Bridging the Gaps in Porous Materials: from Molecular Solids to Ordered Carbon Frameworks

Abstract

The opening chapter provides an introductory, selective overview on the burgeoning field of metal-organic frameworks (MOFs), with examples drawn from the sulfur-functionalized systems to highlight the importance of covalent crosslinking in the field. In general, such crosslinking offers unique opportunities for to convert labile coordination networks into robust covalent frameworks, thus bypassing the obstacles in accessing stable and electroactive metal-organic frameworks. In this connection, crosslinks based on sulfur functions, as exemplified by the reactive yet tractable thiol group (-SH), are of interest. In particular, MOF materials equipped with free-standing thiol functions are versatile: metal guests can be conveniently inserted to install electroactive metal-sulfur bonds, which, as crosslinks, also stabilize the host coordination net.

The sulfur-containing building blocks, however, are sometimes difficult to prepare, and single crystals of the MOF solids can be hard to grow, because of reactivity of the thiol and other sulfur groups. To open up the horizon for the crosslinking of coordination networks, this thesis turned to the alkyne groups, which is relatively under-explored as secondary functional units (e.g., side arms) for coordination frameworks. In particular, a series of backfolded linkers molecules equipped with contiguous alkyne groups and the corresponding MOF products are presented; and the functionalities of alkyne moieties for bridging the gap between coordination networks and covalent networks are highlighted.

Chapter 2 introduced a Sierpinski-like molecule that features four symmetrically backfolded (SBF) side arms with dimethylamino group attached as side groups to increase reactivity of side-arm alkyne units for cycloaddition-retroelectrocyclization (CA-RE) reaction. The CA-RE reaction with TCNE (tetracyanoethylene) afforded a 3-dimensional, conjugated and rigid molecule (SBF-16), based on which an exceptionally stable porous molecular solid was assembled. This molecular porous material is devoid of the common traits of related systems. Namely, the molecule does not rely on directional hydrogen bonds to enforce open packing; and it offers neither large concave faces (i.e., high internal free volume) to frustrate close packing, nor any inherently built-in cavity like in the class of organic cages. Instead, the permanent porosity (as unveiled by the X-ray crystal structure and CO₂ sorption studies) arises from the strong push-pull units built via CA-RE reaction. Unlike the poor/fragile crystalline order of many porous molecular solids, the molecule here readily crystallizes and the crystalline phase can be easily deposited into thin films from solutions. Moreover, both the bulk sample and thin film exhibit excellent thermal stability with the porous crystalline order maintained even at 200 °C. The intermolecular forces underlying this robust porous molecular crystal likely include the strong dipole interactions and the multiple C···N and C···O short contacts afforded by the strongly donating and accepting groups integrated within the rigid molecular scaffold.

Having studied the reactivity of the well-defined molecular system, in Chapter 3 we move on to install the conjugated motif as crosslinks into MOF scaffolds, i.e., by using similar alkyne-based linkers and applying the CA-RE reaction to the MOF scaffold. By so doing, we converted a coordination network into a covalent solid, while maintaining the crystallinity and greatly enhancing the framework rigidity and redox-active and photochemical properties. Specifically, intensely light-absorbing push-pull functions are installed by reacting the electrophilic TCNE guests and the electron-rich alkyne side arms on a microporous Zr-organic framework, generating black microporous crystallites with a bandgap smaller than 1.0 eV. The CA-RE reaction extensively establishes conjugated (polyene) bridges across the linker molecules. The donor (4-methoxyphenyl) and acceptor (dicyanovinyl) couples of the polyene bridges also act as an efficient fluorescent quencher, and can be selectively installed in a thin outer layer of the host crystallite to form a core-shell assembly for turn-on fluorescent sensing of small amine molecules in water solutions.

Have explored the elaborate donor-acceptor crosslinks, we took a step back, and aim for a simple, and potentially wider-scope strategy for converting coordination networks into covalent systems—a simple strategy based on thermal treatment. Specifically, in chapter 4, we described a curious case study of a Zr(IV)-carboxylate framework, which retains significant crystalline order after cascade thermocylization of its linker components, and--more notably--after the crucial carboxylate links were severed by heat. Vigorous heat treatment (e.g., 450 °C and above) benzannulates the multiple alkyne groups on the linker to generate linked nanographene blocks, and to afford real stability. The resultant Zr oxide/nanographene hybrid solid is stable in saturated NaOH and concentrated H₃PO₄, and allows for convenient anchoring of H₃PO₄ into its porous matrix to enable size-selective heterogeneous acid catalysis. The Zr oxide components can also be removed by strong hydrofluoric acid to further enhance the surface area (up to 650 m²/g), without collapsing the nanographene scaffold. The crystallinity order and the extensive thermal transformations were characterized by X-ray diffraction, high resolution transmission electron microscopy (HRTEM), IR, solid state NMR and other instrumental methods.

Chapter 5 builds on the success of the above thermocyclization strategy, and aims to incorporate post-synthetic metalation for further functionalization. Specifically, we constructed a Zr(IV)-carboxyl framework featuring alkyne/thioether functions on the backfolded side arms of the linker molecule for the uptake of metal species. Accidentally, a curious case was found, in which the heavy-atom guests (AgSbF₆) induce order to a host metal-organic framework, but themselves remain highly disordered and invisible to the X-ray diffraction probe. The enhanced order of the framework can be generally ascribed to interaction of the silver guests with the host alkyne and thioether functions, while the invisible guest represents a new phenomenon in the metalation of open framework materials. Thermal treatment (450 °C) partially graphitizes the contiguous alkyne units of the Zr(IV)-carboxyl net to form a porous covalent solid featuring: high electrical conductivity (1.0 S/cm), the rare Ag₃Sb nanoparticle deposit, and cyclable heterogeneous catalysis for making the industrially important azo compounds.

Date of Award	26 Aug 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhengtao XU (Supervisor)