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 Cu(3)Pz(3) (Cu-3, Pz: pyrazolate) cyclic trinuclear complex (CTC) with two different crystalline polymorphs, namely 1a and 1b, was synthesized. The CTC consists of two functional moieties: carbazole (Cz) chromophore and Cu-3 units. In crystals of 1a, discrete Cz-Cu-3-Cu-3-Cz 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 (i.e., from Cu atoms) and metal-ligand charge-transfer promotion. The infinite pi-pi stacking of Cz motifs was observed in 1b and it exhibited good piezochromism as the pressure increased. This work demonstrates a new concept in the design of piezochromic materials to achieve PIPE via combining organic chromophores and metal-organic phosphorescence emitters.

Automated summary

This study demonstrates the design of a new piezochromic material by synthesizing a Cu(3)Pz(3) cyclic trinuclear complex. The complex exhibits an abnormal pressure-induced phosphorescence enhancement in one crystalline polymorph, while good piezochromism is observed in another polymorph with infinite pi-pi stacking of carbazole motifs.