Halide-driven polarity tuning and optimized SHG-bandgap balance in (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I)

Abstract

In the quest for organic–inorganic metal halides (OIMHs) that harmoniously combine large second harmonic generation (SHG) efficiency with broad bandgaps, our study introduces a series of noncentrosymmetric and polar piperazine zinc halides, (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I). The distinct influence of halide anion size on the configuration of ZnNX₃ tetrahedra is uncovered, revealing a design principle for enhancing SHG activity and manipulating bandgap characteristics. Specifically, (C₄H₁₁N₂)ZnCl₃ is the first example of deep-ultraviolet (deep-UV) nonlinear optical (NLO) OIMHs, with UV transparency lower than 190 nm and moderate SHG effect, 0.8 times KH₂PO₄ (KDP). Meanwhile, compared with (C₄H₁₁N₂)ZnI₃ (2.1 times KDP, 4.52 eV), (C₄H₁₁N₂)ZnBr₃ boasts a widened bandgap of 5.53 eV while maintaining a striking SHG response (2.5 times KDP), representing a pinnacle in the SHG-bandgap balance among OIMHs with E_g exceeding 5.0 eV. Computational analyses underscore the critical roles of halide orbitals and ZnNX₃ distortions in dictating SHG efficiency, with increasing halide polarizability correlating with heightened SHG contributions. This study paves the way for a novel approach to designing short-UV NLO crystals based on OIMHs.

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Introduction

The interplay of light and matter at the nexus of laser physics and optical materials science unveils captivating phenomena exemplified by second harmonic generation (SHG).^1–7 This effect empowers nonlinear optical (NLO) crystals to generate a vast laser spectrum, spanning from far-infrared to the elusive deep-ultraviolet (deep-UV, λ < 200 nm) realm. Lasers penetrating the short or even deep-UV regions hold immense potential for ultrafast spectroscopy, high-density information storage, and precision instrument fabrication.^8–17 However, despite their alluring capabilities, commercialization of UV NLO crystals remains sluggish, dominated by established materials like KH₂PO₄ (KDP), β-BaB₂O₄ (BBO), and LiB₃O₅ (LBO). This lag stems from stringent performance requirements for efficient UV laser output. Ideal NLO crystals for this domain must exhibit NCS space groups, robust SHG coefficients (d_ij > KDP), wide bandgaps for transparency in the short or deep-UV region (E_g > 5.0 eV), moderate birefringence for phase matching (Δn > 0.04), and favorable crystal growth habits.^18–25

Anionic groups play a pivotal role in designing NLO crystals. Planar π-conjugated (BO₃)³⁻ and (B₃O₆)³⁻ anions, recognized by anionic group theory, exhibit high anisotropy, large first-order hyperpolarizability, and wide HOMO–LUMO bandgaps – key factors for outstanding NLO properties.^26–31 Non-π-conjugated tetrahedra like (PO₄)³⁻ and (SO₄)²⁻, with their wider HOMO–LUMO gaps, also contribute as NLO building blocks, exemplified by commercially available KH₂PO₄.^32–35 Recent discoveries have yielded novel sulfate- and phosphate-based NLO crystals, such as NH₄NaLi₂(SO₄)₂ (1.1 × KDP),³⁶ Ba₃P₃O₁₀Cl (0.5 × KDP),³⁷ RbNaMgP₂O₇ (1.5 × KDP),³⁸ Li₉Na₃Rb₂(SO₄)₇ (1.3 × KDP),³⁹ and ZrF₂(SO₄) (3.2 × KDP).⁴⁰ However, tetrahedral groups often suffer from low anisotropy and polarizability due to their high symmetry, leading to challenges within NCS space groups and minimal birefringence (Δn < 0.01).^32–35 To address this limitation, researchers have explored disrupting the high symmetry of tetrahedra by partially substituting O atoms with other elements.^3,9,10,41 This strategy, exemplified by the enhanced birefringence of LiSO₃F (0.057) compared to Li₂SO₄ (0.004), demonstrates the effectiveness of introducing distortion.³⁴ Additionally, S–O–Org units, where Org represents an organic group, have emerged as promising SHG functional units, leading to NLO crystals,^42–46 like Ba(SO₃CH₃)₂ (1.5 × KDP),⁴⁵ Cs₂Mg(NH₂SO₃)₄·4H₂O (2.3 × KDP),⁴² and SO₂(NH₂)₂ (4 × KDP).⁴⁴

