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This cumulative dissertation is a compilation of eight peer-reviewed, published scientific papers on the subject of two microscopic techniques of diffusion measurement, namely infrared (IR) microimaging and pulsed field gradient (PFG) NMR. The dissertation contains mainly five chapters. The first chapter introduces diffusion phenomena in general and concisely explains the importance and the current challenges of the investigation of molecular diffusion in nanoporous materials, which are the primary motivations behind the present work. To rise the challenges, it proposes an option of employing IR microimaging in parallel with PFG NMR in the measurement of the molecular diffusion. The second chapter describes the basic principles of the two diffusion measurement techniques and what they are capable of. Chapters 3 and 4 deliver convincing demonstrations of their applicability and potential in diffusion studies. Lastly, Chapter 5 concludes the present work by discussing complementary benefits of the two techniques, along with the novel application of the two-region model for assessing mass transfer in hierarchically porous materials.:Table of Contents CHAPTER 1. Introduction CHAPTER 2. Basics of diffusion measurement techniques 2.1. Introduction to infrared microscopy (IRM) 2.1.1. Working principle 2.1.2. Experimental setup 2.2. Introduction to pulsed field gradient nuclear magnetic resonance (PFG NMR) 2.2.1. Self-diffusion and propagator 2.2.2. Theory of PFG NMR CHAPTER 3. Applicability and potential of IRM • Publication 3.1. Anomaly in the chain length dependence of n-alkane diffusion in ZIF 4 metal-organic frameworks • Publication 3.2. Metal-organic framework Co-MOF-74-based host-guest composites for resistive gas sensing • Publication 3.3. Revealing the transient concentration of CO2 in a mixed-matrix membrane by IR microimaging and molecular modeling • Publication 3.4. IR microimaging of direction-dependent uptake in MFI-type crystals CHAPTER 4. Importance of PFG NMR in diffusion studies • Publication 4.1. NMR diffusometry with guest molecules in nanoporous materials • Publication 4.2. Structural characterisation of hierarchically porous silica monolith by NMR cryo-porometry and -diffusometry CHAPTER 5. Complementary benefits of IR microimaging and PFG NMR • Publication 5.1. Diffusion in nanopores: correlating experimental findings with 'first-principles' predictions • Publication 5.2. Diffusion analysis in pore hierarchies by the two-region model Bibliography Appendix A. Supporting information Appendix B. Author contributions

With reference to these advantages, the poster goes the other way round and identifies a couple of similarities where, on looking at molecular diffusion in nanoporous materials, one is able to recognize features of spreading, which may occur in quite different fields of research. The examples presented include (i) considering molecular uptake and release with nanoporous particles as a model for, respectively, occupation of a habitat by a new species and, vice versa, for the loss of a species in this habitat [2], (ii) the effect of additional highways on overall mass transfer [3,4], (iii) transport impediment (and enhancement!) by diffusant interference [5], (iv) invader-induced changes in the host system [2] and (v) host-induced changes of the invaders [6].

Increasing demands in thefield of sensing, especially for gas detectionapplications, require new approaches to chemical sensors. Metal−organic frameworks (MOFs) can play a decisive role owing to their outstanding performances regardinggas selectivity and sensitivity. The tetrathiafulvalene (TTF)-infiltrated MOF, Co-MOF-74, has been prepared following the host−guest concept and evaluated inresistive gas sensing. The Co-MOF-74-TTF crystal morphology has beencharacterized via X-ray diffraction and scanning electron microscopy, while thesuccessful incorporation of TTF into the MOF has been validated via X-rayphotoemission spectroscopy, thermogravimetric analysis, UV/vis, infrared (IR), andRaman investigations. We demonstrate a reduced yet ample uptake of CO2in thepores of the new material by IR imaging and adsorption isotherms. Thenanocomposite Co-MOF-74-TTF exhibits an increased electrical conductivity incomparison to Co-MOF-74 which can be influenced by gas adsorption from asurrounding atmosphere. This effect could be used for gas sensing.

Through IR microimaging the spatially and temporally resolved development of the CO2 concentration in a ZIF‐8@6FDA‐DAM mixed matrix membrane was visualized during transient adsorption. By recording the evolution of the CO2 concentration, it is observed that the CO2 molecules propagate from the ZIF‐8 filler, which acts as a transport "highway", towards the surrounding polymer. A high‐CO2‐concentration layer is formed at the MOF/polymer interface, which becomes more pronounced at higher CO2 gas pressures. A microscopic explanation of the origins of this phenomenon is suggested by means of molecular modeling. By applying a computational methodology combining quantum and force‐field based calculations, the formation of microvoids at the MOF/polymer interface is predicted. Grand Canonical Monte Carlo simulations further demonstrate that CO2 tends to preferentially reside in these microvoids, which is expected to facilitate CO2 accumulation at the interface.

A systematic NMR cryo-porometry and -diffusometry study using nitrobenzene as a probe liquid is carried out in order to characterise pore structures of hierarchically-organised porous silica monolith possessing mesopores along with a 3D bicontinuous macropore network. The result obtained from NMR cryoporometry shows the presence of a relatively wide mesopore size distribution of 10-35 nm. Furthermore, NMR cryodiffusometry indicates that whilst the mesopores are highly tortuous (Tmeso ≈6), they have little influence on the overall tortuosity of the material (Tmacro ≈1.5), which is largely dominated by the macropores (Toverall ≈1.7).