Chiral Supramolecular Templating Enables Tunable Second‐Harmonic Generation in Hybrid Zinc Thiocyanates

ABSTRACT

Chiral organic molecules have been widely used to construct hybrid nonlinear optical (NLO) materials with second‐harmonic generation (SHG) activity. However, the limited diversity of chiral building blocks has restricted both structural diversity and SHG performance. Herein, we introduce a chiral supramolecular templating strategy for engineering hybrid zinc thiocyanates with tunable SHG responses. Alkali or alkaline‐earth metal ions and natural amino acids cooperatively assemble into one‐dimensional (1D) chiral coordination polymers that serve as supramolecular templates, while the hard‐soft acid‐base principle suppresses undesired coordination between amino acid ligands and zinc ions. This coordination orthogonality enables different chiral supramolecular templates to direct the spatial organization of the SHG‐active [Zn(SCN)₄]²⁻ units. The resulting hybrid solids combine wide optical bandgaps, phase‐matchable birefringence, and diverse SHG responses. Notably, [K₂(L‐ser)][Zn(SCN)₄] exhibits an SHG intensity of 8.0 × KH₂PO₄, representing the strongest response reported to date for homochiral zinc‐based materials derived from natural amino acids. Theoretical analyses reveal that the [Zn(SCN)₄]²⁻ units dominate the SHG response, establishing chiral supramolecular templating as a general strategy for designing high‐performance hybrid NLO materials.

Chiral supramolecular templates derived from natural amino acids direct the spatial organization of SHG‐active [Zn(SCN)₄]²⁻ units, enabling diverse second‐harmonic generation in hybrid zinc thiocyanates.

Nonlinear optical (NLO) crystals are indispensable functional materials for solid‐state photonic technologies, with widespread applications in semiconductor manufacturing, optical communications, and photolithography [1, 2, 3, 4, 5, 6, 7, 8, 9]. According to the anionic group theory, the second‐harmonic generation (SHG) efficiencies of many NLO crystals are primarily determined by their anionic building blocks [10, 11, 12, 13, 14, 15]. A representative example is β‐BaB₂O₄ (BBO), whose strong SHG response (5.9 × KH₂PO₄, KDP) originates from the large microscopic second‐order susceptibility of its π‐conjugated [B₃O₆]³⁻ units [16]. Importantly, the macroscopic SHG response depends not only on the intrinsic NLO activity of the individual anionic groups but also on their spatial organization within the crystal lattice. For instance, CsBaB₃O₆, a structural analogue of BBO, contains the same [B₃O₆]³⁻ units yet exhibits no detectable SHG response [17]. This striking contrast arises from the distinct packing arrangements of the [B₃O₆]³⁻ units: in CsBaB₃O₆, neighboring anions adopt an approximately antiparallel alignment that cancels the microscopic nonlinear polarization, whereas the nearly coplanar, parallel arrangement in BBO enables constructive addition of the microscopic second‐order susceptibilities. These observations underscore that precise control over the spatial organization of NLO‐active anionic groups is essential for maximizing SHG performance.

Alkali and alkaline‐earth metal ions have been widely employed to regulate the spatial organization of various NLO‐active anionic groups, including [B₃O₆]³⁻, NH₂SO₃ ⁻, [C₃N₃O₃]³⁻ (cyanurate), and [C₃N₃S₃]³⁻ (trithiocyanurate) [18, 19, 20, 21, 22, 23, 24]. However, co‐assembly of these anionic groups with spherical metal cations often leads to centrosymmetric crystal structures that are intrinsically SHG‐inactive. Incorporating chiral organic molecules has emerged as an effective strategy for breaking inversion symmetry and constructing hybrid NLO materials [25, 26, 27, 28, 29, 30, 31, 32, 33]. Accordingly, a growing number of SHG‐active metal halides have been realized using chiral amines as extraframework structure‐directing agents. More recently, several SHG‐active metal thiocyanates have been reported by exploiting the coordination interactions between chiral amino acids and metal thiocyanate frameworks [34, 35, 36]. Nevertheless, the scope of chiral building blocks suitable for NLO material design remains limited, restricting both structural diversity and SHG performance. Therefore, the development of new classes of chiral structure‐directing motifs represents a promising strategy for expanding the structural landscape of hybrid NLO materials while enhancing their SHG performance.

