Engineering Multiple Hydrogen‐Bond Sites With D‐π‐A Motifs for Strong Optical Nonlinearity and Giant Birefringence in Organic‐Inorganic Halides

ABSTRACT

Organic‐inorganic metal halides (OIMHs) have emerged as promising nonlinear optical (NLO) materials, but simultaneously achieving large birefringence (Δn) and strong second‐harmonic generation (SHG) remains challenging owing to the propensity of π‐conjugated organic motifs to favor centrosymmetric packing. Herein, we develop a molecular design strategy by constructing a donor‐π‐acceptor (D‐π‐A) long‐chain cation, [DA7ClQ]²⁺, featuring multiple hydrogen‐bonding sites. Hybridization with inorganic [Sb₂Cl₉]³⁻ anions yields the non‐centrosymmetric crystal (DA7ClQ)SbCl₅, which exhibits a record birefringence of 0.46 at 550 nm among OIMHs‐based NLO crystals, coupled with a robust SHG response of 0.4 × KTP at 1550 nm and exceptional thermal and environmental stability. Theoretical calculations reveal that the D‐π‐A motif possesses superior first‐order hyperpolarizability (12.1 × 10⁻²⁹ esu) and polarizability anisotropy (739 a.u.), while a robust hydrogen‐bond network locks the acentric alignment. This work establishes that engineering extended π‐conjugated systems with augmented asymmetry and precise orientational control provides a versatile pathway toward advanced hybrid NLO materials.

A molecular design strategy is proposed by constructing a D‐π‐A long‐chain cation, [DA7ClQ]²⁺, featuring multiple hydrogen‐bonding sites and superior microscopic polarization properties. Hybridization with [Sb₂Cl₉]³⁻ anions yields a non‐centrosymmetric NLO crystal, (DA7ClQ)SbCl₅, which exhibits record birefringence, a strong SHG effect, and excellent thermal and environmental stability within the OIMHs‐type NLO crystal system.

1. Introduction

Second‐harmonic generation (SHG) is one of the most important frequency up‐conversion processes in nonlinear optics (NLO), providing a key route to overcome the intrinsic wavelength limitations of lasers and to access coherent radiation in new spectral regions [1, 2]. The design of SHG‐active crystals must be premised on strict structural non‐centrosymmetry and synergistically optimize core performance metrics such as nonlinear optical (NLO) coefficients and birefringence, thereby meeting the stringent requirements for efficient phase‐matched second harmonic generation output [3]. Organic‐inorganic metal halides (OIMHs) have emerged as an important structural paradigm for exploring novel NLO crystals, owing to their exceptional optoelectronic properties, mild synthesis conditions, and high structural tunability [4, 5, 6, 7, 8]. Since the pioneering work by Miyasaka in 2009 revealed their unique optoelectronic properties [9], OIMHs have demonstrated remarkable performance across a wide range of applications, including high‐energy radiation detection [10], photoluminescence [11], field‐effect transistors [12], and photocatalysis [13], and have thus evolved into one of the most promising families of optoelectronic materials.

Over the past few years, a variety of OIMHs‐based NLO crystals have been reported with relatively large second‐order NLO coefficients (d _ij), exhibiting SHG performances comparable to or even superior to those of conventional inorganic NLO crystals [14, 15, 16]. However, compared to classical inorganic NLO crystals, the introduction of organic components in OIMHs crystals to enhance d _ij often leads to a significant narrowing of the bandgap (E _g), thereby causing a pronounced increase in optical dispersion [17, 18]. This requires OIMHs‐type NLO crystals to have a greater birefringence to meet the requirements of efficient phase matching [19]. As a result, achieving a high birefringence (Δn) becomes a critical factor limiting the practical NLO performance of OIMHs crystals. Only with sufficiently large birefringence can the refractive index mismatch between the fundamental and second‐harmonic waves be effectively compensated, enabling efficient phase matching and high SHG efficiency across a broad wavelength range [20, 21]. Therefore, a key scientific challenge in advancing OIMHs‐based NLO materials toward practical applications is to significantly enhance the birefringence while maintaining a high NLO response through rational crystal structure design.

