Near-infrared luminescence AgPd alloy superatomic clusters

Abstract

Atomically precise superatomic nanoclusters have attracted considerable interest due to their remarkable structures and intriguing photoluminescence properties. Nevertheless, achieving a high photoluminescence quantum yield (PLQY) in the near-infrared (NIR) region of superatomic nanoclusters continues to pose a considerable challenge. Here, we report a novel bimetallic nanocluster prepared utilizing an alloying strategy, formulated as [Ag₁₄Pd(PFBT)₆(TPP)₈](TPP) (abbreviated as Ag₁₄Pd). Notably, this cluster demonstrates a remarkable NIR emission, achieving a PLQY of 15% in the solid state, which is rare for metal nanoclusters. Both experimental and theoretical analyses indicate that Ag₁₄Pd exhibits a characteristic 8-electron superatomic structure with a 1S²1P⁶ electronic shell closure. Furthermore, due to their bright luminescence and exceptional photostability, Ag₁₄Pd clusters hold promising potential for applications as NIR inks for three-dimensional (3D) printing of various objects and models.

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Introduction

Metal nanoclusters, distinguished by their well-defined sizes and meticulously characterized structures, have displayed substantial prospects in multiple application domains, encompassing chemical sensing, bioimaging, and catalysis.^1–5 It is ubiquitously recognized that luminescence constitutes one of the most pivotal photophysical attributes of metal clusters,^6–12 playing an indispensable role in multifarious applications such as 3D printing and light-emitting diodes.^13–17 In recent years, metal nanoclusters that emit within the near-infrared (NIR > 750 nm) region^18–23 have garnered increasing scrutiny due to their potential applications in domains such as optoelectronics, bioimaging, sensors, and near-infrared optics. The atomic-level precision of geometries and well-defined compositions inherent to these metal clusters augment our comprehension of their distinctive photophysical mechanisms,^24,25 facilitating the design of highly photoluminescent materials.

Currently, a substantial number of metal clusters exhibiting high photoluminescence quantum yields (PLQYs) have been reported in the visible light region.^26–35 However, achieving comparable performance in the NIR region has been hindered by the significant loss of excitation energy through non-radiative relaxation as dictated by the energy gap law,³⁶ despite ongoing efforts to develop various strategies for enhancing the PLQY of NIR-emitting metal clusters. For instance, strategies such as ligand tailoring and suppression of surface vibrations have been demonstrated to improve the PLQY of metal nanoclusters.^37,38 Additionally, aggregation of metal nanoclusters has been observed to contribute positively to PLQY enhancement.³⁹ Importantly, alloying represents an effective strategy for improving photoluminescence properties.^40–42 Nevertheless, the PLQY of NIR-emitting metal clusters usually remains below 1%.^43–45 Consequently, synthesizing metal nanoclusters exhibiting a high PLQY and efficient NIR emission remains a challenge.

In this study, we report the synthesis, crystal and electronic structures, and optical properties of bimetallic nanoclusters, formulated as [Ag₁₄Pd(PFBT)₆(TPP)₈](TPP) (referred to as Ag₁₄Pd, PFBT = pentafluorothiophenol and TPP = triphenylphosphine). The crystal structure of Ag₁₄Pd was elucidated through single-crystal X-ray diffraction, electrospray ionization mass spectrometry, thermogravimetric analysis, and X-ray photoelectron spectroscopy. Ag₁₄Pd features a standard icosahedral Ag₁₂Pd metal kernel surrounded by two Ag(PFBT)₃ surface motifs at opposite poles, resulting in a superatom structure characterized by an electronic shell closure of 1S²1P⁶. Remarkably, this cluster exhibits a bright NIR emission in both the solution and solid states. In addition, the optical properties and electronic structure of Ag₁₄Pd were comprehensively investigated using density functional theory (DFT). Given their bright luminescence and excellent photostability, Ag₁₄Pd clusters demonstrate significant potential for applications as NIR inks for the 3D printing of different objects and models.

