Engineering intelligent chiral silver cluster‐assembled materials for temperature‐triggered dynamic circularly polarized luminescence

The development of stimuli‐responsive circularly polarized luminescence (CPL) materials is quite attractive but challenging. Here, a pair of atomically precise enantiomers R/S‐Ag20 nanoclusters has been synthesized using chiral acid ligands. And then, stimuli‐responsive CPL materials were developed by assembling the chiral silver nanoclusters with an achiral bridging ligand. The atomically precise silver cluster‐assembled materials produce CPL with a dissymmetry factor (|glum|) of 1 × 10−3, through the high‐efficiency chiral induction process. More interestingly, the single CPL band at room temperature could quickly transform into highly separated dual CPL emissions at low temperature. This study provides a new strategy for the rational functionalization of chiral silver clusters in preparing cluster‐based CPL emitters and enriches the types of stimuli‐responsive CPL materials.

Atomically precise noble metal clusters with diameters ranging from sub-nanometer to a few nanometers serve as a bridge between atoms and metal nanoparticles, which have attracted widespread attention due to their precise chemical compositions and intriguing photophysical properties. [14,15][25][26][27][28] However, the peripheral ligands of chiral metal clusters are mainly used to protect and regulate the electronic structure of the metal skeleton, resulting in clusters to produce single CPL, which limits its application in the field of chiral sensing to some extent.
Utilizing silver clusters as secondary building blocks to assemble multi-functional silver-cluster-assembled materials (SCAMs) can achieve enhanced stability, maintain the prominent photoluminescence character, and lead to novel hybrid properties.[31][32] Chiral clusters and achiral luminescent organic molecules have been proven to be assembled into CPL materials via a coordination assembly strategy, [33,34] which are expected to modulate the CPL activity of chiral clusters through rational molecular design.SCAMs could be promising candidates for CPL switches because of the temperature-dependent luminescence signals of clusters, [35] which are unexplored yet to date.
In this work, we rationally designed a pair of chiral enantiomers (CO 3 )@Ag 20 ( t BuS) 10 (R/S-THF-COO) 8 (DMF) 4 (abbreviated as R/S-Ag 20 ; DMF = N,N-dimethylformamide), which emitted phosphorescence at low temperatures, by using the chiral tetrahydrofuran formic anion (R/S-THF-COO − ) to modify the reported achiral Ag 20 nanoclusters. [36]onsidering that DMF molecules could be replaced by Ncontaining ligands, linear bridging ligands 4,4′-bipyridine (bpy) and 3-amino-4,4′-bipyridine (bpy-NH 2 ) were used to assemble cluster molecules to explore the influences of coordination assembly on optical properties and global chirality.As shown in Scheme 1, the two-dimensional (2D) frameworks, [(CO 3 )@Ag 20 ( t BuS) 10 (R/S-THF-COO) 8 (bpy-NH 2 ) 2 ] n (R/S-Ag 20 bpy-NH 2 ) with blue-color CPL at room temperature (RT) were successfully achieved.Interestingly, R/S-Ag 20 bpy-NH 2 displays continuous emissive color variation from pink to blue as the temperature increases (83→293 K).The results show that it can be used as a potential visual ratio temperature-sensing fluorescent material within a wide temperature range.Correspondingly, the CPL of R/S-Ag 20 bpy-NH 2 also changes obviously at different temperatures, which is good for preparing CPL switches.This ligand engineering strategy provides an effective way to prepare CPL crystalline cluster-based materials.In addition, an isostructural 2D framework, [(CO 3 )@Ag 20 ( t BuS) 10 (R/S-THF-COO) 8 (bpy) 2 ] n (R/S-Ag 20 bpy), was prepared to further understand the origin of luminescence and CPL.These gave us some insight into the transfer of chirality from the original chiral building block to the global framework at the molecule level, further understanding the relationship between local chirality and global chirality.

