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• The interaction of light with matter significantly alters its frequency, phase, and polarization due to the nonlinear optical (NLO) and birefringent properties of crystals [1-4]. These properties make NLO and birefringent crystals essential optoelectronic materials with broad applications in solid-state laser technology and polarization information processing [5-13]. A promising approach for developing NLO/birefringent crystals is rationally assembling functional units with high anisotropic polarizability (δ) and strong hyperpolarizability (β) [14-21]. The iodate system exemplifies this approach, garnering significant research interest due to the lone pair electrons on I⁵⁺ ions and the high degree of distortion in iodate groups [22-24].

  The exploration of new NLO/birefringent iodates has traditionally followed three main ways [25-33]. Firstly, researchers have focused on inducing the aggregation of iodate anions into polyiodate groups, which possess greater polarizability. This approach has yielded notable materials like SrI₂O₅F₂ (0.203 at 532 nm) [34] and GdI₅O₁₄ (15 × KDP; 0.092 at 1064 nm) [27]. Secondly, combining second-order Jahn-Teller effect units, such as Bi-, V-, or Mo-O/F polyhedra, with IO₃/IO₄ groups has proven effective. This strategy is exemplified by materials like BiF₂(IO₃) (11.5 × KDP; 0.209 at 1064 nm) [35], VO₂(H₂O)(IO₃) (18 × KDP; 0.184 at 546 nm) [29], and Rb(MoO₂)₂O(IO₄) (32 × KDP; 0.339 at 1064 nm) [36], all of which exhibit significant second harmonic generation (SHG) effects and large birefringence. Finally, combining tetrahedral groups like (SO₄)^2- [37], (PO₄)^3- [30] or planar triangular groups like (BO₃)^3- [38], (NO₃)^- [39] with IO₃ groups presents another avenue for developing high-performance materials. This mixed-anion approach allows metal iodates to maintain excellent optical properties while pushing their UV transmission cutoff to shorter wavelengths. A prime example is Sc(IO₃)₂(NO₃), which boasts a wide bandgap (4.15 eV), a strong SHG effect (4 × KDP), and exceptionally high birefringence (0.348 at 546 nm) [39].

  In recent years, protonated guanidinium and pyridine derivatives, with planar π-conjugated structures, have been recognized as emerging SHG-active and birefringent-active groups. This recognition has led to the discovery of a series of high-performance semiorganic NLO/birefringent crystals, including (C₅H₆ON)⁺(H₂PO₄)^- (3 × KDP; 0.25@1064 nm) [40], C(NH₂)₃SO₃F (5 × KDP; 0.133@1064 nm) [41], and [C(NH₂)₃]₆(PO₄)₂·3H₂O (3.8 × KDP; 0.078@1064 nm) [42]. Additionally, significant advancements have been made in semiorganic iodate systems (Table S1 in Supporting information). Mao et al. employed protonated pyrimidine or 2-hydroxypyridine as cations to obtain several SHG-active compounds. Among these, [o-C₅H₄NHOH]₂[I₇O₁₈(OH)]·3H₂O [33] and α-(C₄H₅N₂O)(IO₃)·HIO₃ [43] demonstrate SHG effects more than five times that of KDP. However, their bandgaps and birefringences remain relatively small (E_g < 4.0 eV; Δn < 0.15). Although the use of the guanidinium cation achieves a wide bandgap in metal-free guanidinium iodates (E_g > 4.5 eV), such as C(NH₂)₃(IO₂F₂) (4.81 eV) [44], the reported compounds to date crystallize in centrosymmetric space groups, exhibiting SHG-inertness and low birefringence (Δn ≤ 0.1). Zou et al. discovered that introducing highly distorted MoO₆ octahedra into the C(NH₂)₃⁺-IO₃^- system significantly enhances the SHG effect or birefringence of the compounds. For example, [C(NH₂)₃]₂Mo₂O₅(IO₃)₄·2H₂O (3.55 eV; 5.0 × KDP) [45] and C(NH₂)₃MoO₃(IO₃) (3.33 eV; 0.426 at 546 nm) [46], although their bandgaps remain relatively narrow. Therefore, as shown in Table S1, it is evident that achieving an optimal balance among bandgap, SHG effect, and birefringence in semiorganic iodate systems remain a highly challenging task.

