Research ArticleChemistry

Ultrastable atomically precise chiral silver clusters with more than 95% quantum efficiency

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Science Advances  07 Feb 2020:
Vol. 6, no. 6, eaay0107
DOI: 10.1126/sciadv.aay0107

Abstract

Monolayer-protected atomically precise silver clusters display low photoluminescence (PL) quantum yield (QY) and susceptibility under ambient conditions, and their chiroptical activities also remain underdeveloped. Here, we report enantiomers of an octahedral Ag6 cluster prepared via one-step synthesis using designed chiral ligands at ambient temperature. These clusters exhibit a highest PLQY (300 K) >95.0% and retain their structural integrity and emission up to 150°C in air. Atomically precise structural determination combined with photophysical and computational analysis revealed that thermally activated delayed fluorescence, observed in silver cluster systems, is responsible for the high PLQY, which combines chirality in excited states to generate strong circularly polarized luminescence. These unprecedented findings open up horizons of investigation of monolayer-protected silver clusters for future luminescence applications.

INTRODUCTION

Metal clusters of small size have discrete energy levels, and electronic transitions between them account for the emergence of a new class of luminescent materials (17). To obtain an atomic-level understanding of fundamentally important structure-function relations, monolayer-protected metal clusters with diverse ligands to regulate their structures and emissive properties (35, 817) have been synthesized, and their total structures were elucidated. However, because of the well-known inherent susceptibility of silver, the number of stable atomically precise silver clusters in oxidized or reduced states under ambient conditions or at higher temperatures remains very limited (1113), and all of them suffer from low quantum efficiency or nonluminescence (1625). The reported quantum yield (QY) record is 6.6% for atomically precise Ag(I) clusters from ligand-based emission (24) and 11.7% for mixed-valence ones from aggregation-induced emission in solution (25), preventing understanding of the fundamental cluster structure-photoluminescence relations and restricting the development of their applications as lighting materials. Hence, the designed construction of atomically precise monolayer-protected silver clusters having a satisfactory photoluminescence QY (PLQY) and stability at high temperature in air for future practical application remains a practical challenge, probably because the usual ligand-to-metal charge transfer (LMCT) transition responsible for the PL of silver clusters has intrinsic lower emission efficiency (3, 1625). To resolve the dilemma, exploring a new type of emission mechanism or increasing transition channels in silver cluster systems may be an efficient strategy.

Chiral metal clusters constitute a paramount subset of cluster families that have considerable promise in diverse chiral applications (2628). The combination of chirality and luminescence in metal clusters offers circularly polarized luminescence (CPL) (2830), which can enable widespread potential applications in encrypted transmission, information storage, and chiroptical materials. The transfer of ligand chirality is a simple and efficient method to produce chiral metal clusters (24, 30, 31). However, the reported luminescent chiral silver clusters, including chiral ligand- and DNA-stabilized clusters, consistently suffer from incomplete knowledge of their molecular structure (32, 33). The construction of atomically precise silver clusters with intense CPL response requires concomitant chirality and strong luminescence. A clear understanding of their chiroptical activity at the atomic level presents a great challenge, and, to the best of our knowledge, it has not been achieved.

In this work, we first synthesized a pair of enantiomeric octahedral Ag(I) clusters (Ag6L6/D6) in a single step using the chiral nonemissive ligand (S)-/(R)-4-isopropylthiazolidine-2-thione (abbreviated as L and D) (34). Ag6L6/D6 display intense room temperature (RT) yellow luminescence with QY (300 K) = 56%. By changing isopropyl substituents to a phenyl group in (S)-/(R)-4-phenylthiazolidine-2-thione (hereafter named PL and PD), another enantiomeric pair of Ag6PL6/PD6 with a more compressed Ag6 core is obtained, which achieves a highest RT PLQY (300 K) > 95%. These silver clusters retain their structural integrity and emission up to 150°C in air, and their bright photoluminescence combines with chirality in excited states to generate strong CPL. Atomically precise structural analysis, systematic photophysical measurements, and theoretical calculations revealed that thermally activated delayed fluorescence (TADF) observed in silver cluster systems contributes to this high PLQY. Moreover, chiroptical activities arising from the core and interface, together with strong TADF-based CPL, are unveiled.