RESULTS AND DISCUSSION
The colorless crystalline R/S-Ag 20 was synthesized by the reaction of t BuSAg and R/S-THF-COOAg in acetonitrile and DMF at RT. Single-crystal X-ray diffraction (SCXRD) analysis revealed that a pair of enantiomers crystallize in the chiral monoclinic system with a space group of C 2 (no.5) (Table S1).R-Ag 20 was used here as an example to describe the structure.The silver cluster is similar to the complexes, as previously reported. [36]The silver cluster molecule is a drum-like structure in which the CO 3 2− anion is located on the crystallographic inversion center.The CO 3 2− anion templates, deriving from carbon dioxide in air, play a role in balancing the charge.As shown in Figure 1A, -t BuS is bridged with Ag + ions in a μ 4 binding fashion to form the Ag 20 S 10 structure, in which Ag⋅⋅⋅Ag and Ag-S distances are in the ranges of 2.969 (2) -3.227 (5) Å and 2.422 (5) -2.745 (14) Å, respectively (Tables S2 and S3).The Ag 20 S 10 skeleton can be regarded as a triple-layered structure, where the top and bottom layers are two Ag 5 S 5 pentagrams, and the interlayer is a planar Ag 10 ring.Eight R/S-THF-COO − anions bond to the edge of the Ag 10 ring and two poles of the pentagons adopting μ 2 -η 1 , η 1 and μ 3 -η 2 , η 1 binding modes, respectively, where the Ag─O bond length ranges from 2.244 (16) to 2.560 (30) Å.The introduction of R/S-THF-COO − leads to an asymmetric tensile deformation of the Ag 10 ring, with the length and width changing from 10.563 and 9.423 Å to 11.219 and 9.649 Å, respectively (Figure S1).At the same time, two Ag 5 S 5 pentagrams shrink asymmetrically.These changes induce the chiral transfer from the outside (R/S-THF-COO − ) to the inside (Ag 20 skeleton), causing a pair of chiral asymmetric Ag 20 skeleton.As shown in Figure 1A, chiral R-Ag 20 has a C 2 axis passing through the center of the CO 3 2− .The chiral cluster and six adjacent clusters are grouped together in a hexagonal honeycomb structure to form a layered conformation, which features 2 1 helical axes with a pitch of 21.549 Å (Figure 1B and Figure S2).
Interestingly, four coordinated DMF molecules in R-Ag 20 can be replaced with bridging ligands (bpy/bpy-NH 2 ) to construct SCAMs.SCXRD analysis showed that R/S-Ag 20 bpy were crystallized in a monoclinic system with the chiral P2 1 space group (No. 4) (Table S4).As shown in Figure 1C S5 and S6).The chiral silver clusters are assembled with bidentate linkers through coordinate bonds, which fix their structures, thus leading to a significant decrease in the flack parameter.Unfortunately, the co-assembly of chiral silver clusters with bpy did not significantly adjust the luminescence of the clusters at RT. Based on the ligand engineering strategy, the chiral isostructural SCAM with room-temperature blue luminescence, R/S-Ag 20 bpy-NH 2 , was successfully obtained by assembling silver clusters with the ligand bpy-NH 2 (Table S7).The chiral clusters are also assembled into a chiral (2, 4)-connected 2D layer structure with a sql-type topology (Figure 1D and Figure S3), which features 2 1 helical axes with a pitch of 19.849 Å (Figure S2).
The phase purity of bulk R/S-Ag 20 , R/S-Ag 20 bpy, and R/S-Ag 20 bpy-NH 2 was confirmed by comparison of the observed and simulated powder X-ray diffraction patterns (PXRD) (Figure S4).PXRD indicates that the SCAMs could remain stable for at least 3 months (Figure S5).anion in each cluster center was further confirmed by a Fourier transform infrared (FT-IR) band around 1440 cm −1 (Figure S7).
Crystal structure analysis shows that there are many C-H⋅⋅⋅O (DMF / R/S-THF-COO − ) interactions existing in the crystals.They induce the distortion of R-Ag 20 structure, which realizes the chirality transfer in an outside-in pattern (Figure 2A and Table S8).Similarly, there are C-H⋅⋅⋅O interactions between DMF molecules and R/S-THF-COO − groups in S-Ag 20 (Table S9).The UV−Vis absorption spectrum indicates that R-Ag 20 absorbs UV light with wavelengths from 260 to 370 nm in solution (Figure S8).The circular dichroism (CD) spectra of chiral ligands and R/S-Ag 20 were measured to verify the occurrence of chirality transfer.As shown in Figure 2A and Figure S9, the CD spectra of R/S-THF-COOH in ethanol (EtOH) exhibit the cotton effect in the range of 200-260 nm, whereas the chiral R/S-Ag 20 in EtOH shows good mirror-symmetry signals in the range of 220-370 nm.Therefore, it is speculated that chirality transfer from the chiral ligand (R/S-THF-COO − ) to the Ag 20 S 10 skeleton in this system is due to the emergence of new mirror symmetrical CD signals in the range of 260-370 nm.The UV-Vis absorption spectra of R/S-Ag 20 in the solid state show a broad absorption band in the range of 240-400 nm (Figure S10).The CD spectra of R/S-Ag 20 were tested to verify its ground-state chirality in solids, which was effectively maintained (Figure S11).In addition, the temperature-dependent CD spectra of R/S-Ag 20 were tested in the solid and liquid states, respectively.The results demonstrated that the Cotton effect peaks can be well maintained in the range of −10-50 • C (Figure S12).After assembling R-Ag 20 into frameworks using rigid bidentate linkers (bpy or bpy-NH 2 ), complexes R-Ag 20 bpy and R-Ag 20 bpy-NH 2 produce two new absorption bands that