A covalent organic polymer containing dative B ← N bonds: synthesis, single crystal structure, and physical properties

Abstract

Dative boron ← nitrogen (B ← N) bonds are important in the construction of crystalline covalent organic polymers/frameworks (COPs/COFs). Herein, 9,10-di(4-pyridyl)anthracene (DPA) and 1,4-bis(benzodioxaborolane)benzene (BACT) were employed as building blocks to prepare single crystals of a functional COP (CityU-30). Detailedly, DPA and BACT were connected together through dative B ← N bonds to form zigzag polymer chains, and the neighboring chains interacted with each other through hydrogen bonds to form a pseudo-two-dimensional structure. The observed significant decrease in the fluorescence intensity of CityU-30 compared to that of DPA indicates a pronounced photothermal effect in CityU-30. This research reveals the crucial role of B ← N bonds in designing innovative COPs and guides future materials research studies.

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10th anniversary statement

Prof. Qichun Zhang was appointed as a tenured professor in the Department of Materials Science and Engineering at City University of Hong Kong in September 2020. Previously, he served as an assistant professor at Nanyang Technological University from 2009 and was promoted to tenured associate professor in 2014. His research focuses on crystalline covalent organic polymers/frameworks for applications in optical and electrical devices, energy conversion and storage, and catalysis. He has published >550 papers (H:116). Over the past decade, Prof. Zhang has published 9 papers in Inorganic Chemistry Frontiers (ICF), a journal that plays a pivotal role in advancing chemistry through its commitment to quality and relevance. ICF supports leading researchers and emerging areas in the field, serving as a bridge between scientists worldwide. Congratulations on this landmark publication, and may it continue to inspire the field for years to come.

Introduction

Conventional polymers usually exist as amorphous solids or polycrystalline powders due to the disorder caused by flexible entangled chains.^1,2 This factor poses a big challenge for scientists to deeply understand the atomic structures of covalent organic polymers (COPs) as well as the precise spatial stacking and interactions of polymer chains. To address this issue, growing suitable single crystals of COPs for single crystal X-ray diffraction (SCXRD) analysis is the best solution.^3,4 Besides, the precise structural information of COPs can help us deeply understand the structure–property relationship, and provide guidelines to design and optimize COPs with specific functions.⁵ Thus, growing high-quality single crystals of COPs for such analysis is the key. To realize this, self-correction to mitigate the disorder/mistakes and promote the generation of order arrangement during crystal growth is extremely important.⁶ Obviously, self-correction requires covalent bonds with high reversibility. In fact, several dynamic covalent bonds such as imine, borate, B–O–Te,⁷ P–O–Te,⁸ P–O–B,⁹ and dative B ← N^10–14 bonds have been widely employed to construct single crystals of COPs and covalent organic frameworks (COFs).^15,16

Because the dative B ← N bond is generated by an electron-donating nitrogen-containing ligand interacting with an electron-deficient boron-containing atom (with an empty 2p orbital) to form a Lewis acid–base interaction,¹⁷ this type of bond has high reversibility with medium bond energy, making it ideally suited for the fabrication of robust and reversible supramolecular assemblies.^18–21 Moreover, the covalent character of the dative B ← N bond ensures the stability of the resulting structure, while its directionality allows for precise control of the assembly process.¹² These factors make dative B ← N bonds an excellent choice for the construction of single crystals of complex supramolecular structures and COPs, striking a balance between stability and dynamic assembly.

Herein, single crystals (up to 1.5 millimeters) of a COP (named CityU-30) was prepared through a controllable assembly between 9,10-di(pyridin-4-yl)anthracene (DPA, a nitrogen donor) and 1,4-bis(benzodioxaborole) benzene (BACT, a boron donor) under solvothermal conditions. SCXRD analysis shows that CityU-30 adopts a one-dimensional zigzag chain-like structure, where the neighboring chains interact with each other through hydrogen bonding to a pseudo-two-dimensional structure. Further study indicates that CityU-30 displays a photothermal effect.

