamine vapors. Large blue-shifted emission was observed in all the boron complex spin cast films after acid vapor annealing, and the original emission was partially recovered after base vapor treatment.

Luminescent cyclic amine substituted β-diketones and difluoroboron complexes

256th National Meeting (2018)

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