Unprecedented stacking-dependent piezoluminescence enhancement in atomically precise superatomic gold nanoclusters

Abstract

Deciphering the structure-property relationship between cluster stacking and high-efficiency luminescence of metal nanoclusters is crucial for designing and synthesizing high-performance light-emitting materials and devices. Here, we successfully synthesized two polymorphic gold nanoclusters (Au₈-C and Au₈-P) and investigated their stacking-dependent piezoluminescence based on hydrostatic pressure. Under compression, Au₈-C exhibits notable piezoluminescence enhancement. However, Au₈-P presents monotonic piezoluminescence quenching. High-pressure structural characterizations confirm the existence of stacking-dependent anisotropic compression in Au₈-C and Au₈-P. Under high pressure, the columnar-stacked Au₈-C shrinks faster along the a axis, increasing the aspect ratio (AR) of the fusiform Au₈ core. However, the layered Au₈-P is compressed faster along the c axis, reducing the AR and leading to a flatter Au₈ core. High-pressure femtosecond transient absorption, time-resolved photoluminescence, and Raman spectra collaboratively confirm that differentiated anisotropic compression notably suppresses nonradiative loss caused by low-frequency vibrations of the Au₈ core, which is responsible for the piezoluminescence enhancement in Au₈-C.

Anisotropic compression in columnar-stacked gold nanoclusters drives unprecedented piezoluminescence enhancement.

INTRODUCTION

Superatomic Au nanoclusters (Au NCs) with closed-shell electronic structures typically exhibit excellent structural stability and have broad application prospects in sensing, catalysis, optical imaging, and biomarkers (1–5). However, because of the strong low-frequency vibrations and notable vibrational degrees of freedom of the metal core, superatomic Au NCs typically display low photoluminescence quantum yields (PLQYs) (6, 7), which severely hinders their practical applications. To improve the PLQYs of superatomic Au NCs, strategies such as ligand engineering (8–13) and heterometallic doping (14–19) have been developed. By designing and assembling rigid staple motifs and large sterically hindered ligands onto the surface of Au NCs, the nonradiative relaxation caused by molecular vibrations can be effectively suppressed, considerably improving the PLQYs of Au NCs. Introducing heterometallic atoms into Au NCs to increase their energy gap is also a very effective approach for enhancing the PLQYs of Au NCs. These developed methods have primarily focused on studying the structure-optical property relationships of single-molecule Au NCs in solution and have neglected the study of PLQYs of crystalline superatomic Au NCs (table S1). Crystalline Au NCs with high PLQYs have broad application prospects in high-efficiency displays and solid-state lighting. However, the relationship between the stacking mode of NC molecules and their PLQYs in crystalline Au NCs remains unknown.

To elucidate the effect of the stacking mode of cluster molecules on their PLQYs, polymorphic metal NCs have been developed. The so-called polymorphic metal NCs are clusters with the same chemical composition but different single-crystal structures (20, 21). Polymorphic metal NCs usually show distinct physicochemical properties and are considered ideal models for studying the structure-property relationship in NCs (22, 23). The stacking-dependent luminescence behavior of organic solid materials has been systematically investigated by designing and preparing different types of polymorphic organic single crystals (24–28). Because of the complexity of cluster synthesis and the lack of characterization techniques, the structure-property relationship of polymorphic Au NCs remains unclear. Therefore, it is necessary to urgently develop convenient and efficient strategies to solve the above mentioned key scientific problems. Hydrostatic pressure can be used as a powerful tool to tune the crystal structure, electronic structure, and physicochemical properties of materials by decreasing interatomic distances and manipulating the electron orbital overlap (29–40). The hydrostatic pressure effect has provided valuable insights into the structural transformation and luminescence enhancement of solution-phase monomolecular Au NCs, 2-phenylethylthiol-protected Au₂₅ and cyclohexanethiolate-protected Au₂₁ (41, 42). Therefore, introducing hydrostatic pressure effects into crystalline polymorphic Au NCs is expected to systematically reveal the relationship between the stacking mode of Au NCs and their PLQYs.

Here, a pair of polymorphic superatomic [Au₈(dppp)₄(C≡CPh)₂](NO₃)₂ [dppp = 1,3-bis(diphenylphosphino)propane, PhC≡C⁻ = phenylacetylene substituent] NCs were successfully synthesized by modulating the crystalline solvent system, which crystallize in the C2/c (Au₈-C NC) and P2₁/n (Au₈-P NC) space groups, respectively. Under ambient conditions, Au₈-C (16%) and Au₈-P (20%) display comparable PLQYs. Using a series of in situ high-pressure experiments, we systematically elucidated the structure-property relationship between the stacking modes and PLQYs of Au₈ NCs. Under compression, Au₈-C presents remarkable pressure-induced emission enhancement behavior. When the pressure increases to 1.7 GPa, the PLQY of Au₈-C increases to a maximum of 76%. However, in the same pressure range, Au₈-P only shows pressure-driven emission decay. Angle-dispersive x-ray diffraction (ADXRD) results show that Au₈-C and Au₈-P undergo differential anisotropic compressions under high pressure. During compression, Au₈-C and Au₈-P shrink faster along the a and c axes, respectively. Before 1.7 GPa, Au₈-C shrinks rapidly along the a axis, resulting in a continuous increase in the aspect ratio (AR) of the kernel in Au₈-C, making the Au─Au bonds shorter and uniform. In contrast, under high pressure, the AR of the kernel in Au₈-P continues to decrease, and distortion increases. Raman spectra demonstrate that pressure notably suppresses the low-frequency vibrations of the kernel in Au₈-C. Time-resolved photoluminescence (TRPL) and transient absorption (TA) spectra collaboratively confirm that the nonradiative loss in Au₈-C is considerably reduced under pressure, which is responsible for the emission enhancement of Au₈-C. This study not only provides valuable insights into the relationship between stacking modes and the luminescence properties of superatomic NCs under pressure but also demonstrates the practical applicability of high-pressure studies in nanochemistry.

