Cs3 Na(H2 C3 N3 O3 )4 ·3H2 O: A Mixed Alkali-Metal Hydroisocyanurate Nonlinear Optical Material Containing π-Conjugated Six-Membered-Ring Units

N2H4Zn(HC3N3O3): exceptionally strong second harmonic generation and ultra-long phosphorescence

The discovery and designed synthesis of multifunctional materials is a leading pursuit in materials science. Herein, we report a novel hydro-isocyanurate, N2H4Zn(HC3N3O3), which combines strong second harmonic generation (SHG) and ultra-long room-temperature phosphorescence (RTP). The SHG intensity is the highest within the cyanurate system (13 × KDP), and RTP lifetime extends up to 448 ms, accompanied by a long-lasting afterglow visible to the naked eye for 1.2 s, surpassing most of the current metal-organic complexes. This advancement holds promise for the development of multifunctional optoelectronic devices, particularly leveraging second-harmonic generation (SHG) processes.

Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO materials.

(C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O: Exploring Birefringent Crystals in Hybrid Halide Systems

In this study, new hybrid birefringent crystals of (C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O were successfully synthesized by introducing a new birefringent group [C8H7N2O2]+ by a simple aqueous solution evaporation method. They crystallize in the P21/n space group, and their structure consists mainly of the π-conjugated group [C8H7N2O2]+ and the octahedron centered on Bi3+. By first-principles calculations, the birefringence response comes from the [C8H7N2O2]+ group with a planar π-conjugated structure. Meanwhile, the synthesis, structure, first-principles calculations, and optical properties are reported in this paper.

Rational Combination of π-Conjugated and Non-π-Conjugated Groups Achieving Strong Nonlinear Optical Response, Large Optical Anisotropy, and UV Light-Switchable Fluorescence

Combining π-conjugated and non-π-conjugated groups is an important strategy for synthesizing new nonlinear optical (NLO) crystals. However, the second harmonic generation (SHG) response and optical anisotropy can be limited by improper spatial alignment of these functional groups in the crystal structure. In this work, it is revealed that non-π-conjugated [NH2SO3] group acts as both hydrogen bond donor and acceptor, effectively regulating the 2D planar structure formed by π-conjugated [C4N3H6] groups. The resulting organic-inorganic hybrid crystal C4N3H6SO3NH2 exhibits a strong SHG response (2.5 × KDP), large optical anisotropy (0.233@546 nm), and blue-violet and green fluorescence near 360 and 520 nm, respectively. This work expands the methodology for creating new NLO crystals through organic-inorganic hybridization, while also showcasing the potential of C4N3H6SO3NH2 as a multifunctional optical material.

Mg(C3 O4 H2 )(H2 O)2 : A New Ultraviolet Nonlinear Optical Material Derived from KBe2 BO3 F2 with High Performance and Excellent Water-Resistance

Acquiring high-performance ultraviolet (UV) nonlinear optical (NLO) materials that simultaneously exhibit a strong second harmonic generation (SHG) coefficients, as short as possible SHG phase-matching (PM) wavelength and non-hygroscopic properties has consistently posed a significant challenge. Herein, through multicomponent modification of KBe2 BO3 F2 (KBBF), an excellent UV NLO crystal, Mg(C3 O4 H2 )(H2 O)2 , was successfully synthesized in malonic system. This material possesses a unique 2D NLO-favorable electroneutral [Mg(C3 O4 H2 )3 (H2 O)2 ]∞ layer, resulting in the rare coexistence of a strong SHG response of 3×KDP (@1064 nm) and short PM wavelength of 200 nm. More importantly, it exhibits exceptional water resistance, which is rare among ionic organic NLO crystals. Theoretical calculations revealed that its excellent water-resistant may be originated from its small available cavity volumes, which is similar to the famous LiB3 O5 (LBO). Therefore, excellent NLO properties and stability against air and moisture indicate it should be a promising UV NLO crystal.

NaCl·CH4N2O·H2O: An Organic-Inorganic Hybrid Ultraviolet Nonlinear Optical Crystal with Optimized Comprehensive Properties

Ultraviolet (UV) nonlinear optical (NLO) crystals are a vital component in laser systems to modulate frequency. Organic groups have the advantages of easy synthesis, structural diversity, and large hyperpolarizations (β), which can be combined with inorganic groups to form unique organic-inorganic hybrid NLO crystal materials. Herein, a urea-containing crystal NaCl·CH4N2O·H2O is screened out and large-size crystals have been grown by a simple aqueous solution method. Compared with most reported organic-inorganic hybrid NLO materials, it has optimized comprehensive properties including a large SHG effect of 1.53 × KDP, a moderate birefringence (0.084@1064 nm), and a short UV cutoff edge of 209 nm. First-principles studies reveal that the dominant contributor to the linear and nonlinear optical properties is the urea molecule.

