“Three functions in one”: multifunctional rare-earth cyamelurates with magnetism, luminescence, and giant optical nonlinearity

Abstract

A highly-integrated optical device requires multifunctional crystals, which respond strongly to external stimuli, such as photoluminescence, nonlinear polarization, and magnetoelectric coupling. However, it is usually difficult for these functions to coexist in a single-phase crystal, and can even be incompatible. Here, we propose a ‘unit assembly’ strategy in rare-earth cyamelurates, which combines active rare-earth ions (Gd³⁺, Y³⁺, Lu³⁺) and π-conjugated [H_xC₆N₇O₃]^(3−x)– units into one crystal with an aligned arrangement. For the first time, we synthesized three new rare-earth cyamelurate crystals RE(H_1.5C₆N₇O₃)₂·10H₂O (RE = Y, Gd, Lu) by a facile solution method. Benefitting from the strong polarizability and delocalized electrons of the π-conjugated units, all three crystals display giant second-order optical nonlinearity (12–15.3 × KDP), broadband photoluminescence (300–500 nm), and strong anisotropic birefringence (Δn > 0.22) at 1064 nm. Moreover, the Gd-analogue exhibits paramagnetic behaviour in a wide temperature range. These results highlight rare-earth cyamelurates as promising multifunctional optical crystals with three concurrent functional responses, indicating potential applications in next-generation photonics and optoelectronics.

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Introduction

The integration of multiple functionalities into a single crystal remains a long-standing challenge in the field of inorganic functional materials. Currently, the composition and structure of functional units are key factors affecting the overall performance of functional materials. Rational design of these building units can effectively improve the optical response of the materials under an external field.¹ In the past thirty years, many nonlinear optical (NLO) crystals with excellent performances have been discovered,^2–9 and several anionic groups have been identified as effective fundamental building blocks for enhancing nonlinear responses.^10–16 These include (SeO₃)²⁻ and (IO₃)⁻, which possess stereochemically active lone pairs; (NbO₆)⁷⁻ and (VO₆)⁷⁻, which exhibit Jahn–Teller effects; and (BO₃)³⁻, (CO₃)²⁻, and (B₃O₆)³⁻, which feature planar π-conjugated groups. Among these various kinds of anions, π-conjugated groups with large optical anisotropy and microscopic second-order susceptibility are regarded as some of the most successful. This has led to the discovery of a series of benchmark crystals, e.g. KBe₂BO₃F₂ (KBBF), β-BaB₂O₄ (BBO), LiB₃O₅ (LBO), etc.^17–19 However, these small π-conjugated units typically exhibit an ultrawide bandgap, thus hindering their fluorescent emission and magnetic responses. Therefore, it remains a great challenge to design multifunctional crystals within an inorganic π-conjugated system.

Recently, metal cyanurates with large π-conjugated (C₃N₃O₃)³⁻ unit have been developed as effective nonlinear optical materials,^20–23 including KLi(HC₃N₃O₃)·2H₂O, Cs₃Na(H₂C₃N₃O₃)₄·3H₂O, and RE₅(C₃N₃O₃)(OH)₁₂ (RE = Y, Yb, and Lu). More interestingly, owing to the reduced forbidden gap of the (C₃N₃O₃)³⁻ unit, several cyanurates display a strong photoluminescence response, such as Pb(H₂C₃N₃O₃)F with blue emission around 482 nm.²⁴ Besides, C₄N₃H₆SO₃NH₂ with a π-conjugated [C₄N₃H₆] group also shows blue-violet and green fluorescence near 360 and 520 nm,²⁵ thus indicating that the introduction of a large π-conjugated unit could be a feasible strategy to design multifunctional crystals with concurrent optical nonlinearity and photoluminescence.

