Y(C₆H₅SO₃)₃·9H₂O: strategic integration of ligand substitution and π-conjugated group linkage in sulfates for enhanced ultraviolet nonlinear optical properties

Xiaodong Zhang^ab, Bohui Xu^ab, Deshuai Xiao^c, Xinyuan Zhang^c, Pifu Gong*^a and Zheshuai Lin*^ab
aFunctional Crystals Lab, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
^bUniversity of Chinese Academy of Sciences, Beijing 100049, China
^cTianjin Key Laboratory of Functional Crystal Materials, Institute of Functional Crystals, Tianjin University of Technology, Tianjin 300384, China

First published on 19th August 2025

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Introduction

Nonlinear optical (NLO) crystals are indispensable materials for coherent laser frequency conversion, playing a pivotal role in the rapid development of laser technology and its diverse applications, including ultrafine spectroscopy, entangled photon pair generation, environmental monitoring, and explosive detection.^1–3 To date, commercial NLO materials such as LBO (LiB₃O₅),⁴ β-BBO (β-BaB₂O₄),⁵ KTP (KTiOPO₄),⁶ KDP (KH₂PO₄)⁷ and LN (LiNbO₃)⁸ have demonstrated excellent performance, characterized by suitable transparency ranges, large second-harmonic generation (SHG) responses, and moderate birefringence. These properties stem from the well-arranged NLO-active units within their structures, as elucidated by, e.g., anionic group theory.⁹ However, as laser technology continues to evolve, the discovery of new NLO crystals with good performance has been imperative. Yet, surpassing the performance of commercial crystals using traditional structural units remains a great challenge. Thus, the development of new NLO active units is crucial for the exploration of next-generation NLO materials.^10–15

Very recently, the non-π-conjugated [SO₄]²⁻ unit has garnered significant attention owing to its ability to achieve wide bandgaps, strong SHG responses and rich structural diversity. For example, Zhao et al. reported NH₄NaLi₂(SO₄)₂,¹⁶ which exhibits a bandgap of 5.5 eV and a SHG response of 1.1 × KDP. Similarly, Zou et al. demonstrated CsSbF₂SO₄¹⁷ with a bandgap of 4.76 eV and a SHG response of 3 × KDP. Despite their promising properties, materials whose anionic groups are exclusively composed of non-π-conjugated [SO₄]²⁻ groups typically exhibit low optical anisotropy. This is attributed to their nearly isotropic responses to external optical fields. This limitation typically results in small birefringence (∼0.02), which would hinder the achievement of short phase matching wavelengths.

To address the challenge of low birefringence in sulfates, two primary strategies – typically employed separately – have been developed. The first strategy involves the incorporation of additional units with large structural anisotropy while maintaining the integrity of [SO₄]²⁻ groups. For instance, planar π-conjugated groups, e.g., the [BO₃]³⁻ group, and stereochemically active lone pair (SCALP) cation-centred units, e.g., the [SbO₃Cl₂] group, have been successfully introduced into sulfates, achieving large birefringence values. Notable examples include (NH₄)₂B₄SO₁₀ (0.056 at 1064 nm)¹⁸ reported by Pan et al. and RbSbSO₄Cl₂ (0.112 at 1064 nm)¹⁹ reported by Zou et al. The second strategy focuses on enhancing polarizability anisotropy by modifying the ligands of the [SO₄]²⁻ tetrahedron. Partial substitution of oxygen atoms in [SO₄]²⁻ with other atoms or groups can disrupt the symmetry of the tetrahedron and deform its electron cloud structure, thereby improving polarizability anisotropy. This approach has led to the development of units such as [SO₃F]⁻, [SO₃CH₃]⁻, [SO₃CF₃]⁻ and [SO₃NH₂]⁻, which exhibit relatively large birefringence, enabling short phase-matching wavelengths in NLO sulfates. For example, Ye et al. reported C(NH₂)₃SO₃F²⁰ with a birefringence of 0.133; Lin et al. achieved a birefringence of 0.05 in CsSO₃CF₃²¹ and Luo et al. discovered Sr(SO₃NH₂)₂²² with a birefringence of 0.05.