Results and discussion

Ag₁₄Pd was synthesized using a one-pot synthetic method (for details see the ESI†). In a typical synthesis, silver nitrate and palladium-tetrakis(triphenylphosphine) were initially dissolved in a solvent mixture of methanol, acetone, and dichloromethane at room temperature. Following this, PFBT and TPP were introduced into the mixture under vigorous stirring, leading to the formation of Pd–Ag–S–P complexes. After approximately 10 minutes, a freshly prepared aqueous solution of NaBH₄ was added to the reaction mixture all at once. The reaction mixture was then aged for 5 hours. Subsequently, the aqueous phase was removed and the organic phase was washed with water several times. After about two days, reddish-black block crystals crystallized from the CH₂Cl₂/hexane solution at room temperature.

The purity and chemical composition of the Ag₁₄Pd clusters were rigorously characterized using an array of analytical techniques, including powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), electrospray ionization mass spectrometry (ESI-MS) in negative mode, inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). The phase purity of the Ag₁₄Pd crystals was corroborated by the strong agreement between the experimental and simulated PXRD patterns (Fig. S1†). The TGA curve exhibited a weight loss of 67.56 wt%, closely aligning with the theoretical value of 67.07 wt% (Fig. S2†). Further insights into its composition and charge state were obtained through ESI-MS analysis, which revealed a prominent peak at m/z = 2171.3544 (cal. 2171.3802) in negative-ion mode within the range of 1500–4000, corresponding to [Ag₁₄Pd(PFBT)₆(TPP)₅(CH₂Cl₂)(CH₃OH)(Cl⁻)₂]²⁻, thereby confirming both the charge state and composition of Ag₁₄Pd (Fig. S3†). Additionally, energy-dispersive spectrometry (EDS) and XPS analyses confirmed the presence of Pd, Ag, S, P, F, and C elements in the Ag₁₄Pd clusters (Fig. S4–5†). High-resolution XPS spectra indicated that the oxidation states of Ag and Pd were close to Ag(0) and Pd(0), respectively (Fig. S5†).⁴⁶ Finally, the ICP-MS results demonstrated that the average element ratio of Ag to Pd is 13.96 : 1—remarkably consistent with approximately 14 : 1 determined via single-crystal X-ray analysis (Fig. S6†). Based on superatom theory's electron count rule, its electron count can be computed as 8 (N* = 14–6), suggesting that Ag₁₄Pd is a superatomic cluster with a closed-shell configuration of 1S²1P⁶.⁴⁷

Single-crystal structure determination elucidated that Ag₁₄Pd crystallized in the trigonal space group R3̄c (Table S1†). As illustrated in Fig. 1, Ag₁₄Pd displays an icosahedral Ag₁₂Pd kernel with two silver atoms capping two triangular Ag₃ faces along the three-fold axes of icosahedral Ag₁₂Pd (Fig. 1a and b). These two silver atoms, together with a triangular face of the Ag₁₂Pd icosahedron, give rise to two opposing tetrahedral geometries, wherein each edge of the tetrahedron's waist is bridged by a PFBT ligand (Fig. 1b). Eight TPP ligands are coordinated around the opposite poles of these two silver atoms and six silver atoms arranged in a chair cyclohexane conformation at the waist of the Ag₁₂Pd icosahedron (Fig. 1c, d and S7†). The distances between Ag and Ag_icosahedron within the Ag₁₂Pd kernel range from 2.864 (5) to 2.939 (7) Å (average 2.900 Å, Table S2†), which correlates well with bulk face-centered cubic silver exhibiting an approximate distance of about 2.889 Å, indicative of pronounced argentophilic interactions (Fig. 1e). The average Ag–Pd distance within Ag₁₂Pd is determined to be 2.757 Å, suggesting a stronger metallophilic interaction (Fig. 1e). Conversely, the average distance between adjacent silver atoms in tetrahedral configurations within the Ag₁₄Pd cluster is approximately 3.228 Å (Ag–Ag_tetrahedron), and there is also an Ag–Ag interaction smaller than 3.44 Å (the sum of van der Waals radii of the two Ag atoms), indicating weaker interactions compared to those observed between Ag and Ag_icosahedron. The average bond lengths for both Ag–S and Ag–P are determined to be 2.564 and 2.492 Å, respectively (Fig. 1e). Notably, the structural characteristics of Ag₁₄Pd exhibit similarities to those reported for PdAg₁₄ by Wang et al.⁴⁶

