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. 2026 May 26;65(30):e4675227. doi: 10.1002/anie.4675227

Lock‐and‐Key Assembly Enables Record Birefringence in Monocyclic π‐Conjugated Crystals via Spatial Confinement of Linear Polyhalide

Yun‐Xia Hu 1, Huai Yu Wu 2, Jia‐Jia Li 1, Ming‐Chang Wang 1, Jia‐Min Lian 1, Jin Yu Luo 1, Yi‐Ru Fu 1, Zi‐Yan Chen 1, Yang‐Hang Guo 1, Jin Chen 1,✉, Ke‐Zhao Du 1,✉
PMCID: PMC13383262  PMID: 42189709

ABSTRACT

Birefringent crystals are central to polarization optics, yet pushing birefringence (Δn) beyond 1.0 while retaining transparency (E g > 2.0 eV) has been pursued almost exclusively by extending π‐conjugation from monocyclic to polycyclic aromatics. Monocyclic π‐systems have long been considered intrinsically capped below Δn = 1.0. Here, we challenge this assumption by demonstrating that geometric precision, rather than π‐system enlargement, can unlock this performance ceiling. We introduce a halogenation‐induced dimensional reduction strategy in which halogen substituents redirect the hydrogen‐bonding topology of cytosine from non‐directional 2D networks into wave‐like 1D chains, whose complementary concave pockets form capsule‐shaped cavities. These cavities act as lock‐and‐key templates that confine linear polyhalides into strict collinear alignment while enforcing π‐plane coplanarity. This strategy affords five new birefringent hybrid crystals: (HXCy)2(I2Cl)·Cl (X = Cl, I; Br, II, Cy = cytosine), (HClCy)2(ICl2)·Cl (III), and (HBrCy)(BrCy)·IBr2 (IV) and·Br3 (V). I and II reach calculated Δn = 1.336 and 1.324 at 546 nm, the highest among all π‐conjugated and inorganic crystals reported, while V retains Δn = 1.311 with a widened E g = 2.25 eV, demonstrating that birefringence and transparency can be independently tuned. These results establish geometric precision, rather than π‐system enlargement, as a powerful design route to high‐performance birefringent crystals.

Keywords: birefringence, lock‐and‐key assembly, polyhalide anion, structure‐property relationships, supramolecular frameworks

A lock‐and‐key spatial‐confinement strategy, in which wave‐like cytosine hydrogen‐bonded chains create capsule‐shaped cavities that simultaneously enforce collinear polyhalide alignment and π‐plane coplanarity, resulting in birefringence up to 4–111 times greater than commercial crystals such as MgF2 and α‐BBO.

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1. Introduction

Wave plates, polarizers, and phase‐matched nonlinear frequency converters all rely on optical birefringence, the direction‐dependent refractive index that arises when anisotropic structural units align coherently across a crystal lattice [1, 2, 3, 4, 5, 6]. The magnitude of birefringence (Δn) is governed by two factors: The intrinsic polarizability anisotropy (Δα) of the constituent building blocks and the degree of their orientational order. Despite decades of materials development, widely used crystals such as MgF2 (Δn ≈ 0.012) [7], α‐BBO (0.12) [8], CaCO3 (0.17) [9], and YVO4 (0.23) [10] remain limited to moderate birefringence, constraining device miniaturization and performance in applications that demand strong beam manipulation within compact form factors [11, 12, 13, 14, 15, 16, 17, 18]. This performance ceiling has motivated an intensive search for materials combining giant Δn (>0.3) with broad optical transparency.

Two design strategies have recently pushed birefringence into the giant regime. The first extends π‐conjugation from monocyclic to polycyclic aromatics (Scheme 1a) [19, 20, 21, 22, 23, 24, 25, 26]. As the number of fused rings increases from monocyclic FAHC4O4 [19] and [HPyClB(OH)2]·(NO3) [22] through bicyclic (C12H8N2)SbF3 [24] and (C10H6NO2)2SbF [23] to polycyclic Li3(C9N13)·6H2O [26] and Na2(4‐HPyH)2 (PTS)·H2O [27], the delocalized π‐electron cloud becomes increasingly anisotropic: Highly polarizable within the molecular plane, but weakly polarizable perpendicular to it. The most striking demonstration is the diazapyrene cocrystal series (D27N and D13N), where extensive π‐delocalization yields Δn up to 1.269 at 546 nm [20]. The second strategy exploits linear anisotropic units such as polyhalide anions (IX2 −, X = Cl, Br, I) [13, 28, 29, 30, 31, 32] or Hg2 2+‐centered clusters (Hg2Q2, Q = I, O, S, Se) [33, 34, 35, 36] that generate polarizability contrast between the chain axis and the perpendicular direction. Parallel alignment of these linear units produces giant in‐plane birefringence, such as CsICl2 (Δn = 0.994 at 546 nm), which is built from collinear ICl2 − units [32]. Both strategies achieve impressive performance, yet each optimizes anisotropy along only one dimension, either out‐of‐plane (π‐stacking) or in‐plane (linear alignment), while leaving the orthogonal contribution underdeveloped. This raises a natural question: can the two mechanisms be combined to access even larger birefringence?