RESULTS AND DISCUSSION

Synthesis and characterization

The chiral ligands L/D and PL/PD were prepared from (S/R)-2-amino-3-methyl-1-butanol and (S/R)-2-phenylglycinol, respectively, according to literature procedures (Fig. 1B and fig. S1) (34). Enantiomeric silver clusters (Ag6L6/D6 and Ag6PL6/PD6) were prepared by simple reactions of AgNO3 with ligands (L/D and PL/PD) in a dimethylacetamide/acetonitrile (DMAc/CH3CN) mixture in air, which formed a clear solution that deposited block-like single crystals after 2 days (Fig. 1).

Fig. 1 Silver cluster and ligand structures, images of single crystals, and PXRD patterns.

(A) Ball-and-stick representation of the enantiomers of Ag6L6/D6 and Ag6PL6/PD6. Inset: Schematic of the Ag6 octahedron core in these enantiomers. (B) Structures of L/D and PL/PD. (C) The octahedral Ag6 framework in Ag6L6/D6. Color codes: Ag, green; C, gray; S, yellow; N, blue; H atoms are omitted for clarity. (D) Images of single crystals of Ag6L6 under ambient light (top) and UV light (bottom) at different temperatures. See the images of Ag6PL6 single crystals in fig. S3. Photo credit: Zhen Han, Zhengzhou University. (E) Temperature-dependent PXRD patterns of the as-prepared Ag6L6 powder sample and one sample after exposure to UV light for 60 min.

Single-crystal x-ray diffraction (SCXRD) analysis revealed that the enantiomeric Ag6L6/D6 clusters crystallize in the chiral cubic space group P213 (no. 198) (table S1), with Flack parameters of 0.07 (3) and 0.04 (3) at 150 K, respectively. Ag6PL6/PD6 belongs to the monoclinic chiral space group P21 (no. 4) (table S1), with chiral parameters of −0.025 (5) and −0.019 (4) at 150 K, respectively, indicating homochiral molecular packing in their crystal structure. All of these clusters exhibit a distorted silver octahedron with six faces capped by chiral ligands that alternate in their orientation (Fig. 1A). Every Ag atom is coordinated by two sulfur atoms and a nitrogen atom from three chiral ligands, and each ligand ligates three silver atoms (fig. S2A). Argentophilic interactions (3537), which are prominent in forming the Ag6 octahedron as an integral group (Fig. 1C, fig. S2, and table S2), consolidate the twisted octahedral silver framework. The core and surface of Ag6L6/D6 and Ag6PL6/PD6 deviate from those of a regular octahedral Ag6 skeleton (Fig. 1C and fig. S2), and the role of chiral ligands directing such distortions is clearly discernible. When achiral thiazolidine-2-thione–based ligands such as 2-mercaptothiazoline and 2-mercaptobenzothiazole were used, no octahedral Ag6 cluster formed, further suggesting that the chiral ligands play a key role in the generation of the distorted Ag6 framework.

The formula and phase purity of Ag6L6 and Ag6PL6, which are used as representatives in the following description, are further confirmed by electrospray ionization mass spectrometry (ESI-MS) (fig. S2), elemental analysis, thermogravimetric analysis (fig. S3), and powder x-ray diffraction (PXRD) (Fig. 1E and fig. S3). Temperature-dependent SCXRD measurements of Ag6L6 and Ag6PL6 (Fig. 1D, fig. S3 and table S1) provide high-quality diffraction data in the range of 100 to 370 K, demonstrating good single crystallinity at high temperature. Temperature-dependent PXRD analysis (Fig. 1E and fig. S3) and subsequently discussed temperature-dependent luminescence further corroborate the high thermostability of these enantiomeric silver clusters in air, which is a rare property in ligand-protected silver clusters, including mixed-valence and normal-valence ones (1125).

