Designing promising ultraviolet (UV) birefringent crystals with different hydrogen-bonded phosphate frameworks

Abstract

In this study, we computationally identified using first-principles calculations two pyridine derivatives with high polarizability anisotropy: (C₆H₇N₂O)⁺ (protonated nicotinamide, abbreviated as 3AP⁺) and (C₆H₆NO₂)⁺ (protonated nicotinic acid, abbreviated as 3CP⁺). Subsequently, we synthesized two novel semiorganic crystals: (3AP) (H₂PO₄) (I) and (3CP) (H₂PO₄) (II). Both crystals incorporate hydrogen-bonded phosphate frameworks (HPFs), which are constituted by H₂PO₄⁻ anions linked through O–H⋯O hydrogen bonds. Compound I (Fdd2) features one-dimensional (1D) hydrogen-bonded phosphate frameworks (HPFs) that interconnect with 2D [(3AP)(H₂PO₄)] layers, forming a complex 3D network. In contrast, compound II (Pbca) possesses 2D HPFs that are integrated with 3CP⁺ cations to form a 2D layered network. Compound I exhibits a moderate second-harmonic generation (SHG) effect (1 × KH₂PO₄), and a significant birefringence (Δn_cal.: 0.191@1064 nm). Furthermore, compound II exhibits both a broad bandgap (4.24 eV) but also an exceptional birefringence (Δn_exp.: 0.284@546 nm), which is the highest value reported among all semiorganic phosphates. This suggests that II could be a promising candidate for use as an outstanding ultraviolet (UV) birefringent material.

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Introduction

Birefringent crystals are key materials used in the production of optical devices, including polarizers, optical isolators, circulators, and phase retarders.^1–7 Inorganic phosphate crystals, known for their short ultraviolet cutoff edges and good crystal growth habits, represent a classic optoelectronic material.^8–13 KDP (KH₂PO₄) and KTP (KTiPO₄) are quintessential examples of phosphates, boasting significant application as nonlinear optical (NLO) materials. However, the high symmetry and weak anisotropy of the PO₄³⁻ tetrahedron often limit inorganic phosphates, resulting in low birefringence and phase mismatch.^14–18 Therefore, the search for phosphates with high birefringence remains an active area of research.

To date, researchers have obtained some metal phosphates with high birefringence (Δn > 0.05) and proposed several effective strategies, including: (1) introducing lone pair electron cations (such as Sn²⁺, I⁵⁺) or d⁰ transition metal cations (such as Mo⁶⁺, W⁶⁺).^19–25 These cations can form the building units with high structural distortion and strong polarizability anisotropy, which is beneficial for enhancing the birefringence of phosphates, such as Sn₂PO₄I (Δn_cal. = 0.664@546 nm).²¹ (2) Partial substitution of O atoms with X groups (including F, S, or –NH₃) can break the high symmetry of PO₄³⁻ groups, thereby constructing distorted PO_mX_4−m tetrahedra with strong polarizability anisotropy, leading to enhanced birefringence, for example, NaNH₄PO₃F·H₂O (Δn_exp. = 0.053@589.3 nm),²⁶ KH₂PO₃S (Δn_cal. = 0.1@1 μm),²⁷ and NaPO₃NH₃ (Δn_exp. = 0.062@546.1 nm).²⁸ (3) Combining PO₄³⁻ with traditional planar π-conjugated birefringent functional groups (such as CO₃²⁻ and BO₃³⁻). This method can increase the birefringence of compounds (Δn > 0.1) while also having a wide bandgap (E_g > 6.2 eV), such as Sr₃Y(PO₄)(CO₃)₃ (Δn_cal. = 0.121@532 nm and E_g = 6.9 eV).²⁹ Notably, Rb[PO₂(NH)₃(CO)₂]·0.5H₂O,³⁰ KPO₂(NHCONH₂)₂,³¹ and NaPO₂(NH)₃(CO)₂ ³² can be regarded as products designed using both methods (2) and (3) concurrently. Of these, NaPO₂(NH)₃(CO)₂ exhibits very large birefringence and a wide bandgap (Δn_exp. = 0.280@550 nm; E_g ≥ 6.5 eV).³² (4) The “zipper” arrangement of the PO₄³⁻ tetrahedron with large-angle deviation can also lead to the enhancement of birefringence of metal phosphates. For example, α-YSc(PO₄)₂ exhibits deep ultraviolet transmission and a high birefringence (Δn_cal. = 0.102@1064 nm).³³

