Changes in piezochromic luminescence of a charge transfer complex subjected to grinding and isotropic compression

Functional materials revealing mechanochromism behaviors are in high demand for potential applications. Here, we report a 1 : 1 cocrystal of naphtho[2,1-b]benzofuran (BNF) and 1,2,4,5-tetracyanobenzene (TCNB) that displayed a reversible crystal transition (of α- and β-forms) with a resulting change in emission color. The slight differences in cell dimensions, slipping angles between stacking columns, and TCNB orientation enabled this cocrystal to be a sensitive mechanochromic luminescent material. The α-form cocrystal showed significant bathochromic shifts upon being subjected to grinding or isotropic compression, and returned to the original state upon subsequently being subjected to a heating or fuming treatment. Based on detailed spectroscopic and structural analyses, the observed changes in luminescence were mainly attributed to the crystal having undergone a transformation at low pressure and then having formed an amorphous state as a result of damage to its structure.