Guest Hydrogen Bond Dynamics and Interactions in the Metal-Organic Framework MIL-53(AI) Measured with Ultrafast Infrared Spectroscopy

MIL-53(Al) is among the most "flexible" metal organic frameworks and significantly changes its structure when guest molecules are introduced into the framework's pores. The guest molecules interact with mu(2)-OH, which is a hydroxyl group bridging two aluminums. This interaction is thought to play an important role in the adsorption of molecules into the pores. Here, we report infrared absorption, infrared pump probe, and two-dimensional infrared spectroscopy on a deuterated bridging hydroxyl, mu(2)-OD, in MIL-53(Al) loaded with various guest molecules. Depending on guest, large vibrational frequency shifts, broadening of absorption band, enhancement of absorption coefficient, and acceleration of vibrational relaxation were observed, which are all signatures of hydrogen bonding interactions between the bridging hydroxyls and the guest. 2D IR spectroscopy reveals that the time evolution of the vibrational frequency occurs on multiple time scales: subpicosecond dynamics caused by localized hydrogen bond fluctuations and picosecond dynamics of the framework lattice, followed by significantly slower dynamics likely induced by global reconfigurations of the guest molecules in the pores. When benzonitrile, a strong hydrogen bonding agent, is introduced into the pores, an oscillatory feature was observed in the time evolution of the frequency, which was assigned to underdamped hydrogen bond fluctuations.