Following this design principle, we focused on a large π-conjugated group, (C₆N₇O₃)³⁻, as a building unit for multifunctional optical crystals. Compared to previously studied π-conjugated units, such as (BO₃)³⁻ (conjugated electronic configuration π³₄), (CO₃)²⁻ (π⁴₄), (B₃O₆)³⁻ (π⁹₉), and (C₃N₃O₃)³⁻ (π⁹₉), the (C₆N₇O₃)³⁻ unit possesses the ultra-large π-conjugated electron configuration π¹⁷₁₆, indicating its strong p_π–p_π conjugated interaction and delocalized electron distribution on the planar ring. In 2022, we synthesized the first SHG active alkali cyamelurate, K₃C₆N₇O₃·2H₂O, which exhibited excellent nonlinear optical properties (SHG ∼4 × KDP, Δn_cal > 0.446).²⁶ Subsequently, iso-cyamelurates (H_xC₆N₇O₃)^x−3 (x = 1, 2) were also explored as a promising candidate with both enhanced SHG efficiency and birefringence, including A_0.5H₂C₆N₇O₃·4H₂O (A = Ca²⁺, Sr²⁺, Ba²⁺) (SHG > 5 × KDP, E_g ∼4.0 eV), Cd(H₂C₆N₇O₃)₂·4H₂O (SHG ∼1.4 × KDP, E_g ∼4.0 eV) Sr[H₂C₆N₇O₃]₂·4H₂O, and Er(C₆N₇O₃)·5H₂O.^27–31 However, the integration of strong optical nonlinearity, broadband photoluminescence, and magnetic response has never been realized in the cyamelurate system.

Here, we propose a ‘unit assembly’ strategy to design multifunctional crystals, which combines the active metal ion and π-conjugated [H_xC₆N₇O₃]^(3−x)– unit into rare-earth cyamelurates. The rare-earth ions are considered to enrich the optical response of cyamelurate materials in the following ways: (i) Closed-shell configurations and half-filled 4f orbitals of rare-earth cations (Y³⁺, La³⁺, and Lu³⁺) can inhibit d–d and f–f electronic transitions, expanding the cutoff edge from the visible to the ultraviolet (UV) region and thus broadening the band gap. (ii) Rare-earth cations usually exhibit strong coordination properties, favoring the alignment of ligand anions in non-centrosymmetric arrangements. (iii) Rare-earth ions may impart intriguing magnetic behavior originating from the 4f electronic configuration. In this work, Y³⁺, Gd³⁺, and Lu³⁺ are regarded as key elements to fabricate new SHG-active cyamelurate crystals. Fortunately, three new isostructural rare earth cyamelurates Y(H_1.5C₆N₇O₃)₂·10H₂O (I), Gd(H_1.5C₆N₇O₃)₂·10H₂O (II), and Lu(H_1.5C₆N₇O₃)₂·10H₂O (III) were successfully designed and synthesized by a feasible aqueous solution-cooling method, with strong SHG intensities (>12 × KDP), wide fluorescence, sufficient birefringence (Δn ∼0.22 at 1064 nm), and paramagnetic behaviour over a wide temperature range in II. The structure–property relationships were investigated through theoretical calculations.

Results and discussion

Crystal structure

Colourless plate-like crystals of the three compounds were obtained, and XRD diffraction (Fig. S1†) was used to confirm the purity of the sample powder. The three RE(H_1.5C₆N₇O₃)₂·10H₂O (RE = Y, Gd, Lu) compounds are isostructural, and all the detailed structural information, additional bond lengths, and bond angle information are listed in Tables 1 and S1–S12.† Herein, only compound II is illustrated as an example. Gd(H_1.5C₆N₇O₃)₂·10H₂O (II) crystallizes in the acentric orthorhombic Fdd2 space group (No. 43). As shown in Fig. 1a, the Gd³⁺ cation is coordinated with eight O atoms, of which the Gd–O distances range from 2.34–2.48 Å. Among the eight coordinated O atoms of the Gd–O polyhedron, the two contributed planar iso-cyamelurate [H₂C₆N₇O₃]⁻ and [HC₆N₇O₃]²⁻ anion groups are at the angle of ∼75.74°, and the dihedral angle is slightly different in I and III with values of 76.02° and 76.25°, respectively. The stacking pattern of the Gd³⁺ cation is shown in Fig. 1b, and the distances between Gd⋯Gd interactions are measured as 6.15 Å and 6.58 Å, respectively. Fig. 1c displays the packing diagram with coordinated Gd polyhedra, and the vertical distance between the Gd⋯Gd plane along the c-axis is 5.82 Å, while the nearest anion layers are at a distance of 3.09 Å. Interestingly, as the ligand anions are connected on the two sides of the Gd³⁺ cations like the wings of a butterfly, the crystal structure is like an orderly arrangement of butterfly teams (Fig. 1d).

