Reductant-selected formation of copper nanoclusters with crystallization-induced emission enhancement performance

Abstract

The rational design of emissive copper nanoclusters presents a significant challenge due to their inherent instability and complex coordination chemistry. In this study, we propose a reductant-mediated strategy to precisely regulate the atomic structure and photophysical properties of copper nanoclusters. The strong reductant sodium borohydride (NaBH₄) yields a non-emissive Cu₁₄(DMP)₆(PPh₂py)₈ species, while the mild reductant borane tert-butylamine complex leads to the formation of a tetrahedral Cu₄(PPh₂py)₄Cl₂ cluster, which exhibits a remarkable crystallization-induced emission enhancement (CIEE). Hirshfeld surface analysis of the Cu₄ nanocluster indicates that multiple non-covalent interactions—such as H⋯H, C–H⋯π, and Cl⋯H contacts—stabilize the molecular packing, effectively suppressing non-radiative decay pathways and contributing to the CIEE phenomenon. This work highlights the crucial role of reductants in modulating copper nanocluster structures and offers novel insights into the design of efficient copper-based emitters.

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Introduction

Ligand-protected copper (Cu) nanoclusters represent a critical subclass of metal nanoclusters with immense potential for applications in catalysis, sensing, and photonics.^1–8 Their unique quantum size effects, discrete electronic states, and structure-dependent physicochemical properties contribute to this promise. Recent advances in synthetic chemistry have enabled the preparation of atomically precise Cu nanoclusters with well-defined core–shell architectures, establishing a foundation for rationally designing functional Cu-based nanomaterials.^9–13 However, controllable synthesis and property modulation of Cu nanoclusters remain significant challenges. Unlike gold and silver counterparts, copper clusters exhibit a wide variety of coordination modes and are generally less stable and more prone to oxidation.^12,14–22 These factors hinder precise control over atomic composition and surface coordination structures,^23,24 making the synthesis of stable Cu nanoclusters with superior physicochemical properties essential for both fundamental studies and practical applications.

Photoluminescence (PL) is among the most intriguing features of Cu nanoclusters, characterized by their strong emissions, significant Stokes shifts, and long excited-state lifetimes.^12,25–32 These properties make them promising candidates for optoelectronic and luminescent devices. To enhance the PL performance of Cu nanoclusters, strategies such as ligand modification,^33–35 heteroatom doping,^36–38 aggregation-induced emission (AIE),^39–44 and environmental tuning (e.g., solvent,^15,45,46 pH,⁴⁷ and temperature⁴⁸) have been explored. Crystallization-induced emission enhancement (CIEE), a specialized form of AIE, has proven particularly effective for improving Cu nanocluster emission.^27,49–52 In the crystalline state, ordered molecular packing strengthens intermolecular interactions while restricting intramolecular vibrations/rotations, thereby suppressing non-radiative decay and enhancing radiative recombination.^49–52 Despite this progress, a fundamental question remains unresolved: how can copper cluster formation be precisely directed during nucleation and growth to yield atomically defined structures with tailored luminescence properties? In this regard, the choice of reducing agents—a critical yet underexplored synthetic parameter—may serve as a powerful tool for guiding the structural evolution and emission characteristics of Cu nanoclusters.

Herein, we demonstrate that the reducing agent (sodium borohydride, NaBH₄, vs. borane tert-butylamine complex, TBAB) profoundly influences the formation and PL behavior of Cu nanoclusters. Specifically, NaBH₄ yields non-emissive Cu₁₄(DMP)₆(PPh₂py)₈ (Cu₁₄), while TBAB generates tetrahedral Cu₄(PPh₂py)₄Cl₂ (Cu₄), which exhibits bright emission in both solution and solid states with pronounced CIEE (Scheme 1). Notably, embedding Cu₄ into a poly(methyl methacrylate) (PMMA) matrix via sol–gel processing enhances PL intensity and increases the PL quantum yield (PLQY) nearly 20-fold compared to the solution state. This improvement arises from the reduced non-radiative decay, mediated by intermolecular non-covalent interactions (H⋯H, C–H⋯π, and Cl⋯H interactions). This work highlights the pivotal role of reducing agents in directing Cu nanocluster structures, offering new insights for designing high-performance luminescent copper nanomaterials.

