No abstract

A 3D hydrogen-bonding metal–organic framework shows a low dielectric constant and relaxation dielectric behavior at high temperature.

A rotorlike supramolecular crystal, {[K­(18-crown-6)]­PbI3}∞, is composed of a linear [PbI3]∞ chain acting as a stator and [K­(18-crown-6)]+ cations fastened to the [PbI3]∞ chain and K–I bond like rotators and axes, respectively. A reversible breaking-symmetry phase transition occurs at ∼305 K. Variable-temperature 1H NMR spectra and dielectrics were used for the dynamic analysis of [K­(18-crown-6)]+ cations in the crystal.

In this study, we used the facile solvent evaporation method to achieve the inorganic-organic hybrid crystals of [triethylpropylammonium][PbI], which have been characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and differential scanning calorimetry as well as single-crystal X-ray structure analysis. The hybrid solid crystallizes in the monoclinic space group P2/c at room temperature and is composed of one-dimensional [PbI] chains, where the neighboring PbI coordination octahedra connect together via the face-sharing mode and the organic cations fall in the spaces between [PbI] chains. The hybrid exhibits a dielectric phase transition with a critical temperature of ca. 432 K, dielectric relaxation at frequencies below 10 Hz, and single-ion conducting behavior, the conductivity of which increases rapidly from 9.43 × 10 S cm at 383 K to 4.47 × 10 S cm at 473 K. The variable-temperature single-crystal and powder X-ray diffraction analyses revealed that the dielectric phase transition is related to the disorder-to-order transformation of cations in the lattice. The electric modulus and impedance spectral analyses further disclosed that the dielectric relaxation arises from the ionic displacement polarization and molecular dipole orientation of cations. The single-ion conductance is due to the migration of cations that fall in the spaces of rigid inorganic [PbI] chains. The phase transition gives rise to this hybrid showing switchable ion-conducting nature around the critical temperature of the phase transition. Besides the fascinating functionalities mentioned above, the hybrid also exhibits a thermochromic luminescence feature originating from the electron transition between the valence and conduction bands of the inorganic [PbI] chain.

Two pairs of MOF-based hybrid enantiomorphs, [NH2(CH3)2]3[Pb2X3(BDC)2] (H2BDC = 1,4-benzenedicarboxylic acid, X = Br or I), have been synthesized using the solvothermal reaction and then manually separated, which are labeled as 1a/1b (X = Br) and 2a/2b (X = I). The isomorphic 1a and 2a crystallize in tetragonal space group P43212, and the isomorphic 1b and 2b in the enantiomorphic space group P41212. Twofold interpenetrated three-dimensional (3-D) networks were built from two sets of equivalent I1O2 type hybrid inorganic-organic frameworks in 1a/1b and 2a/2b. Each I1O2 type hybrid inorganic-organic framework constructs by the inorganic pentagonal bipyramid-shape PbX3O4 (X = Br or I) polyhedral chains along the c-axis, which are further connected though bridged BDC2- ligands in the directions perpendicular to the c-axis. Hybrids 1a/1b and 2a/2b have been characterized by elemental analysis (C, H and N elements), thermogravimetric and powder X-ray diffraction techniques, and UV-visible absorption spectroscopy in the solid state. These hybrids show dual emissions at ambient conditions, which arise from the π-π* electron transition within the aromatic rings in the BDC2- ligands and the electron transition in the inorganic polyhedral semiconducting chains, as well as thermochromic luminescence behavior from 10 to 300 K owing to two emission bands displaying different responses to the temperature change.