From the perspective of structural chemistry, the NLO effects and birefringence are mainly determined by two factors: (1) the anisotropy of the polarizability and the first‐order hyperpolarizability of the intrinsic microscopic groups constituted in the crystal lattice; (2) ordered arrangement degree of these microscopic groups within the lattice [22, 23]. Accordingly, current strategies for regulating the NLO effect and birefringence of OIMHs mainly focus on two aspects. At the microscopic level, birefringence‐active motifs with large first‐order hyperpolarizability (β) and pronounced polarizability anisotropy (Δα) are introduced. At the macroscopic level, these groups are induced to align along the same polarization direction and assemble into non‐centrosymmetric structures, thereby enabling the effective accumulation of microscopic β and Δα. Recently, organic π‐conjugated planar units such as aromatic amines [24], pyridine, and quinoline have been recognized as efficient birefringence‐active building blocks and have been successfully employed to construct a series of high‐birefringence OIMHs crystals, including (C₆H₅N₂)HgCl₃ [25], (C₁₀H₉N₂O)₂SnCl₆ [26], PZPbCl₃ (PZ = phenazine) [27], and (C₁₀H₆NO₂)₂SbF [28]. Notably, in 2026, a record birefringence of 1.017 @546 nm was achieved in the BIQCdCl₄ crystal constructed from a biquinoline unit, further highlighting the pivotal role of quinoline‐based π‐conjugated systems in building large‐birefringence crystal structures [29]. Unfortunately, all reported OIMHs crystals with strong birefringence (Δn > 0.4) crystallize in centrosymmetric space groups and therefore lose second‐order NLO activity. As for the reason, this may be attributed to the fact that organic π‐conjugated groups represented by quinoline typically possess relatively small molecular backbones and short molecular chains, which often lead to highly symmetric structural configurations. In addition, such structural characteristics limit the number of hydrogen‐bonding sites of the functional groups, thereby granting the organic molecules high crystallization freedom within the lattice. Consequently, they tend to pack in a centrosymmetric manner around inorganic polyhedra. During crystallization, the intrinsic molecular symmetry is continuously propagated and amplified, making it difficult for the system to escape the symmetry‐locked low‐energy thermodynamic state.

To overcome the structural limitations of the aforementioned groups, this work proposes a structural design strategy in which classical organic π‐conjugated groups are extended through π‐conjugated bridges to construct novel donor‐π‐acceptor (D‐π‐A) type NLO functional groups, with multiple hydrogen‐bonding sites introduced to better match hybrid systems. On one hand, the D‐π‐A long‐chain structure enhances electron delocalization and asymmetric potential distribution, thereby improving the anisotropy polarizability and the first‐order hyperpolarizability of microscopic group [30, 31, 32]. On the other hand, the D‐π‐A long‐chain structure increases the number of hydrogen bonding sites, which helps lock the orientation of molecules in the crystal lattice, thereby promoting the formation of non‐centrosymmetric structures [33, 34]. Based on the above strategy, we designed a D‐π‐A long‐chain group [DA7ClQ]²⁺ featuring multiple hydrogen‐bonding sites, and hybridized it with the inorganic anion [Sb₂Cl₉]³⁻, successfully constructing a novel non‐centrosymmetric OIMHs crystal, (DA7ClQ)SbCl₅. Experimental measurements reveal that it exhibits a SHG response of 0.4 × KTP (@ 1550 nm) and a remarkably large birefringence of 0.46 at 550 nm, representing the highest birefringence reported so far among OIMHs‐type NLO crystals. Meanwhile, the robust hydrogen‐bonding network endows the crystal with excellent thermal and environmental stability. Theoretical calculations further confirm that the designed D‐π‐A long‐chain group [DA7ClQ]²⁺ exhibits an extremely large first‐order hyperpolarizability (β = 12.1 × 10⁻²⁹ esu) and the highest anisotropic polarizability reported in OIMHs systems (Δα = 739 a.u.), which serves as the main source of the giant birefringence and strong NLO properties of (DA7ClQ)SbCl₅. By rationally designing a giant D‐π‐A organic cation, this work achieves the simultaneous enhancement of Δn and d _ij in OIMHs NLO crystals, providing a transferable structural strategy and concrete example for constructing next‐generation hybrid NLO crystals with large birefringence and strong optical nonlinearity.