Scheme 1 Illustration of the reductant-selected formation of copper nanoclusters and their different PL behavior in the crystal state.

Results and discussion

The synthetic protocols for Cu₁₄ and Cu₄ nanoclusters are highly similar, both employing a one-pot reduction strategy using Au–Cu–DMP–PPh₂py complexes as precursors. The critical distinction arises from the choice of reducing agent: Cu₁₄ is synthesized with NaBH₄, while Cu₄ is produced using TBAB (see the SI for detailed procedures). This variation in reductant strength directly influences cluster formation. NaBH₄, a stronger reductant, accelerates metal aggregation, leading to the formation of a larger Cu₁₄ cluster. Conversely, TBAB results in a gentler reduction, forming a smaller tetrahedral Cu₄ cluster. Notably, the synthesis of Cu₁₄ is consistent with our previous work, and the structure of Cu₄ clusters is similar to those reported in earlier studies, despite different synthetic methods.^39,53 Both nanoclusters were crystallized via a CH₂Cl₂/n-hexane diffusion method, yielding high-quality crystals within one week that were suitable for single-crystal X-ray diffraction (SC-XRD) analysis.

The atomic structures of the Cu₄ and Cu₁₄ nanoclusters were determined by SC-XRD. The Cu₄ nanocluster crystallizes in a monoclinic crystal system (space group: P2₁/c), with two pairs of enantiomers present in each unit cell (Fig. S1 and Table S1). As illustrated in Fig. 1a, the nanocluster features a tetrahedral Cu₄ core that is stabilized by four phosphine (PPh₂py) ligands and two chloride (Cl) ligands. Each Cu atom in the core exhibits a mixed coordination environment involving N (from the ring of PPh₂py), P, and Cl ligands, collectively rigidifying the core–shell framework (Fig. 1a). In contrast, the Cu₁₄ nanocluster crystallizes in a triclinic crystal system (space group: P1̄), with one cluster per unit cell (Fig. S2, Table S2). The cubic Cu₁₄ core is protected by six DMP ligands and eight PPh₂py ligands (Fig. 1b), with a simpler coordination sphere dominated by S (from DMP) and P sites. The absence of nitrogen or halogen coordination in Cu₁₄ likely reduces structural rigidity compared to Cu₄. Besides, Cu₄ exhibits longer Cu–Cu bond lengths and shorter Cu–P bond lengths compared to Cu₁₄ (Table S3), which directly influence its photophysical properties. The shorter Cu–P bonds enhance orbital overlap between copper atoms and phosphine ligands, facilitating ligand-to-metal charge transfer (LMCT). The longer Cu–Cu bonds weaken vibrational coupling between copper atoms, reducing non-radiative transitions. Additionally, the Cu–N and Cu–Cl interactions in Cu₄ enhance molecular rigidity and intermolecular interactions, contributing to its stability and superior optical performance.

Fig. 1 Structural anatomy of (a) Cu₄ and (b) Cu₁₄ nanoclusters. Positive-mode ESI-MS spectrum of (c) Cu₄ and (d) Cu₁₄ nanoclusters. Insets: isotope patterns of experimental (black) and simulated (red) spectra of corresponding nanoclusters. Color legend: brown = Cu; yellow = S; pink = P; green = Cl; blue = N. For clarity, C and H atoms are shown in wireframe mode.

Electrospray ionization mass spectrometry (ESI-MS) confirmed the molecular compositions. For Cu₄, the dominant peak at m/z = 1377.9 corresponds to [Cu₄(PPh₂py)₄Cl₂]⁺, while the minor peak at m/z = 1114.9 matches [Cu₄(PPh₂py)₃Cl₂]⁺ (Fig. 1c). Experimental isotope patterns align precisely with simulated patterns, validating the crystal structure. Similarly, ESI-MS analysis of Cu₁₄ verified its composition and purity (Fig. 1d).

