[Cu18H3(S-Adm)12(PPh3)4Cl2]: fusion of Platonic and Johnson solids through a Cu(0) center and its photophysical properties

Structural elucidation of atom-precise thiolate-protected copper nanoclusters (Cu NCs) containing Cu(0) is quite challenging. Here, we report a new adamantane-thiol-protected NC, [Cu18H3(S-Adm)12(PPh3)4Cl2] (Cu18), which represents the first observation of a rare mononuclear Cu(0)-containing Cu10H3Cl2 core that is constructed via kernel fusion through vertex sharing of the Platonic-solid- and Johnson-solid-geometry-like kernels and hydride-bridging. The unique core is surrounded by a Cu8S12P4 metal–ligand motif shell and adopts a butterfly-like structure. In comparison to its closest structural analogue, the predominant effect of the principal Cu atom vacancy-induced structural rearrangement is evidenced. The occupied orbitals of this NC have a major d-orbital contribution to the distorted Cu6 octahedral kernel, whereas unoccupied orbitals owe a contribution to the distorted Cu5 square-pyramidal kernel. Thus, the charge transfer phenomenon is uniquely instigated between the two fused kernels through Cu(d) → Cu(d) transition via the Cu(0) center. This NC exhibits violet emission due to kernel-dominated relaxation at room temperature, which is further enhanced by confining the surface protecting ligands through recognition-site-specific host–guest supramolecular adduct formation by β-cyclodextrin. The unique electronic structure of this NC further facilitates its application toward photocurrent generation. Thus, this study offers a unique strategy for the controllable synthesis of a Cu(0)-containing Cu NC, which enables atomic-level insights into their optoelectronic properties.

Crystal data and structure refinement parameters S7 Table S2 List of reported thiolate protected Cu NC S8 Table S3 The obtained radiative, nonradiative rate constants, quantum yield and lifetime S9 Fig. S1 Optical image of the Cu18 NC S10

Fig. S2
The attachment of hydrides and chloride with the core S11

Synthesis of Cu18 NC
Initially, 50 mg of Cu(CH3CN)4BF4 and 50 mg of PPh3 were dissolved in the mixture solution of 2 mL acetonitrile and 0.5 mL chloroform at room temperature. After 5 min of stirring, 20 mg of Adm-SH was added to the reaction mixture and continued stirring. After the addition of 50 mg NaBH4 dissolved in 2.5 mL methanol, the color of the solution becomes red from colorless. The reaction was kept for another 1 h under continuous stirring. After completion, the reaction mixture was centrifuged and collected the red precipitate. After drying properly, the precipitate was dissolved in the solvent mixture of chloroform/hexane (volume ratio 1:1).
The final clear solution was kept for crystallization at ambient conditions. After 7 days, redcolored cubic crystals were obtained. a one-pot direct reduction strategy had been followed. S4, S5, S6 In the case of Cu15 NC and Cu36 NC, NaBH4 was used and in the case of Cu81 NC, t BuNH2·BH3 was used as reducing agents, though in all cases, Cu(I) salts were taken as a metal precursor. It has been observed that in the case of the mild reducing agent which gives rise to high-nuclearity Cu NC compared to the strong reducing agent under ambient conditions. In both cases, there is no Cu(0) atom. But in this work, Cu18 NC was synthesized by a facile one-pot synthetic strategy. This synthetic procedure is almost the same as the procedure of Cu36 NC. Only we have changed the PET ligand with the bulky adamantane thiol ligand in our synthesis and that bulkiness of the ligand may induce the stability of the Cu(0) atom at the center of the Cu10 core.

Synthesis of Cu18D NC
The same procedure was adopted whatever was mentioned for the synthesis of the Cu18 NC.
Instead of NaBH4, we have used NaBD4 as a reducing agent.

Synthesis of Cu18 ∩ β-CD1
Initially, 3 mg of Cu18 NC was dissolved in 5 mL of tetrahydrofuran and then a 2 mL aqueous solution of β-CD (2 mg/mL) was added. The resultant reaction mixture was then sonicated for 15 min and kept for stirring for 1 h with a recurrent sonication for 2 min at every 15 min intervals. The obtained solution was washed several times with water. Finally, the aqueous phase was removed and the organic phase was dried.