SCHEME 1.

SCHEME 1

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Previous work: Extends π‐conjugation (a); This work: HOMO─LUMO gap and polarization anisotropy (b) as well as localized orbital locator (LOL) (c) of selected units, Lock‐and‐key assembly schematic (d).

The answer, however, is not straightforward. Simply combining π‐conjugated cations with linear polyhalide anions does not guarantee synergy. The fundamental challenge lies in achieving simultaneous coplanarity of π‐planes and collinearity of polyhalide units, a condition that requires directional intermolecular forces strong enough to override competing packing preferences. When geometric constraints are absent or weak, unfavorable orientations emerge: In [H‐4AP][IBr2], a dihedral angle of approximately 69° between the IBr2 − axis and the pyridine plane causes the π‐ring to contribute negatively, limiting Δn to 0.837 at 546 nm [29]. Even when near‐coplanarity is achieved, as in (C4H5N3OI)I2Cl3·2H2O (dihedral angle 7.3 ), the V‐shaped I2Cl3 geometry dilutes the linear anisotropy of individual ICl2 units, yielding Δn = 0.712 at 546 nm [28]. Directional halogen bonding offers partial solutions. In 3,5‐Cl2Py‐ICl, N─I─Cl halogen bonds enforce coplanarity between pyridine rings and ICl units, raising Δn to 0.97 at 546 nm [30]. Yet the modest Δα of ICl limits further improvement. Conversely, C3H8N6I6·3H2O achieves Δn ≈ 1.7 in the NIR through I─I···I halogen‐bonded I3 − chains with large Δα, but the narrow HOMO─LUMO gap of I3 − restricts the bandgap to 1.68 eV, limiting transparency [13]. These examples reveal a persistent design challenge: achieving an optimal balance between giant birefringence (Δn > 1.0) and adequate transparency (E g >2.0 eV) requires not only selecting building blocks with complementary electronic properties, but also engineering intermolecular interactions that enforce the precise geometric alignment needed for constructive anisotropy coupling. This challenge is particularly pronounced for monocyclic π‐systems, whose compact aromatic cores carry only moderate Δα compared to their polycyclic counterparts. A potential solution lies in leveraging the self‐assembly capabilities of hydrogen‐bonded organic frameworks (HOFs): if monocyclic π‐conjugated cations could be pre‐organized through multiple complementary hydrogen bonds into 1D or 2D networks with well‐defined cavities, these cavities could serve as spatial templates to capture and orient linear polyhalide anions, thereby enforcing collinearity while the hydrogen‐bond network simultaneously maintains coplanarity of the π‐planes.

Motivated by this hypothesis, we adopted a computational screening approach to identify building blocks satisfying three criteria simultaneously: (i) High intrinsic Δα, (ii) wide HOMO─LUMO gap, and (iii) chemical functionality enabling strong directional bonding (Scheme 1b, c). Cytosine possesses multiple hydrogen‐bond donor and acceptor sites, enabling the formation of complex hydrogen‐bond networks. Unsubstituted cytosine, such as C4H5N3O·H2O [37], however, tends to form 2D HOFs through multidirectional hydrogen bonding, in which cavities, lack uniform orientation and thus cannot effectively template linear anion alignment. Halogenation fundamentally alters this behavior: the steric bulk of halogen substituents (Cl, Br) disrupts the 2D hydrogen‐bond network and favors the formation of 1D hydrogen‐bonded chains instead. When these chains adopt a wave‐like conformation, adjacent chains create complementary concave pockets that define directional cavities with uniform orientation, well‐suited for capturing and aligning linear polyhalide anions through spatial confinement (Scheme 1d).

To validate this design strategy, we synthesized five cytosine‐based (Cy‐based) hybrid crystals, namely (HXCy)2(I2Cl)·Cl (X = Cl I, Br II), (HClCy)2(ICl2)·Cl III, (HBrCy)(BrCy)·IBr2 IV, and (HBrCy)(BrCy)·Br3 V. In all five compounds, the wave‐like hydrogen‐bonded chains form capsule‐shaped cavities that enforce unidirectional polyhalide alignment, confirming the lock‐and‐key spatial‐confinement mechanism. This dual alignment yields Δn = 1.336 and 1.324 at 546 nm for I and II, the highest values among all monocyclic‐π‐based materials and surpassing all inorganic benchmarks across the visible region. Notably, V retains Δn = 1.311 at 546 nm with a widened bandgap of 2.25 eV, demonstrating that anion substitution enables independent tuning of birefringence and transparency within this framework.