extend to 600 nm, which correspond to their orange-yellow crystal.These indicated that The photoluminescence (PL) properties of R-Ag 20 bpy-NH 2 were studied.As shown in Figure 3, a strong blue emission is observed from R-Ag 20 bpy-NH 2 under UV light excitation at 380 nm, corresponding to the PL band peaked at ∼480 nm at RT.It is interesting to note that as the temperature decreases; R-Ag 20 bpy-NH 2 displays a pink-color emission, in which the emission peak at 457, 571, and 680 nm exhibits significantly different temperature-dependent luminescence behaviors, respectively (Figure 3A).The changing trend of the intensity is more evident for the low-energy (LE) emission (λ em = 571 nm; λ em = 680 nm) than the high-energy emission (λ em = 457 nm) with temperature change (Figure S13a).Correspondingly, continuous emissive color variation from blue to pink was observed in the CIE chromaticity diagram and optical photographs of the crystal (Figure 3B and Figure S14).Because it is easily recognized by the naked eye, it may be a good candidate for the temperature probe.To evaluate the self-calibrated performance, the temperature to the emission intensity ratio (I 457 nm + I 571 nm ) : (I 457 nm + I 571 nm + I 680 nm ) were correlated.The linear correlation is found in the range of 83-193 K and the relative thermal sensitivity (S R ) is 0.0022 % K −1 , which is a relatively wide temperature range and a high sensitivity (Figure 3C).In addition, the temperature to the emission intensity ratio (I 680 nm ) : (I 457 nm + I 571 nm + I 680 nm ), (I 571 nm + I 680 nm ) : (I 457 nm + I 571 nm + I 680 nm ) and (I 571 nm + I 680 nm ) : (I 457 nm + I 571 nm ) were correlated, respectively (Figure S13).The linear correlation of the temperature to the emission intensity ratio (I 571 nm + I 680 nm ) : (I 457 nm + I 571 nm + I 680 nm ) was found in the range of 203-303 K (relative thermal sensitivity 2.451 % K -1 ), which is complementary to the range of temperatures mentioned above.The good luminescence detection performance of R-Ag 20 bpy-NH 2 in a wide temperature region provides the possibility for its application in a self-calibration luminescent thermometer.
Surprisingly, R-Ag 20 bpy-NH 2 shows a low-temperature yellow afterglow, which is detectable by the naked eye (Figure S15).At 83 K, the lifetime of high-energy (HE) emission at 457 nm was calculated to be 7.52 ns (Figure 3D), while the phosphorescence lifetimes at LE were 34.01 and 28.78 ms, respectively (Figure 3E,F).As the temperature increases, the corresponding lifetimes at 183 K are 6.45 ns, 20.56 ms, and 17.76 ms, respectively.The intensity of emission increases as the temperature decreases, which may be related to the enhanced rigidity and suppressed non-radiative transition. [37]Due to the extreme difference in lifetime between the HE and LE in R-Ag 20 bpy-NH 2 , it is speculated that the emission of HE and LE derives from different emission centers.Based on the similar spectrum profile to the bpy-NH 2 ligands (λ em = 425 nm, τ = 1.70 ns) at 83 K (Figures S16 and S17), the fluorescence at ∼457 nm in R-Ag 20 bpy-NH 2 (τ = 7.52 ns) may arise from the ligandcentered (bpy-NH 2 ) singlet excited state.We speculate that the two emission bands in the lower energy region are related to the Ag 20 S 10 skeleton.Therefore, the luminescence spectra of R-Ag 20 were tested.The R-Ag 20 solid is not emissive at RT under ultraviolet irradiation.As the temperature decreases, the bright red emission of R-Ag 20 in the solid state is lit up (Figure S18).R-Ag 20 displays two different emissions (λ ex = 350 nm, λ em = 613 nm; λ ex = 420 nm, λ em = 670 nm) under different excitations at 83 K (Figure S19).In addition, the temperature-dependent lifetimes of the two emission bands were tested.At 83 K, the lifetime of HE (emission at 613 nm) was calculated to be 142.73 μs, and the lifetime at LE (emission at 670 nm) was 157.05 μs (Figure S20).The enhanced luminescence and lifetime of R-Ag 20 may be caused by the reduced non-radiative transitions. [37]At 83 K, the emission of R-Ag 20 bpy-NH 2 at 680 nm under 388 nm excitation is close to the LE emission band (670 nm) of the R-Ag 20 .The LE PL bands (emission at 680 nm) from R-Ag 20 bpy-NH 2 could be tentatively assigned to the cluster-centered transition accompanied by the ligand-to-metal-charge-transfer (LMCT, charge transfer from the S 3p to Ag 5s orbits) transition. [38]The lifetime of R-Ag 20 bpy-NH 2 was calculated to be 28.78 ms at 83 K, which was prolonged by about two orders of magnitude owing to the higher rigidity sustained by the assembled materials.The lifetime of LE (emission at 571 nm) of R-Ag 20 bpy-NH 2 was calculated to be 34.01 ms at 83 K.According to the reported literature, the LE PL bands (emission at 571 nm) of R-Ag 20 bpy-NH 2 were tentatively assigned to the interligand trans-metallic charge-transfer transition (ITCT, S/Ag→bpy-NH 2 ). [30,32,39]It has been reported that the temperature-dependence of bpy-NH 2 was much less than that of the silver cluster in the luminescence, which is consistent with the above.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The supplementary crystallographic data that support the findings of this study can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (CCDC 2307053-2307058).