2. Results and Discussion

2.1. Synthesis and Structural Characterization

First, based on quantum chemical calculations, a comparative analysis was conducted to evaluate the microscopic polarization capability of conventional organic π‐conjugated groups and the long‐chain D‐π‐A groups designed in this work. As shown in Figure 1a, conventional organic π‐conjugated groups possess relatively short molecular chains, which results in limited π‐electron delocalization and a relatively uniform electron density distribution. Consequently, the dipole moment within a single molecular chain is small, leading to low microscopic first‐order hyperpolarizability and weak optical anisotropy. In contrast, the D‐π‐A framework can effectively extend π‐electron delocalization and enhance intramolecular charge‐transfer effects, endowing the molecule with a large intrinsic dipole moment. This significantly improves both the anisotropy of the microscopic polarizability and the first‐order hyperpolarizability of the group. Meanwhile, the steric hindrance generated by the long‐chain backbone helps suppress the tendency toward centrosymmetric packing. Therefore, introducing this molecular design strategy into the construction of OIMHs is expected to enable the development of crystal materials with excellent NLO responses.

FIGURE 1.

(a) Comparison of the geometric structures, electronic structures, and polarization properties of several representative π‐conjugated organic ligands and the [DA7ClQ]²⁺ cation (N, orange; C, gray; O, red; H, white). The polarization properties include the total molecular dipole moment μ _tot (Debay), polarizability anisotropy ∆ ɑ (a.u.), and total first hyperpolarizability β _tot (esu × 10⁻²⁹). More detailed comparisons are provided in Figure S1 and S2 (Supporting Information). (b) Distorted inorganic anionic polyhedral unit [Sb₂Cl₉]³⁻ and the oversized π‐conjugated organic cation [DA7ClQ]²⁺. (c) Structural connectivity of the (DA7ClQ)SbCl₅ crystal (red dashed lines denote hydrogen bonds). (d) Packing framework of (DA7ClQ)SbCl₅ viewed along the c axis.

Guided by the above comparative analysis, the synthesis was subsequently carried out in this work. The preparation of OIMH crystals begins with the synthesis of the organic cation [DA7ClQ]⁺ with a D‐π‐A structure via a condensation reaction between 7‐Cl‐2‐methylquinoline and para‐dimethylamino benzaldehyde. Next, the clarified solution obtained by reacting antimony oxide with concentrated hydrochloric acid is mixed with the organic precursor [DA7ClQ]⁺. The protonated organic [DA7ClQ]²⁺ cation reacts with the inorganic Sb³⁺ ion in an 80°C hydrochloric acid solution for 2 h. Upon cooling, the (DA7ClQ)SbCl₅ crystal material is obtained (detailed preparation steps can be found in Supporting Information S1). Finally, using the synthesized crystal material, single crystals of (DA7ClQ)SbCl₅ are grown by solution cooling (Figure S3).

Single‐crystal x‐ray diffraction (XRD) measurements reveal that (DA7ClQ)SbCl₅ crystallizes in the monoclinic chiral space group C ₂, exhibiting a non‐centrosymmetric structure. As shown in Figure 1a,b, the organic cation [DA7ClQ]²⁺ consists of a D‐π‐A long‐chain structure, where a halogen‐quinoline is connected to protonated para‐dimethylamino benzaldehyde via a π‐conjugated bridge (see Figure S4). This D‐π‐A framework can extend π‐electron delocalization and strengthen the intramolecular charge transfer effect [35, 36], while the steric hindrance of the long‐chain backbone supports to suppress the tendency toward centrosymmetric packing [37, 38]. The inorganic metal anion unit is composed of [Sb₂Cl₉]³⁻ double octahedra. As shown in Figure 1b, influenced by the second‐order Jahn–Teller (SOJT) effect, the Sb‐Cl bond lengths exhibit significant variation (2.82–3.01 Å), causing a certain degree of distortion in the double octahedral unit, which aids in the formation of the non‐centrosymmetric crystal structure [39].

