Two solvent-dependent Al₁₆ nanorings: design, synthesis and nonlinear optical limiting behavior

Abstract

Cyclic compounds are of great interest to chemists for their adjustable organic shells and inorganic cores. Although the abundant bridges provide a bottom-up synthesis approach for the diversity and functionalization of ring compounds, the lack of suitable models still limits us in exploring the influence of bridge types on ring structures. Herein, we demonstrate two Al₁₆ molecular nanorings synthesized via 2 × 8 and 4 × 4 strategies based on the “ligand-induced aggregation and solvent regulation” strategy and discuss the influence of surface bridge flexibility on molecular ring distortion and curvature. Rigid phenol endows AlOC-135 with unique intramolecular π⋯π interactions and higher curvature to increase the delocalization and transfer of electrons, resulting in a superior third-order nonlinear optical (NLO) response. This work provides a platform for explaining the relationship between the structure and optical properties of ring compounds at the molecular level.

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Introduction

Macrocyclic molecules have attracted widespread attention due to their unique symmetry, adjustable supramolecular stacking, and central cavity for guest inclusion.^1–3 Organic–inorganic hybrid macrocycles can be directionally assembled and functionalized, enabling expansion of the applications in specific fields by altering the composition of the organic shell and inorganic metal core.^4–6 In recent decades, numerous innovative hybrid molecular ring models have emerged, such as the transition metal molecular rings Ti₃₂, Fe₃₆, Zr₇₀, Mn₈₄, Mo₁₅₄ and Mo₁₇₆.^7–12 Another big family is rare earth molecular rings such as Ln₄₂ (Yb₄₂ and Sm₄₂) bridged by acetoxy, Ce₇₀ with a sulfate linker, and the Gd₁₄₀ ring linked by 80 acetic acid molecules, 40 myo-inositol and 20 CO₃²⁻ anions.^13–15 Furthermore, heterometallic 3d–4f molecular rings are also the stars of hybrid molecular rings, such as Cr₄Dy₄, Sc₈Gd₈, and Mo₁₂₈Ce₄ with modified delta-chain structures, which are linked via benzoates.^16–18 These macrocyclic molecules exhibit distinct benefits in the optics, magnetism and host–guest chemistry domains.^19–23 Yang et al. reported a {Nd₁₈} molecular ring exhibiting NIR luminescence sensing of antibiotics.²⁴ Zheng et al. synthesized the {Cr₈Dy₈} wheel, which is a single-molecule magnet.²⁵ Sun et al. reported the host–guest catalysis and self-assembly of Pd₂L₂ constructed from a pyridine-bonded macrocyclic ligand (L) and Pd²⁺.²⁶ The excellent performance of macrocyclic molecules in these fields encourages us to further develop more cyclic compounds.

Linkers are essential components of cyclic compounds and can be divided into three main groups (Scheme S1†): (a) Chelating entirely by organic linkers with coordination sites such as O_carboxyl, OH, S, and N. For example, Zhang et al. used N,N′-di[1-(2-hydroxyphenyl)-ethylidene]hydrazone ligand and DMF (N) as linkers to synthesize Ni₁₂.²⁷ Milios et al. synthesized Co₁₆ by salicylaldehyde bridges.²⁸ Besides, Baca et al. constructed a heterometallic ring Fe₆Dy₈ with N-butyl diethanolamine isobutyrate and methanol (O_hydroxyl).²⁹ Yamamoto et al. reported a series of [Pt(μ-SC₈H₁₇)₂]_n (n = 5–13) clusters with the S atom as the connection point.³⁰ (b) Bonding entirely with inorganic bridges. Su et al. synthesized [Nb₄₈V₈(OH)₃₀O₁₃₀]¹⁸⁻ using μ₂-OH⁻ as linkers.³¹ Jacobson et al. and Wang et al. reported [M₈]¹⁶⁻ (M = Cr and Fe) and [Ti₈]¹⁶⁻ with μ₂-OH and μ₂-SO₄ bridges.^32,33 (c) The cooperation between organic connectors and inorganic components constitutes a ring structure. Zhao et al. reported on an Ln₇ ring constructed from 1,1′-cyclopropane-dicarboxylic acid and μ₃-OH inorganic linkers (Ln = Gd and Dy).³⁴ Zheng et al. synthesized Ti₃₂ constructed from acetic acid and μ_n-O (n = 2 or 3) linkers.³⁵ Christou et al. used triethylamine and μ₃-OH to synthesize an Fe₃₆ cluster.¹² Abundant organic/inorganic linkers provide a bottom-up synthesis approach for the diversity of molecular ring types and functionalization. However, exploring the influence of linker types on the ring structures is challenging.

