Stable and Efficient Rare-Earth free Phosphors based on an Mg(II) Metal-Organic Framework for Hybrid Light-Emitting Diodes

Stable and efficient green hybrid light-emitting diodes (HLEDs) were fabricated from a highly emissive Mg(II)-tetraphenyl ethylene derivative metal-organic framework embedded in a polystyrene matrix (Mg-TBC MOF@PS). The photoluminescence quantum yield (ϕ) of the material, >80%, remains constant upon polymer embedment. The resulting HLEDs featured high luminous efficiencies of >50 lm W-1 and long lifetimes of >380 h, making them among the most stable MOF-based HLEDs. The significance of this work relies on the combination of many features, such as the abundance of the metal ion, the straightforward scalability of the synthetic protocol, the great ϕ reached upon phosphor fabrication, and the state-of-the-art HLED performances.


Materials and methods
All the chemicals were purchased from Sigma Aldrich, Tokyo Chemicals Industry, or Fluorochem without any further purification.The solvents were dried from a solvent purification system MB SPS-800/7 from MBraun, using argon as inert gas and a double column solvent filtration system.Deuterated solvents were bought from Eurisotop or Sigma Aldrich and used without any further purification.Nuclear magnetic resonance (NMR) spectra were recorder from a JEOL 400 MHz Year Hold Super Conducting Magnet (400JJYH, JEOL, Tokyo, Japan).Each spectrum was calibrated according to the residual peak of the solvent as internal standard.Liquid-chromatography coupled with high-resolution mass spectrometry (HPLC-HRMS) was performed in positive mode using a Thermo Fisher Ultimate3000 with Scientific Vanquish Flex UHPLC and a Thermo Fisher Orbitrap (Exactive Plus with Extend Mass Range: Source HESI II).The detector was a Vanquish PDA Detector (VF-XX, detection ≤ 5 ppm).Direct injection high-resolution mass spectrometry was used using as eluant HPLC grade acetonitrile.Powder X-ray diffraction (PXRD) data used in this work were obtained from a Miniflex diffractometer (Rigaku, Tokyo, Japan) with a copper source (1.54Å) and a silicon strip detector (D/teX Ultra, Rigaku), a goniometer radius of 150 mm; both Soller slits at 2.5°; a divergence slit fixed at 0.625°; an anti-scatter screen; and a kβ filter of 0.06 mm nickel foil.The powder was placed on a zero-background silicon wafer sample holder (cut 911) and the measurement was performed between 2ϴ values between 3° and 40° at room temperature using a step size of 0.5°/min.Scanning electron microscopy (SEM) images were recorded using a DSM 940A (Zeiss, Germany).The sample holders were mounted by sticking a double-sided adhesive tape on an aluminium stub at the bottom, and a glass disk plate on the top.The sample was prepared by drop-casting a suspension of the material in ethanol on the glass plate and left at ambient conditions until the complete evaporation of ethanol.The samples were then sputter-coated with gold.SEM images were obtained using 25 keV electron energy, using the secondary electron detector.Nitrogen adsorption-desorption curves were recorded from samples degassed overnight at 80 °C, and corrected for the corresponding blank measurements (Surfer Nano, Thermo Fisher, Waltham, USA).From these, the specific surfaces of the materials were determined by the method of Brunauer, Emmett, and Teller (BET) from the relative pressure range 0.05 < P/P 0 < 0.999.s1, s2 Pore size distributions were recorded in the range 1 nm < d <20 nm and were obtained by the method of Broekhoff and de Boer, s3 based on the theory of Barrett, Joyner, and Halenda.s4  Thermogravimetric analyses were carried out with a PT1600 analyser (Linseis, Germany) under ambient atmosphere (TGA, PT1600 Linseis, Selb, Germany) and a heating rate of 15 K·min -1 .For this purpose, the TG detector was mounted.The measurement was related to the original sample mass, established at 100%.Solid-state emission and excitation spectra were recorded with an FS5 spectrofluorometer, (Edinburgh Instruments) and a Xe lamp as a light source.The powder samples were inserted into a quartz tube, and the latter was placed into a quartz cuvette.The measurements were performed using the cuvette holder (SC-05 sample chamber).Regarding films into polymeric matrixes samples, they were performed in the same way, but the films were directly placed in a quartz cuvette.Solid state excited-state lifetime data were recorded using the same spectrofluorometer and the same sample holder.The light source employed for exciting the sample is a pulsed LED EPLED at 375 nm.Photoluminescence quantum yields were determined using a Hamamatsu Quantaurus C11347 absolute photoluminescence quantum yield spectrometer and exciting the samples between 300 and 400 nm.
The synthesis procedure for 1 is modified according to a previous work.s5 Tetraphenylethene (2.5 g, 7.5 mmol, 1.0 eq.) was placed in in a glass petri dish in a desiccator.Bromine (3 mL, 58.5 mmol, 7.8 eq.) was placed in another glass petri dish, side by side to the first petri dish, in the same desiccator.The latter was closed for 120 h at r.t., and was quickly opened from time to time to release the over pressure.The resulting dark brown solid was collected, washed several times with ethanol and recrystallized in methanol in order to obtain 1 as an off-white powder (4.8 g, 7.4 mmol, 99%).Synthesis of tetramethyl 4 ',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)