2. Results and Discussion

PXRD patterns of ground crystalline samples matched well with the simulated patterns from single‐crystal x‐ray diffraction data, confirming phase purity (Figure S3). PXRD analysis of unground crystals revealed preferred growth orientations along the (010) plane for I, II, IV, and V, and along the (011) plane for III (Figures 1a, b; S4). The molecular structures and elemental compositions were further verified by Raman, IR, and FESEM analyses (Figures 1c‐e; S5–S8; Table S10–S12). Raman spectra exhibited characteristic vibrations of the polyhalide units: I─Cl/I stretching modes at 118─182 cm─1 for I─III, I─Br stretching at 180 cm− 1 for IV, and Br─Br stretching at 180 cm−1 for V. Additional features corresponding to C─Cl (587─800 cm−1), C─Br (596─800 cm−1), C═C (1250─1531 cm−1), C═N (1665 cm−1), C═O (1690─1755 cm−1), and N─H (3296─3393 cm− 1) vibrations confirmed the presence of halogenated cytosine moieties (Figure 1c). TGA revealed decomposition temperatures of 156, 153, 164, 227, and 227 C for I─V, respectively (Figures 1f and S9). UV‐Vis‐NIR spectra showed absorption edges at 375, 411, 344, 320, and 357 nm, corresponding to optical bandgaps of 2.14, 2.02, 2.25, 2.28, and 2.25 eV, respectively (Figures 1g–j; S10). Notably, compounds IV and V exhibit superior thermal stability and transparency compared to most previously reported π‐conjugated coupled with polyhalide hybrid materials, such as [H‐4AP][IBr2] (117  C, bandgap 2.11 eV) [29], making them particularly promising for practical optical applications.

FIGURE 1.

FIGURE 1

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PXRD patterns of unground crystals of I (a) and IV (b). Insets show photographs of centimeter‐scale single crystals; Raman spectra of compounds I─V (c). TEM images and EDS elemental mapping of I (d) and IV (e). TGA curves of I─V (f). UV‐Vis‐NIR diffuse reflectance spectrum (g, i) and bandgap determination (h, j) of IV and V, respectively.

2.1. Crystal Structures

All five compounds share a common hierarchical assembly pathway (Figures 2 and S11–S24). Halogenated cytosine cations first self‐assemble into wave‐like one‐dimensional (1D) hydrogen‐bonded organic chains. These chains subsequently capture linear polyhalide anions forming 2D layers through hydrogen bonds and/or halogen bonds. The 2D layers then stack into 3D frameworks via anion─π interactions.

FIGURE 2.

FIGURE 2

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(a,b) 2D layer and 3D packing of I. (c) Lock‐and‐key assembly schematic showing capsule‐shaped cavities that confine polyhalide units. (d,e) 2D layer and 3D packing of III. (f,g) 2D layer and 3D packing of IV.

2.1.1. (HClCy)2(I2Cl)·Cl (I) and (HBrCy)2(I2Cl)·Cl (II)

The two isostructural compounds crystallize in the monoclinic P21/m space group. Taking I as the representative, each asymmetric unit contains two independent HClCy+ cations, one I2Cl─ anion, and one isolated Cl─ anion. Within the I2Cl─ unit, the I─Cl distance (2.8126(15) Å) exceeds both the I─I distance (2.7402(6) Å) and the I─Cl covalent bond lengths reported in related polyhalide systems, such as 3,5‐Cl2Py‐ICl (dI─Cl = 2.479(8) Å) [30] and (C4H5N3OI)(ICl2)·H2O (dI─Cl = 2.540(4) and 2.536(4) Å) [28]. This elongation indicates that the strong I─I covalent bond draws electron density toward the I─I region, rendering the terminal I─Cl interaction more halogen‐bond‐like in character, with a normalized contact ratio Nc(I─Cl) = 0.75, where N c is defined as the ratio of the observed interatomic distance to the sum of the van der Waals radii of the two atoms involved [38, 39, 40, 41, 42]. The opposite terminal iodine further engages an isolated Cl− through a weaker halogen bond (dI···Cl = 3.4368 Å, N c = 0.91), forming a [I2Cl2]2─ aggregate. This interaction is comparable to those reported in 3,5‐I2Py‐ICl (C─I···Cl, N c = 0.85) [30] and C3H8N6I6·3H2O (I─I···I, N c = 0.85─0.88) [13].