Table 1 Crystal data and structure refinements of RE(H_1.5C₆N₇O₃)₂·10H₂O (RE = Y, Gd, Lu)

	Y(H1.5C6N7O3)2·10H2O	Gd(H1.5C6N7O3)2·10H2O	Lu(H1.5C6N7O3)2·10H2O
a R1 = ∑‖Fo| − |Fc‖/∑|Fo|, ωR2 = [∑ω(Fo^2 − Fc^2)^2/∑ω(Fo^2)^2]^1/2.
Formula weight	708.35	776.69	794.41
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic
Space group	Fdd2	Fdd2	Fdd2
a (Å)	67.056(8)	67.4247(9)	66.982(3)
b (Å)	6.1193(7)	6.14740(10)	6.1537(2)
c (Å)	11.4807(13)	11.6319(3)	11.6511(4)
α (deg)	90	90	90
β (deg)	90	90	90
γ (deg)	90	90	90
V/Å^3	4710.9(9)	4821.27(18)	4802.4(3)
Z	8	8	8
ρcalc (g cm^−3)	1.997	2.140	2.197
μ (mm^−1)	2.589	2.862	4.222
F (000)	2880	3080	3136
Flack parameter	0.01(2)	0.04(4)	0.02(3)
Goodness-of-fit on F^2	1.093	1.133	1.146
Reflections collected	27 752	33 858	4777
Final R indexes [I ≥ 2σ(I)] ^a	R1 = 0.0728, ωR2 = 0.2150	R1 = 0.0566, ωR2 = 0.1731	R1 = 0.0492, ωR2 = 0.1496
Final R indexes [all data] ^a	R1 = 0.0801, ωR2 = 0.2251	R1 = 0.0645, ωR2 = 0.1808	R1 = 0.0532, ωR2 = 0.1538
CCDC no.	2385148	2385149	2385150

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Fig. 1 (a) The coordination environment of Gd³⁺; (b) the stacking diagram of Gd³⁺ cations; (c) the stacking pattern of Gd³⁺ polyhedra; and (d) the crystal structure of II. The olive-green, red, gray, blue, and light gray balls represent Gd, O, C, N, and H, respectively.

Chemical phase and thermal stability

Fig. S2† displays the results of thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis of compounds I, II, and III. There are generally two weight loss steps in the process of heating, which correspond to the loss of crystalline water starting from 109 °C and sample decomposition occurring around 230 °C. Besides, the powder XRD diffraction confirmed that the sample can match the calculated diffraction peaks after six months in a dryer, indicating the stability of the three hydrates (Fig. S3†).

Linear and nonlinear optical properties

The IR spectrum (Fig. S4†) indicates the presence of the cyamelurate group, in which the absorption peaks at 1654, 1516, 1406, and 1149 cm⁻¹ can be attributed to aromatic C-N heterocycles, while bending the vibrational mode of s-triazine units can be assigned to peaks at 809 cm⁻¹.³² The UV-vis transmission spectrum of compound II was measured with a transparent crystal plate (Fig. 2a), and the cutoff edge is estimated as 300 nm, corresponding to the band gap of 4.1 eV (Fig. 2b). The diffuse reflection spectra of the three compounds are nearly the same, also indicating the enhanced band gaps ∼ 4.0 eV (Fig. S5†), which are far larger than previously reported alkali cyamelurate of K₃(C₆N₇O₃) and K₃C₆N₇O₃·2H₂O, E_g ∼3.0 eV.³³ The SHG response of the three compounds was measured by the Kurtz–Perry method³⁴ under 1064 nm incident light. Samples of I, II, and III respectively display very strong SHG efficiencies of 13.9, 15.3, and 12 × KDP with phase-matching behavior (Fig. 2c), which is the largest among reported metal cyamelurates, such as Ba(H₂C₆N₇O₃)₂·8H₂O (12 × KDP) and A_0.5H₂C₆N₇O₃·4H₂O (A = Ca²⁺, Sr²⁺) (>5 × KDP). Moreover, all three compounds show a broadband UV photoluminescence around 350 nm under the excitation wavelength of 288 nm (Fig. 2d and S6†). The emission spectra exhibit that the full width at half maximum (FWHM) is > 100 nm for compounds I and II, which is apparently wider than that previously reported in A₃(C₆N₇O₃) (A = Li, Na, K, Rb, Cs) (FWHM ∼50 nm), K₃C₆N₇O₃·2H₂O (∼60 nm), Ca_0.5H₂C₆N₇O₃·4H₂O (∼82 nm), and K_0.5In_0.5(H₂C₆N₇O₃)₂·9H₂O (∼88 nm).^35,36 Such wide photoluminescence could be attributed to the reduced bandgap of the cyamelurate unit and the possible electron-phonon coupling effect between rare-earth ions and their surrounding lattices.

