Solvent-controlled synthesis of an Al₁₂-oxo molecular ring and Al₂₄-oxo truncated metallo-cube

Abstract

Highly symmetrical molecules with beautiful geometries are ubiquitous in nature. It has inspired creative ideas of the perfect combination of geometry and molecular structural chemistry. Some metal clusters with regular geometric polyhedra have been reported, but such highly symmetric polyhedra for Al-oxo clusters are really scarce on account of the fast hydrolysis of Al³⁺ ions. Herein, a [Al₁₂(CH₃O)₂₄(NAP)₁₂]·4DMF·2H₂O·2CH₃OH (Al₁₂, NAP⁻ = 2-naphthoic acid) nanoring was synthesized by the solvothermal reaction of AlCl₃·6H₂O, 2-naphthoformic acid (HNAP) and triethylamine (Et₃N) in CH₃OH and DMF. Interestingly, the regulation from ring-shaped Al₁₂ to [Al₂₄(OH)₃₂(CH₃O)₂₂(CH₃OH)₂(NAP)₁₂]·6Cl·2H₂O·2CH₃OH (Al₂₄) metallocage is realized by only changing the reactive solvents. The Al₂₄ metallocage can be seen as one of 13 Archimedean polyhedra, a truncated cube composed of eight Al₃ triangles and six Al₈ octagons by sharing vertical Al³⁺ ions. In addition, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) reveals that the metallic skeletons of Al₁₂ and Al₂₄ can be maintained stable in CH₃OH and CH₂Cl₂. Furthermore, Al₁₂ and Al₂₄ emit blue luminescence and exhibit photocurrent responses under LED light illumination.

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Introduction

Highly symmetrical polyhedra have intrigued various professional researchers in the fields of mathematics, biology, aesthetics, architecture, and chemistry for thousands of years.^1–6 The classical polyhedra contain 5 platonic polyhedra constructed by one kind of regular convex polygon with the same number of faces at each vertex, and 13 Archimedean polyhedra composed of two or more types of regular polygons.⁷ In chemistry, these exemplifications have been found in coordination molecular nanocages and nanoclusters of coordination chemistry.⁸ For coordination nanocages, based on the construction rules of polyhedra, suitable organic ligands with specific configurations are often selected for their syntheses.^9–14 However, the nanoclusters assembled from simple ingredients are elusory and the acquisition of geometric structures lacks the presupposition. As a famous polyhedral cluster, C₆₀ is constructed by 12 pentagons and 20 hexagons, which is a classical Archimedean polyhedron, truncated icosahedron, inspiring various creative ideas for building fullerene-like nanopolyhedral metal clusters.¹⁵ After long-term unremitting efforts, transition/lanthanide metal clusters with Platonic and Archimedean polyhedra have been isolated, such as V₂₄,¹⁶ Co₃₂,¹⁷ Cd₆₆,¹⁸ Mo₂₄₀,¹⁹ Pd₁₄₅,²⁰ Ag₁₈₀,²¹ and Ln₁₀₄.²²

As the most abundant metal element in the earth's crust, Al clusters with lightweight and high stability have potential applications in environmental science, geochemistry, biology, optical, and adsorption materials.^23,24 Aqueous aluminum chemistry presents a series of typical aluminum oxo-hydroxo polycation aggregations, as exemplified by flat-Al₁₃ [Al₁₃(OH)₂₄(H₂O)₂₄]¹⁵⁺,^25,26 isomers of the Al₁₃-Keggin cluster [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺,^24,27 and larger Al₃₀ clusters [Al₃₀O₈(OH)₅₆(H₂O)₂₆]¹⁸⁺.^24,28 They were produced from hydrolytic processes of Al³⁺ ions in aqueous solutions, which is actually intricate and affected by olation reactions, formation of precursors, aggregation, nucleation and crystallization.^24,27 An effective route is to choose suitable trapping ligands, such as Al₃ and Al₈ protected by the trisilanol ligand,²⁹ a supramolecular zeotype Al₁₅ constructed using hpdta (H₅hpdta = HOCH₂[CH₂N(CH₂COOH)₂]₂).³⁰ Although many efforts have been made to investigate the aqueous aluminum chemistry of hydrolysis and polymerization of Al³⁺ ions, the relationship of solution equilibria and the existence of polynuclear species is still complex and unpredictable.³¹ For this dilemma, recently, a new strategy known as coordination delayed hydrolysis has been adopted by Zhang et al. to synthesize a series of Al-oxo clusters by choosing monodentate carboxylic acid ligands and organic aluminium salts under solvothermal conditions.^32–35 Most of these Al-oxo clusters feature the structures of molecular rings from Al₈ to the largest Al₂₀ constructed using different large conjugated carboxylic acids.^33,34,36 The different peripheral ligands realize luminescence modulation of Al₂₀ molecular rings from blue to green. Besides, other Al clusters also display luminescence properties, which originated from the luminescence of the corresponding ligands.³⁷ Nevertheless, highly symmetric aluminum-oxo clusters are rare because of the rapid hydrolysis of Al³⁺ ions and the difficult precipitation of high-quality and determinable single crystals.³⁶ Many efforts will be made to further explore the assembly and luminescence properties of aluminum-oxo clusters.

