Parallel Alignment of π-Conjugated Anions in Hydroisocyanurates Enhancing Optical Anisotropy

Noncentrosymmetric (C3H7N6)6(H2PO4)4(HPO4)·4H2O and Centrosymmetric (C3H7N6)2SO4·2H2O: Exploration of Acentric Structure by Combining Planar and Tetrahedral Motifs via Hydrogen Bonds

The combination of organic and inorganic nonlinear active units to obtain organic-inorganic hybrid materials has been proved to be a very effective method to obtain nonlinear optical (NLO) materials with excellent properties. Herein we reported two hybrid melamine-based compounds, namely, acentric (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (1) and centrosymmetric (C₃H₇N₆)₂SO₄·2H₂O (2), which were synthesized via facile solvent evaporation method. Compound 1 features a three-dimensional (3D) network structure composed of _∞[(H₂PO₄)₄(HPO₄)(H₂O)₄]^6- layers which are further linked with _∞[(C₃H₇N₆)₆]⁶⁺ layers via hydrogen bonds. Compound 2 displays a 3D structure composed of [(C₃H₇N₆)₂(SO₄)(H₂O)₂]_∞ layers further linked with each other by hydrogen bonds. Compound 1 presents a second harmonic generation signal of about 0.1 × KDP. Furthermore, UV-vis and infrared spectra, thermal analyses, and theoretical calculation were also adopted to evaluate its NLO performance. The theoretical calculations showed that the SHG response and large birefringence of 1 were primarily caused by the (C₃H₇N₆)⁺, (H₂PO₄)^-, and (HPO₄)^2- groups.

Two metal-free cyanurate crystals with a large optical birefringence resulting from the combination of π-conjugated units

Benefiting from the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups, cyanurate crystals have recently become a research hotspot in birefringent materials. Herein, by combining the (H_xC₃N₃O₃)^x-3 (x = 0-3) group with the (CN₃H₆)⁺ cationic group, two metal-free cyanurates, GU(H₂C₃N₃O₃) (I) and GU₃(H₂C₃N₃O₃)₃(H₃C₃N₃O₃) (II), were obtained by the hydrothermal method. These compounds have wide band gaps (∼5 eV) and a large birefringence (∼0.40@400 nm), demonstrating their potential to be ultraviolet birefringent crystals. Moreover, first-principles calculations indicate that their large birefringence values originated from the synergistic effect of the (CN₃H₆)⁺ cations and (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. These findings provide a new design strategy for exploring low-cost UV birefringent crystals with a large birefringence.

Tin Chloride Sulfates A3Sn2(SO4)3-xCl1+2x (A = K, Rb, Cs; x = 0, 1) as Multifunctional Optical Materials

The series of alkali-metal tin chloride sulfates A₃Sn₂(SO₄)_3-xCl_1+2x (A = K, Rb, Cs; x = 0, 1), K₃Sn₂(SO₄)₃Cl, Rb₃Sn₂(SO₄)₂Cl₃, and Cs₃Sn₂(SO₄)₂Cl₃, were successfully synthesized through an improved mild hydrothermal method. Interestingly, in addition to the cation size effect, the structure-directing effect of anions induces different symmetries in the three title compounds, with K₃Sn₂(SO₄)₃Cl being noncentrosymmetric, while Rb₃Sn₂(SO₄)₂Cl₃ and Cs₃Sn₂(SO₄)₂Cl₃ are centrosymmetric. Powder second-harmonic generation (SHG) measurements indicate that K₃Sn₂(SO₄)₃Cl is a nonlinear optical material that is type I phase matchable with a weak SHG response (0.1× KDP). Photoluminescence tests reveal that the three title compounds emit strong greenish yellow, orange, and salmon light, respectively, under UV excitation, indicating that they are promising inorganic solid fluorescent materials. Simultaneously, a detailed structural analysis of all the known tin(II) halide sulfates has been performed, which will guide the systematic exploration of high-performance tin(II)-based functional materials in the future.

