Reversible Luminochromism of an N‐Heterocyclic Carbene‐Protected Carbon‐Centered Hexagold(I) Cluster by Solvent and Mechanical Stimuli

Luminochromic gold clusters have potential applications in sensing due to the unique optical properties arising from their geometric and electronic structures. However, the rational design of such clusters is extremely challenging, especially when the goal is to impart reversible stimuli‐responsive properties to gold clusters. Herein, the reversible multi‐stimuli responsiveness of a carbon‐centered hexagold(I) CAuI6 cluster protected by N,N’‐dibenzylbenzimidazolylidene (BIBn) ligands is reported. Two types of crystals emitting blue (λemmax = 479 nm) and turquoise (λemmax = 497 nm) photoluminescence (PL) are obtained and found to be interconvertible by the addition and removal of solvent. Single‐crystal X‐ray structure analysis has revealed significant differences between the crystal structures, including the conformation of the BIBn ligands. Theoretical calculations suggest that these conformational differences directly affect the photophysical properties and that the flexibility of the BIBn ligands attached to the CAuI6 core makes this possible. Furthermore, when these crystals are grounded with a spatula, they are found to emit green PL (λemmax = 520 nm) following a partial loss of crystallinity. This is a remarkable example of both vapoluminochromism and mechanoluminochromism in an N‐heterocyclic carbene (NHC)‐protected gold cluster and holds promise as a model compound for understanding the principles governing stimuli‐responsiveness in metal clusters.


Introduction
Gold clusters ("gold" includes both gold(0) and gold(I) species, unless otherwise noted) protected by ligands have attracted increasing attention because of their great potential as photoluminescent materials, as their diverse geometric and electronic structures can be precisely tuned to impart intriguing structure-dependent photophysical properties. [1]Luminochromism, [2] such as vapoluminochromism [3] and mechanoluminochromism, [4] which allows changes in the photoluminescence (PL) properties in response to external stimuli, is a fundamental element of smart materials for sensing applications.5i,j] However, the PL properties of stimuli-responsive gold(I) clusters, especially the structure-property correlation of shell ligands in the solid state, have often been difficult to elucidate because of the challenges in uncovering the overall structural changes before and after the stimuli-response.
It Is well known that the nature of the protecting ligands strongly influences the properties of the resulting gold clusters. [6]-heterocyclic carbene (NHC) ligands are increasingly used to protect gold surfaces, [7] nanoparticles [8] and nanoclusters (mixed valence Au 0 /Au I ), [9] and gold(I) clusters, [10] due to their excellent donor properties.[11] Through our own studies of NHC-protected CAu I 6 clusters, [12] we have found that the NHC ligands have a marked effect on the structure as well as the PL and intracellular behavior of the clusters, compared to the phosphine-based counterparts.[13] However, design strategies for imparting stimulus responsiveness to the NHC-protected gold(I) clusters are lacking and remain elusive.
Herein we report the results of experiments and theoretical calculations related to the dynamic structure of the N,N'-dibenzylbenzimidazolylidene (BIBn)-protected carboncentered hexagold(I) cluster [(C)(Au I -BIBn) 6 ](BF 4 ) 2 (1) and its vapoluminochromism and mechanoluminochromism (Figure 1a).This ligand was expected to have a high degree of structural flexibility because of the sterically small methylene groups adjacent to the nitrogen atoms (Figure 1b; Figure S1, Supporting Information). [14]Cluster 1 crystallized in two pseudopolymorphs and showed blue (1 cubic ) and turquoise (1 mono ) PL under UV irradiation.Single-crystal X-ray structure analysis revealed significant differences in the molecule-level conformation and overall packing structure of these crystals.Theoretical calculations suggest that the conformational changes of the BIBn ligands cause a significant excitation energy difference between the two pseudopolymorphs, which affects the associated photophysical properties.Further grinding of 1 cubic and 1 mono with a spatula yielded a solid with low crystallinity and green PL.Overall, the three solid states were interconvertible by solvent and mechanical stimuli (Figure 1a), providing important insight into the effect of the ligand structure for obtaining stimuli-responsive gold(I) clusters.