Based on these considerations, enlightened by the simple one-pot solvothermal synthesis method of transition/lanthanide metal clusters accompanied by adding alkali to control the hydrolysis of metal ions, a ring Al₁₂ dodecagon with the formula, [Al₁₂(CH₃O)₂₄(NAP)₁₂]·4DMF·2H₂O·2CH₃OH was obtained in CH₃OH–DMF by the reaction of monodentate carboxylic acid and inorganic aluminium salt with Et₃N to adjust the alkalinity of the solution. When using CH₃OH–CH₃CN as the solvent, a new Archimedean polyhedron Al₂₄, [Al₂₄(OH)₃₂(CH₃O)₂₂(CH₃OH)₂(NAP)₁₂]·6Cl·2H₂O·2CH₃OH was isolated. It features a truncated-cube metallocage containing 8 Al₃ triangles and 6 Al₈ octagons. This work successfully realizes the regulation of Al-oxo skeletons from molecular ring to metallocage. Moreover, the metallic cores of Al₁₂ and Al₂₄ have good stability in methanol and dichloromethane with the ligand exchange. Furthermore, the photocurrent response and the temperature-responsive luminescence behaviour of the Al₁₂ and Al₂₄ were investigated.

Experimental

Materials and physical measurements

Chemicals and solvents were purchased without further purification. On an ABB Bomen MB 102 series FT-IR (KBr pellets) spectrometer, IR spectra were recorded in the 4000–400 cm⁻¹ region. The elemental analyses were conducted on a Vario EL cube analyzer. Powder X-ray diffraction (PXRD) data were obtained on the Rigaku SmartLab X-ray diffractometer. Thermogravimetric (TG) curves were measured from ambient temperature to 800 °C using the SDT Q600 analyzer. A Puxi Tu-1901 spectrophotometer was used to measure UV-vis spectra. A Bruker Impact II high-definition mass spectrometer quadrupole and time-of-flight (Q/TOF) modules were used to record the mass spectra (HR-ESI-MS). The photocurrent experiments were performed on a CHI660E electrochemistry workstation. The 10 L of naphthol (0.5 wt%) and 5 mg samples of Al₁₂ or Al₂₄ were combined with 0.5 mL of ethanol and then subjected to a 30-minute sonication process. The coated film was produced by pipetting 150 μL solution onto the cleaned ITO glass after evaporation. A Pt wire served as the counter electrode, an Ag/AgCl electrode served as the reference electrode, and the produced ITO glass film served as the working electrode. The medium was an aqueous solution of Na₂SO₄ (0.2 M). An F-4700 fluorescence spectrometer was used to measure the room-temperature fluorescence. The Edinburgh spectrofluorometer (FLS980), with a cryostat to regulate temperature, was used to collect variable-temperature fluorescence data. After a 10-minute homeothermy, each data was collected and quantum yield data were obtained on the integrating sphere, and the luminescence lifetimes were determined on the same device using a time-correlated single-photon counting method (FLS980).

Synthesis of Al₂₄

A mixture of 2-naphthoformic acid (HNAP, 172.2 mg, 1 mmol), AlCl₃·6H₂O (500 mg, 2 mmol), were dissolved in a mixed solvent of CH₃OH (3 mL) : CH₃CN (8 mL) in a 23 mL Teflon-lined reaction vessel, then 0.5 mL triethylamine (Et₃N) were added into the mixture under stirring, finally kept at 100 °C for 4 days. When slowly cooled to room temperature, washed with CH₃OH, and dried in the air, colorless block crystals were obtained (yield 35.62% based on AlCl₃·6H₂O). Elemental analyses calcd. (found) for Al₂₄: C₁₅₈H₂₀₂Al₂₄Cl₆O₈₄: C, 44.08 (43.89); H, 4.73 (4.92)%. Selected IR peaks (cm⁻¹): 3450 (s), 3060 (w), 2950 (m), 2840 (w), 2670 (w),1610 (s), 1570 (m), 1480 (s), 1430 (s), 1070 (m), 982 (m), 870 (m), 796 (s), 594 (s), 501 (w).