  Herein, through density functional theory (DFT) calculations, we have identified a pyridine derivative, protonated 3, 5-dipicolinic acid (HDPA), which has higher anisotropic polarizability compared to some known organic cations (Fig. S1 in Supporting information), making it an ideal birefringent-active group. And then, when combined with the IO₃^- anion, a traditional SHG-active group with strong hyperpolarizability, we synthesized a new polar semiorganic iodate, HDPA(IO₃). Experimental results indicate that HDPA(IO₃) demonstrates a wide bandgap, a significant SHG effect, and exceptionally high birefringence (4.12 eV, 3.6 × KDP; 0.35@546 nm). Structure-property relationship analysis reveals that the synergistic interaction between the large π-conjugated HDPA⁺ cation and the highly distorted IO₃^- trigonal pyramid results in the compound's excellent optical properties.

  HDPA(IO₃) crystallizes in a polar and non-centrosymmetric space group Pna2₁, with cell parameters a = 6.69810(10) Å, b = 10.0169(2) Å, c = 14.1133(2) Å, α = β = γ = 90°, V = 946.92(3) ų, and Z = 4 (Tables S2-S5 in Supporting information). Its asymmetric unit consists of one IO₃⁻ anion and one C₇H₆NO₄⁺ (HDPA⁺) cation, with all atoms in general positions. Each iodine atom is bonded to three oxygen atoms, forming a highly distorted IO₃ triangular pyramid. The I-O bond lengths range from 1.803(3) Å to 1.809(3) Å, and the iodine atom's valence is calculated to be 5.114 (Table S6 in Supporting information), which is consistent with reported values for iodates [26, 30, 31, 47]. All oxygen atoms from the IO₃^- anion act as hydrogen bond acceptors, while the oxygen of the carboxylic acid group and the nitrogen of the pyridine group in the HDPA⁺ cation act as hydrogen bond donors, forming O1-H1···O7 (2.554(5) Å), O2-H2···O6 (2.563(6) Å), and N5-H1A···O5 (2.721(7) Å) hydrogen bonds (Table S7 and Fig. S2 in Supporting information). Through these hydrogen bonds, neighboring iodate and organic groups are linked into a wave-shaped 2D [(C₇H₆NO₄)^⁺(IO₃)^-] infinite layer, parallel to the bc plane (Fig. 1a). Within the layers, the dihedral angle of neighboring organic ligands along the c-axis is 50.374°. The entire complex framework of HDPA(IO₃) is built from the stacking of 2D layers along the a-axis (Fig. 1b), via π-π interactions with an interplanar distance of 5.680(3) Å and dihedral angle of 4.8(3)° (Table S8 and Fig. S3 in Supporting information).

  Figure 1

  

  Figure 1.  (a) 2D neutral layer and (b) 3D overall structure along a direction of HDPA(IO₃). Purple, black, red, blue, and pink spheres represent I, C, O, N, and H atoms, respectively; red, green, and blue arrows represent the a, b, and c axes, respectively.

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  Powder X-ray diffraction (PXRD) analysis confirmed the purity of the synthesized samples (Fig. S4 in Supporting information). Field-emission scanning electron microscopy (FESEM) analyses detected the presence of C, N, O, and I elements in HDPA(IO₃) (Fig. S5 in Supporting information). Elemental analysis provided weight ratios of C: N in HDPA(IO₃) as 7.32:1.09, being consistent with the ratio of 7:1 (Table S9 in Supporting information). Additionally, X-ray photoelectron spectroscopy further investigated the chemical composition of HDPA(IO₃) (Fig. S6 and Table S10 in Supporting information) [48, 49]. The IR spectrum (Fig. S7 and Table S11 in Supporting information) shows absorption peaks between 800 cm^-1 and 500 cm^-1 for the I-O bond, and peaks at 3107 and 3066 cm^-1 for the N-H and C-H stretching vibrations, respectively. TG-DTA curves show that HDPA(IO₃) is stable up to 225 ℃, with decomposition occurring in two stages: A sharp decomposition from 225 ℃ to 300 ℃ and a slower decomposition from 300 ℃ to 600 ℃, corresponding to exothermic/endothermic peaks at 244, 301, and 322 ℃ (Fig. S8 in Supporting information).

