Role of Metal-Chloride Anions in Photoluminescence Regulations for Hybrid Metal Halides.

Organic-inorganic hybrid metal halides with emissive organic cations are of great interest due to their structural diversity and interesting photophysical properties. Here, we assemble emissive organic cations (EnrofloH22+) with different metal-chloride anions (Pb2Cl62- to Bi2Cl104- to SnCl62-) to form the new single crystal phases, and thus the photoluminescence properties of the metal halides, including Stokes shift, full width at half-maximum (FWHM), and photoluminescence quantum yield (PLQY) have been studied accordingly. (EnrofloH2)SnCl6·H2O, as an example, possesses narrow FWHM and high PLQY, which are caused by the strong π-π stacking and inter- and intramolecular hydrogen bonds interactions. Compared with EnrofloH22+ cation in solution, the interactions generate a restraining effect and increase the rigid degree of EnrofloH22+ cation in the bulk single crystals. Our work clarifies the photophysical properties of the EnrofloH22+ organic cations by constructing the inter- and intramolecular interactions and boosts the further study of organic-inorganic hybrid metal halides materials with different luminescence mechanisms.

Realizing Tunable White Light Emission in Lead-Free Indium(III) Bromine Hybrid Single Crystals through Antimony(III) Cation Doping.

Low-dimensional metal halide hybrids (OIMHs) have recently been explored as single-component white-light emitters for use in solid-state lighting. However, it still remains challenging to realize tunable white-light emission in lead-free zero-dimensional (0D) hybrid system. Here, a combination strategy has been proposed through doping Sb3+ enabling and balancing multiple emission centers toward the multiband warm white light. We first synthesized a new lead-free 0D (C8NH12)6InBr9·H2O single crystal, in which isolated [InBr6]3- octahedral units are separated by large organic cations [C8NH12]+. (C8NH12)6InBr9·H2O exhibits dual-band emissions with one intense cyan emission and a weak red emission tail. The low-energy ultrabroadband red emission tail can be greatly enhanced by the Sb3+ doping. Experimental data and first-principles calculations reveal that the original dominant cyan emission is originated from the organic cations [C8NH12]+ and that the broadband red emission is ascribed to self-trapped excitons in [In(Sb)Br6]3-. When the Sb concentration is 0.1%, a single-component warm white-light emission with a photoluminescence quantum efficiency of 23.36%, correlated color temperature of 3347 K, and a color rendering index up to 84 can be achieved. This work represents a significant step toward the realization of single-component white-light emissions in environmental-friendly, high-performance 0D metal halide light-emitting materials.

Synthesis, Crystal Structure and Green Luminescence in Zero-Dimensional Tin Halide (C8H14N2)2SnBr6.

Organic-inorganic hybrid metal halides with broad-band emission are currently receiving an increasing interest for their unique light emission properties. Here we report a novel lead-free zero-dimensional (0D) tin halide, (C8H14N2)2SnBr6, in which isolated [SnBr6]4- octahedrons are cocrystallized with organic cations, 1,3-bis(aminomethyl)benzene (C8H14N22+). Upon photoexcitation, the bulk crystals exhibit broad-band green emission peaking at 507 nm with a full width at half-maximum (fwhm) of 82 nm (0.395 eV), a Stokes shift of 157 nm (1.09 eV), and a photoluminescence quantum yield (PLQY) of 36 ± 4%. Combined structural analysis and density functional theory (DFT) calculations indicate that the excited state structural distortion of [SnBr6]4- octahedral units account for the formation of this green emission. The relatively small Stokes shift and narrow fwhm of the emission are hence caused by the reduced distortion of [SnBr6]4- octahedrons and rigid molecular structure. The discovery of lead-free (C8H14N2)2SnBr6 and insight into the mechanism of green emission provide an essential platform toward unveiling the relationship between structure and property for 0D metal halide perovskites.

Two Covalent Ultraviolet Nonlinear Optical Crystals.

