Diradicals or Zwitterions: The Chemical States of m -Benzoquinone and Structural Variation after Storage of Li Ions

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE5 Aug 2022Diradicals or Zwitterions: The Chemical States of m-Benzoquinone and Structural Variation after Storage of Li Ions Chenyang Zhang†, Yong Zhang†, Kun Fan†, Qian Zou, Yuan Chen, Yanchao Wu, Songsong Bao, Limin Zheng, Jing Ma and Chengliang Wang Chenyang Zhang† School of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics (WNLO), Optics Valley Laboratory, Huazhong University of Science and Technology, Wuhan 430074, Hubei †C. Zhang, Y. Zhang, and K. Fan contributed equally to this work.Google Scholar More articles by this author , Yong Zhang† School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu †C. Zhang, Y. Zhang, and K. Fan contributed equally to this work.Google Scholar More articles by this author , Kun Fan† School of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics (WNLO), Optics Valley Laboratory, Huazhong University of Science and Technology, Wuhan 430074, Hubei †C. Zhang, Y. Zhang, and K. Fan contributed equally to this work.Google Scholar More articles by this author , Qian Zou School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu Google Scholar More articles by this author , Yuan Chen School of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics (WNLO), Optics Valley Laboratory, Huazhong University of Science and Technology, Wuhan 430074, Hubei Google Scholar More articles by this author , Yanchao Wu School of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics (WNLO), Optics Valley Laboratory, Huazhong University of Science and Technology, Wuhan 430074, Hubei Google Scholar More articles by this author , Songsong Bao School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu Google Scholar More articles by this author , Limin Zheng School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu Google Scholar More articles by this author , Jing Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu Google Scholar More articles by this author and Chengliang Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Optical and Electronic Information, Wuhan National Laboratory for Optoelectronics (WNLO), Optics Valley Laboratory, Huazhong University of Science and Technology, Wuhan 430074, Hubei Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101333 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail m-Benzoquinones are often regarded as unstable materials in the form of radicals. Herein, an air-stable small molecular m-benzoquinone [4,6-diamino-1,3-benzoquinone (4,6DA1,3BQ)] without bulky groups or large conjugated systems is reported, and its chemical structure and state are profoundly elucidated by a series of substantial investigations that indicate their presence as zwitterions rather than as diradicals. This deep study indicated that the m-benzoquinone structure of 4,6DA1,3BQ was stabilized through the combination of hydrogen bonding and electron delocalization. Due to the presence of hydrogen bonding and zwitterions, the solubility and electrochemical performance hence were strongly dependent on the intermolecular interactions between the materials and the electrolyte compositions (Li salt, solvent, and concentration). The 4,6DA1,3BQ underwent the reversible transformation from more zwitterion structures to more conjugated benzene nature after storage of Li ions. These results provide insights into the chemistry of 4,6DA1,3BQ and promote the further development of new materials of m-benzoquinone for various applications. Download figure Download PowerPoint Introduction Organic materials have attracted a lot of attention for their applications in metal-ion batteries due to their flexibility, structural designability, recyclability, and potentially low cost and availability from vast natural resources.1–7 Hence, various organic materials have been reported as electrodes for batteries.8–12 Among them, carbonyl-based materials are the most significant because of their high capacity and stable charge/discharge voltage, which stimulate the revival of organic batteries. However, the reported carbonyl materials were mainly based on o- or p-benzoquinones, o-imides or p-carboxylates.13–19 It is well accepted that the m-benzoquinones are not stable materials and typically occur in the form of radicals. Hence, bulky groups or large π-conjugated systems are normally necessary to stabilize the m-benzoquinone analogues.20,21 On the basis of these findings and our previous works on conjugated coordination polymers (CCPs),22–24 in which m-benzoquinone