On the other hand, it has been observed that the creation of M–X–Org (M: metal; X: halide; Org: organic group) asymmetric units can yield NLO OIMHs with a strong SHG effect, such as (C₄H₁₁N₂)CdI₃ (6 × KDP).^47–49 Furthermore, in comparison to SO₃F or S–O–Org units, M–X–Org units are more readily obtained at the experimental level, often through the controlled utilization of HX based on previously obtained MX₄-containing OIMHs.⁴⁷ However, the bandgap of (C₄H₁₁N₂)CdI₃, 4.10 eV, is insufficient for short UV NLO crystal applications.⁴⁷ Besides, in the case of [C₆H₅(CH₂)₄NH₃]₄BiX₇·H₂O, the wider bandgap of the Br compound (3.52 eV) compared to the isomorphous I compound (2.29 eV) does not maintain a stronger SHG effect.⁵⁰ In fact, the SHG effect of the Br compound (0.4 × KDP) is significantly smaller than that of the I compound (1.3 × AGS), in which KDP: d₃₃ = 0.39 pm V⁻¹ and AGS: d₃₆ = 12.6 pm V⁻¹. This reveals that the electronegativity-driven halogen substitution strategy does not ensure both a wide bandgap (E_g > 5.0 eV) and a strong SHG response (d_ij > KDP), thereby presenting a challenge in the exploration of high performance NLO in short UV band.^47–58

To address this challenge, our efforts have been concentrated on the piperazine–Zn–Cl/Br/I system for several reasons. Firstly, the piperazine cation exhibits a wider HOMO–LUMO gap compared to the pyrazine cation (Fig. S1†), despite having a similar N-hybridized structure but lacking a π-conjugated configuration. Furthermore, d¹⁰ transition metals such as Zn²⁺, Cd²⁺, and Hg²⁺ only display weak d–d electron transitions, indicating good transparency in the UV regions. And, in contrast to toxic Cd²⁺ and Hg²⁺, Zn²⁺ is an environmentally friendly cation. Secondly, the Zn²⁺ cation is a typical Soft Lewis Acid, while the N-hybridized piperazine is a Soft Lewis Base. Therefore, they readily form a Zn–N coordination bond, constructing polarizable Zn–X–PIP groups, which can serve as SHG functional units. Finally, it is noteworthy that replacing iodide with the smaller radius bromide can induce changes in both the polar axis and dipole moment.⁵⁹ Consequently, we have grounds to believe that halogen substitution may not only impact optical performance due to electronegativity differences but also potentially enhance the arrangement of SHG motifs through size effects, thereby influencing SHG performance. As a result, the size effect of halides could counterbalance the significant decrease in SHG effect resulting from variances in bandgap during halide substitution.

As a result, we have successfully synthesized three novel piperazine zinc halides, namely (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I). Notably, the UV cutoff edge of (C₄H₁₁N₂)ZnCl₃ falls below 190 nm, and it exhibits a moderate SHG effect at 0.8 times that of KDP, marking it as the first reported deep-UV NLO OIMHs. Additionally, when compared to (C₄H₁₁N₂)ZnI₃ (4.52 eV, 2.1 × KDP), (C₄H₁₁N₂)ZnBr₃ features an unprecedented balance between bandgap and SHG response, 5.53 eV and 2.5 × KDP, the largest value among all reported OIMHs with bandgap wider than 5.0 eV. This unusual relationship is primarily attributed to the ideally arranged ZnNBr₃, whereas ZnNI₃ not.

Experiments

Materials and synthesis

ZnO (>99%), La₂O₃ (>99%), Y₂O₃ (>99%), piperazine (>99%), HCl (36–38%, AR), HBr (48%), and HI (55–58 wt%) were used as purchased from Adamas-beta. For the synthesis of (C₄H₁₁N₂)ZnCl₃, the following starting materials were used: ZnO (162.78 mg, 2 mmol), piperazine (258.42 mg, 3 mmol), La₂O₃ (325.81 mg, 1 mmol), HCl (2 mL), and H₂O (1 mL). The mixture was placed in Teflon pouches (23 mL), sealed in an autoclave, heated at 110 °C for 72 hours, and then cooled to 30 °C at a rate of 1.67 °C h⁻¹. This procedure yielded colorless block-shaped crystals of (C₄H₁₁N₂)ZnCl₃ with approximately 95% yield based on Zn. For the synthesis of (C₄H₁₁N₂)ZnBr₃, the starting materials included ZnO (81.39 mg, 1 mmol), piperazine (129.21 mg, 1.5 mmol), HBr (0.3 mL), and H₂O (2 mL). The synthesis followed the same procedure, resulting in colorless block-shaped crystals of (C₄H₁₁N₂)ZnBr₃ with an approximate yield of 96% based on Zn. For the synthesis of (C₄H₁₁N₂)ZnI₃, the starting materials comprised ZnO (40.69 mg, 0.5 mmol), piperazine (172.28 mg, 2 mmol), Y₂O₃ (112.50 mg, 0.5 mmol), HI (1 mL), and H₂O (2 mL). The synthesis procedure was consistent with the aforementioned methods, leading to colorless block-shaped crystals of (C₄H₁₁N₂)ZnI₃ with an estimated yield of approximately 94% based on Zn.