Synthesis of Al₁₂

The Al₁₂ is similar to Al₂₄ except for CH₃OH : CH₃CN (11 mL, v : v = 3 : 8) was replaced by CH₃OH and DMF (10 mL, v : v = 1 : 1), colorless lamellar crystals were obtained (yield 40.56% based on AlCl₃·6H₂O). Elemental analyses calcd. (found) for Al₁₂: C₁₇₀H₁₉₆Al₁₂N₄O₅₆: C, 58.09 (57.87); H, 5.62 (5.78); N, 1.59 (1.62)%. Selected IR peaks (cm⁻¹): 3434 (s), 2964 (m), 2925 (m), 2856 (m), 2296 (w), 1629 (s), 1381 (m), 1080 (m), 1043 (m), 929 (w), 879 (w), 565 (w).

X-ray crystallography

Single crystals of Al₂₄ and Al₁₂ with the appropriate dimensions were selected under an optical microscope, coated fast into high vacuum grease, and put on a glass fiber for single-crystal data collection with Cu Kα radiation on the Rigaku XtaLAB MM007 CCD diffractometer. OLEX³⁸ and SHELXL-97 were used to directly solve the structures.³⁹ Based on the relevant atoms and refined with predetermined temperature factors, all hydrogen atoms were theoretically hydrogenated. To ensure that no extra symmetry could be given to the models, the Addsym function of PLATON⁴⁰ was used to check each structure. The SQUEEZE command was used to eliminate the disorganized solvent molecules. The Cambridge Crystallographic Data Centre (CCDC) has received the crystallographic information from this article (CCDC: 2163632 (Al₁₂); 2163633 (Al₂₄)).† Table S6† provides the pertinent crystallographic data. Table S7† displays the selected bond lengths and angles.

Results and discussion

Crystal structure of Al₁₂ cluster

Single-crystal X-ray diffraction analysis revealed that the Al₁₂ cluster crystallizes in the triclinic P1̄ space group and it contains 12 Al³⁺, 12 NAP⁻, and 24 CH₃O⁻ (Fig. 1a). Twelve Al³⁺ ions form a complanate dodecagon with the Al–Al distances in the range of 2.8621–2.8781 Å and every Al³⁺ ion is located on the vertex of a dodecagon. The diameter of the Al₁₂ metal ring is about 9.93 Å. The whole Al₁₂ cluster is stabilized by 12 NAP⁻ and 24 CH₃O⁻, which can be divided into three layers (Fig. 1b). The peripheral ligands of the middle layer contain six NAP⁻ ligands and six CH₃O⁻, which alternately array to form a large outer ring parallel to the Al₁₂ metal dodecagon (Fig. 1c). The inner part of the middle layer is filled with parallel six CH₃O⁻, in which three face up and three faces down. The upper and lower two ligand layers vertical to the Al₁₂ metal dodecagon are identical and every layer comprises three NAP⁻ and six CH₃O⁻ (Fig. 1d). Every two methanol molecules are sandwiched between two NAP⁻ ligands. All Al³⁺ ions of Al₁₂ cluster adopt six-coordinated distorted octahedral geometry with six O atoms from 2 NAP⁻ and 4 CH₃O⁻. The NAP⁻ and CH₃O⁻ respectively adopt μ₂–η¹:η¹ and μ₂ connection modes. The Al–O distances and O–Al–O angles are in the range of 1.850–1.946 Å and 77.01–175.27°, respectively.

Fig. 1 (a) and (b) Molecular structures of Al₁₂ at different view directions. (c) and (d) The ligand arrangement of the middle and upper/lower layers of Al₁₂ (green: Al; red: O; grey: C; white: H).