On the organic side, HClCy+ cations are linked by strong N─H···O hydrogen bonds (dD─A = 2.738(7)─3.072(7) Å) into 1D cationic [(HClCy)2]2+ chains propagating along the a‐axis (Figure 2a). These chains adopt a wave‐like conformation, and adjacent chains along the c‐axis are phase‐shifted by half a period, generating complementary concave pockets that define capsule‐shaped cavities (Figure 2a). Each cavity spans 16.560(6) Å along its longest dimension, oriented at approximately 30° to the c‐axis, with interior walls lined by hydrogen‐bond donors that precisely capture the [I2Cl2]2─ aggregates. One end of the cavity coordinates the Cl─ of the strong I─Cl halogen bond through two N─H donors (one from the six‐membered ring, one from the amino group), while the opposite end coordinates the Cl− of the weak I···Cl halogen bond through three N─H donors (two from the ring, one from the amino group), with dD─A = 3.029(5)─3.218(5) Å. This lock‐and‐key matching achieves perfect coplanarity of all components and enforces unidirectional ordering of the polyhalide units through spatial confinement (Figure 2c). The resulting 2D [(HClCy)2(I2Cl)·Cl] layers lie parallel to the ac‐plane and stack along the b‐axis in an ─ABAB─ sequence, consolidated by Cl─···π interactions (Cl···centroid distances of 3.358(2)─3.369(4) Å) with an interlayer spacing of 3.309 Å (Figures 2b and S14).

2.1.2. (HClCy)2(ICl2)·Cl (III)

Compound III crystallizes in the triclinic P‐1 space group, with each asymmetric unit containing two HClCy+ cations, one ICl2 ─ anion, and one isolated Cl─ anion. The 2D layer structure closely resembles that of I and II, retaining the wave‐like 1D hydrogen‐bonded chains, complementary capsule‐shaped cavities, and unidirectional polyhalide ordering. The layers likewise stack in an ─ABAB─ sequence, linked by Cl─···π interactions (Cl···centroid distances of 3.4816(18)─3.7016(18) Å) with an interlayer spacing of 3.293 Å (Figure 2d, e).

The principal structural difference arises from the I2Cl─→ICl2 ─ substitution. The shorter, fully covalent I─Cl bonds in ICl2 − substantially weaken the interaction between the polyhalide terminal atom and the isolated Cl− (dI─Cl···Cl = 3.5635(14) Å, N c = 1.02), effectively eliminating the halogen‐bonded [I2Cl2]2− aggregate. Both terminal Cl atoms of ICl2 ─ consequently require additional N─H donors for stabilization, necessitating a partial reorganization of the hydrogen‐bond network relative to I and II. This reorganization introduces a 4.5 dihedral angle between adjacent HClCy units, though the overall structural integrity and unidirectional alignment are preserved.

2.1.3. (HBrCy)(BrCy)·IBr2 (IV) and (HBrCy)(BrCy)·Br3 (V)

The two isostructural compounds crystallize in the monoclinic C2/c space group. Taking IV as the representative, the BrCy molecule sits on a crystallographic twofold rotation axis, generating a dimer through N─H···O hydrogen bonds. Within the dimer, the proton on the N2 atom of the six‐membered ring (50% occupancy) forms an N2─H···N2 hydrogen bond, giving rise to a [(HBrCy)(BrCy)]+ cation in which N1 is fully protonated (100% occupancy) and the amino nitrogen is designated N3. Adjacent cations alternate in the protonation state of N2. Each asymmetric unit contains one [(HBrCy)(BrCy)]+ cation and one IBr2 ─ anion. Adjacent [(HBrCy)(BrCy)]+ cations are connected by N1─H···O hydrogen bonds along the a‐axis into wave‐like 1D chains. As in I─III, complementary concave pockets from neighboring chains form capsule‐shaped cavities (longest dimension 14.648(8) Å, oriented at approximately 20° to the c‐axis). The terminal Br atoms of IBr2 − are captured within these cavities through two sets of N3─H···Br and two sets of C─H···Br hydrogen bonds, achieving spatial confinement and unidirectional ordering of the polyhalide units to form 2D [(HBrCy)(BrCy)·IBr2] layers (Figure 2f, g). Unlike I─III, the BrCy planes and the IBr2 ─ plane in IV are vertically offset by approximately 0.05 Å, though each component remains internally coplanar and the arrangement is still nearly optimal for optical anisotropy. The 2D layers stack along the b‐axis, linked by Br−···π interactions (Br···centroid distance 3.623(2) Å) with an interlayer spacing of 3.289 Å.

The Interaction Region Indicator (IRI) analysis confirms that N─H···O hydrogen bonds are the dominant interactions organizing the wave‐like 1D chains, while halogen bonds provide additional stabilization at the polyhalide (Figure S25). The Hirshfeld surface analysis shows that H···X contacts constitute the largest proportion of intermolecular contacts, consistent with the lock‐and‐key confinement mechanism (Figures S26─S32). Detailed results and fingerprint plots are retained in the Supporting Information.