Recently, several semiorganic cations with planar π-conjugated configurations, including [C(NH₂)₃]⁺, (C₅H₆ON)⁺, (C₃H₇N₆)⁺, etc., have emerged as high-performance anisotropic functional moieties.^34–37 The combination of π-conjugated organic cations with phosphate tetrahedra has led to the discovery of numerous novel crystals with high birefringence, such as [C(NH₂)₃]₃PO₄·2H₂O (Δn_exp. = 0.055@546 nm),³⁸ [C(NH₂)₃]₆(PO₄)·3H₂O (Δn_exp. = 0.078@546 nm),³⁹ (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (Δn_cal. = 0.220@1064 nm),⁴⁰ and (C₅H₆ON)⁺(H₂PO₄)⁻ (Δn_cal. = 0.25@1064 nm).⁴¹ Clearly, as the π-conjugation in organic cations expands, their contribution to birefringence similarly increases significantly. Therefore, we identified two novel birefringent functional units, characterized by higher π-conjugation and stronger polarizability anisotropy, which are (C₆H₆N₂O)⁺ (protonated 3-carboxamide pyridine, 3AP⁺) and (C₆H₆NO₂)⁺ (protonated 3-carboxypyridine, 3CP⁺) (Scheme 1). And then, our efforts have resulted in the discovery of two novel semiorganic phosphates, namely (3AP) (H₂PO₄) (I) and (3CP)(H₂PO₄) (II). Experimental characterization and theoretical calculations have revealed that I exhibits large birefringence and moderate SHG effect (Δn_exp. = 0.196@546 nm; 1.0 × KDP). II can serve an excellent UV birefringent crystal with the bandgap of 4.20 eV and birefringence of 0.284@546 nm, the highest value among known semiorganic phosphates.

Scheme 1 The HOMO and LUMO maps (a) as well as polarizability anisotropy (b) of related units.

Experimental

Materials and synthesis

C₆H₆N₂O (99%) and H₃PO₄ (85%) were used as purchased from Adamas Beta. Crystals of I were synthesized using a simple evaporation technique in aqueous solution. The raw reactants of C₆H₆N₂O (8.2 mmol, 1.00 g) and H₃PO₄ (0.5 mL) were mixed together in deionized water (3 mL) in a glass beaker. The solution was allowed to reach room temperature and then slowly evaporated, resulting in white flaky crystals that precipitated. For the preparation of II, the starting materials consisted of C₆H₆N₂O (2 mmol, 244.25 mg), H₃PO₄ (0.5 mL), and H₂O (1 mL). A mixture of the starting materials was placed into Teflon pouches (23 mL) and sealed inside an autoclave, which was heated to 110 °C for 3 days, and then cooled to 30 °C at a rate of 1.6 °C h⁻¹. Colorless flake-like crystals I and II were obtained with yields of about 80% and 76% (based on C₆H₆N₂O), respectively.

Single crystal structure determination

Single-crystal X-ray diffraction data for the title compounds were collected using a Rigaku XtaLAB Synergy-DW dual-wavelength CCD diffractometer, equipped with Cu Kα radiation (λ = 1.54184 Å) at 293 K. Data reduction was performed using CrysAlisPro, and absorption correction, based on the multi-scan method, was applied.⁴² The structures of I and II were determined using direct methods and refined by full-matrix least-squares fitting on F² utilizing SHELXL-2014.⁴³ Anisotropic thermal parameters were applied to refine all non-hydrogen atoms. The structure underwent a check for missing symmetry elements using PLATON, and none were found.⁴⁴ Crystallographic data and structural refinements of the compounds are listed in Tables S1–S11.†

Powder X-ray diffraction

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

Thermal analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) data were obtained, via using a Rigaku TG-DTA 8121 unit under an argon (Ar) atmosphere, at a heating rate of 10 °C min⁻¹, ranging from 30 °C to 800 °C.

