(Invited) Integrating Molecules, Surfaces, and Devices to Achieve Emergent Properties in Metal-Organic Frameworks
Authors: Allendorf Mark D. , Foster Michael E. , Léonard François , Stavila Vitalie , Talin A. Alec , Fischer Roland A. 
Published: February 2020

Metal-Organic Frameworks (MOFs) are a rapidly expanding category of crystalline materials comprised of organic electron donors linked to metal cations, creating a rigid structure in which nanoporosity is preserved upon removal of solvent and other “guest” molecules.1 The hybrid inorganic-organic nature of MOFs is inspiring vigorous exploration of their use for applications that traditionally employ porous materials, such as gas storage, separations, and catalysis. However, it has been shown recently that combining MOFs with other materials or infiltrating them with “non-innocent” guest molecules can lead to unexpected properties or functionalities, such as enhanced light absorption, charge mobility, and strain-based transduction mechanisms for chemical sensing.2 This presentation will describe examples in which the integration of MOFs with other materials, either by infiltrating with them with carefully selected guest molecules or by creating heterostructures on surfaces of various types, induces emergent properties. Synthesis of novel MOFs and guest-infiltrated MOFs will be described, coupled with detailed characterization and complementary modeling investigations to elucidate details of the mechanisms leading to emergent properties.
The use of non-innocent guest molecules as a component of MOF design has not been actively investigated. Recent reports show, however, that the presence of guest molecules can lead to dramatic effects, such as increased electrical conductivity or ferroelectric behavior. For example, we demonstrated that the copper-containing MOF known as HKUST-1, when infiltrated with the redox-active molecules TCNQ (TCNQ = 7,7,8,8-tetracyanoquinododimethane) exhibits electrical conductivity that is 107 times higher than the uninfiltrated material.3 This is a p-type material that also displays thermoelectric behavior with a higher Seebeck coefficient than bismuth telluride.4 The advantages of using guests as a design element include the ease of introducing new functionalities, the ability to modify the material properties at will by removing the guest or inserting different ones, and eliminating the need to synthesize frameworks de novo to achieve specific properties, which can be challenging even when the basic topology remains constant. Guest molecules can also be used to enhance the absorption of light. The organic linker molecules in many MOF structures do not absorb strongly in the visible. However, this can be modified by infiltrating with guest molecules such as organic dyes. This concept can be expanded by using multiple guests to enable resonant energy transfer cascades that both harvest light and shift it to other wavelength regions. We demonstrated the utility of this for both exciton capture2a
,
5 and dye-sensitized solar cells.6 Together, these promising results suggest that MOFs can be active components of electronic devices, opening a new application space for these versatile materials.
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