Single crystal structure determination

Single-crystal X-ray diffraction data for the title compounds were collected on a Rigaku XtaLAB Synergy-DW dual-wavelength CCD diffractometer with Cu Kα radiation (λ = 1.54184 Å) at 109 K and 298 K. Data reduction was performed using CrysAlisPro, with absorption correction applied via the multi-scan method.⁶⁰ The structures of (C₄H₁₁N₂)ZnCl₃, (C₄H₁₁N₂)ZnBr₃, and (C₄H₁₁N₂)ZnI₃ were determined by direct methods and refined by full-matrix least-squares fitting on F² using SHELXL-2014.⁶¹ All non-hydrogen atoms were refined with anisotropic thermal parameters. The structures were checked for missing symmetry elements using PLATON, and none were found.⁶² The Flack parameters for (C₄H₁₁N₂)ZnCl₃, (C₄H₁₁N₂)ZnBr₃, and (C₄H₁₁N₂)ZnI₃ are 0.00(4), −0.06(9), and 0.053(14), respectively, confirming the correctness of their absolute structures. Additionally, twinning was observed in (C₄H₁₁N₂)ZnBr₃ and (C₄H₁₁N₂)ZnI₃, with twin laws of (−1.0, 0.0, 0.0, 0.0, −1.0, 0.0, 0.0, 0.0, −1.0). Crystallographic data, structural refinements, and crystal information of the title compounds are listed in Tables S1–S5.†

Powder X-ray diffraction

Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku Ultima IV diffractometer with graphite-monochromated Cu Kα radiation in the 2θ range of 10–70°, with a step size of 0.02°.

Thermal analysis

Thermogravimetric analysis (TGA) was conducted using a Rigaku TG-DTA 8121 unit under an Ar atmosphere, with a heating rate of 10 °C min⁻¹ in the range of 30 to 650 °C.

Optical measurements

Ultraviolet–visible (UV-vis) spectra in the range of 200–800 nm were recorded on a PerkinElmer Lambda 750 UV-vis-NIR spectrophotometer. The reflectance spectrum was converted into an absorption spectrum using the Kubelka–Munk function.⁶³

The infrared (IR) spectrum was recorded on a Thermo Fisher Nicolet 5700 FT-IR spectrometer in the form of KBr pellets in the range from 4000 to 400 cm⁻¹.

Second harmonic generation measurements

Powder SHG measurements were performed using a Q-switched Nd:YAG laser generating radiation at 1064 nm, following the method of Kurtz and Perry.⁶⁴ Crystalline (C₄H₁₁N₂)ZnCl₃, (C₄H₁₁N₂)ZnBr₃, (C₄H₁₁N₂)ZnI₃ samples were sieved into distinct particle-size ranges (45–53, 53–75, 75–105, 105–150, 150–210, and 210–300 μm). Sieved KH₂PO4 (KDP) samples in the same particle-size ranges were used as references.

Elemental analysis

The elemental content was measured using a Vario EL Cube elemental analyzer from Elementar Analysensysteme GmbH, Germany. The combustion temperature was 800 °C.

Energy-dispersive X-ray spectroscope

Microprobe elemental analyses and elemental distribution maps were obtained using a field-emission scanning electron microscope (Phenom LE) equipped with an energy-dispersive X-ray spectroscope (EDS, Phenom LE).

Computational method

The electronic structures were calculated using the plane-wave pseudopotential method within density functional theory (DFT) implemented in the CASTEP code.^65,66 For the exchange–correlation functional, we selected the Perdew–Burke–Ernzerhof (PBE) formulation within the generalized gradient approximation (GGA).⁶⁷ The interactions between the ionic cores and the electrons were described by norm-conserving pseudopotentials.⁶⁸ The following orbital electrons were treated as valence electrons: Cl-3s²3p⁵, Br-4s²4p⁵, I-5s²5p⁵, Zn-3p⁵4s²3d¹⁰, H-1s¹, C-2s²2p², and N-2s²2p³. The basis set was determined using a cutoff energy of 750 eV. Monkhorst–Pack k-point sampling grids of 4 × 5 × 4, 4 × 5 × 4, and 1 × 1 × 1 were used for the numerical integration of the Brillouin zone.

The calculations of second-order nonlinear optical (NLO) susceptibilities were based on the length-gauge formalism within the independent particle approximation.^69,70 The second-order NLO susceptibility can be expressed as:

where the subscript L denotes the length gauge, and χ^mod_abc, χ^inter_abc, and χ^intra_abc represent the contributions to χ_abc from interband processes, intraband processes, and the modulation of interband terms by intraband terms, respectively.