Optical measurements

Infrared (IR) spectra were recorded on a Thermo Fisher Scientific Nicolet 5700 FT-IR spectrometer using KBr pellets over a range from 4000 to 400 cm⁻¹.

Ultraviolet–visible (UV-vis) spectra, ranging from 200 to 800 nm, were recorded on a PerkinElmer Lambda 750 UV-vis spectrophotometer. Reflectance spectra were converted to an absorption spectrum using the Kubelka–Munk function.⁴⁵

Second harmonic generation measurements

Powder SHG measurements were taken using a Q-switched Nd:YAG laser generating radiation at 1064 nm according to the method of Kurtz and Perry.⁴⁶ Crystalline samples of I were sieved into distinct particle-size ranges: 50–70, 70–100, 100–140, 140–200, 200–250, and 250–325 μm. Sieved KDP (KH₂PO₄) samples, within the same particle-size ranges, were used as references.

Birefringence

The optical path difference of the title compounds were characterized using a polarizing microscope (Nikon LV1000) equipped with a Berek compensator.⁴⁷ The average wavelength of the light source was 546 nm. Under orthogonal polarization, the two beams of polarized light pass through the crystal at different speeds, producing interference color phenomena after passing through the analyzer. Polarized light with vibration directions parallel to K1, K2, K1′, K1′′, K2′, K2′′ represents light with different vibration directions. Polarizers are used to filter out light whose vibration directions are not parallel to the analyzer (AA) or polarizer (PP).

The birefringence (Δn) of microcrystals are calculated with the formula: R = d·(n_s − n_f) = T·Δn. Herein, R is the optical path difference. The n_s represents the refractive index of the slow light, and n_f represents the refractive index of the fast light. Here, T represents the thickness of the tested single crystal.³⁵

Elemental analysis

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

Energy-dispersive X-ray spectroscope

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

Computational method

The electronic structures and optical properties of the compounds were computed using the plane-wave pseudopotential method within the framework of density functional theory (DFT), as implemented in the total energy code Cambridge Sequential Total Energy Package (CASTEP).^48,49 For the exchange–correlation functional, we selected the Perdew–Burke–Ernzerhof (PBE) formula within the generalized gradient approximation (GGA).⁵⁰ The interactions between the ionic cores and the valence electrons were modeled using norm-conserving pseudopotentials.⁵¹ The following electrons were considered as valence: C-2s²2p², N-2s²2p³, O-2s²2p⁴, P-3s²3p³, and H-1s¹. The basis set was constructed with plane waves up to a cutoff energy of 750 eV. For both compounds, the self-consistent field (SCF) and optical-property calculations utilized a k-point separation of 0.04 Å⁻¹ for the numerical integration over the Brillouin zone. The k-point samplings employed were 4 × 4 × 4 for compound I and 2 × 1 × 1 for compound II.^48,49

To investigate the polarizability anisotropy and electronic structure of selected birefringent units, a systematic computational approach was undertaken using the Gaussian 09 software suite.⁵² The hybrid B3LYP functional was employed at the 6-31G(d,p) level of theory for these calculations. Subsequent analysis of the computational results was performed using the Multiwfn 3.8 software package.⁵³ The polarizability anisotropy was quantified based on the static polarizability values.⁵⁴

