Design of Photothermal Covalent Organic Frameworks by Radical Immobilization
2021; Chinese Chemical Society; Volume: 4; Issue: 8 Linguagem: Inglês
10.31635/ccschem.021.202101198
ISSN2096-5745
AutoresXiaohui Tang, Zhongxin Chen, Qing Xu, Yan Su, Hong Xu, Satoshi Horike, Huanhuan Zhang, Yuan Li, Cheng Gu,
Tópico(s)Perovskite Materials and Applications
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE5 Aug 2022Design of Photothermal Covalent Organic Frameworks by Radical Immobilization Xiaohui Tang†, Zhongxin Chen†, Qing Xu, Yan Su, Hong Xu, Satoshi Horike, Huanhuan Zhang, Yuan Li and Cheng Gu Xiaohui Tang† State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Zhongxin Chen† State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Qing Xu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS), Shanghai 201210 , Yan Su State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Hong Xu Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084 , Satoshi Horike Institute for Integrated Cell-Material Sciences, Institute for Advanced Study, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501 , Huanhuan Zhang State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Yuan Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 and Cheng Gu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 https://doi.org/10.31635/ccschem.021.202101198 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Covalent organic frameworks (COFs) characterized by structural diversity, face-to-face stacking and open channels exhibit unique advantages as photothermal materials but have rarely been applied in solar-driven water evaporation due to complicated framework design, tedious synthesis, and low solar-to-vapor efficiency. Herein, we report a materials design strategy to produce efficient and robust photothermal COF by anchoring nonemissive radicals to the pore surface by a [2+2] cycloaddition–retroelectrocyclization reaction. The radical COF not only possesses the features of COFs such as crystallinity, porosity, and chemical robustness, but also the characteristics of radicals including high spin density and extended absorption to the near-infrared region, which endow the radical COF with outstanding photothermal properties. The radical COF achieves exceptional temperature increment and good photostability upon irradiation with an 808-nm laser, exhibits solar-to-vapor efficiency up to 90.7%, and is adaptable for efficient seawater desalination. Our results provide a new design strategy for the facile production of COF-based photothermal materials. Download figure Download PowerPoint Introduction Covalent organic frameworks (COFs) are an emerging class of crystalline and porous polymers that allow π-conjugated organic building blocks to be covalently assembled into ordered structures.1–3 The most important feature of COFs is their designability, in which the different functional units can be introduced into COFs directly via de novo syntheses or postsynthetic strategies. These COFs have been shown useful in various applications such as gas capture, chemical sensor, ion conduction, electrocatalysis, and photocatalysis.4–19 COFs are promising candidates for photothermal materials since the face-to-face stacking of COF layers largely decreases the radiative decay and increases the nonradiative decay, while diverse organic units and chemical doping methods can be involved in the COF platforms to encode radical, charge-transfer (CT), and ionic species, which possess broad absorption extending to the near-infrared (NIR) region but are emission-prohibited, to the COF skeletons.20–24 However, up to now, the above-mentioned strategies have encountered several critical obstacles involving complicated framework design, tedious synthesis, low crystallinity, and unstable doped states. Developing a rational and simple design strategy to produce crystalline photothermal COFs with high photothermal efficiency and photostability is long awaited, but the current design approaches are inaccessible for this purpose. Here, we demonstrate a postsynthesis design strategy to produce photothermal COFs by a [2+2] cycloaddition–retroelectrocyclization (CA–RE) reaction. To the best of our knowledge, constructing functional COFs based on the CA–RE reaction has not yet been investigated. The targeted COF, termed GT-COF-3, immobilizing quinoidal radicals on the side chains, displays high crystallinity and chemical stability and converts the initial COF, which barely shows any photothermal performance, to outstanding photothermal COFs. Both the COF synthesis and postmodification are readily achieved and adaptable for scale-up production. The cyanovinyl decorative groups that we introduce exhibit unique radical characters in their ground states and endow the resulting COFs with broad absorption spectra from 250 to 1400 nm. We establish an interfacial heating-evaporation system based on the photothermal COFs, which yields solar-energy-to-vapor efficiency of 90.7% and a water evaporation rate of 1.314 kg m−2 h−1 under 1 sun irradiation. Moreover, the photothermal COFs were efficient for seawater desalination. Our results show that complicated skeleton design is not necessary for high-performance photothermal COFs, and the strategy of introducing appropriate radical side chains by easy synthetic approaches is a promising new option for future photothermal COFs. Experimental Methods Materials 1,3,5-Tri(4-aminophenyl)benzene (TPB), 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) were purchased from Jilin Chinese Academy of Sciences-Yanshen Technology Co., Ltd. (Jilin, China). 4-ethynylanisole (98%, TCI, Chinese Agency Company of Energy Chemical, Shanghai, China), tetracyanoethylene (TCNE) (98%, TCI, Chinese Agency Company of Energy Chemical, Shanghai, China), acetic acid (99.5%, TCI, Chinese Agency Company of Energy Chemical, Shanghai, China), o-dichlorobenzene (o-DCB) (99.8%, Aldrich, Chinese Agency Company of Energy Chemical, Shanghai, China), anhydrous tetrahydrofuran (THF) (99.5%, Energy Chemical, Shanghai, China), n-butanol (n-BuOH) [high-performance liquid chromatography (HPLC), Fisher], anhydrous dichloromethane (DCM) (99.5%, Energy Chemical, Shanghai, China), anhydrous alcohol (99.5%, Energy Chemical, Shanghai, China), N,N-dimethylformamide (99.8%, Energy Chemical, Shanghai, China), and deuterated solvents (Energy Chemical, Shanghai, China) for NMR were purchased and used without further purification. CO2 (99.9999%) was purchased from MULAI Company (Guangzhou, China). Synthesis of molecular analog 1 A mixture of TCNE (1.5 mmol, 192.7 mg), 4-ethynylanisole (1 mmol, 132.1 mg), and DCM (6 mL) was degassed and recharged with nitrogen. After being stirred and refluxed for 16 h, the solvent was evaporated in vacuum, and the product was purified by column chromatography (eluent: DCM) to obtain 260.1 mg of a red solid (yield: 99%). 1H NMR (400 MHz, chloroform-d, δ) (ppm): 8.02 (s, 1H), 7.47–7.43 (m, 2H), 7.11–7.07 (m, 2H), 3.92 (s, 3H). 13C NMR (101 MHz, chloroform-d, δ) (ppm): 55.85, 88.99, 98.14, 108.51, 111.24, 111.64, 111.81, 115.76, 122.36, 131.49, 153.39, 161.44, 164.66. Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) (m/z): [M]+ calcd for C15H8N4O, 260.26; found 261.2. Synthesis of TPB-BPTA-COF An o-DCB/n-BuOH (0.5 mL/0.5 mL) mixture of TPB (0.080 mmol, 28.1 mg) and BPTA (0.120 mmol, 29.1 mg) in the presence of an acetic acid catalyst (6 M, 0.1 mL) in a Pyrex tube (10 mL) was degassed via three freeze–pump–thaw cycles. The tube was flame-sealed and heated at 120 °C for 3 days. The precipitate was collected via centrifugation, washed six times with THF and then subjected to Soxhlet extraction with THF for 1 day to remove trapped guest molecules. The powder was collected and dried at 120 °C under vacuum overnight to produce TPB-BPTA-COF in an isolated yield of 77%. Synthesis of GT-COF-3 TPB-BPTA-COF (100 mg) and TCNE (100 mg) were placed in a 20-mL Schlenk tube, with spatial separation maintained to prevent the two from directly contacting each other (e.g., by holding TCNE in a smaller tube sealed at one end). The Schlenk tube was degassed and then was placed in an oven preheated to 140 °C for 24 h. The solid was washed six times with DMF and THF and then subjected to Soxhlet extraction with THF for 1 day to remove trapped guest molecules. The powder was collected and dried at 120 °C under vacuum overnight to produce GT-COF-3 in an isolated yield of 86%. Large-scale synthesis of GT-COF-3 TPB-BPTA-COF (800 mg) and TCNE (800 mg) were placed in a 50-mL Schlenk tube, with spatial separation maintained to prevent the two from directly contacting each other (e.g., by holding TCNE in a smaller tube sealed at one end). The Schlenk tube was degassed and then placed in an oven preheated to 140 °C for 24 h. The solid was washed six times with DMF and THF, and was then subjected to Soxhlet extraction with THF as solvent for 1 day to remove trapped guest molecules. The powder was collected and dried at 120 °C under vacuum overnight to produce GT-COF-3 in an isolated yield of 80%. Methods 1H and 13C NMR spectra were recorded on a Bruker AVANCE HD III 500M NMR spectrometer (Bruker Inc., Berlin, Germany). Fourier transform Infrared (FT-IR) spectra were recorded on an IFS 66V/S Fourier transform infrared spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA). Solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance spectra (13C CPMAS NMR) were recorded on a JEOL JNM-ECA600 MHz, 3.2 mm rotor, magic angle spinning (MAS) of 20 kHz, recycle delay of 1 s (JEOL Ltd., Japan). UV–vis spectra were recorded on a Shimadzu UV-3600 spectrometer (Shimadzu Ltd., Japan). Elemental analysis was performed on an Elementar Vario EL elemental analyser (Elementar Analysensysteme GmbH, Germany). Thermogravimetric (TG) measurements were performed on a Rigaku Thermo plus EVO2 under N2, by heating to 800 °C at a rate of 5 °C min−1 (Rigaku Corporation, Japan). Field-emission scanning electron microscopy (FE-SEM) was performed on a Hitachi Regulus 8100 operating at an accelerating voltage of 5.0 kV (Hitachi Inc., Japan). High-resolution transmission electron microscopy (HR-TEM) images were obtained on a TEM JEOL 2100F with an acceleration voltage of 300 kV (JEOL Ltd., Japan). Electron spin resonance (ESR) spectra were measured on Bruker ELEXSYS-II E500 CW-ESR spectrometer (Bruker Inc., Berlin, Germany). Photothermal measurements were performed by using an 808-nm Fiber Coupled Laser (Model: MW-GX-808, Changchun Laser Optoelectronics Technology Co., Ltd., Changchun, China), and the temperature response of the samples was measured with an IR thermal camera (FLIR E4, FLIR Systems, Inc., United States). PXRD data were recorded on a Rigaku model RINT Ultima III diffractometer by depositing powder on glass substrate, from 2θ = 2° to 45° with 0.02° increment (Rigaku Corporation, Japan). Thermal conductivities were measured by Quantun Design PPMS-9 (Quantum Design, Inc., United States). XPS measurements were performed with Thermo Scientific ESCALAB XI+ (Thermo Fisher Scientific Inc., Waltham, MA). Power density of sun irradiation were controlled by using Neutral Density filter of Giai photonics Co., Ltd. (Shenzhen, China). Water contact angle were measured with betops DSA-X (betops scientific Co., Ltd., Guangzhou, China). Structural modelling The crystalline structures of COFs were determined using density-functional tight-binding (DFTB).25 The calculations were carried out with the DFTB+ program package version 17.1.26 DFTB25 is an approximate density functional theory (DFT) method based on the tight-binding approach and utilizes an optimized minimal linear combination of atomic orbitals (LCAO) Slater-type all-valence basis set in combination with a two-center approximation for Hamiltonian matrix elements. The Coulombic interaction between partial atomic charges was determined using the self-consistent charge (SCC) formalism. The lattice dimensions were optimized simultaneously with the geometry. Standard DFTB parameters for X–Y element pair (X, Y = C, H, O, and N) interactions were employed from the 3ob set.27–30 Accelrys Materials Studio 7.0 (Accelrys Software Inc., United States) was employed to build inputting files for DFTB simulation and visualize simulating results. All simulation works were performed using the computing resources at the National Supercomputing Center in Shenzhen. The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels were calculated by using DFT calculation, which was carried out using the Gaussian 09 (version D.01) package on a PowerLeader cluster. The ground-state geometry was fully optimized using DFT with a B3LYP hybrid functional at the basis set level of 6–31G(d, p). Gas sorption measurements N2 and CO2 sorption measurements were performed on BELSORP-max (Bel Japan, Inc., Japan) automated volumetric sorption analysers. The desired temperature (77 and 195 K) was controlled by liquid N2 and a mixture of dry ice and isopropanol, respectively. Magnetic measurements Magnetic measurements were carried out on a Quantum Design MPMS 3 SQUID Magnetometer (Quantum Design, Inc., United States). Powder samples with a weight of 20 mg were sealed in a plastic capsule. The magnetic susceptibility was measured in the temperature range of 2–300 K with an applied field of 1000 Oe. Then the data were corrected for the diamagnetic contribution from sample holders and sample themselves using tabulated constants. The high magnetic field at of M(H) at 2 K curve was fitted using the following equation: M = F + A X where M is the magnetic moment per gram, F is the paramagnetic component (emu g−1), A is the diamagnetic background, and X is the applied magnetic field. The diamagnetic background was estimated based on the slope of M(H) at high magnetic field (H > 2 T). The obtained diamagnetic susceptibility M/H for GT-COF-3 was −6.53 × 10−7 emu Oe−1 g−1, which was a reasonable value for typical diamagnetism. Fabrication of photothermal devices 10–30 mg of samples were ground carefully and then ultrasonically dispersed in 10 mL of THF. The COF-loaded polyurethane (PU) foams were obtained by dropping the resulting suspensions onto circular and porous PU foams with diameters of 2 cm and drying under 40 °C under vacuum. Solar steam generation experiments The COF-loaded PU foams were put on a quartz beaker filled with water. The sunlight, generated by a solar simulator with an optical filter for the standard AM 1.5 G spectrum (CEL-S500/350/150), irradiated the sample under specific optical concentrations. The weight loss of water was measured by an electronic mass balance and the temperature over the process was recorded by an IR thermal camera. Calculation of the efficiency for solar to vapor generation The efficiency η of solar energy in photothermal-assisted water evaporation was calculated with the following formula31,32: η = m h LV / C opt P 0 where m refers to the mass flux (evaporation rate) of water, hLV refers to the total liquid-vapor phase-change enthalpy [i.e., the sensible heat and the enthalpy of vaporization (hLV = Q + Δℎvap)], Q is the energy provided to heat the system from the initial temperature T0 to a final temperature T; Δℎvap is the latent heat of vaporization of water; P0 is the nominal solar irradiation value of 1 kW m−2; and Copt represents the optical concentration. The schematic for the vaporization enthalpy of the vapor was as follows: Q = C liquid × ( T − T 0 ) Δ h vap = Q 1 + Δ h 100 + Q 2 Q 1 = C liquid × ( 100 − T ) Q 2 = C vapor × ( T − 100 ) In this work, Cliquid, the specific heat capacity of liquid water is a constant of 4.18 J g−1 °C−1. Cvapor, the specific heat capacity of water vaper is a constant of 1.865 J g−1 °C−1. Δℎ100 is the latent heat of vaporization of water at 100 °C, taken to be 2260 kJ kg−1. For example, the surface temperature of GT-COF-3 loaded PU (20 mg) was 43.6 °C during the evaporation process, therefore T is 43.6 °C. According to the above formulas, Q = C liquid × ( T − T 0 ) = 4.18 × ( 43.6 − 20.6 ) = 96.14 kJ kg − 1 Δ h vap = Q 1 + Δ h 100 + Q 2 = 4.18 × ( 100 − 43.6 ) + 2260 + 1.865 × ( 43.6 − 100 ) = 2390.566 kJ kg − 1 Δ h LV = Q + Δ h vap = 96.14 + 2390.566 = 2486.706 kJ kg − 1 m = 1.314 kg m − 2 h − 1 P 0 = 1 kW m − 2 C opt = 1 As a result, evaporation efficiency η = mhLV/CoptP0 = 90.7% when the latent heat of water vaporization at 43.6 °C (2390.566 kJ kg−1) is used in calculation. By the way, in this system, the solar evaporation was applied at temperatures above the environmental temperature, thus it was unnecessary to deduct the dark evaporation (<0.001 kg m−2 h−1). Desalination of seawater The real seawater sample (from the South Sea, China) was used for desalination by our system. The concentrations of all the five primary ions (Na+, Mg2+, Ca2+, K+, and Cl−) originally present in the sea water were examined by inductively coupled plasma optical emission spectroscopy (ICP-OES; Agilent 5110) before and after desalination. Then, the impact of ion concentrations on the efficiency, water evaporation rate, and photostability were evaluated. Long-term durability We carried out the long-term stability tests to check the durability in water evaporation. The cycle performance of COF-loaded PU evaporation system under fixed optical illumination (1 sun) was investigated under air for a continuous 2 h as one cycle (total 12 cycles). The mass change was recorded every 5 min. During the water evaporation process, the temperature of the surface of COF-loaded PU foam was monitored by an IR camera to study the photostability of the COF-loaded PU foam. Results and Discussion We chose an established imine-linked COF, TPB-BPTA-COF, as a platform to anchor the radical side chains because of its superior crystallinity, mesopores with high Brunauer–Emmett–Teller (BET) surface area, chemical robustness, and facile synthesis.33 We employed a classic type of radical generator, para-quinodimethanes, which has a typical closed-shell quinoidal and an open-shell radical resonance structure. The cyanovinyl decorated COF, termed GT-COF-3, was synthesized via a [2+2] CA–RE reaction of TCNE with the ethynyl groups suspended on the TPB-BPTA-COF (Figures 1a–1c and Supporting Information Figure S1).34 GT-COF-3 was thus transformed to radical COF by the structural resonance of carbonitrile-equipped buta-1,3-dienes and their radicals. Notably, the synthesis of TPB-BPTA-COF and GT-COF-3 was high-throughput with good yields: more than 1 g of COFs could be obtained from one batch of synthesis, compatible with large-scale production. We also synthesized a cyanovinyl quinoidal molecular analog 1 via a similar CA–RE reaction to investigate the radical property in such kind of quinoidal molecules ( Supporting Information Figures S1–S4). The structure of GT-COF-3 was unambiguously characterized by various analytical methods ( Supporting Information Figures S5–S10). GT-COF-3 showed an absence of ethynyl stretching vibration peak at 2119 cm−1 and an occurrence of cyano stretching vibration peak at 2212 cm−1 in the IR spectrum ( Supporting Information Figure S5 and Table S1). Solid-state 13C NMR of GT-COF-3 revealed the presence of a peak belonging to the cyano group at 116.2 ppm together with the disappearance of the peak ascribed to the ethynyl group at 69.9 and 76.6 ppm ( Supporting Information Figure S6 and Table S2). Elemental analysis performed on the guest-free GT-COF-3 was in good agreement with the expected formula ( Supporting Information Table S3). XPS confirmed that the nitrogen atom ratio of cyano to imine groups was 1:3.7 ( Supporting Information Figure S7 and Table S4), which is close to the theoretical one (1:4) and thus demonstrating the high yield of the CA–RE reaction and successful incorporation of six cyanovinyl groups within one pore. Figure 1 | Synthesis and structures of the radical COF. (a) The mechanism of CA–RE reaction between TCNE and ethynyl groups. (b) Synthesis of TPB-BPTA-COF and GT-COF-3 through condensation reaction and [2+2] CA–RE reaction, respectively. Inset: The structure of the edge unit of GT-COF-3 and its resonance structure. (c) Graphic view of TPB-BPTA-COF and GT-COF-3 (red, O; blue, N; grey, C; white, H). Download figure Download PowerPoint The crystallinity of the COFs was characterized by PXRD. TPB-BPTA-COF displayed the most intensive peak at 2.72° together with several relatively weak peaks at 4.75°, 5.51°, 7.32°, 9.61°, and 25.14°; and these peaks were assigned to the (100), (110), (200), (210), (220), and (001) reflection planes, respectively (Figure 2a). On the other hand, GT-COF-3 showed decreased crystallinity; the peaks located at 2.77°, 5.05°, 5.70°, 7.52°, 9.92°, and 25.24° were attributed to the reflection facets of (100), (110), (200), (210), (220), and (001) (Figure 2a). We constructed eclipsed AA (AA = completely eclipsed stacking) stacking models for the two COFs by using their optimized monolayer structures (Figures 2b and 2c). The two COFs adopted a space group of P6 with α = β = 90°, and γ = 120°; the lengths of unit cells were a = b = 37.5842 Å, c = 3.7406 Å for TPB-BPTA-COF, and a = b = 37.5186 Å, c = 3.8329 Å for GT-COF-3, respectively ( Supporting Information Tables S5 and S6). Notably, the PXRD results combined with the simulation modeling clearly revealed that the crystal lattice structures of GT-COF-3 remained unchanged in the postsynthetic process, whereas the crystallinity decrement in GT-COF-3 was probably because of the alignment disorder and local motion of the carbonitrile group in the channels. Figure 2 | Crystallinity, porosity stability, and photophysics of the radical COF. (a) PXRD patterns of TPB-BPTA-COF (experimental results in purple curve, simulated AA stacking mode in black curve) and GT-COF-3 (experimental results in orange curve, and simulated AA stacking mode in grey curve). (b) Top view of the unit cell and side view of layers of TPB-BPTA-COF (O, red; N, blue; C, grey; H, white). (c) Top view of the unit cell and side view of layers of GT-COF-3 (O, red; N, blue; C, grey; H, white). (d) CO2-sorption isotherm profiles of TPB-BPTA-COF (purple) and GT-COF-3 (orange). Filled and open circles represent adsorption and desorption. (e) UV–vis–NIR absorption spectra of TPB-BPTA-COF (purple) and GT-COF-3 (orange). Inset: The photos of TPB-BPTA-COF and GT-COF-3. Download figure Download PowerPoint Because of the highly polar pore surface, GT-COF-3 did not adsorb nonpolar N2 ( Supporting Information Figure S11). CO2-sorption isotherms of TPB-BPTA-COF measured at 195 K exhibited a type IV sorption profile with rapid uptake at a low pressure of P/P0 < 0.45 followed by a sharp step at 0.45 < P/P0 < 0.55, a characteristic of mesoporous materials (Figure 2d). In contrast, GT-COF-3 displayed a type I isotherm with rapid uptake at a low pressure of P/P0 < 0.1 and slight increment at a high pressure of P/P0 > 0.1, demonstrating that postsynthesis has transferred GT-COF-3 to a microporous material (Figure 2d and Supporting Information Figure S12). GT-COF-3 showed obviously decreased BET surface area of 144 m2 g−1 compared to that of TPB-BPTA-COF (645 m2 g−1), indicative of the occupation of channels by the decorated side chains. To investigate the stability of the radical COFs, we dispersed them for 1 week in water (25 and 50 °C). GT-COF-3 exhibited almost no weight loss (<10 wt %) in 1-week dispersion in water ( Supporting Information Figure S13a). The unchanged peak positions in PXRD after being soaked in water for one week (25 and 50 °C) confirmed that GT-COF-3 still retained its intact crystalline structure ( Supporting Information Figure S13b). The BET surface areas were 239 and 145 m2 g−1 for GT-COF-3 treated for 1 week in water at 25 and 50 °C, respectively ( Supporting Information Figure S13c); these values are very close to that of the pristine COFs. All these results demonstrated that the radical COF possessed outstanding water stability, a prerequisite for water-related photothermal applications, such as solar-driven water evaporation. TPB-BPTA-COF showed a weak absorption from 250 to 950 nm with a peak located at 406 nm, as revealed by UV–vis–NIR diffused reflectance spectroscopy (Figure 2e). Thus, TPB-BPTA-COF was brownish yellow in color. In sharp contrast, GT-COF-3 exhibited substantially enhanced absorbance and extended absorption spectra ranging from 250 to 1400 nm (Figure 2e). Therefore, GT-COF-3 was pure black. Through the Kubelka–Munk equation, the optical band gaps of TPB-BPTA-COF and GT-COF-3 were estimated to be 1.66 and 1.18 eV, respectively ( Supporting Information Figure S14). DFT calculations of the COF fragments revealed a HOMO of −5.45 eV and a LUMO of −1.60 eV for TPB-BPTA-COF, corresponding to a bandgap of 3.85 eV ( Supporting Information Figure S15). By contrast, GT-COF-3 exhibited simultaneously decreased HOMO of −5.86 eV and LUMO of −4.65 eV, corresponding to a substantially narrowed bandgap of 1.21 eV. The broadened and enhanced adsorption originated from the radical characteristic of decorating groups, CT interactions between the framework and the decorating groups, and excitonic coupling. The low band gap of GT-COF-3 provided more opportunity to generate radicals and thus strengthened light harvesting of radical COF.35 To investigate the feature of radical species in the COFs, we measured the COF samples in sealed quartz tubes with ESR spectroscopy. TPB-BPTA-COF showed a very weak ESR signal with g-factor = 2.0047, probably originated from unreacted terminal amino or aldehyde groups at the grain boundary (Figure 3a). By contrast, GT-COF-3 exhibited an obvious ESR signal appearing at g-factor = 2.0031, which indicated that the carbon radical generated also possesses a spin degree of freedom (Figure 3a).36 As a control, the molecular analog 1 showed an ESR intensity equal to 6‰ that of GT-COF-3 ( Supporting Information Figure S16), which confirmed that the observed spin functions in GT-COF-3 originated from their extended crystalline structure.20 Temperature-dependent ESR measurements from 285 to 110 K showed that the peak-to-peak heights decreased in GT-COF-3 as the increment of temperatures (Figures 3b and Supporting Information Figure S17), indicative of reduced spin parallelism, a feature of localized radicals with weak spin-spin interactions.37 The temperature dependence of the spin susceptibility (χspin) determined by integrating the ESR signal intensity showed that the spin susceptibility gradually increased as the decrement of temperature ( Supporting Information Figure S18). Temperature-dependent ESR linewidth (ΔHpp) showed constant profiles for GT-COF-3 above 120 K ( Supporting Information Figure S19). Since the temperature-dependent ESR linewidth is dominated by the spin–spin exchange interaction through space between spins in ne
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