In the (DA7ClQ)SbCl₅ crystal, protonation extends the hydrogen‐bond donor capability of the D‐π‐A long‐chain cation [DA7ClQ]²⁺ to three sites. These sites, together with free Cl⁻ ions in the acidic medium and the inorganic bioctahedral [Sb₂Cl₉]³⁻ groups, enable the construction of a highly directional hydrogen‐bonding network. This precisely locks the non‐centrosymmetric packing arrangement, resulting in a stable organic‐inorganic hybrid crystal structure. Structural analysis further confirms that the cations and anions in (DA7ClQ)SbCl₅ are interconnected through three distinct types of hydrogen bonds, forming a unique 0D layered stacking structure (Figures 1c and S5, with hydrogen bond types denoted as 1, 2, and 3 in Figure 1c; specific hydrogen bond data can be found in Table S2, Supporting Information). The crystal structure is centered around the [Sb₂Cl₉]³⁻ unit, with the organic cation [DA7ClQ]²⁺ stacked around the [Sb₂Cl₉]³⁻ anion. The free Cl⁻ ions regulate the interlayer spacing through Type 1 hydrogen bonds, bonding with the dimethylamino group on [DA7ClQ]²⁺. The [DA7ClQ]²⁺ cations at positions A and B also connect to [Sb₂Cl₉]³⁻ through bifurcated Type 2 hydrogen bonds and Type 3 hydrogen bonds, respectively, via hydrogen bonding sites on their quinoline and benzene rings. Ultimately, the hydrogen‐bond network constructed by three types of hydrogen bonds between the organic D‐π‐A long‐chain group [DA7ClQ]²⁺, inorganic Cl⁻ ions, and [Sb₂Cl₉]³⁻ units locks the crystal into a non‐centrosymmetric structure. Observing along the c‐axis, the (DA7ClQ)SbCl₅ crystal maintains a head‐to‐tail layered arrangement, resulting in enhanced nonlinear and birefringent properties (Figure 1d).

The powder XRD of the (DA7ClQ)SbCl₅ crystal samples is consistent with theoretical computational simulations, indicating that the synthesized crystal samples possess good phase purity (Figure S6). Powder XRD and variable‐temperature XRD after 90 days of exposure to an open environment confirm that (DA7ClQ)SbCl₅ exhibits excellent crystallinity (Figures S6 and S7). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) demonstrate that the sample has good thermal stability, with a decomposition temperature above 200°C (Figures S8 and S9).

2.2. Linear and Nonlinear Optical Properties

Figure 2a presents the ultraviolet–visible–near‐infrared (UV‐Vis‐NIR) transmission spectrum of the ‌(DA7ClQ)SbCl₅ crystal (derived from the diffuse reflectance spectrum using the Kubelka–Munk function). As shown in Figure 2a, the crystal exhibits an ultraviolet cutoff wavelength of approximately 413 nm, and the optical bandgap is estimated to be about 2.78 eV based on the Tauc plot (inset of Figure 2a). In the organic‐inorganic hybrid metal halide ‌(DA7ClQ)SbCl₅ crystal, the highly electronegative inorganic halide ions Cl⁻ are favorable for the formation of a relatively wide bandgap. Meanwhile, the protonation of the organic moiety induces a torsional distortion of the p‐dimethylamino group, preventing it from adopting a coplanar configuration with the conjugated chromophore. This weakens the conjugation effect of the cation, which also leads to a blueshift in the absorption peak, thereby broadening the optical bandgap and the transparent window of the crystal [40, 41].

FIGURE 2.