2.2. Birefringence

Benefiting from the lock‐and‐key assembly that enforces near‐ideal geometric alignment of both π‐conjugated planes and linear polyhalide units, crystals I─V are expected to exhibit exceptional birefringence. To quantify their optical performance, we first performed first‐principles calculations to determine the linear optical responses, including refractive indices and birefringence. Since all five compounds belong to triclinic or monoclinic crystal systems and are biaxial crystals, we performed principal axis transformation (Table S13) to align the maximum refractive index direction with the principal optical axis Z [29], with the remaining axes X and Y satisfying n Z > n Y > n X. Birefringence was then defined as Δn = n Z ─ n X (Figures 3a and S33). As shown in Figure 3a, crystals I─V exhibit calculated birefringence values of 1.336, 1.324, 1.046, 1.177, and 1.311 at 546 nm and 0.862, 0.858, 0.815, 0.824, and 0.878 at 1064 nm, respectively. These values are approximately 4 to 111 times higher than those of commercial birefringent crystals (Figure 3b), identifying I─V as promising birefringent materials across the visible to near‐infrared region.

FIGURE 3.

FIGURE 3

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Birefringence curves for I─V across the visible to near‐infrared region (a) and the comparison with commercial birefringent crystals; (b) Schematic optical path of the compensator method (c). Polarizing microscope images showing brightness and extinction upon crystal rotation. (d) Experimental birefringence determination for crystals I─V (e─i) via linear fitting of retardation (R) versus crystal thickness (t).

2.2.1. Experimental Birefringence Measurement

The practical significance of giant birefringence lies in its ability to reduce the physical dimensions of polarization optical devices. A birefringent crystal of thickness t placed between crossed polarizers induces a phase retardation R = Δn × t converting linearly polarized light into elliptically polarized light that can pass through the analyzer (Figure 3c). A larger Δn allows thinner crystals to achieve the same retardation, enabling more compact optical components. For instance, achieving a quarter‐wave retardation (λ/4 = 137 at 546 nm) requires a crystal thickness of only ∼100 µm for Δn = 1.3 (as in I, II, and V), compared to ∼1100 µm for commercial calcite (Δn = 0.17), representing an order‐of‐magnitude reduction in device thickness. To experimentally verify the calculated birefringence values, we employed the compensator method under a polarizing microscope [43, 44]. A birefringent crystal was placed between crossed polarizers, with a calibrated compensator inserted between the crystal and the analyzer to measure the induced retardation R (Figure 3c; detailed optical setup and extinction images in Figures 3d and S34). For each compound, seven crystals of varying thickness were measured to minimize experimental error. The relationship R = Δn × t Rpredicts a linear correlation between retardation and thickness, with the slope directly yielding Δn (Table S18). As shown in Figure 3e─i, the experimental data for all five crystals exhibit excellent linearity (R 2> 0.98), with fitted slopes corresponding to birefringence values of 1.158, 1.155, 0.919, 1.051, and 1.167 at 546 nm for I─V, respectively. These experimental values are somewhat lower than the calculated results, which is primarily attributed to crystal orientation uncertainty: for biaxial crystals I─V, the crystallographic axes do not fully coincide with the principal optical axes, such that the crystal may not be cut strictly along the optimal optical axis direction, and the measured retardation therefore does not strictly correspond to the maximum birefringence.

2.2.2. Comparative Analysis

The birefringence values of I─V are 4 to 94 times higher than those of commercial birefringent crystals, including YVO4 (0.204 at 532 nm), TiO2 (0.256 at 546 nm), CaCO3 (0.172 at 546 nm), and α‐BBO (0.12 at 546 nm) (Figure 3b) [11, 12, 13, 14, 15]. In the near‐infrared region, where E g requirements are less stringent, linear anisotropic units are particularly effective building blocks. As shown in Figure 4a, crystals I─V exhibit birefringence values of 0.862, 0.858, 0.815, 0.824, and 0.878 at 1064 nm, respectively, exceeding those of previously reported linear‐unit‐based compounds such as [H‐4AP][IBr2] (0.762 at 1064 nm) [29], SnCN2 (0.730 at 1064 nm) [45], and CsICl2 (0.719 at 1064 nm) [32] (Table S14). Among monocyclic π‐systems, only croconate‐based crystals such as NaRbC5O5 (0.815 at 1064 nm), NaCsC5O5 (0.756 at 1064 nm), and Na2C5O5·2H2O (0.737 at 1064 nm) have achieved Δn > 0.5 at 1064 nm [21], apart from pyridine derivatives combined with linear polyhalides [21, 46, 47]. All five compounds I─V (Δn = 0.815─0.878 at 1064 nm) surpass these benchmarks (Figure 4c; Table S14, S15), demonstrating the effectiveness of the lock‐and‐key strategy for achieving large near‐infrared birefringence with compact monocyclic building blocks.