Results and discussion

Single crystals of (H₁₁C₄N₂)ZnX₃ (X = Cl, Br, I) were prepared through simple hydrothermal reactions. In designing the H₂O to HX ratio, we referred to the preparation conditions of (C₄H₁₂N₂)ZnX₄ (X = Cl, Br), which utilized 0 mL H₂O and 3 mL HX, as previously reported by our research group.⁷¹ We found that only when the H₂O to HX ratio exceeds 2 does it facilitate the formation of coordination bonds between Zn²⁺ cations and halide anions. Powder XRD results indicate that the obtained samples are phase-pure (Fig. S2†). EDS analysis reveals that the Zn : X ratios in the Cl, Br, and I compounds are 1 : 2.86, 1 : 2.99, and 1 : 3.03, respectively (Fig. S3†). Elemental analysis of C, N, and H atoms was performed. For (C₄H₁₁N₂)ZnCl₃, the weight percentages were C 6.77%, H 1.63%, and N 4.02%. For (C₄H₁₁N₂)ZnBr₃, the weight percentages were C 8.30%, H 1.85%, and N 4.53%. For (C₄H₁₁N₂)ZnI₃, the weight percentages were C 8.30%, H 1.85%, and N 4.53%. IR spectra are provided in Fig. S4,† and the detailed assignments of the absorption peaks (Table S6†) are comparable with previously reported compounds.^47–49 These values are in good agreement with the crystal structure solutions.

Crystal structures

Three title compounds crystallize in different space groups (Table S1†), but they all exhibit the same topology (Fig. 1 and 2). They shared structure consists of a 2D [(C₄H₁₁N₂)ZnX₃]_∞ neutral layer, which is comprised of 0D (C₄H₁₁N₂)ZnX₃ units connected by N–H⋯X hydrogen bonds. Within this structure, the 0D (C₄H₁₁N₂)ZnX₃ units are formed by the ZnNX₃ tetrahedra and piperazine cations via the bridging of Zn–N bonds.

Fig. 1 The structures of 0D (H₁₁C₄N₂)ZnX₃ units, in which X = Cl (a), Br (b) and I (c–d).

Fig. 2 Views of the structures of 2D [(C₄H₁₁N₂)ZnCl₃]_∞ neutral layer along a direction (a), 3D (C₄H₁₁N₂)ZnCl₃ along c direction (b), 2D [(C₄H₁₁N₂)ZnI₃]_∞ neutral layer along a direction, and 3D (C₄H₁₁N₂)ZnI₃ along b direction (d). The spheres of pink, violet-blue, blue, black, green, and dark purple respectively represent the atoms of H, Zn, N, C, Cl, and I.

(C₄H₁₁N₂)ZnCl₃ and (C₄H₁₁N₂)ZnBr₃ are isomorphic compounds of each other, crystallizing in P1 space group, and their unit cells have and only one asymmetric unit, containing one Zn, three Cl or Br atoms and one (C₄H₁₁N₂)⁺ organic cation (Fig. 1). Whereas (C₄H₁₁N₂)ZnI₃ crystallizes in Cc space group, and its unit cell has four asymmetric unit, which is composed of a pair of (C₄H₁₁N₂)ZnI₃ molecule with different orientation (Fig. 1). Within their crystal structures, the six-membered (C₄H₁₁N₂)⁺ ring shows a chair-shaped configuration with each C and N atom exhibiting sp³ hybridization, suggesting that (C₄H₁₁N₂)⁺ is non-π-conjugated. Each Zn²⁺ cation bonds with three halide anions and one N atom from organic cation, forming a tetrahedral ZnNX₃ groups (Fig. 1). The lengths of Zn–N are comparable with each other, with the values of 2.055(6) Å, 2.076(9) Å, and 2.074(11)–2.083(11) Å within ZnNCl₃, ZnNBr₃, and ZnNI₃ units. The size effect of halide anions induces the difference of Zn–X bonds, with the magnitude of Zn–I (2.568(2)–2.6067(19) Å) > Zn–Br (2.360(2)–2.4068(18) Å) > Zn–Cl (2.2291(19)–2.2735(19) Å). Obviously, Zn–N distances are smaller than those of Zn–X. Each ZnNX₃ tetrahedron is linked with (C₄H₁₁N₂)⁺ organic cation via the bridging of Zn–N coordination bond, forming a 0D (C₄H₁₁N₂)ZnX₃ neutral unit (Fig. 1).