(a) UV‐Vis‐NIR transmission spectrum of the (DA7ClQ)SbCl₅ crystal; the inset shows the Tauc plot of (αhν)² versus hν. (b) SHG phase‐matching curves of (DA7ClQ)SbCl₅ measured at 1064 and 1550 nm. (c) Comparison of SHG intensities of (DA7ClQ)SbCl₅ with KTP and AGS at 1550 nm under the same particle‐size range. (d) Polarized optical microscopy image of the (DA7ClQ)SbCl₅ crystal under crossed polarizers. (e) Complete extinction observed for the same crystal, which was further used for thickness determination, and (f) corresponding thickness measurement. (g) Theoretical birefringence curves of (DA7ClQ)SbCl₅ and, based on real‐space atom‐cutting, those of [DA7ClQ]²⁺, [Sb₂Cl₉]³⁻, and Cl⁻ and the refractive‐index ellipsoid with the three principal refractive indices at 550 nm for (DA7ClQ)SbCl₅ is also shown. (h) Comparison of birefringence and NLO coefficients of OIMHs‐type NLO crystals @550 nm (detailed data are summarized in Table S3).

Using the Kurtz–Perry method, the SHG responses of (DA7ClQ)SbCl₅ powders were measured under 1064 and 1550 nm laser excitation under identical experimental conditions [42]. As shown in Figure 2b, at both 1064 and 1550 nm, the SHG intensity of (DA7ClQ)SbCl₅ increases with increasing particle size, indicative of phase‐matching behavior (Detailed information can be found in S10). As shown in Figure 2c, at 1550 nm, with KTiOPO₄(KTP) and AgGaS₂(AGS) as reference crystals, the SHG intensity of (DA7ClQ)SbCl₅ at the same particle‐size range (350–450 µm) is approximately 0.4 times that of KTP and 1.8 times that of AGS. Notably, 1550 nm is close to the edge of the operating band of AGS, which may partially affect its SHG intensity. A full comparison of the SHG intensities of AGS, KTP, and (DA7ClQ)SbCl₅ over different particle‐size ranges is provided in Figure S10. It is worth noting that the increase in SHG intensity shows no obvious tendency to level off with increasing particle size, suggesting that (DA7ClQ)SbCl₅ may exhibit an even stronger SHG response at larger sizes.

The large birefringence is a key factor in achieving efficient phase matching between the fundamental and second‐harmonic waves in NLO crystals, significantly enhancing the SHG effect and optical conversion efficiency. In this study, the birefringence properties of (DA7ClQ)SbCl₅ crystals at a wavelength of 550 nm were measured using a polarizing microscope combined with a polarization interference method. Experimentally, during the positive and negative rotations of the compensator, distinct interference color changes and extinction phenomena were observed in the crystal (Figure 2d,e). Based on the measured optical path difference Δ = 1662.47 nm and crystal thickness d = 3.6 µm (Figure 2f), the birefringence at 550 nm was calculated to be Δn = Δ/d, yielding an experimental birefringence of 0.46 for the crystal. This value represents the highest reported birefringence in metal halide‐based NLO crystals, significantly surpassing similar compounds such as [DASH]Cd₂Cl₆ (Δn ≈ 0.36@550 nm) [19] and (C₆H₅N₂)HgCl₃ (Δn ≈ 0.36@546 nm) [25], and also outperforming several commercial inorganic birefringent crystals, such as LiNbO₃ (Δn = 0.074@546 nm) [43] and α‐BaB₂O₄ (Δn = 0.122@546 nm) [44]. First‐principles calculations confirmed that the birefringence at 550 nm is 0.47, with the corresponding principal refractive indices shown in Figure 2g. Based on the dispersion relation between refractive index and wavelength, we further calculated the anisotropic refractive index dispersion curve and phase‐matching wavelength of the crystal (Figure S11) [45]. The results indicate that the crystal can achieve phase matching at a wavelength of approximately 526 nm, confirming that (DA7ClQ)SbCl₅ is capable of phase matching under 1064 nm laser radiation. The excellent agreement between theoretical and experimental data verifies the accuracy of the measurements in this study. Additionally, we systematically compared the NLO coefficients and birefringence properties of (DA7ClQ)SbCl₅ with other reported OIMHs‐type NLO crystals (Figure 2h, Tables S3 and S4, Supporting Information) [19, 25, 46, 47, 48, 49, 50, 51, 52, 53, 54]. The results show that (DA7ClQ)SbCl₅ simultaneously exhibits excellent NLO coefficients and the highest birefringence in its material class.

