Ag 44 (EBT) 26 (TPP) 4 Nanoclusters with Tailored Molecular and Electronic Structure

: Although atomically precise metalloid nanoclusters (NCs) of identical size with distinctly different molecular structures are highly desirable to understand the structural effects on the intriguing optical and photophysical properties, their synthesis remains highly challenging. Herein, we employed phosphine and thiol capping ligands featuring appropriate steric effects and synthesized a charge-neutral Ag NC with the formula, Ag 44 (EBT) 26 (TPP) 4 (EBT: 2-ethylbenzenethiolate; TPP: triphenylphosphine). The single-crystal X-ray structure reveals that this NC has a hollow metal core of Ag 12 @Ag 20 and a metal-ligand shell of Ag 12 (EBT) 26 (TPP) 4 . The presence of mixed ligands and long V-shaped metal-ligand motifs on this NC has resulted in the enhancement of NIR-II photoluminescence quantum yield by >25-folds compared to an all-thiolate-stabilized anionic [Ag 44 (SR) 30 ] 4― NC (SR: thiolate). Time-dependent density functional calculations show that our Ag 44 NC is an 18-electron superatom with a modulated electronic structure as compared to the [Ag 44 (SR) 30 ] 4― anion, significantly influencing its optical properties.


Introduction
Ligand-protected metalloid nanoclusters (NCs), [1] representing a new class of functional nanomaterials with a precise metal-and ligand-count, have attracted tremendous attention for not only their aesthetic atomic arrangements but also for the fascinating optical, photophysical and chemical properties. [2]2b, 4] Furthermore, the NCs promise potential applications in catalysis, [5] optoelectronics, [6] energy conversion [7] and bioimaging. [8]The coinage metals (Au, Ag and Cu) [3,9] and main group metals [10] have largely been explored for synthesizing NCs comprising tens to hundreds of metal atoms. [11]5b, 12b] Post synthetic ligand-exchange has also been found to result in the NCs of novel size and molecular structure. [18]Recently, metals [19] and ligands [17,20] are found to assist the synthesis of NCs without being present in the final product.Although it is highly challenging to understand the exact role of metals and ligands that mediate the synthesis of NCs, they are anticipated to form suitable intermediates, directing or facilitating the formation of NCs.
12b, 21] Unfortunately, there are no hard and fast rules to attain NCs of desired characteristics since the molecular mechanism of the formation of NCs is still unknown.As a result, NCs of different metals with identical size, as well as NCs of the same metal with identical size but different atomic arrangements are rarely observed, [22] preventing the thorough exploration of fundamental aspects of NCs, such as size-and structure-dependencies on the optical, catalytic and photophysical properties.The [Ag44(SR)30] 4-NC (SR: thiolate) is one of the early discoveries in thiolated Ag NCs, [1b, 23] which exhibits multiple absorption features in the entire UV-vis window.Its structure comprises a Ag32 core protected by six equivalent Ag2(SR)5 motifs.Synthesizing NCs analogues to this size and 10.1002/anie.202015907

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structure would be very interesting.While there are some maingroup metalloid NCs possessing the same core and different surface structures, [24] those of noble metal-based remain unavailable due to synthetic challenges, restricting the studies of surface-structure effects on their properties.
In this work, we report on the synthesis of a Ag NC by the combined use of a phosphine (triphenylphosphine, TPP) and a thiol (2-ethylbenzenethiol, EBTH) ligand with a bulky alkyl group adjacent to sulfur binding site.In the presence of a Cu source, Ag and these ligands together produce a novel Ag44(EBT)26(TPP)4 NC, which is much desired and analogous to the reported [Ag44(SR)30] 4-NC [1b, 23] in terms of total metal and ligand number.The X-ray crystal structure shows that our Ag44 has the metal core similar to [Ag44(SR)30] 4-, however with a vividly different surface structure, making Ag44(EBT)26(TPP)4 a promising candidate for the investigation of structure-dependency on the properties of NCs.Interestingly, despite similarities in the core structures, the optical properties are completely modified due to alterations in the electronic structure as described here by time-dependent density functional theory (TDDFT) results.Furthermore, the Ag44(EBT)26(TPP)4 NCs exhibit enhanced near-infrared-II (NIR-II, 1000-1700 nm) photoluminescence (PL).