X-ray Crystallography details
Single-crystal data of Cu18 NC was collected on a Rigaku Oxford Diffraction XtaLAB Synergy-S diffractometer equipped with a HyPix-6000HE Hybrid Photon Counting (HPC) detector operating in shutter less mode and an Oxford Cryosystems Cryostream 800 Plus at 173 K using Cu Kα (λ = 1.54184 Å) from PhotonJet micro-focus X-ray Source. Data were processed using the CrystAlis Pro software suite. S7 The structure was solved using the chargeflipping algorithm, as implemented in the program SUPERFLIP S8 and refined by full-matrix S5 least-squares techniques against Fo 2 using the SHELXL program S9 through the OLEX2 interface. S10

Computational details
The DFT calculations were done using Gaussian 09 D.01 program. S11 B3LYP functional with Pople's 6-31G* basis set S12, S13 was used for non-metal elements, and LANL2DZ-ECP (effective core potential) S14, S15 was employed for Cu atoms, respectively. Chloroform solvent was considered to simulate the experimental non-polar solvent scenario. The TD-DFT calculations considered 300 singlet-to-singlet excitation energies. The -PPh3 ligand is simplified to -PMe3 during the calculation. Kohn-Sham orbital analysis has been performed for identifying the orbital contribution in molecular orbitals and corresponding energies by using the multi wave function 3.6 program. S16, S17 The NMR spectrum is simulated by using the GIAO method implemented in Gaussian 09 D.01 program. S18 The projected density of states (PDOS) calculation for the compounds at the bulk level is carried out using Vienna Ab-Initio Simulation Package (VASP) by using Generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) functional. S19, S20 Projector augmented wave (PAW) method is used for treating ion-electron interactions. S21 The ionic relaxations have been carried out using a Conjugate gradient algorithm with convergence criteria of 10 -4 eV for minimum energy and 0.05 eV Å -1 for Hellmann-Feynman forces on atoms. Due to the large size of the unit cells of the compounds, the Brillouin zone was sampled at the Gamma point (1×1×1). For the PDOS calculation, a higher (2×2×2) K-point is used. The atomic charges are calculated using Bader charge analysis. S22

Instrumentation
A SHIMADZU UV-3800 spectrometer was used for measuring the absorbance spectra.
Emission measurements were performed using a Fluorolog-3 spectrofluorimeter from Horiba Jobin Yvon. The relative quantum yield was determined by the best match of the excitation wavelength of a well-known chromophore such as pyrene with the NC and the excitation was fixed at 317 nm. The concentration of the chloroform solutions was fixed by adjusting the absorption 0.05 OD and measured the quantum yield at room temperature. A picosecond timecorrelated single photon counting (TCSPC) system (Horiba Jobin Yvon-IBH) was utilized for S6 the measurement of emission lifetime. Transmission electron microscopy (TEM; FEI Tecnai G2 F30 S-Twin; 300 kV), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS; FEI Nova NANOSEM 450) were used. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements have been done by using the Omicron Nanotech instrument (MgKα radiation at 1253.6 eV). All binding energies were referenced to the neutral C 1s peak at 284.8 eV. The thermo-gravimetric analysis (TGA) was done on SDT Q600 (Shimadzu) instrument with a heating rate of 10 ο C min -1 under N2 atmosphere. Cyclic Voltammogram was done by using a CH instrument with a three-electrode system where glassy carbon was used as a working electrode, graphite electrode served as a counter electrode and silver wire was used as a reference electrode in dichloromethane solvent.
In this organic solvent, 0.5 M [( n Bu)4N]PF6 salt was added as a supporting electrolyte. We have calibrated against the standard reference electrode (Ag/Ag + ). Bruker Avance III, 500 MHz, NMR was used for the 1 H and 31 P studies. Electron paramagnetic resonance (EPR) spectrum was recorded using MiniScope MS 5000 Magnettech EPR spectrometer. Waters Q-TOF mass spectrometer equipped with a Z-spray source was used for the electrospray ionization (ESI) mass spectrometry measurement in positive mode. Samples were dissolved in chloroform (1 mg/mL) and diluted by acetonitrile (1:1). The solution was infused at 20 µL/min. The spectrometer was operated in the mass range of m/z 1500-6000, capillary voltage was 2.9 kV, sampling cone 60, source temperature 70 °C source offset 28, desolvation temperature 100 °C, cone gas flow 20 L/Hr, desolvation gas flow 244 L/Hr. The chloroform solution of the synthesized NCs is drop-casted between the patterned gold electrodes deposited on the ceramic plate to form a thin layer of the material. The electrical and photoresponse measurements were observed using a Keithley 6712 electrometer using a laser of 360 nm wavelength and 10 mW.

The radiative and nonradiative rate constants calculation
The radiative rate (kr) and nonradiative rate (knr) constants are calculated using the following equations where, ɸ = Quantum yield; τav = Average emission lifetime.  Table S2. List of reported thiolate protected Cu NCs.

Fig. S1
The optical images of the Cu18 NC with cubic shape.