X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of expected elements in Cu₄ and Cu₁₄ nanoclusters (Fig. S3 and S4). High-resolution Cu 2p spectra reveal mixed Cu⁺/Cu⁰ species in Cu₁₄, with peaks at 932.5 eV and 952.4 eV. In contrast, Cu₄ exhibits higher binding energies in Cu 2p spectra (Fig. S5), indicative of elevated Cu oxidation states. Cu LMM Auger electron spectroscopy further confirms that Cu₄ contains more oxidized Cu species than Cu₁₄ (Fig. S6). These differences correlate with reductant strength: the milder reducing environment of TBAB promotes partial oxidation in Cu₄, while NaBH₄ drives Cu₁₄ toward reduced states. By combining the ESI-MS and XPS results, the Cu₄ nanocluster should be in a “+2”-charge state while Cu₁₄ is an electrically neutral molecule. However, both clusters presented “+1”-charged signals in the mass spectra, suggesting a potential electron-gaining or electron-losing behavior of the two clusters in the mass spectrometry environment.

Optical properties were investigated via UV-vis and PL spectroscopy. The UV-vis spectrum of Cu₄ shows a shoulder peak at 265 nm and two prominent bands at 315 and 390 nm. In contrast, Cu₁₄ displays a dominant absorption peak at 265 nm, accompanied by a weaker shoulder at 350 nm (Fig. 2a). The main absorption peak of Cu₄ at 390 nm represents a significant 40 nm redshift compared to Cu₁₄, reflecting structural differences in core–shell geometry and electronic configurations of the two nanoclusters.

Fig. 2 (a) UV-vis spectra and (b) PL spectra of Cu₄ (red) and Cu₁₄ (blue) nanoclusters in CH₂Cl₂ solution (λ_ex = 365 nm). All samples were measured at the same optical density of 0.05.

PL measurements reveal stark contrasts of the two nanoclusters in the solution state: Cu₁₄ is non-emissive in solution, while Cu₄ exhibits strong emission with the emissive peak centered at 573 nm (Fig. 2b). In the crystalline state, Cu₁₄ remains virtually non-emissive, while Cu₄ shows a significant increase in PL intensity (Fig. S7). In this context, the crystalline state of Cu₄ nanocluster demonstrates markedly enhanced PL emission, with emission intensity greatly surpassing that observed in solution, demonstrating its CIEE behavior.

To evaluate the PL behavior of Cu₄ in monodisperse (solution), amorphous (film), and crystalline states, we fabricated a Cu₄-based composite film using PMMA as the polymer matrix. Under 365 nm UV excitation, all samples emitted visible emission with distinct color shifts dependent on their physical state (Fig. 3a). The corresponding CIE chromaticity coordinates confirmed a progressive color shift from yellow to green, with coordinates of (0.46, 0.51) for the solution, (0.42, 0.52) for the film, and (0.37, 0.53) for crystalline Cu₄ (Fig. 3b). Besides, steady-state fluorescence tests of Cu₄ crystals demonstrated that such a nanocluster was singly emissive (Fig. S8). Quantitative photophysical measurements revealed that the solution-state Cu₄ exhibited weak emission, with a PLQY of only 1.2%. However, when embedded in the PMMA matrix, the PLQY increased substantially to 21.2%. This enhancement is attributed to the restriction of intramolecular motion through AIE, even in the amorphous state (Fig. 3c). Notably, the crystalline form exhibited the highest PLQY of 35.4%, indicative of a strong CIEE effect. Additionally, a PL emission intensity test was conducted to compare the stability of the Cu₄ cluster in both crystal and film states. The results demonstrate that both forms retain their PL properties after exposure to the atmosphere for three months (Fig. S9).

Fig. 3 (a) Photographs of Cu₄ samples under UV illumination (365 nm) in solution, film, and crystal states. (b) CIE chromaticity coordinates of Cu₄ samples. (c) PL intensity and (d) fluorescence lifetime decay profiles of Cu₄ samples.

To elucidate the emission mechanism behind the Cu₄ nanocluster, we calculated the radiative (k_r) and non-radiative (k_nr) rate constants based on the measured PLQY and lifetime data (Table S4). Time-resolved fluorescence measurements revealed progressively increased lifetimes: 0.1 μs (solution), 1.8462 μs (film), and 2.6133 μs (crystal) (Fig. 3d). Notably, the k_nr values for both the Cu₄-based film and crystalline samples decreased by approximately one order of magnitude compared to the solution state. This dramatic reduction confirms that Cu₄ aggregation effectively suppresses non-radiative decay pathways, significantly enhancing the PL efficiency of the cluster materials.