Our group has been dedicated to the isolation of Earth-abundant element related aluminum-based molecular rings and developed the “ligand-induced aggregation and solvent regulation” strategy toward customizing ring compounds.^36,37 In previous research, we successfully expanded the Al₈ ring to the Al₁₆ ring by tuning the carbon chain length of the flexible primary alcohols and developing the Al₂₀ ring using a rigid phenol linker.^38,39 As a continuous work, we herein report two rings with the same nuclearity but different degrees of conjugation and study their optical properties. Considering that a ligand with a high degree of conjugation will facilitate photo-related properties, we introduced 2-naphthoic acid (2-NA) instead of the previously used benzoic acid. To investigate the effects of different auxiliary ligands (solvents) on the ring structure, we chose flexible n-propanol and rigid phenol (melting point 43 °C). With this in mind, we successfully isolated two Al₁₆ molecular nanorings constructed using a 2-NA ligand but with different auxiliary bridges. The introduction of rigid phenol brings intramolecular π–π interactions, increasing the ring curvature and further influencing the intermolecular stacking, resulting in photoluminescence and nonlinear optical (NLO) changes. This study provides a platform for explaining the structure–function relationship of macrocyclic molecules at the molecular level.

Results and discussion

Synthesis discussion

2-Naphthoic acid (2-NA) plays an essential role in inducing the aggregation of Al³⁺ to form molecular rings, while phenol can be viewed as a solvent and auxiliary bridge like n-propanol. A transparent square block crystal [Al₁₆(OⁿPr)₁₆(μ₂-OH)₈(2-NA)₂₄] (AlOC-134) was prepared through the one-step self-assembly method by heating Al(OⁱPr)₃ and 2-naphthoic acid (2-NA) under the regulation of a methylaminoethanol solution and hexamethylenetetramine in a mixture of n-propanol, 1,4-dioxane and a DMF solution. When we directly added 6 mL phenol and increased the reaction temperature (100 °C to 120 °C), a light yellow oval shaped block crystal [Al₁₆(phenol)₂₀(μ₂-OH)₈(2-NA)₂₀] (AlOC-135) was successfully synthesized (Fig. 1). Although AlOC-134 and AlOC-135 are both 16-membered molecular rings, they exhibit different linkage modes (2 × 8 and 4 × 4 strategy).

Fig. 1 “Ligand-induced aggregation and solvent regulation” synthesis of AlOC-134 and AlOC-135.

Aluminum molecular rings supported by flexible n-propanol (2 × 8 strategy)

Single-crystal X-ray diffraction (SCXRD) showed the compound AlOC-134 crystallizing in the monoclinic space group C2/c with a total diameter of 3.1 nm, a height of about 1.9 nm, and a central hole of 0.5 nm (Fig. 2a and c). The entire molecular ring includes 16 Al³⁺ ions, 24 2-NA ligands, 16 ⁿOPr, and eight bridged hydroxyl connections (Fig. 2a). Each aluminum ion adopts a six-coordinated octahedral configuration with three carboxylate oxygens (O_COO) from three 2-NA ligands, two hydroxyl oxygens from four ⁿOPr ligands and one μ₂-OH (Fig. S1†), which is similar to the previously reported coordination mode of the Al₁₆ ring bridged by alcohol.³⁸ In AlOC-134, the 2-NA ligand adopts the μ₂–η¹:η¹ coordination mode linking two neighbouring Al³⁺ ions to form an edge-sharing [Al₂(ⁿOPr)₂(2-NA)₃] (Al₂) dimer (Fig. 2b and S2†). Eight Al₂ dimers are connected with eight μ₂-OH to form a coronal configuration ring through the vertex sharing mode (2 × 8, Fig. S3†).