RESULTS

In our work, by adjusting the crystallization solvent system, two crystalline polymorphs Au NCs that crystallize in C2/c (Au₈-C) and P2₁/n (Au₈-P) space groups, respectively, were successfully obtained (Fig. 1). In detail, by diffusing ether in a solution of CH₂Cl₂/CH₃OH (v/v = 1/1) containing Au₈ NCs, Au₈-C crystals were obtained (43). By contrast, Au₈-P crystals were obtained by diffusing ether in CH₂Cl₂ containing Au₈ NCs (tables S2 to S4). To the naked eye, Au₈-C (purple) and Au₈-P (red) crystals have notably different colors. The analysis of single crystal x-ray diffraction data reveals that Au₈-C and Au₈-P have similar [core + exo]-type Au₈ frameworks composed of a bitetrahedral Au₆ core and two Au atoms attached at exo positions, which are decorated by four dppp and two PhC≡C⁻ (fig. S1). CH₃OH molecules are included in the lattice unit of Au₈-C, notably altering the cluster stacking in the unit cell and resulting in a different space group for Au₈-C compared to Au₈-P. The stacking patterns of Au₈-C and Au₈-P are shown in Fig. 2 (A and C), respectively. Au₈-C exhibits a columnar configuration along the c axis (Fig. 2B), whereas Au₈-P displays a lamellar arrangement along the same direction (Fig. 2D) (44). Along the a axis, CH₃OH molecules are intercalated into a void formed by Au₈-C, some CH₃OH molecules are connected to Au₈-C via C─H⋯O hydrogen bonding interactions (2.602 to 2.677 Å; fig. S2). Noncovalent interaction analysis (45) further demonstrates the presence of C─H⋯O hydrogen bonding interactions in Au₈-C (fig. S3). Thus, the formation of polymorphic superatomic Au₈ NCs may be mainly dictated by the C─H⋯O hydrogen bonding. Different molecular stacking patterns result in distinct intermolecular interactions in Au₈ NCs (fig. S4). Au₈-C displays C─H⋯π (2.769 to 3.052 Å) and π⋯π (3.655 to 3.664 Å) interactions (fig. S5A), whereas Au₈-P exhibits only C─H⋯π (2.782 to 2.996 Å) interactions (fig. S5B). This variation in molecular stacking also leads to minor differences in Au₈ cores, with Au─Au bond lengths ranging from 2.618 to 3.168 Å in Au₈-C and from 2.622 to 3.110 Å in Au₈-P. The well overlapping powder x-ray diffraction (PXRD) patterns of the as-synthesized samples and simulated single-crystal data confirm the crystal phase purity of Au₈-C and Au₈-P (fig. S6).

Fig. 1. Synthetic route for Au₈-C and Au₈-P NCs.

Fig. 2. Single-crystal structures and simplified molecular stacking modes of Au₈-C and Au₈-P.

Single-crystal structures of (A) Au₈-C and (C) Au₈-P with hydrogen atoms omitted for clarity. The simplified molecular stacking modes of (B) Au₈-C and (D) Au₈-P are shown schematically.

To investigate the optical properties of Au₈-C and Au₈-P crystals under ambient conditions, the absorption, PL, and TRPL spectra were measured. Absorption spectra show that the absorption peaks of Au₈-C and Au₈-P crystals are different, located at 528 and 519 nm, respectively, and both present isolated absorption bands, corresponding to characteristic energy gaps of 2.18 and 2.25 eV, respectively (fig. S7). The photoluminescence (PL) spectra of Au₈-C and Au₈-P crystals reveal that they both exhibit red emission (fig. S8) with slightly different emission centers at 697 nm (PLQY: 16%) and 687 nm (PLQY: 20%), respectively (fig. S9A). The TRPL spectra show that Au₈-C (3.6 μs) and Au₈-P (5.4 μs) crystals have microsecond-order lifetimes under ambient conditions (fig. S9B), indicating that both crystals have phosphorescence-type emission properties. To investigate the effect of low temperature on the optical properties of gold NCs, the temperature-dependent PL and TRPL spectra of Au₈-C and Au₈-P were performed. As the temperature decreases from 300 to 80 K, the marked enhancement of the PL intensity and much longer lifetimes can be observed for Au₈-C and Au₈-P (fig. S10), indicating a notable suppression of nonradiative relaxation, which is attributed to the suppression of ligand vibrations at low temperatures. The consistent electrospray ionization mass spectrometry (fig. S11), absorption (fig. S12), emission spectra (fig. S13A), and decay lifetimes (6.35/6.40 ns; fig. S13B) of the solutions containing Au₈-C and Au₈-P indicate their consistency in the solution state. In addition, there were no obvious changes in absorption, emission, and Raman spectra within 25 days (fig. S14), indicating the well stability of the prepared crystals under ambient conditions.

