Mass transport of molecular compounds through porous solids is a decisive step in numerous, important applications like chromatography or heterogeneous catalysis. It is a multi-scale, hierarchical phenomenon: macrodiffusion (>μm) is influenced, in addition to parameters like grain boundaries and particle packing, by meso-scale (>10 nm, <μm) factors like particle size and the connectivity of pores. More importantly, meso-scale diffusion and macro-scale diffusion are first and foremost determined directly by processes on the molecular scale (<10 nm), which depend on numerous factors like pore-size, interactions of the host with the solid surfaces and with the solvent. Due to the high complexity of the latter and the fact that current analytical techniques enable only limited insights into solvent-filled pores with sufficient spatial and temporal resolution, the knowledge about the molecular origins of diffusive processes in porous materials is still restricted. The main focus of the current paper is on the development of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy into a tool shedding some new light on molecular diffusion inside mesoporous silica materials differing systematically in pore size and surface functionalities. The advantages of CW-EPR are that its spatial resolution fits ideally to the size of mesopores (2-10 nm), it is fast enough for spotting molecular processes, and any conventional solvent and the porous matrix are EPR silent. Diffusion coefficients have been calculated considering spin exchange occurring from the diffusive collision of radicals, and are compared to complementary analytical techniques like MAS PFG NMR (sensitive for meso-scale) and EPR-imaging (sensitive to macroscale diffusion). Our results show that the choice of surface bound functional groups influences diffusion much stronger than pore-size. There are indications that this is not only due to different guest-surface interactions but also due to an altered mobility within the solvent under confinement.

The existence of more than one functional entity is fundamental for materials, which are desired of fulfilling complementary or succeeding tasks. Whereas it is feasible to make materials with a homogeneous distribution of two different, functional groups, cases are extremely rare exhibiting a smooth transition from one property to the next along a defined distance. We present a new approach leading to high-surface area solids with functional gradients at the microstructural level. Periodically ordered mesoporous organosilicas (PMOs) and aerogel-like monolithic bodies with a maximum density of azide groups were prepared from a novel sol-gel precursor. The controlled and fast conversion of the azide into numerous functions by click chemistry is the prerequisite for the implementation of manifold gradient profiles. Herein we discuss materials with chemical, optical and structural gradients, which are interesting for all applications requiring directionality, for example, chromatography.

We used spatially and time-resolved electron paramagnetic resonance (EPR) spectroscopy to study diffusion of guest molecules within solvent filled aerogel monoliths. We experimentally obtained the time-dependent spin density of EPR active guest molecules ρ 1d (y,t), numerically solved the diffusion equation to simulate ρ 1d (y,t), and determined the macroscopic translational diffusion coefficients for different aerogels and guest molecules. Simultaneously, we determined the microscopic diffusion coefficient by spectral simulation. We show that diffusion in the aerogels under study is dominated by the tortuosity of the pore system but not by surface effects.

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In electron paramagnetic resonance (EPR) distance distributions between site-directedly attached spin labels in soft matter are obtained by measuring their dipole-dipole interaction. The analysis of these distance distributions can be misleading particularly for broad distributions, because the most probable distance deviates from the distance between the most probable label positions. The current manuscript studies this effect using numerically generated spin label positions, molecular dynamics simulations, and experimental data of a model system. An approach involving Rice distributions is proposed to overcome this problem.