Photoluminescence and yellow-emitting phosphor

Both pairs of enantiomeric silver clusters display bright photoluminescence under ambient conditions. Solid-state Ag6L6/D6 exhibit broad ultraviolet-visible (UV-vis) absorption with a band edge of 500 nm, which extends to 550 nm for solid-state Ag6PL6/PD6 (fig. S4). Ag6L6/D6 exhibit bright yellow emission centered at 556 nm, which can be excited in the 300- to 480-nm range (Fig. 2, A and B, and fig. S4), with a PLQY of 56% and an observed decay lifetime (τobs) of 18.6 μs under ambient conditions (Fig. 3). Compared to the emission spectra of Ag6L6/D6, those of Ag6PL6/PD6 red-shifted to 575 nm with excitation at 300 to 550 nm and a τobs of 16.3 μs in air (fig. S4). The PLQY of Ag6PL6/PD6 surpasses 95% at RT. In addition, these silver clusters are stable after 60 min of exposure to UV light (Fig. 1E). The microsecond lifetimes indicate that the transitions are spin-forbidden, being similar to those reported in other silver(I) clusters (16, 17, 19, 2123). Such a high QY is unprecedented for monolayer ligand-protected silver(I) clusters (1624), making them suitable candidates for light-based applications.

Fig. 2 Photoluminescence and WLED assembly.

(A) Normalized excitation and emission spectra of Ag6L6 and Ag6PL6 in the solid state at RT. (B) Solid-state emission spectra of Ag6L6 at different excitation wavelengths at RT. (C) Comparison of emission spectra between Ag6L6 and a commercial yellow phosphor, cerium-doped yttrium aluminum garnet (YAG: Ce3+), which are nearly identical at RT. The CIE coordinates of Ag6L6 (0.41, 0.55) overlapped those of YAG: Ce3+ (0.41, 0.56). (D) Photographs of WLED assembly: 1, blue LED is turned off (emission range, 450 to 480 nm; OSRAM LED Lighting Company); 2, the same blue LED coated with Ag6L6 powders is turned off; 3, LED coated with Ag6L6 powders emits white light when it is turned on. The green solid line is the monitored emission spectrum. Photo credit: Zhen Han, Zhengzhou University.

Fig. 3 Temperature-dependent emission spectra of Ag6L6 and the proposed PL process.

(A) Normalized temperature-dependent solid-state emission in the range of 50 to 400 K upon excitation at 370 nm. (B) Temperature-dependent solid-state emission intensity in the range of 50 to 400 K. (C and D) Temperature dependence of the excited-state lifetimes in the range of 50 to 300 K. (E) Plot of emission decay lifetime against temperature (50 to 300 K); the red line represents the fit according to the TADF equation (eq. S1 in section S1). (F) Energy diagram of Ag6L6 indicating TADF and phosphorescence (Ph) emission processes. See the main text for details of the processes.

The emission spectra of these enantiomeric silver clusters in dilute solution with a maximum wavelength at 574 nm nearly overlap and are solvent independent (fig. S5), suggesting that the emission could originate from the inner Ag6 core and interfacially coordinated S6N6 atoms, being slightly influenced by the outer shell layer (phenyl and isopropyl groups). Compared to those in the solid state, their emission lifetimes in solution considerably decreased (fig. S5). As a representative, Ag6L6 is mainly discussed in the subsequent photoluminescence analysis. The emission peak at 556 nm of solid-state Ag6L6 is independent of the excitation wavelength at RT (Fig. 2B), indicating that the emission originates from the lowest excited states. Ag6L6 crystals still shine brightly at 150°C in air with approximately one-fifth intensity of RT emission (fig. S6). This behavior is similar to that of Ag6PL6 (figs. S5 and S6).

The bright yellow emission of solid-state Ag6L6 with Commission Internationale de l’Éclairage (CIE) coordinates of (0.41, 0.55) is very close to that of commercial cerium-doped yttrium aluminum garnet (YAG:Ce3+; 0.41, 0.56), and their RT emission spectra are nearly identical (Fig. 2C and fig. S6). A solution process was used for the preparation of Ag6L6 polymethylmethacrylate films, which demonstrated similar emissive spectra and lifetimes with those of crystalline Ag6L6 with a 16-nm red shift (fig. S6). The blue-excitable yellow emission combined with its thermal stability raises the possibility of Ag6L6 as a yellow-emitting phosphor. Next, we fabricated an archetype of white light-emitting diode (WLED) by using a commercial blue LED panel with an emission peak approximately at 465 nm whose top surface is coated with a Ag6L6 powder sample. As shown in Fig. 2D, bright white light was generated when the blue LED was turned on with a voltage of 3.0 V, and its white light emission spectrum is demonstrated.