Fig. 2 Structure of AlOC-134 and AlOC-135. (a) The space-filling view of AlOC-134 (total diameter of 3.1 nm and central hole of about 0.5 nm); (b) the ball-and-stick view of the 2 × 8 ring of AlOC-134 (Al₂ connected with μ₂-OH); (c) the side view of the corona-shaped ring of AlOC-134 (presenting the intramolecular hydrogen bond interactions); (d) nanotube stacking diagram of AlOC-134; (e) the space-filling view of AlOC-135 (total diameter of 2.7 nm and central hole of about 0.4 nm); (f) the ball-and-stick view of the 4 × 4 ring of AlOC-135 (Al₄ connected with μ₂-OH); (g) the side view of the corona-shaped ring of AlOC-135 (presenting the intramolecular hydrogen bond interactions and π⋯π interaction); (h) nanotube stacking diagram of AlOC-135; and (i) curvature changes of the aluminum molecular ring; currently AlOC-135 has the largest curvature aluminum molecular ring. Color code: Al, green; O, red; C, black; H, grey; π⋯π interaction, blue dashed line in (g); and intramolecular hydrogen bonds, orange dashed lines in (c) and (g).

Aluminum molecular rings supported by rigid phenol (4 × 4 strategy)

When we replace the flexible n-propanol connector with rigid phenol, the Al₁₆ crown-core is transformed into a twisted saddle-core (AlOC-135). AlOC-135 crystallizes in the tetragonal space group P4₂/n with a slightly smaller diameter (2.7 nm), a central hole of 0.4 nm and a height of about 2.3 nm (Fig. 2e and g). AlOC-135 consists of 16 Al³⁺ ions, 20 NA⁻ ligands, 20 phenols and eight bridged hydroxyl groups (Fig. 2e). As shown in Fig. S1,† all Al³⁺ ions in this Al₄ tetramer have a distorted octahedral geometry: for Al₁, the coordination oxygen comes from two O_COO and four phenols; for Al₂, the coordination comes from two O_COO, three phenols, and one μ₂-OH; and for Al₃, the coordination comes from three O_COO, one phenol, and two μ₂-OH. Four 2-NA ligands adopt the same mode bridging bidentate μ₂-linking four neighbouring Al³⁺ ions to construct an edge-sharing [Al₄(phenol)₅(2-NA)₅] tetramer (Fig. 2f and S2†). Four independent μ₂-OH connect two adjacent Al₄ tetramers to form a saddle-shaped Al₁₆ ring (4 × 4, Fig. S3†) with a bigger bending degree.

Ring curvature discussion

We tried to introduce rigid ligands with larger sizes to adjust the structural curvature, such as benzyl alcohol, 1-naphthol and 2-naphthol. Unfortunately, no related compounds were isolated, which may be attributed to their larger steric hindrance. The degree of curvature of the AlOC-134 (crown-type) is caused by significant intramolecular hydrogen bond interactions (C–H⋯O, 2.691 Å to 3.288 Å) (Fig. 2c and Table S3†). Compared to AlOC-134 and previous aluminum molecular rings, AlOC-135 exhibits the most significant curvature and a rare saddle structure (Fig. 2i).^38–44 In solvent systems containing conjugated ligands, phenols with stronger polarity can easily interact with them via π⋯π interaction, thereby distorting the Al₁₆ nucleus. This is also the reason why the amount of the linker phenol (20) is greater than that of n-propanol (16). Besides, the saddle-shaped conformation provides stronger supramolecular interactions within AlOC-135, manifested in strong hydrogen bonding interactions (C–H⋯O, 2.722–2.831 Å, Table S4†) and intramolecular π⋯π stacking interactions between phenols (4.549–4.562 Å, Fig. S4†). The two factors combine to cause the large curvature of AlOC-135. In both structures, all nanorings are connected to adjacent molecular rings by rich π⋯π interactions (4.129–4.869 Å, Fig. S4 and Table S5†) to form a dense array of supramolecular nanotubes. The Al–O bond lengths of two Al₁₆ rings fell within the range of 1.837–1.943 Å (Table S2†), comparable with those reported in the literature.^38,40,45 The conjugated system is instrumental in exploring the application of the two compounds in optics.