Results and Discussion
12a,15] Electrospray ionization time-of-flight (ESI-TOF) mass spectroscopy identified the major dicationic peak at m/z 1491.36 corresponding to a molecular ion of 1 as [(C)(Au I -BIBn) 6 ] 2+ (theoretical value: m/z 1491.34). 1 H and 13 C NMR spec- tra of 1 in CD 3 CN showed a single set of NHC ligand signals, suggesting that all ligands are in the same chemical environment in solution at 300 K (Figures S8 and S9, Supporting Information).Crystallization of cluster 1 in two different solvents yielded crystals with two completely different structures: crystallization from CH 2 Cl 2 yielded colorless octahedral crystals of cubic space group Fd-3 (phase 1 cubic ), while crystallization from CH 3 CN yielded yellow blocky crystals of monoclinic P2 1 /c space group (phase 1 mono ). [15]These two crystals were found to differ significantly from each other in the molecule-level conformation and overall packing structure.
The core part of the CAu I 6 clusters was expected to adopt an octahedral geometry in both crystals.The CAu I 6 core of cluster 1 cubic (Figure 2a) exhibited octahedral O h symmetry, and all bond lengths and angles were equivalent to each other.The twelve Au I ⋅⋅⋅Au I distances (corresponding to the sides of the octahedra) were 2.9924(3) Å, indicating the presence of aurophilic interactions, and the Au I -C center -Au I angles were 90°.The CAu I 6 core in cluster 1 mono (Figure 2b) was similar, but in a slightly distorted octahedral conformation.All gold atoms were crystallographically inequivalent, with Au I ⋅⋅⋅Au I distances ranging from 2.9142(2) to 3.0697(2), but the average value was close to that of 1 cubic (2.9942 Å).The Au I -C center -Au I angles were also similar, ranging from 93.4(2) to 86.7(2)°(average: 90.0°).Overall, the above parameters of 1 cubic and 1 mono showed no significant differences between the CAu I 6 core parts.On the other hand, the ligand molecules in clusters 1 cubic and 1 mono were found to have different conformations, as shown in Figure 2. Specifically, the benzyl moieties located on opposite sides of the CAu I 6 motifs were found to have different propellerlike arrangements.In 1 cubic , both the upper (Figure 2c) and lower (Figure 2e) faces exhibited a clockwise arrangement (blue arrows) of the benzyl groups.Conversely, in 1 mono clusters, the upper face exhibited a clockwise arrangement (Figure 2d) and the lower face an anti-clockwise arrangement (Figure 2f, red arrows).Furthermore, all ligands in 1 cubic showed a perfectly linear orientation to the attached Au I ion, as shown in Figure S18 (Supporting Information).In contrast, the ligands in 1 mono were slightly tilted toward the Au I ions, resulting in non-linear coordination of the NHC moieties to Au I (Figure S19, Supporting Information).These conformational differences between the two pseudopolymorphs reflect the flexibility of the BIBn ligand.
Moreover, these molecules were aggregated by multipoint intermolecular interactions (Figure S20, Supporting Information).In cubic crystal 1 cubic , the cluster molecules occupied the vertices of interconnected tetrahedra and were arranged in a diamond-like packing structure typical of cubic space groups (Figure S21, Supporting Information).This arrangement created large pockets that were easily accessible to solvent molecules, where BF 4 − counter-anions were encapsulated (Figures S21 and S22, Supporting Information). [16]The large solvent accessible volume suggested the presence of unidentified crystallization solvent molecules, which was supported by 1 H NMR quantification (Figure S25, Supporting Information) and a large residual electron density in the crystal structure (Table S2, Supporting Information). [17]A more orthodox dense molecular arrangement was observed in monoclinic crystal 1 mono (Figure S23, Supporting Information).No voids were found in 1 mono that could contain crystallization solvent molecules (Figures S23 and S24, Supporting Information).The densities of these two crystals highlight the significant differences in their packing structures (1 cubic :  = 1.37 g cm −3 ; 1 mono :  = 1.92 g cm −3 , Table S1, Supporting Information).
