Solid-State Structures and Photoluminescence of Lamellar Architectures of Cu(I) and Ag(I) Paddlewheel Clusters with Hydrogen-Bonded Polar Guests

Two hexanuclear paddlewheel-like clusters appending six carboxylic-acid pendants have been isolated with the inclusion of polar solvent guests: [Cu6(Hmna)6]·7DMF (1·7DMF) and [Ag6(Hmna)6]·8DMSO (2·8DMSO), where H2mna = 2-mercaptonicotininc acid, DMF = N,N’-dimethylformamide, and DMSO = dimethyl sulfoxide. The solvated clusters, together with their fully desolvated forms 1 and 2, have been characterized by FTIR, UV–Vis diffuse reflectance spectroscopy, TG-DTA analysis, and DFT calculations. Crystal structures of two solvated clusters 1·7DMF and 2·8DMSO have been unambiguously determined by single-crystal X-ray diffraction analysis. Six carboxylic groups appended on the clusters trap solvent guests, DMF or DMSO, through H-bonds. As a result, alternately stacked lamellar architectures comprising of a paddlewheel cluster layer and H-bonded solvent layer are formed. Upon UV illumination (λex = 365 nm), the solvated hexasilver(I) cluster 2·8DMSO gives intense greenish-yellow photoluminescence in the solid state (λPL = 545 nm, ΦPL = 0.17 at 298 K), whereas the solvated hexacopper(I) cluster 1·7DMF displays PL in the near-IR region (λPL = 765 nm, ΦPL = 0.38 at 298 K). Upon complete desolvation, a substantial bleach in the PL intensity (ΦPL < 0.01) is observed. The desorption–sorption response was studied by the solid-state PL spectroscopy. Non-covalent interactions in the crystal including intermolecular H-bonds, CH⋯π interactions, and π⋯π stack were found to play decisive roles in the creation of the lamellar architectures, small-molecule trap-and-release behavior, and guest-induced luminescence enhancement.

In this work, we succeeded in determining two new crystal structures of copper(I) and silver(I) paddlewheel clusters as solvated forms: [Cu6(Hmna)6]·7DMF (1·7DMF) and [Ag6(Hmna)6]·8DMSO (2·8DMSO). Chemical structures of 1 and 2 are shown in Figure 1. We show that solvated copper cluster 1·7DMF is, upon UV excitation, a low-energy emitter in the solid state showing near-IR phosphorescence while the silver(I) counterpart 2·8DMSO vividly emits in the visible region. Significantly, the intense emission observed for both of the solvated clusters is virtually quenched upon complete desolvation, and the resultant tiny emission is then greatly enhanced after gas sorption.

Synthesis
The synthesis and solid-state structures of 1 and 2 have been reported previously by Nomiya et al. [36] and Kund et al. [38]. Herein, we successfully isolated and structurally characterized their solvated clusters, 1·7DMF and 2·8DMSO, in which the solvation number and location were unequivocally determined. The hexacopper(I) cluster with seven DMF guests 1·7DMF was obtained as orange block-shaped crystals in a test tube by softly layering and slowly diffusing metal-and ligand-precursor solutions (e.g., copper(II) acetate in CH3CN and H2mna in DMF, at room temperature). The crystalline solid 1·7DMF,

Synthesis
The synthesis and solid-state structures of 1 and 2 have been reported previously by Nomiya et al. [36] and Kund et al. [38]. Herein, we successfully isolated and structurally characterized their solvated clusters, 1·7DMF and 2·8DMSO, in which the solvation number and location were unequivocally determined. The hexacopper(I) cluster with seven DMF guests 1·7DMF was obtained as orange block-shaped crystals in a test tube by softly layering and slowly diffusing metal-and ligand-precursor solutions (e.g., copper(II) acetate in CH 3 CN and H 2 mna in DMF, at room temperature). The crystalline solid 1·7DMF, which is insoluble in most organic solvents, gave bright red photoluminescence (PL) under UV illumination. The hexasilver(I) cluster with eight DMSO guests 2·8DMSO was obtained as pale-yellow block-shaped crystals by recrystallization of 2 from DMSO/Et 2 O at room temperature. Complex 2·8DMSO, also sparingly soluble in most organic solvents, was photoluminescent in intense greenish yellow in the solid state.
For both solvated complexes, the solvation number was identified by single-crystal X-ray diffraction analysis (see below). The thermogravimetric (TG) analysis revealed that the solvent loss began in a heating process around room temperature for both crystalline samples and ended at ca. 400 K ( Figure S1, Supplementary Materials). On a bench-top experiment, the fully-desolvated solids, 1 and 2, were prepared by the vacuum-pump method at 353 K. As described later, the intense solid-state PL from both solvated clusters was significantly diminished upon desolvation, whose dramatic PL change is easily recognized by the naked eye.