2.3. Theoretical Calculations and Mechanism Analysis

2.3.1. D‐π‐A: Enabled Strong Optical Nonlinearity and Giant Birefringence

To elucidate the microscopic origin of the strong NLO response and large birefringence of the (DA7ClQ)SbCl₅ crystal, we calculated the electronic structure and polarization characteristics of the key building unit—the organic D‐π‐A long‐chain cation [DA7ClQ]²⁺, including the total molecular dipole moment (μ _tot), polarizability anisotropy (Δɑ), and total first‐order hyperpolarizability (β _tot). Electrostatic‐potential simulations reveal a highly pronounced asymmetric potential distribution for [DA7ClQ]²⁺: the protonated dimethylamino group acts as a strong acceptor, whereas the halogenated quinoline serves as a strong donor; the two are connected through a π‐conjugated bridge to form a highly polarized D‐π‐A architecture (Figure S4). This architecture endows [DA7ClQ]²⁺ with an exceptionally large intrinsic dipole moment and markedly reduces its symmetry. The calculations show that the highly polarized D‐π‐A long‐chain [DA7ClQ]²⁺ not only exhibits an extremely large first‐order hyperpolarizability of 12.1 × 10⁻²⁹ esu, but also an extraordinarily high polarizability anisotropy of 739 a.u., exceeding any π‐conjugated building unit reported to date in the OIMHs system (Figures 1a, Figures S1 and S2). Such outstanding microscopic polarizability renders the organic D‐π‐A unit [DA7ClQ]²⁺ an ideal motif for simultaneously inducing strong NLO effects and giant birefringence. Furthermore, using a mixed basis set of 6–311g⁺ and Lanl2DZ, we calculated the first‐order hyperpolarizability β, dipole moment μ, static polarizability α, and polarizability anisotropy Δɑ of the (DA7ClQ)SbCl₅ molecular entity that includes the inorganic polyhedra (Table S5). The results indicate that the (DA7ClQ)SbCl₅ molecule effectively inherits and further amplifies the excellent properties of the organic [DA7ClQ]²⁺ unit, with the molecular first‐order hyperpolarizability increasing to 26.44 × 10⁻²⁹ esu, the polarizability anisotropy reaching 976.77 a.u., and the dipole moment rising substantially to 1882.6 Debye.

The NLO coefficient is a key descriptor of the macroscopic NLO performance of a crystal and provides a more direct assessment of the NLO response after periodic packing of (DA7ClQ)SbCl₅ units. As summarized in Table S6, (DA7ClQ)SbCl₅ crystallizes in the C ₂ space group, and its second‐order susceptibility tensor contains four non‐zero components. The largest coefficient reaches d ₂₂ = 5.67 pm/V, which is approximately 0.5 times that of KTP (10.7 pm/V) and is essentially consistent with the experimental powder SHG results.

To further clarify the macroscopic additive effects of the organic and inorganic components in the crystal, we calculated the refractive indices of each sublattice using the real‐space atom‐cutting method (Figures 2g and S12) together with the corresponding SHG coefficients (Table S6). The results show that the organic D‐π‐A unit [DA7ClQ]²⁺ contributes far more to the crystal birefringence than the inorganic anionic units [Sb₂Cl₉]³⁻ and Cl⁻, and it nearly determines the overall birefringence. Likewise, for the second‐order NLO coefficients, the organic [DA7CIQ]²⁺ unit also dominates the contribution to the d ₂₂ component. These results indicate that the giant birefringence and excellent NLO coefficients of the (DA7CIQ)SbCl₅ crystal predominantly originate from the microscopic Δα and β of the organic D‐π‐A unit, whose contributions are efficiently accumulated into a strong macroscopic response.