  The ultraviolet-visible (UV–vis) absorption spectrum indicates that HDPA(IO₃) exhibits a UV cutoff edge at 269 nm, corresponding to a wide bandgap of 4.12 eV (Fig. 2a). This bandgap is significantly larger than many reported metal iodates, including β-Ba₂[VO₂F₂(IO₃)₂]IO₃ (2.59 eV) [31], Bi(IO₃)F₂ (3.97 eV) [35], KMoO₃(IO₃) (3.34 eV) [50], Rb(MoO₂)₂O(IO₄) (3.33 eV) [36], and comparable to several pure inorganic iodates with other π-conjugated anions, such as La(IO₃)₂(NO₃) (4.23 eV) [51], and Sc(NO₃)(IO₃) (4.15 eV) [39]. Importantly, its bandgap is wider than most of semiorganic iodates, including α-(C₄H₅N₂O)(IO₃)·HIO₃ (3.65 eV)[43], [C₅H₆O₂N₃]₂[IO₃]₂ (2.43 eV) [52], (C₅H_6.16N₂Cl_0.84)(IO₂Cl₂) (3.38 eV) [53], C(NH₂)₃MoO₃(IO₃) (3.33 eV) [46], C(NH₂)₃Rb(I₃O₈)(IO₃)(I₂O₆H₂) (3.54 eV) [54] and [C(NH₂)₃]₂Mo₂O₅(IO₃)₄·2H₂O (3.55 eV) [45] and [o-C₅H₄NHOH]₂[I₇O₁₈(OH)]·3H₂O (3.9 eV) [33], and only smaller than three metal-free guanidinium iodates, including (C(NH₂)₃)₂(I₂O₅F)(IO₃)(H₂O) (4.49 eV) [44], C(NH₂)₃IO₂F₂ (4.87 eV) [44], and C(NH₂)₃IO₃ (4.57 eV) (Table S1) [44]. Additionally, laser-induced damage threshold (LDT) measurements reveal a high LDT value of 191.5 MW/cm², which is approximately 48 times that of AGS (AgGaS₂, 4.0 MW/cm²). Therefore, HDPA(IO₃) is a promising candidate for high-power optical materials in the short-wave UV region.

  Figure 2

  

  Figure 2.  (a) UV–vis diffuse reflectance spectrum (F(R) is absorption coefficient/scattering coefficient). The SHG response of HDPA(IO₃), KDP served as (b) brandmark, (c) original crystal, (d) crystal achieving complete extinction. (e) SHG, Birefringence, bandgap comparison between title compound and reported semiorganic iodates.

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  Since HDPA(IO₃) is polar and non-centrosymmetric, its NLO performance was measured using method of Kurtz and Perry [55]. As shown in Fig. 2b, the SHG effect of HDPA(IO₃) is 3.6 times that of KDP and can achieve type-I phase matching. Its SHG effect is superior to most inorganic iodates with different and isolated SHG-active units, such as Ba₄Ag₅(IO₃)₆(I₃O₈)₃(I₄O₁₁)₂ (2.5 × KDP) [56], Bi₄O(I₃O₁₀)(IO₃)₃(SeO₄) (1.1 × KDP) [57], Na₇(SeO₄)₃(IO₃), (0.6 × KDP) [58], and La(IO₃)₂(NO₃) (0.6 × KDP) [51]. Moreover, compared to reported semiorganic iodates, its SHG effect is better than that of β-(C₄H₅N₂O)(IO₃)·HIO₃ (0.9 × KDP) [43], [C₅H₆O₂N₃]₂[IO₃]₂ (2.4 × KDP) [52] and C(NH₂)₃(I₃O₈)(HI₃O₈)(H₂I₂O₆)(HIO₃)₄·3H₂O (2.1 × KDP) [59], but less than that of [o-C₅H₄NHOH]₂[I₇O₁₈(OH)]·3H₂O (8.5 × KDP) [33] and α-(C₄H₅N₂O)(IO₃)·HIO₃ (6.4 × KDP) [43].

