Process-dependent reversible mechanochromic luminescence of bismuth based polymorphs

The two polymorphs a-[BiBr3(bp2mo)2] (1) and g-[BiBr3(bp2mo)2] (2), which are aggregation induced phosphors (AIP), exhibit mechanochromic luminescence (MCL) properties: the grinding (g) of samples involves a crystal to amorphous state transition together with a change in luminescence from green (1: lem = 525 nm, 2: lem = 503 nm) to red (1-g: lem = 611 nm, 2-g: lem = 614 nm). The process is reversible by heating, exposure to vapor (H2O, ACN) or by recrystallization in a few drops of ACN. Interestingly, the heating and fuming processes of both ground samples 1-g and 2-g allow the recovery of the thermodynamic phase 1, while the fast recrystallization of 1-g and 2-g leads to the kinetic phase 2. Several applications can be envisaged for this unprecedented ''one compound/three types of luminescence (g, 1, 2)'' system, in the fields of organic compound detection, and humidity and thermal sensing.