Quinoid-Thiophene-Based Covalent Organic Polymers for High Iodine Uptake: When Rational Chemical Design Counterbalances the Low Surface Area and Pore Volume

A novel 2D covalent organic polymer (COP), based on conjugated quinoid-oligothiophene (QOT) and tris(aminophenyl) benzene (TAPB) moieties, is designed and synthesized (TAPB-QOT COP). Some DFT calculations are made to clarify the equilibrium between different QOT isomers and how they could affect the COP formation. Once synthetized, the polymer has been thoroughly characterized by spectroscopic (i.e., Raman, UV–vis), SSNMR and surface (e.g., SEM, BET) techniques, showing a modest surface area (113 m2 g–1) and micropore volume (0.014 cm3 g–1 with an averaged pore size of 5.6–8 Å). Notwithstanding this, TAPB-QOT COP shows a remarkably high iodine (I2) uptake capacity (464 %wt) comparable to or even higher than state-of-the-art porous organic polymers (POPs). These auspicious values are due to the thoughtful design of the polymer with embedded sulfur sites and a conjugated scaffold with the ability to counterbalance the relatively low pore volumes. Indeed, both morphological and Raman data, supported by computational analyses, prove the very high affinity between the S atom in our COP and the I2. As a result, TAPB-QOT COP shows the highest volumetric I2 uptake (i.e., the amount of I2 uptaken per volume unit) up to 331 g cm–3 coupled with a remarkably high reversibility (>80% after five cycles).


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The A Micromeritics ASAP 2020 volumetric apparatus was used to measure both N 2 and CO 2 adsorption isotherms at 77 K and 273 K, respectively from vacuum up to 1100 mbar. Prior to the measurement, powders were degassed overnight at 373 K reaching a residual pressure of 10 -4 mbar.
Measurements with CO 2 at 273 K were collected employing a home-made patented apparatus Before measuring the samples have been sputtered with 20 nm of gold.
Thermogravimetric analysis (TGA) data was recorded with a TA instruments Q600 thermobalance in dry N 2 flow (100 mL/min) with a ramp of 10 K/min from 300 to 1073 K.
Raman spectra have been collected on an inVia Raman Microscope and adopting a 514 nm Ar + exciting LASER line. LASER light/backscattered light have been focused/collected on/from samples through an Olympus 20x ULWD objective (NA = 0.40). In the case of solid powders, 0.5% of the total LASER power has been admitted reaching the samples. For solutions (measured in a Helma QS cuvette and subjected to a magnetic stirring), 10% of the total laser power has been adopted. In any case, stability of samples under the LASER light have been carefully investigated. Backscattered light S-34 (after Rayleigh light removal through an edge filter) have been analysed by an 1800 l/mm grating and collected through a CCD Peltier cooled detector. For powders, each presented spectrum is resulting from the average of spectra (30 x 20" acquisitions) collected on three different points. For solutions, the presented spectra results from the average of three spectra recorded consecutively (20 x 20" acquisitions).
The iodine uptake/release kinetic was investigated as follows: TAPB-QOT polymer (10 mg) was placed into a small open vial and the whole system was weighed; then, this small vial was placed into a larger one where excess iodine solid (500 mg) were added, and the whole system was closed (Photo S2). Then, the system was heated in an oven at 348 K to promote the I 2 sublimation. After a fixed amount of time, it was removed, cooled to room temperature, and the small vial was weighed, comparing its weight before and after iodine adsorption. The following equation calculated the I 2loading weight of samples: α = (m 2 -m 1 )/m 1 *100, being α the iodine uptake, m 1 the mass of polymer before adsorption of Iodine and m 2 = the mass of polymer after adsorption of Iodine). To confirm the reproducibility of the approach, adsorption studies were repeated 5 times (for five nominally identical aliquots) in the same experimental conditions.
Prior to using the TAPB-QOT polymer in recycling studies, iodine was released by immersing the material (5 mg) in EtOH (3 ml) and stirring vigorously on a plate at room temperature. This process continued until the solvent was transparent (240 minutes were enough to assure maximum I 2 release).

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Then, the polymer was re-activated under vacuum oven at 393 K to remove solvent entrapped within the pores. This process was repeated 5 times. Iodine adsorption was also performed in a hexane solution (i.e. liquid phase): 5 mg of the polymer was added to an iodine solution of hexane (20 mg/2 ml). Uptake kinetics were monitored by UV-Vis absorption and for each measurement the adsorption capacity was determined according to the Beer-Lambert Law equation.

Computational methods
All density functional calculations presented here were performed with the Gaussian 09 program. 2 The ground-state geometries were fully optimized using a DFT level without any symmetry constrains using B3LYP hybrid exchange-correlation functional 3 and 6-311++G(d,p) basis set for QOT and TAPB-QOT systems and 6-31G basis set for COF structural unit. All found stationary points were identified as minima (no normal vibrations with imaginary frequency were detected). The graphical representations of the molecular structures were made using ChemCraft software (Version 1.6). 4 The binding energy was calculated as follows: (1) Where E unit is the total energy of the studies isomer, E In is the total energy of iodine form, E unit+In is the total energy of the corresponding iodine absorbing isomer system. At the beginning of modeling, S-36 the geometry of the isomer unit+iodine form was optimized by using B3LYP/6-311++G(d,p) and LanL2DZ for the iodine atom level of theory. Further, the one-shot calculation was done with the dispersion corrected density functional theory (D3BJ) correction and obtained the total energy used for binding energy calculation. 5 All total energy in equation 1 is used with D3BJ correction.

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To a 50 mL two-neck flask, anhydrous hexane (25 mL), N,N,N',N'-tetramethylethylenediamine (TMEDA) (1.55 g, 2.0 mL, 13.3 mmol), thiophene (0.45 g, 0.43 mL, 5.4 mmol), n-butyllithium (8 ml, 12.8 mmol) were added dropwise to the stirred solution at room temperature. The reaction mixture was refluxed for 1 h and then cooled to −40 °C. Then, a solution of phenyl thienyl ketone 1 (2.44 g, 13 mmol) in anhydrous diethyl ether (25 ml) was added dropwise to the reaction mixture. The mixture was warmed up to room temperature and stirred overnight. The reaction was then quenched with 1 M NH 4 Cl aq (20 mL) and the organic phase was extracted with chloroform and ethyl acetate. Finally, the combined organic phases were dried over Na 2 SO 4 , filtered , and the solvent was evaporated under vacuum to afford thiophene-2,5-diylbis(phenyl(thiophen-2-yl)methanol) (2), which was used for the next step without further purification. The obtained crude alcohol product (2) was dissolved in toluene and a solution of Na 2 S 2 O 4 (13.8 g, 79 mmol) and 57% HI (13.5 mL) in distilled water (50 mL) was added. The two-phase system was vigorously stirred at room temperature for 24 h. An orangecoloured reaction mixture was neutralized with NaHCO 3 and extracted several times with diethyl ether. The combined organic phases were dried over Na 2 SO 4 and filtered. After evaporation of the solvent, the product was purified through flash column chromatography on silica column (DCM/PE