Results and discussion

Compound I, akin to many previously reported semiorganic phosphates, was synthesized by dissolving a specific stoichiometric ratio of nicotinamide (C₆H₆N₂O) and phosphoric acid (H₃PO₄) in an aqueous solution, followed by slow evaporation. It should be noted that nicotinamide is easily hydrolyzed to nicotinic acid in aqueous solutions (Fig. S1†), a process that can be accelerated by heating.^55–57 This hydrolysis facilitated the hydrothermal synthesis of compound IIvia the reaction involving nicotinamide and H₃PO₄ at 110 °C. The transparent single crystals of II, with dimensions of approximately 13 × 7 × 3 mm³, were observed (Fig. 1). Powder X-ray diffraction (PXRD) analysis confirmed the purity of the synthesized samples (Fig. S2†). Field-emission scanning electron microscopy (FESEM) analyses of both I and II detected the presence of C, N, O and P elements (Fig. S3†). Elemental analysis provided weight ratios of C : H : N in I and II as 5.97 : 1.95 : 8.76 and 5.61 : 0.99 : 7.90, respectively, which are congruent with the crystal structure analysis of 6 : 2 : 9 and 6 : 1 : 8, respectively (Table S12†). This agreement corroborates the presence of (C₆H₇N₂O)⁺ (3AP⁺) and (C₆H₆NO₂)⁺ (3CP⁺) as the organic cations in the structures of I and II, respectively. The infrared (IR) spectra for I and II are presented in Fig. S4,† with the detailed assignments of the absorption peaks provided in Table S13,† showing consistency with those of previously reported semiorganic phosphates.^39–41 Thermal analysis demonstrated that the thermal stability of both compounds decompose above 150 °C (Fig. S5†), comparable to other reported crystals, such as (C₅H₆ON)⁺(H₂PO₄)⁻ (166 °C),⁴¹ (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (120 °C),⁴⁰ [C(NH₂)₃]₆(PO₄)·3H₂O and (100 °C).³⁹

Fig. 1 As-grown crystals of I (a) and II (b).

Crystal structure

Compound I crystallizes in the non-centrosymmetric space group Fdd2, with each unit cell housing 16 asymmetric units (Table S1†). Each asymmetric unit consists of one (C₆H₆N₂O)⁺ (3AP⁺) cation and one (H₂PO₄)⁻ anion (Fig. 2a). Within the structure of I, each 3AP⁺ cation is interconnected with three (H₂PO₄)⁻ anions through three N–H⋯O and one O–H⋯O hydrogen bonds (Fig. S6a†). Conversely, each (H₂PO₄)⁻ anion is coordinated with three 3AP⁺ cations via three N–H⋯O hydrogen bonds and is additionally linked to two axial (H₂PO₄)⁻ anions through two O–H⋯O hydrogen bonds (Fig. S7a†). Consequently, adjacent (H₂PO₄)⁻ anions are interconnected via O–H⋯O hydrogen bonds, resulting in the formation of one-dimensional [H₂PO₄]⁻ chains, denote as 1D hydrogen-bonded phosphate frameworks (1D HPFs) (Fig. 2b). Furthermore, neighboring 3AP⁺ cations and (H₂PO₄)⁻ anions are connected through hydrogen bonds, creating a 2D neutral layer of [(3AP)(H₂PO₄)] parallel to the ac plane (Fig. 2c). Thus, the overall three-dimensional (3D) architecture of I can be envisioned as a network of 2D [(3AP)(H₂PO₄)] layers interconnected by 1D HPFs (Fig. 2d).

Fig. 2 The asymmetric unit (a), 1D HPFs (b), 2D neutral layer (c) along b direction, and 3D overall structure along c direction (d).

Compound II crystallizes in the centrosymmetric space group Pbca, with each unit cell comprising 8 asymmetric units (Table S1†). Each asymmetric unit consists of one (C₆H₆NO₂)⁺ (3CP⁺) cation and one (H₂PO₄)⁻ anion (Fig. 3a). Within the structure of II, pairs of (H₂PO₄)⁻ anions are interconnected via O–H⋯O hydrogen bonds, forming a (H₄P₂O₈)²⁻ dimer (Fig. S7b†). These dimers further associate through O–H⋯O hydrogen bonds to create two-dimensional hydrogen-bonded phosphate frameworks (2D HPFs), which are aligned parallel to the ab plane (Fig. 3c). Additionally, adjacent 3CP⁺ cations are linked into one-dimensional [C₆H₆NO₂]⁺ chains through weaker C–H⋯O hydrogen bonds (Fig. 3b and S6b†). These chains are then integrated with the 2D HPFs through N–H⋯O and O–H⋯O hydrogen bonds (Fig. S7b†). Consequently, each pair of cationic 1D [C₆H₆NO₂]⁺ chains is sequentially embedded on either side of the 2D HPFs, generating a neutral 2D [(3CP)(H₂PO₄)] layer. These 2D layers interlace with each other along the c-axis, constituting the overall architecture of compound II (Fig. 3d).

