High-contrast mechanofluorochromic properties of TPE-modified difluoroboron β-diketonate complexes and doped LDPE

In this paper, three novel tetraphenylethylene (TPE) based difluoroboron β-diketonate complexes (TPEDKBF₂Me, TPEDKBF₂Tf, and TPEDKBF₂Cy) were synthesized, which were composed of different substituents. All the complexes were essentially fluorescent dyes that exhibited aggregation-induced emission (AIE) properties and mechanofluorochromic (MFC) behavior. The UV-Vis and fluorescence (FL) spectra were measured to ascertain the photophysical properties of the fluorescent dyes. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were conducted, and the results showed that the MFC behavior originated from the transformation from the initial crystal state to the ground amorphous state. It was found that the MFC behaviors of the dyes were influenced by various substituents. Among the three dyes studied, TPEDKBF₂Me exhibited the most prominent red-shift from 507 nm to 578 nm, which corresponds to a wavelength shift of the emission peak of 71 nm. The as-prepared dyes were doped into low density polyethylene (LDPE) by melt-blending process. The FL emission of the prepared splines of LDPE showed a discoloration effect under UV light after being stretched. A mechanism of MFC based on the variation of molecular arrangement upon the application of force in polymer was proposed. These MFC polymers with low concentration (0.06 %) dye can be applied more expediently in fields such as damage detection, anti-counterfeiting, and information storage.