Crystal Structures
The detailed crystallographic data on 1·7DMF and 2·8DMSO are given in Table 1, and the molecular structures of 1 and 2 are shown in Figure 2. Selected interatomic distances and bond angles are collected in Table 2.    (Figure 3a), six of the seven DMF molecules per cluster was H-bonded to COOH via COOH···O=C(H)-(DMF) contact with dO···O = 2.53-2.56 Å, while the remaining DMF molecule (not shown) was located on the crystallographic -3 axis without any assistance by intermolecular contacts. In 2·8DMSO (Figure 3b), six DMSO guests were H-bonded to COOH via COOH···O=S (DMSO) contact with dO···O = 2.54-2.56 Å. Two of these six DMSO had an additional contact through Hbonds from the S=O group (labeled as O8* and O8*** atoms in Figure 3b) to the pyridyl moiety in Hmna − with dC···O = 3.18 Å. The remaining two solvent molecules were encapsulated in the cavity without any significant contacts. 1·7DMF and 2·8DMSO crystallize in a triclinic space group P−1. The cluster molecules are located on an inversion center in the unit cell, in which a half of the molecule (three metal ions and ligands) is crystallographically independent. We found that the crystal lattice constant of 2·8DMSO was virtually consistent with that reported previously for [Ag 6 (Hmna) 6 ] [36]. Both clusters had six-bladed paddlewheel structures in which a 3-fold rotoinversion axis (−3) penetrates the midpoint of the paddlewheel (Figure 2a,b). Each Cu(I) or Ag(I) center adopts a trigonal-planar, distorted three-coordinate geometry with a {NS 2 } donor set. A total of six carboxylic groups (-COOH) per cluster were oriented in an alternate αβαβαβ conformation with respect to the paddlewheel planes, affording trigonally-oriented three -COOH arms on each side. Table 2. Selected interatomic distances (Å) and bond angles ( • ) for 1·7DMF and 2·8DMSO at 150 K.
Six M(I) centers form highly distorted octahedral cores (Figure 2c,d) due to unsymmetrical N,S-bridging mode of Hmna − and the crystal packing, thus providing inequivalent metal-metal separations. For 1, the Cu· · · Cu separations ranged from 3.0190(6) to 3.3315(6) Å for triangles perpendicular to the −3 axis, whereas the others showed shorter separations ranging from 2.6819(6) to 2.9899(6) Å ( Figure 2c and Table 2). These Cu· · · Cu separations were comparable to those reported previously such as neutral [Cu 6 (Hmna) 6 ] [38] and hexaanionic [Cu 6 (mna) 6 [38]. Similar to 1, cluster 2 ( Figure 2d and Table 2) also showed an unsymmetrical structural feature in the {Ag 6 } skeleton where the Ag· · · Ag separations were identical to those of a series of [Ag 6 (mna) 6 ] 6− clusters in the literature [32,33,37,39,40]. Solvent guest molecules located around the paddlewheel clusters are shown in Figure 3. Six guests (per cluster) were trapped to the -COOH side arms from the paddlewheel cluster through intermolecular H-bonds. In 1·7DMF (Figure 3a), six of the seven DMF molecules per cluster was H-bonded to COOH via COOH· · · O=C(H)-(DMF) contact with d O· · · O = 2.53-2.56 Å, while the remaining DMF molecule (not shown) was located on the crystallographic -3 axis without any assistance by intermolecular contacts. In 2·8DMSO (Figure 3b), six DMSO guests were H-bonded to COOH via COOH· · · O=S (DMSO) contact with d O· · · O = 2.54-2.56 Å. Two of these six DMSO had an additional contact through H-bonds from the S=O group (labeled as O8* and O8*** atoms in Figure 3b) to the pyridyl moiety in Hmna − with d C· · · O = 3.18 Å. The remaining two solvent molecules were encapsulated in the cavity without any significant contacts.

