Exploring the Luminescence, Redox, and Magnetic Properties in a Multivariate Metal–Organic Radical Framework

Persistent neutral organic radicals are excellent building blocks for the design of functional molecular materials due to their unique electronic, magnetic, and optical properties. Among them, triphenylmethyl radical derivatives have attracted a lot of interest as luminescent doublet emitters. Although neutral organic radicals have been underexplored as linkers for building metal–organic frameworks (MOFs), they hold great potential as organic elements that could introduce additional electronic properties within these frameworks. Herein, we report the synthesis and characterization of a novel multicomponent metal–organic radical framework (PTMTCR@NR-Zn MORF), which is constructed from the combination of luminescent perchlorotriphenylmethyl tricarboxylic acid radical (PTMTCR) and nonemissive nonradical (PTMTCNR) organic linkers and Zn(II) ions. The PTMTCR@NR-Zn MORF structure is layered with microporous one-dimensional channels embedded within these layers. Kelvin probe force microscopy further confirmed the presence of both organic nonradical and radical linkers in the framework. The luminescence properties of the PTMTCR ligand (first studied in solution and in the solid state) were maintained in the radical-containing PTMTCR@NR-Zn MORF at room temperature as fluorescence solid-state quenching is suppressed thanks to the isolation of the luminescent radical linkers. In addition, magnetic and electrochemical properties were introduced to the framework due to the incorporation of the paramagnetic organic radical ligands. This work paves the way for the design of stimuli-responsive hybrid materials with tunable luminescence, electrochemical, and magnetic properties by the proper combination of closed- and open-shell organic linkers within the same framework.


General methods and materials
All reagents and solvents employed in the syntheses were of high purity grade and were purchased from Sigma-Aldrich Co., or TCI. 1 H liquid-state NMR spectra were recorded on a Bruker AVANCE 300 spectrometer (300 MHz).Tetramethylsilane was used as an internal reference.Chemical shifts (δ) are quoted in ppm from TMS and the coupling constants (J) in Hz.Positive-ion ESI mass spectra were acquired using a Q-TOF 2 instrument [Nitrogen was used as nebulizer gas and argon as collision gas.The needle voltage was set at 3000 V, with the ion source at 80 °C and desolvation temperature at 150°C.The cone voltage was 35 V].Infrared spectra were recorded using powdered samples in an ATR FT-IR GALAXY SERIES FT-IR 7000 (Mattson Instruments) spectrometer in the 4000-400 cm -1 range.EPR measurements were performed in a EMX 300 equipment (Bruker) at room temperature.Magnetic susceptibility measurements were performed using a MPMS3 SQUID-VSM Magnetometer (7 Teslas) (Quantum Design) or PPMS-9 equipment (9 Teslas) (Quantum Design).TGA was measured in a Q5000 IR thermobalance (TA instruments).
Photoluminescence spectroscopy: The emission and excitation spectra were recorded on a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a near-infrared R928 Hamamatsu photomultiplier, using the front face acquisition mode.The excitation source was a 450 W Xe arc lamp.Both recorded emission and excitation spectra were corrected with the spectrofluorimeter optical spectral response and the spectral distribution of the lamp intensity using a photodiode reference detector, respectively.Absolute photoluminescence quantum yields (PLQY) were measured with a quantum yield measurement system Quantaurus-QY (C13534, Hamamatsu), equipped with a 150 W Xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as sample chamber and two multi-channel analyzers for signal detection in the visible and in the NIR spectral ranges.
The excitation wavelength was 378 nm.
Electrochemical measurements: The electrochemical experiments were performed using an Autolab electrochemical workstation (PGSTAT302N with FRA32M Module) connected to a personal computer that uses Nova 2.1 electrochemical software.A typical three-electrode experimental cell equipped with a platinum wire as the counter electrode and a silver wire as the pseudoreference electrode was used for the electrochemical characterization of the working electrodes.The electrochemical properties were studied measuring the cyclic voltammogram at different scan rates in previously N2 purged 0.1 M TBAPF6/CH2Cl2 solution.Ferrocene was added as an internal standard upon completion of each experiment.All potentials are reported in V versus Ag/AgCl.Electrode preparation: The powdered materials (2 mg) were mixed in 2 mL of Nafion and ethanol (1:3).100 µL were deposited on a 3 mm diameter glassy carbon disc working electrode, which was previously polished with 0.3, 0.1, and 0.05 µm alumina powders.Afterwards, the solvent was evaporated at room temperature.