The assembly mode of the 0D building block is consistent across the crystal structures of the three compounds (Fig. 2). In each compound, the 0D (C₄H₁₁N₂)ZnX₃ units are connected to each other through hydrogen bonds (Fig. 2a and b and Table S5†). Specifically, two halogen anions from one unit are linked to another pair of 0D (C₄H₁₁N₂)ZnX₃ units, while two H atoms from different N–H bonds bridge another pair of units. The distances of d(D–A) in the hydrogen bonds are 3.224(7)–3.282(7) Å, 3.398(12)–3.464(10) Å, and 3.645(15)–3.740(11) Å for N–H⋯Cl, N–H⋯Br, and N–H⋯I, respectively. Therefore, each 0D (C₄H₁₁N₂)ZnX₃ unit forms connections with four surrounding 0D units through four differently oriented hydrogen bonds (Fig. 2a and b). These interactions lead to the formation of two-dimensional [(C₄H₁₁N₂)ZnX₃]_∞ infinite layers (Fig. 2a and b). Finally, the stacking of these 2D layers results in the overall 3D framework of (C₄H₁₁N₂)ZnX₃, in which the stacking form are –A–A–A– for Cl and Br compounds whereas –A–A′–A–A′– for (C₄H₁₁N₂)ZnI₃ (Fig. 2c and d).

Structure comparation

We delve into the structural disparities and their underlying causes between Cl, Br, and I compounds (Fig. 3a–c). Initially, the computations reveal that the distortion index of Δd_tetr for ZnNCl₃, ZnNBr₃, and ZnNI₃ are 0.034, 0.050, and 0.078, respectively, based on the formular of . The distortion degree of ZnNI₃ significantly exceeds that of ZnNCl₃ and ZnNBr₃, aligning with the relative disparity in Zn–X and Zn–N bond lengths in their respective units. As the ionic radius gradually increases from Cl⁻ to I⁻, it induces a progressive flexibility of the halide anion orbital. Consequently, with minimal alteration in the Zn–N radius, the elongation of Zn–X bond lengths gradually amplifies the tetrahedral distortion degree.

Fig. 3 The comparisons of radius of halide anions (a), as well as volume (b) and distortion index (c) of ZnNX₃ tetrahedra. The arrangement of ZnNX₃ tetrahedra in (C₄H₁₁N₂)ZnCl(Br)₃ (d) and (C₄H₁₁N₂)ZnI₃ (e). The green and blue arrow means the orientation of polarization. The symmetry of the space group of P1 (f) and Cc (g).

Subsequently, the ZnNCl₃ and ZnNBr₃ tetrahedra exhibit ideal alignment, with each tetrahedron's distortion direction being consistent, whereas neighboring ZnNI₃ not (Fig. 3). This results in the polarizability of ZnNCl₃ and ZnNBr₃ tetrahedra being able to align along a single polarization direction, while the polarizability of ZnNI₃ is partially counteracted. This phenomenon stems from the distinct space groups to which they belong (Fig. 3f and g). Cl and Br compounds crystallize in the P1 space group with the lowest symmetry and the fewest symmetry operations, with each unit cell containing only one ZnNX₃ tetrahedron, ensuring consistent orientations of adjacent ZnNX₃ groups. Conversely, (C₄H₁₁N₂)ZnI₃ crystallizes in the monoclinic Cc space group, leading to the orientations of adjacent (C₄H₁₁N₂)ZnI₃ molecules crossing each other, with the consistent components aligning along the polar axis. This observation is likely due to the size effect of the halide anions. The significantly larger ionic radius of I⁻ causes the volume of the ZnNI₃ tetrahedron to increase, rendering it unfeasible for the (x, y, z) symmetry operations in P1 space group to accommodate the (H₁₁C₄N₂)ZnI₃ molecules in space. Consequently, the latter can only be arranged in the Cc space group with more symmetry operations and energy-favorable.

Properties

TG-DTA analysis indicates that the initial decomposition temperatures of the Cl, Br, and I compounds are 300 °C, 334 °C, and 338 °C, respectively (Fig. S5†). These temperatures suggest substantial thermal stability, surpassing that of reported semi-organic NLO crystals or NLO organic–inorganic metal halides (OIMHs), such as C(NH₂)₃SO₃F (162 °C),⁷² [C(NH₂)₃]₆(PO₄)₂·3H₂O (100 °C), (C₅H₆ON)(H₂PO₄) (166 °C),⁷³ and (H₇C₃N₆)(H₆C₃N₆)HgCl₃ (225 °C).⁴⁸ Additionally, these three crystals exhibit air stability, showing no signs of weathering or moisture absorption even after being stored in air at room temperature for several weeks (Fig. S2†).

