Sensitive mechanofluorochromism based on conversion of paired and unpaired enantiomer packing modes

Abstract Mechanofluorochromic organic compounds have attracted increasing attention owing to their application in various fields. Here, dimethyl 1,3-bis(4-bromophenyl)-2-phenyl-1,2,3,6-tetrahydropyrimidine-4,5-dicarboxylate (Me-THP-2Br), a racemic C6-unsubstituted tetrahydropyrimidine with highly sensitive mechanofluorochromic characteristics based on a new mechanism is reported. Me-THP-2Br has excellent aggregation-induced emission (AIE) characteristics, that is, no emission in solution but becomes highly emissive upon aggregation. It can unusually form four stable solid forms, namely, three polymorphic crystals PC, BC and CC emitting purple, blue, cyan fluorescence respectively, and one amorphous state AS with 18–52% fluorescence quantum yields. We find that all these as-prepared solid forms can emit the same green fluorescence (490 nm) when they are ground into powder, and revert to purple fluorescence (425 nm) upon fuming/heating. The green and purple emissions can be switched reversibly by grinding-fuming/heating process. In addition, the ground powder of Me-THP-2Br can be used as an excellent rewritable material. More importantly, its sensitive mechanofluorochromism proves to originate mainly from conversion between the paired RS -packing and unpaired RR/SS -packing of enantiomers, which is first reported. Since racemic organic compounds can be easily obtained by chemical synthetic methods, more racemic organic fluorophores with sensitive mechanofluorochromism are expected to be developed.