Structural dynamics inside a functionalized metal–organic framework probed by ultrafast 2D IR spectroscopy

• Published in: Proceedings of the National Academy of Sciences - PNAS 2014-12, Vol.111 (52), p.18442-18447
• Main Authors: Nishida, Jun, Tamimi, Amr, Fei, Honghan, Pullen, Sonja, Ott, Sascha, Cohen, Seth M., Fayer, Michael D.
• Format: Article
• Language: English
• Subjects: 2D IR spectroscopy; Anisotropy; Deformation; Dynamic structural analysis; Infrared radiation; Infrared spectroscopy; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Materials elasticity; metal-organic framework; Molecular evolution; Molecules; Oscillators; Physical Sciences; Pumps; Scattering; solvent confinement effect; Solvents; Spectrum analysis; Time constants; UiO-66 MOF; ultrafast structural fluctuations; Vibration analysis; Vibrational spectra
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1422194112

The structural elasticity of metal–organic frameworks (MOFs) is a key property for their functionality. Here, we show that 2D IR spectroscopy with pulse-shaping techniques can probe the ultrafast structural fluctuations of MOFs. 2D IR data, obtained from a vibrational probe attached to the linkers of UiO-66 MOF in low concentration, revealed that the structural fluctuations have time constants of 7 and 670 ps with no solvent. Filling the MOF pores with dimethylformamide (DMF) slows the structural fluctuations by reducing the ability of the MOF to undergo deformations, and the dynamics of the DMF molecules are also greatly restricted. Methodology advances were required to remove the severe light scattering caused by the macroscopic-sized MOF particles, eliminate interfering oscillatory components from the 2D IR data, and address FÅöörster vibrational excitation transfer. Significance A unique aspect of metal–organic frameworks (MOFs) is their structural flexibility coexisting with a degree of regularity. Adsorbed guest molecules can cause MOF pore shapes to deform. Pore shape changes may be related to the high capacity and selectivity of the MOFs for gas adsorption and other processes. MOF flexibility and other properties are influenced by fast dynamics of the framework. Direct measurements to characterize fast motions of the MOFs have not been applied previously. We show that 2D IR spectroscopy can be performed on functionalized MOFs. We use 2D IR and other ultrafast IR methods to elucidate the timescales for ultrafast structural fluctuations and how they are influenced by a solvent filling the pores.