  The single crystal of HDPA(IO₃) was analyzed using a polarizing microscope equipped with a Berek compensator, which facilitated the measurement of its optical path difference (R). This type of measurement is crucial for identifying the transition of the crystal polarization from an orthogonal state (Fig. 2c) to complete extinction (Fig. 2d). And then, the formula R = Δn × T was applied, where Δn is the birefringence and T is the thickness of the sample, to calculate the experimental birefringence (Δn_exp.) for HDPA(IO₃). Hence, an optical path difference of 1153.11 nm was observed, corresponding to a sample thickness of 3.11 μm (Fig. S11 in Supporting information). As a result, the birefringence of HDPA(IO₃) was determined to be 0.35 at 546 nm. The experimental birefringence of HDPA(IO₃) is larger than those form commercial crystals, including α-BaB₂O₄ (0.122@532 nm) [60], YVO₄ (0.204@532 nm) [61], and CaCO₃ (0.172@532 nm) [62]. It is superior to recently reported inorganic birefringent crystals such as Bi(SO₄)(NO₃)·3H₂O (Δn_exp. = 0.124@546 nm), K(GeHPO₃)₂Br (Δn_exp. = 0.247@546 nm) [63], and NaPO₂(NH)₃(CO)₂ (Δn_exp. = 0.280@550 nm) [64], and is larger than some of the compounds containing pyridine derivatives that have been reported, such as, (C₅H₆N)SbF₂SO₄ (Δn_exp. = 0.179@546 nm) [12] and (3AP)₂(Sb₄F₁₃) (Δn_exp. = 0.258@546 nm) [65]. In contrast to some SHG-active inorganic iodates, it is superior to VO₂(IO₃)(H₂O) (Δn_exp.. = 0.184@546 nm) [29], [Zn(IO₃)(I₂O₅(OH))] (Δn_exp. = 0.054@46 nm) [66], Cs₂VOF₄(IO₂F₂) (Δn_exp. = 0.112@546 nm) [67] and is consistent with Sc(IO₃)₂(NO₃) (Δn_exp. = 0.348@546 nm) [39]. Although smaller than (NH₄)₂(I₅O₁₂)(IO₃) (Δn_exp. = 0.431@546 nm) [32], HDPA(IO₃) has a wider bandgap and a shorter UV absorption edge, making it more competitive in the UV and short-wave UV bands.

  There is significant interest in combining SHG, birefringence, and bandgap. HDPA(IO₃) was compared with the reported semiorganic iodates (Fig. 2e). Among the compounds with SHG effect, only HDPA(IO₃) has a bandgap greater than 4.0 eV, and its experimental birefringence is the largest among the semiorganic iodates with SHG properties. Furthermore, although the birefringence of HDPA(IO₃) is smaller than those of C(NH₂)₃MoO₃(IO₃) and (C₅H_6.16N₂Cl_0.84)(IO₂Cl₂), these compounds are SHG-inert and have narrower bandgaps (< 4.0 eV). In summary, HDPA(IO₃) features the best-optimized balance of SHG, bandgap, and birefringence among all reported semiorganic iodates, indicating its strong potential as a promising short-wave UV optical crystal.

  DFT calculations were employed. HDPA(IO₃) is an indirect bandgap compound with a theoretical value of 3.105 eV (Fig. S9b in Supporting information), smaller than the measured bandgap of 4.12 eV. A scissor operation of 1.015 eV was utilized to assess the optical properties. Density of states (DOS) calculations (Fig. S12 in Supporting information) show that the top of the valence band is mainly occupied by C-2p and O-2p orbitals, while the conduction band is mainly contributed by C-2p, I-5p, and O-2p orbitals.

  The calculated birefringence of HDPA(IO₃) was obtained via the calculation of refractive indices, with the order of n_X < < n_Z < n_Y. Thus, through the formula Δn_cal = n_max − n_min = n_Y − n_Z, the theoretical birefringence at 546 nm for HDPA(IO₃) is 0.35. This value is in high agreement with the experimental value (0.35), indicating the reliability of the experimental birefringence. Furthermore, by examining the n(ω) and n(2ω), it can be observed that n_max(ω) is always greater than n(2ω)_min, suggesting that the theoretically type-I phase-matched wavelength for HDPA(IO₃) can extend to the near-UV or even shorter regions.