α- and β-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and β-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and β-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

Perfectly Encoding π-Conjugated Anions in the RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE₅ (C₃ N₃ O₃ )(OH)₁₂ (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH₂ PO₄ ), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C₃ N₃ O₃ )^3- anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

K3C6N7O3·2H2O: A Multifunctional Nonlinear Optical Cyamelurate Crystal with Colossal π-Conjugated Orbitals

The delocalized π-conjugated units are considered as an advantageous gene for improving the optical nonlinearity of acentric crystals. For the first time, we synthesized a new acentric SHG-active metal cyamelurate crystal K₃C₆N₇O₃·2H₂O (I) by a facile solution method, containing a colossal planar π-conjugated (C₆N₇O₃)^3- unit. It displays a strong second-order harmonic generation (SHG) of 4 × KDP and a giant anisotropic birefringence of 0.446 at 1064 nm. The theoretical calculations reveal that such substantial improvement is contributed from the strong molecular susceptibility of (C₆N₇O₃)^3- units and their near-perfect coplanar arrangement. Moreover, I exhibits a broadband ultraviolet photoluminescence at 366 nm, suggesting its multifunctional capacity and great potential for compact highly integrated optoelectronic devices.

Directional Construction of New Nonlinear Optical Bifunctional Units through Molecular Engineering Design Inspired by the B3O7-Typed Configuration

Studies on new functional structural units with both large hyperpolarizability and high anisotropy are essentially important for finding high-performance nonlinear optical (NLO) materials and enriching the material systems. Under the guidance of the "structure-analogue" strategy, the work utilizes the molecular engineering approach to direct the construction of target units, BC₂N₅O₂ and B(C₂N₅)₂ units. The BC₂N₅O₂ unit with a highly analogous structure to the B₃O₇ group and its derivate B(C₂N₅)₂ unit with a configuration of B₅O₁₀ group are designed as NLO-active units. Furthermore, two compounds with these new NLO-active units, BC₂N₅H₆(OH)₂·H₂O (I) and B(C₂N₅H_6.5)₂(NO₃)₂ (II), are synthesized, successfully. These compounds exhibit excellent properties with second-harmonic generation (SHG) responses ranging from 0.5 to 5.9 times that of KDP and large birefringence (Δn_I = 0.181 @ 546.1 nm and Δn_II = 0.148 @ 546.1 nm). Theoretical calculations prove that the BC₂N₅O₂ and B(C₂N₅)₂ units make great contributions to the SHG effects and birefringence, which confirms that the BC₂N₅O₂ and B(C₂N₅)₂ units are novel NLO bifunctional units and could be excellent fundamental building blocks to construct amounts of novel NLO and birefringence crystals. Our studies would enlighten the research studies on biguanide complexes of boron.

A flexible functional module to regulate ultraviolet optical nonlinearity for achieving a balance between a second-harmonic generation response and birefringence

Rigid planar π-conjugated groups are adopted for designing ultraviolet (UV) nonlinear optical (NLO) materials extensively. However, for these UV NLO crystals, the realization of a strong second harmonic generation (SHG) response is commonly accompanied by undesired overlarge birefringence. Herein, we propose a new functional gene, the flexible π-conjugated (C3H2O4)2- group, for designing a UV NLO crystal with a balance between the SHG response and birefringence. Furthermore, the combination of low-coordinated and high-coordinated alkali cations with the flexible (C3H2O4)2- group results in finding a new mixed alkali malonate, KLi(C3H2O4)·H2O (KLMW). As expected, KLMW exhibits a strong SHG efficiency (3 × KDP) and moderate birefringence (0.103 @ 1064 nm). In addition, it has a short UV cut-off edge of 231 nm and can be conveniently grown from solution. More importantly, it realized fourth harmonic generation with type-I phase-matching. Therefore, these excellent properties make KLMW a potential practical UV NLO material.