In the Cu₄ crystal lattice, most PPh₂py phenyl rings engage in intercluster interactions: H⋯H (2.173–2.736 Å), C–H⋯π (2.883–3.146 Å), and Cl⋯H (2.705 Å) interactions (Fig. S10). To probe the molecular origins of the CIEE in Cu₄, we conducted Hirshfeld surface analysis and 2D fingerprint mapping to investigate the intermolecular interactions that influence the crystal packing. The d_norm surface reveals close short-range contacts, particularly C–H⋯π (red dots) and Cl⋯H interactions (red circle) (Fig. 4a and Fig. S11). The shape index map displays complementary red/blue triangular features, indicative of C–H⋯π stacking interactions (Fig. 4b), while the curvedness map shows extended flat regions that further support the presence of these C–H⋯π contacts (Fig. 4c). Fingerprint analysis quantifies all intermolecular interactions (Fig. 4d), with C⋯H/H⋯C interactions dominating, emphasizing the crucial role of C–H⋯π interactions in stabilizing the crystal lattice (Fig. 4e). Additionally, Cl⋯H/H⋯Cl contacts suggest that chlorine–hydrogen interactions also serve as an auxiliary stabilizing factor (Fig. 4f). This dense network of non-covalent interactions rigidifies the structure, restricting intramolecular ligand rotation/vibration. By elevating the energy barrier for vibrational relaxation, these constraints suppress non-radiative decay pathways, thereby enhancing PL quantum efficiency and driving the pronounced CIEE behavior in Cu₄. Furthermore, Hirshfeld surface analysis of Cu₁₄ reveals that its surface is mostly governed by weak intermolecular interactions, such as H⋯H and C–H⋯π interactions (Fig. S12). Compared to the Cu₄ cluster, interactions like Cl⋯H bonds were not present in the Cu₁₄ surface, suggesting its significantly weaker intermolecular interactions and non-CIEE behavior.

Fig. 4 Hirshfeld surface plots and 2D fingerprint plots of the Cu₄ cluster. Short contacts are shown in red. Long contacts are shown in blue. The white area represents the contacts with lengths equivalent to the sum of the van der Waals radii of interacting atoms. Hirshfeld surface analysis of Cu₄ in (a) d_norm, (b) shape index, and (c) curvedness. 2D fingerprinting plots of Cu₄ showing the (d) total interaction and proportion of (e) C⋯H/H⋯C and (f) Cl⋯H/H⋯Cl in the fingerprint.

Conclusion

In summary, we have developed a reductant-mediated strategy to control the structures and photophysical properties of copper nanoclusters. By varying the reducing agent from NaBH₄ to TBAB, we successfully obtained two structurally and optically distinct Cu nanoclusters: non-emissive Cu₁₄ and highly luminescent Cu₄. Notably, Cu₄ exhibits a pronounced CIEE phenomenon, which is attributed to its rigid molecular environment in the crystalline state. This rigidity suppresses non-radiative decay through synergistic C–H⋯π and Cl⋯H interactions. Hirshfeld surface analysis and 2D fingerprint plots reveal that these non-covalent interactions form a compact and ordered network, providing structural insights into the mechanism behind the enhanced PL intensity. In addition to understanding structure–property correlations, the impressive luminescence and stability of Cu₄ in both crystalline and film forms emphasize its potential applications in light-emitting devices, chemical sensors, and other optoelectronic systems. This work highlights the critical role of reductants in tailoring the structure–property correlations of Cu nanoclusters and presents a feasible approach for the rational design of high-performance luminescent copper nanomaterials.

Author contributions

Y. Tian, W. Xu and J. Zhu conceived and carried out experiments. J. Lu, Z. Chen and F. Alam assisted in the synthesis and optical spectral measurements. H. Shen and X. Kang analyzed the data and wrote the paper. M. Zhu supervised the project.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

The data that support the findings of this study are available in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nr03478g.

CCDC 2468154 contains the supplementary crystallographic data for this paper.⁵⁴

Acknowledgements

We acknowledge the financial support from the NSFC (22371003, 22101001, and 22471001), the China Postdoctoral Science Foundation Funded Project (2024M760007), and the Scientific Research Program of Universities in Anhui Province (2022AH030009).

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Footnote

† These authors contributed equally to this work.

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