  We further explored the origins of HDPA(IO₃)'s excellent birefringence and SHG performance. Through calculations using the Gaussian 09 program, we determined the polarizability anisotropy (δ) and the first-order hyperpolarizability (β) for the HDPA⁺ and IO₃^- functional groups in HDPA(IO₃), which are 72 and 64 for HDPA⁺ as well as 13 and 80 for IO₃^-, respectively (Table S13 in Supporting information) [68-70]. Considering the relationship between the macroscopic properties of the crystal (birefringence, SHG) and the microscopic properties (δ, β), we referred to the perspectives of the Chen and Pan research groups on the effects of the arrangement of planar π-conjugated groups and distorted polyhedra with lone pairs on the macroscopic optical properties of crystals [71-73]. By evaluating the actual contribution of the groups to the birefringence and SHG through cosγ and cosθ, where γ₁ represents the dihedral angle between the π-conjugated plane and the plane where the n_in-plane is located, γ₂ is the angle between the polarization direction of the distorted polyhedron with lone pairs and the optical principal axis where n_min is located, and θ is the angle between the polarization direction of the functional group's with polar axis (here, the 2₁ screw axis) (Figs. 3a-d). Through the formula (n × δ × cosγ)/Z or (n × β × cosθ)/Z, the effective contributions can be obtained, as shown in Table S13. After normalizing these effective contributions, as shown in Fig. 3e, HDPA⁺ contributes significantly more to the crystal birefringence than IO₃^- (87.7% vs. 12.3%), while Fig. 3f shows that IO₃^- contributes slightly more to SHG than HDPA⁺ (60.2% vs. 39.8%). These indicate that introducing planar π-conjugated units with large anisotropy of polarizability into the iodate system can indeed significantly enhance the birefringence, and the high distortion degree of IO₃^- can break the parallel arrangement preferred by the planar π-conjugated units, resulting in a polar crystal and having a strong SHG effect. On the other hand, 2D ELF maps were provided with crystal planes parallel to HPDA⁺ and IO₃^-. The high degree of charge localization observed on the 2D plane of HDPA⁺ suggests that there are strong covalent bonds between C, N, and O in organic cation (Fig. 3g), besides, the existence of a lone pair of electrons on I⁵⁺ could be visually comprehended from Fig. 3h. Therefore, we believe that the π-lone pair synergistic interactions between HDPA⁺ and IO₃^- are the main reasons for the optimized balance of SHG response, birefringence, and bandgap in HDPA(IO₃).

  Figure 3

  

  Figure 3.  (a, b) Birefringence analysis of HDPA⁺ and IO₃^-. (c, d) SHG analysis of HDPA⁺ and IO₃^-. (e, f) Contributions of HDPA⁺ and IO₃^- to birefringence and SHG. (g, h) 2D ELF sections projected on crystal planes parallel to HDPA⁺ cation and the lone pair of IO₃^-.

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  In conclusion, this study successfully synthesized and characterized a novel semiorganic iodates, HDPA(IO₃). The crystal exhibits an exceptional balance of SHG efficiency, birefringence, and bandgap properties, with 3.6 × KDP, 0.35@546 nm, and 4.12 eV. Additionally, it displays a short cutoff edge of 269 nm and a large LIDT of 191.5 MW/cm², indicating its potential as a multifunctional optical crystal in the short-wave UV window. Structural analysis and DFT calculations reveal that the synergistic interactions between the large π-conjugated HDPA⁺ cations and the highly distorted IO₃^- anions significantly enhance its optical properties.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Miao-Bin Xu: Writing – review & editing, Writing – original draft, Investigation, Data curation, Conceptualization. Qian-Qian Chen: Data curation. Bing-Xuan Li: Data curation. Ke-Zhao Du: Supervision. Jin Chen: Writing – review & editing, Supervision.

  Acknowledgments

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

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110513.

