SnCl₂[SC(NH₂)₂]: lone-pair and hydrogen-bonding triggered chromophore assembling for dual optical optimization

Abstract

Designing birefringent crystals with a wide bandgap presents significant challenges, especially within chalcogenide systems. Herein, motivated by the high structural anisotropy of thiourea, along with the ultraviolet compatibility and birefringence activity of halogenated tin(II)-based anionic groups, a molecular crystal-SnCl₂[SC(NH₂)₂] has been screened out from crystal database. SnCl₂[SC(NH₂)₂] was characterized by a bench-shaped molecular structure, which is formed by the sharing of a sulfur anion between a distorted SnCl₂S trigonal pyramid and a planar π-conjugated thiourea unit. Through the synergistic interactions among multiple functional groups and the auxiliary regulation of hydrogen bonds, SnCl₂[SC(NH₂)₂] succeeded in breaking the “3 eV wall” with a bandgap of 3.362 eV and achieved a large birefringence of 0.20@546 nm, demonstrating a facile design strategy for the development of chalcogenides with dual optical enhancement.

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1. Introduction

Birefringent crystals serve as essential functional materials for manipulating the polarization state of light, demonstrating significant applications in optical isolators, optical measurements, and biomedical imaging.¹ In recent years, the rapid development of photonic technologies has intensified the demand for high-performance birefringent crystals.² Regrettably, commonly used birefringent crystals, including α-BaB₂O₄,³ YVO₄ ⁴ and MgF₂,^5,6 still face several limitations. For example, despite the excellent ultraviolet (UV) transmittance exhibited by MgF₂, practical applications of MgF₂ are severely restricted by its relatively small birefringence (about 0.01). Conversely, YVO₄ has been characterized by a giant birefringence (about 0.22), while its UV cut-off wavelength is dramatically limited to be about 400 nm. Hence, the exploration of novel birefringent crystals is still active.

Hitherto, chalcogenides have emerged as prominent candidates due to their structural diversity and excellent optical properties,^7–9 while representative crystals of BaTeS₃,¹⁰ K₂BaGeS₅ ¹¹ and β-Pb₃P₂S₈ ¹² have been characterized by large birefringences of 0.19@550 nm, 0.19@1064 nm and 0.26@550 nm, respectively. Nevertheless, the bandgaps of chalcogenides are commonly restricted to the region below 3 eV, which poses significant challenges for their application in the UV region. To achieve transparency in the UV wavelength range, an effective approach is the introduction of strong electronegative anions (Cl⁻, F⁻, O²⁻, N³⁻, etc.) into chalcogenides, which has led to the development of UV-transparent chalcogenides like La₂Zn₃(SeO₃)₆,¹³ [Ba₄Cl₂][CdGa₄S₁₀],¹⁴ [Ba₂K][Cl][SiS₄],¹⁵ Ae₃[TO₃][SnOQ₃] (Ae = Sr, Ba; T = Si, Ge; Q = S, Se)¹⁶ and Ba₂GeF₂S₃.¹⁷ However, the mutual exclusivity between bandgap and birefringence leads to their birefringence being smaller than 0.1, highlighting the significance of developing chalcogenides with both large bandgaps (E_g > 3 eV) and birefringences (Δn > 0.1).

As is well known, the properties of birefringent crystals are mainly determined by the fundamental building blocks (FBBs) adopted.¹⁸ Among various FBBs, planar π-conjugated groups have garnered much attention due to their notable structural anisotropy, potential for UV applications and tunable spatial arrangement.^19–23 Similarly, halogenated FBBs with lone pair electrons (LPE) have attracted widespread interest owing to their structural anisotropy, UV transparency and structural tunability.^24–29 Thus, we propose that the utilization of LPE-type halogenated groups to modulate planar π-conjugated sulphur-containing groups holds promise for achieving simultaneous regulation of birefringence and bandgap. Based on this hypothesis, we screened out SnCl₂[SC(NH₂)₂]³⁰ (CCDC 1120335) from the crystal database. The uniqueness of this crystal lies in the LPE-type halogenated SnCl₂S triangular pyramids and planar π-conjugated sulphur-containing SC(NH₂)₂ molecules. Specifically, Sn²⁺ is a main group cation with stereochemically active LPE, which contributes to the structural anisotropy and spatial orientation of SnCl₂S groups. Meanwhile, SC(NH₂)₂ groups are in a quasi-coplanar alignment due to the structural modulation of SnCl₂S groups, which is beneficial for optical anisotropy. Further considering the UV capability of SnCl₂S groups, SnCl₂[SC(NH₂)₂] is expected to achieve simultaneous regulation of birefringence and bandgap.