Crystal structure of Al₂₄ cluster

Similar to the synthesis of Al₁₂, one new Al₂₄ cluster with the formula [Al₂₄(OH)₃₂(CH₃O)₂₂(CH₃OH)₂(NAP)₁₂]·6Cl·2H₂O·2CH₃OH was obtained by using the solvent of CH₃OH–CH₃CN. It crystallizes in the monoclinic P21/n space group and the asymmetric unit contains half an Al₂₄ cluster. As shown in Fig. 2a, the Al₂₄ molecular skeleton is composed of eight [Al₃(OH)(CH₃O)₃] in the truncated positions and six open [Al₈(OH)₄(CH₃O)₄(NAP)₄] windows. The [Al₃(OH)(CH₃O)₃] building block is coordinated by 3 CH₃O⁻ capped on the Al–Al sides and 1 OH⁻ stamped over the centre of the triangle (Fig. 2b). The [Al₈(OH)₄(CH₃O)₄(NAP)₄] consists of four NAP⁻ and OH⁻, both capped on the interval four sides, and the remaining four sides are fixed by four CH₃O⁻ (Fig. 2c). The Al₂₄ is a cation cluster with six free Cl⁻ ions in the centre of octagonal windows bonded to the Al₂₄ frame by hydrogen-bond interactions, which are formed with four μ₂-OH⁻ in the interior of the octagon with the Cl⋯O distances in the range of 3.174–3.302 Å and the Cl⋯H–O angles in the range of 161.16–165.06° (Table S4†). This Cl⁻ template interaction is frequently found in the lanthanide clusters, such as Dy₇₆,⁴¹ Ln₄₈,^42–44 and Ln₁₅.^45–47 The Al₂₄ metallic inner features a truncated cube (Fig. 2d) with a small 7 Å diameter, though the total molecular Al₂₄ cluster with a larger 2.0 nm diameter due to the peripheral big NAP⁻ ligands. The Al–Al interactions are in the range of 2.853(3)–2.976(3) Å.

Fig. 2 (a) Molecular structures of Al₂₄. (b) and (c) The [Al₃(OH)(CH₃O)₃] and [Al₈(OH)₄(CH₃O)₄(NAP)₄] building blocks. (d) The 24-nuclearity Al metallic skeleton with a truncated hexahedral geometry (green: Al; red: O; grey: C; dark green: Cl; white: H).

The Al clusters are also a kind of metallic hydroxide clusters. The simplified Al/O core of Al₂₄ (Fig. 3c) is composed of the simplified trigonal [Al₃(OH)O₃] and octagonal [Al₈(OH)₈O₄] (Fig. 3a and b) by sharing the sides of the triangles. As depicted in Fig. 3f, the further simplified metal skeleton belongs to one of the 13 Archimedean polyhedra, a truncated cube comprising 8 Al₃ triangles on the truncated positions of the cube (Fig. 3d) and 6 Al₈ octagons on the six faces (Fig. 3e) with the strong Al–Al interactions in the range of 2.850–2.978 Å. This Archimedean polyhedron is rare in the metal nanoclusters, compared with the truncated tetrahedron (Ag₃₇),⁴⁸ truncated octahedron (Co₃₂),⁴⁹ truncated icosahedron (Ln₆₀),⁵⁰ whereas some similar M₂₄ coordination molecular cages with the organic ligands as the linkers have been reported.^51,52

Fig. 3 (a) and (b) [Al₃(OH)O₃] and [Al₈(OH)₈O₄] building units. (c) The Al/O core of Al₂₄. (d) and (e) The simplified Al₃ triangle (blue) and Al₈ octagon (purple). (f) The Archimedean polyhedron of Al₂₄ metallic core (green: Al; red: O).

All Al³⁺ ions in the Al₂₄ cluster adopt six-coordinated distorted octahedral geometry with six O atoms from 3 OH⁻, 1 NAP⁻ and 2 CH₃O⁻. The Al–O contacts and O–Al–O angles are in the range of 1.823–1.987 Å and 74.97–171.3°, respectively. All 12 NAP⁻ ligands are situated at the sides of the octagons by adopting a μ₂–η¹:η¹ connection mode (Fig. S1a†). The 32 OH⁻ can be divided into two types, namely, 24 μ₂-OH⁻ is uniformly oriented to the centre of 6 octagon (Fig. S1b†), which are at the opposite positions of every NAP⁻ ligand, whereas the residual 8 μ₃-OH⁻ are capped on the centre of the 8 trigonal Al₃ building blocks (Fig. S1c†). For the CH₃O⁻ ligands, all are located on the sharing Al–Al sides of triangular Al₃ and octagonal Al₈ building blocks (Fig. S1d†).