In this work, we combine the two aforementioned strategies by substituting one oxygen atom in the [SO₄]²⁻ tetrahedron with an additional group exhibiting large structural anisotropy, aiming to develop a new NLO active unit. Specifically, given its substantial hyperpolarizability, the planar π-conjugated benzene ring is chosen as the additional group. The [C₆H₅SO₃]⁻ group, which fuses the π-conjugated benzene ring with the non-π-conjugated [SO₄]²⁻ tetrahedron, is then constructed and subjected to theoretical investigation. This innovative group exhibits both large hyperpolarizability and significant polarizability anisotropy. Based on this optimized structural unit, we successfully synthesize the first UV NLO benzenesulfonate, Y(C₆H₅SO₃)₃·9H₂O. This compound crystallizes in a non-centrosymmetric space group and exhibits a wide UV cutoff edge of 280 nm, a relatively large powder SHG response of 0.8 × KDP, a large birefringence of 0.16 at 1064 nm and a short phase matching wavelength of 300 nm, indicating its potential as a UV NLO material. Furthermore, first principles calculations are performed to elucidate its NLO mechanism, highlighting the [C₆H₅SO₃]⁻ group as a promising NLO active unit for further exploration and material design.

Experimental section

Reagents

Benzenesulfonic acid (98%, Aladdin) and YCl₃·6H₂O (97%, Aladdin) were used directly in the synthesis process without further processing.

Synthesis

Benzenesulfonic acid (7.11 g, 4.5 mmol) was dissolved in 15 mL of deionized water. YCl₃·6H₂O (2.925 g, 1.5 mmol) was then added to the solution, and the mixture was stirred thoroughly until complete dissolution was achieved. The solution was permitted to evaporate gradually at a controlled temperature of 25 °C, which effectively facilitated the crystallization process. After a period of ten days, the formation of single crystals of Y(C₆H₅SO₃)₃·9H₂O was distinctly observed, as they precipitated and settled at the bottom of the beaker.

Single-crystal structure analysis

Single crystals of suitable dimensions (0.01 × 0.01 × 0.01 mm³) were carefully selected for X-ray diffraction (XRD) analysis. Data collection was performed using a Bruker D8 Quest diffractometer equipped with a Mo Kα radiation source (λ = 0.71073 Å). Indexing and data integration were carried out using the APEX3 software suite, employing the difference vectors method. Absorption corrections were applied using the multi-scan method implemented in the SADABS program. The space group was determined using XPREP, which is integrated into the APEX3 package.²³

The crystal structures were initially solved using SHELXT and subsequently refined with SHELXL-2014, employing full-matrix least-squares refinement on F². Anisotropic displacement parameters were included in this refinement process. The structural frameworks of Y(C₆H₅SO₃)₃·9H₂O were visualized and analyzed using the OLEX2 software package. The positions of individual atoms were refined by least-squares minimization using the SHELXL-2014 refinement package.²⁴ Finally, the PLATON program was used to verify the structural symmetry and authenticity.

Powder X-ray diffraction

The powder XRD (PXRD) measurements of Y(C₆H₅SO₃)₃·9H₂O were carried out on a Bruker D8 focus diffractometer (λ = 1.5418 Å and 2θ = 5–80°). The scanning step was set to 0.05°, and the fixed counting time was set to 0.5 s per step.

UV-vis-NIR diffuse reflectance spectra

The UV-vis diffuse reflection data were recorded at room temperature using powder samples and an Agilent Cary 7000 UV/vis/NIR spectrophotometer over 200–2000 nm. BaSO₄ was chosen as the back base.

Birefringence

Birefringence was measured using a Nikon Eclipse polarizing microscope E200MV POL under a visible light filter. The birefringence was calculated using the formula Δn = ΔR/T, where ΔR, Δn, and T represent the optical path difference, the birefringence, and the thickness of the crystal, respectively. A transparent strip crystal was chosen to ensure accuracy. The thickness of the sample was measured using a Bruker Smart Apex II.

Thermal analysis

Thermogravimetric analysis (TGA) was performed on a HITACHI STA 200 instrument. The crystal sample was heated in the temperature range of 30–800 °C at a heating rate of 10 °C min⁻¹.