Herein, we present a chiral supramolecular templating strategy for constructing hybrid zinc thiocyanates with tunable SHG responses. Unlike the conventional strategy, in which amino acids directly coordinate to metal thiocyanate frameworks as structural ligands, the present approach spatially separates the structure‐directing component from the SHG‐active [Zn(SCN)₄]²⁻ units (Scheme S1). In this approach, alkali or alkaline‐earth metal ions serve as structural nodes, while natural amino acids act as bridging ligands to assemble chiral coordination polymers that function as supramolecular templates. Compared with chiral organic molecules, these supramolecular templates offer substantially greater structural diversity arising from the wide range of metal‐ion valence states and ionic radii, together with the versatile coordination modes between metal ions and amino acid ligands. Furthermore, coordination‐driven self‐assembly generates additional chiral motifs, providing expanded opportunities for the rational design of high‐performance NLO materials.

A series of hybrid zinc thiocyanates incorporating distinct chiral supramolecular templates were successfully synthesized by solvent evaporation. During crystallization, the hard carboxylate groups of the amino acids preferentially coordinate to hard alkali or alkaline‐earth metal ions, whereas the soft thiocyanate ligands selectively bind to the moderately soft zinc ions in accordance with the hard‐soft acid‐base principle. This coordination selectivity prevents undesired coordination of the amino acid ligands to the zinc centers, thereby preserving discrete [Zn(SCN)₄]²⁻ units and allowing different chiral coordination polymers to direct their spatial organization. As a consequence, the resulting hybrid zinc thiocyanates exhibit diverse SHG responses ranging from 0.8 to 8.0 × KDP under 1064 nm laser radiation (Figure S1).

Single‐crystal x‐ray diffraction (XRD) analysis reveals that all synthesized compounds crystallize in chiral space groups (Table 1). In every structure, each zinc center adopts a tetrahedral coordination geometry defined by four thiocyanate ligands through Zn‐N bonds, forming discrete [Zn(SCN)₄]²⁻ units. Meanwhile, all amino acid molecules exist in their zwitterionic forms, featuring deprotonated carboxylate groups and protonated amino groups. Coordination between the alkali or alkaline‐earth metal ions and the amino acid ligands gives rise to infinite 1D chiral coordination polymers (Figure 1), which are further stabilized through weak S···H hydrogen bonding interactions with the discrete [Zn(SCN)₄]²⁻ units to form the hybrid architectures. In compound 1, each calcium center is coordinated by six proline ligands, while each proline ligand bridges two adjacent calcium centers. This connectivity generates a propane‐like chiral motif with a Δ configuration (Figure S2a). Replacing L‐proline with D‐proline affords the corresponding enantiomeric motif with a Λ configuration (Figure S2b). In compounds 2 and 3, each calcium center is coordinated by four amino acid ligands and two water molecules, with the amino acid ligands bridging neighboring calcium centers to construct 1D coordination polymers. Despite their similar coordination environments and connectivity, compounds 2 and 3 exhibit distinct Zn/Ca ratios of 1:1 and 2:1, respectively. This difference arises from the additional guanidinium group present in the amino acid ligand of compound 3, which alters the overall charge balance. Compounds 4 and 5 are isostructural and feature tetrahedrally coordinated lithium ions as the structural nodes. Unlike the μ₂‐bridging mode observed in compounds 1‐3, the amino acid ligands in compounds 4 and 5 function as μ₃‐bridging ligands through their carboxylate groups, linking adjacent lithium centers into 1D architectures composed of helical Li‐O‐Li chains. Compound 6 exhibits an even higher degree of connectivity, in which each L‐serine ligand acts as a μ₄‐bridging linker. Specifically, its carboxylate group coordinates to three potassium centers, whereas the hydroxyl group binds to an additional potassium ion.

TABLE 1.

Crystallographic data and structure refinement details for compounds 1–6.