Synthesis of Mg-TBC MOF
In a pressure tube, anhydrous MgCl 2 (300 mg, 3.1 mmol, 17.5 eq) TBC (150 mg, 0.18 mmol, 1 eq), acetic acid (5 mL, 0.087 mmol, 0.44 eq) and dimethylformamide (DMF, 50 mL) were mixed together.The tube was sonicated for 2 min, and was heated at 150°C for 48 h.The crystalline yellow crystalline powder was purified by a Soxhlet extraction in ethanol for 24 h.The powder was dried under vacuum in order to obtain the final material (88 mg).

Coating & HLEDs preparation
The photon down-converting coatings were prepared and optimized as followed.A 200 mg·mL -1 polystyrene (PS, average Mw=350000 g/mol) solution in toluene was made by mixing 4 g of PS in 20 mL toluene, prior to heating at 80°C and stirring for 2 h.6 mg of Mg-TBC MOF, was put in a vial and 900 µL of the PS solution was poured in the vial.The resulting suspension was heated at 80°C and stirred at 300 rpm for 10 minutes.The final suspension was allowed to cool down to r.t., and 300 µL of it were put in a well-shaped Teflon mould.The suspension was left to dry for 48h at r.t., and the resulting dry, dome-shaped coating was recovered.The thickness of the coating was measured using a Helios-Preisser electronic outside micrometre.The HLEDs fabrication was performed as followed: the prepared coatings were placed according to either an on-chip (zero distance to the emitting chip) or a remote (2 cm distance to the emitting chip) configuration on a 380 nm LED (HotRed 1 W).Luminous efficiency was determined at different applied currents (10-200 mA), while device stability was monitored at constant 200 or 20 mA under ambient conditions.The employed power source was a Keithley 2231-A-30-3.The electro-luminescence spectra were recorded by an Avantes 2048L spectrometer (300 VA grating, 200 µm slit, CCD detector) coupled with an AvaSphere 30-Irrad integrated sphere.The temperature of the photon down-converting coatings under operation condition was monitored using a thermographic camera FLIR ETS320.The stability of the air-free coatings was performed the same way, but in a glove box, using as inert gas grade 5 nitrogen (99.999% purity).

Figure S3 .
Figure S3.Top: 1 H NMR spectrum of TBC; middle: 13 C NMR spectrum of TBC; bottom: HR-MS spectrum of TBC.

Figure S5 .
Figure S5.Left: volume adsorbed vs. relative pressure plot of Mg-TBC MOF, right: Pore size distribution of Mg-TBC MOF.

Figure S11 .
Figure S11.Emission intensity over time of the Mg-TBC MOF-based HLED in an on-chip configuration at 200 mA under nitrogen atmosphere.

Figure S12 .
Figure S12.Left: PXRD pattern; and right: emission of spectra of the fresh (green line) and of the post-mortem (orange line) Mg-TBC MOF sample in its powder state.Inset: zoom of the post-mortem sample.