Indolylbenzothiadiazoles with varying substituents on the indole ring: a systematic study on the self-recovering mechanochromic luminescence

Self-recovering mechanochromic luminophores are able to autonomously regain their original color at room temperature after exposure to mechanical stimuli. In spite of recent extensive studies on mechanochromic luminophores, the generation of mechanochromic fluorophores with specific sel-frecovering behavior remains a challenging task owing to a lack of systematic studies on self-recovering mechanochromic luminophores. Herein, a series of self-recovering mechanochromic fluorophores with different solid-state emission colors and recovery times is described. The introduction of different substituents onto the indole ring of 4-(1H-indol-2-yl)-2,1,3-benzothiadiazole afforded control over the mechanochromic luminescence properties of the fluorophores. X-ray diffraction analyses revealed slipped stacks or no intermolecular stacking of the benzothiadiazole rings in the crystalline state of these dyes, which is indicative of the relative lability of the crystal structures upon exposure to mechanical stimuli. Remarkably, the fluorescence spectra after mechanical stimulus (grinding) were in good agreement with those of the molten fluorophores. Based on powder X-ray diffraction and DSC analyses of crystalline and amorphous samples, we propose an amorphization of the crystalline fluorophores to be responsible for the change of emission color in response to mechanical stimuli. Accordingly, the self-recovering nature should be rationalized in terms of a spontaneous recrystallization of the fluorophores from the amorphous states, which would be facilitated by unchanged partial crystalline phases in the immediate vicinity.