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

Abstract

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 μ2-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, μ2-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 fluctuati...