TADF contributing to the bright photoluminescence

To fully understand the underlying principles of QY enhancement in the silver clusters, we performed temperature-dependent measurements of the luminescence properties of Ag6L6/Ag6PL6 in the range of 8 to 400 K (Fig. 3 and fig. S6). Upon decreasing the temperature from 400 to 8 K, the emission energy of Ag6L6 decreased, causing the emission maximum to red-shift from 550 nm (400 K) to 594 nm (8 to 40 K) under excitation at 370 nm (Fig. 3A). The emission maximum remained approximately at 594 nm from 8 to 40 K, and its intensity is also nearly consistent. When the temperature increased from 50 to 125 K, the intensity of the emission peaks increased and subsequently continuously decreased as the temperature further rose to 400 K (Fig. 3B). This trend of temperature-dependent emission energies is also found with Ag6PL6 (fig. S6). This observation is reminiscent of TADF materials (3841), where delayed fluorescence is obtained owing to efficient reverse intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited state (S1). In the past few years, some strategies have been developed to achieve TADF in low-cost Cu(I) and Ag(I) compounds (3841). We tentatively assigned the higher energy emission band (556 nm) of Ag6L6 at RT, which crosses over its excitation spectrum (Fig. 2A), mainly to TADF, while the energy-lowered emissive bands centered at 594 nm between 8 and 40 K were assigned to phosphorescence emitted from the lowest triplet excited state (T1).

To obtain a deeper understanding of the photophysical properties of these silver clusters, we measured the emission decay curves of Ag6L6 and Ag6PL6 at different temperatures ranging from 8 to 300 K (Fig. 3, C and D, and fig. S6). At low temperature (8 to 40 K), a constant value of approximately 48 μs, which mainly consists of the phosphorescence decay time τ(T1) for the T1 → S0 transition, was observed. Above 50 K, the decay lifetime gradually decreased to 18 μs at 300 K. Fitting the temperature-dependent photoluminescent decay lifetimes τ(T, 50 to 300 K) in Fig. 3E of Ag6L6 to the modified Boltzmann equation (eq. S1 in section S1) (39) gave an activation energy of ΔE(S1 − T1) = 0.096 eV, a radiative rate of phosphorescence k(T1) = 2.26 × 104 s−1, and a radiative rate fluorescence k(S1) = 4.42 × 106 s−1 that is smaller than that of commonly observed fluorescence, which may be a factor in explaining why phosphorescence and TADF can compete with slow fluorescence, leading to high QY at RT. Prompt fluorescence was not observed in the emission process of Ag6L6. According to the emission QYs and the decay times, one can determine the radiative decay rate kr = ΦPL/τ (300 K) = 3.0 × 104 s−1, suggesting that RT emission contains contributions of delayed fluorescence and phosphorescence, which can also be reflected by a continuous increase in emissive energy from 300 to 400 K. In the PL spectra at 400 K, the TADF component is dominated by a very small phosphorescence contribution.

With the isopropyl unit replaced by the phenyl group, Ag6PL6 displayed a similar trend of temperature-dependent photoluminescence spectra and decays to that of Ag6L6 (fig. S6). Fitting the plots of the emission lifetime of Ag6PL6 against temperature with eq. S1 (section S1) (39) gave ΔEST = 0.041 eV, which is smaller than 0.096 eV for Ag6L6; k(T1) = 3.91 × 104 s−1, which is a little larger than 2.26 × 104 s−1 of Ag6L6; and k(S1) = 5.17 × 105 s−1, which is much smaller than 4.42 × 106 s−1 for Ag6L6. Therefore, the reduced activation energy from T1 to S1 for RISC, the faster rate of radiative ISC from T1 to the ground state (S0), and the much decreased radiative rate from S1 to S0 collectively lead to a substantial increase in QY from 56% for Ag6L6 to 95% for Ag6PL6 at RT. Considering that photoluminescence originates from the cluster core and interface (vide supra), we propose that enhancement in QY is also related to the reduction in nonradiative decay rates of the emissive excited state induced by rigidification of the system arising from the introduction of phenyl groups, which compress the Ag6 core with shorter averaged Ag-Ag contacts (3.13 Å) in crystalline Ag6PL6 compared to 3.27 Å in Ag6L6 (table S2) (42, 43).

