The role of lattice vibration in the terahertz region for proton conduction in 2D metal–organic frameworks† † Electronic supplementary information (ESI) available. See DOI: 10.1039/c9sc05757a
ABSTRACT: We studied the relationship between proton conductivity and the terahertz-regime vibrations of two-dimensional MOFs. The results of spectroscopy studies clarified the essential role played by the collective motions in the terahertz region in 2D layers for efficient H+ conduction. Ab initio calculations suggested the collective motion to be predominantly determined by the valence electronic structure, depending on the identity of the metal ion. Terahertz-regime vibrations of 2D MOFs dominate the intrinsic proton conductivity, and the motions depend on the valence electronic structures.
Project description:Biological polymers are expected to exhibit functionally relevant, global, and subglobal collective modes in the terahertz (THz) frequency range (i.e., picosecond timescale). In an effort to monitor these collective motions, we have experimentally determined the absorption spectrum of solvated bovine serum albumin (BSA) from 0.3 to 3.72 THz (10-124 cm(-1)). We successfully extract the terahertz molar absorption of the solvated BSA from the much stronger attenuation of water and observe in the solvated protein a dense, overlapping spectrum of vibrational modes that increases monotonically with increasing frequency. We see no evidence of distinct, strong, spectral features, suggesting that no specific collective vibrations dominate the protein's spectrum of motions, consistent with the predictions of molecular dynamics simulations and normal mode analyses of a range of small proteins. The shape of the observed spectrum resembles the ideal quadratic spectral density expected for a disordered ionic solid, indicating that the terahertz normal mode density of the solvated BSA may be modeled, to first order, as that of a three-dimensional elastic nanoparticle with an aperiodic charge distribution. Nevertheless, there are important detailed departures from that of a disordered inorganic solid or the normal mode densities predicted for several smaller proteins. These departures are presumably the spectral features arising from the unique molecular details of the solvated BSA. The techniques used here and measurements have the potential to experimentally confront theoretical calculations on a frequency scale that is important for macromolecular motions in a biologically relevant water environment.
Project description:Several years ago, strong coupling between electronic molecular transitions and photonic structures was shown to modify the electronic landscape of the molecules and affect their chemical behavior. Since then, this concept has evolved into a new field known as polaritonic chemistry. An important ingredient in the progress of this field was the demonstration of strong coupling with intra-molecular vibrations, which enabled the modification of processes occurring at the electronic ground-state. Here we demonstrate strong coupling with collective, inter-molecular vibrations occurring in organic materials in the low-terahertz region ([Formula: see text]10<sup>12</sup>?Hz). Using a cavity filled with ?-lactose molecules, we measure the temporal evolution and observe coherent Rabi oscillations, corresponding to a splitting of 68?GHz. These results take strong coupling into a new class of materials and processes, including skeletal polymer motions, protein dynamics, metal organic frameworks and other materials, in which collective, spatially extended degrees of freedom participate in the dynamics.
Project description:The interaction between intramolecular and intermolecular degrees of freedom in liquid water underlies fundamental chemical and physical phenomena such as energy dissipation and proton transfer. Yet, it has been challenging to elucidate the coupling between these different types of modes. Here, we report on the direct observation and quantification of the coupling between intermolecular and intramolecular coordinates using two-dimensional, ultra-broadband, terahertz-infrared-visible (2D TIRV) spectroscopy and molecular dynamics calculations. Our study reveals strong coupling of the O-H stretch vibration, independent of the degree of delocalization of this high-frequency mode, to low-frequency intermolecular motions over a wide frequency range from 50 to 250?cm<sup>-1</sup>, corresponding to both the intermolecular hydrogen bond bending (??60?cm<sup>-1</sup>) and stretching (??180?cm<sup>-1</sup>) modes. Our results provide mechanistic insights into the coupling of the O-H stretch vibration to collective, delocalized intermolecular modes.