S C H E M E 1
Schematic representation of the coordination assembly of Ag 20 clusters to induce the chirality of framework and prepare dual-emissive circularly polarized luminescence materials.
, similar to R/S-Ag 20 , R/S-Ag 20 bpy retains the skeleton structure of Ag 20 S 10 , where Ag⋅⋅⋅Ag and Ag-S distance range F I G U R E 1 (A) X-ray structure of R/S-Ag 20 .(B) Chiral spiral arrangement of the R-Ag 20 cluster based on 2 1 helix axes.(C) X-ray structures of R/S-Ag 20 bpy and R/S-Ag 20 bpy-NH 2 .(D) 2D network structure of R-Ag 20 bpy and R-Ag 20 bpy-NH 2 .H atoms are omitted for clarity.The green dotted lines indicate that the Ag⋅⋅⋅Ag distances are longer than 3.440 Å.

from 2 .
878 (7) to 3.376 (2) Å and 2.405 (5) to 2.680 (30) Å, respectively (Tables The chemical formulas of R-Ag 20 , R-Ag 20 bpy, and R-Ag 20 bpy-NH 2 were verified by thermogravimetric analysis (TGA) and elemental analysis (EA) (Figure S6).TGA shows that R-Ag 20 contains four coordinated DMF molecules in an isolated cluster (a rapid weight loss in the range of 25-135 • C).In addition, R-Ag 20 bpy and R-Ag 20 bpy-NH 2 have significant weight loss below 150 • C, corresponding to the loss of free solvent molecules (H 2 O molecules and THF molecules).Subsequently, the existence of one CO 3 2−

F
I G U R E 2 (A) C-H⋅⋅⋅O interactions (orange dashed lines) among N,N-dimethylformamide (DMF) and R-THF-COO − in R-Ag 20 and CD spectra of R/S-Ag 20 in EtOH (10 −5 M). (B) C-H⋅⋅⋅O (orange dashed lines) interactions between R-THF-COO − and bpy in R-Ag 20 bpy and the circular dichroism (CD) spectra of R/S-Ag 20 bpy in solid.(C) C-H⋅⋅⋅O (orange dashed lines) and N-H⋅⋅⋅O (blue dashed lines) interactions between R-THF-COO − and bpy-NH 2 in R-Ag 20 bpy-NH 2 and the CD spectra of R/S-Ag 20 bpy-NH 2 in solid.Color code: Ag, green; S, yellow; C, gray; O, red; N, blue; H, turquoise.a new charge transfer occurred between bidentate linkers and Ag 20 S 10 skeleton.Can the assembly of chiral clusters with bidentate linkers induce the framework to be a chiral structure?On the one hand, many C-H⋅⋅⋅O and N-H⋅⋅⋅O interactions exist in the assemblies (Tables S10-S13) that ensure high chiral-transmission fidelity.On the other hand, the solid CD spectra of R/S-Ag 20 bpy and R/S-Ag 20 bpy-NH 2 were measured as shown in Figure 2B,C.They exhibit mirrorimage CD signals in the range of 250-500 nm, corresponding to the absorption region of the SCAMs.Compared with R/S-Ag 20 , CD signals of R/S-Ag 20 bpy and R/S-Ag 20 bpy-NH 2 in the range of 370-500 nm may be derived from the inductive effect of cluster chirality to the overall framework chirality.

