A_0.5H₂C₆N₇O₃·4H₂O (A = Ca²⁺, Sr²⁺) iso-cyamelurates with ultra-large π-conjugated group and excellent nonlinear optical properties

Abstract

Nonlinear optical (NLO) crystals are the core components of high-performance coherent sources. Their properties can be modulated and improved by molecular design of constituent functional motifs. In this study, inspired by a classical π-conjugated borate group (B₃O₆)³⁻, we discovered a π-conjugated isocyamelurate [H₂C₆N₇O₃]⁻ anion with colossal π-conjugated orbitals, thus giving ultra-strong anisotropic first-order polarizability and ultralarge second-order susceptibility. Two new alkaline-earth metal isocyamelurates, namely, Ca_0.5H₂C₆N₇O₃·4H₂O (I) and Sr_0.5H₂C₆N₇O₃·4H₂O (II), were successfully synthesized by a facile aqueous solution method. Both I and II show excellent nonlinear optical properties, including wide band gap (>4.05 eV), large birefringence (Δn ∼ 0.24), and a strong second order harmonic generation response (>5 × KDP). Moreover, I and II exhibit a broadband ultraviolet photoluminescence around 400 nm, which indicate that they are potential multifunctional optical materials with photon-emission and photon-conversion properties. The first-principles calculations reveal that the (H₂C₆N₇O₃)⁻ anionic group plays a dominant role in optimizing and enhancing the optical performance of the crystals. This study confirms that there are plenty of opportunities for (iso)-cyamelurates in designing functional crystals with superior properties.

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Introduction

Nonlinear optical (NLO) crystals can extend the wavelength of laser to new spectral regions by the use of the second harmonic generation (SHG), DFG, and OPO processes, which are widely used in precision micromanufacturing, laser spectroscopy, optical communication, etc.^1–4 The famous anionic group theory has for long time been the research guide in the structural design and screening for high performance NLO materials, and it is of great significance to find high efficiency functional structural units (FBUs), which can simultaneously be provided with large microscopic nonlinear susceptibility and optical anisotropy.^5–10 In this regard, the π-conjugated group is probably the most important and successful group, which promotes the discovery of some commercial crystals, such as KBe₂BO₃F₂ (KBBF) with (BO₃)³⁻ group, β-BaB₂O₄ (BBO) with (B₃O₆)³⁻ group, and LiB₃O₅ with (BO₃)³⁻ group.^11–15 Recently, organic FBUs of π-conjugated (C₃N₃O₃)³⁻, structurally similar to inorganic traditional (B₃O₆)³⁻ rings, have been proven to have large nonlinear optical response due to the delocalized p_π electrons and enhanced p_π–p_π interaction.^16–18 Some intriguing crystals with excellent optical properties have been reported, including KLi(HC₃N₃O₃)·2H₂O, RbLi(HC₃N₃O₃)·2H₂O, K₂Pb(H₂C₃N₃O₃)₄·4H₂O, and RE₅(C₃N₃O₃)(OH)₁₂ (RE = Y, Yb, and Lu),^19–22 and these metal cyanurates can exhibit excellent performance with the largest measured SHG responses up to ∼5 × KDP and birefringence > 0.18.²³ As a result, this has greatly pushed the development of functional crystals containing π-conjugated groups.

Returning back to anionic group theory, we find that the number of p_π electrons is very important for the conjugacy of a planar group. In order to find more possibilities in conjugated systems, the cyamelurate (C₆N₇O₃)³⁻ group was revisited recently, which has an impressive ultra-large π-conjugated ring with sixteen coplanar atoms. This can be viewed as an extended cyanurate (C₃N₃O₃)³⁻ by inserting a (C₃N₄) motif. As a result, the (C₆N₇O₃)³⁻ anion has a colossal π-conjugated electron configuration of π¹⁷₁₆ and exhibits strengthened p_π–p_π interaction and more delocalized electron distribution than (C₃N₃O₃)³⁻ (π⁹₉), thereby leading to a large SHG intensity (>4 × KDP) and birefringence (0.446 @ 1064 nm), as demonstrated in the alkali metal cyamelurate of K₃C₆N₇O₃·2H₂O.²⁴ Moreover, iso-cyamelurates have possibilities in the existence of hydroisocyamelurate (HC₆N₇O₃)²⁻ and dihydroisocyamelurate (H₂C₆N₇O₃)⁻, similar to the structural transformation in (iso)-cyanurate groups of (H_xC₃N₃O₃)^x−3 (x = 0, 1, 2), which can also be predicted to have great potential in creating enhanced SHG efficiency and birefringence concurrently by rational design.^25–28 However, to the best of our knowledge, there are a few reports about their related NLO properties and potential applications in this field.