Results and Discussion
The synthesis of Ag44(EBT)26(TPP)4 is described in the experimental section and Figure S1 in Supporting Information (SI).Briefly, Ag and Cu precursors along with tetraphenylphosphonium tetraphenylborate are reacted with EBTH in THF to form metalthiolates.Upon addition of TPP, insoluble metal-thiolates are solubilized by complexation with TPP.Subsequently, borohydride reduction of these metal-ligand complexes has produced Ag44(EBT)26(TPP)4 NCs.Thoroughly washed final product of NCs is dissolved in DCM and layered with n-hexane at 4 o C, resulting black rectangular single-crystals (Figure S2) of appropriate quality, which are used for single-crystal X-ray diffraction (SCXRD) to determine the molecular structure of NCs.The SCXRD analysis of a suitable single-crystal reveals that the synthesized NC consists of 44 Ag atoms and 26 EBT and four TPP ligands (Figure 1). [25]No counterions are located in the molecular structure, indicating that its electronic charge is zero.Thus, the chemical formula of the NCs determined through the SCXRD is Ag44(EBT)26(TPP)4.The number of metal atoms and ligands is identical to that of an all-thiolate-protected Ag NC of [Ag44(SR)30] 4-, [23b] wherein four thiolates of [Ag44(SR)30] 4-are substituted by four phosphines and the 4-charge is switched to zero.23b] The Ag44(EBT)26(TPP)4 NC is found to crystallize in a triclinic system with a space group of P-1 (Table S1 in SI).
A detailed structural analysis of the Ag44(EBT)26(TPP)4 NC shows the presence of a non-centered Ag12 icosahedron inner core (Figure 2A).Subsequently, an Ag20 dodecahedron (magenta layer) completely encapsulates this Ag12 icosahedron to form an Ag32 (Ag12@Ag20) metal core (Figure 2B).There are two types of surface motifs on this NC (Figure 2C): (i) simple ligand motifs (bridging thiolates) and (ii) metal-ligand motifs (three-dimensional Ag2(EBT)5 and long V-shaped Ag4(EBT)7(TPP)2)).Binding of these motifs with the surface of the Ag32 metal core, such that two of each type are positioned oppositely, generates the total structure of Ag44(EBT)26(TPP)4 NC with a large box-like shape (Figure 2D).