Fig. 2 (a) The grown crystal of II. (b) UV-vis-NIR transmittance spectrum of II. (c) SHG measurements of the three compounds. (d) Photoluminescent measurements of II. (e) χ versus temperature of II for FC data (green symbol line) and the plot of 1/χ versus temperature (light green symbol line). (f) Magnetic susceptibility as a function of the applied field for the II samples with Gd.

Magnetic properties

The temperature-dependent magnetic susceptibility (χ) and its inverse (1/χ) for compound II are presented in Fig. 2e. The magnetic susceptibility data of compound II is fitted to a modified Curie–Weiss (C–W) law: χ = C/(T − Θ), where C is the Curie constant and Θ is the Weiss temperature. As shown, compound II obeys the C–W law and is paramagnetic. The effective magnetic moment μ_eff is 6.97 μ_B for the high-temperature region (100–280 K), which matches the theoretical value for Gd³⁺. For the low-temperature range (2–10 K), the derived value of Θ for compound II is −0.54 K, which suggests weak magnetic exchange interactions among the adjacent Gd³⁺ cations.^37–39 The magnetization saturates around 5 T when the temperature is 2 K in compound II (Fig. 2f). Combining the strong optical nonlinearity, ultraviolet fluorescence, and paramagnetic behaviour, the Gd(H_1.5C₆N₇O₃)₂·10H₂O (II) crystal is expected to be a promising multi-functional material in optoelectronic applications.

Theoretical calculations

To explore the origin of the second-order nonlinear optics, the electronic structure of compound II was calculated. As shown in Fig. 3a, it is a direct-gap semiconductor with a spin-up gap of 2.71 eV and a spin-down gap of 1.67 eV. Furthermore, the spin-up density of states (DOS) and partial density of states (PDOS) of II are analyzed. Fig. 3b depicts that the contribution from crystalline water molecules is negligible, and the valence band maximum and the conduction band minimum were primarily occupied by N 2p orbitals together with O 2p and C 2p orbitals, indicating the contribution of conjugated π-bonds. Interestingly, not only unoccupied C p, N p, and O p orbitals but also Gd d and f orbitals contribute to the bottom of the conduction band, corresponding to the clear participation of the rare earth Gd in the construction of energy band structures. Therefore, it should be the synergistic effect of the rare-earth ion and the large π-conjugated group that determines the optical response of Gd(H_1.5C₆N₇O₃)₂·10H₂O. For the Y and Lu-analogues, their band structure and PDOS were plotted in Fig. S7 and S8.†

Fig. 3 (a) Calculated band structure of II and (b) DOS and partial DOS of II. (c) The calculated birefringence of II. (d) The original and the extinction state of the crystal plane (001) of II.

Furthermore, the calculated optical anisotropy of compound II exhibits a large variation from the visible to near-IR region with n_z > n_y > n_x. As shown in Fig. 3c, the calculated birefringence n_z – n_x is 0.22 at 1.06 μm, which is comparable to the commercialized birefringent crystal α-BBO (Δn ∼0.12) and calcite (Δn ∼0.17).^40,41 The calculated refractive index of I and III was given in Fig. S9† and the Δn_cal is around 0.2. In addition, the in-plane birefringence of single crystals I, II, and III was estimated experimentally by using orthogonal polarization microscopy with a halogen lamp.⁴² The measured values of I, II, and III are all around 0.20 (Fig. 3d and S10†), which also agree well with the calculated values. This suitable optical anisotropy could be due to the large π-conjugated interaction of the cyamelurate (C₆N₇O₃)³⁻ groups.