Characterization

The experimental PXRD peaks of AlOC-134 and AlOC-135 match well with the simulated peaks based on their space groups and cell parameters obtained using Berrar–Baldinozzi refinement. Negligible difference curves (R_wp and R_p values less than 5%) demonstrate the high purity of synthesized crystals (Fig. 3a and b). The energy dispersive spectroscopy (EDS) (Fig. S5 and S6†) and Fourier transform infrared spectroscopy (FT-IR) demonstrate the introduction of ligands (Fig. S7 and S8†). The vibration at 3054 cm⁻¹ (AlOC-134) and the vibration at 3058 cm⁻¹ (AlOC-135) are assigned to the C–H vibration of the aromatic ring. The absorption band of 3618 cm⁻¹ (AlOC-135) is attributed to the ν(O–H) stretching on phenol; the bands at 1550 cm⁻¹ (AlOC-134) and 1556 cm⁻¹ (AlOC-135) can be ascribed to –CO₂⁻ asymmetric stretching. The bond valence sum (BVS) calculation shows that all Al ions have a +3 oxidation state (Tables S6 and S7†). The contact angle test reflects their excellent hydrophobicity and stability (Fig. S9†). Their thermal stability was evaluated by thermogravimetric analysis (TGA) (Fig. S10 and S11†). The solvent and air stability of the compounds are demonstrated by PXRD under different conditions, single crystal diffraction patterns and crystal pictures (Fig. 1 and S12–S14†). The excellent stability limited the dissolution of the compound in the organic solvent, which prevented further solution stability studies such as mass spectrometry.

Fig. 3 (a) Simulated PXRD patterns (blue) and experimental PXRD patterns (red) of AlOC-134. (b) Simulated PXRD patterns (blue) and experimental PXRD patterns (red) of AlOC-135. (c) UV–vis diffuse reflectance spectra of AlOC-134 and AlOC-135 (insets are solid-state crystal samples). (d) Room-temperature photoluminescence spectra of AlOC-134 and AlOC-135 (inset is solid-state crystal samples under 365 nm UV light; λ_em is measured at an excitation wavelength of λ_ex = 300 nm).

Luminescence properties

The large π-conjugated systems and abundant supramolecular forces encouraged us to investigate their optical properties. We first explored the light absorption properties of AlOCs by measuring the UV-vis diffuse reflectance spectra. As shown in Fig. 3c, the results show that the absorption spectra of both AlOC-134 and AlOC-135 exhibit broad profiles ranging from 250 nm to 450 nm (Fig. S15 and S16†), which are similar to the absorption bands of 2-NA ligands (Fig. S17†). Subsequently, the fluorescence properties of the solid-state samples were measured at room temperature. As shown in Fig. 3d and S18,† the maximum emission peaks show a red shift (28 nm for AlOC-134, 3 nm for AlOC-135) at the same excitation wavelength (300 nm). Besides, AlOC-134 has a longer lifetime and higher quantum yield (9.76 ns, 17.87%) than AlOC-135 (2.57 ns, 7.85%) (Fig. S19 and S20†). The comparative emission spectra show that the luminescence of the nanoring can be attributed to the coordination of the 2-NA ligand with inorganic aluminum atoms.⁴⁶ The difference in their emission intensity may be related to the quantity of the 2-NA ligands (24 in AlOC-134vs. 20 in AlOC-135) (Fig. 3d).

Third-order nonlinear optical (NLO) properties

Third-order nonlinear optics has shown potential applications in fields such as laser protection and optical switches.^47,48 The large conjugated system of Al₁₆ rings encouraged us to combine crystalline materials with third-order NLO properties. The hydrophobic shell of the ring makes it less soluble and dispersible in solvents. Thus, we chose polydimethylsiloxane (PDMS, one chemically inert material with transmittance and excellent flexibility) as a solid dispersion substrate and made flexible AlOCs@PDMS films (30 mm in diameter).^49,50 All sample films had similar thicknesses and high linear transmittance (inset pictures in Fig. 4a). Open aperture (OA) Z-scan measurements were performed on solid samples under a nanosecond laser at 532 nm and a pulse energy at 80 μJ (Fig. 4a). As shown in Fig. 4b, the Z-scan curves of AlOC-134 and AlOC-135 display typical reverse saturation absorption (RSA) responses. The minimum normalised transmittance (T_min) values at the focal point were 0.84 and 0.79 for AlOC-134 and AlOC-135, respectively. Such responses were reproducible and presented consistent T_min values, showing that the prepared samples featured excellent chemical and physical stability (Fig. S21 and S22†). According to eqn (4) and (5), the light fluence F_in(z) at the corresponding position z is calculated from the input laser pulse energy E_in. To compare the optical limiting (OL) performance, we extracted the representative OL parameter values from OL curves. Their optical limiting threshold values (F_OL, defined as the input fluency at 50% linear transmittance) were 3.98 J cm⁻² (AlOC-134) and 0.62 J cm⁻² (AlOC-135) (Fig. 4c). The curves of output fluence versus input fluence of such samples showed that the output fluence linearly increased at low-incident fluence. At high-incident fluence, the output fluence deviated from linearity and showed a typical OL response (Fig. 4d). The nonlinear absorption coefficient (β) (eqn (1)–(3)) of AlOC-134 and AlOC-135 was 22 cm GW⁻¹ and 41 cm GW⁻¹, respectively. Usually, higher α and β values and lower OL values are key parameters for considering optical limiting performance. These values are not as good as those of some porphyrins and phthalocyanine materials, such as 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyaninato (H₂TPPc) with detonation nanodiamonds in DMSO and single-walled carbon nanotube-5,10,15,20-tetraphenylporphyrin (SWN₇-TPP) in DMF, but superior to those of some known transition metal cluster materials, which include but are not limited to, KBr-based AgSn₁₂ cluster thin films, graphene oxide (GO) in DMF and [(Tp*WS₃Cu₃)₄(μ₄-S)₄(Ag)]OTf (3@PVBC) thin films (Fig. 4e and Table S9†).^51–55