Powder SHG measurement

The powder SHG of Y(C₆H₅SO₃)₃·9H₂O was measured using the Kurtz–Perry approach²⁵ on a Q-switched Nd:YAG laser (1064 nm). Polycrystalline samples of Y(C₆H₅SO₃)₃·9H₂O were ground into different particle size ranges (10–60, 60–100, 100–150, and 200–250 μm). Accordingly, KH₂PO₄ (KDP) samples with the same particle size ranges were tested for reference.

Computational method

The electronic and optical properties of the [C₆H₅SO₃]⁻ group and related [SO₄]²⁻ based groups, including the LUMO–HOMO (lowest unoccupied molecular orbital–highest occupied molecular orbital) gaps, hyperpolarizability (β) and polarization anisotropy (δ) were investigated using Gaussian 16 software.^26,27 The calculations were performed using the B3LYP functional with the 6-31G basis set, a computational setting widely validated and employed in quantum chemical calculations.²⁸

The CASTEP program based on density functional theory (DFT)²⁹ was employed to calculate the linear and nonlinear optical properties in Y(C₆H₅SO₃)₃·9H₂O. The exchange–correlation (XC) functional is described by the generalized gradient approximation (GGA) with Perdew, Burke and Ernzerhof (PBE) functionals.³⁰ The ion/electron interactions are modelled using norm-conserving pseudopotentials for all the constituent elements. In this model, C 2s₂2p₂, H 1s₁, O 2s₂2p₄, S 3s₂3p₄ and Y 4d₁5s₂ electrons are treated as the valence electrons. A kinetic energy cutoff of 750 eV and Monkhorst–Pack k-point meshes spanning less than 0.07 Å⁻³ in the Brillouin zone were chosen to ensure sufficient accuracy for the present purposes.

The SHG coefficients were calculated using the method developed by our group. In addition, the band-resolved SHG coefficient and SHG-weighted density analyses were performed to intuitively visualize the electronic orbitals and structural motifs contributing to SHG in real space.³¹ Our previous study revealed that the occupied-state contribution via virtual electronic transitions dominates the SHG effect in materials.³²

Results and discussion

The [C₆H₅SO₃]⁻ group can be viewed as a benzene ring replacing one O atom in the [SO₄]²⁻ group. We theoretically investigated its LUMO–HOMO gap (E_LUMO–HOMO), hyperpolarizability (β) and polarization anisotropy (δ), and compared it with those in related [SO₄]²⁻ based groups, including [SO₄]²⁻, [SO₃NH₂]⁻, [SO₃F]⁻, and [SO₃CH₃]⁻. Their structures and calculated results are shown in Fig. 1. As shown, the [SO₄]²⁻, [SO₃NH₂]⁻, [SO₃F]⁻, and [SO₃CH₃]⁻ groups exhibit a tetrahedral configuration, considering the small size of the H atom, while the [SO₃CF₃]⁻ group adopts a distorted tetrahedral geometry. In comparison, the [C₆H₅SO₃]⁻ group features a planar ring connected to a [SO₃] triangular pyramid. These structural differences lead to significant variations in their optical properties. Notably, the NLO-related properties of the [C₆H₅SO₃]⁻ group are studied for the first time, although its polarizability anisotropy has been previously highlighted.³³

The results demonstrate that the [C₆H₅SO₃]⁻ group exhibits superior β and δ, which are approximately two and ten times larger, respectively, than those of the related tetrahedral groups. Meanwhile, a wide E_LUMO–HOMO can be maintained even after the introduction of the π-conjugated benzene ring, which is conducive to applications in the UV range. These findings indicate that the incorporation of the [C₆H₅] moiety significantly enhances the β and δ of the [SO₄]²⁻ group, highlighting the [C₆H₅SO₃]⁻ group as a promising UV NLO active group.

Based on the benzenesulfonic [C₆H₅SO₃]⁻ unit, the first UV NLO benzenesulfonate, Y(C₆H₅SO₃)₃·9H₂O, was successfully synthesized. Single crystals of Y(C₆H₅SO₃)₃·9H₂O were grown using a simple solution evaporation method and a single crystal with the dimensions of 10 × 10 × 2 mm³ was obtained, as shown in Fig. 2a, which demonstrated a good crystal growth habit. Single-crystal XRD measurements were performed to solve and refine the crystal structure of Y(C₆H₅SO₃)₃·9H₂O.