Fig. 1 (a) Overall structure of Ag₁₄Pd. (b) Icosahedral Ag₁₂Pd core. (c) Ag₁₄Pd(PFBT)₆ structure with two Ag(PFBT)₃ motif structures. (d) Ag₁₄Pd(PFBT)₆(TPP)₈ structure with six waist and two top TPP ligands. (e) Bond lengths of Ag–Pd, Ag–Ag_icosahedron, Ag–Ag_tetrahedron, Ag–P and Ag–S in Ag₁₄Pd. Color labels: cyan and dark green, Ag; red, Pd; yellow, S; purple, P; bright green, F; and grey, C. All the H atoms are omitted for clarity.

Importantly, the photoluminescence of Ag₁₄Pd was systematically investigated in the solid and solution states. Solid state Ag₁₄Pd displayed intense NIR emission at 780 nm with a wide excitation range of 400–650 nm and a photoluminescence quantum yield of 15% (Fig. 2a). The photoluminescence lifetime measured for Ag₁₄Pd was on the microsecond scale, specifically at 5.5 μs (Fig. 2b), indicating that its luminescence behavior is characteristic of phosphorescence.⁸ However, in CH₂Cl₂ solution, Ag₁₄Pd also displayed NIR emission centered at 768 nm, with a PLQY of 6% and a lifetime of 4.1 μs. The PLQY in the solid state (15%) significantly surpasses that observed in the CH₂Cl₂ solution (6%), attributed to the restriction of intermolecular motion inherent to the solid state. Specifically, the packing structure of the Ag₁₄Pd cluster reveals an ABCABC stacking pattern characterized by numerous intermolecular interactions such as C–H⋯F and C–H⋯π between the clusters within each layer and those adjacent to it (Fig. S8†).

Fig. 2 (a and c) NIR emission spectra and (b and d) lifetime decay curves of Ag₁₄Pd in the solid and solution states.

To verify this result, we conducted aggregation-induced emission and dynamic light scattering experiments of the clusters in the solvent systems of dichloromethane and n-hexane, and the results showed that the emission of Ag₁₄Pd in the dilute solution of dichloromethane was weak. The emission intensity gradually enhanced upon further increasing the n-hexane fraction. When the n-hexane fraction increased to 90%, the solution showed strong red emission, indicating the possible presence of a larger aggregate,^48–51 which was further demonstrated by dynamic light scattering (Fig. S9†). Meanwhile, the emission intensity at the maximum of solid-state Ag₁₄Pd remains largely unchanged after irradiation for 3 h with a 400 nm xenon lamp (Fig. S10†), demonstrating exceptional photostability and opening new avenues for practical applications including 3D printing, cell imaging, and biological labeling.

The ultraviolet-visible (UV-vis) absorption spectrum of the Ag₁₄Pd cluster was recorded in CH₂Cl₂, as illustrated in Fig. 3. The UV/vis spectrum of Ag₁₄Pd shows four prominent optical absorption bands at 330 (a), 421 (b), 496 (c), and 679 (d) nm (Fig. 3a). To gain further insights into the electronic structures and absorption characteristics of Ag₁₄Pd, we conducted time-dependent density functional theory (TD-DFT) calculations (Fig. 3b–d and S11†).

Fig. 3 (a) Theoretical (black) and experimental (red) UV-vis spectra of Ag₁₄Pd. (b) Energy alignment of the MOs of Ag₁₄Pd. (c) HOMO to HOMO-2 of Ag₁₄Pd showing P-type orbital character. (d) LUMO to LUMO+4 of Ag₁₄Pd showing D-type orbital character.

