Two types of two-step mechanochromic luminescence of phenanthroimidazolylbenzothiadiazoles†
The reversible color change of the solid-state emission upon exposure to mechanical stimuli is called mechanochromic luminescence (MCL). In spite of the recent growing interest in MCL materials, only a limited number of organic crystalline materials exhibit two-step MCL that responds to the mechanical stimuli of different intensities. Herein, we report two types of two-step MCL as well as bicolor MCL of phenanthroimidazolylbenzothiadiazole derivatives that bear a substituted phenyl group on the nitrogen atom of the phenanthroimidazole ring. One-way type two-step MCL was observed for a p-bromo-substituted derivative, whereas an o-bromo-substituted derivative displayed a rare back-and-forth type two-step MCL. The other derivatives including the m-bromo-substituted compound showed typical bicolor MCL. Specifically, three types of MCL behaviors are exhibited by changing the position of the bromo group. Single-crystal X-ray diffraction analyses revealed that the crystal system is altered by introducing different substituents. The mode of intermolecular interactions differs depending on the crystal system, which should account for the difference in emission wavelength and MCL properties of the crystalline samples. Based on powder X-ray diffraction and differential scanning calorimetry measurements, one-way type MCL can be rationalized in terms of the crystal-to-crystal transitions followed by crystal-to-amorphous transitions. The mechanism of the back-and-forth type MCL is proposed as the destruction of extended intermolecular interactions upon crushing the crystals followed by amorphization upon strong grinding. These results provide useful insights into the future development of multi-step MCL systems with potential applications in advanced mechano-sensing technologies.