Fig. 3 The asymmetric unit (a), 1D [C₆H₆NO₂]⁺_∞ chain (b), 2D HPFs (c) along c direction, and overall structure along b direction (d).

We conducted a detailed analysis to discern the differences in π–π and dipole–dipole interactions stemming from the dimensional variation of hydrogen-bonded phosphate frameworks (HPFs). In compound I, a π–π interaction along the 1D HPFs is evident, characterized by a plane-to-plane distance of 4.5654(15) Å and a dihedral angle of 15.10(13)° (Fig. S8†). Moreover, adjacent 3AP⁺ cations bordering the 1D HPFs exhibit larger dihedral angles of 24.295°. In contrast, compound II displays a π–π interaction along one side of the 2D HPFs with a plane-to-plane distance of 5.0811(13) Å and a dihedral angle of 9.06(11)° (Fig. S9†). Additionally, neighboring 3CP⁺ cations from distinct 2D [(3CP)(H₂PO₄)] layers are nearly antiparallel, with a dihedral angle of 0.12(11)° and a plane-to-plane distance of 5.1088(13) Å (Fig. S9†). Notably, this π–π interactions act as the binding force for the stacking of the 2D neutral layers in II. Consequently, compound II demonstrates stronger dipole–dipole interactions, which may contribute to an enhanced birefringence performance.

Transmittance and bandgaps

Ultraviolet–visible (UV-Vis) absorption spectra indicate that compounds I and II both exhibit bandgaps of 4.24 eV, with cutoff wavelengths at 265 nm and 270 nm, respectively (Fig. 4a and b). Electronic structure calculations substantiate that the two compounds possess indirect and direct bandgaps, with theoretical values of 2.279 eV and 1.919 eV, respectively (Fig. S10†). To mitigate the systematic underestimation inherent in the Generalized Gradient Approximation (GGA) method when calculating optical properties, scissor operators of 1.921 eV and 2.281 eV are applied. Density of states (DOS) analysis reveals that for both compounds, the top of the valence band predominantly originates from O-2p orbitals, while the conduction band minimum is largely composed of C-2p orbitals, with a minor contribution from N-2p and O-2p orbitals (Fig. 4c and d). This suggests that the bandgaps are mainly shaped by the organic constituents, a finding consistent with previous research on semiorganic phosphates.^39–41

Fig. 4 UV–vis diffuse reflectance spectra (the inset show the bandgaps and F(R) is absorption coefficient/scattering coefficient.) for I (a) and II (b), as well as the density of states for I (c) and II (d).

SHG property

Given that compound I crystallizes in a polar space group, we assessed its SHG effect. Upon 1064 nm laser excitation, the SHG intensity of I was found to be on par with that of KH₂PO₄ (KDP), utilizing type I phase matching (Fig. 5). Compound I demonstrates a superior SHG response compared to certain reported semiorganic phosphates, such as (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (0.1 × KDP)⁴⁰ and (C₅H₁₂NO)H₂PO₄ (0.15 × KDP),⁵⁸ and is comparable to [C(NH₂)₃]₂HPO₄·H₂O (1.2 × KDP)⁵⁹ and [C(NH₂)₃]₂PO₃F (1.0 × KDP).⁶⁰ However, it underperforms in SHG effect relative to [C(NH₂)₃]PO₄·2H₂O (1.5 × KDP),³⁸ [C(NH₂)₃]₆(PO₄)·3H₂O (3.8 × KDP),³⁹ and (C₅H₆ON)⁺(H₂PO₄)⁻ (3.0 × KDP).⁴¹ Additionally, theoretical NLO coefficients for compound I were calculated, identifying five non-zero independent tensor components: d₃₁ = 0.262 pm V⁻¹, d₁₅ = 0.262 pm V⁻¹, d₃₂ = 0.087 pm V⁻¹, d₂₄ = 0.087 pm V⁻¹, and d₃₃ = −0.426 pm V⁻¹. Since the space group of II is Fdd2, in the mm2 point group, its effective tensor should be d₃₁, which is comparable with the measured result.^61–63