Compounds a	Space group	a (Å)	b (Å)	c (Å)	α (°)	β (°)	γ (°)	R(F)
[Ca(L‐pro)3][Zn(SCN)4] (1)	P63	23.4874(3)	23.4874(3)	9.2039(2)	90	90	120	0.0503
[Ca(L‐ala)2(H2O)2][Zn(SCN)4]·H2O (2)	P1	4.9218(6)	10.5583(5)	11.5701(6)	94.496(6)	95.454(5)	100.128(6)	0.0653
[Ca(L‐Harg)2(H2O)2][Zn(SCN)4]2·2H2O (3)	C2	33.421(2)	14.0594(8)	4.9932(2)	90	90.578(3)	90	0.0432
[Li(L‐ala)(H2O)]2[Zn(SCN)4] (4)	C2	29.0895(8)	5.0274(2)	21.2085(7)	90	132.506(2)	90	0.0319
[Li(L‐val)(H2O)]2[Zn(SCN)4] (5)	C2	29.0116(16)	5.0988(2)	22.1598(14)	90	127.411(2)	90	0.0443
[K2(L‐ser)][Zn(SCN)4] (6)	P212121	10.4597(4)	11.9821(5)	13.8806(5)	90	90	90	0.0326

^a pro = proline; ala = alanine; arg = arginine, val = valine; ser = serine.

FIGURE 1.

(a) Compositional tunability of hybrid zinc thiocyanates constructed from discrete [Zn(SCN)₄]²⁻ units and chiral 1D coordination polymers. (b–g) Crystal structures of compounds 1‐6, showing the packing of the [Zn(SCN)₄] [2‐] units and the corresponding chiral 1D coordination polymers assembled through Ca²⁺‐L‐proline (1), Ca²⁺‐L‐alaline (2), Ca²⁺‐L‐arginine (3), Li⁺‐L‐alanine (4), Li⁺‐L‐valine (5), and K⁺‐L‐serine (6) coordination.

Previous studies have suggested that neighbouring [Zn(SCN)₄]²⁻ units can engage in supramolecular interactions when the S···S separation is shorter than 4.06 Å [37]. According to this criterion, the [Zn(SCN)₄]²⁻ units in compounds 1‐6 assemble into distinct supramolecular architectures. Specifically, the [Zn(SCN)₄]²⁻ units in compounds 1 and 2 are organized into tubular and layered supramolecular arrays, respectively (Figures S3 and S4). In compound 3, each [Zn(SCN)₄]²⁻ unit is connected to five neighboring anions, giving rise to a 3D sqp‐type supramolecular network (Figure S5). In compounds 4 and 5, each [Zn(SCN)₄]²⁻ unit interacts with four neighboring anions to generate a five‐fold interpenetrating dia network (Figure S6). Compound 6 exhibits the highest degree of connectivity, with each [Zn(SCN)₄]²⁻ unit interacting with eight neighboring anions to form a 3D bcu network (Figure S7). These diverse supramolecular architectures indicate that the chiral coordination polymers direct the spatial organization of the [Zn(SCN)₄]²⁻ units through N/O─H···S hydrogen‐bonding interactions between the coordination polymers and thiocyanate ligands (Figure S8 and Table S1). To further elucidate the templating effect, the electrostatic potential (ESP) distributions of the chiral coordination polymers were calculated (Figure S9). The distinct ESP surfaces reveal that different templates provide different electrostatic environments, which are expected to influence the orientation of the thiocyanate ligands during crystal growth and thereby promote the formation of distinct packing motifs.

Powder XRD patterns confirm the phase purity of all as‐synthesized compounds. Moreover, the unchanged PXRD patterns after one week of exposure to ambient air demonstrate the good stability of compounds 1–6 under ambient conditions (Figure S10). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) show that compounds 1 and 6 remain stable without detectable weight loss up to 188 and 173°C, respectively, whereas compounds 2–5 exhibit initial weight losses between 40 and 100°C due to the release of lattice water molecules. The corresponding DSC curves are fully consistent with the TGA results (Figure S11). Their infrared spectra exhibit similar characteristic absorption features (Figure S12). Using compound 6 as a representative example, the intense absorption band at 2052 cm⁻¹ is assigned to the C≡N stretching vibration of the thiocyanate group. The characteristic bands at 1616 and 1334 cm⁻¹ are attributed to the C = O and C−O stretching vibrations of the amino acid ligand, respectively. UV/vis diffuse‐reflectance spectroscopy reveals wide bandgaps ranging from 4.27 to 4.57 eV for compounds 1‐6 (Figure S13 and Table S2). Compounds 1‐6 exhibit high powder laser‐induced damage thresholds (LIDTs) of 82.18–99.01 MW/cm², corresponding to 20.8–25.0 times that of the benchmark AgGaS₂ (3.96 MW/cm²) measured under identical conditions (Table S3). Birefringence measurements performed on optically transparent single crystals using a polarizing microscope yielded values ranging from 0.047 to 0.255 at 546 nm (Figure S14 and Table S2).

