Vapor-, thermo-, and mechanical-grinding-triggered tri-stimuli-responsive luminescence switching of cycloplatinated(II) complex bearing 8-quinolinol derivatives

Abstract

Four novel cycloplatinated(II) complexes, Pt(ppy)(L1) (1), Pt(ppy)(L2) (2), Pt(ppy)(L3) (3) and Pt(ppy)(L4) (4) based on 5-[2-(4-substituent-phenyl)ethynyl]-8-quinolinol ligands {substituent = H (L1), Me(L2), Et(L3), t-Bu(L4)} were synthesized and characterized. Complex 4 has three main pseudopolymorphs, including orange 4·CH3CH2OH, red 4R, and dark-brown 4·½(CCl4). Complex 4 exhibits interesting tri-stimuli-responsive luminescence switching properties. Upon exposure to different VOC vapors, three pseudopolymorphs display unusual irreversible vapoluminescence that 4R and 4·½ (CCl4) can be exchanged with each other and be irreversibly converted to 4·CH3CH2OH. Furthermore, 4·½(CCl4) shows reversible thermochromic luminescence behaviour and can be reversibly converted to 4R upon heating. No detectable change in color and luminescence of 4·CH3CH2OH and 4R can be found during the heating process, although 4·CH3CH2OH lost its solvent and changed to desolvated 4O. In addition, 4·CH3CH2OH, and 4·½ (CCl4) also show the mechanoluminescent property that both of them will be converted to the same red amorphous 4G peaked at 742 nm after grinding. Systematic studies revealed that the tri-stimuli-responsive luminescence switching property of 4 is due to the structural conversion and resultant changes of the lowest-energy excited states. In contrast, complexes 1–3 have not any luminescent switching property. The reason is most likely due to the compact stacking structures of them which prevent their structural conversion triggered by external stimuli.