structures or analogues might be stabilized by the π-d conjugation ( Supporting Information Figure S1), herein, we report an air-stable small molecular m-benzoquinone [4,6-diamino-1,3-benzoquinone (4,6DA1,3BQ), Figure 1a] without bulky groups or large conjugated systems. Basically, the small molecule should be unstable. However, through controlling the growth of single crystals, various characterizations of the chemical structures as well as the magnetic characterizations and theoretical calculations, we concluded that the chemical states of 4,6DA1,3BQ were mainly in the form of zwitterions, and the easily observed electron paramagnetic resonance (EPR) signals in 4,6DA1,3BQ samples were probably from the presence of minor diradicals and the defects of single crystals (Figure 1b). Compared with its analogues, that is, the oxidation states of m-dihydroxybenzene (m-DHB, Figure 1a), the m-benzoquinone structure of 4,6DA1,3BQ was stabilized through the combination of hydrogen bonding and electron delocalization (Figure 1c). Due to the presence of hydrogen bonding and zwitterions, the solubility and electrochemical performance were strongly dependent on the intermolecular interactions between the materials and the electrolyte composition (Li salt, solvent, and concentration). The 4,6DA1,3BQ underwent the reversible transformation from more zwitterion structures to more conjugated benzene nature after storage of Li ions. These results provide insights into the chemistry of 4,6DA1,3BQ and would promote the further development of new materials of m-benzoquinone for various applications. Figure 1 | Design of stable small molecular m-benzoquinone. (a) Diagram of the deprotonation and oxidation of m-DHB and 4,6DA1,3DHB. (b) The possible resonant chemical states of 4,6DA1,3BQ. (c) The proposed strategy of the combination of hydrogen bonding and electron delocalization for achieving stable small molecular m-benzoquinone (Orange dotted lines, possible intramolecular hydrogen bonds; cyan dotted lines, possible intermolecular hydrogen bonds). Download figure Download PowerPoint Experimental Methods Synthesis of 4,6DA1,3BQ powders All commercially available reagents and solvents were purchased from Energy Chemical (Shanghai, China) or Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without further purification. First, 42.6 mg (0.2 mmol) of 4,6-diamino-1,3-dihydroxybenzene dihydrochloride (4,6DA1,3DHB•2HCl) was dissolved in 20 mL of deionized water. Subsequently, ammonium hydroxide (10 equiv to 4,6DA1,3DHB•2HCl) was slowly dropped into the solution as a proton scavenger. The mixture was stirred for 12 h at room temperature. The resulting dark purple suspension was filtered and washed several times with deionized water and methanol, and dried under vacuum at 80 °C for 10 h. Finally, the 4,6DA1,3BQ powder was obtained with a yield of 93%. 1H NMR (300 MHz, dimethyl sulfoxide (DMSO), δ): 4.91 (s, 1H) 5.62 (s, 1H) 8.44 (s, 2H) 9.19 (s, 2H). High-resolution mass spectrometry (HR-MS) (m/z): [M + H]+ calcd for C6H7N2O2+, 139.05; found, 139.05016. Synthesis of 4,6DA1,3BQ crystals 213 mg (1 mmol) of 4,6DA1,3DHB•2HCl was dissolved in 24 mL of deionized water. Subsequently, the as-obtained solution was transferred to a 12-well culture plate, and excess ammonium hydroxide was added to the interval of culture plate. This culture plate was maintained at room temperature without shock for 10 h. Finally, large-scale filiform 4,6DA1,3BQ crystals were collected by filtration. Materials characterizations 1H NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer (Bruker, Switzerland), using DMSO-d6 as solvent and tetramethylsilane (TMS) as internal standard. MS spectra were acquired from a orbitrap liquid chromatography-mass spectrometry (LC/MS) (Q Exactive) spectrometer (Agilent, United States). The Fourier transformed infrared (FT-IR) spectra were recorded by a Bruker ALPHA spectrometer (KBr pellets; Bruker, Germany). A Pyris1 thermogravimetric analyzer (PerkinElmer Instruments, United States) was used to perform thermogravimetric analysis (TGA) tests at Ar atmosphere with a heating rate of 10 °C min−1. The morphology characterizations were carried out on a scanning electron microscope (ZEISS Gemini 300, Carl Zeiss, Germany). X-ray photoelectron spectroscopy (XPS) was collected on a Thermo Fisher ESCALAB 250Xi (Thermofisher, United States) using a monochromic Al X-ray source (hν = 1486.6 eV). The single-crystal X-ray diffraction (XRD) study of as-obtained 4,6DA1,3BQ was carried out using XtaLAB PRO MM007HF (Rigaku, Japan). UV–vis absorption spectra were recorded using SolidSpec-3700 (Shimadzu, Japan). The saturated solutions of 4,6DA1,3BQ in different electrolytes were collected by carefully filtering supersaturated solution through filters. These saturated solutions were properly diluted for further measurement. Furthermore, these standard solutions of 4,6DA1,3BQ were prepared with various concentrations. The solubility of 4,6DA1,3BQ in different electrolytes was determined according to the calibration curve obtained by linking the maximum absorbance of the standard solutions to their concentration. EPR spectra were carried out on a Bruker A300 spectrometer (Bruker, Germany). Microwave frequency was set at 9.854 GHz with a power of 20.23 mW. Magnetic measurements were performed using a Quantum Design SQUID VSM magnetometer (Quantum Design, United States) with a field of 0.1 T. Electrochemical measurements To prepare the working electrodes, the 4,6DA1,3BQ powder was blended with super P (conductive additive) and sodium salt of carboxyl methyl cellulose (binder) in a mass ratio of 5∶4∶1, using deionized water as solvent. The resulting slurry was coated on Cu or Al foil and dried at 80 °C for overnight under vacuum, giving areal loading of 4,6DA1,3BQ active material of 1.0∼2.0 mg cm−2. The electrochemical performance of 4,6DA1,3BQ electrode was evaluated using 2032 type coin cells with Li foil as the counter electrode. Glass fiber membrane (Whatman, GF/B) was used as separator. Various electrolytes formulations were homemade. These cells were assembled in an Ar-filled glove box with a low level of H2O (<0.1 ppm) and O2 (<1.0 ppm). The battery performance was evaluated on the Landt CT2001A battery testing system (Wuhan, China) at room temperature. The cyclic voltammetry (CV) measurements were performed on the BioLogic VMP3 potentiostat. The ex situ measurements were carried out by disassembling the batteries in the Ar-filled glove box. The electrodes were washed by 1,2-dimethoxyethane (DME), dried in vacuum, and then sealed in bottles for further tests. Theoretical capacity calculation The 4,6DA1,3BQ small molecule can undergo reversible redox reaction and gain or lose one electron. According to the calculation equation: C capacity = ( n × 26 , 801 ) / M where n is the number of electrons gained or lost, and M is the relative molecular mass the 4,6DA1,3BQ molecule. Therefore, the theoretical specific capacity is 194.2 mAh g−1. Electronic structure calculation To better understand the fundamental electronic structures of 4,6DA1,3BQ, we used density functional theory (DFT) to obtain the electronic-spin structures, which were performed with Guassian 16 package suite. The UB3LYP functional was used for the geometry optimization, and the def2svp level basis set was selected for all atoms.25–27 Frequency calculations were also performed to identify that all the optimized structures were local minima. As illustrated below, there are three different singlet electronic states of 4,6DA1,3BQ. Calculations showed that the lowest singlet state of 4,6DA1,3BQ is the close-shell singlet (S0) state, which was 10.38 kcal/mol lower than that of the open-shell singlet (S0′) state and 31.38 kcal/mol lower than that of the S0(TS) state. However, the diradical signal can only be detected in S0′ state and the main part of the spin density is located on the C=O fragment in the unit cell. The reason why the energy of S0′ is lower than that of S0(TS) is easy to understand. In the S0′ state, the two nonbonding electrons are largely localized on the exocyclic carbonyl groups, leaving the six-membered ring with an aromatic, benzenoid π system. In contrast, in the S0(TS) state, there is substantial π bonding to the exocyclic carbonyl groups at the expense of the aromaticity of the π system of the six-membered ring in 4,6DA1,3BQ. On the basis structure of S0, open-shell triplet state (T1) was also calculated. Compared to the S0 state, a significant difference exists: there is a stronger radical signal in T1 and the electrons are located on the whole system. However, the electronic energy is calculated to be 23.99 kcal/mol higher than that of the S0 ground state, which means that the T1 state is less stable than S0. The electrostatic potential surfaces (EPS) of 4,6DA1,3BQ at the UB3LYP/def2svp level were calculated using Multiwfn program.28 The S0 state is a typical zwitterion structure whose negative charges are delocalized between the oxygen atoms of 4,6DA1,3BQ, but the positive charges are delocalized between the nitrogen atoms of 4,6DA1,3BQ, which means no radical exists in the system and hence could lower the energy of the system. However, for T1 state, there is no clear