Fig. 5 Phase-matching curve of I with 1064 nm laser radiation (insert is oscilloscope traces of the SHG signals for powders of I and KDP (150–210 μm) with 1064 nm laser radiation).

Birefringence property

Using a polarization microscope equipped with a Berek compensator, we measured the optical path difference (R). This measurement is required to determine the transition of the crystals from orthogonal polarization to complete extinction. By applying the formula R = Δn × T, where Δn represents the birefringence and T the sample thickness, we obtained the experimental birefringence values for compounds I and II (Fig. 6). Using a 546 nm laser source, we observed optical path difference of 1007.41 nm for I and 1139.46 nm for II, corresponding to sample thicknesses of 5.13 μm and 4.01 μm, respectively (Fig. S11†). The resulting birefringence (Δn_exp.) were measured as 0.196 at 546 nm for I and 0.284 at 546 nm for II. The enhanced birefringence of II is attributed to the intensified π–π and dipole–dipole interactions stemming from the smaller dihedral angles between neighboring 3CP⁺ cations within its structure. Notably, the birefringence exhibited by I and II significantly exceeds the experimental values of recently reported semiorganic phosphates, such as [C(NH₂)₃]₃PO₄·2H₂O (0.055@546 nm),³⁸ [C(NH₂)₃]₆(PO₄)·3H₂O (0.078@546 nm),³⁹ [C(NH₂)₃]₂Sb₃F₃(HPO₃)₄ (0.027@546 nm),⁶⁴ [C(NH₂)₃]SbFPO₄·H₂O (0.151@546 nm),⁶⁴ NaIn(C₂O₄)(HPO₄)(H₂O)₅ (0.098@546 nm),⁶⁵ [Te(C₆H₅)₂][PO₃(OH)]_n (0.133@550 nm)⁶⁶ (Fig. 7 and Table S14†). The increased birefringence of I and II is primarily due to the enhanced π-conjugation of the 3AP⁺ or 3CP⁺ cations, which introduces greater anisotropy compared to guanidinium or oxalate. Moreover, the experimental birefringence of II surpasses that of commercial birefringent crystals, including α-BaB₂O₄ (0.122@532 nm), CaCO₃ (0.172@532 nm), and YVO₄ (0.204@532 nm), highlighting its potential as a novel high-performance UV birefringent material.

Fig. 6 Original crystal, crystals achieving complete extinction and triaxial ellipsoid of three refractive indices along crystallographic axes for I (a–c) and II (d–f).

Fig. 7 Birefringence comparison between title compounds and reported phosphates or sulfates containing organic groups.

