Stimuli Responsive Furan and Thiophene Substituted Difluoroboron β-Diketonate Materials

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.