Polymorphism, Mechanofluorochromism, and Photophysical Characterization of a Carbonyl Substituted Difluoroboron-β-Diketone Derivative

Difluoroboron-β-diketones are known to exhibit very promising mechanochromic luminescence but a greater understanding of these materials and their mechanor-esponsive properties is still desirable. Here we demonstrate that a combination of spectroscopic and physicochemical techniques is necessary to understand the variation of solid-state fluorescence observed under mechanical stress and thermal annealing. For this study, we decided to focus on a new fluorescent compound showing a polymorphic behavior. Our investigation on the mechanofluorochromic properties is based on a thorough spectroscopic study (steady-state and time-resolved) on powder samples and thin films deposited on paper and coverglass substrates. Three different states were identified: two crystalline states (a stable green-emissive and a metastable yellow-emissive one) and an amorphous phase (yellow orange emission). The detailed photophysical properties highlight the dynamic excimer formation processes, which can take place in the yellow-emissive crystalline and amorphous states, along with the effect of the substrate on the thin films composition. These results were confirmed by studying the thermal annealing process with a combined AFM and fluorescence microscope. The switching between these states under mechanical and thermal treatments underlies the fluorescence color changes.