The calculated absorption spectrum of Ag₁₄Pd reveals four distinct absorption peaks. The slight shift in the positions of these peaks compared to the experimental data can be attributed to an overestimation of the excitation energy (Fig. 3b). Peak d, corresponding to the highest energy absorption band, primarily arises from transitions between the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), predominantly involving metal-to-metal charge transfer (MMCT) transitions associated with Ag and Pd kernel-based orbitals (Fig. 3a and S11†). Similarly, peak c within this higher energy absorption band exhibits MMCT transitions such as HOMO → LUMO+3 and HOMO−1 → LUMO+1 (Fig. 3a and S11†). Peak b in the lower energy absorption band encompasses both HOMO → LUMO+5 and HOMO−1 → LUMO+4 transitions, while peak a is primarily generated from the transition between HOMO−13 and LUMO, which mainly involves ligand to metal charge transfer (LMCT) transitions (Fig. 3a and S11†). Furthermore, the occupied orbitals HOMO, HOMO−1, and HOMO−2 display pronounced superatomic P-like character and are predominantly composed of Ag and Pd atoms (Fig. 3c), whereas the unoccupied orbitals LUMO, LUMO+1, LUMO+2, LUMO+3, and LUMO+4 demonstrate distinct superatomic D-like character largely derived from Ag and Pd atoms (Fig. 3d). Consequently, Ag₁₄Pd qualifies as a typical superatomic cluster.

Photo-curing 3D printing involves the layer-by-layer construction of three-dimensional objects through the polymerization or cross-linking of photo-curing resin monomers under UV laser irradiation.⁵² This method offers significant advantages in terms of resolution, cost-effectiveness, and production efficiency compared to traditional subtractive manufacturing technologies, making it highly attractive. Although a variety of materials can be utilized in photo-curable 3D printing techniques,⁵³ there have been no reports focusing on the application of superatomic clusters with NIR luminescence as inks for 3D printing. Given the excellent solubility, light stability, and bright NIR luminescence exhibited by Ag₁₄Pd clusters, we were motivated to explore their potential for fabricating macroscopic models via 3D printing (Fig. 4a). Initially, the Ag₁₄Pd clusters are thoroughly mixed with photocuring resin and loaded into the liquid storage tank of the printer for subsequent use. Subsequently, a series of model files representing various shapes (such as rings, tetrahedra, cubes, spheres, and icosahedra) are designed using drawing software and converted into the “.CWS” format compatible with 3D printers through slicing software before being uploaded to the printer. The parameters of the printer are adjusted to produce regular and varied macroscopic stereo models (Fig. 4b). Notably, these printed models retain the original NIR luminescence characteristics of the Ag₁₄Pd clusters (Fig. 4c), indicating that superatomic clusters may hold important potential for applications in optoelectronic devices.

Fig. 4 (a) Schematic illustration of Ag₁₄Pd based 3D printed monoliths with different objects and printed models by using photo-curing 3D printing technology in the solid state. (b) Photographs of the 3D printed model under daylight and UV light, respectively. (c) NIR emission spectrum of the 3D printed model.

Conclusions

In summary, we have synthesized and characterized a novel superatom alloy cluster that demonstrates robust NIR luminescence both in solid and solution states, achieving a maximum PLQY of 15% and exhibiting a microsecond lifetime. For the first time, this near-infrared luminescent superatomic cluster has been employed as an ink for 3D printing of various three-dimensional models. Concurrently, we performed a comprehensive analysis of the electronic structure of the cluster through theoretical calculations. This work expands the application scope of superatomic clusters with near-infrared luminescence and offers a new direction for the application of these clusters.

Author contributions

X.-Y. D. conceived and designed the project. X.-H. M. and F.-F. W. conducted the synthesis, characterization, and analysis of experimental results. P. L. performed the calculations. X.-H. M., J.-H. Q., and X.-Y. D. co-wrote the manuscript.

Data availability

The X-ray crystallographic coordinates for the structure reported in this work have been deposited at the Cambridge Crystallographic Data Centre (CCDC 2352112†).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. U21A20277 and 22271130), the Excellent Youth Foundation of Henan Scientific Committee (232300421022), and the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (IRTSTHN, 25IRTSTHN001).

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Footnote

† Electronic supplementary information (ESI) available: additional material synthesis, experimental methods, data analysis, and supporting tables and figures. CCDC 2352112. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02655a

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