The impact of alkyl- and alkoxy-functionalization on the responsive behaviour of metal-organic frameworks: from conventional to frustrated flexibility

Abstract

The investigation of functional materials, which can change their physical properties depending on external triggers is of ongoing interest for various fields of materials research. Metal-organic frameworks (MOFs) are porous coordination polymers constructed of inorganic building units (e.g., metal-ions, metal-oxo-clusters) joined by organic building units (e.g., multidentate carboxylates), also called linkers. A particular subclass of MOFs are flexible MOFs, which undergo structural changes as a function of external stimuli. This behaviour arises from a delicate balance between enthalpic (e.g., dispersion interactions) and entropic (e.g., vibrational motions) contributions, which can be modified and controlled by the introduction of additional functional groups at the organic building unit of the framework. In this work, alkyl-, alkoxy-, and methoxy-alkoxy group functionalization of the organic building unit was utilized to modulate and study the influence of sidechain length and polarity on the structural responsivity of two fundamentally different MOF platforms. Of these, one is intrinsically flexible (DMOF-1), while the other one is structurally rigid (MOF-5). For the DMOF-1-based materials, it has been shown in the past that the implementation of alkoxy groups induces a guest- and temperature-depending switching between contracted and expanded phases. In this work, a series of purely alkyl-functionalized DMOF-1 derivatives was studied and structurally characterized in great detail by means of single crystal and powder X-ray diffraction. Furthermore, their sorption behaviour towards N2, CO2 and C3 and C4 hydrocarbons was investigated and subsequently compared to their alkoxy counterparts, which revealed significant differences that strongly relate to the different polarities of the functional groups. Particular highlights are path-depending multi-step CO2 sorption behaviours and an interesting propane/propylene gating behaviour with potential for an application in the separation of the gases, both phenomena were studied by in-situ X-ray diffraction techniques. Upon thermal treatment, the new alkyl functionalized DMOF-1 derivatives exhibit a significantly softer behaviour than their alkoxy counterparts indicating a much flatter free energy landscape for these materials in connection with weaker intra-framework interactions of the less polar alkyl groups. In the second part of this thesis, the concept of frustrated flexibility of MOFs is introduced. Due to the incompatibility of a rigid, non-responsive MOF structure type (here MOF-5) with intra-framework dispersion forces demanding a densification of the structure a new type of responsive behaviour evolves. Controlled by chemical functionalization of the organic linkers with dispersion energy donating (DED) alkoxy groups, a series of materials is obtained, which reversibly switch between a cubic crystalline and either a non-crystalline or a rhombohedral phase. These transitions are either driven enthalpically through guest adsorption/desorption or by vibrational entropy at elevated temperatures. Importantly, frustratedly flexible behaviour is shown to be tuneable by adjusting the length and polarity of the DED groups. Overall, the results presented herein demonstrate that linker functionalization is a powerful tool to modulate the free energy landscape of MOF materials. For intrinsically flexible MOFs this approach allows targeted fine-tuning of their flexible behaviour, while for rigid MOFs it allows for the generation of completely new and exotic responsive behaviour.