In this study, we explored iso-cyamelurates by introducing alkaline-earth metal cations into a crystal structure with the [H₂C₆N₇O₃]⁻ anion. We successfully synthesized two new isocyamelurates, Ca_0.5(H₂C₆N₇O₃)₂·4H₂O (I) and Sr_0.5(H₂C₆N₇O₃)₂·4H₂O (II), which showed excellent nonlinear optical properties. Both of them have strong second-order harmonic generation response (>5 × KDP), wide band gaps (>4.0 eV), and large birefringence (Δn ≈ 0.25 at 1064 nm). Moreover, their structure–property relationship, especially the role of conjugated isocyamelurate group, is elucidated by first-principles calculations.

Experimental section

Reagents

All reagents, melamine (C₃H₆N₆, Aladdin, 99%), KOH (Aladdin, ACS), CaCO₃ (Aladdin, 99.99%), SrCO₃ (Aladdin, AR), Rb₂CO₃ (Aladdin, 99%), were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd and used without further treatment.

Synthesis

Potassium cyamelurate was obtained according to the literature.²⁹ The potassium cyamelurate (1 mmol, 0.275 g), hydrochloric acid (37%, 2 mL), Rb₂CO₃ (0.5 mmol, 0.116 g), CaCO₃ (1 mmol, 0.1 g)/SrCO₃ (1 mmol, 0.084 g), and deionized water (40 mL) were mixed and heated to boiling, and then the hot solution was filtered. Transparent crystals were precipitated from the obtained solution when cooled to room temperature (Fig. S1†).

Single crystal X-ray determination

Bruker SMART APEX II 4K CCD single crystal diffractometer was used to collect the single-crystal X-ray diffraction data for I and II under Mo Kα radiation (λ = 0.71073 Å) at 298 K. Data collection, reduction, and cell refinement were performed by using the software APEX3. The crystal structures were solved by the direct method by intrinsic phasing with the ShelXT structure solution program, and refined using least squares minimization with the ShelXTL refinement package in Olex2.^30,31 Potential missing symmetry of crystal data was checked with the program PLATON30 and no higher symmetry was found.³² The detailed crystallographic data for Ca_0.5H₂C₆N₇O₃·4H₂O and Sr_0.5H₂C₆N₇O₃·4H₂O are listed in Table 1 and Tables S1–S8.†

Table 1 Crystal data and structure refinements of Ca_0.5H₂C₆N₇O₃·4H₂O (I) and Sr_0.5H₂C₆N₇O₃·4H₂O (II)

	I	II
a R 1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo^2 − Fc^2)2/∑w(Fo^2)^2]^1/2.
Formula weight	312.25	336.02
Crystal system	Orthorhombic	Orthorhombic
Space group	Fdd2	Fdd2
a (Å)	11.749(7)	11.898(2)
b (Å)	63.01(4)	63.422(9)
c (Å)	5.978(4)	6.0255(10)
V/Å^3	4426(5)	4546.8(13)
Z	16	16
ρ calc. (g cm^−3)	1.874	1.963
μ (mm^−1)	0.392	2.477
F(000)	2576.0	2720.0
Flack parameter	0.05(4)	0.07(2)
Reflections collected	22 594	15 400
Final R indexes [I ≥ 2σ(I)]^a	R 1 = 0.0519, wR2 = 0.1007	R 1 = 0.0626, wR2 = 0.1404
Final R indexes [all data]^a	R 1 = 0.1059, wR2 = 0.1216	R 1 = 0.1140, wR2 = 0.1635

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Powder X-ray diffraction

A Smart Lab 9 kW X-ray diffractometer was used to measure the powder X-ray diffraction with Cu Kα radiation (λ = 1.5418 Å) under room temperature and the 2θ scan range was from 5° to 80° (Fig. S1†).