To gain further insights into the intrinsic relation between the stacking mode of Au₈ NCs and their luminescent properties, in situ high-pressure PL, TRPL, and absorption spectra were performed. As shown in Fig. 3A, as the pressure increases, the PL spectra of Au₈-C display enhanced emission. When the pressure reaches 1.7 GPa, the PL intensity of Au₈-C is enhanced by more than six times compared with the initial intensity. Upon further compression, the PL signal of Au₈-C is gradually quenched (fig. S15). The PL photographs at different pressures demonstrate the change in emission brightness of Au₈-C and Au₈-P with pressure. By contrast, the emission intensity of Au₈-P continuously decreases with pressure (Fig. 3D). Meanwhile, the change in the emission of Au₈-C and Au₈-P was quantitatively evaluated in combination with their PLQYs under pressure (46). Figure 3C and fig. S16 show the variation trend of the PL intensity and PLQY of Au₈ NCs under pressure. At 1.7 GPa, Au₈-C exhibits the strongest PL intensity and its PLQY reaches a maximum of 76%. To elucidate the excited-state dynamics process of the PL evolution, the pressure-dependent TRPL spectra of Au₈-C and Au₈-P were collected. As shown in Fig. 3 (B and F), the average lifetime of Au₈-C increases from 3.6 to 6.6 μs when the pressure increases to 1.7 GPa. Thereafter, the PL lifetime of Au₈-C decreases (fig. S17), which is consistent with the trend of its PL signal under pressure. However, the average lifetime of Au₈-P only continuously decreases under pressure (Fig. 3E and fig. S18). The absorption spectra of Au₈-C and Au₈-P show that their absorption band edges are slightly red shifted with an increase in pressure and the energy gap value decreases (figs. S19 and S20), which is consistent with the change in the emission wavelengths of the corresponding PL spectra under pressure (fig. S21). Moreover, at different pressures, the absorption intensity of Au₈-C at an excitation wavelength of 450 nm does not show notable changes (fig. S22), indicating that the enhanced luminescence of Au₈-C is not contributed by changes in absorbance. After the pressure was completely released, the PL, absorption, and TRPL spectra of Au₈-C and Au₈-P are similar to those under ambient conditions, illustrating that compression and decompression processes are reversible (figs. S23 to S25). These results indicate that Au₈-C and Au₈-P exhibit stacking-dependent luminescence enhancement phenomena under pressure.

Fig. 3. In situ high-pressure spectroscopic measurements of Au₈-C and Au₈-P.

In situ PL spectra of (A) Au₈-C and (D) Au₈-P crystals at different pressures. The illustrations show the optical micrographs of Au₈-C and Au₈-P at selected pressures. Pressure-dependent TRPL spectra of (B) Au₈-C and (E) Au₈-P crystals. (C) PL intensity and (F) PL lifetime of Au₈-C and Au₈-P crystals as a function of pressure.

To reveal the correlation between stacking-dependent luminescence enhancement and structural evolution, in situ high-pressure ADXRD experiments were performed. As shown in Fig. 4 (A and D), with an increase in pressure, all diffraction peaks shift monotonically to higher diffraction angles, implying that pressure drives lattice contraction in Au₈-C and Au₈-P (tables S5 and S6). Meanwhile, no other notable peak or peak splitting was observed, indicating that no structural phase transition occurs in Au₈-C and Au₈-P under pressure. In addition, ADXRD can be reversibly returned to the initial test state, demonstrating that the structural changes of Au₈ NCs are reversible. The Rietveld refinement results for Au₈-C and Au₈-P at 1 atm and 1.7 GPa are shown based on ADXRD data. The refinement results are in agreement with experimental data, confirming that Au₈-C and Au₈-P belong to the monoclinic crystal system at 1 atm and 1.7 GPa, corresponding to the C2/c and P2₁/n space groups, respectively (Fig. 4, B and C, and fig. S26). The variations in the cell parameters of Au₈-C and Au₈-P upon compression are shown in Fig. 4 (E and F) and fig. S27. Under compression, Au₈-C and Au₈-P compress faster along the a and c axes, respectively. These considerably different anisotropic compressions may be closely related to their stacking modes. Figure S28 shows the variation in the cell volume of Au₈ NCs upon compression. Meanwhile, the relationship between the cell volume and pressure was fitted by the Birch-Murnaghan equation of state to obtain the bulk modulus (B₀) of Au₈-C (21.1 GPa) and Au₈-P (22.2 GPa) (fig. S29), indicating that the compressibilities of Au₈-C and Au₈-P are comparable (47, 48).

Fig. 4. In situ structural characterization of Au₈-C and Au₈-P under high pressure.