F
I G U R E 3 (A) Temperature-dependent emissive spectra of R-Ag 20 bpy-NH 2 .(B) CIE coordinates of R-Ag 20 bpy-NH 2 in the range of 83-293 K. (C) The working curve and equation of R-Ag 20 bpy-NH 2 .(D-F) Temperature-dependent time-resolved decay traces of R-Ag 20 bpy-NH 2 at 457, 571, and 680 nm, respectively.
Since the chiral cluster induced the chirality of the framework structure of R/S-Ag 20 bpy-NH 2 , which also displayed bright luminescence, the CPL spectra of R/S-Ag 20 bpy-NH 2 were measured to investigate their chirality in the excited state.First, the R/S-Ag 20 bpy-NH 2 crystals displayed blue emissive mirror-image CPL signals in the range of 430-530 nm under 340 nm excitation at RT (Figure 4A), which are in accordance with the PL spectra of R/S-Ag 20 bpy-NH 2 , indicating that chirality transfer does occur between chiral molecules and luminescent molecules.Luminescent dissymmetry factor (g lum ) is an important parameter of CPL properties, and the g lum values of R/S-Ag 20 bpy-NH 2 corresponded to −1.1 × 10 -3 and +1.2 × 10 -3 , respectively (Figure S21).Second, from the above results, the temperature-dependent red (LE) and blue (HE) dual-emission behavior was observed in R/S-Ag 20 bpy-NH 2 .The CPL signals of R/S-Ag 20 bpy-NH 2 at 83 K were tested to verify the excited-state chirality of the LE emission.To note, there was no CPL in LE emissions at RT. Yet, the CPL of R/S-Ag 20 bpy-NH 2 at LE bands was switched on at 83 K (Figure4B).After assembling Ag 20 S 10 with bpy-NH 2 , the CPL signals at low temperature covered almost the entire visible region, and the CPL spectra corresponded well with the emission spectra.The mechanism of temperature-induced CPL switching is plainly expounded in Figure4C,D.It could be seen that the luminescence in LE emission comes from LMCT or ITCT, which is closely linked with the silver cluster core, proving that chirality transfers from the chiral ligands to the silver cluster.Cluster-based materials with adjustable CPL activity are rarely reported.It is expected to produce multiplecolor CPL materials through the coordination assembly of chiral ligands, luminescent molecules, and metalclusters.

F
I G U R E 4 (A,B) Circularly polarized luminescence (CPL) spectra of R/S-Ag 20 bpy-NH 2 solid at room temperature and low temperature.(C,D) Schematic illustration of CPL for R/S-Ag 20 bpy-NH 2 at room temperature and low temperature, respectively.

3 CONCLUSION
In summary, a pair of chiral clusters was obtained by replacing the nitrate on the centrosymmetric Ag 20 nanoclusters with chiral carboxylic acid ligands.By anchoring R/S-Ag 20 with N-containing bridge ligands via coordination assembly, the luminescence of clusters-based materials was modulated.Moreover, compared with R/S-Ag 20 and R/S-Ag 20 bpy, chiral R/S-Ag 20 bpy-NH 2 displayed an infrequently visualized ratio fluorescence temperature sensing performance and multiple CPL activities under temperature stimuli.Crystallographic analysis, CD, and CPL spectra revealed that chiral acids induced the distortion of the Ag 20 S 10 skeleton, and the overall framework, especially the chirality of bridging ligands, was endowed with chirality.This study provides a new strategy for the construction of stimuli-responsive CPL materials with clear structures, which is beneficial to understand the origin of the chirality, thereby allowing control of their related physical and chemical properties.A C K N O W L E D G M E N T S This work was supported by the National Natural Science Foundation of China (No. 92061201, U21A20277), Thousand Talents (Zhongyuan Scholars) Program of Henan Province (234000510007), and the Excellent Youth Foundation of Henan Scientific Committee (232300421022).