The Role of a Confined Space on the Reactivity and Emission Properties of Copper(I) Clusters

Metal clusters have gained a lot of interest for their remarkable photoluminescence and catalytic properties. However, a major drawback of such materials is their poor stability in air and humidity conditions. Herein we describe a versatile method to synthesize luminescent Cu(I) clusters inside the pores of zeolites, using a sublimation technique with the help of high vacuum and high temperature. The porous materials play an essential role as a protecting media against the undesirable and easy oxidation of Cu(I). The obtained clusters show fascinating luminescence properties, and their reactivity can be triggered by insertion in the pores of organic monodentate ligands such as pyridine or triphenylphosphine. The coordinating ligands can lead to the formation of Cu(I) complexes with completely different emission properties. In the case of pyridine, the final compound was characterized and identified as a cubane-like structure. A thermochromism effect is also observed, featuring, for instance, a hypsochromic effect for a phosphine derivative at 77K. The stability of the encapsulated systems in zeolites is rather enthralling: they are stable and emissive even after several months in the air.

monochromators (2.1 nm mm −1 of dispersion; 1200 grooves mm −1 ) and a TBX-04 single photon-counting detector. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Time-resolved measurements were performed using the time-correlated single-photoncounting (TCSPC) PicoHarp300 or the Multi Channel Scaling (MCS) electronics NanoHarp 250 of the PicoQuant FluoroTime 300 (PicoQuant GmbH, Germany), equipped with a PDL 820 laser pulse driver. A pulsed laser diode LDH-P-C-405 (λexc= 405 nm) was used to excite the sample and mounted directly on the sample chamber at 90°. The photons were collected by a PMA-C-192 photomultiplier(PMT) single-photon-counting detector. The data were acquired by using the commercially available software EasyTau (PicoQuant GmbH, Germany), while data analysis was performed using the commercially available software FluoFit (PicoQuant GmbH, Germany). PLQY measurements were performed by using an absolute photoluminescence quantum yield spectrometer Quantaurus C11347 (Hamamatsu, Japan) exciting the sample at λexc = 350 and 400 nm. Confocal microscopy analyses were performed by using Zeiss LSM 710 confocal microscope system with 63X magnification, numerical aperture 1.3 of Zeiss LCI Plan-NEOFLUAR water immersion objective lens (Zeiss GmbH). The samples were excited by laser at 405 nm. All image processing was performed by ZEN 2011 software. XPS measurements were done by a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer using a monochromatic AlKα radiation (hν= 1486.6 eV; λ= 8.340113Å). Element scans were performed with a 50 eV analyzer pass energy and a 0.1eV energy step size obtain the chemical state information. All the obtained binding energies were referenced from carbon 1s peak, coming from the residual CO2, at 284.80eV. 1 H and DOSY NMR spectra were recorded on a Bruker Avance III 600MHz, using as solvent deuterated dichloromethane (CD2Cl2). The residual CH2Cl2 present in the solvent was used as internal standard (δ = 5.35 ppm).
Liquid-chromatography coupled with high-resolution mass spectrometry (HPLC-HRMS) was performed in positive mode using a ThermoFisher Ultimate3000 with Scientific Vanquish Flex UHPLC and a ThermoFisher Orbitrap (Exactive Plus with Extend Mass Range: Source HESI II The detector is a Vanquish PDA Detector (VF-XX, detection ≤ 5 ppm). Direct injection highresolution mass spectrometry was used using as eluant HPLC grade dichloromethane 1 : 9 methanol.
Nitrogen Adsorption-Desorption analyses of the samples were performed using a Micromeritics porosimeter (model ASAP-2020). The samples were degassed at 150 °C for 3h and N2 adsorption/ desorption measurement was done at -196 °C. The surface areas and pore volume were calculated by BET method and the pore size distributions were calculated by DFT methods.
The loading ratio was determined using ICP-AES with a Varian 720 ES instrument at 324.754 nm for Cu. Quantification was performed by a calibration curve established with standards (0, 0.025, 0.1, 0.5, 2, 10 mg/L) prepared from certified standards (1000 mg/L; CPI International) after dilution of the samples.

Synthesis of MCM-41
In a 250ml round bottom flask Hexadecyl-trimethyl-ammonium bromide (CTAB; 0.5 g) was (Pluronic® P123) was dissolved into 42 mL water, then 4.36 g Na2SiO3·9H2O and 0.292 mL APTES was added. The solution was then heated up to 40˚C with vigorous stirring. After the temperature was stable, 10.92 mL of 37% HCl was added and the stirring was kept at 40˚C for 1 h. The solution was transferred into a sealed glass bottle and kept at 100˚C for 24 h. The final product was centrifuged and washed by water 3 times, then dried in vacuum, and calcined at 550˚C for 6h to remove the remaining surfactant. The material was obtained as a white powder.

Synthesis of CuI clusters inside porous materials
The porous materials and the zeolites were always 100 mg, and placed inside a glass ampoule in the presence of the follow amount of CuI. For the Zeolite Y (1.2 nm pore), MCM-41 (2 nm pore size), and SBA-15 (10 nm pore size) 100 mg of CuI was sublimed. For the zeolite L different amount of CuI was sublimed and in particular the following amount of CuI was sublimed for the different samples: A vacuum was applied to the ampoule (10 -9 bar), and the latter was sealed before being placed in a rotating oven. After heat treatment at 200°C for 2h together with a rotation of the tube, the sealed ampoule was opened, and, for each sample, a white, red emissive powder was obtained.

Loading of CuI inside the zeolites
The loading ratio was determined using ICP-AES with a Varian 720 ES instrument at 324.754 nm for Cu.

Synthesis of CuIPy clusters inside porous materials
To a suspension of CuI inside porous materials (100 mg) inside ethanol (25 mL), was added pyridine (85 µL, 1.04 mmol) and the reaction mixture was stirred overnight at room temperature. The mixture was then centrifuged and washed 3 times with cold ethanol in order to yield the final material, CuIPy@porous material as an off-white and yellow emissive powder.

Synthesis of of CuIPPh3 clusters inside porous materials
To a suspension of CuI inside porous materials (100 mg) inside ethanol (25 mL), was added pyridine (273 mg, 1.04 mmol) and the reaction mixture was stirred overnight at room temperature. The mixture was then centrifuged and washed 3 times with cold ethanol in order to yield the final material, CuIPy@porous material as an off-white and yellow emissive powder. Figure S1. Comparison between emission of bulk copper iodide and copper iodide sublimed in presence of non-porous silica.