Detailed structural information including crystallographic data and refinement parameters, crystal data and structure refinement, atomic coordinates and equivalent isotropic displacement parameters, anisotropic displacement parameters, selected bond lengths, selected bond angles, and hydrogen atom coordinates with isotropic displacement parameters is provided in Table 1 and Tables S1–S6.

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The PXRD pattern of the title compound is shown in Fig. 2b, which is in good agreement with the calculated results of the single crystal XRD data, indicating the high purity of the obtained samples. The crystal structure of Y(C₆H₅SO₃)₃·9H₂O is displayed in Fig. 2c. This compound crystallizes in the noncentrosymmetric monoclinic space group of P2₁ (no. 4). The asymmetric unit comprises 18, 33, 18, 3 and 1 crystallographically independent C, H, O, S and Y atoms, respectively. Six C and five H atoms constitute a benzene ring, which is further connected with [SO₃] and forms a benzenesulfonic [C₆H₅SO₃]⁻ unit. The Y atom is coordinated by eight O atoms, two of which originate from the [C₆H₅SO₃]⁻ units, while the remaining six are provided by the H₂O molecule. As shown in Fig. 2c, the [C₆H₅SO₃]⁻groups are connected to the Y atom in two ways: one is by sharing the oxygen atom on the [C₆H₅SO₃]⁻ group, and the other is by hydrogen bonding mediated by water molecules nearby. The [C₆H₅SO₃]⁻ groups are alternately arranged on the a–c plane, forming a porous structure. The [YO₈H₁₂] groups occupy the space between coordination chains and hydrogen bonding in stabilizing the crystal structure.

Optical properties

The UV-vis diffuse spectrum of the title compound is displayed in Fig. 3a. Clearly, Y(C₆H₅SO₃)₃·9H₂O exhibits good transmittance extending into the UV region. Its UV cutoff edge is located at 280 nm, corresponding to a wide band gap of 4.35 eV,³⁴ indicating its potential for applications in the UV range. The UV cutoff edge of this compound is comparable to those of other UV NLO materials, such as Li₂(H₂C₄N₂O₃)·2H₂O (260 nm),³⁵ [C(NH₂)₃]₂SO₃S (254 nm),³⁶ and [C(NH₂)₃]₃PO₄·2H₂O (250 nm),³⁷ while it is smaller than those of (NH₄)₂PO₃F (177 nm),³⁸ Cs₃[(BOP)₂(B₃O₇)₃] (165 nm),³⁹ and Rb₃B₁₁P₂O₂₃ (168 nm).⁴⁰

The SHG response of Y(C₆H₅SO₃)₃·9H₂O was evaluated using the Kurtz–Perry method under 1064 nm laser irradiation, with KDP as the reference. As shown in Fig. 3b, the SHG signal of Y(C₆H₅SO₃)₃·9H₂O increases with particle size in the range from 10 to 250 μm, confirming its phase-matching capability. Finally, the SHG response reaches approximately 0.8 × KDP, a value comparable to those of Ba₅P₆O₂₀ (0.8 × KDP),⁴¹ β-(C₂H₅N₄)(NO₃) (0.8 × KDP),⁴² and K₇CaY₂(B₅O₁₀)₃ (1 × KDP),⁴³ and smaller than those of Rb₃Hg₂(SO₄)₃Cl (1.5 × KDP)⁴⁴ and NH₄B₄O₆F (3 × KDP).⁴⁵ It should be emphasized that, despite the large β of the [C₆H₅SO₃]⁻ unit, the NLO-unfavorable arrangement of these units in Y(C₆H₅SO₃)₃·9H₂O may limit further enhancement of the SHG response.

