Electrostatic Design of Polar Metal–Organic Framework Thin Films

Abstract

In recent years, optical and electronic properties of metal–organic frameworks (MOFs) have increasingly shifted into the focus of interest of the scientific community. Here, we discuss a strategy for conveniently tuning these properties through electrostatic design. More specifically, based on quantum-mechanical simulations, we suggest an approach for creating a gradient of the electrostatic potential within a MOF thin film, exploiting collective electrostatic effects. With a suitable orientation of polar apical linkers, the resulting non-centrosymmetric packing results in an energy staircase of the frontier electronic states reminiscent of the situation in a pin-photodiode. The observed one dimensional gradient of the electrostatic potential causes a closure of the global energy gap and also shifts core-level energies by an amount equaling the size of the original band gap. The realization of such assemblies could be based on so-called pillared layer MOFs fabricated in an oriented fashion on a solid substrate employing layer by layer growth techniques. In this context, the simulations provide guidelines regarding the design of the polar apical linker molecules that would allow the realization of MOF thin films with the (vast majority of the) molecular dipole moments pointing in the same direction.

Avoiding the Center‐Symmetry Trap: Programmed Assembly of Dipolar Precursors into Porous, Crystalline Molecular Thin Films

“…Such flipping processes are unlikely to be triggered by lateral electrostatic interaction, since DFT calculations as well as electrostatic considerations reveal only small repulsive interactions between adjacent linkers of less than 2 meV. [ 41 ] But, of course, entropic effects come into play, which also favor an up/down ratio of 1.…”

“…Bulk MOF compounds have been fabricated from asymmetric nondipolar subunits to yield NC structures. [ 22,39,40 ] To our knowledge, dipolar subunits have not yet been successfully used to assemble MOFs with a macroscopic electrostatic field, although theoretical predictions [ 41 ] have indicated the huge potential of such assemblies. Since the ability to produce thin films is a prerequisite for fabricating NC layers with a permanent electric field normal to the surface, we initiated a systematic experimental research effort to reach this goal.…”

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering

“… 51 As an example, Figure 10 d shows the gradient of the electrostatic energy that is generated in a MOF in which the apical bipyridine linkers have a dipole moment due to partial fluorination. 50 The key challenge for the experimental realization of such systems is that one must realize an asymmetric bonding of the linkers in order to align their dipoles (see discussion in ref ( 50 ) for details). The first successful steps in this direction have recently been made for 1,4-biphenyldicarboxylate-linked Cu-paddlewheel sheets connected by polar apical linkers, 52 but in these experiments the realized degree of dipole alignment was still rather low.…”

“…Copyright 2020 American Chemical Society (ref ( 48 )). Copyright 2015 (ref ( 49 )) and 2020 (ref ( 50 )) The Authors. Published by Wiley and MDPI under a Creative Commons Attribution 4.0 International (CC BY 4.0) License.…”

“…(d) Evolution of the DFT-calculated electrostatic energy for a MOF thin film consisting of 1,4-benzenedicarboxylate-linked Zn-paddlewheel sheets connected by seven layers of apical, polar (partly fluorinated) bipyridine linkers (see top-right inset). Adapted with permission from refs − . Copyright 2020 American Chemical Society (ref ).…”