Density functional theory calculations

To further understand the complex photophysical properties of Ag6L6 and Ag6PL6, we performed density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The calculated highest occupied molecular orbital (HOMO) frontier orbitals of Ag6L6 and Ag6PL6 are mainly localized on Ag and the coordinated S and N atoms with a small percentage of contribution from C in amine of ligand; their lowest unoccupied molecular orbitals (LUMOs) are mainly delocalized over the central region surrounded by six silver atoms, as shown in Fig. 4A and fig. S7, which may originate from argentophilic Ag∙∙∙Ag interactions (35). See below for the detailed percentage of atomic contribution. The calculated UV-vis absorption spectra of Ag6L6 and Ag6PL6 matched well with the experimental spectra in dichloromethane (DCM) solution (Fig. 4B and fig. S7). The lowest-energy absorption band at 383 nm mainly originated from the HOMO → LUMO transition (95%), the band centered at 316 nm is attributed to the HOMO-13 → LUMO (86%) and HOMO-12 → LUMO (42%) transitions, and the deep UV band at 238 nm originates from the transitions of HOMO → LUMO+5 (21%) and other deeper energy levels. The orbitals related to notable transitions mainly involved silver atoms in the core and coordinated atoms at the interface. Compared to those in solution, the absorption bands of crystalline Ag6L6 and Ag6PL6 display broadening in the visible-light region, which is thought to be related to interactions between the transition dipole moment of the individual cluster and the induced dipole moments (20). These calculations indicate that the emissions of these silver clusters mainly originate from ligand-to-metal-metal charge transfer transitions (19, 2123). DFT calculations of Ag6PL6 and Ag6L6 reveal that the theoretical ΔE(S1 − T1) values are 0.026 and 0.094 eV, respectively, based on the structures optimized at the lowest excited singlet state S1 and the triplet state T1, supporting the involvement of thermally assisted RISC in delayed fluorescence in these silver clusters.

Fig. 4 DFT, optical absorption, CD, and CPL.

(A) Selected frontier molecular orbital representations for Ag6L6 in optimized structures of S0. (B) Experimental optical absorption spectrum (red) of Ag6L6 in CH2Cl2 (10−5 M) compared to the calculated spectrum (black). Gray bars show the individual transitions (delta function–like peaks showing the relative oscillator strengths). (C) Schematic photoinduced transitions in Ag6L6. The core consists of octahedral Ag6 and coordinated N and S atoms, and the shell layer includes the outer noncoordinated atoms of ligand L. (D) CD spectra of clusters Ag6L6/D6 and Ag6PL6/PD6, together with ligands L/D and PL/PD in CH2Cl2 in the wavelength range of 200 to 450 nm. (E) CPL spectra (top) and the corresponding emission spectra (bottom) of Ag6L6 and Ag6D6 in CH2Cl2 (10−5 M). The CPL spectra of Ag6PL6/PD6 in solution are presented in fig. S8.

