Temperature dependence of radiative and non-radiative decay in the luminescence of one-dimensional pyridinium lead halide hybrids.

The photoluminescence properties of organic-inorganic pyridinium lead bromide [(pyH)PbBr3] and iodide [(pyH)PbI3] compounds were investigated as a function of temperature. The inorganic substructure consists of face-sharing chains of PbX6 octahedra. Diffuse reflectance spectra of the compounds show low energy absorption features consistent with charge transfer transitions from the PbX3 chains to the pyridinium cations. Both compounds display extremely weak luminescence at room temperature that becomes strongly enhanced upon cooling to 77 K. Broad, featureless low energy emission (λem > 600 nm) in both compounds have large Stokes shifts [1.1 eV for (pyH)PbBr3 and 0.46 eV for (pyH)PbI3] and are assigned to transitions from self-trapped excitons on the inorganic chains whereas emission at higher energy in (pyH)PbBr3 (λem = 450 nm) is assigned to luminescence from a free exciton state. Analysis of data from temperature-dependent luminescence intensity measurements gives activation energies (Ea) for non-radiative decay of the self-trapped excitons in (pyH)PbBr3 and (pyH)PbI3, (Ea = 0.077 eV and 0.103 eV, respectively) and for the free exciton in (pyH)PbBr3 (Ea = 0.010 eV). Analysis of temperature dependent luminescence lifetime data indicates another non-radiative decay process in (pyH)PbI3 at higher temperatures (Ea = 0.17 eV). A large increase in the luminescence lifetime of (pyH)PbI3 below 80 K is consistent with thermalization between triplet sublevels. Analysis of the luminescence power dependence for (pyH)PbI3 shows superlinear response suggestive of quenching by static traps.

Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids.

We report the growth and photophysical characterization of two polar hybrid lead halide phases, methylenedianiline lead iodide and bromide, (MDA)Pb2I6 and (MDA)Pb2Br6, respectively. The phases crystallize in noncentrosymmetric space group Fdd2, which produces a highly oriented molecular dipole moment that gives rise to second harmonic generation (SHG) upon excitation at 1064 nm. While both compositions are isostructural, the size dependence of the SHG signal suggests that the bromide exhibits a stronger phase-matching response whereas the iodide exhibits a significantly weaker non-phase-matching signal. Similarly, fluorescence from (MDA)Pb2Br6 is observed around 630 nm below 75 K whereas only very weak luminescence from (MDA)Pb2I6 can be seen. We attribute the contrasting optical properties to differences in the character of the halide sublattice and postulate that the increased polarizability of the iodide ions acts to screen the local dipole moment, effectively reducing the local electric field in the crystals.

Reversible Intercalation of Li Ions in an Earth-Abundant Phyllosilicate Clay.

The phyllosilicate family of clays is an intriguing collection of materials that make ideal models for studying the intercalation of alkali ions due to their layered topology and broadly tunable composition space. In this spirit, we present a hydrothermal method to prepare a layered iron phyllosilicate clay, Fe2Si4O10(OH)2, and an evaluation of its electrochemical performance for the (de)insertion of Li ions. Through careful structural refinement, we determined that this iron clay contains a 2:1 stacking sequence, which is directly analogous to the widely studied mineral montmorillonite, with the crystallites adopting a platelike morphology. Cyclic voltammetry and galvanostatic cycling reveal reversible insertion of lithium into the interstitial layers via a solid solution mechanism. Comparison of ion (de)intercalation with reports on other clay systems like muscovite, KFe2.75Si3.25O10(OH)2, which features a rigidly bound interlayer cation, demonstrates that controlling the net charge on the layers with phyllosilicate minerals is a route to enabling reversible cationic intercalation within the structure.

Influence of Dimethyl Sulfoxide on the Structural Topology during Crystallization of PbI2.

Hybrid metal-organic halides are an exciting class of materials that offer the opportunity to examine how fundamental aspects of chemical bonding can influence the structural topology. In this work, we describe how solvent adducts of lead halides can influence the crystallization and subsequent annealing of these hybrid phases. While the size and shape of organic molecules are known to govern the final topology of the hybrid, we show that the affinity of solvent molecules for Pb ions may also play a previously underappreciated role.

Four Lead-free Layered Double Perovskites with the n = 1 Ruddlesden-Popper Structure.

Herein we report the synthesis, structure, and band gaps of four layered halide double perovskites, i.e., BA2Cu0.5In0.5Cl4, BA2Ag0.5In0.5Cl4, BA2Ag0.5Sb0.5Cl4, and BA2Ag0.5Sb0.5Br4 [BA = butylammonium = CH3(CH2)3NH3+], each of which has the n = 1 Ruddlesden-Popper structure. In addition, the crystal structure of BA2Ag0.5Bi0.5Br4 is revisited and that of BA2PbCl4 is reported for the first time. Only BA2Ag0.5Sb0.5Cl4 has the tetragonal I4/mmm symmetry of the undistorted Ruddlesden-Popper structure. The other five compounds have orthorhombic structures due to tilts of the octahedra and orientational ordering of the butylammonium groups. As the lateral dimensions of the inorganic layer decrease, the c/a ratio increases due to decreased interdigitation of the alkyl ends of the butylammonium cations. This structural feature may help to explain the increased stability of the bromide phases with respect to the chloride phases. There are features in the diffraction patterns of BA2Ag0.5Bi0.5Br4 and BA2Cu0.5In0.5Cl4 that suggest ordering of octahedral cations within the layers, but in those compounds there appears to be a high concentration of stacking faults between layers that limits long-range, three-dimensional ordering of cations. In the other cases the scattering powers of the cations (Ag/Sb and Ag/In) are too similar to say anything definitive about cation ordering. The band gaps of these compounds range from 2.65 to 4.27 eV, with the bromide compositions possessing smaller band gaps than the chlorides. The band gaps of layered BA2M0.5M'0.5X4 compositions studied here are roughly 0.5-0.8 eV larger than analogous Cs2MM'X6 cubic double perovskites due to a combination of dimensional reduction (3D → 2D), distortions of the octahedral environment around the M/M' ions, and octahedral tilting distortions.

Postsynthetic Metal Exchange in a Metal-Organic Framework Assembled from Co(III) Diphosphine Pincer Complexes.

A Zr metal-organic framework (MOF) 1-CoCl3 has been synthesized by solvothermal reaction of ZrCl4 with a carboxylic acid-functionalized CoIII-PNNNP pincer complex H4(L-CoCl3) ([L-CoCl3]4- = [(2,6-(NHPAr2)2C6H3)CoCl3]4-, Ar = p-C6H4CO2-). The structure of 1-CoCl3 has been determined by X-ray powder diffraction and exhibits a csq topology that differs from previously reported ftw-net Zr MOFs assembled from related PdII- and PtII-PNNNP pincer complexes. The Co-PNNNP pincer species readily demetallate upon reduction of CoIII to CoII, allowing for transmetalation with late second and third row transition metals in both the homogeneous complex and 1-CoCl3. Reaction of 1-CoCl3 with [Rh(nbd)Cl]2 (nbd = 2,5-nobornadiene) results in complete Rh/Co metal exchange at the supported diphosphine pincer complexes to generate 1-RhCl, which has been inaccessible by direct solvothermal synthesis. Treating 1-CoCl3 with PtCl2(SMe2)2 in the presence of the mild reductant NEt3 resulted in nearly complete Co substitution by Pt. In addition, a mixed metal pincer MOF, 1-PtRh, was generated by sequential substitution of Co with Pt followed by Rh.