Hydrogen Bonding Rylene Diimides for Electron Transfer in Framework Materials and Flow Batteries

Abstract

Supramolecular chemistry can be defined as the assembly of multiple molecular components that are generally held together by weak (non-covalent) forces. By understanding how these weak intermolecular forces work and designing a suitable building block (molecule), researchers can control these weak bindings and direct the building blocks to assemble in an architecture that helps to form new materials with diverse properties. Hydrogen-bonded organic frameworks (HOFs) are crystalline materials that self-assemble primarily through hydrogen bonds. H-bonding interactions are characterized by their weak, flexible, directional, and reversible nature. This thesis explores supramolecular chemistry, with a particular focus on hydrogen-bonded organic frameworks. A key objective of this thesis is to design H-bond building blocks with interesting optical and electronic properties for the construction of charge-assisted HOFs. In order to achieve this goal, a member of the rylene diimide family (naphthaline diimide (NDI) or perylene diimides (PDIs)) was used as a tecton.

A charge-assisted HOF was constructed using NDI substituted with isophthalate moieties as an anion and a bisamidinium molecule as a cation. Families of porous HOF materials can be formed, either as two-dimensional (2D) frameworks (HOFs2 and HOFs3) or as three dimensional (3D) frameworks (HOFs1), with crystallisation conditions, in particular the pH of the crystallisation solution, playing a large part on the crystallisation product. The optical and electronic properties of HOFs3 are of particular interest. HOFs3 exhibit distinct photosensitive properties owing to the photoinduced radicals generated by NDIs. A UV-Vis study has confirmed that HOFs3 exhibits unique absorbance properties since it can absorb light in both the UV and visible regions. Powder X-ray diffraction analysis has also confirmed that this system forms a stable framework even after vacuum activation. In this study, it was found that carboxylate⋯amidinum interactions provide a robust framework that is further supported by non-covalent interactions (𝜋 − 𝜋 stacking) that are derived from the extended aromatic backbone of NDI. In the same manner, charge-assisted HOFs were also formed using perylene dihydride as the hydrogen bond acceptor, which resulted in robust porous HOFs. Through its extended aromatic backbone, PDI provides further support for this robustness through non-covalent interactions (𝜋 − 𝜋 stacking).

The chemistry of charge assisted HOFs was further enhanced by using two different anion building blocks with different lengths and flexibility, namely NDI and biphenyl. The two anion components were functionalized with a flexible phosphonate group. Combining biphenyl or NDI anions with bisamidinium produces HOFs5 and HOFs6, respectively, driven by H-bond interactions between phosphonate and bisamidinium. In general, HOFs5 was more readily formed within two days after mixing, whereas HOFs6 was more difficult to produce. When sunlight and warm weather are present, HOFs6 can be formed within a few hours. There was a significant difference between phosphonate and carboxylate H-bond sites in terms of their structural characteristics. Compared to carboxylates, phosphonate groups show densely packed structures and shorter hydrogen bonds, resulting in layered, poreless (or small pore) structures. In addition, HOF derived from phosphonate⋯amidinium interactions exhibit notable stability limitations, including a tendency to undergo phase transformations or structural collapse once the solvent is removed.

A variety of soluble redox active building blocks were synthesized in chapter four. The synthesis of six disubstituted PDIs and the difficult separation of the 1,6- and 1,7-regioisomers has been undertaken. It has been shown that there is little difference between the two isomers regarding physical properties as they have a small difference in the E1/2 values of the two reduction processes, forming a radical anion and dianion, even though they have a significantly different oxidation behaviour. In addition, there is also a difference between the two isomers regarding the compound’s optical properties. The 1,7- PDI isomer shows a very stable redox process, even after 100 cycles. All of these electronic characteristics make these PDIs excellent candidates for use in battery applications. Some of these PDIs are arranged in solid state to form porous HOFs. A variety of scientific and technological fields can benefit from these building blocks in the form of catalysts, energy storage materials, and electronic components.

PDI HOFs hold significant promise as versatile materials with an unusual combination of properties. A combination of their crystalline nature and exceptional electrochemical properties could find application in a variety of fields. These frameworks may be explored in the context of energy storage technologies, where their ordered structure may enhance the efficiency of charge storage and transfer.

Pilar[5]arene with ten methylbenzoate substituent Pill2 was successfully synthesized. A variety of techniques have been used to confirm the synthesis, including MALDI, 1H NMR, and single crystal analysis. Pill2's electron-rich cavity can successfully be utilized to thread an electron-poor imidazolume rod through it. For this large Pill2, an alkane rod with a chain length of 12 carbon atoms was adequate. To prevent the rod from dethreading, 2-(iodomethyl)-1,3,5-trimethylbenzene was sufficient as a stopper group. Simple techniques such as MALDI and 1H NMR can be used to confirm the formation of these mechanically interlocked materials and to investigate Pill2's shuttling behaviour.