Solid-State Spectroscopy and DFT Calculations
UV-Vis diffuse reflectance spectra of desolvated clusters 1 and 2 exhibited shoulders at ca. 450 and 420 nm, respectively ( Figure S2, Supplementary Materials). DFT calculations on 1 ( Figure S3, part a, Supplementary Materials) revealed that the HOMO (162A u ) could be attributed to a Cu-3d/S-2p orbital in the {Cu 6 S 6 } core with a non-bonding character, while the LUMO (163A u ) was localized on the ligand pyridyl ring with a π* character. Therefore, the observed absorption shoulder was assigned to a copper/sulfur-to-ligand charge-transfer transition. For 2 ( Figure S2, part b), the HOMO (189A g ) was comprised of Ag-4d/S-2p orbital, while the LUMO (190A g ) was localized on the {Ag 6 S 6 } core ( Figure  S2, part b). An unoccupied π* ligand orbital LUMO+1 (190A u ) was also found at a slightly higher level to the LUMO. Accordingly, the absorption shoulder for 2 was tentatively assigned to a {Ag 6 S 6 } cluster-centered transition and/or a silver/sulfur-to-ligand chargetransfer transition.
It was found that the clusters formed 2D sheet motifs, as shown in Figure 4. In 1·7DMF (Figure 4a), a 2D monolayer sheet was comprised of hexacopper(I) paddlewheels held together by multiple intermolecular non-covalent interactions such as CH···O and CH···π interactions, while in 2·8DMSO (Figure 4b), it can be seen as intermolecular CH···O H-bonds and π···π contacts. The solvent guest molecules form layers on both sides of the paddlewheel sheet. As a result, the lamellar structure comprises an alternate stack of two distinct layers (Figure 4c,d).

Solid-State Spectroscopy and DFT Calculations
UV-Vis diffuse reflectance spectra of desolvated clusters 1 and 2 exhibited shoulders at ca. 450 and 420 nm, respectively ( Figure S2, Supplementary Materials). DFT calculations on 1 ( Figure S3, part a, Supplementary Materials) revealed that the HOMO (162Au) could be attributed to a Cu-3d/S-2p orbital in the {Cu6S6} core with a non-bonding character, while the LUMO (163Au) was localized on the ligand pyridyl ring with a π* character. Therefore, the observed absorption shoulder was assigned to a copper/sulfur-to-ligand charge-transfer transition. For 2 ( Figure S2, part b), the HOMO (189Ag) was comprised of Ag-4d/S-2p orbital, while the LUMO (190Ag) was localized on the {Ag6S6} core ( Figure S2,  part b). An unoccupied π* ligand orbital LUMO+1 (190Au) was also found at a slightly higher level to the LUMO. Accordingly, the absorption shoulder for 2 was tentatively assigned to a {Ag6S6} cluster-centered transition and/or a silver/sulfur-to-ligand chargetransfer transition.
In the FTIR spectra ( Figure S4 and Table S1, Supplementary Materials), the presence of solvent molecules in 1·7DMF and 2·8DMSO [42] was clearly confirmed. Moreover, vibration peaks ascribed to C=O and C-OH in the COOH pendants for solvated solids were observed at ca. 1710 and ca. 1390 cm −1 , respectively. We observed that these frequencies were apparently affected by desolvation, but the direction of the frequency shift depended on the compounds. This ambiguity probably comes from complexity arising from crystalpacking change or amorphization occurring in the solvent desorption process. In order to address this issue, a powder X-ray diffraction study is required, which is in progress in our laboratory.