Moreover, Fig. 4 displays the reported SHG intensities of the compounds containing a single kind of FBUs of large π-conjugated (B₃O₆)³⁻,^43–47 (C₃N₆)⁶⁻,^48–50 (C₃N₃O₃)³⁻,^51–55 and (C₄N₂O₃)²⁻ groups,^56,57 and large π-conjugated groups of [C₆N₇(NCN)₃]³⁻,⁵⁸ [C₆N₇(NH)₂],⁵⁹ and (C₆N₇O₃)³⁻.^26–31 The commercialized NLO crystal BBO was plotted as a benchmark crystal with a strong SHG response of 5.6 × KDP. As classical (B₃O₆)-type units, the contribution of the FBUs could favor the SHG response reaching up to 5.6 × KDP for recently discovered borate of CsYB₃O₆F, 5.3 × KDP for (iso)-cyanurates of KLi(HC₃N₃O₃)·2H₂O/RbNa(HC₃N₃O₃)·2H₂O, and 10.8 × KDP for the barbiturate of Li₂(H₂C₄N₂O₃)·2H₂O. In contrast, the compounds with large π-conjugated FBUs, such as [C₆N₇(NCN)₃]³⁻, [C₆N₇(NH)₂] and (C₆N₇O₃)³⁻, generally exhibit larger SHG effects with a greater possibility than metal borates/cyanurates (>4 × KDP).

Fig. 4 SHG response of the compounds containing six-member ring B₃O₆-type π-conjugated groups and large C₆N₇-type π-conjugated groups.

Among the large π-conjugated FBUs constructed compounds, only Cd(H₂C₆N₇O₃)₂·4H₂O displays small SHG signals < 2 × KDP, while A₃[C₆N₇(NCN)₃]·8H₂O (A = Rb, Cs)/Ba(H₂C₆N₇O₃)₂·8H₂O can even reach the SHG intensities around 10 × KDP. Remarkably, benefiting from the synergistic effects of the large π–conjugated (C₆N₇O₃)³⁻ group and rare earth metals, compounds I, II, and III exhibit extraordinary second-order harmonic generation (SHG) properties (13.9, 15.3, 12 × KDP, respectively) far beyond those reported previously for π-conjugated NLO compounds. To the best of our knowledge, this represents the strongest SHG effect among all π-conjugated NLO crystals, indicating the great contribution of (H_xC₆N₇O₃)^3−x units for strengthening the nonlinear optical response.

Conclusions

Three new cyamelurate crystals of RE(H_1.5C₆N₇O₃)₂·10H₂O (RE = Y, Gd, Lu) containing large π-conjugated orbitals were successfully synthesized. The three compounds displayed very strong SHG activity, sufficient birefringence, paramagnetic behaviour in the Gd-analogue, and broadband ultraviolet photoluminescence concurrently, suggesting that this series of compounds could be promising multi-functional optical materials. We believe this work provides a feasible approach for designing novel high-performance (iso)-cyamelurates and enriching new multi-functional integrated optoelectronic crystals, pointing to applications such as self-frequency doubling, magneto-optical switches, magnetoelectric coupling, and broadband fluorescent response.

Data availability

All data are available in the article and the ESl† or can be made available from the corresponding authors upon reasonable request. CCDC deposition numbers 2385148–2385150 (for compounds I, II, and III) contain the supplementary crystallographic data for this paper. By emailing E-mail: data_request@ccdc.cam.ac.uk, or by contacting the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK: fax: +44 1223 336033.

Author contributions

Jing Zhang: Investigation, writing – original draft; Yuxiao Liu: Investigation; Fangyan Wang: Investigation; Pifu Gong: Investigation; Zhaoyi Li: Investigation; Xinyuan Zhang: Investigation, writing-review & editing; Fei Liang: Theoretical calculations, writing-review & editing; Shu Guo: Investigation, formal analysis; Zhanggui Hu: Supervision; Yicheng Wu: Supervision.

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, 52422201, 52372010, 22275201, and 22205091), National Key Research and Development Program of China (2021YFA0717800), and Natural Science Foundation of Shandong Province (ZR2023ZD53).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, crystallographic data, PXRD patterns, thermal analysis, IR spectrum, UV–visible–NIR diffuse reflectance spectrum, and band structures. CCDC 2385148–2385150. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc01230a

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