Preparation, Characterization, and Application of Functional Tellurium-Based Single-Crystal Materials

Abstract

Single-crystal structure analysis of polymers is essential as it provides precise information on macromolecular interactions, packing modes, crystallization physics, intrinsic properties, and structure-property relationships. Furthermore, single crystals of polymers hold potential applications across various fields. However, compared to other main-group elements, the formation of tellurium-based polymer crystals is rare, as most tellurium-based polymers typically exist as films or powders. Tellurium, the heaviest non-radioactive element in the chalcogen family, possesses unique properties that are increasingly significant in inorganic and organic chemistry, as well as materials science. In this study, we introduce a novel method for growing single-crystal tellurium-based covalent polymers, including one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) polymers. By functionalizing the backbones of these polymers, we successfully enhance their performance as optical materials and photocatalytic materials.

Firstly, single crystals of a covalent helical polymer (CityU-10) with a non-carbon backbone and organic side-chain were prepared through solvothermal conditions. Characterization by single-crystal X-ray diffraction (SCXRD) uncovered that the polymer consisted of a tellurium-oxygen-phosphorus (-Te-O-P-) backbone and organic side chains with phenyl rings. A collection of strong hydrogen-bonding (OH···O=P) interactions between same-handed polymer chains endows it with a two-dimensional layer structure. The helical direction of the chains in adjacent layers is opposite, and weak hydrogen-bonding (phenyl-H…O=P) interactions extend the layers into a three-dimensional structure. The uniqueness of structure prompts the helical polymer with the properties of inorganic and organic polymers. Besides, the helical polymer crystal is found to be a prospective optical material with quite a large birefringence, which is two orders of magnitude larger than previously reported polymer single crystals with confirmed structures through SCXRD. This strategy clarifies the relationship between the properties and structure of hybridized covalent helical polymers, which could be a novel bridge for designing functional materials in the future.

Secondly, we designed and prepared single-crystal covalent organic polymers (COP, CityU-21) through the reaction between tellurinyldibenzene and [1,1'-biphenyl]-4,4'-diylbis (phosphonic acid). SCXRD analysis demonstrates that biphenyl units were covalently connected with Te(Ph₂) moieties via [HPO₃] parts to form a one-dimensional organic polymer and adjacent polymer chains via multiple hydrogen-bonding interactions, promoting CityU-21 to become a two-dimensional framework. With the effect of aromatic P=O parts in the backbone, the heavy atoms (tellurium), and hydrogen bonds, the single-crystal CityU-21 possesses room-temperature phosphorescence (RTP) behavior with a lifetime of 179 ms @540 nm and 158 ms @565 nm, the photoluminescence quantum yield of 84.69% and an afterglow time of 1.2 s, which for the first time, an RTP peculiarity is realized in a single-crystal COP. In addition, because of its unique structure, CityU-21 can keep its crystalline and RTP properties with an afterglow time of up to 0.8 s in different solvents, which remedies the problem that most small molecule crystals do not keep their crystalline and RTP properties in solvents and create the possibility for wider applications of RTP materials.

Thirdly, we developed a variety of 2D and 3D COPs (CityU-17, CityU-18, CityU-19, and CityU-20) using tellurium to construct linkages. Furthermore, we harvested single crystals of these 2D and 3D COPs in 2 days with crystal sizes of up to 500 µm. The high-quality crystals uncovered the detailed structural information for this new family member of 2D/3D COPs via SCXRD analysis with resolutions of up to 0.83 Å. In addition, these new networks showed promising performance as photocatalysts for superoxide anion radical-mediated coupling of (arylmethyl)amines. Among them, CityU-18 and CityU-20 show nearly 100% selectivity and yield for oxidative coupling of benzylamine as photocatalysts. Our strategy not only enriches the family of COPs but also sheds light on the development of various large single crystals of 2D/3D COPs, especially for the growing of 2D COPs.

Finally, we extended 3D COPs into crystalline carbon-free covalent backbones to endow these materials with more exotic functions. By linking 3-connected nods (BO₃) and 2-connected organic building blocks (Te(Ph)₂) together, colorless single crystals (size up to 400 µm) of an organic-inorganic hybrid 3D chiral covalent framework (CityU-22) were prepared. SCXRD analysis reveals that CityU-22 has a non-carbon Te-O-B bonds-based network with the srs topology. The chiral CityU-22 displays good stability under the treatment of different common solvents or heat (the decomposition temperature above 300 °C). Due to its non-π-conjugated backbone (-Te-O-B-O-), CityU-22 shows an ultraviolet nonlinear optics (NLO) behavior with a second-harmonic generation (SHG) response similar to KH₂PO₄ (KDP). This work firstly realizes metal-free 3D covalent organic-inorganic hybrid framework crystals used as an NLO material, providing another alternative strategy to solve instability and environmentally unfriendly issues in the NLO field and broaden the applications of covalent frameworks as well as open up a new pattern for functionalizing COPs.

Date of Award	11 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yun CHI (Supervisor), Qichun ZHANG (Supervisor) & Jr-Hau HE (Co-supervisor)