2.3.2. Multiple Hydrogen‐Bond Sites

Effectively inducing and locking the uniform orientational alignment of the microscopic D‐π‐A motif [DA7ClQ]²⁺ is crucial for the constructive accumulation that gives rise to strong macroscopic NLO responses and giant birefringence. To evaluate the binding strength between the organic cation [DA7ClQ]²⁺ and the inorganic anionic units [Sb₂Cl₉]³⁻ and Cl⁻, as well as the stability of the hybrid architecture, we calculated the binding energy of the system based on density functional theory (DFT) (Supporting Information S8). The binding energy of (DA7ClQ)SbCl₅ is −1.398 eV, and this negative value indicates that the organic–inorganic interaction is thermodynamically favorable. The multiple hydrogen‐bonding sites engineered on the D‐π‐A motif not only modulate the electronic structure of the material but also assemble into a hydrogen‐bond network that robustly locks the orientational ordering of the motifs. More directly, Hirshfeld surface analysis and the corresponding fingerprint plots show that three types of hydrogen bonds between the organic cation and the inorganic double‐octahedral unit as well as the free Cl⁻ account for 33.4% of the total intermolecular interactions; in particular, two sets of N‐H···Cl interactions formed upon protonation are exceptionally strong, tightly binding the cations and anions together (Figure 3a) [55]. The electron localization function (ELF) maps further reveal highly localized electrons (> 0.7) around the isolated Cl⁻ and around the hydrogen atoms on the two protonated dimethylamino groups. The strongly distorted electron density indicates that Cl⁻ not only promotes denser packing and a more stable spatial framework, but that its high electronegativity also draws electron density toward itself, thereby strengthening organic–inorganic electronic transfer and further enhancing polarization. In addition, the electrons around the H atom on C17 of the benzaldehyde fragment and the nearby Cl atoms are likewise highly localized (Figure 3b,c), implying strong short‐range interactions between adjacent atoms. Collectively, these results further substantiate that the dual‐protonation‐enabled design of the D‐π‐A long‐chain motif can yield a stable organic‐inorganic hybrid compound.

FIGURE 3.

(a) Hirshfeld surface mapped over hydrogen‐bonding interactions and the corresponding H···Cl fingerprint plots of the (DA7ClQ)SbCl₅ crystal (N, orange; C, gray; O, red; H, white; Cl, green; Sb, blue). (b) ELF maps illustrating the interactions between the protonated dimethylamino sites of the [DA7ClQ]²⁺ cation and Cl⁻ anions. (c) ELF maps showing the interactions between the protonated quinoline sites and phenyl rings of [DA7ClQ]²⁺ and the inorganic polyhedral units. (d) Electronic band structure and PDOS of (DA7ClQ)SbCl₅. (e) SHG‐weighted density of d ₂₂ for (DA7ClQ)SbCl₅ during the virtual‐electron (Ve) and virtual‐hole (Vh) processes.

2.3.3. Elucidating the Electronic Structure and Optical Response Mechanism

In most reported organic‐inorganic hybrid metal halides, the optical bandgap is dominated by the inorganic sublattice and is therefore typically tuned by varying the metal or halogen composition, whereas the organic component exerts only a minor influence. In (DA7ClQ)SbCl₅, however, the deliberately designed oversized D‐π‐A organic cation introduces multiple hydrogen‐bonding sites, and the resulting hydrogen‐bond network markedly strengthens the cation–anion interactions, thereby enabling the organic component to participate in bandgap regulation. Using hybrid density functional theory (HSE) for improved accuracy, the calculated electronic band structure (Figure 3d) reveals a direct bandgap of 1.86 eV [56]. The partial density of states (PDOS) shows that the valence‐band maximum (VBM) is dominated by Cl‐3p orbitals, whereas the conduction‐band minimum (CBM) primarily originates from the C‐2p orbitals of the organic component. It is noteworthy that we have performed separate density of states (DOS) calculations for the Cl atoms in the organic cation, free Cl ions, and the inorganic polyhedron (Figure S13a). The results indicate that the Cl‐3p orbitals contributing to the VBM are predominantly derived from the Cl atoms in the [Sb₂Cl₉]³⁻ polyhedron. In the energy window from −6 to 0 eV, the overlap between Sb‐5s and Cl‐3p states (Figure S13b) indicates Sb‐Cl covalent interactions that connect into the double‐octahedral [Sb₂Cl₉]³⁻ unit. The use of highly electronegative Cl⁻ helps maintain a relatively wide bandgap, while incorporation of the long‐chain D‐π‐A cation [DA7ClQ]²⁺ provides stronger interactions and enhanced polarizability, exerting a beneficial influence on the optical properties. The partial charge‐density distributions of the VBM and CBM obtained from first‐principles calculations further support this conclusion (Figure S14). Moreover, band‐projection analysis indicates that the dominant contribution to the largest SHG coefficient, d ₂₂, arises from the organic moiety. Collectively, these results demonstrate that the bandgap of (DA7ClQ)SbCl₅ is jointly governed by both organic and inorganic components, endowing hybrid metal halides with greater structural and property design flexibility.