Subsequently, we computationally determined the dielectric function and refractive indices for compounds I and II, from which we calculated the theoretical birefringence (Δn_cal.) using the relationships ε(ω) = ε₁(ω) + iε₂(ω) and n²(ω) = ε(ω). Both I (Fdd2) and II (Pbca) are characterized as biaxial crystals, with their refractive indices ordered as n_X > n_Y > n_Z. Specifically, for I and II, the refractive indices are arranged such that n[010] > n[001] > n[100]. To align with the standard crystallographic axes, we performed the transformations [010] → X, [001] → Y, and [100] → Z (Fig. 6 and S12†). At a wavelength of 546 nm, the static refractive indices are as follows: for I, n[100] = 1.14, n[010] = 1.35, n[001] = 1.32; and for II, n[100] = 1.41, n[010] = 1.69, n[001] = 1.66. The calculated birefringence (Δn_cal.), derived using the formula Δn_cal. = n_max − n_min, is 0.209 at 546 nm for I and 0.284 at 546 nm for II, which are consistent with their experimental counterparts. Additionally, the calculated birefringence values at 1064 nm are 0.191 for I and 0.264 for II (Fig. S12†). Importantly, the calculated birefringence of II (Δn_cal. = 0.286@532 nm and 0.264 nm@1064 nm) exceeds that of currently reported semiorganic phosphates, such as (C₅H₆ON)⁺(H₂PO₄)⁻ (0.25@1064 nm),⁴¹ [C(NH₂)₃]₂PO₃F (0.039@532 nm),⁶⁰ (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (0.220@1064 nm),⁴⁰ and (N₂H₆)[HPO₃F]₂ (0.077@1064 nm)⁶⁷ as well as most semiorganic sulfates, including [C₅H₆O₂N₃][HSO₄]·H₂O (0.25@1064 nm),⁶⁸ C(NH₂)₃SO₃F (0.133@1064 nm)⁶⁹ and Te(CS(NH₂)₂)₄SO₄·2H₂O (0.210@546.1 nm)⁷⁰ (Fig. 7 and Table S14†).

To elucidate the mechanism behind the exceptional birefringence observed in compound II, we undertook a computational analysis of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) distributions, complemented by electron localization function (ELF) mapping, as shown in Fig. S13† and Fig. 8. The HOMO is predominantly constituted by O-2p orbitals from the (H₂PO₄)⁻ group, while the LUMO is significantly influenced by the vacant π-conjugated orbitals of the 3CP⁺ cation, a finding that aligns with the DOS plots. The 2D ELF maps, depicted in Fig. 8a and b, are oriented parallel and perpendicular to the planar 3CP⁺ cation, respectively. These maps reveal significant variations in electron cloud density around the 3CP⁺ cation across different crystallographic planes, in contrast to the relatively uniform electron cloud density observed around the (H₂PO₄)⁻ anion. By synthesizing the information derived from the LUMO and ELF analyses, we infer that the in-plane and inter-plane anisotropy is predominantly due to the π-conjugated bonds within the 3CP⁺ organic ligands, which are aligned parallel to the (100) crystal plane. This alignment leads to a substantial difference in refractive indices, with n_in-plane ≫ n_out-plane, consistent with the observed refractive index profile (n[010] ≈ n[001] ≫ n[100]). Therefore, the 3CP⁺ organic ligands are identified as the key contributors to the outstanding birefringence properties of compound II.

Fig. 8 2D ELF sections projected on crystal planes parallel (a) to and perpendicular (b) to 3CP⁺ cation.

Conclusions

In summary, through the strategic modification of organic cations, we have successfully synthesized two novel semiorganic crystals: (C₆H₇N₂O)⁺(H₂PO₄)⁻ (I) with 1D hydrogen-bonded phosphate frameworks (HPFs) and (C₆H₆NO₂)⁺(H₂PO₄)⁻ (II) with 2D HPFs. Both compounds exhibit an expansive bandgap of 4.24 eV and remarkable birefringence (Δn_exp. = 0.196 and 0.284@546 nm for I and II, respectively). These characteristics position them as prospective candidates for applications in UV birefringent crystals. Structural and computational analyses indicate that the 2D HPFs present in compound II augment dipole–dipole interactions and minimize the dihedral angles among the (C₆H₆NO₂)⁺ cations, subsequently enhancing the birefringence. Given the structural diversity achievable with inorganic frameworks and the compositional flexibility inherent in organic ligands within hybrid organic–inorganic compounds, this study paves the way for further exploration and development of novel high-performance birefringent crystals.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22205037 and 22031009) and Natural Science Foundation of Fujian Province (2023J01498).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2343554 and 2343555 for (3AP) (H₂PO₄) (I) and (3CP) (H₂PO₄) (II). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01220h

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