To understand the possible reason that TADF occurs in these silver clusters, we carefully compared them to previously reported luminescent silver(I) clusters, which are mostly protected by S-, Se-, Te-, and P-based ligands and invariably have very low PLQY at RT (1625, 36, 37). The phosphorescence was generally assigned to spin-forbidden transitions from triplet excited states, which are an admixture of metal-centered d-s and LMCT (S → Ag) states modified by metal-metal interactions (36). In contrast, the core framework of Ag6L6/D6 and Ag6PL6/PD6 consists of an (AgI)6N6S6 unit in which, apart from two S atoms, each Ag atom in the Ag(I)6 octahedron is coordinated by one imine N atom that forms π bonds with the neighbor C atoms (C═N bond in the range of 1.2 to 1.3 Å) in a thiazolidine ring, as it has chemically active groups ─N(H)─C(═S)─ in equilibrium with its thiol form ─N═C─S(H)─. The calculated orbital compositions of HOMO of Ag6L6 contain 25.3% of Ag atoms, 51.5% of six coordinated S atoms, 15.3% of imine (C═N) contributions, and LUMO including 52.9% Ag atoms, 31.0% coordinated S atoms, and 8.6% of imine (C═N) contributions. Therefore, we propose that the introduction of non-negligible contribution of imine (C═N) resulted in its distinct electronic structure, which would further promote the separation of HOMO and LUMO, minimize the singlet-triplet energy gap (ΔEST) that activates the RISC process, optimize fluorescence process, and eventually enable highly efficient TADF. In addition, the TADF mechanism of these silver(I) clusters is different from that of reported mononuclear Ag(I) and Cu(I) complexes, which mainly involves transitions of ligand π* orbitals dominating S1 and T1 states (40, 41). As the electronic structure of metal clusters is much more complicated compared to that of small organic molecules, the TADF origin of the silver clusters will be subjected to detailed investigation in the future. Such study is expected to provide an avenue to modulate the electronic structure of metal clusters with protecting ligand for high-performance luminescence.

On the basis of the above experimental and theoretical analysis, we proposed an energy diagram (Fig. 3F) based on Ag6L6 showing the TADF and phosphorescence emission processes. Below 40 K, the emission with a consistent decay time of 48 μs stems from the lowest triplet state T1. From 50 to 125 K, TADF comes into effect and surpasses nonradiative decay, resulting in an enhancement in emission intensity. With increasing temperature, TADF gradually becomes dominant with a decay time of approximately 18 μs and a high QY in RT range.

Chirality and CPL

Atomically precise chiral silver clusters remain very scarce and have rarely been investigated with regard to chiral electronic transitions related to the origin of luminescence (46). Considering the chirality of these Ag clusters known from their crystallographic structures (Fig. 1 and table S1) together with their bright luminescence (vide supra), we examined the circular dichroism (CD) and CPL of ligands (L/D and PL/PD) and clusters (Ag6L6/D6 and Ag6PL6/PD6) in dilute solution (Fig. 4, D and E) and selected L/D and Ag6L6/Ag6D6 as representatives for detailed analysis. In comparison to the CD spectra of the L/D ligands, which have two CD signal peaks at 300 nm (weak) and 344 nm (strong), the CD spectra of clusters Ag6L6/Ag6D6 in solution showed the appearance of a series of new bands and more intense signals at approximately 378 nm stretching to 420, 302, 267, and 230 nm and some at deeper UV regions, which were basically consistent with their UV-vis absorption spectra (Fig. 4B). Therefore, these differences in transitions can be used as fingerprints to investigate chiroptical properties. Combined with DFT and TD-DFT studies (vide supra), these absorption profiles were assumed to preferentially involve the S/N-Ag core and Ag (d-sp) intracluster transitions. We postulate that the HOMO-LUMO transition is optically active and endowed with ligand character through the Ag-S/Ag-N interface. Overall, the chiroptical properties resulted from a mixing of ligand orbitals with the silver atoms in the clusters. In other words, chiroptical activity is generated both inside the asymmetric Ag6 core and the chiral Ag-S/Ag-N interface, and to a lesser extent, a small percentage from the chiral outer organic group shell, as manifested in the CD response from metal-based electronic transitions other than the ligand’s own transition(s).