  
  
• 1. [1]

     Y. Yang, J.W. Henke, A.S. Raja, et al., Science 383 (2024) 168–173. doi: 10.1126/science.adk2489

  2. [2]

     J. Lu, W. Wu, F.M. Colombari, et al., Nature 630 (2024) 860–865. doi: 10.1038/s41586-024-07455-4

  3. [3]

     X. Liu, Y.C. Yang, M.Y. Li, et al., Chem. Soc. Rev. 52 (2023) 8699–8720. doi: 10.1039/d3cs00691c

  4. [4]

     M. Mutailipu, K.R. Poeppelmeier, S.L. Pan, Chem. Rev. 121 (2021) 1130–1202. doi: 10.1021/acs.chemrev.0c00796

  5. [5]

     M. Luo, X. Li, X. Jiang, et al., Chin. Chem. Lett. 35 (2024) 109108.

  6. [6]

     Y. Zhou, Z. Guo, H. Gu, et al., Nat. Photon. 18 (2024) 922–927. doi: 10.1038/s41566-024-01461-8

  7. [7]

     M. Mutailipu, J. Han, Z. Li, et al., Nat. Photon. 17 (2023) 694–701. doi: 10.1038/s41566-023-01228-7

  8. [8]

     P.F. Li, C.L. Hu, J.G. Mao, et al., Coord. Chem. Rev. 517 (2024) 216000.

  9. [9]

     W. Wang, D. Mei, S. Wen, et al., Chin. Chem. Lett. 33 (2022) 2301–2315.

  10. [10]

      Y. Lan, J. Ren, P. Zhang, et al., Chin. Chem. Lett. 35 (2024) 108652.

  11. [11]

      Y. Zhou, Y. Li, Q. Ding, et al., Chin. Chem. Lett. 32 (2021) 263–265. doi: 10.1109/mems51782.2021.9375424

  12. [12]

      P. Zhang, X. Mao, X. Dong, et al., Chin. Chem. Lett. 35 (2024) 109235.

  13. [13]

      P.F. Li, C.L. Hu, J.G. Mao, et al., Coord. Chem. Rev. 517 (2024) 216000.

  14. [14]

      H. Fan, N. Ye, M. Luo, Acc. Chem. Res. 56 (2023) 3099–3109. doi: 10.1021/acs.accounts.3c00575

  15. [15]

      Y. Li, J. Luo, S. Zhao, Acc. Chem. Res. 55 (2022) 3460–3469. doi: 10.1021/acs.accounts.2c00542

  16. [16]

      M. Mutailipu, Z. Yang, S. Pan, Acc. Mater. Res. 2 (2021) 282–291. doi: 10.1021/accountsmr.1c00028

  17. [17]

      J. Chen, M.B. Xu, H.Y. Wu, et al., Angew. Chem. Int. Ed. (2024) e202411503.

  18. [18]

      J. Cheng, G. Yi, Z. Zhang, et al., Angew. Chem. Int. Ed. 63 (2024) e202318385.

  19. [19]

      P. Qian, Y. Li, J. Cheng, et al., Inorg. Chem. 63 (2024) 8013–8017. doi: 10.1021/acs.inorgchem.4c01397

  20. [20]

      P.F. Li, C.L. Hu, J.G. Mao, et al., Chem. Sci. 15 (2024) 7104–7110. doi: 10.1039/d4sc01376j

  21. [21]

      P.F. Li, C.L. Hu, B.X. Li, et al., Inorg. Chem. 63 (2024) 4011–4016. doi: 10.1021/acs.inorgchem.4c00033

  22. [22]

      J. Chen, C.L. Hu, F. Kong, et al., Acc. Chem. Res. 54 (2021) 2775–2783. doi: 10.1021/acs.accounts.1c00188

  23. [23]

      C.L. Hu, J.G. Mao, Coordin. Chem. Rev. 288 (2015) 1–17.

  24. [24]

      D. Gao, H. Wu, Z. Hu, et al., Chin. J. Struct. Chem. 42 (2023) 100014.

  25. [25]

      Y. Hou, H. Wu, H. Yu, et al., Angew. Chem. Int. Ed. 60 (2021) 25302–25306. doi: 10.1002/anie.202111780

  26. [26]

      J. Chen, C.L. Hu, X.H. Zhang, et al., Angew. Chem. Int. Ed. 59 (2020) 5381–5384. doi: 10.1002/anie.202000587

  27. [27]

      J. Chen, C.L. Hu, F.F. Mao, et al., Angew. Chem. Int. Ed. 58 (2019) 11666–11669. doi: 10.1002/anie.201904383

  28. [28]

      J. Chen, C.L. Hu, F.F. Mao, et al., Angew. Chem. Int. Ed. 58 (2019) 2098–2102. doi: 10.1002/anie.201813968

  29. [29]

