Strategies for Controlling Through-Space Charge Transport in Metal-Organic Frameworks via Structural Modifications.

Christian Winkler,Egbert Zojer

doi:10.3390/nano10122372

Abstract

In recent years, charge transport in metal-organic frameworks (MOFs) has shifted into the focus of scientific research. In this context, systems with efficient through-space charge transport pathways resulting from π-stacked conjugated linkers are of particular interest. In the current manuscript, we use density functional theory-based simulations to provide a detailed understanding of such MOFs, which, in the present case, are derived from the prototypical Zn2(TTFTB) system (with TTFTB4− corresponding to tetrathiafulvalene tetrabenzoate). In particular, we show that factors such as the relative arrangement of neighboring linkers and the details of the structural conformations of the individual building blocks have a profound impact on bandwidths and charge transfer. Considering the helical stacking of individual tetrathiafulvalene (TTF) molecules around a screw axis as the dominant symmetry element in Zn2(TTFTB)-derived materials, the focus, here, is primarily on the impact of the relative rotation of neighboring molecules. Not unexpectedly, changing the stacking distance in the helix also plays a distinct role, especially for structures which display large electronic couplings to start with. The presented results provide guidelines for achieving structures with improved electronic couplings. It is, however, also shown that structural defects (especially missing linkers) provide major obstacles to charge transport in the studied, essentially one-dimensional systems. This suggests that especially the sample quality is a decisive factor for ensuring efficient through-space charge transport in MOFs comprising stacked π-systems.

Highlights

Metal-organic frameworks (MOFs) are porous, highly crystalline solids consisting of inorganic nodes connected by organic linkers [1,2,3]
We use density functional theory-based simulations to provide a detailed understanding of such metal-organic frameworks (MOFs), which, in the present case, are derived from the prototypical Zn2(TTFTB) system
We will focus on the latter approach, where a large overlap of neighboring π-electron systems can result in the formation of bands displaying a large dispersion and generating through-space charge transport pathways

Summary

Introduction

Metal-organic frameworks (MOFs) are porous, highly crystalline solids consisting of inorganic nodes connected by organic linkers [1,2,3] They have been investigated extensively for various applications in fields such as gas storage, [4,5,6] catalysis, [7,8,9], and gas separation [10,11]. The electronic (and the optical) properties of a solid are determined (in a first approximation) by its electronic band structure, where MOFs usually show rather flat bands [27] This is a consequence of the commonly observed weak hybridization between states localized on the organic linkers and states localized on the inorganic secondary building units. We will focus on the latter approach, where a large overlap of neighboring π-electron systems can result in the formation of bands displaying a large dispersion and generating through-space charge transport pathways

Methods

Results

Conclusion