Fig. 4 (a) Schematic representation of the Z-scan setup (inset is a picture of the PDMS substrate and AlOCs@PDMS under natural light); (b) the OA Z-scan curves of input laser pulse energy 80 μJ of AlOCs@PDMS; (c) the plots of normalized transmittance versus input fluence; (d) curves of output fluence versus input fluence; and (e) the plots of comparisons of the nonlinear absorption coefficients.

The optical limiting properties of molecular rings are closely related to the supramolecular interactions in the structure. As shown in Table S8,†AlOC-135 has larger β and Im χ⁽³⁾ (eqn (6) and (7)). To find their correlation, we compared the third-order NLO effects of aluminum molecular rings with different curvatures.^39,40 However, there is insufficient evidence to prove that increasing the curvature increases the NLO effect. In this work, the introduction of phenol not only resulted in significant intramolecular π⋯π interactions but also affected intermolecular stacking, which resulted in the saddle-shaped configuration of AlOC-135. The synergistic interaction created by a richer conjugation interaction may directly affect the surrounding environment of the ring and its electron density, increase the delocalization and transfer of electrons, and thus produce a more obvious OL response, providing a new path for the research of enhanced light-limiting materials.

Conclusions

In summary, we demonstrated the specific application of the “ligand-induced aggregation and solvent regulation” strategy in nanoring-type regulation. Flexible n-propanol can generate a greater number of the components in the rings (2 × 8 strategy, AlOC-134). Increasing the rigidity of the solvent can reduce the crystalline symmetry and produce larger but fewer bricks (4 × 4 strategies, AlOC-135). Furthermore, their luminescence and third-order NLO were studied. AlOC-134 containing more chromophores has a stronger emission intensity. Replacing n-propanol with phenol regulates AlOC-135 intermolecular stacking, increases structural curvature and provides more intramolecular π⋯π interactions. These characteristics increase the delocalization and transfer of electrons, resulting in a more pronounced NLO response. This work provides a platform for us to further understand the binding mechanism of molecular rings and their structural influence on properties at the molecular level.

Experimental

Solvothermal synthesis of (Al₁₆(ⁿOPr)₁₆(μ₂-OH)₈(2-NA)₂₄) (AlOC-134)

A mixture of Al(OⁱPr)₃ (153 mg, 0.75 mmol), 2-naphthoic acid (2-NA) (137 mg, 1.0 mmol), hexamethylenetetramine (50 mg, 0.35 mmol), methylamine solution in alcohol (50 μL), 1,4-dioxane (3 mL), n-propanol (5 mL), and DMF (2 mL) was sealed in a 20 mL vial and transferred to a preheated oven at 100 °C for 3 days. When cooled to room temperature, colorless transparent thin pieces of crystals were obtained (yield: 22% based on Al(OⁱPr)₃). FT-IR (cm⁻¹): 3058 (w), 2954 (w), 2360 (w), 1635 (m), 1556 (m), 1473 (m), 1421 (s) 1240 (w), 1070 (m), 965 (m), and 788 (s). The crystals were rinsed with n-propanol and preserved in a sealed and dry environment.

