Light-promoted synthesis of highly-conjugated crystalline covalent organic framework

Soyoung Kim,Hee Cheul Choi

doi:10.1038/s42004-019-0162-z

Abstract

Low electrical conductivity caused by low degree of π-conjugation and structural disorders induces critical limitations in electronic applications of covalent organic frameworks (COFs). Here we focus on improving the electrical properties of COFs by synthesis of highly conjugated and crystalline COF (hcc-COF). Addition of a small amount of water and acetic acid induces the efficient reversible dynamic imine condensation reaction that is required to obtain uniform and crystalline products. Also, simulated sunlight irradiation facilitates the reversible imine condensation reaction, and achieves formation of hcc-COF with high-yield within short reaction time. The obtained hcc-COF has an extended π-conjugated structure along the lateral direction and an inclined stacking structure. The intrinsic electron transport properties along the in-plane direction are evaluated using a hcc-COF film grown on a water surface from a polarity-controlled precursor solution. To the best of our knowledge, our hcc-COF film shows the highest reported electrical conductivity for highly conjugated organic porous polymers.

Highlights

Low electrical conductivity caused by low degree of π-conjugation and structural disorders induces critical limitations in electronic applications of covalent organic frameworks (COFs)
The highly conjugated and crystalline COF (hcc-COF) was produced by mixing precursors, 1,2,4,5-benzenetetramine (BTA) and hexaketocyclohexane (HCH) in a lab-made quartz vial that is designed to minimize unintended light absorption (Fig. 1a and Supplementary Fig. 1a), dissolving them in methanol/ mesitylene mixed solvent, and adding a small amount of deionized (DI) water and acetic acid to induce an efficient reversible imine condensation reaction
Due to the efficient reversible condensation reaction that was induced by the added acid catalyst and water, our product showed high crystallinity, which is clearly different from previously reported amorphous Aza-CMPs23

Summary

Introduction

Low electrical conductivity caused by low degree of π-conjugation and structural disorders induces critical limitations in electronic applications of covalent organic frameworks (COFs). Because of electron localization and abundant defects generated by the low molecular conjugation of π-electrons and insufficient reversible self-healing process[19,20], COFs have low electric conductivity, so their value in electrical applications is limited To improve their electrical and optical properties, highly conjugated COFs or doped COFs have been suggested[19,21,22]. We reported a light-promoted synthesis of poly-iminebased COF (pi-COF) and prove that photon energy accelerate imine condensation reaction and facilitate transformation from amorphous imine containing precipitates to crystalline COF through fast reversible dynamic imine condensation reaction[17,31] We adapt this synthesis method, which yields hcc-COF at room temperature within 3 h. We synthesize atomically thin hcc-COF 2D film on a water surface; the film has highly conjugated π-electrons along the in-plane direction, so electrons can move freely and the films show outstanding electrical conductivity

Methods

Results

Conclusion