Efficient and versatile mechanochromic luminescence of phenanthroimidazolylbenzothiadiazoles: tricolor switching and directional control over the chromism†
Mechanochromic luminescence (MCL) refers to the mechanical-stimuli-responsive reversible color change of solid-state-emissive dyes. In spite of recent extensive studies on the development of MCL dyes, most of these display bicolor MCL that usually manifests in bathochromic shifts of the emission color in response to mechanical stimuli. Herein, phenanthroimidazolylbenzothiadiazoles are described as a new class of highly emissive solid-state fluorophores that exhibit more versatile MCL properties. Depending on the substituent on the phenyl group of the benzothiadiazole ring, bathochromically or hypsochromically shifted bicolor MCL as well as tricolor MCL are observed for these dyes. Powder X-ray diffraction and differential scanning calorimetry measurements for the bicolor MCL systems indicated that the mechanism of the emission-color change upon grinding is based on typical crystalline-to-amorphous phase transitions. To investigate the tricolor MCL system in detail, single-particle-level fluorescence microscopy has been applied for the first time, which allowed distinguishing three states with different emission properties. The versatility of the present MCL system was attributed to the formation of different crystal structures by introducing various substituents on the aforementioned phenyl group. These results provide useful insights into generating diverse MCL dyes in the future and thus promote the practical applications of MCL systems.