Advanced Functional Single-Crystal Materials Based on Boron-Containing Covalent Polymer

Abstract

Boron’s distinct electron structure has attracted significant interest from researchers owing to its exceptional performance in luminescent, optical, or catalytic materials. Notably, single crystals (SCs) derived from boron-containing polymers harness boron’s excellent properties and benefit from the advantages of crystallinity. Moreover, the advent of single crystal X-ray diffraction (SCXRD) allows for the precise identification of atomic structures in crystals, revealing the structure–function relationship and aiding in the design and fine-tuning of functional materials. This dissertation presents the successful synthesis of single crystals featuring B–O covalent bonds, Si–O–B covalent bonds, and dative B←N bonds using the solvothermal method. Accurate structural information was obtained via SCXRD analysis, and the unique optical and energy storage properties were also investigated.

First, a metal-free two-dimensional (2D) covalent inorganic framework (H₂en)[Si(B₄O₉)], designated as CityU-12 (where "en" refers to ethylenediamine), was successfully synthesized using the solvothermal method. The crystal structure of CityU-12 was determined using SCXRD and low-dose high-resolution transmission electron microscopy (LD-HRTEM). SCXRD revealed that CityU-12 crystallized in the monoclinic space group P2₁/n, which was corroborated by LD-HRTEM observations showing nearly identical interplanar distances for the (200) and (011 ̅) crystal planes. These distances closely matched with the double peaks at 2θ = 14.26° and 15.44°, observed in the powder X-ray diffraction (PXRD) patterns. CityU-12 also exhibited interesting optical properties, including long-lived room-temperature phosphorescence (RTP) at 530 nm with a lifetime of 1,528.84 ms and an afterglow duration of 2.5 s in air.

Second, a metal-free three-dimensional (3D) covalent inorganic framework (denoted as CityU-11) was successfully synthesized using a modified solvothermal method. By introducing a small amount of F⁻ and using SiO₂ as the silicon source, SCs of CityU-11 were prepared under solvothermal conditions. The crystal structure of CityU-11 was determined using SCXRD and LD-HRTEM. SCXRD revealed that CityU-11 crystallizes in the non-centrosymmetric space group Pnn2. LD-HRTEM observations indicated similar interplanar distances for the (200) and (020) crystal planes, which corresponded well with the double peak at 2θ = 15° observed in the PXRD patterns. Additionally, CityU-11 exhibited an interesting optical property, displaying moderate birefringence of 0.0258@550 nm.

Finally, dative boron–nitrogen (B←N) bonds have shown great potential for constructing crystalline organic frameworks. The moderate binding energy of these bonds allows for favorable reversibility, which is crucial for the growth of single-crystalline polymers (SCPs). This paves the way for creating diverse functional SCPs using functionalized boron- and nitrogen-containing building blocks. In this study, 1,4-bis(benzodioxaborole) benzene (BACT) and N,Nʹ-Di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxdiimide (DPNTCI) were used as building blocks to synthesize a dative B←N SCP denoted CityU-25. CityU-25 exhibited robust stability in air and various solvents such as N,N-dimethylformamide, ethanol, and methanol. Additionally, incorporating functional 1,4,5,8-naphthalenetetracarboxylic dianhydride in the building block endowed CityU-25 with excellent electrochemical properties for lithium batteries. The results showed that the battery maintained an ultrahigh specific capacity of 673 mAh/g after 720 cycles at a high current of 0.5 A/g and 455 mAh/g after 170 cycles at 0.1 A/g. This work demonstrates the potential of dative B←N SCPs in design and application.

Date of Award	30 Jul 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jr-Hau HE (Supervisor) & Qichun ZHANG (Supervisor)