Chemical stability and TG analyses

The stability of Al₂₄ and Al₁₂ was measured by immersing their single crystals in water and typical organic solvents. According to the comparison of the experimental and simulated powder X-ray diffraction (PXRD) patterns (Fig. S9†), Al₂₄ can be kept stable in CH₃OH, EtOH, CH₃CN, and H₂O at least for a day. For Al₁₂, it displays very good stability in CH₃OH, EtOH, CH₃CN, H₂O, and DMF for a month (Fig. S10†). The above findings demonstrate that Al₂₄ and Al₁₂ exhibit high chemical stability in some solvents and H₂O. As shown in Fig. S8c and S8d,† the experimental IR spectra of Al₁₂ and Al₂₄ after immersing their single crystals in water and typical organic solvents were measured to further show their stabilities. Additionally, dry solid samples of Al₂₄ and Al₁₂ were used to study the thermal stability under the N₂ environment from 30 to 800 °C (Fig. S11†). Two H₂O and two CH₃OH molecules were removed from Al₂₄ during a slight weight loss of 2.33% up to 210 °C (calcd 2.35%). The mass loss from 210 to 285 °C is due to the departure of 24 coordinated methanol molecules (exp 17.98%, calcd 17.34%). As the temperature increased to 615 °C, the naphthalene rings of the organic NAP⁻ ligands began to break down and the mass fraction of 39.50% was lost (calcd 39.00%). For Al₁₂, the first weight loss below 185 °C was ascribed to the removal of four DMF, two H₂O, and two CH₃OH molecules (exp 11.16%, calcd 11.21%). Next, the weight loss of 47.86% from 260 to 440 °C is ascribed to the removal of the naphthalene rings from the NAP⁻ ligands (calcd 47.51%). The third weight loss from 500 to 615 °C corresponds to the decomposition of coordinated methanol molecules.

Solution behaviours of Al₂₄ and Al₁₂ clusters

The high-resolution electrospray ionization mass spectrometry (ESI-MS) can be used to determine the fragment composition and charge state of the metal clusters.^21,53–60 The negative-mode ESI-MS of Al₂₄ was investigated by dissolving the crystals in CH₃OH–CH₂Cl₂ to analyse its solution stability. As shown in Fig. 4, there are two sets of peaks in the mass-to-charge (m/z) ratio ranges of 2000–2100 (1) with −2 charge and 4000–4200 (2) with −1 charge, respectively. For 1 group, the distances between two neighbouring peaks are disparate with 6.51, 7.51, 6.55, 7.47, 7.00, and 7.51 having no uniform variation trend for the seven species (1a–1g). Their compositions, at the positions: m/z = 2045.78, 2052.20, 2059.79, 2066.22, 2073.79, 2080.79, and 2088.20, can be identified by matching the experimental and simulated isotopic distributions with the corresponding formulas, [Al₂₄(OH)₃₄(CH₃O)₂₆(NAP)₁₁(H₂O)₄Cl₃]²⁻, [Al₂₄(OH)₃₄(CH₃O)₂₃(NAP)₁₁(H₂O)₄Cl₆]²⁻, [Al₂₄(OH)₃₂(CH₃O)₂₈(NAP)₁₁(H₂O)₄Cl₃]²⁻, [Al₂₄(OH)₃₂(CH₃O)₂₅(NAP)₁₁(H₂O)₄Cl₆]²⁻, [Al₂₄(OH)₃₁(CH₃O)₃₀(NAP)₁₁(H₂O)₁Cl₄H₂]²⁻, [Al₂₄(OH)₃₁(CH₃O)₃₁(NAP)₁₁Cl₄H₃]²⁻, [Al₂₄(OH)₃₄(CH₃O)₂₃(NAP)₁₁(H₂O)₆Cl₇H₁]²⁻ (Table S1†). This set of signals indicates that the metal skeleton of Al₂₄ cluster is stable in CH₃OH–CH₂Cl₂, whereas the protection groups of OH⁻, CH₃O⁻, and NAP⁻ exist in the coordination–disassociation equilibrium. The most dominant peak (1d) with the composition of [Al₂₄(OH)₃₂(CH₃O)₂₅(NAP)₁₁(H₂O)₄Cl₆]²⁻ at m/z = 2066.22 is approximate to the parent cluster except for one NAP⁻ replaced with one CH₃O⁻. For the 2 groups, 2a–2c has similar distances of 14.02 corresponding to [+CH₃O⁻-H₂O + H⁺] with the formulas, [Al₂₂(OH)₂₃(CH₃O)₂₅(NAP)₁₂(H₂O)₂Cl₇]⁻, [Al₂₂(OH)₂₃(CH₃O)₂₆(NAP)₁₂(H₂O)₁Cl₇H₁]⁻, [Al₂₂(OH)₂₃(CH₃O)₂₇(NAP)₁₂Cl₇H₂]⁻ at m/z = 4097.45, 4111.47 and 4125.48, suggesting the presence of the Al₂₂ fragments in the CH₃OH–CH₂Cl₂ of Al₂₄.