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It is very important to compare the structural features of Ag44(EBT)26(TPP)4 NC with those of [Ag44(SR)30] 4-NC to understand their effect on the optical properties.Notably, both the NCs share the same metal core of Ag12@Ag20 with a non-centered icosahedron inner core.However, upon close inspection, the distinct differences are apparent.The Ag-Ag bonds of Ag12 inner core and its encapsulating layer (Ag20 dodecahedron) of Ag44(EBT)26(TPP)4 NC are significantly elongated (Table S2) with a range of 2.755-2.980and 2.951-3.887Å , respectively, compared to those of [Ag44(SR)30] 4-NC (2.805-2.870and 3.078-3.262Å , respectively, Table S3).Such a large distribution of longer metal-metal bonds clearly indicates the critical role of ligands in distorting the Ag32 metal core of Ag44(EBT)26(TPP)4 NC.
Note that the EBT ligand comprises a bulky ethyl group adjacent to S binding site.Ultimately, EBT ligands have to stabilize the overall NC along with the secondary TPP ligands by overcoming the intrinsic steric constraints.As a result, not only the metal core would have been distorted to accommodate EBT ligands but also modified the surface structure through two different types of ligand and metal-ligand motifs (Figure 2C).Recall that the [Ag44(SR)30] 4-NC has only one-type of six Ag2(SR)5 metal-ligand motifs due to the absence of any secondary ligands, imparting more spherical nature to this cluster, whereas the Ag44(EBT)26(TPP)4 NC appears as a box (Figure 2D) mainly due to long V-shaped Ag4(EBT)7(TPP)2 motifs.The Ag-S bond length range for Ag20 layer-S in Ag44(EBT)26(TPP)4 NC is 2.446-2.960Å , showing the elongated Ag-S bonds compared to those (2.539-2.674Å ) of [Ag44(SR)30] 4-NC.The TPP ligands are a part of V-shaped metal-ligand motifs with a Ag-P bond length range of 2.388-2.412Å .
The molecular formula of the Ag44(EBT)26(TPP)4 NCs obtained from the SCXRD is further supported by various characterization techniques.The matrix-assisted laser desorption ionization mass spectrometry of single crystals (Figure S4) shows a high-mass peak at ~8.3 kDa corresponding to Ag44(EBT)26, indicating the loss of TPP ligands during the ionization process.Due to the weak binding of labile ligands such as phosphines, they are detached from the cluster surface in the gas phase, which is a common observation in the mass spectrometry of noble metal NCs. [26]evertheless, the presence of TPP has been unambiguously confirmed by the solution-state 1 H and 31 P nuclear magnetic resonance (NMR) spectroscopy (Figures S5,S6).A clear shift and broadening of 31 P NMR peak of the NCs, compared to pristine TPP, confirm the binding of TPP with the NCs. [26] 1H NMR spectroscopy substantiated the presence of not only TPP but also EBT ligands.Furthermore, the thermogravimetric analysis (Figure S7) shows a weight loss of 49.15%, which is in good agreement with the ligand content (49.3%) of the Ag44(EBT)26(TPP)4 NC.The observation of NCs through transmission electron microscopy (TEM) is challenging due to their small size and sensitivity to electron beam.However, it reveals the absence of any large nanoparticles, which is a characteristic of the NCs (Figure S8).The scanning TEM energy dispersive X-ray spectroscopy of aggregates of NCs shows the presence of elements of NCs, including Ag, S and P (Figure S9).
The superatom theory [27] provides a free valence electron count of 18 for Ag44(EBT)26(TPP)4 NC, i.e., [(44 x 1) − ( 26x 1) − (4 x 0)], as each Ag contributes one s electron and each EBT thiolate withdraws one electron and neutral phosphine does not receive any electron.DFT analysis of symmetries of several frontier orbitals confirms this picture and establishes a HOMO-LUMO gap of 0.78 eV (Figures S10,S11) at the PBE level (see computational details in the SI).The optical absorption spectrum of Ag44(EBT)26(TPP)4 NC shows two prominent peaks at 535 and 391 nm along with a shoulder peak around 735 nm (Figure 3, black trace).1b, 23b] These spectra clearly suggest that the electronic structure of Ag44(EBT)26(TPP)4 NC is strongly modified due to distortions in the metal core and differences in the molecular structure.To understand the experimental observations, we performed TDDFT calculations and analyzed the computed absorption spectrum of Ag44(EBT)26(TPP)4 (technical details are in the SI).As shown in Figure 3, the computed spectrum (blue trace) is in a rather good agreement with the experimental data, showing peaks at 415, 530 and 775 nm (as compared to previously mentioned experimental peaks at 391, 535 and 735 nm).We analyzed the computed absorption peaks by using the so-called transition contribution maps (TCMs) (Figure 4).23b] The lowest-energy peak of Ag44(EBT)26(TPP)4 at 775 nm has contributions both from the superatom 1D → 1F transition and from ligand-to-metal transitions (Figure 4A), while in the case of [Ag44(SR)30] 4-the peak consisted solely of superatom 1D → 1F.Transition-induced hole and electron density (Figure 4B) confirms transfer of electrons in ligands to metal core as well.The peak at 530 nm has contributions from superatom 1D → ligand states, particularly to pi-electrons of the phenyl rings in phosphine ligands (Figures 4C,  4D), and the peak at 415 nm has solely ligand-to-ligand transitions ( Figures 4E, 4F).
Comparison of the electronic ground state of Ag44(EBT)26(TPP)4 with that of the [Ag44(SR)30] 4-shows that while the calculated HOMO-LUMO energy gap is very similar in both the clusters, the mixed EBT-TPP ligand layer lowers the 10.1002/anie.202015907