The SHG responses of compounds 1‐6 were evaluated using the Kurtz‐Perry powder technique [38]. As shown in Figure 2a, the SHG intensities increase progressively with increasing particle size (25−212 µm), indicating that all compounds satisfy the phase‐matching condition. Under 1064 nm laser radiation, compounds 1‐6 exhibit diverse SHG intensities ranging from 0.8 to 8.0 × KDP in the particle‐size range of 150−212 µm (Figure 2b and Table S2). The relatively weak SHG intensity of compound 1 (0.8 × KDP) may be attributed to less effective cooperative alignment of the microscopic nonlinear polarizations, together with the lowest packing density of the [Zn(SCN)₄]²⁻ units in this series. To benchmark their SHG performance, the measured values were compared with those of previously reported chiral zinc‐based materials derived from natural amino acids (Table S4), including (C₅H₉NO₂)₂ZnCl₂ (1.0 × KDP), Zn(HPO₃)(alanine) (1.2 × KDP), C₅H₁₁NO₂ZnSO₄·7H₂O (1.3 × KDP), Zn(lp)₂Br₂ (1.5 × KDP), Zn(C₉H₁₁NO₃)₂CO₃ (2.1 × KDP), Zn₂(BO₃)(C₃H₆NO₂) (2.5 × KDP), and Zn(SCN)(L‐C₅H₈NO₂)(H₂O) (3.8 × KDP) [34, 39, 40, 41, 42, 43, 44, 45, 46]. Among this class of materials, [K₂(L‐ser)][Zn(SCN)₄] (6) exhibits the strongest SHG response reported to date, reaching 8.0 × KDP (Figure 2c). It is also instructive to compare compound 6 with inorganic zinc thiocyanates containing discrete [Zn(SCN)₄]²⁻ units. For instance, K₂Zn(SCN)₄·3H₂O exhibits a moderate SHG response of 1.74 × KDP, whereas (NH₄)₂Zn(SCN)₄·3H₂O reaches 8 × KDP [6, 47]. In contrast, replacing K⁺ or NH₄ ⁺ with Na⁺ affords the centrosymmetric compound Na₂Zn(SCN)₄·8H₂O, which is SHG‐inactive. Furthermore, when the discrete [Zn(SCN)₄]²⁻ units are interconnected by Zn²⁺ ions to form a dense diamond‐type framework, Zn(SCN)₂ exhibits a giant SHG response of 30 × KDP. These comparisons demonstrate that the macroscopic SHG response is governed not only by the intrinsic NLO activity of the [Zn(SCN)₄]²⁻ units but also by their spatial organization and packing density. In this context, chiral supramolecular templating provides an effective strategy for directing the noncentrosymmetric assembly and structural diversification of SHG‐active [Zn(SCN)₄]²⁻ units.

FIGURE 2.

(a) Particle‐size dependence of the SHG intensities of compounds 1‐6 and KDP under 1064 nm radiation. (b) Comparison of the SHG intensities of compounds 1‐6 and KDP for particles with sizes of 150–212 µm. (c) Comparison of the SHG intensities of representative zinc‐based hybrid materials derived from natural amino acids and two representative nonhomochiral zinc thiocyanates.