Thermal analysis

A NETZSCH STA thermal analyzer instrument was used to study the thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) of crystalline samples under the flow of N₂. The powdered compound was placed in Al₂O₃ crucibles ranging from 40 °C to 900 °C.

UV-Vis-NIR diffuse reflectance spectrum

UV-Vis-NIR diffuse reflectance spectrum was measured using a UH4150 UV-Vis-NIR spectrophotometer, and the wavelength range was from 200 nm to 1000 nm. The reflection spectrum was converted to the absorption spectrum according to the Kubelka–Munk function, F(R) = (1 − R)²/2R, where R is the reflectance coefficient.^33,34 The band gap was deduced using a straightforward extrapolation method.³⁵

Birefringence measurement

The birefringence was measured using a Nikon Eclipse polarizing microscope E200MV POL under a visible light filter. The calculated birefringence formula was as follows: ΔR = Δn × T, where ΔR, Δn, and T represent the optical path difference, the birefringence, and the thickness of the crystal, respectively.^36,37 Transparent strip crystals were chosen to insure the accuracy. The thickness of the sample was measured by the Bruker Smart Apex II.

Power SHG measurement

SHG responses of the samples were measured using a Q-switched Nd:YAG laser under 1064 nm wavelength by the Kurtz–Perry technique at room temperature.³⁸ The crystalline powder was sieved into several different standard sizes: 26–74 μm, 74–100 μm, 100–125 μm, 125–154 μm, 154–180 μm, and 180–200 μm, and KH₂PO₄ (KDP) was used as the reference sample with the same particle size ranges.

Fluorescence measurement

The fluorescence spectra of I and II were collected using an Edinburgh fluorescence spectrometer at room temperature. A xenon lamp light with a wavelength of 280–310 nm was used as the incident source. The excitation and emission slit functions were set to be 16 and 0.5. The wavelength step is 0.1 nm and the dwell time is 0.02 s. Collection of the spectral lines was repeated twice.

Calculation method

The first-principles calculations for the metal cyamelurates I and II were performed using the CASTEP package³⁹ based on density functional theory,⁴⁰ which is similar to previous studies on K₃C₆N₇O₃·2H₂O and other metal cyanurates.^24,41 An energy cutoff of 900 eV was adopted and a Monkhorst–Pack k-point meshes⁴² (2 × 2 × 2) in the first Brillouin zone was selected to ensure sufficient accuracy of the calculated results. The exchange–correlation functionals were described by a generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBEsol) functional⁴³ and the ion–electron interactions for all constituent atoms were modelled by the norm-conserving pseudopotentials.⁴⁴ The refractive indices and birefringence values were obtained based on the electronic structures. In the electronic structure calculations, both the lattice constants and atom sites are fully optimized.

Results and discussion

The colorless plate crystals Ca_0.5H₂C₆N₇O₃·4H₂O (I) and Sr_0.5H₂C₆N₇O₃·4H₂O (II) were synthesized by simple evaporation from aqueous solution (Fig. S1†). The measured powder XRD patterns closely matched the diffraction peaks calculated from the crystallographic data, confirming the purity of the sample powder (Fig. S1†). I and II are isomorphic and both crystallize in the non-centrosymmetric Fdd2 space group (no. 43). Herein, only the structure of I is taken as an example. As seen in Fig. 1a, the main part of the asymmetric unit of I is consisted of a central metal Ca²⁺ coordinated with two O atoms from (H₂C₆N₇O₃)⁻ anions and six O atoms from six H₂O molecules, and the two (H₂C₆N₇O₃)⁻ anion planes are in the angle of 61.45°. The coordinated Ca²⁺ cations are aligned along the a-axis and the nearest distance of (H₂C₆N₇O₃)⁻ layers is 3.086 Å (Fig. 1b). In the crystal packing diagram, the isolated asymmetric unit is further interconnected by hydrogen bonds to build a 3D structure (Fig. 1c) with symmetry operator of glide plane (n), and crystal water molecules arranged between the layers. As for Sr_0.5H₂C₆N₇O₃·4H₂O (II), the two (H₂C₆N₇O₃)⁻ anion planes are in the angle of 61.68° in the main part of the asymmetric unit, and the nearest distance between the anion planes is 3.106 Å, which are very similar to I (Fig. S2†) but different in the content of water of crystallization in the unit cell reported for Sr[H₂C₆N₇O₃]₂·4H₂O.²⁵

