Mechanofluorochromic Properties of an AIEE‐Active 2‐Phenylbenzoxazole Derivative: More Than Meets The Eye?

The presence of a dicyanovinyl group on the robust 2‐phenylbenzoxazole framework led to the emergence of aggregation‐induced emission enhancement (AIEE) characteristics. A mechanofluorochromic (MFC) behaviour was observed. The pristine microcrystals emitted yellow‐green light, which became golden yellow upon grinding, with the reverse process observed upon heating or solvent fuming. This phenomenon was accompanied by a reversible evolution of the photoluminescence quantum yields and lifetimes. These variations were attributed to the formation of different types of emissive species in the crystalline phases and in the ground material, which is a mixture of very small microcrystals and amorphous matter. Remarkably, evidence was also given for other spectroscopic effects, which depend on the size, compactness and heterogeneity of the solid samples. These effects are presumably linked to the generation of regions rich in low‐energy exciton traps in polycrystalline and pressed samples, and to light reabsorption. They are enhanced by the fact that the pristine platelets exhibit self‐waveguided edge emission, and they are all the more significant as the MFC effect is rather weak.