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symmetry of the electronic state distributions.This shows up particularly in the broader dispersion of the empty 1F orbitals: Figure S10 shows that each of the seven 1F states is split in energy, while in the [Ag44(SR)30] 4-case [23b] the seven states are grouped in 3, 1, 3 manifolds.Another qualitative difference is that the LUMO state of Ag44(EBT)26(TPP)4 is of dominantly 1F symmetry, while in [Ag44(SR)30] 4-it is 2S.Next, we investigate the structural effects of Ag44 NCs on their photophysical properties.The PL emission spectrum of Ag44(EBT)26(TPP)4 NCs displays two broad NIR-II peaks centered at ~1124 and ~1270 nm along with another broad feature spread in the 1400-1600 nm region (Figure 5).The prominent PL peaks are blue-shifted from those of [Ag44(EBT)30] 4-NCs.This may be due to the qualitative differences in the spread and nature of the frontier orbitals in these two clusters as discussed above.23b, 28] The PL excitation spectrum for emission at ~1270 nm well matches with the absorption spectrum (Figure S12), indicating the origin of the PL from the NCs.Furthermore, the PL quantum yield (PLQY) of Ag44(EBT)26(TPP)4 is found to be ~0.26%,which is enhanced by >25-and >10-folds compared to the reported [Ag44(SR)30] 4- (~0.01%) [1b] and [Ag44(EBT)30] 4-(~0.02%),respectively.Importantly, the PLQY of Ag44(EBT)26(TPP)4 is >5-folds higher compared to that of the standard dye IR26 (0.05%) and also comparable to PLQY of cyclodextrin-protected gold NCs (~0.11%). [29]The increase of PLQY of Ag44(EBT)26(TPP)4 NCs is attributed to the presence of secondary TPP ligands, which are known to play important role in the PL enhancement of the metal NCs. [30]2a, 21a]  We note the absence of Cu in the Ag44(EBT)26(TPP)4 NC (Figure 1 and Table S4) although the Cu precursor Cu(acac)2 (acac: acetylacetonate) is used during the synthesis of this NC.The product synthesized in the absence of Cu source shows an absorption spectrum (Figure S14A) with a broad single peak at ~447 nm corresponding to typical plasmonic Ag nanoparticles (Figure S14B), indicating that Cu is necessary for the synthesis of Ag44(EBT)26(TPP)4 NCs in our experimental conditions.It has been observed that Pd and Cu assist the synthesis of Ag33 and Ag23 NCs, respectively. [19]However, the exact mechanism of the metal-mediated formation of NCs is still unknown due to challenges in the characterization of the atomic structures and

