Functional π-Conjugated Two-Dimensional Covalent Organic Frameworks

Fingerprints of pi-conjugated compounds are ubiquitous in nature and play a crucial part in human existence. For instance, cis-retinal, an endogenous H-conjugated molecule present in the eye, performs a vital role in the function of visual perception. pi-Conjugated molecules have also received a great deal of attention owing to their intriguing optical properties and created a surge in optoelectronics. Varieties of pi-conjugated molecules/oligomers have been developed and explored for a number of applications such as organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and sensors, among others. While the extended pi-delocalization in one-dimensional (1D) polymers versus oligomers produce superior optical and electronic properties, further extension of pi-delocalization to the second dimension (2D) is expected to give rise even more intriguing properties as revealed by theoretical studies. As a matter of fact, graphene is the best example of 2D-conjugated polymers, but its zero-band-gap behavior is a major impediment for semiconducting applications. In contrast, it was challenging to prepare 2D crystalline polymers until the discovery of boroxine/boronate ester linked covalent organic frameworks (COFs) by Yaghi and co-workers. COFs are a new class of porous crystalline polymers in which organic building blocks are held together by covalent bonds. These polymers exhibit potential applications in gas storage, energy storage, photocatalyst, heterogeneous catalysis, sensors, etc. However, the first pi-conjugated COF was realized in 2009 via the introduction of imine linker (-C=N-) between the building blocks. Since then, wide varieties of COFs with various pi-delocalization promoting spacers have been developed and explored their electronic and optical properties and pertinent applications. In this review, we will highlight the importance of 2D pi-conjugated COFs and their achievements in developing novel functionalities.