Herein, SnCl₂[SC(NH₂)₂] crystals were synthesized using a solution volatilization method. Studies on the properties of SnCl₂[SC(NH₂)₂] have indicated a large birefringence (0.2@546 nm) and a wide bandgap (3.362 eV) of SnCl₂[SC(NH₂)₂], offering an effective design strategy for the development of novel UV chalcogenides with balanced bandgap and birefringence.

1.1. Synthesis

Analytical grade thiourea and tin(II) chloride dihydrate were procured from Shanghai Aladdin Biochemical Technology Co., Ltd and were employed without further purification. The SnCl₂[SC(NH₂)₂] complex was synthesized using an aqueous solution evaporation method. Stoichiometric amounts of SnCl₂·2H₂O (0.02 mol) and SC(NH₂)₂ (0.013 mol) were weighed into separate beakers to afford a 2 : 1.3 molar ratio. SnCl₂·2H₂O was dissolved in 4 mL of deionized water, whereas thiourea was dissolved in the minimum volume of deionized water required for complete solvation. The two solutions were merged and subjected to magnetic stirring for 50 minutes. Subsequently, the mixture was left undisturbed in the dark at ambient temperature for a period of one week. Following this, colorless block SnCl₂[SC(NH₂)₂] crystals were isolated.

1.2. Powder X-ray diffraction

The powder X-ray diffraction datas for SnCl₂[SC(NH₂)₂] crystals were acquired under ambient conditions using a TongDa TD-3500 X-ray diffractometer, employing CuKα radiation (λ = 1.54056 Å). The 2θ angle was scanned from 10° to 70°, with a step size of 0.02° and a fixed counting time of 0.1 s per step. The SnCl₂[SC(NH₂)₂] crystal powder was analyzed, and its experimental XRD pattern was compared against standard data to confirm the product's purity (Fig. 1).

Fig. 1 PXRD patterns of SnCl₂[SC(NH₂)₂].

1.3. Infrared absorption spectrum

The infrared (IR) absorption spectrum of the powder sample in the wavenumber range of 400–4000 cm⁻¹ was acquired using a PerkinElmer Spectrum Two Fourier Transform Infrared spectrometer.

1.4. UV-Vis diffuse reflectance spectroscopy

The UV-Vis reflectance spectra of the powder sample were collected in the wavelength range from 200 nm to 800 nm using a UV-2600 UV-Vis-NIR spectrophotometer, while BaSO₄ was selected as the reference standard.

1.5. Birefringence measurement

The birefringence of SnCl₂[SC(NH₂)₂] crystals was determined using a polarizing microscope equipped with a Berek U-CTB compensator and a 546 nm filter, while the following equation was adopted for the assessment of the birefringence:

ΔL = Δn·d,

ΔL represents the optical path difference, Δn denotes the birefringence, and d indicates the crystal thickness.

1.6. Theoretical calculation

Theoretical predictions were conducted using plane-wave pseudopotential density functional theory (DFT).³¹ The generalized gradient approximation (GGA) delineated by Perdew–Burke–Ernzerhof (PBE) was selected as the exchange–correlation functional.³² A cutoff energy of 750 eV was applied to ascertain the number of plane-wave basis sets, while the Monkhorst–Pack k-point sampling for numerical integration in the Brillouin zone was established at 5 × 2 × 3. In the band structure calculations, 56 empty bands were factored in, whereas 168 empty bands were harnessed for the density of states (DOS) calculation.