      T. Wu, X. Jiang, K. Duanmu, et al., Angew. Chem. Int. Ed. 63 (2024) e202318107.

  30. [30]

      M. -B. Xu, J.J. Li, H.Y. Wu, et al., Dalton Trans. 53 (2024) 10536–10543. doi: 10.1039/d3dt04343f

  31. [31]

      H. Yu, M.L. Nisbet, K.R. Poeppelmeier, J. Am. Chem. Soc. 140 (2018) 8868–8876. doi: 10.1021/jacs.8b04762

  32. [32]

      N. Ma, J. Chen, B.X. Li, et al., Small 19 (2023) 2304388.

  33. [33]

      Q.Q. Chen, C.L. Hu, J. Chen, et al., Angew. Chem. Int. Ed. 60 (2021) 17426–17429. doi: 10.1002/anie.202106335

  34. [34]

      M. Gai, T. Tong, Y. Wang, et al., Chem. Mater. 32 (2020) 5723–5728. doi: 10.1021/acs.chemmater.0c01452

  35. [35]

      F.F. Mao, C.L. Hu, X. Xu, et al., Angew. Chem. Int. Ed. 56 (2017) 2151–2155. doi: 10.1002/anie.201611770

  36. [36]

      H. Liu, H. Wu, Z. Hu, et al., Chem. Mater. 34 (2022) 3501–3508. doi: 10.1021/acs.chemmater.2c00428

  37. [37]

      H.X. Tang, Y.X. Zhang, C. Zhuo, et al., Angew. Chem. Int. Ed. 58 (2019) 3824–3828. doi: 10.1002/anie.201813122

  38. [38]

      G. Peng, C.S. Lin, H.X. Fan, et al., Angew. Chem. Int. Ed. 60 (2021) 17415–17418. doi: 10.1002/anie.202105777

  39. [39]

      C. Wu, X. Jiang, Z. Wang, et al., Angew. Chem. Int. Ed. 60 (2020) 3464–3468.

  40. [40]

      J. Lu, X. Liu, M. Zhao, et al., J Am. Chem. Soc. 143 (2021) 3647–3654. doi: 10.1021/jacs.1c00959

  41. [41]

      M. Luo, C. Lin, D. Lin, et al., Angew. Chem. Int. Ed. 59 (2020) 15978–15981. doi: 10.1002/anie.202006671

  42. [42]

      C. Wu, X. Jiang, Z. Wang, et al., Angew. Chem. Int. Ed. 60 (2021) 14806–14810. doi: 10.1002/anie.202102992

  43. [43]

      Q.Q. Chen, C.L. Hu, M.Z. Zhang, et al., Inorg. Chem. 62 (2023) 12613–12619. doi: 10.1021/acs.inorgchem.3c02207

  44. [44]

      J. Cui, S. Wang, A. Tudi, et al., Inorg. Chem. 63 (2023) 661–667. doi: 10.1007/s11663-022-02716-x

  45. [45]

      W. Zeng, X. Dong, Y. Tian, et al., Chem. Commun. 58 (2022) 3350–3353. doi: 10.1039/d2cc00134a

  46. [46]

      W. Zeng, Y. Tian, X. Dong, et al., Chem. Mater. 36 (2024) 2138–2146. doi: 10.1021/acs.chemmater.3c03296

  47. [47]

      J. Chen, C.L. Hu, J.G. Mao, Sci. China Mater. 64 (2020) 400–407.

  48. [48]

      A.N. Mansour and C.A. Melendres, Surf. Sci. Spectra 3 (1994) 287–295.

  49. [49]

      C. Wang, X. Ji, J. Liang, et al., Angew. Chem. Int. Ed. 63 (2024) e202403187.

  50. [50]

      Q. Li, H. Liu, H. Yu, et al., Inorg. Chem. 62 (2023) 3896–3903. doi: 10.1021/acs.inorgchem.2c04282

  51. [51]

      F.F. Mao, C.L. Hu, B.X. Li, et al., Inorg. Chem. 56 (2017) 14357–14365. doi: 10.1021/acs.inorgchem.7b02508

  52. [52]

      L. Zhang, X. Zhang, F. Liang, et al., Inorg. Chem. 62 (2023) 14518–14522. doi: 10.1021/acs.inorgchem.3c02659

  53. [53]

      Q.Q. Chen, C.L. Hu, M.Z. Zhang, et al., Chem. Sci. 14 (2023) 14302–14307. doi: 10.1039/d3sc05770d

  54. [54]

      D. Yan, Y. Ma, R.L. Tang, et al., J Solid State Chem. 315 (2022) 123530.

  55. [55]

      S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798–3813.