Notably, 1 cubic and 1 mono showed different PL colors under 365 nm light irradiation (Figure 3a,b).Specifically, the ascrystallized 1 cubic exhibited blue PL ( em max = 479 nm), and the as-crystallized 1 mono exhibited turquoise PL ( em max = 497 nm) with high quantum yields (58% and 60%, respectively).The PL lifetimes of 1 cubic and 1 mono are 0.484 and 0.499 μs, respectively, suggesting that the PL is phosphorescent (Table S3, Supporting Information).However, PL was not observed in solution, suggesting the predominance of nonradiative relaxation pathways in solution. [18]t is noteworthy that these two PL colors in the solid state are interconvertible (Figure 3a).When fresh crystals of 1 cubic were dried under vacuum, their PL color changed from blue to turquoise under 365 nm light irradiation.When 1 mono showing turquoise PL was treated with a small amount of CH 2 Cl 2 , it immediately reverted to blue PL under 365 nm light irradiation.When 1 mono was fumigated with CH 2 Cl 2 vapor, this conversion took a few minutes.Thus, the PL changes between 1 cubic and 1 mono indicate that this material is vapoluminochromic.
To better understand the relationship between the PL colors and the crystalline morphologies of 1 cubic and 1 mono , the vapoluminochromic processes were monitored by powder X-ray diffraction (PXRD) (Figure 3c).First, when the 1 mono crystals were exposed to liquid CH 2 Cl 2 , the turquoise PL of 1 mono immediately changed to blue PL, and its PXRD pattern (Figure 3c,B) matched well with the simulated pattern of cluster 1 cubic (Figure 3c,E), indicating a crystal phase change from 1 mono to 1 cubic .Next, the crystals of 1 cubic were soaked in chlorobenzene for 10 min to achieve the conversion from blue-emissive to turquoise-emissive (PXRD spectra change from B to C in Figure 3c).The PXRD pattern of the resulting turquoise-emissive crystals was in good agreement with the simulation pattern of 1 mono .We further examined several solvents for the 1 mono -to-1 cubic conversion and found that small solvent molecules with electronegative atoms (O, F, and Cl) caused the PL changes (Table S4, Supporting Information).
Density functional theory (DFT) and time-dependent (TD) DFT calculations were then performed based on the single crystal structures of clusters 1 cubic and 1 mono .These calculations were performed for a single cluster molecule in the gas phase to clarify the conformational effects of the ligands on their geometric and electronic structures.The optimized structures show excellent agreement with the crystal structures (Figure S26, Supporting Information).Meanwhile, the gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are very close for these two structures (4.20 and 3.95 eV for 1 cubic and 1 mono , respectively), indicating the similar electronic structures (Figure 4).Compared to 1 cubic , the LUMO energy level of 1 mono is stabilized (≈0.18 eV) while its HOMO energy level is destabilized (≈0.07 eV), leading to a smaller HOMO-LUMO gap in 1 mono .Note that 1 cubic has the high symmetry point group of T, while 1 mono exhibits the C 3 point group.Thus, triply degenerate HOMOs and LUMOs are identified in 1 cubic .
The TD-DFT calculations for clusters 1 cubic and 1 mono reproduced well the observed solid-state UV-vis absorption spectra (Figure S27, Supporting Information).The observed spectrum of 1 cubic showed a large peak ≈350 nm, while 1 mono has a lower energy absorption as a shoulder at 400 nm in addition to the 350 nm absorption band.The simulated spectrum of 1 cubic also showed one peak ≈340 nm, while 1 mono showed two peaks at 367 and 332 nm.The lowest excited states with considerable intensity are located at ≈340 nm for 1 cubic , which is mainly due to the tran-sition from triply degenerate HOMOs to triply degenerate LU-MOs (Table S5, Supporting Information).In the case of 1 mono , the transition from HOMO to LUMO also contributes significantly to the lowest excited state of ≈367 nm (Table S6, Supporting Information).The UV-vis absorption of 1 cubic is blue-shifted relative to that of 1 mono , which is consistent with the experimental results.The slightly larger HOMO-LUMO gap of 1 cubic also explains well the characteristic blue-shift.The second lowest excited states of 1 cubic are found at ≈297 nm, which is mainly due to the HOMO→LUMO+1 transitions.For 1 mono , peaks at ≈361 and ≈350 nm were identified, attributed to the HOMO-1→LUMO and HOMO→LUMO+1 transitions, respectively.We further analyzed the orbital compositions of 1 cubic and 1 mono (Table S7, Supporting Information).The results show that in 1 cubic , the HOMO orbitals are mainly localized on the carbon center (25.9%) and Au I atoms (53.0%), with a small contribution from the ligands (21.1%); the LUMO orbitals are mainly localized on the ligands (57.0%) and Au I atoms (42.8%).A similar orbital composition was found in 1 mono .Therefore, it can be concluded that in both 1 cubic and 1 mono , the ligands make up the majority of the unoccupied orbitals and slightly affect the occupied orbitals, which further contributes to their distinct absorption properties.These results clearly show that the conformational changes of the ligands in the single cluster molecules of 1 cubic and 1 mono have a significant effect on the ground states, and we speculate that similar ligand effects may occur in the excited states, ultimately tuning the photoluminescent properties.In the calculated triplet excited states of clusters 1 cubic and 1 mono , the triplet structures were highly distorted and the emission energies were underestimated, but these energies reproduced the experimental trends (Figure S30 and Table S9, Supporting Information).