2. Results and discussion

2.1. Crystal structure

SnCl₂[SC(NH₂)₂] crystallized in the monoclinic space group P2₁/c with cell parameters a = 5.951 Å, b = 13.937 Å, c = 8.883 Å, β = 111.13°, and V = 687.212 ų. As illustrated in Fig. 2, SnCl₂[SC(NH₂)₂] is a molecular crystal with the asymmetric unit constituted by one Sn atom, one S atom, one C atom, two Cl atoms, and two N atoms (hydrogen atoms were omitted). As illustrated in Fig. 2, all S atoms are bonded to C atoms by double bonds, and each C atom is bonded to two –NH₂ groups by single bonds, ultimately forming a nearly planar SC(NH₂)₂ group. All Sn²⁺ ions are connected to two Cl⁻ ions and one SC(NH₂)₂ group to form SnCl₂[SC(NH₂)₂] groups with the Sn–Cl bond lengths of 2.492(2)–2.608(3) Å and the elongated Sn–S bond length of 2.703 Å. The SnCl₂[SC(NH₂)₂] group is the basic repeating unit of the SnCl₂[SC(NH₂)₂] crystal. Within the SnCl₂[SC(NH₂)₂] group, benefiting from the strong repulsion of the lone pair electrons from Sn²⁺ and the significant bond length difference between Sn–Cl and Sn–S bonds, the Sn²⁺ ion, two bonded Cl⁻ ions, and one S atom form a markedly distorted SnCl₂S trigonal pyramidal group, which is likely to make a substantial contribution to the birefringence of SnCl₂[SC(NH₂)₂]. On the other hand, featuring a planar π-conjugated structure, the thiourea group always acts as a birefringent active moiety. Within the crystal lattice of SnCl₂[SC(NH₂)₂], all thiourea groups were linked through directional N–H⋯Cl hydrogen-bonding interactions with the SnCl₂S trigonal pyramidal units, resulting in an ordered arrangement relative to the ab plane, and the bond lengths of these hydrogen bonds span 3.318(22)–3.428(4) Å, while the corresponding bond angles range from 97.0(8)° to 120.9(8)°. Besides, the dihedral angle between the thiourea molecular plane and the ab plane is 38.8°. This spatial packing dominated by the hydrogen bond parameters effectively amplifies the intrinsic optical anisotropy of the thiourea groups. In addition, the interconnections between groups within the SnCl₂[SC(NH₂)₂] crystal endow it with a certain degree of thermal stability, and the thermal stability temperature of the SnCl₂[SC(NH₂)₂] crystal can reach 160 °C (Fig. S1). Furthermore, within each SnCl₂[SC(NH₂)₂] molecule, the molecular plane of the SnCl₂ moiety lies nearly parallel to that of the coordinated thiourea group, which is supposed to further enhance the overall birefringence of SnCl₂[SC(NH₂)₂].

Fig. 2 Structures of SnCl₂[SC(NH₂)₂]: (a) the asymmetric unit of SnCl₂[SC(NH₂)₂], (b) the overall structure of SnCl₂[SC(NH₂)₂], and (c) illustration of the structural configuration of thiourea groups.

2.2. IR spectra analysis

The Fourier transform infrared (FT-IR) spectrum of the as-synthesized crystal exhibits characteristic vibrational modes corresponding to the coordination environments of Sn, S and Cl. Comparison with the IR spectrum of pure thiourea (Fig. 3) confirms the presence of thiourea groups in the structure of the SnCl₂[SC(NH₂)₂] crystal. The assignments of the characteristic absorption peaks are as follows: in the high-frequency region, the absorption bands at 3382 cm⁻¹, 3290 cm⁻¹, and 3192 cm⁻¹ are assigned to the asymmetric and symmetric N–H stretching vibrations of the NH₂ group in the thiourea moiety. Upon complexation, these vibrational bands all shift to higher wavenumbers relative to free thiourea, which is attributed to the formation of the Sn–S coordination bond that enhances the contribution of highly polar resonance structures within the thiourea molecule, thereby increasing the double-bond character of the C–N bond and rendering the C=S bond more single-bond-like.³³ The strong absorption band at 1612 cm⁻¹ is assigned to the NH₂ bending vibration,³³ while the band in the 1500–1550 cm⁻¹ region corresponds to the N–C–N skeletal stretching vibration of the thiourea group. This band shifts from 1465 cm⁻¹ in free thiourea to a higher wavenumber upon coordination, also due to the enhanced double-bond character of the C–N bond.³³ The band at 1392 cm⁻¹ arises from the coupling of NH₂ wagging, N–C–N stretching, and C=S stretching vibrations. The band at 1101 cm⁻¹ represents a composite mode involving symmetric C–N stretching, NH₂ wagging, and C=S stretching vibrations; it shifts from 1093 cm⁻¹ in free thiourea to 1101 cm⁻¹ upon coordination. This blue shift can be explained by the significant reduction in the contribution of C=S stretching upon sulfur coordination, causing symmetric C–N stretching to dominate this vibrational mode. Furthermore, the absorption band at 700 cm⁻¹ is assigned to a coupled mode of C=S stretching and symmetric C=N stretching vibrations; its notable downshift from 730 cm⁻¹ in free thiourea indicates that coordination weakens the double-bond character of the C=S bond. In addition, the two weak absorption peaks of 1766 cm⁻¹ and 1726 cm⁻¹ mainly originate from the C=O stretching vibration of a small amount of urea, which is a minor hydrolysis product of thiourea.