Synthesis of PTMTC R
First, tris (2,3,5,6-tetrachlorophenyl) 1,2,4,5-tetrachlorobenzene (9.6 g, 44 mmol) was mixed with aluminum chloride (AlCl3) (1.66 g, 12.48 mmol) and chloroform (CHCl3) (1.0 mL, 12.48 mmol) in a glass pressure vessel.The mixture was heated at 165 °C for 24 hours.Then, the mixture was cooled with ice and hydrochloric acid (HCl) (1 M, 50 mL) was added.The mixture was extracted three times with CHCl3.The organic layer was then washed with water and aqueous sodium bicarbonate (NaHCO3) dried over sodium sulfate (Na2SO4) and the solvent was removed by evaporation under reduced pressure.The residue was purified by flash chromatography on silica gel using hexane as an eluent.The result of the purification process yielded 2.15 g of white powder (67 % yield) (1).Characterization:   Tris(2,3,5,6-tetrachlorophenyl)methane (1) (500 mg, 0.76 mmol) and N,N,N',N-Tetramethylethylenediamine (TMEDA) (1.15 ml, 7.6 mmol) were dissolved in 50 mL of anhydrous tetrahydrofuran (THF) under inert atmosphere and cooled to -78 °C.Then, a solution of 2.5 M n-BuLi in n-hexane (3.95 mL, 10.0 mmol) was added in a single step, and the mixture was stirred at this temperature for one hour.Subsequently, ethyl chloroformate (0.94 mL, 10.0 mmol) was added, and the reaction mixture was allowed to reach room temperature.After 16h, the solvent was evaporated, and the resulting residue was dissolved in dichloromethane (CH2Cl2).The organic layer was washed with water and dried with anhydrous Na2SO4.The solvent was removed under vacuum, and the remaining residue was purified over silica gel chromatography using CH2Cl2/hexane (1/1) as eluent to obtain 540 mg (81 % yield) of 2 as a white solid.Characterization:   Compound 2 (250 mg, 0.29 mmol) was mixed with 30 mL of concentrated sulfuric acid (H2SO4) (97 %) and heated at 90 °C for 12 hours.Then, the mixture was cooled down to room temperature, ice was added, and the resulting aqueous phase was extracted with diethyl ether (Et2O).The organic phase was concentrated and then extracted with an aqueous solution of Na2CO3.The resulting aqueous phase was acidified using 5 M HCl and extracted multiple times with Et2O.The organic phase was dried with anhydrous Na2SO4, and the solvent was removed under vacuum.The crude product was dissolved in Et2O and precipitated multiple times with hexane.PTMTC NR was obtained as a white powder (200 mg, 88 % yield).Characterization:            3. Preparation of PTMTC R @PTMTC NR films.
Preparation of thin films with different radical concentration.PTMTC R @PTMTC NR films with different radical concentrations (1%, 2%, 3% and 4%) were prepared by spin coating in the dark.
Solutions of 0.05 mM of PTMTC R @PTMTC NR in CHCl 3 were prepared and deposited on quartz substrates.The spin coating was performed at 1000 rpm for 1 minute.This procedure was repeated again to obtain 2 layers for each PTMTC R @PTMTC NR film.