In situ ADXRD patterns of (A) Au₈-C and (D) Au₈-P under pressure. Rietveld refinements of ADXRD patterns were collected at 1 atm of (B) Au₈-C and (C) Au₈-P. The insets show the corresponding molecular stacking modes in the monoclinic lattice. Pressure-dependence lattice parameters of (E) Au₈-C and (F) Au₈-P.

To further reveal the intrinsic relationship between the stacking mode and optical properties of Au₈ NCs, the crystal structures of Au₈-C and Au₈-P at different pressures were optimized via density functional theory (DFT). For Au₈-C, as the pressure increases, the compression rate along the a axis is faster than that along the b and c axes (Fig. 5A). By analyzing the intercluster distances of Au₈-C along the a, b, and caxes (18.08, 14.53, and 19.40 Å), it was found that although the intercluster distance is largest along the c axis, the presence of stronger C─H⋯π and π⋯π intermolecular interactions result in larger steric hindrance, which is not favorable for compression (fig. S30). Because the intercluster distances along the a axis are relatively larger and there are no intermolecular interactions, contraction occurs more rapidly in Au₈-C. It is inferred that anisotropic compression correlates with the tightness of the cluster molecular stacking. Similarly, because the largest intercluster distance is along the c axis and intermolecular interactions are absent (fig. S31), Au₈-P exhibits an anisotropic compression tendency that makes it shrink faster along the c axis than a and b axes (Fig. 5B). Anisotropic compression strongly influences the distortion of the Au₈ core. As shown in Fig. 5C and fig. S32, under pressure, Au₈-C and Au₈-P tend to compress faster along the horizontal (a axis) and longitudinal (c axis) direction of the Au₈ core, respectively. To further quantify the structural deformation of Au₈ core under pressure, the distance between two Au_center atoms in the longitudinal direction and two Au_exo atoms in the horizontal direction were defined as L₁ and L₂, respectively (for illustrations see Fig. 5D). The change of L₁/L₂ (AR) (49) was used to represent the structural deformation of the Au₈ core (tables S7 and S8). Figure 5D shows that the AR of Au₈-C gradually increases during compression, reaching a maximum value of 0.275 at 1.7 GPa compared to 0.270 at 1 atm. Upon further compression, the AR value of Au₈-C gradually decreases. By contrast, the AR of Au₈-P decreases monotonically under pressure. Therefore, the relationship between the structural evolution and optical properties of Au₈ NCs under pressure can be inferred as follows: When pressure is applied to Au₈ NCs, different stacking modes lead to distinct deformation trends of the Au₈ core. When the AR value increases, the Au₈ core becomes more compact and the bond length becomes more uniform, leading to an improvement in the PLQYs (Fig. 5E). Conversely, as the AR value decreases, the Au₈ core becomes more flattened and the length of metallic bonds varies more differently, resulting in the gradual quenching of the PLQYs (Fig. 5F).

Fig. 5. The evolution of anisotropic lattice compressibilities and core structure in Au₈-C and Au₈-P under pressure.

Anisotropic lattice compressibilities of (A) Au₈-C and (B) Au₈-P. (C) Anisotropic Au core compressibility of Au₈-C and Au₈-P. (D) Pressure-dependent the change in the AR of Au₈-C and Au₈-P. Schematic diagram of the stacking-dependent emission enhancement in (E) Au₈-C and (F) Au₈-P crystals under pressure.

To further investigate the effect of pressure on the kernel structure of Au₈-C and Au₈-P, in situ high-pressure Raman spectra were performed. With an increase in pressure, the vibration peaks of Au₈-C and Au₈-P gradually blue shift due to continuous lattice contraction (Fig. 6, A and C). In the Raman spectra of Au₈-C, the vibrational peaks in the low-frequency region (72.8 and 131.4 cm⁻¹) are attributed to the asymmetric stretching vibrations and breathing modes of the Au₈ kernel (50). These vibration peaks gradually broaden as the pressure increases and almost disappear when the pressure reaches 1.7 GPa (Fig. 6, A and B), indicating that the degrees of freedom and low-frequency vibrations of the Au₈ kernel are notably suppressed. This is associated with the pressure-induced enhancement of the PLQYs of Au₈-C. However, below 2.0 GPa, the vibrational peaks of Au₈-P do not show notable changes, indicating that the vibrations of its Au₈ core are not easily suppressed by pressure (Fig. 6, C and D). The effect of pressure on ligand vibrations is also unknown; therefore, pressure-dependent high-frequency Raman spectra of Au₈-C and Au₈-P were collected (fig. S33). Upon compression, the intensity of the main Raman signals gradually decreases, indicating that the vibrations of the ligand in Au₈-C and Au₈-P are suppressed. To study the evolution of noncovalent interactions in Au₈ NCs under high pressure, in situ high-pressure IR spectra of Au₈-C and Au₈-P were obtained (fig. S34). In Au₈-C, the O─H stretching vibration [ν(O─H)] shows an obvious redshift, indicating that the C─H⋯O hydrogen bonds are strengthened. The strengthening of hydrogen bonds may limit the benzene ring vibration and reduce the nonradiative loss, which contributes to the enhancement of piezoluminescence of Au₈-C. However, no notable redshift of the vibration peak was observed in Au₈-P, indicating that there was no strengthening of hydrogen bonds in Au₈-P.

