Dual Emissions and Thermochromic Luminescence Behavior of Chloroplumbate Organic–Inorganic Hybrid Crystals

Two hybrids, [BMIP][Pb₂Cl₆] (1) and [BMIP][Pb_1.5Cl₅]·H₂O (2) {BMIP²⁺ = 1,3-bis(1-methylimidazolium)propane}, are achieved by mutual diffusion of inorganic and organic components in anhydrous DMF and DMF with a trace amount of water (the trace amount of water originates from MnCl₂·4H₂O), respectively. Single-crystal X-ray diffraction data are collected for 1 in 300–100 K and 2 in 370–100 K, revealing that 1 and 2 crystallize in monoclinic space group P2₁/c and triclinic space group P1, respectively. Crystal 1 contains [Pb₄Cl₁₂]_∞ twin chains, rarely reported, constructed from edge-sharing PbCl₆ octahedra, and dications surround the [Pb₄Cl₁₂]_∞ twin chains. Crystal 2 includes [Pb₃Cl₁₀]_∞ single chains, comprised of distorted PbCl₆ octahedra linked together in face- and edge-sharing manners, with dications occupying the space among the [Pb₃Cl₁₀]_∞ chains. The solid-state UV/Visible absorption spectra at ambient temperature, and the excitation and emission spectra in 10–300 K for 1 and 2 are investigated, showing dual emissions and thermochromism, originating from the distinct response of the two emission bands to temperature. In particular, the plots of the intensity ratio between the two emission bands versus the temperature display a linear relationship in 10–85 K and 150–300 K for 2, respectively, indicating its potential application in luminescent thermometers, a phenomenon which has scarcely been reported in lead–halide-based hybrids.