boundary between positive and negative charges, which leads to the instability of the T1 state. We also considered resonance structures and their electronic structures. Both the S2 and T2 state can be detected with a radical signal. However, the electronic energy of S2 and T2 is calculated to be 58.57 and 67.89 the kcal/mol higher than that of the S0 state, respectively, which leads to the instability of the resonance structures at room temperature. Theoretical calculation on the electron transfer number (ETN) To investigate why only one rather than two electrons were accepted, DFT calculations were performed with the CASTEP module in Materials Studio software.29 The generalized gradient approximation (GGA) and the Perdew–Burke–Ernzerhof (PBE) functional were employed in geometry optimizations with Grimme’s correction, and the ultrasoft pseudopotential was carried out.30 The cutoff energy for the plane wave expansion was 500 eV. The Brillouin zone (BZ) was sampled with a 4 × 2 × 2 k-point. In every supercell, four molecules were present. The geometry optimization was carried out until the total energy was <2.0 × 10−5 eV/atom, max force: 0.05 eV/Å, max stress: 0.1 GPa, max displacement: 0.002 Å. The binding energy per Li ion, Eb, of the 4,6DA-1,3BQ was calculated by using the following equation: E b = E A − n Li − E n Li − E A 4 n where EA−nLi and EA are the energies of the reduction states and pristine 4,6DA1,3BQ, respectively; EnLi is the energy of Li atom. Results and Discussion Synthesis and characterizations of the stable small molecular m-benzoquinone (4,6DA1,3BQ) As shown in Figure 1a, m-DHB underwent a one-pot deprotonation and oxidation reaction and then transformed into a radical intermediate state that bears two free lone electrons within the single molecule of benzene ring. Given the fact that the radical intermediate state of m-DHB was extremely unstable,31–33 amino groups were hence introduced to construct intra- and inter-molecular hydrogen bonding,34 which resembled the coordination bonds in our previously reported CCPs,22,35 forming a cross-linking network (Figure 1c). Simultaneously, the electron delocalization (Figure 1b) further enhanced the stability through resonance.36 The stable 4,6DA1,3BQ was then synthesized via one-pot in situ deprotonation and oxidation of 4,6DA1,3DHB in a basic environment ( Supporting Information Figures S2 and S3). Large crystals were obtained through controlling the temperature and reaction speed and selected for single-crystal X-ray analysis. The results strongly confirmed the proposed chemical structure of 4,6DA1,3BQ (Figure 2a). The reactant was deprotonated and oxidized, leading to deprotonated phenoxyl structure. No water molecules were observed in the crystal structure, which was different from the previous reports (4,6DA1,3BQ•H2O).37 The length of four C–C bonds (C1–C2 and C3–C4) was about 1.38∼1.39 Å, which was slightly longer than the typical C=C double bonds; while the other two (C2–C3) were about 1.52 Å, slightly shorter than the typical C–C single bonds. Besides, the length of C–O bonds was about 1.25 Å, which was between the values of typical single and double C–O bonds ( Supporting Information Table S1). Furthermore, only one hydrogen atom was connected to the C1 and C4; and no hydrogen atoms were connected to the oxygen atoms. The whole molecule was planar. Intramolecular hydrogen bonds (2.276 Å, Figure 2b and Supporting Information Table S2) could be observed between O1 and H1B. All of these results indicated the conjugated nature of the small molecule 4,6DA1,3BQ. The single-crystal structure of 4,6DA1,3BQ belonged to an orthorhombic space group Pbcn with a = 5.3320 (1) Å, b = 11.0053 (2) Å, c = 10.2294 (2) Å, and V = 600.26 (2) Å3 ( Supporting Information Table S3). It displayed a layer-by-layer packing motif along the b axis through intermolecular hydrogen bonds (N1-H1A···O1, 2.037 Å, Figure 2b). The molecules in every layer packed in the same orientation with two kinds of π–π interfacial spacing of 3.23 and 4.13 Å, respectively. Another type of intermolecular hydrogen bond (N1-H1B···O1, 2.166 Å, Figures 2b and 2c) was present within the layer, forming one-dimensional molecular chains. The molecules in the adjacent layers showed different orientations, leading to a herringbone structure with a dihedral angle of 78.38° (Figure 2d). The intermolecular hydrogen bonds and the π–π interactions resulted in the three-dimensional cross-linking networks. Figure 2 | The solved single-crystal structure of 4,6DA1,3BQ. (a) The solved molecular structure of 4,6DA1,3BQ. (b) View of inter- and intra-molecular