The energy difference between clusters 1 cubic and 1 mono (Table S8, Supporting Information) was small, leading us to speculate that mechanical stress could cause further PL emission color changes.Grinding 1 cubic and 1 mono crystals with a spatula again changed the PL color, yielding a new solid 1 lc emitting green light (Figure 5a,b,  em max = 520 nm).Its quantum yield and PL lifetime were 59% and 0.454 μs, respectively, comparable to 1 cubic and 1 mono (Table S3, Supporting Information).No changes were observed when the crystals were pressed from above, indicating that shearing force must be applied to cause mechanoluminochromism.The PXRD profile of 1 lc showed two broad peaks ≈7°and no other peaks of notable intensity, suggesting that it is a poorly crystalline solid (Figure 5c, bottom).The two peaks appeared at similar angles to the main PXRD peaks of 1 mono and had similar relative intensities.Since these two peaks originate from the (111) and (11 " 1) crystal planes of 1 mono , some order along these planes should be maintained in 1 lc , while the other planes should be broken by mechanical action.Indeed, the clusters in 1 mono were found to be connected by intermolecular interactions such as CH- and - interactions along the (111) and (11 " 1) crystal planes (Figures S31 and S32, Supporting Information).This also indicates stability to pressure in the ( 111) and (11 " 1) plane directions. [19]Furthermore, the solvent-responsive behavior was maintained in 1 lc .Blue PL was restored when soaked in CH 2 Cl 2 , and turquoise PL was restored when soaked in chlorobenzene.These results indicate that cluster 1 is the first NHC-protected gold(I) cluster that exhibits a reversible response behavior to multiple stimuli.

Conclusion
In conclusion, we have successfully synthesized an NHCprotected gold(I) cluster, [(C)(Au I -BIBn) 6 ](BF 4 ) 2 (1), that exhibits multi-stimuli responsive reversible luminochromism.Cluster 1 was found to form two interchangeable pseudopolymorphs, 1 cubic and 1 mono , with different PL colors in response to solvent stimulation.Theoretical calculations suggest that the conformational changes of the protective ligands affect the electronic structures of cluster 1, resulting in changes in its PL.Thus, in contrast to previous studies that focused on metal-metal, metal-solvent, and packing interactions, we were able to show the effect of the ligand conformations on the vapoluminochromism of gold(I) cluster 1. [5] Furthermore, the spatula grinding of 1 cubic and 1 mono yielded a solid phase 1 lc with low crystallinity and red-shifted PL emis-sion.1 lc could also revert to the two crystalline states by solvent stimulation.This study identifies key ligand design guidelines for imparting stimuli-responsive properties to NHC-protected Ccentered gold(I) clusters, and this is expected to have implications for the development of new gold(I) cluster-based materials for sensing applications.

Figure 2 .
Figure 2. The CAu I 6 cores of single cluster molecules in a) 1 cubic and b) 1 mono ; c) upper faces of 1 cubic and d) 1 mono showing a clockwise propellerlike arrangement (blue arrows) of the benzyl groups; and lower faces of e) 1 cubic and f) 1 mono showing clockwise and anti-clockwise (red arrows) propeller-like arrangements of the benzyl groups, respectively.In each BIBn ligand, the benzyl groups on the right side of the benzimidazole moieties are colored in light blue and the benzyl groups on the left side are colored in pink.

Figure 5 .
Figure 5. a) Schematic representation of reversible mechano-induced luminochromism between 1 cubic (top left), 1 mono (bottom left) and 1 lc (right) by grinding; b) PL excitation (dashed line) and emission (solid line) spectra of 1 lc (excitation spectrum at  em = 520 nm; emission spectrum at  ex = 365 nm); c) The PXRD pattern of 1 lc and the simulation pattern of 1 mono .The PXRD peaks corresponding to the (11 " 1) and (111) planes are shown.