Mechanochromic luminescent property and anti-counterfeiting application of AIE-active cyclometalated platinum(II) complexes featuring a fused five-six-membered metallacycle

Abstract

Two couples of pinene-containing N^C*N and C*N^N-coordinated Pt(II) complexes featuring a fused five-six-membered metallacycle have been developed, and their structures were confirmed by NMR, MS and single crystal X-ray crystallography. Cyclometalated complex (−)-2 has two polymorphs involving yellow-orange form (−)-2·CH3COCH3 and orange form (−)-2·CH3SOCH3, revealing different conformations and intermolecular π−π interactions. Complexes (−)-1 and (−)-2 display prominent aggregation-induced emission (AIE) in THF-water solution system with emission quantum yield (Φem) strengthening from 0.2% to 37%. Despite the structural similarity of complexes (−)-1 and (−)-2, different mechanochromic luminescent phenomena have been distinguished. No detectable change in color and luminescence of yellow solids (−)-1 can be found during the grinding process, whereas yellow solids of complex (−)-2 show a reversible mechanochormic luminescent behavior and a simultaneous crystal-to-amorphous transformation. It is speculated that dimeric structures containing weak π−π interactions are formed under the mechanical force, and simultaneously the molecular conformation becomes more planar, improving the conjugation, inducing the emissive state variance with more 3MLCT/3ILCT character. Moreover, anti-counterfeiting application based on complex (−)-2 has been explored. This research provides a new design strategy for exploring AIE-active Pt(II) complexes with high-contrast mechanochromic luminescence.