  56. [56]

      J. Chen, Q.Q. Chen, F.F. Mao, et al., Inorg. Chem. Front. 9 (2022) 5917–5925. doi: 10.1039/d2qi01817a

  57. [57]

      F.F. Mao, J.Y. Hu, B.X. Li, et al., Dalton Trans. 49 (2020) 15597–15601. doi: 10.1039/d0dt03399e

  58. [58]

      Y.X. Song, M. Luo, C.S. Lin, et al., Chem. Mater. 29 (2017) 896–903.

  59. [59]

      D. Yan, M.M. Ren, F.F. Mao, et al., Inorg. Chem. 62 (2023) 1323–1327. doi: 10.1021/acs.inorgchem.3c00002

  60. [60]

      X.J. Zhou, G. Chen, X. Zhong, et al., J. Cryst. Growth 191 (1998) 517–519. doi: 10.2307/1592678

  61. [61]

      T.T.H.T. Luo, E.L. Dereniak, Opt. Lett. 31 (2006) 616.

  62. [62]

      G. Ghosh, Opt. Commun. 163 (1999) 95–102.

  63. [63]

      Y. Tian, W. Zeng, X. Dong, et al., Angew. Chem. Int. Ed. 63 (2024) e202409093.

  64. [64]

      B.P. Yang, X. Xu, C. Huang, et al., CrystEngComm 15 (2013) 10464–10469. doi: 10.1039/c3ce41653d

  65. [65]

      J.H. Wu, C.L. Hu, Y.F. Li, et al., Chem. Sci. 15 (2024) 8071–8079. doi: 10.1039/d4sc01716a

  66. [66]

      J. Chen, P. Yang, H. Yu, et al., ACS Mater. Lett. 5 (2023) 1665–1671. doi: 10.1021/acsmaterialslett.3c00239

  67. [67]

      M. Ding, H. Wu, Z. Hu, et al., J. Mater. Chem. C 10 (2022) 12197–12201. doi: 10.1039/d2tc02489f

  68. [68]

      M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Revision (2010) B.01.

  69. [69]

      T. Lu, F. Chen, J. Comput. Chem. 33 (2012) 580–592. doi: 10.1002/jcc.22885

  70. [70]

      A. Alparone, Chem. Phys. 410 (2013) 90–98.

  71. [71]

      J. Lu, Y.K. Lian, L. Xiong, et al., J. Am. Chem. Soc. 141 (2019) 16151–16159. doi: 10.1021/jacs.9b08851

  72. [72]

      S. Han, A. Tudi, W. Zhang, et al., Angew. Chem. Int. Ed. 62 (2023) e202302025.

  73. [73]

      J. Guo, A. Tudi, S. Han, et al., Angew. Chem. Int. Ed. 60 (2021) 3540–3544. doi: 10.1002/anie.202014279

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  Figure 1  (a) 2D neutral layer and (b) 3D overall structure along a direction of HDPA(IO₃). Purple, black, red, blue, and pink spheres represent I, C, O, N, and H atoms, respectively; red, green, and blue arrows represent the a, b, and c axes, respectively.

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  Figure 2  (a) UV–vis diffuse reflectance spectrum (F(R) is absorption coefficient/scattering coefficient). The SHG response of HDPA(IO₃), KDP served as (b) brandmark, (c) original crystal, (d) crystal achieving complete extinction. (e) SHG, Birefringence, bandgap comparison between title compound and reported semiorganic iodates.

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  Figure 3  (a, b) Birefringence analysis of HDPA⁺ and IO₃^-. (c, d) SHG analysis of HDPA⁺ and IO₃^-. (e, f) Contributions of HDPA⁺ and IO₃^- to birefringence and SHG. (g, h) 2D ELF sections projected on crystal planes parallel to HDPA⁺ cation and the lone pair of IO₃^-.

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