Fig. 1 Crystal structure of Ca_0.5H₂C₆N₇O₃·4H₂O (I). (a) The coordination environment of Ca²⁺; (b) the arrangement of (H₂C₆N₇O₃)⁻ anions; (c) the packing diagram of I viewed along the [101] direction. Green, red, blue, black, and pink balls represent Ca, O, N, C, and H atoms, respectively.

The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves show the weight loss process of I and II. As seen in Fig. S3,† there are two steps for I in the range of 110–900 °C in nitrogen atmosphere. The first weight loss is from 110 to 205 °C corresponding to 4 molecules of H₂O (exp. 23.22%/cal. 23.08%) and the continuous mass loss starts at 205 °C, indicating thermal decomposition of the sample. II also displays similar steps of weight loss in the range of 110–900 °C. Such a weight loss process is similar to LnC₆N₇O₃·7H₂O (Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, and Tm) which have been reported previously.⁴⁵

Fig. 2 shows the UV-Vis-NIR diffuse reflection spectra of I and II. Both I and II have rather wide band gaps (E_g) of 4.05 eV and 4.07 eV, respectively, corresponding to the cutoff edge of 306 nm and 304 nm. As expected, with the introduction of alkaline-earth metal cations, the obvious enhancement of E_g is realized by comparison with the previously reported alkali metal cyamelurate K₃C₆N₇O₃·2H₂O (2.9 eV). The band gaps of I and II are comparable or larger than some other compounds featuring π-conjugated FBUs, such as K₂Pb(H₂C₃N₃O₃)₄·4H₂O (3.9 eV), NH₄Sb₂(C₂O₄)F₅ (3.85 eV), Na₃C₆N₉·3H₂O (4.16 eV), KC₉H₅O₆(H₂O) (3.91 eV), and [C(NH₂)₂NHNO₂][C(NH₂)₃](NO₃)₂ (3.58 eV).^21,46–49

Fig. 2 The UV-Vis-NIR diffuse reflectance spectra of (a) I and (b) II.

Since I and II crystallize in the NCS space group of Fdd2, their NLO optical properties were investigated by SHG measurements using incident laser irradiation at a wavelength of 1064 nm. As shown in Fig. 3a, the SHG intensities of both I and II increase with the increase of particle size, and finally I and II exhibit large SHG response of 5.3 times and 6.5 times of KDP, respectively, with phase matching behaviour. The stronger SHG response in the Sr-analogue is similar to that in Ca₃(C₃N₃O₃)₂–Sr₃(C₃N₃O₃)₂ solid solutions.^16,17 As far as we know, except alkaline-earth metal isocyanurates, e.g. Sr(HC₃N₃O₃)·2.5H₂O and Sr(HC₃N₃O₃)·2H₂O ^50,51 without measured SHG values, the NLO properties under 1064 nm incident light of I and II as isocyamelurates attributed to the extended (H₂C₆N₇O₃)⁻ anions are comparable or larger than some those of isocyanurates as KLi(HC₃N₃O₃)·2H₂O (5.3 × KDP), LiRb(HC₃N₃O₃)·2H₂O (2.7 × KDP), NaRb_0.84Cs_0.16HC₃N₃O₃·2H₂O (3.0 × KDP), as well as other compounds featured with 6-MR π-conjugated groups, such as CsAlB₃O₆F (2.0 × KDP), (C₅H₆ON)⁺(H₂PO₄)⁻ (3.0 × KDP).^19,20,52,53 In addition, both I and II exhibit a broadband UV photoluminescence (∼400 nm) under the excitation of 290 nm (Fig. 3b and S4†). Especially for II, the emission spectrum displays that its full width at half maximum (FWHM) is ∼82 nm, which is larger than that of previously reported K₃(C₆N₇O₃)·2H₂O (FWHM ∼ 60 nm).²⁴ Considering II also has strong SHG response and large birefringence, it can be regarded as a potential multi-functional material for photonic fields.