Fig. 4 The negative-ion ESI-MS of Al₂₄ in CH₃OH/CH₂Cl₂. Inset: the experimental isotropic pattern (block) and simulated (red) data.

The positive-ion ESI-MS data of Al₂₄ was also monitored at m/z = 500–6000 (Fig. 5) by dissolving its crystals in CH₃OH–CH₂Cl₂. There are four groups (1a–1f, 2a–2g, 3a–3g, and 4a–4e) of experimental peaks at m/z = 1950–2250 with +2 charge (see inset in Fig. S2†). By matching the experimental and simulated isotopic distributions, all formulas of fragments are listed in Table S2.† The formulas of 2c–2f ([Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₂(H₂O)_x(CH₃OH)_3−x]²⁺, x = 3, 2, 1, 0), 3a–3f ([Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₂(H₂O)₅(CH₃CN)₁(CH₃OH)₁]²⁺ (3a); [Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₂(H₂O)₃(CH₃CN)₃]²⁺ (3b); [Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₂(H₂O)₂(CH₃CN)₃(CH₃OH)₁]²⁺ (3c); [Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₃(H₂O)₄(CH₃OH)₃H₁]²⁺ (3d); [Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₄(H₂O)₁(CH₃OH)₄H₂]²⁺ (3e); [Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₄(CH₃OH)₅H₂]²⁺ (3f)) and 4a–4e ([Al₂₄(OH)₃₂(CH₃O)₂₄(NAP)₁₂Cl₂(H₂O)_x(CH₃CN)₄(CH₃OH)_6−x]²⁺, x = 3, 2, 1, 0) are consistent with the molecular formulas of Al₂₄ ([Al₂₄(OH)₃₂(CH₃O)₂₂(CH₃OH)₂(NAP)₁₂]·6Cl·2H₂O·2CH₃OH) except for the different number of Cl⁻ and solvent molecules, indicating the stability of total Al₂₄ structure in CH₃OH and CH₂Cl₂ again. Meanwhile, 1b–1f ([Al₂₄(OH)₃₂(CH₃O)₂₆(NAP)₁₀Cl₅(H₂O)_x(CH₃OH)_4−xH₃]²⁺, x = 4, 3, 2, 1, 0), 2a ([Al₂₄(OH)₃₂(CH₃O)₂₆(NAP)₁₀Cl₅(H₂O)₁(CH₃CN)₂(CH₃OH)₃H₃]²⁺), 2b ([Al₂₄(OH)₃₂(CH₃O)₂₆(NAP)₁₀Cl₅(CH₃CN)₂(CH₃OH)₄H₃]²⁺) and 3g ([Al₂₄(OH)₃₂(CH₃O)₂₅(NAP)₁₁Cl₄(H₂O)₃(CH₃CN)₄(CH₃OH)₃H₂]²⁺) also show the ligand exchange between CH₃O⁻ and NAP⁻.

Fig. 5 The positive-ion ESI-MS of Al₂₄ in CH₃OH/CH₂Cl₂. Left inset: the experimental isotropic pattern. Right inset: the experimental isotropic pattern (block) and simulated (red) data.