The ultraviolet absorption spectrum shows that the transmittance of (C₄H₁₁N₂)ZnCl₃ at 190 nm is greater than 80% (Fig. 4a). And the wavelength-depended F(R) (absorption coefficient/scattering coefficient) values are nearly close to zero (Fig. 4a). These are consistent with most reported deep-UV crystals, such as (NH₄)₂Na₃Li₉(SO₄)₇,³⁶ HfF₂(SO₄)⁴⁰ and Ba(NH₂SO₃)₂,⁴⁵ indicating that (C₄H₁₁N₂)ZnCl₃ is able to transmit in the deep-UV region and possesses a wide bandgap larger than 6.20 eV. The absorption cutoff edges for (C₄H₁₁N₂)ZnBr₃ and (C₄H₁₁N₂)ZnI₃ are 207 nm and 222 nm, respectively, corresponding to bandgaps of 5.53 eV and 4.52 eV (Fig. 4b and c). Notably, the halogen substitution from I to Cl increases the compound's bandgap and enhances its UV transmittance due to the difference in electronegativity of the halide anions. Importantly, the bandgaps of (C₄H₁₁N₂)ZnCl₃ and (C₄H₁₁N₂)ZnBr₃ are wider than all reported NLO-active OIMHs, including (C₃N₆H₇)(C₃N₆H₆)HgCl₃ (4.40 eV),⁴⁸ (C₄H₁₀NO)₂Cd₂Cl₆ (5.45 eV),⁷⁴ (L/D-C₁₀H₂₀N₂O₄)Cd₅Cl₁₂ (5.42 eV),⁷⁵ (C₂₀H₂₀P)CuBr₂ (3.62 eV)⁵² and L/D-C₆H₁₀N₃O₂ZnBr₃ (5.02 eV).⁷⁶ This highlights the exceptional deep-UV/UV transmittance and optical properties of (C₄H₁₁N₂)ZnCl₃ and (C₄H₁₁N₂)ZnBr₃. Their wide bandgaps also ensure good resistance to laser stability, with powder laser damage thresholds being 205.4 MW cm⁻², 150.2 MW cm⁻², and 87.0 MW cm⁻² for Cl, Br, and I compounds, respectively, which are 51, 38 and 22 times that of AgGaS₂ (AGS: 4.0 MW cm⁻²).

Fig. 4 UV transmittance spectra (a, b, c for Cl, Br, and I compounds, respectively). The inset shows the bandgaps and as-grown crystals. F(R) is the absorption coefficient/scattering coefficient. Oscilloscope traces of the SHG signals (150–210 μm) with 1064 nm laser radiation (d) and Phase-matching curves (e). KDP was used as references for the SHG measurements.

The SHG responses of (C₄H₁₁N₂)ZnX₃ were measured under 1064 nm laser radiation, and the results are presented in Fig. 4d. All compounds exhibit SHG activity, with efficiencies of 0.8, 2.5, and 2.1 times that of KH₂PO₄ (KDP) for (C₄H₁₁N₂)ZnCl₃, (C₄H₁₁N₂)ZnBr₃, and (C₄H₁₁N₂)ZnI₃, respectively, within the particle size range of 150–210 μm. Besides, Fig. 4e demonstrates that all compounds are phase-matchable. Combining the mentioned UV spectra, as shown in Fig. 6, we can draw some exciting conclusions. Firstly, (C₄H₁₁N₂)ZnCl₃ is the first reported deep-UV OIMHs with SHG activity (Fig. 5c), and its SHG response is comparable with those of known deep-UV compounds, including (NH₄)₂Na₃Li₉(SO₄)₇ (0.5 × KDP),³⁶ Ba₃P₃O₁₀Cl (0.6 × KDP)³⁷ and HfBa₃M₂F₁₄Cl (0.9 × KDP).⁷⁸ Secondly, the SHG effect of (C₄H₁₁N₂)ZnBr₃ surpasses that of currently reported OIMHs with a bandgap greater than 5.0 eV (Fig. 6a), including L/D-C₁₂H₂₀N₆O₄Cd₂Cl₅ (0.2 × KDP),⁷⁶ (C₄H₁₀NO)₂Cd₂Cl₆ (0.73 × KDP),⁷⁴ and (L/D-C₁₀H₂₀N₂O₄)Cd₅Cl₁₂ (0.25 and 0.3 × KDP).⁷⁶ Additionally, the SHG effect of (C₄H₁₁N₂)ZnBr₃ is comparable to some recently reported semi-organic short UV NLO crystals, such as (C₅H₆ON)(H₂PO₄) (3 × KDP),⁷⁷ [C(NH₂)₃]₃PO₄·2H₂O (1.5 × KDP)⁷³ and Ba(SO₃NH₂)₂ (2.7 × KDP).⁴⁶ Finally and most importantly, the substitution of I with Br in (H₁₁C₄N₂)ZnI₃ (2.1 × KDP, 4.52 eV) not only significantly increases the bandgap but also enhances its SHG effect. The anomalous relationship between bandgap and SHG effect due to halogen substitution is unprecedented, distinguishing it from previously reported OIMHs, such as (C₆H₅(CH₂)₄NH₃)₄BiBr₇·H₂O (0.4 × KDP, 3.52 eV) with (C₆H₅(CH₂)₄NH₃)₄BiI₇·H₂O (1.3 × AGS, 2.29 eV),⁵⁰ and Cs₃Pb₂(CH₃COO)₂Br₅ (4 × KDP, 3.26 eV) with Cs₃Pb₂(CH₃COO)₂I₅ (8 × KDP, 2.55 eV).⁵⁵

Fig. 5 SHG response and bandgap of selected NLO-active OIMHs with d¹⁰ cation.