Solid-State Photoluminescence
Solvated hexacopper(I) complex 1·7DMF exhibited PL at λPL = 765 nm with a micro- In the FTIR spectra ( Figure S4 and Table S1, Supplementary Materials), the presence of solvent molecules in 1·7DMF and 2·8DMSO [42] was clearly confirmed. Moreover, vibration peaks ascribed to C=O and C-OH in the COOH pendants for solvated solids were observed at ca. 1710 and ca. 1390 cm −1 , respectively. We observed that these frequencies were apparently affected by desolvation, but the direction of the frequency shift depended on the compounds. This ambiguity probably comes from complexity arising from crystal-packing change or amorphization occurring in the solvent desorption process. In order to address this issue, a powder X-ray diffraction study is required, which is in progress in our laboratory.
For solvated hexasilver(I) complex 2·8DMSO, the observed PL (λ PL = 545 nm, τ PL = 9.3 µs, and Φ PL = 0.17) was ascribable to the 3 CC (Ag 6 S 6 ) excited state and/or a Ag/S-toligand charge-transfer triplet excited state, as predicted by our DFT calculations. Here, the contribution of the 3 CC excited state to phosphorescence may be rationalized by energylowering of the CC orbital (Ag-5s,p/S-3s,p) to become LUMO, which is located slightly below the ligand π* orbitals in energy ( Figure S3, part b). For reference, the PL from the hexaanionic [Ag 6 (mna) 6 ] 6− complexes with various cationic salts was assigned to LMCT mixed with the CC excited state [32,33,40].
In sharp contrast to intense PL for the above-mentioned solvated clusters, desolvated 1 and 2 produced significantly weakened PL with Φ PL < 0.01 (Table 3). It was also observed that the desolvation allowed the PL lifetimes to be shortened.

Vapochromic Luminescence
Vapochromic response was studied by solid-state PL spectroscopy, as shown in Figure 5. Desolvated 1 and 2 used for the vapochromic experiments were prepared by vacuum pumping of 1·7DMF and 2·8DMSO, respectively, for 0.5 h at 353 K. The vapor sorption was then achieved by standing the desolvated samples under the atmosphere of saturated DMF or DMSO vapor for 24 h at 296 K.
Under a white LED light, the color of desolvated 1 was yellow (panel A in Figure 5a), which changed into orange under the DMF vapor (panel B). For the same sample as above, the PL of desolvated 1 was faint red (panel C), but the PL was then remarkably enhanced to vivid red after DMF sorption (panel D). For desolvated 2, the pale-yellow solid under the white light (panel E) became white under the DMSO vapor (panel F). Upon UV exposure, an almost invisible PL from desolvated 2 (panel G) became intensely brightened with a vivid greenish yellow upon sorption of the DMSO vapor (panel H). We confirmed that the initial faint PL for the desolvated solids was completely regenerated from the vividlycolored gas-sorbed solids by conducting the vacuum pumping at elevated temperature (0.5 h, 353 K). Chromaticity color coordinate plots [44] (Figure 5b) also characterize two distinct states ("solvated" and "desolvated") for 1 and 2.
Finally, desorption-sorption cycles were examined for 2·8DMSO by probing the PL intensity. A moderate degree of reversibility was shown (Figure 5d).
An appealing advantage has been demonstrated for complexes 1 and 2 as solid-state turn-on detectors toward polar VOCs by use of visible (Ag6) or near-IR (Cu6) optical windows. Further study is in progress on the gas-sorption/desorption ability of these paddlewheel clusters in terms of gas selectivity, response speed, and reproducibility for multiple operations.  [44]. (c) Solid-state PL spectra (298 K, λex = 365 nm) of desolvated 2 (red solid curve) with its 150-times-magnified trace (red dots) and those under a DMSO atmosphere kept for 24 h (blue solid curve). An excitation profile (monitored at 545 nm) is also shown (blue dots). (d) The PL intensity change during two cycles for desorption and sorption procedures for 2·8DMSO (monitored by the intensity at the peak top of PL).
Step 1: PL peak intensity of 2·8DMSO. Steps 2-5: Relative PL peak intensity monitored at desolvated (steps 2 and 4) and solvated states (steps 3 and 5).  [44]. (c) Solid-state PL spectra (298 K, λ ex = 365 nm) of desolvated 2 (red solid curve) with its 150-times-magnified trace (red dots) and those under a DMSO atmosphere kept for 24 h (blue solid curve). An excitation profile (monitored at 545 nm) is also shown (blue dots). (d) The PL intensity change during two cycles for desorption and sorption procedures for 2·8DMSO (monitored by the intensity at the peak top of PL).
The solid-state PL spectroscopy (Figure 5c and Figure S5 for 2 and 1, respectively) revealed that the very weak luminescence of desolvated 2 (red solid curve with red dots for the magnified trace) was significantly enhanced after the desolvated solid was stood under a DMSO atmosphere (blue solid curve, Figure 5c). The enhanced PL spectrum was no longer changed under continuous exposure to the DMSO vapor, suggesting that the sorption was completed within 24 h. Desolvated 1 also showed a similar behavior toward DMF in the PL spectra, in which ca. 40-times enhancement was observed under DMSO ( Figure S5). Such a vapor-induced PL enhancement occurs by trapping guest molecules to form intermolecular H-bonds as shown by X-ray crystallography. The guest trapping through Hbonds may restrict the freedom of molecular motions or thermal vibrations in the crystalline state, resulting in suppression of the probability of the non-radiative decay of the excited states and, as a consequence, the solid-state luminescence is enhanced. In the literature, issues on the effects of non-covalent interactions to solid-state luminescence have been addressed for aggregation-induced emission systems [45][46][47].
Finally, desorption-sorption cycles were examined for 2·8DMSO by probing the PL intensity. A moderate degree of reversibility was shown (Figure 5d).
An appealing advantage has been demonstrated for complexes 1 and 2 as solid-state turn-on detectors toward polar VOCs by use of visible (Ag 6 ) or near-IR (Cu 6 ) optical windows. Further study is in progress on the gas-sorption/desorption ability of these paddlewheel clusters in terms of gas selectivity, response speed, and reproducibility for multiple operations.