Nonlinear optical (NLO) crystals are the vital components of laser science and technology, as they can convert lasers in common wavelengths into new wavelength bands for ultraviolet (UV), IR, and even terahertz laser output. Known UV NLO crystals mainly focus on crystals containing cations, but covalent crystals have rarely been reported. Here we report two covalent NLO crystals, B2 O3 I and B2 O3 II. According to the first-principles calculations, B2 O3 I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d22 =1.38 pm/V and d24 =0.702 pm/V, as well as sufficient birefringences of 0.037 and 0.031, respectively. Notably, the absorption edges are almost the shortest among NLO crystals. Meanwhile, the NLO coefficients are evidently larger than that of another well-known covalent NLO crystal α-SiO2 and are comparable to those of the commercial UV NLO crystal LiBO3 with Li+ cation. Furthermore, the birefringences are significantly larger than that of α-SiO2 , which are favorable to the phase matching for both crystals. These results reveal that B2 O3 I and B2 O3 II are excellent candidates for UV NLO applications. In-depth calculations are carried out to reveal the origin of excellent NLO properties. These covalent crystals provide a new direction for the research of UV NLO crystals.

Lead-Free Tin(IV)-Based Organic-Inorganic Metal Halide Hybrids with Excellent Stability and Blue-Broadband Emission.

Lead-free zero-dimensional (0D) organic-inorganic metal halide hybrids have recently attracted special attention as luminescent materials. However, their structural stability is still a challenge for the further development. Here, we select Sn4+ as the B-site inorganic cation and obtain a new tin(IV)-based organic-inorganic metal halide hybrids (C6N2H16Cl)2SnCl6 with remarkable air stability. (C6N2H16Cl)2SnCl6 exhibits a blue broadband emission originating from self-trapping excitons (STEs) and the emission intensity remains stable for over three months. When the temperature rises to 450 K, the intensity of photoluminescence can maintain about 50%, indicating the good thermal stability of (C6N2H16Cl)2SnCl6. This work presents a new strategy toward the tin(IV)-based photoluminescent organic-inorganic metal halide hybrids with environmentally friendly, high stability characteristics.

An unprecedented planar π-conjugated [B2P5]5- group with ultra-large birefringence and nonlinearity: an ab initio study.

Searching for new nonlinear optical active (NLO-active) units would greatly promote the development of NLO materials. Herein, we theoretically investigate a newly reported [B2P5]5- group which consisted of two corner-sharing [BP3]3- trigonal planar units. Attributed to the coplanar π-conjugated configuration and the unique connection mode, the [B2P5]5- group can achieve both the largest birefringence (Δn∼ 0.68) and nonlinearity (d15∼ 57.14 pm V-1) among all inorganic planar NLO-active groups, which makes it a good NLO-active unit. Our analysis also reveals that the modulation of the electronegativity of constituent elements upon replacing with the elements in the same period can effectively improve the NLO responses, which would inspire a new direction for designing high performing NLO materials.

KNa4B2P3O13: A Deep-Ultraviolet Transparent Borophosphate Exhibiting Second-Harmonic Generation Response.

A new deep-ultraviolet transparent alkali metal borophosphate KNa4B2P3O13 is synthesized by high-temperature flux method, and its properties were characterized by a combination of experimental and ab initio methods. This compound crystallizes in the noncentrosymmetric Pna21 space group and its framework is formed of one-dimensional infinite [B2P3O13]∞ chains consisting of apex-sharing [BO4] and [PO4] tetrahedra. KNa4B2P3O13 had a short ultraviolet edge (<190 nm) and exhibited a moderate powder second-harmonic generation signal (0.4 × KDP). The discovery of KNa4B2P3O13 enriches the structural diversity of borophosphates and would bring a more comprehensive understanding of the structure-properties relationship in deep-ultraviolet nonlinear optical crystals.

Lone-Pair Enhanced Birefringence in an Alkaline-Earth Metal Tin(II) Phosphate BaSn2 (PO4 )2.

The first alkaline-earth metal tin(II) phosphate, BaSn2 (PO4 )2 , has been discovered, which consists of layered structures constructed from strictly alternating [SnO3 ]4- and [PO4 ]3- moieties. This compound is expected to have a large birefringence with Δn≈0.071 at 1064 nm, owing to the presence of stereochemically active lone pair metal cations.

Na3B7O11F2: a new sodium-rich fluorooxoborate with a unique [B14O24F4] ring and a short ultraviolet absorption edge.

A new sodium-rich fluorooxoborate Na3B7O11F2 features a novel large [B14O24F4] ring formed by two corner-shared [B7O13F2] groups, each containing two [B3O7] anionic units linked by a [BO2F2] group. The band gap of the title compound calculated by using first-principles calculations is 7.69 eV (∼161 nm) and the birefringence refractive index is 0.083 at 193 nm. The results show that it is potentially an excellent deep-ultraviolet (DUV) birefringence crystal.