Fig. 6 The comparisons of dipole moment (a and b) and SHG contributions (c) of ZnNX₃ tetrahedra as well as bandgaps (d) and SHG effects (e) in (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I).

Structure–property relationship

We further analyzed the structure–property relationship of the three title compounds, especially the intrinsic reason why the Br compound can achieve SHG-bandgap balance optimization. Firstly, the size difference of halogens results in the volumes of ZnNBr₃ and ZnNCl₃ being much smaller than that of ZnNI₃, leading to different arrangement modes of this distorted tetrahedron in the structure (Fig. 3). Secondly, although the difference in halogen polarizability leads to a much larger distortion degree of ZnNI₃ than those of ZnNBr₃ and ZnNCl₃ (Fig. 3c), the gap between ZnNI₃ and ZnNBr₃ narrows in terms of dipole moment (Fig. 6a). The dipole moments of ZnNCl₃, ZnNBr₃ and ZnNI₃ are 2.065, 5.113, and 6.692–7.306 D, respectively (Table S7†). Therefore, after further considering the arrangement of structural units and the unit cell volume, we found that the order of dipole moment per unit volume is (C₄H₁₁N₂)ZnBr₃ (0.020 DÅ⁻³) > (C₄H₁₁N₂)ZnI₃ (0.018 D Å⁻³) ≫ (C₄H₁₁N₂)ZnCl₃ (0.009 D Å⁻³) (Fig. 6b). This order is consistent with the experimentally measured SHG effects, (C₄H₁₁N₂)ZnCl₃ (0.8 × KDP) ≪ (C₄H₁₁N₂)ZnI₃ (2.1 × KDP) < (C₄H₁₁N₂)ZnBr₃ (2.5 × KDP) (Fig. 6e). Therefore, considering that the SHG effect and bandgap are contradictory conditions for NLO crystals, we arrive at the following conclusions: ① chloride anion has the strongest electronegativity and the weakest polarizability. And, due to the perfectly aligned arrangement of ZnNCl₃ along the polarization direction in the structure, (C₄H₁₁N₂)ZnCl₃ can guarantee a moderate SHG effect (0.8 × KDP) while maintaining deep-UV transparency with wide bandgap (>6.2 eV); ② iodide anion has the weakest electronegativity and the strongest polarizability. Therefore, although the polarity of ZnNI₃ is partially cancelled out due to its non-ideal arrangement, its smaller bandgap ensures a strong SHG effect (2.1 × KDP; 4.52 eV); ③ the slightly larger radius of bromide anion compared to chloride allows ZnNBr₃ to adopt the same arrangement mode as ZnNCl₃, with perfectly superimposed polarity. Therefore, compared with (C₄H₁₁N₂)ZnCl₃, the SHG effect of (C₄H₁₁N₂)ZnBr₃ is dramatically enhanced at the expense of a partial bandgap sacrifice (Fig. 6d and e). More importantly, compared with (C₄H₁₁N₂)ZnI₃, (C₄H₁₁N₂)ZnBr₃ exhibits an enhanced SHG effect while significantly widening the band gap, thus achieving optimization of the SHG-bandgap balance (Fig. 6d and e).

Systematic theoretical calculations were conducted to analyze the band structures and densities of states (DOS) of the title compounds. The calculated band structures for these compounds are illustrated in Fig. S6.† The theoretical bandgaps (4.815 eV, 4.404 eV, and 4.304 eV for Cl, Br, and I compounds, respectively) roughly align with experimental values. For (C₄H₁₁N₂)ZnCl₃ and (C₄H₁₁N₂)ZnBr₃, the theoretical bandgaps deviate significantly from experimental values due to the discontinuity of the exchange–correlation function. As shown in Fig. 7a–c, The Cl-3p, Br-4p, and I-5p states predominantly contribute to the top of the valence band (VB), while the Zn-4s state is the primary contributor to the bottom of the conduction band (CB). Additionally, with the substitution of Cl by Br and I, the contribution of X-ns²np⁶ orbitals to the bottom of the CB becomes significant and shows an increasing trend.

