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

Abstract

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 Förster vibrational excitation transfer.2D IR spectroscopy | metal-organic framework | UiO-66 MOF | ultrafast structural fluctuations | solvent confinement effect M etal-organic frameworks (MOFs) are molecular architectures in which metal clusters are connected by organic linkers to yield relatively regular 3D coordination polymers with nanometer-sized pores (1, 2). MOFs have been investigated for a wide variety of chemical applications, such as adsorption of gases (3) and heterogeneous catalysts (4). Among many types of porous materials, MOFs are unique because of their structural elasticity coexisting with a high degree of spatial regularity. In some sense, MOFs are like crystals and polymers. They have relatively regular structures like a crystal, but the metal clusters are joined by organic linkers that, in some systems, produce significant structural mobility. The elasticity of MOFs is intimately related to their physical properties and behavior (5).An important goal for understanding the nature of MOFs and how their chemical composition influences their properties and applications is to develop and apply an experimental method that can measure the ultrafast structural motions of MOFs. The relatively slow motions of the framework occurring in submicrosecond to millisecond scales have been studied by NMR and neutron scattering (6). However, until now, quantitative measurements that can characterize the time dependence of MOF structural motions in ultrafast regimes have not been possible because of the lack of appropriate techniques. Here, we have accomplished the goal by applying ultrafast 2D IR spectroscopy (7) to the study of MOF structural dynamics. 2D IR is akin to 2D NMR, but it operates on the ultrafast timescales necessary to characterize the time dependence of MOF structural motions. In addition, there is the important question of the effects on MOF dynamics when guest molecules fill the MOF nanopores. The guest molecules will affect the structural fluctuations of the framework by interacting with the linkers and the metal units. Furthermore, because of the confinement of guest molecules in MOF nanopores, the dynamics of these molecules are expected to be very different from t...

Distinguishing Population and Coherence Transfer Pathways in a Metal Dicarbonyl Complex Using Pulse-Shaped Two-Dimensional Infrared Spectroscopy

“…Fayer and coworkers used a similar approach to reduce QB oscillations in their analysis of center line slope changes in 2DIR spectra of a metal-organic framework. 21 We control the contributions to our spectra by shaping the spectrum of the pump pulses so that only one transition can be resonant with our excitation. The interactions leading to QB and double-CT always have their first and second interactions with different transitions, but this type of interaction cannot occur in our pulse-shaped experiment.…”

Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational Spectroscopy

“…71,75,[83][84][85][86]138,[275][276][277][278][279][280] More recently, a couple of works have been presented, in which properties and dynamics of molecules immobilized on surfaces have been analyzed in a similar manner. 23,50,133,134,147,148,173,174,[181][182][183]185,263,269,274,281,282 It is of pivotal importance to elucidate the differences in molecular properties at interfaces compared to solution and relate them to the distinct properties of an interface. Here we present a first overview of recent examples from surface-sensitive and surface-specific 2D vibrational spectroscopy, which will then be discussed in more detail in the upcoming sections.…”

“…2D IR is ideal to study delocalized vibrations in closely-packed molecular MLs 121,249,286,287 , and to investigate the effect of coupling in dependence of surface properties. Vibrational coupling between chromophores has for instance been identified in dye-sensitized systems 286,287 resonance energy transfer 92,281 or diffusive and ballistic transfer over sub-nm distances [288][289][290] .…”

Smart Metal‐Organic Frameworks (MOFs): Switching Gas Permeation through MOF Membranes by External Stimuli

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