Fig. 3 IR spectra of SnCl₂[SC(NH₂)₂] and pure thiourea.

2.3. Optical properties

With the presence of the UV-applicable FBB of SnCl₂ and the UV-detrimental unit of SC(NH₂)₂, the optical transparency of SnCl₂[SC(NH₂)₂] is of extreme significance for its practical applications. As shown in Fig. 4, the optical reflectance rate of SnCl₂[SC(NH₂)₂] remains above 80% that of benchmark BaSO₄ in the wavelength region from 400 nm to 800 nm, indicating good transparency of SnCl₂[SC(NH₂)₂] in the visible region. In contrast, the reflectance rate dropped sharply in the UV region, while the cut-off wavelength was about 320 nm. Also, with the utilization of KM fitting as shown in the inset of Fig. 4, the actual bandgap of SnCl₂[SC(NH₂)₂] was determined to be 3.362 eV, a value that is larger than those of typical chalcogenides like CdS,³⁴ α-HgS,³⁵ and ZnTe³⁶ and rivals those of oxides like SrTiO₃,³⁷ Nb₂O₅,³⁸ and SrSnO₃.³⁹ In particular, despite the molecular crystal nature of SnCl₂[SC(NH₂)₂], the bandgap of SnCl₂[SC(NH₂)₂] is larger than that of pure thiourea crystals,⁴⁰ suggesting the importance of SnCl₂ in contributing to the wide bandgap of SnCl₂[SC(NH₂)₂].

Fig. 4 UV-Vis diffuse reflectance spectrum of SnCl₂[SC(NH₂)₂].

To get an idea of the optical dispersion ability of SnCl₂[SC(NH₂)₂], a piece of the SnCl₂[SC(NH₂)₂] crystal was selected for the measurement of the optical path difference and the corresponding crystal thickness using a polarization microscope equipped with a Bereck compensator and a 546 nm light filter. As shown in Fig. 5(a), the OPD was recorded to be 1.139 μm using the Bereck compensator when the top surface of the crystal was subjected to be in the state of total light extinction, while the crystal thickness was found to be 5.69 μm as depicted in Fig. 5(b). Hence, the birefringence (Δn) of SnCl₂[SC(NH₂)₂] was calculated to be 0.20 which is much larger than those of common UV birefringent crystals like α-BBO and MgF₂. Also, compared with tin-based UV crystals like Sn₂B₅O₉Cl (0.168@546 nm),⁴¹ Sn₂OSO₄ (0.004@546 nm)⁴² and Sn₃O₂(OH)(HSO₄) (0.169@546 nm)⁴² as well as thiourea-containing crystals like Hg[CS(NH₂)₂]₄(SiF₆) (0.145@546 nm),⁴³ [C(NH₂)₃]₂SiF₆ (0.16@546 nm)⁴⁴ and Zn[CS(NH₂)₂]₃SO₄ (0.09@546 nm),⁴⁵ the birefringence of SnCl₂[SC(NH₂)₂] has much advantage. This finding demonstrates the effectiveness of combining thiourea and LPE-type Sn²⁺ in the construction of birefringent crystals for better performance.

Fig. 5 Assessment of the birefringence of SnCl₂[SC(NH₂)₂]: (a) complete light extinction of the crystal with a 546 nm light filter, (b) the crystal under polarized illumination without a light filter, and (c) the thickness of the crystal.