Kelvin probe force microscopy (KPFM)
KPFM experiments were done at rarefied atmosphere (air pressure below 5×10 -1 Torr) and room temperature (about 25°C) using Park NX HiVac microscope (Park Systems).A supersharp AFM probe SSS-NCHR (Nanosensors, Switzerland) with the tip curvature radius below 2 nm was used.The first resonance frequency of the cantilever (298 kHz) was used for the non-contact feedback with the set point fixed at 90% of the amplitude of free vibrations (7 nm) that corresponds to a tip-sample distance about 18 nm.A single-pass KPFM mode was implemented using AC voltage of amplitude 0.1 V and frequency of 17 kHz.No DC voltage bias was applied between the tip and the sample.The registered KPFM signal was analyzed by a built-in lock-in amplifier with time constant 30 ms and sensitivity 0.1 V. Scan size was set 25×25 nm with 256×256 points.In total, 18 scans at different places of one PTMTC NR -Zn MOF and two PTMTC R@NR -Zn MORF crystals were measured.For each scan line, forward and backward KPFM signals were collected and analyzed jointly to insure the reproducibility of the measurements.However, raw scans contain much amount of noise.Therefore, the following pre-treatment scheme was performed using Gwyddion software.First, high frequency noise of electrical origin was removed from the scans using FFT filtration.The instability of the mechanical system especially prominent at small size scans leads to the mismatch even between consequent scans.Therefore, the cross-correlation analysis between forward and backward scans was applied to reveal the regions with high correlation (correlation score higher than 70% of the maximum).The obtained mask was applied to the FFT filtered scans and the average and median values of the KPFM potential were extracted for each region.Such pre-treatment was applied for all pairs of forward and backward scans measured for each crystal.Then, the results were combined together and plotted as histograms.Such analysis allows separating the reproducible signals only thus related to the effect of the sample.The sample's work function, φ s , depends on the work function of the tip, φ t , and the KPFM amplitude, V KPFM : φ s = φ t -eV KPFM , where eis an elementary charge.Therefore, for the same tip, the measured KPFM potential signal is proportional to the sample's work function: V KPFM = (φ t -φ s )/e.

Figure S9 .
Figure S9.Evolution of the reaction for the synthesis of PTMTC R monitored by UV-vis spectroscopy.

Figure S10 .
Figure S10.UV-vis spectra of the PTMTC R at different concentrations in CHCl 3 .

Figure S14 .
Figure S14.Magnetic susceptibility of PTMTC R as a function of temperature in the 5-200 K range.a) •T versus T at H = 5000 Oe).b)  -1 versus T and Curie-Wiess linear fit.

Figure S15 .
Figure S15.Emission spectra of the PTMTC R (0.05 mM) in CHCl 3 and anhydrous THF excited at 378 nm.

Figure S16 .Figure S17 .
Figure S16.PLQY dependence on the concentration of PTMTC R of PTMTC R @PTMTC NR films and PLQY of PTMTC R in CHCl 3 (0.05 mM) as a reference.

Figure S18 .
Figure S18.Pictures of PTMTC R (0.05 mM) solution in CHCl 3 under constant UV irradiation taken every 10 min.

Figure S24 .
Figure S24.EPR spectrum of PTMTC R@NR -Zn MORF single crystals at room temperature.

Figure S25 .
Figure S25.Solid-state CV of PTMTC R@NR -Zn MORF crystals in CH 2 Cl 2 using TBAPF 6 0.1 M as electrolyte and different scan rate.Platinum wire was used as the counter electrode and silver wire as the pseudoreference electrode.Ferrocene was added as internal standard.All potentials are reported versus Ag/AgCl.The inset shows the linear relationship of cathodic peak current vs. the square root of the scan rate.

Figure S28 .
Figure S28.The distributions of KPFM potential measured over the surface of PTMTC NR -Zn MOF (green) and PTMTC R@NR -Zn MORF (red) crystals for different scanning directions.Solid curves show the histogram fitting with Gauss function.

Figure S29 .
Figure S29.Time dependence of the normalized area calculated from the images of a PTMTC R@NR -Zn MORF crystal taken with an optical microscope at different times.The crystal was initially in contact with ethanol (0 s) that in the end completely evaporated (180 s) reducing its area by about 15 %.Scalebars correspond to 0.5 mm.

Figure S30 .
Figure S30.Optical images of PTMTC R@NR -Zn MORF crystal after being exposed to one drop of ethanol at different times.

Figure S31 .
Figure S31.Calculated areas for ethanol-soaked and dried crystals of PTMTC R@NR -Zn MORF after three solvation-evaporation cycles using the same crystal.

Figure S34 .
Figure S34.Time evolution of the emission intensity of PTMTC R@NR -Zn MORF single crystal over irradiation time by 378 nm excitation.