From Pb(H2C3N3O3)(OH) to Pb(H2C3N3O3)F: Homovalent Anion Substitution-Induced Band Gap Enlargement and Birefringence Enhancement

Birefringent materials capable of modulating the polarization of light have attracted intensive studies because of their wide utilization in optical communication and the laser industry. Herein, two new lead(II)-based cyanurates, namely, Pb(H₂C₃N₃O₃)X (X = OH, F), were synthesized by hydrothermal methods, and the first halogen-containing metal cyanurate Pb(H₂C₃N₃O₃)F was successfully obtained by the rational substitution of a homovalent anion. Pb(H₂C₃N₃O₃)X (X = OH, F) belong to space group P1̅, and their structures display a neutral [Pb(H₂C₃N₃O₃)X] (X = OH, F) layer. The Pb²⁺ ions in Pb(H₂C₃N₃O₃)(OH) are interconnected by hydroxyl groups and oxygen atoms of cyanurate anions into a 1D [PbO(OH)]^- chain, whereas the Pb²⁺ ions in Pb(H₂C₃N₃O₃)F are interconnected by F^- anions and oxygen atoms of cyanurate anions into a 2D [PbOF]^- layer. The π-π interactions between adjacent hydroisocyanurate rings and the hydrogen bonds between neighboring neutral layers provide additional stability to the structures. Luminescent studies show that Pb(H₂C₃N₃O₃)(OH) and Pb(H₂C₃N₃O₃)F emit yellow-green and blue light, respectively. Theoretical calculations unveiled their birefringences of 0.079 and 0.203@1064 nm and their band gaps of 3.96 and 4.96 eV, respectively, for OH^- and F^- containing materials. Obviously, the substitution of OH^- by F^- with the largest electronegativity can simultaneously improve both the birefringence and band gap.

Co-crystal AX·(H3C3N3O3) (A = Na, Rb, Cs; X = Br, I): a series of strongly anisotropic alkali halide cyanurates with a planar structural motif and large birefringence

Birefringent crystals with strong anisotropy are important components in modern optical devices. The newly discovered planar π-conjugated cyanurate group (H_xC₃N₃O₃)^x-3 (x = 0-3) has been demonstrated as an effective functional motif for improving birefringence in the ultraviolet region. Here, single co-crystals of alkali halide cyanurates, RbBr·(H₃C₃N₃O₃) (I), RbI·(H₃C₃N₃O₃) (II), and CsBr·(H₃C₃N₃O₃) (III) were synthesized by the ethanol solution method, and NaBr·(H₃C₃N₃O₃) (IV) was obtained via the solvent-drop grinding method. These four co-crystals feature a planar (H₃C₃N₃O₃) arrangement and exhibit wide band gaps (> 4.90 eV), tunable birefringence (Δn_exp∼ 0.124-0.256), and high thermal stability (156 °C-349 °C). In addition, first principles calculations were also carried out to evaluate the relationship between molecule density, spatial arrangement and optical birefringence, and suggested a great tailoring effect of the alkali metal and halogen species on regulating the optical anisotropy of co-crystal cyanurates.

AZn4(OH)4(C3N3O3)2 (A = Mg, Zn): Two Zn-Based Cyanurate Crystals with Various Cation Coordination and Large Birefringence

Cyanurate crystals have recently become a research hot spot in birefringent materials owing to the large structural and optical anisotropy of the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. In this study, two new Zn-based cyanurate crystals, Zn₅(OH)₄(C₃N₃O₃)₂ and MgZn₄(OH)₄(C₃N₃O₃)₂, were synthesized by the hydrothermal method. In Zn₅(OH)₄(C₃N₃O₃)₂, the d¹⁰ Zn²⁺ cations have three different coordinating environments, which has never been found in cyanurates. These compounds have wide band gaps (∼5 eV) and large birefringence (∼0.32 at 400 nm), indicating their potential as ultraviolet birefringence crystals.

Zn(H2C3N3O3)2·3H2O: the first single-d10 transition metal based ultraviolet hydroisocyanurate crystal with large birefringence

The first single-d¹⁰ transition metal hydroisocyanurate crystal was designed and synthesized successfully with large band gaps and optical anisotropy.

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.

