Copper(i)-iodide based coordination polymers: bifunctional properties related to thermochromism and PMMA-doped polymer film materials

Abstract

Poly(methyl methacrylate) (PMMA) films doped with Cu(I)-based imidazole derivative coordination polymers [Cu₃I₃(bib)_1.5]_n (1) and [Cu₄I₄(bix)₂]_n (2) [bib = 1,4-bis(1-imidazolyl)benzene and bix = 1,4-bis(imidazol-1-ylmethyl)benzene] were synthesized and their photophysical properties were studied. 1 and 2 were prepared by a solvothermal method and structurally characterized by single-crystal X-ray diffraction, IR spectroscopy, ¹H NMR, PXRD and thermal gravimetric analyses. Molecular structural analysis reveals that 1 exhibits a unique one-dimensional (1D) infinite triplex chain and 2 is built from a [Cu₄I₄]_n cluster which possesses an interesting two-dimensional (2D) (4,4)-connected sql (square lattice) network. Detailed structural characterization of the supramolecular organization of 1 and 2 revealed overall three-dimensional (3D) interlinked networks driven by extensive π⋯π stacking interactions. Both 1 and 2 display remarkable narrow band emission with a smaller full width at half-maximum (FWHM) (77 K, 34.63 and 60.07 nm; 298 K, 121.95 and 126.83 nm) in the solid state at 77 K, which leads to excellent monochromaticity. The combination of such a narrow FWHM and the large red-shift of 62 nm from 298 K to 77 K endows 2 with a more prominent thermochromism effect than 1, with emissions strongly depending on temperature and tunable from yellow to red by changing the temperature from 298 K to 77 K. Here, the role of the [Cu₄I₄]_n cluster in controlling the performance of thermochromic luminescence is highlighted. Meanwhile, 1 and 2 demonstrate stronger and longer lifetime yellow luminescence emissions at concentrations of 0.8% (τ = 156.62 μs) and 1.0% (τ = 92.28 μs) in poly(methyl methacrylate) (PMMA). Furthermore, development of easy-to-prepare hybrid materials 1–PMMA and 2–PMMA leads to bright yellow luminescence polymer film materials with outstanding thermal stability in daily applications (321 °C and 500 °C).