Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks

Covalent
organic frameworks (COFs), formed by reversible condensation
of rigid organic building blocks, are crystalline and porous materials
of great potential for catalysis and organic electronics. Particularly
with a view of organic electronics, achieving a maximum degree of
crystallinity and large domain sizes while allowing for a tightly
π-stacked topology would be highly desirable. We present a design
concept that uses the 3D geometry of the building blocks to generate
a lattice of uniquely defined docking sites for the attachment of
consecutive layers, thus allowing us to achieve a greatly improved
degree of order within a given average number of attachment and detachment
cycles during COF growth. Synchronization of the molecular geometry
across several hundred nanometers promotes the growth of highly crystalline
frameworks with unprecedented domain sizes. Spectroscopic data indicate
considerable delocalization of excitations along the π-stacked
columns and the feasibility of donor–acceptor excitations across
the imine bonds. The frameworks developed in this study can serve
as a blueprint for the design of a broad range of tailor-made 2D COFs
with extended π-conjugated building blocks for applications
in photocatalysis and optoelectronics.