The strong symmetric CPL response of Ag6L6/D6 in solution occurs in the same wavelength region as their emission band centered at 574 nm (Fig. 4E) in notable contrast to non–CPL-active ligands L/D, indicating that light emitted from the lowest excited states in clusters Ag6L6/D6 is polarized in opposite directions, which originated from the combination of the delocalized electron density of LUMO induced by the asymmetric Ag6 core and an electronic coupling between the Ag6 cluster and chiral ligands involving HOMO. The luminescence anisotropy factor (glum) can be used to quantify the performance of CPL-active materials. The dissymmetric extent of emission was quantified by the dissymmetric factor, glum [glum = 2(ILIR)/(IL + IR) (2730), where IL and IR are the intensities of left- and right-handed CPL, respectively]. The maximum glum values for clusters Ag6L6/D6 in dilute solution were measured to be approximately ±4.42 × 10−3 (fig. S8), which are comparable to that of chiral gold clusters (27) and inorganic quantum dots (30). A similar phenomenon occurred for PL/PD ligands and Ag6PL6/PD6 clusters in solution (fig. S8). Therefore, the CPL activities are intrinsic for these chiral silver clusters. Moreover, solid-state samples of Ag6L6/D6 and Ag6PL6/PD6 also displayed mirror image CD and CPL signals (fig. S8), indicating the chiral arrangement of these chiral clusters in crystalline state.

The consistency between CD and UV-vis spectra, and between CPL and PL spectra, together with the definite crystallographic structure enables us to understand the intrinsic optical activity of the chiral silver clusters at the atomic level. Overall, the origins of chirality elucidated in Ag6L6/Ag6D6 and Ag6PL6/Ag6PD6 clusters open an avenue toward the design and preparation of chiral metal-based nanospecies by ligand modification for sustaining the evolution of chiral functionality, such as chiral catalysis, separation, and sensing, as well as polarized photonic devices. To the best of our knowledge, this work reports the first examples of chiral ligand-induced atomically precise CPL-active silver clusters. Considering the TADF emission of these Ag(I) clusters, they are also the first chiral TADF-based luminescent silver clusters.

CONCLUSION

In summary, two enantiomeric pairs of hexanuclear silver clusters that feature bright photoluminescence with high thermal stability under ambient conditions have been synthesized and well characterized. Their high QYs (56% for Ag6L6/D6 and 95% for Ag6PL6/PD6) set a record in reported silver clusters protected by monolayer ligands. As a representative, Ag6L6 with blue-excitable bright yellow luminescence comparable with that of commercial YAG:Ce3+ serves as a standard yellow phosphor for white lighting. TADF was revealed in multinuclear silver(I) clusters, which combined phosphorescence and resulted in high QY at RT. Strong chiroptical activities including CD and CPL are observed in atomically precise silver clusters and are used to probe chiral electronic transitions and the origin of luminescence. This work achieved groundbreaking progress in ligand-protected silver clusters that have remarkable luminescence properties for further applications and unambiguously electronic properties that elucidate the origin of silver cluster luminescence by virtue of their atomically precise structure. These investigations will greatly stimulate great interest in silver clusters and other metal clusters protected by monolayer ligands for future optical functional applications.

MATERIALS AND METHODS

Synthesis of L/D and PL/PD

(S)-(+)-2-amino-3-methyl-1-butanol, (R)-(−)-2-amino-3-methyl-1-butanol, (S)-(+)-2-phenylglycinol, and (R)-(−)-2-phenylglycinol were added to 50 ml of aqueous 1 N potassium hydroxide before CS2 (50 mmol, 3 ml) was added. The reaction mixture was stirred at 100°C for 16 hours. After cooling to RT, the reaction mixture was extracted with DCM (2 × 50 ml). The combined organic layers were dried over anhydrous Na2SO4 and then evaporated under reduced pressure to obtain the desired products. Proton nuclear magnetic resonance (1H NMR) spectra demonstrate the structure of L/D and PL/PD (fig. S1).

Preparation of Ag6L6/D6 and Ag6PL6/PD6 single crystals

L/D or PL/PD (0.1 mmol) and silver nitrate (0.1 mmol) were dissolved in mixed solvent of DMAc/CH3CN (3:1) to form a clear solution, which evaporated slowly in darkness at RT for 2 days to afford yellow block crystals of Ag6L6/D6 and Ag6PL6/PD6 suitable for x-ray diffraction analysis, respectively. Yield: 63.8% for Ag6L6 and 67.4% Ag6PL6 (based on Ag). Elemental analysis for Ag6L6 (calculated): C, 26.87%; H, 3.75%; N, 5.22%; and S, 23.91%; found: C, 27.01%; H, 3.82%; N, 5.11%; and S, 23.93%. Elemental analysis for Ag6PL6 (calculated): C, 35.77%; H, 2.67%; N, 4.63%; and S, 21.22%; found: C, 35.38%; H, 2.67%; N, 4.46%; and S, 21.06%.