hydrogen bond of 4,6DA1,3BQ. (c and d) Molecular stacking of 4,6DA1,3BQ: single layer (c) and double layer (d). Download figure Download PowerPoint As expected, the XRD patterns of 4,6DA1,3BQ powders with mass production were consistent well with the simulated XRD results using the single-crystal structure (Figure 3a), indicating that all of the resultant materials were of the same molecular arrangement with the single crystals of 4,6DA1,3BQ. The chemical structure of 4,6DA1,3BQ was further confirmed by energy-dispersive spectroscopy (EDS), elemental analyses (EA), mass spectrum (MS), 1H NMR spectra, FT-IR spectroscopy, and TGA. The EDS spectrum indicated the presence of C, N, and O elements and the elemental mapping revealed their homogeneous distribution of them in the as-prepared products ( Supporting Information Figures S4 and S5). EA results proved that the contents of C, H, N, and O were extremely close to theoretical values in the proposed formula ( Supporting Information Table S4). Additionally, the MS spectrum (Figure 3b) clearly demonstrated that the material was deprotonated with accurate molecular weight with the proposed structure. The material was not easily soluble in DMSO, probably due to the strong intermolecular interactions (hydrogen bonding and π–π interactions). Nevertheless, the 1H NMR spectra of 4,6DA1,3BQ in DMSO clearly reconfirmed that no hydrogen atoms were connected to oxygen atoms. Furthermore, due to the conjugation in the whole molecule and the stronger electronegativity of oxygen, the chemical shift of H4 shifted down-field (high chemical shift) compared with H1. On the other hand, because of the intramolecular hydrogen bonds, the chemical shift of H1B shifted down-field compared with H1A (Figure 3c, Supporting Information Figure S6). Figure 3 | Characterization of 4,6DA1,3BQ powders. (a) XRD patterns of 4,6DA1,3BQ and 4,6DA1,3DHB•2HCl powders, the simulated XRD patterns based on the single-crystal structure of 4,6DA1,3BQ and 4,6DA1,3BQ•H2O.37 (b) MS spectrum of 4,6DA1,3BQ. (c) 1H NMR spectrum of as-prepared 4,6DA1,3BQ in DMSO. (d) TGA curves of 1,4-benzoquinone (BQ), m-PDA and 4,6DA1,3BQ. (e) The variable-temperature EPR spectra of 4,6DA1,3BQ powders. (f) The variable-temperature 1H NMR spectrum of 4,6DA1,3BQ in DMSO. Download figure Download PowerPoint To further confirm the chemical structure of 4,6DA1,3BQ, the FT-IR spectra of the products as well as 4,6DA1,3DHB•2HCl and the m-phenylenediamine (m-PDA) were conducted. The characteristic vibrations of primary amine salt (–NH3+) at 3155 cm−1 disappeared, and the vibrations of N–H bonds at 3282 cm−1 could be obviously observed in 4,6DA1,3BQ, indicating that the hydrochloride was thoroughly eliminated, and the amino groups remained stable ( Supporting Information Figure S7). As expected, significant attenuation of C–O stretching vibration around 1209 cm−1 along with the clear signal of C=O stretching vibration at 1575 cm−1 in 4,6DA1,3BQ compared to 4,6DA1,3DHB were observed, which indicated the conjugated nature of the products.38 We also noted that the stretching vibration peak of proton acceptor (C=O) underwent a bathochromic shift compared with the typical carbonyl materials; while the stretching vibration peak and the bending vibration peak of the proton donor (N–H) suffered a bathochromic and hypsochromic shift, respectively, compared with those of m-PDA. These phenomena corroborated the formation of intermolecular hydrogen bonds between the –NH2 and C=O groups. In addition, both single and double characters of C–C/C=C and C–N/C=N bonds wereobserved. All of these results indicated the resonant structure of the obtained 4,6DA1,3BQ. The TGA results (in Ar) also supported the formation of strong intermolecular hydrogen bonds. Compared with similar compounds without intermolecular hydrogen bonds, such as BQ and m-PDA, the thermal stability of 4,6DA1,3BQ was significantly enhanced. The onset of rapid weight loss increased to 250 °C for 4,6DA1,3BQ, which was much higher than those of BQ and m-PDA (Figure 3d). In addition, the weight remained as high as 40% after being heated above 700 °C, which was similar with the materials [2,5-diamino-1,4-benzoquinone (2,5DA1,4BQ)] with intermolecular hydrogen bonds.38 The undegradable residual of 4,6DA1,3BQ after annealing at 800 °C was further analyzed by scanning electron microscopy (SEM) and XRD ( Supporting Information Figure S8). The morphology remained similar to the pristine 4,6DA1,3BQ. However, the XRD pattern indicated the undegradable residual could be attributed to carbon (JCPDS card no. 26-1080). These results demonstrated that