The two-stepwise luminescent switching properties of triple-stimuli-responsive platinum(II) complexes bearing 4,4′-bis(2-phenylethynyl)-2,2′-bipyridine ligand

Abstract

Three new square-planar platinum(II) complexes, Pt(PhC≡CbpyC≡CPh)(C≡CC6H4-X-2)2 [PhC≡CbpyC≡CPh = 4,4′-bis(2-phenylethynyl)-2,2′-bipyridine; X = F(1), Cl(2), and Br(3)] were synthesized and characterized. Upon exposure of the ground sample to volatile organic compound (VOC) vapors, a series of metastable structures of three complexes, including yellow solvent-free species 2 and 3, orange species 1, 2·ClCH2CH2Cl and 3·ClCH2CH2Cl, red species 1·1.5(CHCl3), 2·½(CH2Cl2), 2·½(ClCH2CH2Cl), 3·½(Toluene), and 3·½(ClCH2CH2Cl) were isolated. All three complexes exhibit vapor-, thermo-, and mechanical-grinding-triggered triple-stimuli-responsive luminescent switching properties. More interestingly, upon exposure to ClCH2CH2Cl vapor, the red species 2·½(CH2Cl2) or 3·½(Toluene) undergo an unusual time-dependent two-stepwise luminescent switching process and convert to the orange species 2·ClCH2CH2Cl or 3·ClCH2CH2Cl via intermediate 2·½(ClCH2CH2Cl) or 3·½(ClCH2CH2Cl). Furthermore, 1·1.5(CHCl3) also displays a two-stepwise thermoluminescent property and is transformed into orange species 1via intermediate 1·⅛(CHCl3). Furthermore, both 2 and 3 present the irreversible vapochromic and vapoluminescent properties as the yellow and orange species could not be converted to each other or transformed to the red species by absorbing the corresponding VOC vapors. Systematic studies revealed that the vapor-, thermo-, and mechanical-grinding-triggered triple-stimuli-responsive luminescence switching properties of 1–3 are due to the structural conversion and resultant changes of the lowest-energy excited states. The crystal structure, 1H NMR, luminescence spectra and PXRD measurements demonstrate that the two-stepwise luminescent switching behavior of complexes are associated with the stacking structure of species with same kind and different amount of solvent molecule. The irreversible vapoluminescent property of 2 and 3 are most likely due to the closed stacking of species which prevent the inserting of solvent molecules into the crystal lattice. In addition, complexes 1–3 have good application prospects in the visual identification of CHCl3, CH3CN and 1,2-dichloroethane vapors.