Mechanochromic luminescence of halide-substituted difluoroboron β-diketonate dyes

Abstract

Many difluoroboron β-diketonate (BF₂bdk) compounds display mechanochromic luminescence (ML) and substituents can influence their optical properties. For example, the difluoroboron iododibenzoylmethane derivative, BF₂dbm(I)OC₁₂H₂₅, exhibits mechanochromic luminescence quenching (MLQ). When annealed films of this dye are smeared under air, the perturbed region becomes dim under UV light. In this study, effects of differing halide substituents on ML were investigated. A series of dyes of the form BF₂dbm(X)OC₁₂H₂₅, where X = H, F, Cl, Br, and I, were synthesized. The luminescence properties of these dyes were studied in CH₂Cl₂ solution, as films on weighing paper, spin-cast films on glass substrates, and as bulk powders. All of the dyes exhibit ML and MLQ. Solid-state emissions from these dyes are also sensitive to annealing temperature. The F, Cl, and Br dyes required higher annealing temperatures than the H and I analogues to achieve ordered emissive states. Spin-cast films of the dyes on glass were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD), revealing a change from an amorphous material in the as-spun state to an ordered, crystalline material after annealing, with the F and Cl dyes showing a propensity to form large lamellar crystallites. Powders were also investigated using XRD and showed changes in diffraction patterns corresponding to thermal annealing and grinding. Finally, halogen substituents greatly diminished the ability of the dye films to recover their ordered emissive states after smearing under ambient conditions. These dyes could be useful for applications in which a more permanent inscription is desirable.