Difluoroboron β-diketonate (BF2bdk) dyes display impressive optical properties in solution, polymer matrices and as solid-state molecular materials, and can report on the surrounding milieu. In solution, fluorescence color changes in response to solvent polarity and viscosity. In the latter case, emission intensities may be enhanced by restricting intramolecular motions in more viscous environments, for example, upon cytosol-to-membrane transitions of dyes in biological environments. In addition to liquid solvents, polymer matrices can also control the dielectric constant or rigidity of the dye environment. Data suggest that polymer glass transition temperature and crystallinity, along with dye/polymer loading are all important factors. In fact, polymers such as poly(lactic acid) (PLA) enable room temperature phosphorescence, which, in conjunction with fluorescence, can be used for ratiometric oxygen sensing. This has been exploited for biomedical O2 imaging in tumor, wound and brain contexts. Increasing dye-dye interactions with higher dye loading is associated with redshifted emission, providing a different way, besides bdk ligand design, for tuning emission color. Similar trends are also noted as dyes, or even BF2-free bdk ligands, are gradually precipitated by non-solvents in aggregation induced emission (AIE) experiments. That is, amorphous aggregates in liquid suspension or in spin cast films often show redshifted emission compared to the corresponding monomeric dyes in dilute solution or polymer matrices. Amorphous states may also be accessed mechanically, via scratching or smearing thermally annealed spin cast dye films. Thermally annealing these polymer- and solvent-free dye films generates more ordered states that are perturbed upon shearing. In addition to mechanical activation of boron dye molecular solids, interesting optical phenomena have also been noted upon thermal treatment or acid/base exposure. Progress in the study of stimuli responsive and environment sensitive BF2bdk dyes will be presented. Synthesis, properties, and applications will be discussed.

Stimuli responsive difluoroboron β-diketonate dyes in different environments

256th National Meeting (2018)

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