2(C3H7N6)+·2Cl-·H2O: an ultraviolet nonlinear optical crystal with large birefrigence and strong second-harmonic generation

Large birefringence (Δn = 0.277 at 546 nm) and strong second-harmonic generation (SHG) intensity (4.3 × KDP) achieved a remarkable balance in a potential ultraviolet nonlinear optical crystal: 2(C₃H₇N₆)⁺·2Cl^-·H₂O. Theoretical calculations showed that planar benzene-like π-conjugated [C₃N₆] groups had important optical properties.

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.

An Exceptional Peroxide Birefringent Material Resulting from d-π Interactions

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb₂ VO(O₂ )₂ F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb₂ VO(O₂ )₂ F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first-principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O₂ ^2- anions and V⁵⁺ 3d orbitals, which may be also favorable to the stability in the air for Rb₂ VO(O₂ )₂ F. These findings distinguish peroxides as a brand-new class of birefringent materials that may possess birefringence superior to the traditional oxides.

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.

K4Cu3(C3N3O3)2X (X = Cl, Br): strong anisotropic layered semiconductors containing mixed p–p and d–p conjugated π-bonds

Metal cyanurates K₄Cu₃(C₃N₃O₃)₂X (X = Cl, Br) containing π-conjugated anions are synthesized in flame-sealed silica tubes and they exhibit 2D graphene-like layered structures and intriguing semiconductor behaviors.

RbNa(HC3N3O3)·2H2O exhibiting a strong second harmonic generation response and large birefringence as a new potential UV nonlinear optical material

A new hydro-isocyanurate, RbNa(HC₃N₃O₃)·2H₂O, with a strong SHG response and large birefringence, has been successfully grown by a hydrothermal method.

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.

Inorganic planar π-conjugated groups in nonlinear optical crystals: review and outlook

Nonlinear optical crystals with various inorganic planar π-conjugated groups are reviewed based on their bandgaps.

“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.

Second-order nonlinear optical materials with a benzene-like conjugated π system

This feature article highlights the strategies used to design UV/DUV NLO materials based on benzene-like π-conjugated units.

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.

RE(H2C3N3O3)2·(OH)·xH2O (RE = La, Y and Gd): potential UV birefringent materials with strong optical anisotropy originating from the (H2C3N3O3)- group

A new series of dihydro-isocyanurates, namely, RE(H₂C₃N₃O₃)₂·(OH)·xH₂O (RE = La, Y and Gd) was readily obtained by a mild hydrothermal technique using lithium hydroxide as a deprotonation agent. La(H₂C₃N₃O₃)₂·OH·2H₂O and Re(H₂C₃N₃O₃)₂·OH·5H₂O (Re = Y, Gd) crystallized in the centrosymmetric (CS) space groups P2₁/c and P1[combining macron], respectively. Their intricate 3D frameworks were built by cationic polyhedrons (LaO₈N/ReO₈) and (H₂C₃N₃O₃)^- groups. The experiments and theoretical studies demonstrated that these compounds possessed short ultraviolet cut-off edges (≤240 nm) and large birefringences (Δn≥ 0.257@546.1 nm). In addition, the theoretical studies further revealed that the (H₂C₃N₃O₃)^2- group plays a pivotal role in strengthening the optical anisotropy for all compounds.

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.

Co-crystal LiCl·(H3C3N3O3): a promising solar-blind nonlinear optical crystal with giant nonlinearity from coplanar π-conjugated groups

Co-crystal LiCl·(H₃C₃N₃O₃) with the largest SHG coefficient among all solar-blind nonlinear optical crystals was studied by first principles calculations.

Intrinsic zero thermal expansion in cube cyanurate K6Cd3(C3N3O3)4

The novel cubic cyanurate K₆Cd₃(C₃N₃O₃)₄ exhibits zero thermal expansion behavior with a very low thermal expansion coefficient of 0.06 MK⁻¹ in a broad temperature range from 10 to 130 K.

A rich structural chemistry in π-conjugated hydroisocyanurates: layered structures of A2B(H2C3N3O3)4·nH2O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) with high ultraviolet transparency and strong optical anisotropy

Mixed alkali/alkali-earth metal hydroisocyanurates A₂B(H₂C₃N₃O₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Ca, n = 4, 10) with strong optical anisotropy were grown.

“Two in one”: an unprecedented mixed anion, Ba2(C3N3O3)(CNO), with the coexistence of isolated sp and sp2 π-conjugated groups

The first mixed cyanate–cyanurate double salt Ba₂(C₃N₃O₃)(CNO) containing two types of π-conjugated groups was successfully synthesized and systematically characterized.