Multiple Flexibilities Trigger Luminescent Piezochromism of Closely Packed Cu(I) Coordination Polymers

The packing states of luminescent molecules can be affected by external stimuli, leading to changes in the emission energy. Compared with the traditional organics and metal-organic frameworks, closely packed coordination polymers (CPs) have more transition types and better stabilities, respectively. However, little attention is paid to the closely packed CPs in the field of piezochromism, and much less to the corresponding optical mechanism. For studying the effect of molecular structure and the energy transfer on piezochromism, ten [Cu2I2] cluster-based CPs with bridging aromatic ligands and terminal triphenylphosphine ligands are constructed to explore the pressure-dependent luminescence, especially the influences of bridging ligand length, supramolecular interactions, and halogen atoms on the wavelength shift for luminescent piezochromism. In these CPs, the conformational and structural flexibilities can be effectively increased by lengthening the bridging ligand and addition of pendent methyl groups in the phosphine ligands, respectively, resulting in more obvious piezochromism. In addition, the XLCT/MLCT ratios are efficiently modulated by adjusting the flexibility of halogen electron cloud, which further affects the piezochromic property. Upon the combined effect of multiple flexibilities, a closely packed CP with a large wavelength shift (77 nm) and reversible piezochromism is found.