FIGURE 4.

FIGURE 4

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Birefringence (at 1064 nm) versus bandgap for isolated linear units containing compounds (a) and monocyclic π‐conjugated crystals (c); Experimental and calculated birefringence (at 546 nm) of representative high‐performance birefringent crystals with π‐conjugated (b) and without π‐conjugated (d) organic units (Δn> 0.3).

In the visible region, crystals I, II, and V surpass all reported inorganic compounds, including CsICl2 (0.994 at 546 nm) [32], BaSbBS4 (0.952 at 546 nm) [48] and Sn2PO4I (0.664 at 546 nm) (Figure 4d; Table S16) [49], as well as π‐conjugated/polyhalide hybrids such as [H‐4AP][IBr2] (0.836 at 546 nm) [29], (C4H5N3OI)Cl2·H2O (0.746 at 546 nm), and (C4H5N3OI)2Cl3·2H2O (0.712 at 546 nm) [28]. These compounds also outperform the majority of recently reported π‐conjugated crystals (Figure 4b; Table S17), including monocyclic systems such as I+(C6H4NO2)─ (0.778 at 550 nm) [31], (C5H6.16N2Cl0.84)(IO2Cl2) (0.67 at 546 nm) [50], [(C7N2H11)+(C5N4H)─]·H2O(0.629 at 546 nm) [51] and CsH2C6N9·H2O(0.550 at 550 nm) [52], bicyclic systems such as (C12H8N2)SbF3 (0.79 at 546 nm) [24] and (C10H6NO2)2SbF (0.87 at 546 nm) [23], and polycyclic systems such as Cd(H2C6N7O3)2·8H2O(0.60 at 550 nm) [53], Na2(4‐HPyH)2 (PTS)·H2O (0.811 at 546 nm) [27], (C18H14N2)CdCl4(1.017 at 546 nm) [54] and Li3(C9N13)·6H2O (1.031 at 546 nm) [26]. Among all π‐conjugated materials with bandgaps in the range of 2.0─2.5 eV, I, II, and V achieve the highest experimental birefringence, representing a favorable combination of large Δn (> 1.0) and sufficient transparency (E g > 2.0 eV) [55, 56, 57, 58, 59, 60]. These results establish halogenated cytosine─polyhalide hybrids as strong candidates for next‐generation birefringent applications across both the visible and near‐infrared regions.

2.3. Electronic Structure and Structure‐Property Relationships

To elucidate the electronic origin of the exceptional birefringence in crystals I─V, we performed electronic structure calculations using the GGA‐PBE functional [61]. Band structure analysis reveals that all five compounds are indirect bandgap compounds with calculated bandgaps of 2.483, 2.446, 2.732, 2.310, and 2.112 eV, respectively (Figure S35). These values are systematically higher than the experimental optical bandgaps (2.02─2.28 eV) for I─IV, consistent with the underestimation of GGA‐PBE arising from the discontinuity of the exchange‐correlation energy functional. As for V, its calculated bandgap (2.112 eV) falls below the experimental value (2.25 eV), and a scissor operator of 0.138 eV was therefore applied in the optical calculations to correct for this discrepancy.

The partial density of states (PDOS) and total density of states (TDOS) provide further insight into the orbital contributions to optical properties (Figures 5a–c, S36). For all five compounds, the top of the valence band is dominated by Cl‐3p (I─III), I‐5p (I─IV), and Br‐4p (IV─V) orbitals, with moderate contributions from the C‐2p and N‐2p orbitals of the halogenated cytosine units. At the bottom of the conduction band, I‐5p (I─IV) and Br‐4p (IV─V) orbitals remain dominant, while the C‐2p and N‐2p contributions are markedly enhanced relative to the valence band. This orbital distribution indicates that the linear polyhalide units (I2Cl−, ICl2 −, IBr2 −, and Br3 −) are the primary contributors to the optical properties of I─V, with the halogenated cytosine moieties providing secondary, but non‐negligible contributions. Together, these results support the lock‐and‐key design rationale: the coplanar 1D hydrogen‐bonded organic chains define directional cavities that enforce collinear polyhalide alignment, enabling synergistic coupling of π‐stacking anisotropy (from cytosine) and linear‐unit anisotropy (from polyhalides).

FIGURE 5.

FIGURE 5

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Total and partial density of states of I (a), II (b), and V (c). Polarizability tensor visualizations of all functional units in I─V (d). (e) Stacked bar chart of individual birefringence contributions (Δn cut) for I─V, obtained from polarizability‐decomposed unit‐cutting analysis.