Building on the DOS analysis, we further performed SHG‐weighted density calculations to provide a more intuitive, qualitative assessment of the atomic contributions to the NLO response (Figure 3e). The results show that the SHG activity is most pronounced in the virtual‐electron occupied states (veocc) and virtual‐hole unoccupied states (vhunocc), with the SHG‐relevant electron density mainly localized on the two types of Cl⁻ in the inorganic sublattice as well as the C and N atoms in the [DA7ClQ]²⁺ cation. This observation is consistent with the DOS analysis, corresponding to the Sb‐5s and Cl‐3p states near the CBM and the C‐2p (π‐conjugated) states near the VBM, and it confirms a pronounced energy transfer between cations and anions enabled by the multiple hydrogen‐bonding sites. Collectively, these results further substantiate that designing and incorporating oversized D‐π‐A organic motifs into OIMHs constitutes an effective strategy for simultaneously achieving giant birefringence and strong optical nonlinearity.

3. Conclusions

In summary, this study addresses a key challenge in OIMHs for NLO applications by proposing a rational design strategy that incorporates D‐π‐A long‐chain cations bearing multiple hydrogen‐bonding sites. Through the successful synthesis and characterization of the non‐centrosymmetric crystal (DA7ClQ)SbCl₅, we demonstrate a record‐high birefringence of 0.46 at 550 nm among OIMHs‐based NLO crystals, coupled with a robust SHG response of 0.4× KTP at 1550 nm and outstanding thermal stability exceeding 200°C. Theoretical analyses reveal that the [DA7ClQ]²⁺ motif plays a pivotal role in delivering exceptional first‐order hyperpolarizability (β = 12.1 × 10⁻²⁹ esu) and polarizability anisotropy (Δα = 739 a.u.). Furthermore, the robust hydrogen‐bond network locks the orientation of the D‐π‐A motifs, enabling the constructive accumulation of microscopic responses into a stable non‐centrosymmetric lattice with superior macroscopic optical performance. These findings not only validate the efficacy of extending π‐conjugated systems to achieve large birefringence and strong NLO responses, but also provide a transferable blueprint for the design of next‐generation hybrid NLO materials.

Author Contributions

Yangkai Zhang: writing – review and editing, writing – original draft. Yuchao Li: writing – review and editing. Yuekai Wu: methodology, writing – review and editing. Bing Teng: supervision, funding acquisition. Hongting Lu: software. Xingxing Jiang: resources, writing – review and editing. Shijia Sun: conceptualization, supervision. Chen Hu: supervision. Dongwei Zhai: supervision. Lifeng Cao: supervision. Jinkang Ma: data curation. Run Li: data curation. Degang Xu: supervision. Zheshuai Lin: software, investigation, validation, methodology. Degao Zhong: funding acquisition, resources, supervision.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

CCDC 2536558 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Supporting File 1: anie72850‐sup‐0001‐SuppMat.docx.

Supporting File 2: anie72850‐sup‐0002‐DataFile.cif.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.