Similarly, the positive-mode ESI-MS obtained by dissolving the crystals of Al₁₂ in CH₃OH–CH₂Cl₂ was also monitored at m/z = 1000–6000 (Fig. 6a). Three groups of experimental peaks at m/z = 1950–2020 (1a–1j, +2), 2100–2160 (2a–2f, +2) and 2290–2330 (3a–3f, +2) can be clearly assigned by matching the experimental and simulated isotopic distributions (Fig. 6b–d). Two neighbouring peaks of the three groups of experimental data have the same distances with 14.02 corresponding to [+CH₃O⁻-H₂O + H⁺] and all formulas of fragments are listed in Table S3.† Regrettably, the molecular ion signal was not detected. 1a–1e assigned to [Al₁₂(CH₃O)₁₇(NAP)₁₇(H₂O)₈]²⁺ (1a); [Al₁₂(CH₃O)₁₆(NAP)₁₈(H₂O)₁]²⁺ (1b); [Al₁₂(CH₃O)₁₇(NAP)₁₈H₁]²⁺ (1c); [Al₁₂(CH₃O)₂₀(NAP)₁₇(H₂O)₅H₃]²⁺ (1d); [Al₁₂(CH₃O)₂₁(NAP)₁₇(H₂O)₄H₄]²⁺ (1e), and 2c–2f assigned to [Al₁₂(CH₃O)₁₄(NAP)₂₀(H₂O)₅]²⁺ (2c); [Al₁₂(CH₃O)₁₅(NAP)₂₀(H₂O)₄H₁]²⁺ (2d); [Al₁₂(CH₃O)₁₆(NAP)₂₀(H₂O)₃H₂]²⁺ (2e); [Al₁₂(CH₃O)₁₇(NAP)₂₀(H₂O)₂H₃]²⁺ (2f) coincide with the metallic core of Al₁₂ whereas the numbers of CH₃O⁻ and NAP⁻ are different with the molecular formulas, indicating the occurrence of ligand exchange between CH₃O⁻, NAP⁻ and H₂O. 1f–1i assigned to [Al₁₁(CH₃O)₁₂(NAP)₁₉(H₂O)₃]²⁺, [Al₁₁(CH₃O)₁₃(NAP)₁₉(H₂O)₂H₁]²⁺, [Al₁₁(CH₃O)₁₄(NAP)₁₉(H₂O)₁H₂]²⁺, and [Al₁₁(CH₃O)₁₅(NAP)₁₉H₃]²⁺ lost a metal ion compared with Al₁₂ and 1j assigned to [Al₁₀(CH₃O)₈(NAP)₂₀(H₂O)₅]²⁺ has two fewer metal ions than Al₁₂. Interestingly, the fragments of 2a–2b with the corresponding attribution of [Al₁₃(CH₃O)₂₀(NAP)₁₉(H₂O)₁H₂]²⁺ and [Al₁₃(CH₃O)₂₁(NAP)₁₉H₃]²⁺, and 3a–3f with the corresponding attribution of [Al₁₄(CH₃O)₂₀(NAP)₂₀(H₂O)₉]²⁺, [Al₁₄(CH₃O)₂₁(NAP)₂₀(H₂O)₈H₁]²⁺, [Al₁₄(CH₃O)₂₂(NAP)₂₀(H₂O)₇H₂]²⁺, [Al₁₄(CH₃O)₂₃(NAP)₂₀(H₂O)₆H₃]²⁺, [Al₁₄(CH₃O)₂₄(NAP)₂₀(H₂O)₅H₄]²⁺, [Al₁₄(CH₃O)₂₅(NAP)₂₀(H₂O)₄H₅]²⁺ display the increase of a and two Al³⁺ ions for Al₁₂. The above analysis result indicates that the metal skeleton of the Al₁₂ cluster can be partly stable in CH₃OH and CH₂Cl₂, while some fragments with adjacent nuclear numbers, e.g., Al₁₀, Al₁₁, Al₁₃, Al₁₄ exist in CH₃OH and CH₂Cl₂. Alternatively, the number of CH₃O⁻ is less than the theoretical 24, whereas that of NAP⁻ is more. Thus, we surmise that NAP⁻ firstly coordinated with Al³⁺ ions in the assembly process of Al₁₂, and was later replaced by CH₃O⁻. Besides, the negative-ion ESI-MS data of Al₁₂ obtained by dissolving its crystals in CH₃OH–CH₂Cl₂ was monitored at m/z = 500–6000 (Fig. S2†). Only a small fragment peak at m/z = 863.15 was detected at m/z = 500–6000 and there are no more pieces of information.

Fig. 6 (a) The positive-ion ESI-MS of Al₁₂ in CH₃OH/CH₂Cl₂. The experimental isotropic pattern (block) and simulated (red) data for 1 group (c), 2 group (d), and 3 group (b).

UV–Vis spectra and photocurrent measurements of Al₂₄ and Al₁₂

The solid-state ultraviolet-visible absorption spectra (UV-Vis) of HNAP, Al₂₄, and Al₁₂ were recorded at room temperature. As shown in Fig. S3a and S3b,† the HNAP ligand exhibits absorption at 210 nm, whereas Al₂₄ and Al₁₂ have a similar broad absorption band at 250–400 nm. The red-shifted phenomenon may be caused by Al-perturbed π → π* transition of the ligand.⁶¹ The band gaps (E_g) of Al₂₄ and Al₁₂ are 3.48 and 3.44 eV calculated based on the Kubelka–Munk function, respectively (Fig. S4†),⁶² which are much smaller than 9 eV of Al₂O₃.⁶³ This reveals that the aggregation of aluminum ions into aluminum-oxo clusters affects the band gap structure, including the widening of the absorption edge and the narrowing of the band gap.

