Dynamic 2D covalent organic frameworks

• Applicant: Dr. Florian Auras
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Preparatory and Physical Chemistry of Polymers
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525243720

Covalent organic frameworks (COFs) are two-dimensional (layered) or three-dimensional crystalline, porous polymer networks with potential applications in gas storage and separation, photocatalysis, and optoelectronics. While large efforts have been undertaken to optimise their crystallinity and stability, strategies to equip COFs with a controlled degree of structural flexibility have remained underexplored, especially for 2D COFs. This project will develop dynamic 2D COFs – partially flexible COFs that can be switched reversibly between two (or more) crystalline phases of different unit cell parameters and porosity without breaking chemical bonds. These dynamic phase transformations will be triggered by external stimuli such as the uptake or removal of gas or solvent molecules. The COF design will focus on balancing flexibility and stability by employing a combination of rigid and flexible elements. This will be realised with a “wine rack” layout where rigid 1D pillars are interconnected in two dimensions via flexible bridges. The rigid pillars will be formed by strongly π-stacked columns of aromatic building blocks such as perylene diimides (PDIs). The key change in the proposed COF design compared to reported non-flexible COFs will be to introduce small gaps in the framework to achieve a controlled reduction of the interactions between adjacent bridge units – just enough to allow them to bend and slide at low activation energies, but not too flexible to avoid any uncontrolled collapse of the frameworks. This goal will be achieved by two approaches: In the first approach, a substantial lateral offset between adjacent COF layers will be introduced to engineer the bridge-to-bridge interactions. The second approach will focus on controlling the distance of the COF layers along the stacking direction via the introduction of co-crystallised interlayers. Opening or contraction of the COF pores can significantly change the distance between parts of the frameworks and thus alter their electronic coupling. This provides a unique handle for introducing stimuli-responsive optical, electronic, and magnetic properties. To demonstrate this potential, the project will focus on controlling and manipulating the coupling between unpaired spins in the framework. In a first step, the long-range ordering of chemically generated radical spins will be investigated. This will enable COFs with switchable magnetism, e.g., COFs that are paramagnetic when the pores are open but ferromagnetic when the pores are contracted. In a second step, the geometry-dependent interactions between photogenerated high-spin states such as spin triplets will be investigated. This will deepen the fundamental understanding of how spins interact in highly ordered fully organic frameworks and explore the particular strengths of the new dynamic 2D COFs for studying quantum phenomena, e.g., for potential future applications in quantum information technology.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professorin Dr. Karena Chapman