Fig. 6. Raman spectra and excited-state dynamics of Au NCs under pressure.

Raman spectra of (A) Au₈-C and (C) Au₈-P at different pressures. Raman shift of (B) Au₈-C and (D) Au₈-P as a function of pressure. (E) Transient absorption data map of Au₈-C at 1 atm upon excitation at 530 nm. Evolution-associated spectra of Au₈-C at (F) 1 atm and (G) 1.7 GPa were obtained from global fitting on the TA data.

To gain further insights into the underlying photophysical mechanism of the enhancement of PLQYs in Au₈ NCs under pressure, femtosecond transient absorption (fs-TA) spectra were performed on Au₈-C crystals at 1 atm and 1.7 GPa, respectively. Upon excitation with a 530-nm laser pulse, Au₈-C displays notable excited-state absorption signals in the 587- to 735-nm range (Fig. 6E and fig. S35). The TA dynamics of Au₈-C crystals at 1 atm and 1.7 GPa can be decomposed into three components via global fitting (Fig. 6, F and G), and their corresponding kinetic traces at 700 nm are shown in fig. S36. The ultrafast component τ₁ of the Au₈-C crystals at 1 atm (5.38 ps) and 1.7 GPa (5.82 ps) may be assigned to internal conversion from S_n to S₁ states, and the second decay time τ₂ at 1 atm (29.01 ps) and 1.7 GPa (72.36 ps) may be ascribed to the structural relaxation of the Au₈-C crystal (51). The third decay process of Au₈-C is long-lived and exceeds the detection limit of the fs-TA. Combining these results with the microsecond lifetimes obtained by TRPL, the long lifetimes τ₃ of Au₈-C crystals are assigned to the relaxation from the T₁ state to the ground state to emit phosphorescence. Above all, the notably longer τ₂ of Au₈-C at 1.7 GPa (compared to its value at 1 atm) implies that its nonradiative relaxation is reduced under pressure (52, 53), which is consistent with the suppression of the low-frequency vibrations of its kernel. The radiative decay rate (k_r) and nonradiative decay rate (k_nr) of Au₈ NCs under pressure were further calculated. According to equations

$$\text{PLQY}=\frac{k_{\mathrm{r}}}{(k_{\mathrm{r}}+k_{\text{nr}})}$$ (1)

$$\begin{array}{c}\mathrm{\tau }=\frac{1}{k_{\mathrm{r}}+k_{\text{nr}}}\end{array}$$ (2)

Either increasing k_r or reducing k_nr will increase the PLQY. Based on the PLQY and lifetime data of Au₈ NCs, the k_r and k_nr rates at each pressure point were obtained. At 1.7 GPa, the k_r rate of Au₈-C exhibits threefold, and the k_nr rate declined sevenfold (table S9). However, Au₈-P only shows a decrease in k_r and a rapid increase in k_nr under pressure (table S10).

To further investigate the relationship between the electronic structure and optical properties of Au₈-C and Au₈-P, time-dependent DFT (TDDFT) calculations were performed. The simulated absorption spectra of Au₈-C (Fig. 7, A and D) and Au₈-P (fig. S37A) correlate well with experimental spectra, indicating the accuracy of computational results. The molecular orbitals [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)] of Au₈-C and Au₈-P present the typical superatomic configuration (fig. S38). To analyze the molecular orbital composition of Au₈-C and Au₈-P, the transitions of their molecular orbitals are presented in figs. S39 and S40. The isolated absorption peaks of Au₈-C (a) and Au₈-P (α) correspond to the transitions from the highest occupied molecular orbital to the lowest unoccupied molecular orbital (HOMO → LUMO). Notably, in Au₈-C, the LUMO distribution is more delocalized under high pressure than the initial orbital distribution (Fig. 7, C and F), which is correlated with enhanced luminescence (54). The Kohn-Sham molecular orbitals of Au₈-C (Fig. 7, B and E) and Au₈-P (fig. S37B) show that their HOMO and LUMO are predominantly constituted by Au (6sp) atomic orbitals. Therefore, the red luminescence of Au₈-C and Au₈-P originates from the metal-centered state. In addition, the hole-electron pairs of the T₁ state are mainly distributed on the kernels of Au₈-C and Au₈-P (figs. S41 to S44), further confirming metal-centered state luminescence. Meanwhile, the spin-orbit coupling constant (SOC) of Au₈-C is higher at high pressure than at 1 atm (Fig. 7H), which will contribute to the promotion of the intersystem crossing rate. However, the SOC of Au₈-P does not exhibit noticeable changes under pressure (fig. S46).

Fig. 7. Pressure-induced PL enhancement mechanism of Au₈-C.

Experimental and calculated absorption spectra of Au₈-C at (A) 1 atm and (D) 1.7 GPa. The Kohn-Sham molecular orbital energy level diagram and associated populations of atomic orbitals in each molecular orbital for Au₈-C at (B) 1 atm and (E) 1.7 GPa. Frontier orbitals including the HOMO and LUMO of Au₈-C at (C) 1 atm and (F) 1.7 GPa. (G) Radiative and nonradiative recombination rates of Au₈-C at different pressures. (H) Schematic diagram of the PL mechanism of Au₈-C at 1 atm and 1.7 GPa. Ex., excitation; N.R., nonradiative; Phos., phosphorescence; a.u., arbitrary unit.