strong intermolecular hydrogen bonds significantly enhanced the thermal stability of 4,6DA1,3BQ and enabled its carbonization rather than direct sublimation. Chemical states of 4,6DA1,3BQ: diradicals? Considering the possible formation of diradicals after deprotonation and oxidation of 4,6DA1,3DHB, we further probed the chemical states of 4,6DA1,3BQ using EPR spectra. Interestingly, the powders of 4,6DA1,3BQ did indeed display strong EPR signal with a g-factor of 2.001, suggesting the possible presence of unpaired electrons ( Supporting Information Figure S9). Meanwhile, as control experiments, negligible EPR signal was observed for the reactant (4,6DA1,3DHB•2HCl) and the p-benzoquinone analogues [e.g., benzoquinone (BQ) and 2,5DA1,4BQ]. Impressively, signal was so stable that it still could be detected after being stored in air for more than 8 months. These results indicated the possible chemical states of 4,6DA1,3BQ in its diradical states (Figure 1b). The variable-temperature EPR and 1H NMR spectroscopy were widely adopted as evidence of the presence of diradicals.39–42 We therefore conducted the EPR and 1H NMR measurements of 4,6DA1,3BQ at different temperatures. The EPR signal of 4,6DA1,3BQ powders indeed increased with increasing temperature (Figure 3e), which was often regarded as the excited triplet states at elevated temperature.39,41,42 Moreover, as shown in Figure 3f, with the increase of temperature, the peaks of H1A and H1B in 1H NMR spectra became weaker and broader, which were also often used to verify the presence of diradicals.39,40 All of these results indicated that 4,6DA1,3BQ seemed to be present in the diradical states. Chemical states of 4,6DA1,3BQ: zwitterions? However, another possibility was proposed about 15 years ago, suggesting that the 4,6DA1,3BQ may be in the form of zwitterions,37 although little evidence of this was presented. Therefore, we further conducted the magnetic measurements of 4,6DA1,3BQ powders to verify their chemical states. The superconducting quantum interference device (SQUID) measurements indicated that the magnetic susceptibility of 4,6DA1,3BQ powders was always very low, indicating its diamagnetism (Figure 4a). What is more, the magnetization curves quantificationally reconfirmed that the diradicals were not the major states in the powders: less than 3‰ of the molecules might be in the diradical states (Figure 4b). Owing to the high sensitivity of EPR measurements, the EPR signal might come from the presence of minor diradical states of 4,6DA1,3BQ or defects in the crystal powders. These results indicated that the major states of 4,6DA1,3BQ should be the zwitterions. In this case, the stronger intensity of the EPR signals of 4,6DA1,3BQ powders at higher temperatures should be ascribed to the activation of the radicals (Figure 3e). On the other hand, the weaker peaks in the 1H NMR spectra at higher temperatures should contribute to the vibration of H atoms and the variation of intermolecular interactions of hydrogen bonds (Figure 3f). The extensive diffusion of molecules in the solvents at higher temperatures weakened the hydrogen bonds.43,44 Such variation could also be supported by movements of the chemical shifts: the chemical shifts of H1A and H1B shifted up-field; while the chemical shifts of H1 and H4 shifted down-field, which could be readily ascribed to the attenuation of hydrogen bond interactions. Figure 4 | Magnetic measurements and theoretical calculations. (a) Temperature-dependent plots of χMT for 4,6DA1,3BQ powders from 2 to 300 K in the SQUID measurements. (b) Magnetization curves of 4,6DA1,3BQ powders at 2 K. (c) The EPR signals of 4,6DA1,3BQ powders and crystals. (d) Six possible electron states of 4,6DA1,3BQ and the corresponding electron energy. S: single state, T: triplet state, TS: transition state. (e) The structure of six states and the selected bond lengths are given in Å. (f) The spin isodensity surfaces of six states (isovalue = 0.008 a.u.). Atoms are colored as follows: brown (C), white (H), red (O), and blue (N). (g) The calculated EPS of 4,6DA1,3BQ at the UB3LYP/def2svp level (isoface = 0.001 a.u.). Red indicates positive charge density, and blue indicates negative charge density. Download figure Download PowerPoint Chemical states of 4,6DA1,3BQ: DFT calculations DFT calculations were then performed to further explore the electronic structures of 4,6DA1,3BQ. The crystal structure of 4,6DA1,3BQ was used as the starting geometry for optimization of the possible electronic states. Several minimal electronic states were obtained at the UB3LYP/def2svp level (isoface = 0.001 a.u.), i