Fig. 3 (a) SHG intensity curves of I and II and (b) photoluminescent measurement of II. Excitation spectrum (E_m = 420 nm) and emission spectrum (E_x = 290 nm).

Multi-color source orthogonal polarization microscopy was used to estimate the in-plane birefringence of the extinction of I and II single crystals, and the results are shown in Fig. S5 and S6.† The observed interference colors are second order pink for I and second order green for II, respectively. According to the Michal-Levy diagram, the corresponding delay values are 1050 nm and 1350 nm, and the measured crystal thickness is about 4.2 μm and 5.5 μm, respectively. Thus, the measured birefringence of I and II is 0.250 and 0.245, respectively. These estimated values are slightly larger than another hydro-isocyamelurate of Ba(H₂C₆N₇O₃)·8H₂O (0.24 at 550 nm),²⁷ and show enhancement compared with typical birefringent materials, such as CaCO₃ (0.172 at 589 nm) and MgF₂ (0.013 at 253.7 nm).^36,37 Regarding materials with other π-conjugated motifs, e.g. β-BBO (0.119 at 546 nm),¹³ Ca₃(BO₃)₂ (0.097 at 589 nm),⁵⁴ and KLi(HC₃N₃O₃)·2H₂O (0.186 at 514 nm),¹⁹I and II maintain a comparable birefringent value, indicating their potential application advantages.

To deeply reveal the origin of the second-order nonlinearity of these two compounds, the electronic structures were calculated. As shown in Fig. S7,† both I and II are direct gap semiconductors with simulated bandgaps of 3.74 eV and 3.62 eV, respectively, which are close to the experimental values. Fig. 4a and c depict that the Ca²⁺/Sr²⁺ cations and water molecules show no effect on the electronic states near the forbidden gap, and the main contribution is provided by C 2p, N 2p, and O 2p orbitals from the (H₂C₆N₇O₃)⁻ anion group. This result conforms to the anionic group theory, which also indicates that the ultra-large conjugated (H₂C₆N₇O₃)⁻ motif plays a significant role in the optical properties of I and II.

Fig. 4 Total and partial DOS for (a) I and (c) II; calculated refractive indices for (b) I and (d) II.

Furthermore, the optical anisotropy of I and II was calculated and analyzed. Since I and II share the same crystal structure and molecular arrangement, they exhibit a large difference in refractive index from the visible to near-IR region with almost the same value. As displayed in Fig. 4b and d, the birefringence n_y–n_x reaches up to 0.24 at 1.06 μm, which is in good agreement with experimental values. For planar (H₂C₆N₇O₃)⁻ groups, localized C–N/C–O σ-bonds prevail over the in-plane polarizabilities, and delocalized p_π–p_π orbitals around C–N–O rings dominate the out-of-plane polarizabilities, so that the p_π–p_π interaction of iso-cyamelurate groups successfully make the main contribution to the enhancement of the optical anisotropy.

Conclusions

In summary, two new iso-cyamelurate crystals, namely, A_0.5H₂C₆N₇O₃·4H₂O (A = Ca²⁺, Sr²⁺) containing colossal π-conjugated orbitals were discovered and investigated in detail. They concurrently showed a strong SHG response (5–6.5 × KDP), sufficient birefringence (∼0.25 at 1064 nm), wide band gaps (>4.0 eV), and broadband UV photoluminescence, suggesting that they are very promising multi-functional photonic materials. We believe this study would not only be a feasible strategy for designing novel (iso)-cyamelurates with high performance but also enrich new NLO crystals pointing to multi-functional applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 52272007, 52002220, 51890864), and the National Key Research and Development Program of China (2021YFA0717800). X. Y. Zhang and F. Liang thank the support from Open Funding of the Key Laboratory of Functional Crystals and Laser Technology, TIPC, CAS.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional crystallographic data and experimental and theoretical results, additional tables and figures. CCDC 2277765 and 2277766. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01251d

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