In summary, under high pressure, the different molecular stacking arrangements of Au₈-C and Au₈-P NCs lead to distinct anisotropic compression behaviors. The rapid shrinkage of the lattice a axis in Au₈-C leads to a more uniform Au─Au bond length in the Au₈ kernel, notably suppressing low-frequency vibrations and thus decreasing nonradiative loss. This is the source of the pressure-induced PL enhancement in Au₈-C (Fig. 7, G and H). However, the kernel of Au₈-P becomes more flattened under compression and its low-frequency vibrations are not considerably suppressed, resulting in pressure-induced PL quenching (figs. S45 and S46). To validate the correctness of the discovered stacking-dependent efficient luminescence mechanism, we designed and synthesized an eight-core Au NC, protected by the 4-ethynylbenzaldehyde ligand (Au₈-CHO), which displayed pressure-induced luminescence enhancement. The crystal structure of Au₈-CHO (fig. S47, tables S11 to S13) and its cluster molecular stacking pattern (fig. S48) are almost identical to those of Au₈-C. The pressure-dependent microscopic optical photos, luminescence phenomenon, and Raman spectra of Au₈-CHO are similar to the evolution in the optical behavior of Au₈-C under high pressure (figs. S49 and S50). In addition, the Au₈-TPE (55) and Au₈-CzPA (56) were also synthesized to further confirm the piezoluminescence enhancement stemmed from NCs stacking in the unit cell. Upon compression, the packing arrangement of Au₈-TPE and Au₈-CzPA leads the lattice units to shrink faster along the c and a axes, respectively. Thus, the AR of Au₈-TPE and Au₈-CzPA show opposite changing trends under pressure (figs. S51 and S52). Ultimately, the Au₈-TPE exhibits the piezoluminescence enhancement, whereas Au₈-CzPA shows only pressure-induced luminescence quenching. The absove results further prove the correctness of our discovered structure-property relationship between the stacking mode and PLQYs.

DISCUSSION

In conclusion, we designed and synthesized a pair of polymorphic superatomic Au NCs (Au₈-C and Au₈-P) and systematically elucidated the stacking-dependent efficient luminescence mechanism of metal NCs via the hydrostatic pressure effect and a series of in situ characterization methods. Under compression, columnar-stacked Au₈-C exhibits notable luminescence enhancement, with its PLQY reaching a maximum of 76% at 1.7 GPa, whereas lamellar-stacked Au₈-P only displays luminescence quenching. Pressure-dependent ADXRD results reveal that Au₈-C and Au₈-P show anisotropic trends with faster compression along the lattice a axis and c axis, respectively. The faster shortening of the a axis leads to a more compact core in Au₈-C, whereas the faster compression along the c axis results in a more flattened core in Au₈-P. TRPL, TA, and Raman spectra collaboratively demonstrated that pressure effectively suppresses the low-frequency vibrations of the kernel in Au₈-C, leading to a decrease in nonradiative loss. This suppression is responsible for the pressure-induced emission enhancement in Au₈-C. This study provides a deeper understanding of the relationship between cluster stacking and luminescence properties, providing valuable insights for designing highly efficient luminescent superatomic NCs in the future.

MATERIALS AND METHODS

Materials and reagents

All chemicals and reagents for synthesis were obtained from commercial sources and used without any further purification. HAuCl₄·4H₂O were purchased from Civic Chemical. The dppp (97%) and sodium borohydride (99%) were purchased from Sigma-Aldrich. 4-ethynylbenzaldehyde (98%), 9-(4-ethynylphenyl)carbazole (98%), (2-(4-ethynylphenyl)ethene-1,1,2-triyl)tribenzene (96%), and phenylacetylene (98%) were purchased from Macklin. Methanol (99.5%), dichloromethane (99%), and ether (99%) were purchased from Macklin.

Synthesis of Au₈-C and Au₈-P crystals

The [Au₈(dppp)₄(C≡CPh)₂](NO₃)₂ clusters were synthesized using [Au₈(dppp)₄](NO₃)₂ (57) as precursor. A methanolic solution (50 ml) of [Au₈(dppp)₄](NO₃)₂ (30.0 mg, 9 μmol) was added to phenylacetylene (2.0 μl, 18 μmol) and sodium methoxide (145 mg, 2.7 mmol), and the mixture was stirred at room temperature for 10 hours. The obtained mixture was treated with water and then extracted with dichloromethane (20 ml × 3). The organic phase was dried by anhydrous Na₂SO₄, filtered, and evaporated to dryness to give a pinkish solid; purified by recrystallization by vapor diffusion of ether into a dichloromethane/methanol (~50/50 v/v) solution to give Au₈-C as purple crystals; and purified by recrystallization by vapor diffusion of ether into a dichloromethane solution to give Au₈-P as red crystals. The synthesis yields of Au₈-C and Au₈-P are 46 and 48%, respectively. Au₈-CHO was prepared using the same synthetic route as described above, with the synthesis yield of 52%.