2.4. Theoretical calculation

To elucidate the origin of the bandgap and birefringence of SnCl₂[SC(NH₂)₂], first-principles calculations of the band structure, total and projected DOS, and electron-localization function (ELF) were conducted. An indirect bandgap of 2.722 eV is predicted for SnCl₂[SC(NH₂)₂] (Fig. 6a), which slightly underestimates the experimental value of 3.362 eV. This underestimation of bandgap should be attributed to the discontinuity of GGA–PBE exchange–correlation potentials.⁴⁶ According to the total and projected DOS maps presented in Fig. 6b, the upper region of the valence band of SnCl₂[SC(NH₂)₂] is dominated by hybridized Sn 5s–5p, S 3p and Cl 3p states, whereas the valence band maximum (VBM) is localized almost exclusively on the S 3p orbitals. The bottom of the conduction band of SnCl₂[SC(NH₂)₂] is constructed primarily from Sn 5p and C 2p contributions, with the conduction band minimum (CBM) occupied by Sn 5p states. Hence, the pronounced separation between the VBM and CBM is primarily attributed to the ionic bonds between Sn and S atoms, which effectively lowers the VBM, while the high-energy nature of the Sn 5p states also dictates the position of the CBM. Although Cl⁻ anions do not directly contribute to bandgap formation, the robust Sn–Cl bonding interactions are distinctly revealed by the significant overlap of the Sn 5s–5p and Cl 3p states within the upper region of the valence band, which collaboratively facilitates the downward shift of the VBM.

Fig. 6 Calculated band structure (a), total and projected DOS maps (b), and ELF (c) for SnCl₂[SC(NH₂)₂].

As illustrated in Fig. 6c, the electron localization function (ELF) reveals pronounced anisotropy in the electron density distribution surrounding the thiourea molecular plane. This anisotropic feature highlights the critical role of thiourea in dictating the optical anisotropy of SnCl₂[SC(NH₂)₂]. The electron cloud behavior around the S²⁺ cations shows significant anisotropy, which further substantiates thiourea's substantial contribution to the material's birefringence properties. Turning to the SnCl₂ moiety, no notable anisotropy is observed in the electron density surrounding the Cl⁻ anions; however, the region adjacent to the Sn²⁺ cations exhibits distinct anisotropic electron localization. This finding identifies Sn²⁺ as the other key contributor to the optical dispersion capabilities of SnCl₂[SC(NH₂)₂], a phenomenon attributed to the stereochemical activity of its lone pair electrons. While Cl⁻ anions do not exert a significant influence on birefringence, strong electron localization is evident in their vicinity. This characteristic confirms that Cl⁻ anions act as electron acceptors from Sn atoms, which facilitates the expansion of the material's bandgap. Clearly, this electron acceptance phenomenon aligns well with the DOS maps discussed earlier. Subsequently, SnCl₂ emerges as the critical factor in overcoming the “3 eV wall” barrier while simultaneously maintaining a well-balanced birefringence. Additionally, the calculated refractive index (Fig. S2) revealed that the birefringence of SnCl₂[SC(NH₂)₂] is primarily dominated by the disparity between the refractive indices n_y and n_x, which is consistent with the arrangement of thiourea groups within the SnCl₂[SC(NH₂)₂] structure, further underscoring the pivotal role of thiourea in generating substantial birefringence.

3. Conclusion

In summary, the molecular crystal —SnCl₂[SC(NH₂)₂] has been investigated as a promising candidate for birefringent applications. This compound successfully overcomes the “3 eV wall” barrier through the unique bonding interactions and electron transfer processes between Sn²⁺ cations and S²⁻/Cl⁻ anions. Additionally, the cooperative structural arrangement of π-conjugated thiourea planes and LPE-type SnCl₂S trigonal pyramidal units played a critical role in contributing to a large birefringence of Δn = 0.20@546 nm. When compared to typical chalcogenide birefringent crystals (as shown in Fig. 7), SnCl₂[SC(NH₂)₂] exhibits a well-balanced performance between bandgap and birefringence. Consequently, the discovery of SnCl₂[SC(NH₂)₂] offers an effective design strategy for developing high-performance chalcogenide birefringent crystals. Further related research efforts are currently underway.

Fig. 7 Illustration of the birefringence and bandgap of some chalcogenide crystals.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data supporting this article have been included as part of the supplementary information (SI). Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6dt00144k.

Acknowledgements

We gratefully acknowledge the support from the Sichuan Science and Technology Program (No. 2023NSFSC0957), the Natural Science Foundation of Sichuan Province (No. 2022NSFSC1854), the PhD Research Startup Foundation of China West Normal University (No. 20E070 and 22kE011) and the Key Laboratory of Functional Crystals and Laser Technology, TIPC, CAS (No. FCLT202204).

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Footnote

† These authors contributed equally to this work.

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