Experimental

CityU-30 was synthesized in two steps (Scheme 1). Firstly, BACT was pre-synthesized according to the reported methods (Fig. S1†).²² Then, 4 mg BACT and 3 mg DPA were added into a vial containing a mixed solvent of toluene and methanol (4 mL, v/v 1 : 1). The vial was sonicated for 3 min and then heated at 85 °C. The mixed solvent was allowed to slowly evaporate overnight. Yellow single crystals of CityU-30 were observed on the wall of the vial. The as-obtained product was harvested and washed with toluene to remove unreacted monomers to provide the pure single crystals (CityU-30: {(BACT)_0.5(DPA)_0.5}_n, 3.0 mg, yield: 52% based on DPA).

Scheme 1 The synthetic route to CityU-30. The brown spheres and blue spheres represent boron and nitrogen atoms, respectively.

Results and discussion

As indicated in the picture (Fig. 1a), the average size of CityU-30 crystals could reach 1.5 mm with a parallel hexahedron shape. The microscopic features of CityU-30 crystals were further explored using a scanning electron microscope (SEM) (Fig. 1b). Meanwhile, the elemental distribution of CityU-30 was characterized through energy dispersive X-ray spectroscopy (EDX) mapping. As shown in Fig. 1c–f, the O and B elements from BACT and the N element from DPA are evenly distributed on the surface of CityU-30 crystals. In addition, X-ray photoelectron spectroscopy (XPS) was used for elemental analysis of CityU-30. The results show that CityU-30 consists of four elements C, O, B, and N (Fig. S2†). In detail, the peaks at 284.8 eV, 286.3 eV, 285.8 eV, and 283.9 eV in the C1s spectrum can be assigned to the carbon of C–C/C=C, C–O, C–N, and C–B bonds, respectively (Fig. S3a†).²³ The peak at 532.9 eV in the O1s spectrum and the peak at 191.3 eV in the B1s spectrum belong to the oxygen and boron of B–O–C, respectively (Fig. S3b and c†).^24,25 The peaks at 400.5 eV in the N1s spectrum come from the N atoms of the pyridyl N=C groups (Fig. S3d†).²⁶

Fig. 1 (a) Optical images of single crystals of CityU-30; (b) the SEM image of the single crystal of CityU-30; and (c–f) the corresponding mapping images of C, O, N, and B elements.

SCXRD analysis revealed that CityU-30 {(BACT)_0.5(DPA)_0.5}_n crystallized in the triclinic P1̄ space group with lattice constants of a = 6.4167(1) Å, b = 13.1842(4) Å, c = 13.6585(4) Å, α = 116.729(3)°, β = 92.641(2)°, γ = 103.286(2)°, and a unit-cell volume of 989.45(6) ų. The asymmetric unit contains a half of BACT and a half of DPA molecule (Fig. 2a). The low discrepancy factor R (4.11%) suggested that the information on the structure of CityU-30, including atomic positions, bond lengths, bond angles, and so on (Tables S1–S3†), is more accurate. In CityU-30, DPA and two BACTs are connected by dative B ← N bonds to form a zigzag chain along the [011] direction (Fig. 2b). The lengths of dative B ← N bonds were found to be 1.689 Å, consistent with previously reported dative B ← N bond lengths (1.627 Å to 1.691 Å).^27,28 The distance between layer chains is 6.417 Å (Fig. S4†). Between the neighboring chains, there are strong hydrogen bond interactions formed by the O atoms in the BACT and the hydrogen atoms in the pyridine moiety (O⋯H–C bond), with a bond length of about 2.578 Å (Fig. 2c). The close and continuous hydrogen bonds between the chain structures extend them into a pseudo-two-dimensional structure.

Fig. 2 (a) The asymmetric unit of CityU-30. Thermal ellipsoids are drawn at the 50% probability level; (b) the zigzag chain of CityU-30; (c) the stacking model of CityU-30. View along the c axis.