Crystallographic data collection and structural refinement

Single-Crystal x-ray Diffraction (SCXRD) measurements of Au₈-P clusters were performed at 200 K on a Rigaku XtaLAB Pro diffractometer with Cu-Kα radiation (λ = 1.54184 Å). Data collection and reduction were performed using the program CrysAlis^Pro (58). The structure of Au₈-P was solved with direct methods (SHELXS) (59) and refined by full-matrix least squares on F² using OLEX2 (60), which uses the SHELXL-2015 module (61, 62). All atoms were refined anisotropically, and hydrogen atoms were placed in their calculated positions with idealized geometries and assigned fixed isotropic displacement parameters. The detailed information of the crystal data and refinement results are summarized in table S2.

Instrumentation

PXRD patterns of the samples were collected at room temperature in the air using a Rigaku MiniFlex diffractometer (Cu-Kα, λ = 1.54178 Å; 2θ range of 3° to 50°). Simulated powder patterns were obtained with Mercury software and a crystallographic information file from a single-crystal x-ray experiment. Mass spectra (MS) were recorded on an X500R QTOF spectrometer.

Photophysical measurements

Absorption spectra were recorded using a Hitachi UH4150 ultraviolet-visible spectrophotometer in the range of 200 to 800 nm. PL spectra were recorded with a HORIBA FluoroLog-3 fluorescence spectrometer. TRPL decay spectra were measured on a HORIBA FluoroLog-3 fluorescence spectrometer equipped with a 370-nm laser operating in time-correlated single-photon counting mode. The PLQYs in the solid state were operated using an integrating sphere on the HORIBA FluoroLog-3 fluorescence spectrometer.

High-pressure generation

High pressure was generated by a symmetric diamond anvil cell (DAC) through a pair of diamonds with 500-μm culet size. The Au₈-C, Au₈-P crystals, and a small ruby ball were loaded into the 180-μm-diameter hole of a DAC, which was made of a T301 steel gasket pre-indented to a thickness of 45 μm. All the high-pressure experiments used the silicon oil as a pressure transmitting medium (PTM). The PTM remains quasi-hydrostatic up to 4 GPa but can be safely used in the pressure range covered by this study (63) and did not have any detectable effect on the behavior of Au₈-C/Au₈-P crystals under pressure. All the experiments were conducted at room temperature.

In situ high-pressure optical experiments

The in situ high-pressure PL spectra were measured using a 450-nm laser excitation. The PL micrographs were captured using an Olympus DP74 camera equipped on the microscope (Olympus BX53) with the same exposure time. High-pressure absorption experiments were carried out using the deuterium-halogen light source (Ocean Insight DH-2000-BAL) and a fiber spectrometer of Ocean Insight QEP03490. The TRPL spectra were measured in a PMA Hybrid spectrometer using a 450-nm picosecond laser with a resolution time of 512 ps. The Raman spectra were collected in a spectrometer (Princeton Instruments, HRS-500) with a cryogenic detector (Andor, iVac 316). The excitation source is a laser with a wavelength of 785 nm, and the grating used is 2400 lines/mm Bragg grating. In situ high-pressure ADXRD experiments were carried out at beamline 15 U1, Shanghai Synchrotron Radiation Facility (SSRF) >and 4 W2 HP-Station in the Beijing Synchrotron Radiation Facility (BSRF). Before high-pressure ADXRD measurements, CeO₂ was applied for calibration. The wavelength of the incident x-ray beam was adjusted to be 0.6199 Å. The one dimensional ADXRD spectra were obtained using the Dioptas software. All high-pressure experiments were conducted at room temperature.

The fs-TA measurements were carried out on a homebuilt pump-probe system used with a regeneratively amplified Ti:sapphire laser system (Coherent Legend Elite HE+USP-1 K-III, 35 fs, 1 kHz). The central wavelength of the output pulse is 800 nm, and the output pulse is split into two beams. The first beam passes through the optical parametric amplifier (Light Conversion, TOPAS Prime) to generate 530 nm. The second beam is focused on sapphire to produce continuous white light (from 450 to 1000 nm) for visible near-infrared detection. The time delay between the pump and probe pulse is controlled by the motorized delay stage. The transmission changes of the probe light were collected by a fiber spectrometer (AvaSpec-ULS2048CL-EVO, Avantes).

Computational details

DFT calculations

All-electron DFT calculations have been carried out by the latest version of ORCA quantum chemistry software (64) (version 5.0.4). The calculated structures were built from their single-crystal structures. The position of the H atoms was optimized, and the other atoms kept their positions unchanged. The Perdew-Burke-Ernzerhof (PBE) functional (65) and the def2-SVP basis set (66) were adopted for geometry optimization calculation. The DFT-D3 dispersion correction with BJ damping (67, 68) was applied to correct the weak interaction to improve the calculation accuracy. The excited states calculation and spin-orbit coupling (SOC) calculation were performed with TPSSH functional (69) and mixed basis set (SARC-DKH-TZVP basis set for Au atoms and DKH-def2-SVP basis set for other atoms). SOC calculation was performed by the spin-orbit mean-field method (70).

