Pressure-induced phosphorescence enhancement and piezochromism of a carbazole-based cyclic trinuclear Cu(i) complex.

Interest in piezochromic luminescence has increased in recent decades, even though it is mostly limited to pure organic compounds and fluorescence. In this work, a CuPz (, Pz: pyrazolate) cyclic trinuclear complex (CTC) with two different crystalline polymorphs, namely and , was synthesized. The CTC consists of two functional moieties: carbazole () chromophore and units. In crystals of , discrete --- stacking was found, showing abnormal pressure-induced phosphorescence enhancement (PIPE), which was 12 times stronger at 2.23 GPa compared to under ambient conditions. This novel observation is ascribed to cooperation between heavy-atom effects (, from Cu atoms) and metal-ligand charge-transfer promotion. The infinite π-π stacking of motifs was observed in and it exhibited good piezochromism as the pressure increased. This work demonstrates a new concept in the design of piezochromic materials to achieve PIPE combining organic chromophores and metal-organic phosphorescence emitters.