The purity of CityU-30 was tested through powder X-ray diffraction (PXRD). As shown in Fig. S5,† the experimental PXRD pattern matches well with the pattern simulated from single crystal data. The more prominent peaks located at 13.00°, 17.18°, and 18.24° originate from the (0, 1, 1), (−1, −1, 1) and (1, 1, 0) crystal planes, respectively. Fourier-transform infrared spectroscopy (FTIR) was applied to further demonstrate the formation of dative B ← N bonds (Fig. S6†). The peaks located at 1067 cm⁻¹ and 1032 cm⁻¹ belong to the generated dative B ← N bonds.²⁹ Besides, the peak at 1363 cm⁻¹ corresponds to the B–O stretching vibrations of the boronate ester.³⁰ The thermogravimetric analysis (TGA) result demonstrates the thermal stability of CityU-30 (Fig. S7†), where the crystals remain stable until 273 °C with 5% weight loss.

The optical properties of CityU-30 were investigated using UV-visible (UV-Vis) spectroscopy, photoluminescence (PL) and time-resolved photoluminescence (TRPL) analyses. The UV-Vis spectrum shows that CityU-30 has a broad absorption peak of around 380 nm (Fig. S8†), and the band gap of CityU-30 was calculated to be 2.67 eV through the Tauc-plot method, and it is smaller than the band gap (2.82 eV) of the isolated DPA monomer (absorption peak at 400 nm, Fig. S9†). The fluorescence emission peaks of DPA and CityU-30 are located at 452 nm and 467 nm. The absolute fluorescence quantum yields (PLQYs) of DPA and CityU-30 are 27.7% and 1.31%, respectively. Comparing the PL profiles between DPA and CityU-30 (Fig. 3a), the fluorescence intensity of CityU-30 was found to significantly decrease. Besides, as shown in Fig. 3b, the lifetime of CityU-30 and DPA is 2.99 ns and 6.27 ns, respectively. Considering that the thermal deactivation through nonradiative decay is in a competitive relationship with the fluorescence emission,³¹ the decrease of fluorescence intensity usually implies an enhancement of nonradiative decay. Thus, an infrared thermal imaging camera was used to monitor real-time changes in core temperature. CityU-30 crystals were uniformly loaded on slides and irradiated with a 450 nm laser. Fig. 3c shows the infrared (IR) images of CityU-30 at 0 min, 1 min, 2 min, 5 min and 10 min of laser irradiation. Obviously, the temperature of CityU-30 increased from room temperature to 31.8 °C in 10 minutes. Concurrently, under continuous UV irradiation at 396 nm for 10 minutes, the Raman spectra remain unchanged, indicating the stability of CityU-30 during exposure (Fig. S10†).

Fig. 3 (a) Fluorescence spectra of DPA (purple) and CityU-30 (orange) upon 375 nm excitation; (b) time-resolved fluorescence decay profiles of DPA and CityU-30; and (c) corresponding thermal images of CityU-30 recorded with an IR camera.

Conclusions

In conclusion, we have successfully prepared single crystals of a COP (CityU-30) based on dative B ← N bonds under solvothermal conditions. Since there is a significant decrease in the fluorescence intensity of CityU-30 when compared to that of DPA, CityU-30 exhibits a certain photothermal effect, which is firstly discovered in single crystals based on dative B ← N bonds. This result opens new avenues for the development of photothermal materials utilizing the unique properties of dative B ← N bonds. This study paves the way for further exploration of the functionalization of dative B ← N bonds in COPs.

Data availability

The data that support the findings of this study are available in the ESI of this article. CCDC 2363533.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Q. Z. is thankful for the funding support from the City University of Hong Kong (9380117 and 7020089) and Hong Kong Institute for Advanced Study, City University of Hong Kong, Hong Kong, P. R. China. D. Y. L. acknowledges the financial support of the Research Grants Council of Hong Kong through a Collaborative Research Equipment Grant (C1015-21EF).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2363533. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02121e

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