The photocurrent responses of Al₂₄ and Al₁₂ under LED light were further examined by the typical three-electrode system. The working electrode was indium tin oxide (ITO) glass, the auxiliary electrode was platinum wire, and the reference electrode was Ag/AgCl, maintaining the bias voltage of 0.6 V in the meantime. As shown in Fig. 7, upon on–off cycling exposure with LED light (λ = 365 nm and 420 nm; 10 s intervals; 50 W), obvious photocurrent reactions were observed, and the photocurrent densities exhibit a promptly rising or fast falling under light irradiation or no irradiation conditions. This indicates that Al₁₂ and Al₂₄ both have fast response speeds to LED light. No matter whether the irradiations wavelengths are at 420 nm or 365 nm, Al₁₂ generates higher photocurrent values than these of Al₂₄, which demonstrates that more efficient generation and separation of photoinduced electron/hole pairs were observed in ITO electrodes of Al₁₂ under the LED light. This could be explained on the basis of the lower band gap of Al₁₂ at 3.44 eV.⁶⁴ The above experimental results exhibiting Al₁₂ and Al₂₄ are a kind of potential photoelectric materials.

Fig. 7 Transient photocurrent responses of Al₂₄ (a) and Al₁₂ (b).

Luminescence properties of Al₂₄ and Al₁₂

At room temperature, the solid-state luminescence properties of HNAP, Al₂₄, and Al₁₂ were observed (Fig. S5†). The maximum emission of HNAP was at 372 nm under a maximum excitation wavelength of 319 nm (Fig. S5a and 5b†), which originated from the intramolecular π–π* transition of the HNAP organic ligand. As illustrated in Fig. S5c and 5d,† the Al₂₄ cluster exhibits the ligand-centered blue fluorescence with emission at 374 nm under 319 nm excitation. A similar blue fluorescence from ligand-centered emission is also observed in Al₁₂ with the maximum emission wavelength at 389 nm under 352 nm excitation (Fig. S5e and 5f†).

To determine their potential applications as optical thermometers, the operating range for the temperature-dependent luminescence spectra of Al₂₄ and Al₁₂ was from 300 to 120 K. The temperature was changed from 300 K to 120 K, but the emission location of the Al₂₄ cluster remained fixed at x = 0.15 and y = 0.06 in the CIE space (Fig. S6a and Table S5†) in the blue-light region (Fig. 8a). Furthermore, the emission position of Al₁₂ is slightly blue-shifted from 394.5 nm to 387.5 nm, as the temperature drops from 300 K to 120 K (Fig. 8b) while the CIE coordinates of the emission positions change from x = 0.16, y = 0.05 to x = 0.17, y = 0.07 (Fig. S6b and Table S5†). The probable reason is the restricted rotation of ligands in Al₁₂ at lower temperatures. In addition, the emission intensity of Al₂₄ and Al₁₂ gradually increased with decreasing temperature on account of the reduction of non-radiative decay.

Fig. 8 The temperature-dependent luminescence spectra of Al₂₄ (a) and Al₁₂ (b) from 300 K to 120 K. Luminescence lifetimes of Al₂₄ (c, λ_em = 379 nm) and Al₁₂ (d, λ_em = 389 nm) at room temperature.

The time-resolved decay curves of the Al₂₄ and Al₁₂ clusters were recorded at room temperature, and their fitting results show the double exponential functions. The lifetimes of Al₂₄ and Al₁₂ are 17 μs (λ_em = 379 nm; τ₁ = 4.44, 37%; τ₂ = 24.4, 63%; Fig. 8c) and 7 μs (λ_em = 389 nm; τ₁ = 1.26, 35%; τ₂ = 10.26, 65%; Fig. 8d). The absolute solid-state quantum yields of the Al₂₄ and Al₁₂ are 25.11% and 18.42%, respectively. The different luminescence behaviours of Al₂₄ and Al₁₂ can be attributed to the different ligand environments in the crystalline structures, generating the different constraints on the rotation of the NAP⁻ ligands.

Conclusions

In summary, we synthesized an Al₁₂ molecular ring and a rare Al₂₄ metallocage based on the NAP⁻ ligand obtained by modulating the solvent in a simple one-pot solvothermal reaction. The Al₁₂ is a dodecagonal molecular ring and the Al₂₄ metallocage can be seen as a truncated cube composed of eight Al₃ triangles and six Al₈ octagons formed by sharing vertical Al³⁺ ions, realizing the complete change of Al-oxo skeletons. Interestingly, the Al₁₂ and Al₂₄ clusters exhibited the photocurrent-generating capacity and luminescent behaviour, indicating that the Al clusters are potentially photocatalytic and luminescent materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 22101148) and the Natural Science Foundation of Shandong Province (no. ZR2021QB008).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2163632 and 2163633. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02461f

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