Materials
Chemical reagents and solvents used in this study were purchased from TCI, WAKO, and Nakalai, and used as received. All the synthetic reactions in this study were performed under an Ar atmosphere unless otherwise stated. Complex 2 was prepared by the reported method [36].

Physical Measurements
Fourier transform infrared (FTIR) spectra were obtained on a JASCO FT/IR-4000 with the attenuated total reflectance (ATR) method for the solid samples. Solid-state UV-Vis diffuse reflectance UV-Vis spectra were measured by a HITACH U-3500 UV-Vis-NIR spectrometer with an integrating sphere at 296 K. Thermogravimetry and differential thermal analyses (TG-DTA) were performed on a Rigaku Thermoplus EV02. Elemental analysis was carried out by the Laboratory for Organic Elemental Microanalysis, Kyoto University.

Photoluminescence and Photophysical Measurements
Solid-state PL spectra were measured by a Hamamatsu PMA-11 (C7473-46) spectrometer (190 to 900 nm). The crystalline samples were placed in a Linkam THMS 600 temperature-controlled (78 to 293 K) microscope stage. A Panasonic UJ-30 UV-LED system was used as an excitation light source (λ PL = 365 nm). The PL from the sample was introduced to the optical fibers of the spectrometer through a microscope. The excitation spectra for solids were measured by a JASCO FP-8500 spectrofluorometer with an FPA-810 powder sample cell block. The absolute PL quantum yields (PLQYs) were measured by a Hamamatsu C9920-02 system with an integrating sphere. The PL lifetimes were measured on a Hamamatsu Quantaurus-Tau C11367-02 fluorescence lifetime spectrometer.
Single crystals of 2·8DMSO suitable for X-ray crystallography were obtained by slow vapor diffusion of Et 2 O into a DMSO solution of 2 for 24 h at 293 K.