Fig. 7 The DOS (a–c) as well as the SHG density distribution in VB (d–f) and CB (g–i) for (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I).

We further calculated the optical properties (birefringence and second-order nonlinear optical coefficients) of the title compounds. A principal axis transformation was first conducted; for the triclinic systems of (C₄H₁₁N₂)ZnCl₃ and (C₄H₁₁N₂)ZnBr₃, the rotation angles between the crystallographic axes and the principal dielectric axes are shown in Table S8.† For the monoclinic system of (C₄H₁₁N₂)ZnI₃, the rotation angle between the crystallographic axes and the principal dielectric axes is 25.173°. As depicted in Fig. S7,† the birefringence values for the Cl, Br, and I compounds are 0.04@546 nm, 0.06@546 nm, and 0.08@546 nm, relating to the shortest type-I phase-matchable wavelength of 371 nm, 335 nm, and 451 nm, respectively. It is worth noting that from (C₄H₁₁N₂)ZnI₃ to (C₄H₁₁N₂)ZnBr₃, halide substitution has achieved a triple increase in SHG effect, bandgap, and phase-matching capability. We also calculated the effective second-order nonlinear optical coefficients (d_eff), which are 0.522 pm V⁻¹, 0.847 pm V⁻¹, and 1.344 pm V⁻¹ for Cl, Br, and I compounds, respectively. Notably, the d_eff of the (C₄H₁₁N₂)ZnBr₃ is 2.2 times that of KDP (0.39 pm V⁻¹), aligning closely with the experimental value (2.5 × KDP).

The origins of the SHG effects for the three title compounds were elucidated through computational analysis of the SHG density distribution (Fig. 6d–i). In the VB, the Cl-3p, Br-4p, and I-5p orbitals predominantly contribute to the SHG effect in their respective compounds. For the Cl and Br analogs, the Zn-3d and N-2p orbitals associated with the Zn-N bonds also play a significant role in the SHG effect, but a contribution that is negligible in the I analog. Furthermore, within the CB, the Zn-3s orbital is identified as the primary contributor to the SHG, with additional contributions from the Cl-3p/Br-4p/I-5p and N-2p orbitals on the Zn–N bonds. The quantitative contributions from the piperazine cation and the distorted tetrahedral ZnNX₃ anion to the SHG were assessed, with the ZnNCl₃, ZnNBr₃, and ZnNI₃ units exhibiting SHG contribution percentages of 87.8%, 95.4%, and 96.5%, respectively (Fig. 6e). This trend indicates a progressive increase in SHG contribution as the halogen substitution proceeds from chlorine to bromine to iodine, correlating with a decrease in electronegativity and an increase in polarizability of the halogen anions. Concurrently, the impact of the Zn–X bonds on the SHG effect of the compounds becomes more pronounced.

Conclusions

This study culminates in the discovery of three novel noncentrosymmetric piperazine zinc halide compounds, (C₄H₁₁N₂)ZnX₃ (X = Cl, Br, I), achieved through strategic halide substitution. Notably, (C₄H₁₁N₂)ZnCl₃ stands as the pioneering deep-UV nonlinear optical OIMH with a SHG effect of 0.8 × KDP. Our findings underscore the exceptional case of (C₄H₁₁N₂)ZnBr₃, which achieves a remarkable bandgap expansion to 5.53 eV without compromising on a potent SHG response (2.5 × KDP), setting a new benchmark among OIMHs with bandgaps greater than 5.0 eV. The progression from chlorine to bromine and iodine in halide substitution is shown to progressively enhance SHG contributions, correlating with reduced electronegativity and heightened polarizability of the halide ions. These findings not only broaden the scope of OIMHs with tailored optical properties but also highlight (C₄H₁₁N₂)ZnBr₃ as a prime candidate for short-wavelength UV NLO crystal applications. The work underscores the importance of halide choice in engineering OIMHs with optimized SHG-bandgap balance and sets the stage for further explorations into high-performance NLO materials.

Author contributions

Chen Jin and Wu Huai-Yu: conceptualization, methodology, writing – original draft, data curation, visualization; Xu Miao-Bin, Wang Ming-Chang, Chen Qian-Qian and Bing-Xuan Li: data curation; Hu Chun-Li: formal analysis; Hu Chun-Li, Du Ke-Zhao and Chen Jin: writing – review & editing, supervision.

Data availability

Crystallographic data for (H₁₁C₄N₂)ZnX₃ (X = Cl, Br, I) have been deposited with the CCDC under deposition numbers 2313121–2313123† and can be obtained from https://www.ccdc.cam.ac.uk/. Other data are available from the authors upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Our work has been supported by the National Natural Science Foundation of China (No. 22205037 and 22373014) and Natural Science Foundation of Fujian Province (2023J01498).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2313121–2313123. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01515k

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