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compositions of the reaction intermediates.The Cu is also shown to cause the size-transformation of Au NCs from Au25 to Au44. [31] series of experiments was performed to understand the formation pathway of Ag44(EBT)26(TPP)4 NCs.Time-dependent absorption spectroscopy and elemental analysis suggest that the Ag44(EBT)26(TPP)4 NCs are formed through intermediates of AgCu alloy NCs (Figures S15-S17 and Table S4).Given the unfavorable steric and stability constraints for AgCu NCs under our reaction conditions, they would have transformed to thermodynamically and electronically stable Ag44(EBT)26(TPP)4 NCs.Thus released Cu species would further combine with Ag to form byproducts of insoluble Ag-rich, AgCu nanoparticles and soluble AgCu/Cu-ligand complexes, which are discarded during the purification (see experimental section in the SI for details) of Ag44 NCs (Figures S18,S19 and Table S4).However, the yield of the Ag44(EBT)26(TPP)4 NCs is still ~52% (Ag atom basis).
The effect of anion of the Cu source (acac) on the formation of Ag44(EBT)26(TPP)4 NCs is found to be absent because the use of CuCl2 has resulted the NC with the same absorption features (Figure S20).Nevertheless, the precise compositional and structural details of the intermediates of AgCu alloy NCs along with a detailed computational study are indeed necessary to gain molecular-level mechanistic insights into the formation of pure Ag44 NCs, which is beyond the scope of this work.The generality of our synthetic approach to obtain Ag44 or a different-size solely Ag NC is further verified by replacing the Cu source with that of Au. Surprisingly, the synthesized product is not a pure Ag NC, but is an AgAu alloy NC exhibiting NIR emission features, which are blue-shifted from those of Ag44(EBT)26(TPP)4 NCs (Figures S21,S22).This indicates that our method is specific to Cu for Ag44 synthesis.Furthermore, for the metal combination Ag and Cu, different thiols such as 1,3-benzenedithiol and 2,4dichlorobenzenethiol provide AgCu alloy NCs, Ag17Cu12 and Ag28Cu12, respectively. [32]These experimental results together establish that the choice of ligands and metals plays crucial role in obtaining NCs of specific size, structure and composition.

Conclusion
In conclusion, by judicious choice of an aromatic thiol, containing a bulky alkyl group adjacent to sulfur binding site, and a phosphine coligand, a silver nanocluster Ag44(EBT)26(TPP)4 is synthesized and its single crystal X-ray structure is determined.Its core structure (Ag12@Ag20) resembles that of well-known [Ag44(SR)30] 4-cluster, while the surface structure is different, since four negative thiolates of [Ag44(SR)30] 4-are replaced by four neutral phosphines.Significant distortions in the metal core as well as novel surface structure of our nanocluster are found to significantly modify the electronic structure and optical properties.Furthermore, the presence of phosphine and rigidified surface through interligand interactions have resulted in the enhancement of NIR-II photoluminescence quantum yield by >25-folds.The TDDFT calculations revealed important differences in the details of the electronic structure of Ag44(EBT)26(TPP)4 as compared to the previously documented [Ag44(SR)30] 4-which underlie the observed photophysical properties.We hope that these two Ag44 nanoclusters of identical size and different molecular structures will enable researchers to conduct more fundamental studies to understand the structure-property relationships.

Entry for the Table of Contents
In this work, synthesis and total structure determination of a novel Ag44(EBT)26(TPP)4 nanocluster, the first analogue of [Ag44(SR)30] 4- nanocluster, are reported.The identical size and similar core-and different surface-structures of these two nanoclusters enable the study of the structural effect on the optical and photophysical properties.The novel surface structure plays crucial role in the enhancement of NIR-II photoluminescence.

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Figure 2 .
Figure 2. Structural anatomy of Ag44(EBT)26(TPP)4 NC. (A) Ag12 icosahedron inner core and (B) Ag20 dodecahedron (magenta) encapsulating Ag12 inner core, producing the Ag32 metal core.(C) Three types of ligand and metal-ligand motifs.(D) Mounting of these motifs on the Ag32 core forms the complete structure of NC.Carbon atoms of the ligands are not shown for a better visualization of the structure.

Figure 4 .
Figure 4. Analysis of the computed optical spectrum of Ag44(EBT)26(TPP)4.Panels (A, C, and E) show the transition contribution maps (TCMs) for peaks at 775, 530 and 415 nm, respectively, and panels (B, D, and F) show the corresponding transition-induced hole-electron densities.In each TCM panel, the horizontal and vertical PDOS panels show the occupied and unoccupied Kohn-Sham electronic density of states as projected to spherical harmonics according to the color labels on top of the Figure.In the map, the contributions of the single transitions to the absorption (marked peak on the lower right corner) are shown by the red contours (high density of contours showing the higher weights).