2.3.1. Structure─Property Relationship

To understand the microscopic origin of the large birefringence in I─V, we analyzed the polarizability anisotropy of individual building blocks, ClCy, BrCy, I2Cl─, ICl2 ─, IBr2 ─, and Br3 ─, using unit sphere and vector representation methods (Figure 5d) [62, 63, 64]. For the linear polyhalide units, the largest polarizability tensor components (α Z ) are predominantly distributed along the linear axis. Since all polyhalide units in I─V adopt collinear alignment through spatial confinement, their polarizability anisotropy is fully expressed, contributing substantially to the macroscopic birefringence. For the halogenated cytosine moieties, α Z  is slightly larger than αY and substantially exceeds αX, indicating that the maximum polarizability lies within the molecular plane along the C─Cl/Br bond direction. Although the cytosine units are coplanar within each compound, the orientation of the C─Cl/Br bonds vary between crystallographically independent cytosine units, leading to distinct individual contributions to the overall birefringence.

To quantify these contributions, we applied a polarizability‐decomposed unit‐cutting approach to decompose the total calculated birefringence into individual contributions (Δn cut) from each structural unit (Figure 5e;Table S19) [65]. For I─V, the Δn cut are 0.796/0.303/0.236, 0.772/0.322/0.232, 0.561/0.254/0.231, 0.644/0.254/0.279, and 0.758/0.266/0.287, corresponding to the linear polyhalide unit, the first halogenated cytosine, and the second halogenated cytosine, respectively. Several trends emerge from this analysis. First, the linear polyhalide units consistently dominate the birefringence, accounting for 54%─60% of the total Δn in I─V. The variation in total birefringence across I─V primarily reflects differences in the intrinsic Δα of the polyhalide units: the higher contribution of I2Cl− (0.796 and 0.772 in I and II) relative to ICl2 −(0.561 in III), IBr2 − (0.644 in IV), and Br3 − (0.758 in V) is consistent with the computed Δα ranking (Scheme 1b). In contrast, the contributions from halogenated cytosine units remain relatively stable across all five compounds (0.231─0.322), as all achieve near‐ideal coplanar packing through the lock‐and‐key assembly. This indicates that while the structural design ensures optimal geometric alignment for both building blocks, the total birefringence is primarily governed by the choice of polyhalide unit. Second, the cytosine contributions account for 45%─47% of the total birefringence in III and IV, somewhat higher than in I, II and V (40%─42%), reflecting the relatively greater weight of the organic component when the polyhalide Δα is lower. The two crystallographically independent cytosine moieties exhibit slightly different contributions owing to differences in their C─Cl/Br bond orientations relative to the principal optical axes. In II, for instance, the BrCy unit with the more favorably oriented C─Br bond contributes Δn cut = 0.322, compared with 0.232 for the less favorably oriented unit.

2.3.2. Structural Origin of Giant Birefringence

We further investigate the structural origin of outstanding optical anisotropy (Figures 6, and S37–S40). Taking I as the representative, Figure 6a, b reveal the key geometric features responsible for the large optical anisotropy. The maximum polarization direction of each I2Cl─ unit is nearly parallel to the principal optical axis Z, with a deviation of approximately seven (Figure 6c). This near‐perfect alignment, achieved through lock‐and‐key spatial confinement, ensures that all I2Cl─ units contribute constructively to the macroscopic birefringence. All constituent atoms lie within a single plane perpendicular to the b‐axis, which coincides with the minimum optical axis X. This coplanarity results in reduced electron localization along the b‐direction relative to the (010) plane, accounting for the minimum refractive index along X. The refractive index ellipsoids in the YZ‐plane (Figure 6d) and XZ‐plane (Figure 6e) quantify the anisotropic optical response: the collinear alignment of I2Cl− units generate substantial in‐plane anisotropy, with n Z = 2.968 substantially exceeding n Y = 2.366. The combined effect of coplanar and collinear assembly produces synergistic coupling of in‐plane and out‐of‐plane anisotropies, yielding Δn = n Z ─ n X = 2.968─1.632 = 1.336.

FIGURE 6.

FIGURE 6

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(a,b) Crystal structure viewed perpendicular to (a) and along (b) the b‐axis, showing wave‐like [HClCy]+ chains, capsule‐shaped cavities (ellipses), and I2Cl─ alignment. Crystallographic axes (a, b, c; solid arrows) and principal optical axes (X, Y, Z; dashed arrows) are indicated. (c) Angular deviation between I2Cl− polarization direction and Z‐axis. (d, e) Refractive index ellipsoids in the YZ‐ and XZ‐planes. (f,g) Two‐dimensional electron localization function (ELF) maps within (f) and perpendicular to (g) the 2D layer. Yellow ellipses in (f) highlight capsule‐shaped cavities (ELF ≈ 0) with I2Cl─ units embedded.