K2Pb(H2C3N3O3)4(H2O)4: a potential UV nonlinear optical material with large birefringence

K₂Pb(H₂C₃N₃O₃)₄(H₂O)₄ (I) features a 2D [K₂PbO₈(H₂O)₄]^12- anionic layer and reveals a moderate SHG signal of approximately 2.6 × KDP.

Enhanced optical anisotropy via dimensional control in alkali-metal chalcogenides

Crystals with both large birefringence and wide transparent range are suitable for broad applications in the areas of optical communications, the laser industry and modulation of the light polarization requirement. In this work, to assist the design of urgently needed crystals with large birefringence in the infrared (IR) region, typical alkali-metal chalcogenides, KPSe6, Na2Ge2Se5, and Li2In2GeSe6 have been studied. They exhibit a hierarchical characteristic in the calculated birefringence by about 0.21, 0.11, and 0.04, respectively. To explore the origin of the birefringence difference, the polarizability anisotropy and the effect of electron distribution anisotropy are analyzed. The alkali-metal chalcogenides KPSe6, Na2Ge2Se5, and Li2In2GeSe6 feature infinite one-dimensional (1D) chains of [PSe6], 2D anionic framework of [Ge2Se5] layers and 3D [In2GeSe9] networks, respectively. It is found that the anionic group with low-dimensional configuration could enhance polarizability anisotropy and render large birefringence for the macroscopic structure. This provides evidence that a low-dimensionality configuration in the structure would be beneficial for the enhancement of optical anisotropy, which can motivate the exploration and design of novel IR birefringent materials.

A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy

The first alkali-metal nitrate isocyanurates, A(H₃C₃N₃O₃)(NO₃) (A = K, Rb), were synthesized by the tactic of introducing (NO₃)^- into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P2₁/c, which featured a 2D [(H₃C₃N₃O₃)(NO₃)]_∞ layered structure separated by K⁺ and Rb⁺ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λ_cutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO₃)^- into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.

Optimal arrangement of π-conjugated anionic groups in hydro-isocyanurates leads to large optical anisotropy and second-harmonic generation effect

Optimal arrangements of π-conjugated anions in alkaline earth metal hydro-isocyanurates with rich structures result in large birefringence and a giant nonlinear optical effect.

Rb3BaTeB7O15: a novel [B7O16] fundamental building block in a new telluroborate with [TeO3] polyhedra

A new telluroborate Rb₃BaTeB₇O₁₅ with a new [B₇O₁₆] fundamental building block has been synthesized, and it is the first telluroborate that is constructed only by using [TeO₃] polyhedra.

Hydroisocyanurates X2Y(H2C3N3O3)4·4H2O (X = K, Cs; Y = Zn, Cd) with large birefringence stemming from π-conjugated (H2C3N3O3)− anions

A series of new hydroisocyanurates X₂Y(H₂C₃N₃O₃)₄·4H₂O (X = K, Cs; Y = Zn, Cd) with large birefringence resulting from the π-conjugated (H₂C₃N₃O₃)⁻ unit have been obtained.

“Old dog, new tricks”: the lone pair effect inducing divergent optical responses in lead cyanurates containing π-bonds

The stereochemically active lone pair effect of Pb²⁺ and induced narrower bandgaps were elaborated in Pb₃(HC₃N₃O₃)₂(OH)₂ for the first time.

Rb3Na(H2C3N3O3)4·3H2O with Large Birefringence

A mixed alkali-metal nonlinear optical (NLO) dihydro-cyanurate crystal Rb₃Na(H₂C₃N₃O₃)₄·3H₂O has been synthesized via the hydrothermal method. Its calculated birefringence is about 0.368, which is very large, its ultraviolet (UV) cutoff edge is down to 230 nm, and the powder second harmonic generation (SHG) intensity is about 0.2 × KDP. In addition, a first-principles investigation of the electronic properties on Rb₃Na(H₂C₃N₃O₃)₄·3H₂O was carried out. The calculated band gap and SHG coefficient values agree well with the experimental ones. These results suggest that it could be applied as a UV birefringent material.

LiO4 tetrahedra lock the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates

The “lock-in effect” of LiO₄ tetrahedra contributes to the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates.