A Three-Dimensional Silicon-Diacetylene Porous Organic Radical Polymer

Abstract

Open AccessCCS ChemistryCOMMUNICATIONS6 Dec 2022A Three-Dimensional Silicon-Diacetylene Porous Organic Radical Polymer Chengcheng Dong, Juan Chu, Linzhu Cao, Fengchao Cui, Shuang Liang, Xintong Zhang, Xin Tao, Heng-guo Wang and Guangshan Zhu Chengcheng Dong Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Juan Chu Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Linzhu Cao Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Fengchao Cui Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Shuang Liang Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, School of Physics, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Xintong Zhang Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, School of Physics, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Xin Tao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author , Heng-guo Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author and Guangshan Zhu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education, School of Chemistry, Northeast Normal University, Changchun 130024, Jilin Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202351 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Radical-containing porous organic polymers (POPs) have drawn great interest in various applications. However, the synthesis of radical POPs remains challenging due to the unstable nature of organic radicals. Here, a persistent and stable three-dimensional silicon-diacetylene porous organic radical polymer was synthesized via a classic Eglinton homocoupling reaction of tetraethynylsilane. The presence of carbon radicals in this material was confirmed by electron paramagnetic resonance, and its paramagnetic behavior was analyzed by a superconducting quantum interference device. This unique material has a low-lying lowest unoccupied molecular orbital (LUMO) energy level (−5.47 eV) and a small energy gap (ca. 1.46 eV), which shows long-term cycling stability and excellent rate capability as an anode material for lithium-ion batteries, demonstrating potential application in energy fields. Download figure Download PowerPoint Introduction Porous organic radical polymers (PORPs) are a new class of porous materials, which introduce open-shell organic radical species into porous organic network structures. PORPs are of great interest in synthesis and various practical applications due to their unique properties in magnetic, electronic, optoelectronic, and biologic fields.1–8 However, targeted synthesis of persistent and stable PORPs with high surface areas, good thermal and chemical stability, and tunable functionalization is particularly challenging. A variety of protocols have been employed to achieve this goal, including the control of steric,9–11 electronic,12 and aggregated states13 to avoid covalent bonding of the organic radicals. So far, very few PORPs have been successfully synthesized.14–22 In 2015, Jiang’s group14 immobilized nitroxide radicals on the pore walls of a conventional imine-linked covalent organic framework (COF), which gave rise to radical-containing COFs for outstanding energy storage. Then, they found that oxidation of the pyrene unit of an sp2 carbon-conjugated COF, by iodine vapor, resulted in a pyrene radical cation-containing COF.15 Fang’s group16 has decorated 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical on the channel walls of three-dimensional (3D) COFs via a bottom-up approach, which resulted in the formation of 3D radical COFs. Another desirable strategy is using persistent and stable organic radicals as building blocks to construct PORPs, such as polychlorotriphenylmethyl-covalent organic radical framework (PTM-CORF),17 polychlorotriphenylmethyl radical-linked covalent triazine framework (PTMR-CTF),18 and phenanthroline-based cationic radical porous hybrid polymer (Phenc·+-PHP-2Br).19 The unique π-conjugated PORPs,20–22 were synthesized through a strategy to trap the spins in the organic network structure, which avoided the delocalizing and recombining processes. So far, the majority of PORPs reported are two-dimensional layered materials. We endeavor to design 3D PORPs because of their rigid framework, high surface area, and good physical and chemical stability. The above properties could potentially favor the stabilization of organic radical species in these materials, leading to the development of new PORPs with unique functionalities.23–26 From a structural point of view, sp3 hybridized C, Si, and Ge atoms, along with conjugated diacetylene derivatives, have been selected to serve as nodes and linear linkers, respectively, to construct new 3D porous organic polymers (POPs) by covalent bonds.27–29 The resulting acetylene-rich porous materials serve as promising electrode materials in lithium-ion batteries (LIBs).30–35 However, to the best of our knowledge, the radical characteristics of these materials have not yet been uncovered. In contrast, conjugated diacetylene-derived small-molecule compounds were reported to exhibit diradical resonance structure in the solid state.36,37 Based on these reports, we predict that a new 3D PORP could be constructed if the conjugated diacetylene units were introduced into 3D organic network structures. This turned out to be the case. Herein, we report the facile synthesis of a 3D Si-diacetylene PORP in the powder state via Eglinton38,39 homocoupling reaction of tetraethynylsilane. This unique material has a low-energy lowest unoccupied molecular orbital (LUMO), a strong extended absorption band up to 2000 nm, a small energy gap, and ferromagnetism at low temperature. The obtained 3D PORP exhibits excellent rate capability and long cycling stability as an anode material in LIBs. Results and Discussion We have initially designed three sp3 hybridized silicon-based POPs with similar structures, in which diacetylene ( DA), biphenyl ( BP), and phenylene diacetylene ( PDA) serve as linear linkers (Scheme 1). The 3D silicon-diacetylene POP Si-DA is synthesized via catalytic Eglinton homocoupling reaction of tetraethynylsilane. The analogous polymer Si-PDA is synthesized via Sonogashira cross-coupling reaction40 of tetraethynylsilane and 1,4-diidobenzene (Scheme 1). Polymer Si-BP (reported as porous aromatic framework [ PAF-3]25 or porous polymer network [ PPN-4]26) is prepared from a Yamamoto-type Ullman homocoupling reaction of tetrakis(bromophenyl)silane according to the procedures described in the literature. The full synthesis and characterization details for Si-DA, Si-PDA, and Si-BP are depicted in Supporting Information Schemes S1–S3 and Figures S1–S20. Scheme 1 | Synthesis of porous organic polymers Si-DA, Si-PDA, and Si-BP. Download figure Download PowerPoint The powder X-ray diffraction analysis showed that Si-DA is amorphous in nature (see Supporting Information Figure S5). According to Fourier transform infrared (FT-IR) analysis of Si-DA (Figure 1a), the stretching vibration of terminal C(alkyne)–H bond of tetraethynylsilane at 3284 cm−1 disappeared, indicating the complete coupling reaction of terminal alkyne carbons. The weak and broad stretching vibration peak of the C≡C bond was located around 2150 cm−1. The X-ray photoelectron spectroscopy (XPS) analysis of Si-DA (Figure 1b) shows a C 1s peak at 284.82 eV with a shoulder peak towards higher energies, which was deconvoluted into three peaks at 284.66, 285.39, and 288.19 eV. The peaks observed at 284.66 and 285.39 eV were assigned to C (sp2) and C (sp), respectively. The peak at 288.19 eV could be ascribed to C–O and C=O probably because the surface of the samples absorbed oxygen, which would subsequently react with exposed, uncoupled terminal acetylenic bonds.27–29,41,42 In addition, the signal of copper catalyst was not observed in the XPS analysis (see Supporting Information Figure S9). In the solid-state 13C NMR spectrum of Si-DA, a broad peak with low intensity was observed at around 126 ppm, which we assigned to C (sp2) due to the paramagnetic effect of the radical material and the low symmetry of the amorphous framework (see Supporting Information Figure S10). In the solid-state 29Si NMR spectra of Si-DA, a peak at −111 ppm was observed, which is similar with that of the monomer tetraethynylsilane (see Supporting Information Figure S11). This indicates that the chemical environment surrounding the silicon atoms was retained after the C–C coupling reaction. Thermogravimetric analysis plots revealed that Si-DA maintained its structural integrity under air atmosphere until 180 °C (see Supporting Information Figure S12). To investigate the pore-size distribution in Si-DA, N2-sorption isotherm measurements were performed at 77 K (Figure 1c). The isotherms are typical type II curves with a Brunauer–Emmett–Teller surface area of 779 m2 g−1, showing a microporous nature. The pore sizes calculated from the corresponding N2 isotherms using density functional theory (DFT) were mainly 1.8 and 4.2 nm. The morphology of Si-DA obtained from scanning electron microscopy images are irregular spherical particles (see Supporting Information Figure S16). The element distribution of carbon and silicon in Si-DA was clearly seen through transmission electron microscopy elemental mapping images (see Supporting Information Figure S17). The ultraviolet–visible-diffuse reflectance (UV–vis-DR) spectrum of Si-DA exhibits a stronger and more extended absorption ranging from 200 to 2000 nm (Figure 1d). The optical band gap of Si-DA was evaluated to be 1.30 eV as can be seen in Figure 1d.43 Figure 1 | (a) FT-IR spectra of tetraethynylsilane and Si-DA. (b) C 1s XPS spectra of Si-DA. (c) N2-adsorption isotherms and the pore-size distribution of Si-DA. (d) UV–vis-DR absorption spectrum of Si-DA. Download figure Download PowerPoint Si-DA was also characterized by electron paramagnetic resonance (EPR) in sealed quartz tubes under ambient conditions. The EPR spectrum of Si-DA powder shows a strong signal at g-factor ≈ 2.003,12,13,22,44 indicating the existence of carbon-centered radical species (Figure 2a). In contrast, analogous POP Si-PDA shows a very weak EPR signal, while Si-BP is EPR silent under similar measurement conditions. To further confirm the presence of the unpaired electrons in Si-DA, magnetization measurements were performed using superconducting quantum interference device (SQUID) magnetometry. The temperature-dependent magnetization is shown in Figure 2b. The magnetic susceptibility (χm) is significantly enhanced with decreasing temperature, which demonstrates a Curie-like paramagnetic character where the spin orientations become ordered at low temperature.8,20 From 300 to 50 K, the spins are paramagnetic and randomly oriented in Si-DA. This is a typical characteristic of paramagnetic materials. The measured magnetization (M) versus applied field (H) curves for Si-DA powder sample at 2 K illustrate the nonlinear M–H plots (Figure 2c), which became linear at 300 K (see Supporting Information Figure S20). These results reveal an enhanced magnetism at low temperature, which further confirms the radical feature of Si-DA. Furthermore, the magnetization at 2 K showed a substantial increase and saturation behavior at high magnetic fields, which also denotes a ferromagnetic phase transition at very low temperature.20,21,45,46 Figure 2 | (a) Solid-state EPR spectra of Si-DA, Si-PDA, and Si-BP at room temperature. (b) Temperature-dependent spin susceptibility (χm) of Si-DA determined by the SQUID. (c) M–H profiles of Si-DA at 2 K. (All magnetic data are processed by background correction. M, magnetization; H, applied field.) (d) Calculated spin density of the representative fragment of Si-DA. (e) The optimized geometries of nonradical (left, singlet) and diradical (right, triplet) structures with the corresponding bond length (Å). The isosurface of the spin density was drawn with an isovalue of 0.01 electrons/Å3. The α-spin and β-spin electrons are shown with the green and blue isosurfaces, respectively. Carbon, silicon, and hydrogen atoms are light gray, cyan, and white, respectively. Download figure Download PowerPoint To further understand the radical nature of Si-DA, DFT was used to calculate spin density and electronic structure. As reported by Ogawa et al.,36,37 the conjugated diacetylene units could exhibit open-shell diradical resonance structures (the triplet state) in the solid state. Here, we investigated the spin density distribution and geometry of the open-shell resonance structure with DFT calculations. We found for the open-shell resonance structure that the spin density is mainly distributed on C atoms neighboring Si atoms on the polymer skeleton (Figure 2d). In the optimized open-shell resonance structure, the original C≡C bond length changed from 1.216 to 1.287 Å, while the original C–C bond length is shortened from 1.358 to 1.286 Å, both indicating double bond characteristics (Figure 2e). The calculated energy levels of the singly occupied molecular orbital (−6.93 eV) and the LUMO (−5.47 eV) deliver a low energy gap (Egap) of 1.46 eV (see Supporting Information Figure S21). The unique features of the as-obtained 3D PORP Si-DA, including persistent radicals, permanent porosity, robust framework, and low energy gap, makes it a potential candidate for electrochemical energy storage.47,48 Consequently, we explored the potential application of Si-DA as the anode material for LIBs and demonstrated the important role of persistent radicals for Si-DA in comparison with nonradical POPs Si-BP and Si-PDA (see Supporting Information Figures S21–S26). The electrochemical property of Si-DA as the anode material for LIBs was first studied using cyclic voltammetry (CV) at a scan rate of 0.1 mV s−1 (Figure 3a). In the first cathodic sweep, peaks at 0.7, 0.9, and near 0 V, indicated a sequential lithiation process.49,50 The well-defined peak at 0.7 V is associated with the decomposition of the electrolyte and the formation of a solid electrolyte interface (SEI) layer.51–53 In the first anodic sweep, two dominant peaks at 0.15 and 0.98 V can correspond to the delithiation process.54 In the subsequent sweeps, as the formation of the SEI layer diminished, the two anodic peaks shifted to 0.13 and 0.93 V, respectively, and the CV curves overlapped, indicating the highly reversible redox reactions of Si-DA.55 The galvanostatic discharge/charge curves of Si-DA were further recorded at 50 mA g−1 (Figure 3b), which were in line with the CV curves. High initial discharge/charge capacities of 1308 and 503 mA h g−1 were obtained, corresponding to 38.5% of the initial Coulombic efficiency resulting from the formation of SEI on the surface of the electrode.56–59 Interestingly, significantly improved capacity (946 mA h g−1) was achieved with increased Coulombic efficiency (99.2%) up to 60 cycles after the activation process.60–62 Moreover, when using a higher activation rate of 200 mA g−1 (Figure 3c), Si-DA still showed the increasing capacity for the initial 80 cycles and a saturated capacity of 1161 mA h g−1 for an additional 220 cycles, which are higher than those of Si-PDA (493 mA h g−1) and Si-BP (490 mA h g−1). Even at an extremely higher current density of 20 A g−1, the capacity of Si-DA can still reach 200 mA h g−1 with excellent cycling stability for at least 10,000 cycles with Coulombic efficiency of >99.6% (Figure 3e). The possible activation process can be determined by the electrochemical impedance spectroscopy results of Si-DA anodes, in which the charge-transfer resistance (253 Ω for the first cycle) decreased to 45 Ω after 250 cycles (see Supporting Information Figure S23). Furthermore, Si-DA also possesses superior reversible capacities of 709, 649, 604, 520, 447, 382, 281, 212,and 159 mA h g−1 at current densities from 50 mA g−1 to 20 A g−1, which are significantly higher than the two counterparts (Figure 3d). The binding energy between the diradical structure and two Li ions is −6.13 eV, which is lower than that between the nonradical structure and two Li ions (−2.97 eV) (see Supporting Information Figure S27). These results imply that the presence of carbon centered radicals in Si-DA could enhance electrical conductivity and electrochemical properties, providing binding sites for lithium ions more rapidly, resulting in better LIB performances. Figure 3 | Electrochemical performance of Si-DA, Si-PDA, and Si-BP as anode materials in LIBs. (a) CV curves of Si-DA for the initial three cycles at 0.1 mV s−1. (b) The typical galvanostatic charge/discharge profiles between 3 and 0.01 V at 50 mA g−1. (c) Cycle performance of Si-DA, Si-PDA, and Si-BP at 200 mA g−1. (d) The rate performance of Si-DA, Si-PDA, and Si-BP at different current densities. (e) Cycling performance of Si-DA at 20 A g−1. Download figure Download PowerPoint Conclusion A novel 3D silicon-diacetylene PORP Si-DA has been synthesized via a classic Eglinton homocoupling reaction of tetraethynylsilane. This method provides a facile synthesis of amorphous diacetylene-linked silicon POPs with unique radical features such as high stability, low-energy LUMO, strong extended absorption up to 2000 nm, narrow band gap, and ferromagnetism at low temperature. Benefiting from the unique radical features, 3D PORP Si-DA exhibits high capacity, promising rate capability and is a potentially stable anode material for LIBs. Overall, our research could broaden the horizons for future 3D PORPs designs, which additionally helps the understanding of structure-property relationship for energy storage applications. Supporting Information Supporting Information is available and includes additional experimental details of POPs, various characterization experiments and results, computational experiments and results, and battery performance measurements and results. Conflict of Interest The authors declare no conflict of interest Acknowledgments Financial support from the National Natural Science Foundation of China (grant nos. 22131004, U21A20330, and 52173195), the “111” project (grant no. B18012), Jilin Provincial Department of Science and Technology (grant no. 20210508048RQ) and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.