X-ray Crystallography
Single-crystal X-ray diffraction analysis for 1·7DMF and 2·8DMSO was performed on a Rigaku Rapid image plate diffractometer at 150 K using Mo-Kα radiation (λ = 0.71073 Å). The crystal was attached to a thin Nylon loop and kept under a stream of cold N 2 gas from a He gas-expansion cryostat. The structures were solved by a direct method with SHELXT-2018/2 [48] and refined by using the full-matrix least-squares method based on F 2 with the SHELXL-2018/3 refinement program [49]. An empirical absorption correction was applied. All non-hydrogen atoms, except some disordered oxygen atoms, were refined anisotropically. Hydrogen atoms were located by calculations. All the calculations for structural determination and refinements were carried out using the WinGX [50].
For 1·7DMF, two disordered DMF molecules, DMF-A (comprising of C25, C26, N6, C27, and O9) and DMF-B (comprising of C28, C29, N7 C30, and O10), were seen. The amide group in DMF-A (N6-C27=O9) was located in two orientations where the positions of N6 and C27 were completely disordered. In DMF-B, N7 was located on a crystallographic inversion center and the other atoms were located in two positions. For 2·8DMSO, two DMSO molecules, DMSO-A (C23, C24, S6, and O9) and DMSO-B (C25, C26, S7, and O10), were disordered. For DMSO-A, the S atom was located above and below the C23-C24-O9 plane with an uneven occupancy factor of 0.9:0.1. The molecule DMSO-B was disordered in two orientations and located with an occupancy factor of 0.8:0.2. For these disordered atoms, the major and minor component atoms were labelled "a" and "b", respectively, as in S6a and S6b. The minor components, labelled "b", are omitted in Figure 3 for the sake of clarity.

DFT Calculations
DFT calculations were carried out with the SCM ADF2019 suite of programs [51,52]. The initial geometrical parameters for 1 and 2 were taken from the corresponding crystal structures at 150 K, symmetrizing the molecules to adapt C i symmetry as in the crystal structures, and used for geometrical optimization. The exchange-correlation (XC) energy function of the PW91 method was applied for calculations. The valence shell atomic orbitals of Cu, Ag, S, N, C, and H atoms were described by triple-zeta Slater-type basis sets with two polarization functions (ADF database TZ2P).

Photophysical Measurements
Completely desolvated samples 1 and 2 for use in the gas-sorption experiments were prepared by vacuum pumping (0.5 h at 353 K) of the corresponding solvated samples 1·7DMF and 2·8DMSO, respectively, which were loaded on an aluminum sample holder. The gas sorption experiments were then achieved by standing the desolvated samples under vapor in a Petri dish-like seal container at 298 K. The in situ solid-state PL measurements were carried out in the optical window between 190 and 950 nm while allowing for the solid samples to maintain the vapor-saturated conditions during the PL measurements. The PL spectra were recorded at a constant interval, typically at every 2 h, to trace the time course of the sorption process until the PL spectra reach the saturation level.

Conclusions
The paddlewheel-like hexanuclear clusters [Cu 6 (Hmna) 6 ] (1) and [Ag 6 (Hmna) 6 ] (2) were isolated as solvated forms 1·7DMF and 2·8DMSO. Single-crystal X-ray diffraction analysis revealed the formation of lamellar architectures composed of an alternate stack of two distinct layers: "paddlewheel cluster layer" (supported with intermolecular CH· · · π and π· · · π interactions) and "solvent guest layer", where DMF or DMSO guests are bound through intermolecular H-bonds to carboxylic pendants from the paddlewheel molecule. The 1·7DMF and 2·8DMSO exhibited microsecond-order, intense PL in the solid state in the near-IR and visible wavelength regions, respectively. In contrast, desolvated forms 1 and 2, prepared by vacuum drying for the solvated forms, were tiny emitters due to vibrational non-radiative decay. Remarkable recovery for intense PL was observed by spontaneous gas sorption. The vapor-induced luminescence enhancement may promise the future development of novel VOC-sensitive solid-state sensors and switching devices. Along this line, the paddlewheel clusters described here, which were synthesized from inexpensive metal and ligand sources commercially available, should be an elaborate molecular motif. Further study is in progress to improve their response ability in terms of sensitivity, selectivity, and response speed in order to develop more advanced materials for sensing VOCs and hazardous chemicals in the atmosphere.