2D covalent organic frameworks (COFs) are the most common type of COFs, though they are structurally less diverse compared to potential 3D structures. The focus on 2D COFs is largely due to the ease of synthesizing their building blocks and the high predictability of their structures, making the design of new materials more straightforward. However, certain building block geometries can still lead to unpredictable outcomes due to polymorphism. In 3D COFs, the challenge lies in the vast number of possible structures (or nets) that can form from a single building block type. In 2D COFs, net polymorphism is primarily seen in the competition between the kagome (kgm) and square lattice (sql) nets when using planar four-connected (4-c) linkers. Despite the synthetic challenges that arise due to polymorphism, it can also be a source of structural versatility and to fine tune the properties of COF materials. We investigate systematic and reliable means of both controlling polymorphic systems to harness the structural versatility for applications, but also design systems to introduce addressable polymporphism into systems that are normally not polymorphic. To better control the polymorphism of kgm vs. sql in 2D COFs, we have recently developed a linker design strategy that uses bulky functional groups to introduce steric hindrance, to bias the formation of the kgm structure over the sql structure [1]. This strategy is not only able to favor the formation of one or the other net, it is strong enough to drive the system to the formation of a 3D polymorph from a normally 2D system. In another system, we have designed linker sizes and geometries to precisely match each other to allow for the formation of dual net of square triangle tilings. By this precise matching, a host of nets become theoretically feasible. We observe evidence of disordered linker mixing in the form of solid solutions and the formation of lattice matched heteroepitaxial interfaces. This linker-matching strategy offers new possibilities for creating structurally complex 2D COFs, potentially enabling the design of materials with tailored properties and topologies.
References:
[1] A. Winter, F. Hamdi, A. Eichhöfer, K. Saalwächter, P. L. Kastritis, F. Haase, Chemical Science 2024, DOI:10.1039/D4SC03461A.
2D covalent organic frameworks (COFs) are the most common type of COFs, though they are structurally less diverse compared to potential 3D structures. The focus on 2D COFs is largely due to the ease of synthesizing their building blocks and the high predictability of their structures, making the design of new materials more straightforward. However, certain building block geometries can still lead to unpredictable outcomes due to polymorphism. In 3D COFs, the challenge lies in the vast number of possible structures (or nets) that can form from a single building block type. In 2D COFs, net polymorphism is primarily seen in the competition between the kagome (kgm) and square lattice (sql) nets when using planar four-connected (4-c) linkers. Despite the synthetic challenges that arise due to polymorphism, it can also be a source of structural versatility and to fine tune the properties of COF materials. We investigate systematic and reliable means of both controlling polymorphic systems to harness the structural versatility for applications, but also design systems to introduce addressable polymporphism into systems that are normally not polymorphic. To better control the polymorphism of kgm vs. sql in 2D COFs, we have recently developed a linker design strategy that uses bulky functional groups to introduce steric hindrance, to bias the formation of the kgm structure over the sql structure [1]. This strategy is not only able to favor the formation of one or the other net, it is strong enough to drive the system to the formation of a 3D polymorph from a normally 2D system. In another system, we have designed linker sizes and geometries to precisely match each other to allow for the formation of dual net of square triangle tilings. By this precise matching, a host of nets become theoretically feasible. We observe evidence of disordered linker mixing in the form of solid solutions and the formation of lattice matched heteroepitaxial interfaces. This linker-matching strategy offers new possibilities for creating structurally complex 2D COFs, potentially enabling the design of materials with tailored properties and topologies.
References:
[1] A. Winter, F. Hamdi, A. Eichhöfer, K. Saalwächter, P. L. Kastritis, F. Haase, Chemical Science 2024, DOI:10.1039/D4SC03461A.