The birefringence of Y(C₆H₅SO₃)₃·9H₂O was measured with a polarizing microscope (Fig. 3c). The optical path difference R was found to be 880 nm and the crystal thickness T was about 6.0 μm. Using the equation Δn = ΔR/T, the birefringence was determined to be 0.147. This value exceeds those of commercial UV NLO materials, such as BBO⁴⁶ and LBO,⁴ and is comparable to those of recently reported UV NLO materials, e.g., Zn(SCN)₂ (0.164 at 546 nm),⁴⁷ Na[BF₂(CN)₂] (0.152 at 546.1 nm),⁴⁸ and Rb₂SbFP₂O₇ (0.15 at 546 nm).⁴⁹ From a structural perspective, the large birefringence can be attributed to the planar arrangement of the π-conjugated benzene rings in Y(C₆H₅SO₃)₃·9H₂O.

First-principles calculations were performed to evaluate the phase-matching behavior of Y(C₆H₅SO₃)₃·9H₂O (Fig. 4b). The calculated birefringence value at 1064 nm was 0.16, in good agreement with the experimental value. Based on the calculated refractive index dispersion curves, the shortest phase-matching wavelength was predicted to be 300 nm, indicating its potential for NLO applications in the UV region. According to the Kleinman symmetry restriction, Y(C₆H₅SO₃)₃·9H₂O possesses four independent SHG coefficients, i.e., d₁₄, d₁₆, d₂₂ and d₂₃. The first-principles values were 0.07 pm V⁻¹, 0.29 pm V⁻¹, −0.50 pm V⁻¹ and 0.04 pm V⁻¹ for these SHG coefficients, respectively, with the maximum value of d₂₂ = −0.50 pm V⁻¹, which agree well with the experimental result. To further elucidate the relationship between structure and NLO performance, the electronic band structure and partial density of states (PDOS) in Y(C₆H₅SO₃)₃·9H₂O were calculated, and the band-resolved analysis of the d₂₂ coefficient was performed. As shown in Fig. 4a, Y(C₆H₅SO₃)₃·9H₂O is a direct bandgap compound. The orbitals located at both the conduction band minimum and the valence band maximum are occupied by O and C from the [C₆H₅SO₃]⁻ group. Meanwhile, the band-resolved plot shows that the orbitals nearby the band gap play dominant roles in determining the SHG response, d₂₂ in this case, indicating that the [C₆H₅SO₃]⁻ group makes a dominant contribution to the NLO properties. To further clearly show the contributions of these orbitals, the SHG weighted densities are shown in Fig. 4c. Clearly, the orbitals are mainly from the [C₆H₅SO₃]⁻ group. For further exploration, [C₆H₅SO₃]⁻ groups may be used as good UV NLO active units and both larger SHG responses and birefringence are expected if they are arranged in an orderly manner.

Conclusions

In conclusion, the integration of a π-conjugated benzene ring into a non-π-conjugated [SO₄]²⁻ tetrahedron led to the discovery of the [C₆H₅SO₃]⁻ group as an exceptional UV NLO active unit, characterized by its large hyperpolarizability and significant polarizability anisotropy. Capitalizing on this discovery, we successfully synthesized the first UV NLO benzenesulfonate, Y(C₆H₅SO₃)₃·9H₂O. This compound exhibits a wide bandgap, a relatively strong SHG response, a significant birefringence, and a short phase-matching wavelength, collectively highlighting its potential as a promising UV NLO material. Additionally, first-principles calculations were conducted to unravel the underlying NLO mechanism. This work not only introduces a new candidate for UV NLO materials but also establishes the [C₆H₅SO₃]⁻ group as a high-performance UV NLO active unit for further exploration and material design.

Conflicts of interest

There are no conflicts of interest to report.

Data availability

The data supporting this article have been included as part of the SI.

TG/DTG curves of Y(C₆H₅SO₃)₃·9H₂O, crystal data and structure refinement, atomic coordinates (×10⁴) and equivalent isotropic displacement parameters (Ų × 10³), anisotropic displacement parameters (Ų × 10³), selected bond lengths, selected bond angles (deg), hydrogen atom coordinates (Å × 10⁴) and isotropic displacement parameters (Ų × 10³) of Y(C₆H₅SO₃)₃·9H₂O. CCDC 2366861 contains the supplementary crystallographic data for this paper. See DOI: https://doi.org/10.1039/d5qi01311a.

CCDC 2366861 contains the supplementary crystallographic data for this paper.⁵⁰

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 22275201 and 22133004). P. G. acknowledges support from the Youth Innovation Promotion Association CAS.

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