Density functional theory (DFT) calculations were performed to elucidate the electronic structures of compounds 1‐6 [48, 49]. The calculated band structures reveal indirect bandgaps of 3.91–4.36 eV, which are slightly smaller than the experimental values because conventional DFT methods are known to underestimate bandgaps (Figure S15). The total and partial density of states (DOS) provide further insight into the origin of the optical transitions (Figures 3a and S16). The valence band maximum is dominated by the S 3p and N 2p orbitals of the thiocyanate groups, whereas the conduction band minimum is primarily composed of states derived from the amino acid ligands together with the C 2p and N 2p orbitals of the thiocyanate groups. These results indicate that the band‐edge electronic transitions arise from cooperative contributions of both the amino acid ligands and the thiocyanate groups. The calculated SHG coefficients are in good agreement with the experimentally measured SHG intensities (Figure S17). As a representative example, compound 6 possesses a single nonzero independent SHG tensor component under Kleinman symmetry conditions [50]. Its calculated SHG coefficient, d ₃₆, reaches 3.26 pm/V at 1064 nm (1.165 eV), corresponding to approximately 8.4 × KDP (d ₃₆ = 0.39 pm/V for KDP). Furthermore, the calculated refractive index dispersion curves reveal pronounced optical anisotropy, resulting in birefringence values of 0.060–0.266 at 546 nm (Figure S18 and Table S2). These values are sufficient to satisfy the phase‐matching condition.

To clarify the origin of the SHG response, SHG‐weighted electron density analyses were performed for compounds 1‐6 by examining the occupied and unoccupied electronic states involved in virtual electron (VE) transitions. As shown in Figure 3b, the SHG‐weighted electron density of the occupied states in compound 6 is predominantly localized on the N and S atoms of the thiocyanate ligands, with minor contributions from the zinc atoms and the hydroxyl oxygen atoms of the L‐serine ligands. In contrast, the unoccupied states are primarily distributed over the C and N atoms of the thiocyanate ligands together with the carboxylate groups of the L‐serine ligands (Figure 3c). Real‐space atomic cutting analysis reveals that the [Zn(SCN)₄]²⁻ units account for 84.0% of the overall SHG response of compound 6, whereas the chiral coordination polymer template contributes only 16.0% [51, 52]. Similar trends are observed for compounds 1–5, in which the [Zn(SCN)₄]²⁻ units contribute 59.5–80.6% of the total SHG responses (Figures S19–S23). These results demonstrate that the variation in SHG performance among compounds 1–6 is governed primarily by differences in the spatial organization of the [Zn(SCN)₄]²⁻ units rather than by the intrinsic SHG activity of the chiral coordination polymer templates. For the isostructural compounds 4 and 5, the difference in SHG intensity can be attributed to the packing density of the [Zn(SCN)₄]²⁻ units. Specifically, the larger molecular volume of L‐valine relative to L‐alanine reduces the volume fraction of the [Zn(SCN)₄]²⁻ units in compound 5, leading to a weaker SHG response.

FIGURE 3.

(a) Total and partial DOS for compound 6. SHG‐weighted electron densities of the (b) occupied and (c) unoccupied states involved in the virtual electron process.

In conclusion, we have developed a chiral supramolecular templating strategy for constructing hybrid zinc thiocyanates through a simple solvent evaporation approach. The resulting materials comprise discrete [Zn(SCN)₄]²⁻ units and chiral 1D coordination polymers assembled from natural amino acids and alkali or alkaline‐earth metal ions. These hybrid solids exhibit wide optical bandgaps, sufficient birefringence for phase matching, and diverse SHG responses. Structural analyses combined with first‐principles calculations reveal that the SHG activity originates predominantly from the [Zn(SCN)₄]²⁻ units, whereas the chiral coordination polymer templates primarily direct their spatial organization. Consequently, the NLO response can be modulated through supramolecular templating, which provides a versatile strategy for expanding the structural diversity of hybrid NLO materials and a new design principle for tailoring second‐order NLO properties.

Author Contributions

Xuemei Wen: investigation, formal analysis, visualization, writing – original draft. Qinglan Zhong: formal analysis, data curation. Gangji Yi: formal analysis, data curation. Guohong Zou: writing – review and editing. Jianli Wang: supervision, writing – review and editing. Zhien Lin: conceptualization, funding acquisition, writing – original draft, writing – review and editing, supervision. Kang Min Ok: conceptualization, funding acquisition, writing – review and editing, supervision.

Conflicts of Interest

The authors declare no conflicts of interest

Supporting information

Supporting File: anie74287‐sup‐0001‐SuppMat.docx.

Data Availability Statement

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