Tuning the electrostatic energy landscape within the pores of covalent organic frameworks: post-synthetic modification reactions and structural imperfections

Abstract

It is well established that collective electrostatic effects resulting from the periodic arrangement of polar entities determine the energy-level alignment at conventional, 2D extended interfaces by introducing a discontinuity in the electrostatic energy. This suggests that exploiting the assemblies of (di)polar entities should be a useful way for tuning the electronic properties of materials. While realizing such structures has turned out to be a formidable challenge in most cases, assemblies of polar substituents can be elegantly realized in the pore walls of covalent organic frameworks. In the current manuscript state-of-the art dispersion-corrected density functional theory simulations supported by electrostatic considerations are employed to show (i) how polar groups decorating the pores present in stacked 2D COFs can be used to control the electrostatic energy within the pores, (ii) how variations in the stacking motifs impact the obtained results, (iii) what role localized chemical defects play in the described effects, and (iv) to what extent post-synthetic modification reactions described in the literature for the studied COFs bear the potential of an a posteriori tuning of the electrostatic energy. Notably, in the latter case additional complications arise from the bulky, yet flexible nature of the substituents produced in the said reactions. Additionally, it is suggested how heterostructures of differently substituted groups of COF layers could be used to spatially modulate the energy landscape within pores, providing a vision for realizing materials in which any type of charge-transfer reaction can be confined to specific regions and in which the localization of differently charged ionic species can be controlled.