The electronic origin of this optical anisotropy is visualized through two‐dimensional ELF maps (Figure 6f, g). Within the 2D layer (Figure 6f), strong electron localization (ELF = 0.5─1.0) is observed along the I─I covalent bonds, reflecting substantial orbital overlap, while the I─Cl interactions exhibit markedly weaker localization, confirming their halogen‐bonding character. Within the halogenated cytosine units, pronounced localization along the C─C, C─N, and C═O bonds reflect π‐conjugation. The electron clouds from adjacent cytosine rings enclose cavities with ELF ≈ 0 (highlighted by yellow ellipses), corresponding to the capsule‐shaped voids occupied by I2Cl─ units. The isolated Cl─ anions display nearly spherical electron density, indicating a negligible contribution to birefringence. Perpendicular to the 2D layer (Figure 6g), the electron density shows pronounced anisotropy between the in‐plane (along the I2Cl−axis) and out‐of‐plane (perpendicular to the layer) directions, with strong localization within the layer and markedly weaker localization in the interlayer region. This microscopic electronic anisotropy is the direct origin of the large macroscopic birefringence, confirming that the lock‐and‐key spatial‐confinement assembly successfully translates the intrinsic polarizability anisotropy of individual building blocks into high optical performance through precise geometric control.

3. Conclusion

In summary, we have established a lock‐and‐key spatial‐confinement assembly that leverages halogen‐induced dimensional control of hydrogen‐bonded networks to achieve simultaneous coplanarity of π‐planes and collinearity of linear polyhalide units, the two geometric prerequisites for constructive anisotropy coupling. Guided by computational screening of building blocks with complementary polarizability anisotropy and HOMO─LUMO gaps, we synthesized five halogenated‐cytosine (XCy) polyhalide crystals: (HClCy)2(I2Cl)·Cl (I), (HBrCy)2(I2Cl)·Cl (II), (HClCy)2(ICl2)·Cl (III), (HBrCy)(BrCy)·IBr2 (IV), and (HBrCy)(BrCy)·Br3 (V). In all five compounds, wave‐like 1D hydrogen‐bonded cationic chains create capsule‐shaped cavities that spatially confine the polyhalide anions into unidirectional alignment while simultaneously enforcing near‐perfect coplanarity of the π‐planes. The I2Cl─‐based crystals I and II, further rigidified by an additional I···Cl halogen bond, achieve perfect coplanarity (dihedral angle = 0 ) and birefringence values of Δn = 1.336 and 1.324 at 546 nm, the highest among all monocyclic‐π‐based materials and surpassing all inorganic benchmarks. Compound V retains Δn = 1.311 at 546 nm with a widened bandgap of 2.25 eV, showing that anion substitution enables independent tuning of birefringence and transparency within this framework. First‐principles calculations and polarizability‐decomposed unit‐cutting analysis confirm that this performance originates from synergistic coupling of out‐of‐plane (π‐stacking) and in‐plane (polyhalide chain) anisotropies, both simultaneously maximized by the lock‐and‐key design. These findings show that compact monocyclic π‐systems can achieve high optical performance when precise geometric control is enforced through spatial confinement, a capability previously considered exclusive to polycyclic aromatics. The lock‐and‐key spatial‐confinement strategy is expected to extend to other hydrogen‐bond‐forming aromatic systems paired with diverse linear anisotropic units, providing a general route for the rational design of high‐performance birefringent materials.

Author Contributions

Yun‐Xia Hu: conceptualization, methodology, data curation, writing – original draft, and formal analysis. Huai‐Yu Wu: data curation, formal analysis, and software. Jia‐Jia Li: data curation, formal analysis. Ming‐Chang Wang: data curation, formal analysis. Jia‐Min Lian: data curation, formal analysis. Jin‐Yu Luo: data curation, formal analysis. Yi‐Ru Fu: data curation, formal analysis. Zi‐Yan Chen: data curation, formal analysis. Yang‐Hang Guo: data curation, formal analysis. Jin Chen: conceptualization, supervision, writing – review and editing, and funding acquisition. Ke‐Zhao Du: supervision, writing – review and editing, and funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: Deposition Numbers 2544188‐2544192 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

ANIE-65-e4675227-s001.docx (24.3MB, docx)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 22573015 and 22373014) the Natural Science Foundation of Fujian Province (No. 2026J009040 and 2026J002026).

Contributor Information

Jin Chen, Email: cj2015@fjnu.edu.cn.

Ke‐Zhao Du, Email: duke@fjnu.edu.cn.

Data Availability Statement

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

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File: Deposition Numbers 2544188‐2544192 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

ANIE-65-e4675227-s001.docx (24.3MB, docx)

Data Availability Statement

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

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