Single-crystal x-ray diffraction analyses

SCXRD measurements of clusters Ag6L6/D6 and Ag6PL6/PD6 were performed at 150 K. Ag6L6 and Ag6PL6 were also measured at different temperatures (100, 143, 163, 213, 253, 293, 330, and 370 K) on a Rigaku XtaLAB Pro diffractometer with Cu-Kα radiation (λ = 1.54184 Å). Data collection and reduction were performed using the program CrysAlisPro (44, 45). All the structures were solved with direct methods (SHELXS) (46) and refined by full-matrix least squares on F2 using OLEX2 (47), which uses the SHELXL-2015 module (48). All atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions with idealized geometries and assigned fixed isotropic displacement parameters. Detailed information about the x-ray crystal data, intensity collection procedure, and refinement results for all cluster compounds is summarized in table S1.

Quantum chemical calculations

DFT and TD-DFT calculations were performed with Gaussian 16 combined with the ORCA 4.0.1 software package (49, 50). All Gaussian 16 calculations were conducted under the Perdew-Burke-Ernzerhof functional (51) using 6-31g* basis set for H, B, C, N, O, F, and S atoms (52, 53) and Lanl2DZ effective core potentials for Ag atoms (54, 55). ORCA, which can overcome the poor performance of Gaussian in optimizing a nanocluster system, was used to optimize the S0, S1, and T1 structures of the silver clusters. The Perdew-Burke-Ernzerhof functional was adopted with a double-zeta valence basis set. The resolution of identity approximation with the AUTOAUX approximation was performed as implemented in ORCA (56). The single-crystal structure was chosen as the initial guess for ground-state optimization, and all reported stationary points were verified as true minima by the absence of negative eigenvalues in the vibrational frequency analysis. The optimized S0 structure was further used as the starting structure for S1 and T1 optimization. The calculated absorption spectra were obtained from GaussSum 2.1 (57).

Photophysical measurements

Instrumentation and photophysical measurements are provided in the Supplementary Materials.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/6/eaay0107/DC1

Section S1. Instrumentation and Photophysical Measurements

Fig. S1. Synthesis and 1H NMR spectra of ligands.

Fig. S2. Coordination configurations and ESI-MS.

Fig. S3. TG analyses, PXRD patterns, and images of single crystals of Ag6PL6.

Fig. S4. UV-vis diffuse reflectance, luminescence, and temperature-dependent solid-state emission lifetimes.

Fig. S5. PL spectra and decay lifetimes of silver clusters in diluted solution.

Fig. S6. Photographs of solid-state samples and the fabricated film, and temperature-dependent emission and lifetimes of solid-state Ag6PL6.

Fig. S7. DFT and TDDFT calculation of Ag6PL6.

Fig. S8. CD and CPL spectra.

Table S1. Crystal data and structure refinements.

Table S2. Bond lengths.

Dataset S1. Ag6D6-150 K

Dataset S2. Ag6L6-150 K

Dataset S3. Ag6PD6-150 K

Dataset S4. Ag6PL6-150 K

Dataset S5. Ag6L6-100-370 K

Dataset S6. Ag6PL6-100-370 K

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REFERENCES AND NOTES

Acknowledgments: Funding: This work was supported by the National Science Fund for Distinguished Young Scholars (no. 21825106), the National Natural Science Foundation of China (no. 21671175), the Program for Science & Technology Innovation Talents in Universities of Henan Province (164100510005), the Program for Innovative Research Team (in Science and Technology) in Universities of Henan Province (19IRTSTHN022), and Zhengzhou University. Author contributions: S.-Q.Z. conceived and designed the experiments. Z.H., S.L., and Z.-Y.W. conducted the synthesis and characterization. P.L. performed the calculations. S.-Q.Z., Z.H., and X.-Y.D. analyzed the experimental and calculation results. Z.H., X.-Y.D., S.-Q.Z., and T.C.W.M co-wrote the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors. Also, cif.data have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC (table S1).
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