Solvothermal synthesis of (Al₁₆(phenol)₂₀(μ₂-OH)₈(2-NA)₂₀) (AlOC-135)

A mixture of Al(OⁱPr)₃ (550 mg, 2.7 mmol), 2-NA (200 mg, 1.16 mmol) and phenol (6 mL) was sealed in a 20 mL vial and transferred to a preheated oven at 120 °C for 3 days. When cooled to room temperature, colorless spheroidal crystals were obtained (yield: 13% based on Al(OⁱPr)₃). FT-IR: 3618 (w), 3054 (w), 1550 (m), 2360 (w), 1550 (m), 1481 (m), 1423 (s), 1224 (m) 1002 (w), 833 (m), and 769 (m) cm⁻¹. The crystals were rinsed with acetonitrile and preserved in a sealed and dry environment.

Caution: To avoid direct skin contact with corrosive phenol, we placed it at 80 °C for 60 minutes before use in the reaction as a solvent.

Single-crystal X-ray crystallography

Crystallographic data of AlOC-134 and AlOC-135 was collected on a Hybrid Pixel Array detector equipped with Ga Kα radiation (λ = 1.3405 Å) at about 298 K. The structures were determined by direct methods using Olex2 and refined with the full-matrix least-squares technique on F² using SHELXL. Non-hydrogen atoms were refined anisotropically.

Contact angle measurements

Contact angles were measured on powder samples using a contact angle meter with a rotatable substrate holder. 10 mg powder samples of the clusters were pressed using a glass slide to make a flat surface. A 20 μL water droplet was released slowly on the flat surface of the powder samples. Later, the droplet image was captured using a high-performance charge-coupled device (CCD) sensor. Five-point simulation analysis was used to perform an analysis of the contact angles of all the powder samples.

Fluorescence spectroscopy measurement

Photoluminescence spectra (PL) and luminescence decay curves were performed on an Edinburgh FLS1000 fluorescence spectrometer at room temperature. All test samples were solid crystalline particles (5 mg). Emission wavelength, quantum yield and fluorescence lifetime were all measured at maximum excitation.

Preparation of AlOCs@PDMS samples

Sylgard 184 (Dow Corning) is a kit product consisting of liquid A and B components, including basic components and a curing agent. PDMS samples were fabricated using Sylgard 184 (Dow Corning) by thoroughly mixing ten parts base with one part curing agent. The fully ground AlOC samples were added to the PDMS mixture and stirred for 3 h to form AlOC dispersed PDMS suspension. Then the mixed suspension was added to a round mould, and the template was placed in a vacuum oven at 60 °C for six hours to obtain transparent and flexible AlOCs@PDMS films. The thickness of the films was measured using a digital caliper (MNT951101 manufactured by Shanghai Minette).

Third-order NLO measurement

The NLO properties of the samples were evaluated using the open-aperture (OA) Z-scan technique. The excitation light source was an Nd:YAG laser with a repetition rate of 10 Hz. The laser pulses (period, 8.5 ns; wavelength, 532 nm) were split into two beams with a mirror. The pulse energies at the front and back of the samples were monitored using energy detectors D1 and D2. All of the measurements were conducted at room temperature. The measured Z-scan curves were fitted using the following expressions:

q₀(Z,0) = βI₀L_eff (2)

where T is the normalized transmittance, β is the nonlinear absorption coefficient, I₀ is the peak on-axis irradiance of the laser beam at the focus, L_eff is the effective thickness of the sample, α is the linear absorption coefficient at the laser wavelength , z is the Z-scan displacement, ε₀ is the permittivity of vacuum (8.85 × 10⁻¹² F m⁻¹), c is the speed of light and l denotes the thickness of the glass cell.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.† CCDC reference numbers 2289659 (AlOC-134) and 2289660 (AlOC-135) contain the X-ray crystallographic data.†

Author contributions

All authors contributed extensively to the work presented in this paper. J. Zhang and W.-H. Fang conceived the research project. S.-T. Wang and X. Qi performed the synthesis, characterization, fluorescence property analysis and third-order NLO properties analysis. R.-Q. Chen assisted with the data collection and analysis. W.-H. Fang, X. Qi and S.-T. Wang wrote the manuscript and the ESI with input from the other authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22371278 and 92061104), Funding of Fujian Provincial Chemistry Discipline Alliance, the Natural Science Foundation of Fujian Province (2021J06035) and the Youth Innovation Promotion Association CAS (Y2018081).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2289659 and 2289660. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi02008h

‡ These authors contributed equally to this work.

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