Carbazole-containing difluoroboron β-diketonate dyes: two-photon excited fluorescence in solution and grinding-induced blue-shifted emission in the solid state

Mechanochromic luminescent (MCL) materials have attracted considerable attention in recent years because of their potential applications in various fields and the fundamental importance of understanding the mechanism of mechanochromic luminescence. The most reported MCL materials show a bathochromic shift after external mechanical stimulus. Here we report new MCL materials with hypochromic shift of the emission after grinding. These materials are based on carbazole-containing difluoroboron beta-diketonate complexes. These complexes show strong fluorescence in both the solid state and organic solutions. Our study reveals that the emission of structural symmetrical complexes is less influenced by the polarity of the solvent than those of unsymmetrical complexes and they retain strong fluorescence in highly polar solvents. In addition, the good planarity of the carbazole group and the ICT character of the complexes make them exhibit obvious two-photon excited fluorescence. In the solid state, their emission colours can be tuned reversibly by external stimuli, and they exhibit an unusual hypochromic shift after grinding. Single crystal X-ray analysis of the complexes reveals that the initial close molecular packing is responsible for the blue-shifted emission. Interestingly, both hypsochromic and bathochromic shifts of the emission are realized by different treatments of the samples. Our work provides information on the mechanism of the mechanochromic luminescent properties of organic luminescent materials and expands the potential applications of MCL materials.