ABSTRACT
Metal-organic frameworks (MOFs) draw increasing attention as nanoenvironments for chemical reactions, especially in the field of catalysis. Knowing the specifics of MOF cavities is decisive in many of these cases; yet, obtaining them in situ remains very challenging. We report the first direct assessment of the apparent polarity and solvent organization inside MOF cavities using a dedicated structurally flexible spin probe. A stable ß-phosphorylated nitroxide radical was incorporated into the cavities of a prospective MOF ZIF-8 in trace amounts. The spectroscopic properties of this probe depend on local polarity, structuredness, stiffness and cohesive pressure and can be precisely monitored by Electron Paramagnetic Resonance (EPR) spectroscopy. Using this approach, we have demonstrated experimentally that the cavities of bare ZIF-8 are sensed by guest molecules as highly non-polar inside. When various alcohols fill the cavities, remarkable self-organization of solvent molecules is observed leading to a higher apparent polarity in MOFs compared to the corresponding bulk alcohols. Accounting for such nanoorganization phenomena can be crucial for optimization of chemical reactions in MOFs, and the proposed methodology provides unique routes to study MOF cavities inside in situ, thus aiding in their various applications.
ABSTRACT
Understanding the heterogeneous nano/microscopic structures of various organic glasses is fundamental and necessary for many applications. Recently, unusual structural phenomena have been observed experimentally in various organic glasses near their glass transition temperatures (Tg), including dibutyl phthalate (DBP). In particular, the librational motion of radical probe in the glass is progressively suppressed upon temperature increase. In this work, we report in-depth molecular dynamics studies of structural anomalies in DBP glass, that revealed insights into the general mechanism of these phenomena. In particular, we have evidenced that the two types of solvation within alkyl chains coexist, allowing only small-angle wobbling of the solute molecule (TEMPO radical), and another favouring large-angle rotations. The former solvation assumes constrained location of the solute near carboxyl groups of DBP, while the latter is coupled to the concerted movement of butyl chains. Remarkably, excellent qualitative and quantitative agreement with previous experimental results were obtained. As such, we are certain that the above-mentioned dynamic phenomena explain the intriguing structural anomalies observed in DBP and some other glasses in the vicinity of Tg.
