The dual-emissive properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdkPLA) materials have been utilized for biological oxygen sensing. In this work, BF2dbm(X)PLA materials were synthesized, where X = H, F, Cl, Br, and I. The effects of changing the halide substituent and PLA polymer chain length on the optical properties in dilute CH2Cl2 solutions and solid-state polymer films were studied. These luminescent materials show fluorescence, phosphorescence, and lifetime tunability on the basis of molecular weight, as well as lifetime modulation via the halide substituent. Short BF2dbm(Br)PLA (6.0 kDa) and both short and long BF2dbm(I)PLA polymers (6.0 or 20.3 kDa) have fluorescence and intense phosphorescence ideal for ratiometric oxygen sensing. The lighter halide-dye polymers with hydrogen, fluorine, and chlorine substitution have longer phosphorescence lifetimes and can be utilized as ultrasensitive oxygen sensors. Photostability was also analyzed for the polymer films.

Though the effects of arene substituents on difluoroboron βdiketonate (BF 2 bdk) mechanochromic luminescence (ML) are known, substitution at the α carbon of the dioxaborine ring has yet to be carefully investigated. Substitution at the α position can force these dyes out of a planar conformation, causing materials to take on unique aggregation-induced enhanced emission (AIEE) character. Furthermore, if ML arises from the formation of H-aggregates upon smearing, as postulated, substitution at the α position could hinder this process and influence ML. To test this hypothesis, luminescent BF 2 bdk derivatives containing substituents at the α position were synthesized and their optical and morphological properties were characterized by fluorescence spectroscopy, UV/vis spectroscopy, atomic force microscopy (AFM), differential scanning calorimetry (DSC), and density functional theory (DFT) calculations. Specifically, a difluoroboron dibenzoylethane derivative with a methyl group at the α position exhibits bright blue emission in the solid state, but it is practically nonemissive in solution (Φ F < 1% in CH 2 Cl 2 ). Additionally, it shows clear AIEE behavior in varying water fractions (f w ) with either dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF) as the solvent. In comparison, a difluoroboron 2-benzoyl-1-tetralone derivative is much brighter in solution (Φ F = 61% in CH 2 Cl 2 ) and only a decrease of emission intensity was observed upon inducing aggregation. Furthermore, a methoxyphenyl-substituted dye exhibited bright, observable emission from a twisted intramolecular charge transfer (TICT) state in solution (Φ F = 16% in CH 2 Cl 2 ) and unique attenuation of emission intensity in low f w and AIEE at higher f w with DMSO as the solvent. We demonstrate that stereoelectronic perturbations of this kind can be used to tune the stimuli-responsive emission properties of these dyes.

No abstract

Difluoroboron β-diketonate (BF2bdk) compounds exhibit solid-state switchable luminescence under excitation by UV light. This property is usually manifested as a blue-shift in emission when dye films are thermally annealed followed by a red-shift in response to mechanical shear (i.e. mechanochromic luminescence). Here we report thiophene and furan heterocycle-substituted dyes bearing short methoxy and long dodecyloxy chain substituents. Optical properties of the dyes were investigated in solution, pristine powders, and films on paper and glass. The structural and thermal properties of the dyes were also investigated by powder x-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The methoxy-substituted dyes exhibited neither thermal nor mechano-responsive behavior, however, addition of a longer C12 alkoxyl chain substituent resulted in stimuli-responsive behavior. The furan and thiophene dodecyloxy-substituted dyes both exhibit high-contrast reversible luminescence switching between crystalline, blue-shifted and amorphous red-shifted emissive states. For the furan dye, gentle heating produced a green emissive form while melting followed by rapid cooling produced an orange emissive form. The thiophene dye, on the other hand, exhibited blue-shifted emission when annealed below its melting temperature and red-shifted emission when smeared with a cotton swab (mechanochromic luminescence). These transformations for both dyes were found to be completely reversible, making them potential candidates for applications requiring reusable, functional materials.