Theoretical calculation of weak interaction calculations

The calculation was carried out using Gaussian 09 software package. First, the PBE/def2tzvp functional basis set was used to optimize the structure of Au₈-C cluster under normal pressure using DFT optimization only for hydrogen atoms. Then, based on the minimum structure of ground state, TPSSH/def2-SVP level TDDFT calculations were performed; Multiwfn software was used combined with VMD program to perform IGMH analysis, and the desired hydrogen bonds were drawn (71).

Refinements methods and procedures

Pawley and Rietveld refinements of XRD patterns were accomplished using the Reflex module combined with the Materials Studio program. All Pawley and Rietveld refinements were performed using four refinement cycles and fine convergence criteria. First, the pattern was indexed by means of the peak picking option of the software package. Potential solutions for cell parameters were found using the X-cell methods. Then, a Pawley profile-fitting procedure was applied to refine cell parameters and search space groups. The final Rietveld refinement (including Pawley refined parameters, atomic positions, preferred orientations, and overall isotropic factor) was performed to obtain the crystal structural parameters. The quality of the fitting between the experimental and calculated profile is assessed by the various R parameters like R_p (profile factor) and R_wp (weighted profile factor)

$$\begin{array}{c}{R}_{\mathrm{p}}=\left\{\frac{{\sum }_i\mid I_i^{\text{obs}}-I_i^{\text{cal}}\mid }{{\sum }_i{I}_i^{\text{obs}}}\right\}\end{array}$$ (3)

$$\begin{array}{c}{R}_{\text{wp}}={\left\{\frac{{\sum }_i{w}_i{(I_i^{\text{obs}}-I_i^{\text{cal}})}^2}{{\sum }_i{w}_i{\left(I_i^{\text{obs}}\right)}^2}\right\}}^{\frac{1}{2}}\end{array}$$ (4)

where $I_i^{\text{obs}}$, $I_i^{\text{cal}}$, and “i” indicates the experimental, calculated, and total number of points, respectively, and the “w_i” is the reciprocal of the variance of observation $I_i^{\text{cal}}$.

Bulk modulus determination using the Birch-Murnaghan equation of state

The pressure-volume (P-V) data were fitted by the third-order Birch-Murnaghan (B-M) equation of state

$$\begin{array}{c}\begin{array}{c}P\left(V\right)=\frac{3B_0}{2}[{\left(\frac{V_0}{V}\right)}^{\frac{7}{3}}-{\left(\frac{V_0}{V}\right)}^{\frac{5}{3}}]\left\{1+\frac{3}{4}(B\prime -4)[{\left(\frac{V_0}{V}\right)}^{\frac{2}{3}}-1]\right\}\end{array}\hfill \end{array}$$ (5)

where V₀ is the zero-pressure volume, B₀ is the bulk modulus, and B′ is the parameter for the pressure derivative. For the structural comparisons, B′ is fixed at 4.

Calibration of pressure using emission line of ruby sphere

The measurement of ruby fluorescence and sample photoluminescence at the same time. There are two emission lines in ruby fluorescence, and the prime one is the so-called R₁ ruby fluorescence line (72). The R₁ line shows a roughly linear dependence on pressure. Therefore, the spectral position of the R₁ line can be used to obtain the pressure using the empirical equation as follows (73, 74)

$$\begin{array}{c}P\left(\text{GPa}\right)=\frac{1904}{B}[{(1+\frac{\mathrm{\Delta }\mathrm{\lambda }}{{\mathrm{\lambda }}_0})}^B-1]\end{array}$$ (6)

where λ₀ is the wavelength of the R₁ line near 694.25 nm under ambient conditions and ∆λ is the difference value between the measured wavelength λ and the initial wavelength λ₀.

Details of PLQYs calculation under pressure

The PLQY can be calculated using the following equation (46)

$$\begin{array}{c}\mathrm{\Phi }={\mathrm{\Phi }}_R\frac{\int F\left({\mathrm{\lambda }}_{\text{em}}\right)}{\int F_R\left({\mathrm{\lambda }}_{\mathit{em}}\right)}\frac{A_R\left({\mathrm{\lambda }}_{\mathit{ex}}\right)}{A\left({\mathrm{\lambda }}_{\mathit{ex}}\right)}\frac{n^2}{n_R^2}\end{array}$$ (7)

where Φ is the PLQY, ∫F(λ_em) is the integrated intensity of emission, A is the absorbance at the excitation wavelength, n is the refractive index, and the subscript R denotes the reference data at ambient pressure.

n can be estimated from the Clausius-Mossotti equation and Lorentz-Lorenz equation

$$\begin{array}{c}\frac{n^2-1}{n^2+2}·\frac{1}{\mathrm{\rho }}=\frac{4\mathrm{\pi }}{3}·N_{\mathrm{A}}·\mathrm{\alpha }=R_{\text{LL}}\end{array}$$ (8)

in which the density ρ can be calculated from the cell volume. R_LL is called the Lorentz-Lorenz constant. R_LL is related to polarizability α.

Supplementary Materials

This PDF file includes:

Figs. S1 to S52

Tables S1 to S13

References