Project description:Two-dimensional (2D) ?-conjugated metal-organic frameworks (?MOFs) are a new class of designer electronic materials that are porous and tunable through the constituent organic molecules and choice of metal ions. Unlike typical MOFs, 2D ?MOFs exhibit high conductivity mediated by delocalized ?-electrons and have promising applications in a range of electrical devices as well as exotic physical properties. Here, we develop a growth method that generates single-crystal plates with lateral dimensions exceeding 10 ?m, orders of magnitude bigger than previous methods. Synthesis of large single crystals eliminates a significant impediment to the fundamental characterization of the materials, allowing determination of the intrinsic conductivity and mobility along the 2D plane of ?MOFs. A representative 2D ?MOF, Ni-CAT-1, exhibits a conductivity of up to 2 S/cm, and Hall measurement reveals the origin of the high conductivity. Characterization of crystalline 2D ?MOFs creates the foundation for developing electronic applications of this promising and highly diverse class of materials.
Project description:Our study of tunnelling in proton-coupled electron transfer (PCET) oxidation of ascorbate with hexacyanoferrate(III) follows the insights obtained from ultrafast 2D IR spectroscopy and theoretical studies of the vibrational water dynamics that led to the proposal of the involvement of collective intermolecular excitonic vibrational water dynamics in aqueous chemistry. To test the proposal, the hydrogen tunnelling modulation observed in the PCET reaction studied in the presence of low concentrations of various partial hydrophobic solutes in the water reaction system has been analyzed in terms of the proposed involvement of the collective intermolecular vibrational water dynamics in activation process in the case. The strongly linear correlation between common tunnelling signatures, isotopic values of Arrhenius prefactor ratios ln AH/AD and isotopic differences in activation enthalpies ??H‡ (H,D) observed in the process in fairly diluted water solutions containing various partial hydrophobic solutes (such as dioxane, acetonitrile, ethanol, and quaternary ammonium ions) points to the common physical origin of the phenomenon in all the cases. It is suggested that the phenomenon can be rooted in an interplay of delocalized collective intermolecular vibrational dynamics of water correlated with vibrations of the coupled transition configuration, where the donor-acceptor oscillations, the motions being to some degree along the reaction coordinate, lead to modulation of hydrogen tunnelling in the reaction.
Project description:Nearly all protein functions require structural change, such as enzymes clamping onto substrates, and ion channels opening and closing. These motions are a target for possible new therapies; however, the control mechanisms are under debate. Calculations have indicated protein vibrations enable structural change. However, previous measurements found these vibrations only weakly depend on the functional state. By using the novel technique of anisotropic terahertz microscopy, we find that there is a dramatic change to the vibrational directionality with inhibitor binding to lysozyme, whereas the vibrational energy distribution, as measured by neutron inelastic scattering, is only slightly altered. The anisotropic terahertz measurements provide unique access to the directionality of the intramolecular vibrations, and immediately resolve the inconsistency between calculations and previous measurements, which were only sensitive to the energy distribution. The biological importance of the vibrational directions versus the energy distribution is revealed by our calculations comparing wild-type lysozyme with a higher catalytic rate double deletion mutant. The vibrational energy distribution is identical, but the more efficient mutant shows an obvious reorientation of motions. These results show that it is essential to characterize the directionality of motion to understand and control protein dynamics to optimize or inhibit function.
Project description:Crystalline, electrically conductive, and intrinsically porous materials are rare. Layered two-dimensional (2D) metal-organic frameworks (MOFs) break this trend. They are porous crystals that exhibit high electrical conductivity and are novel platforms for studying fundamentals of electricity and magnetism in two dimensions. Despite demonstrated applications, electrical transport in these remains poorly understood because of a lack of single crystal studies. Here, studies of single crystals of two 2D MOFs, Ni3(HITP)2 and Cu3(HHTP)2, uncover critical insights into their structure and transport. Conductivity measurements down to 0.3 K suggest metallicity for mesoscopic single crystals of Ni3(HITP)2, which contrasts with apparent activated conductivity for polycrystalline films. Microscopy studies further reveal that these MOFs are not isostructural as previously reported. Notably, single rods exhibit conductivities up to 150 S/cm, which persist even after prolonged exposure to ambient conditions. These single crystal studies confirm that 2D MOFs hold promise as molecularly tunable platforms for fundamental science and applications where porosity and conductivity are critical.
Project description:We studied the low-frequency terahertz spectroscopy of two photoactive protein systems, rhodopsin and bacteriorhodopsin, as a means to characterize collective low-frequency motions in helical transmembrane proteins. From this work, we found that the nature of the vibrational motions activated by terahertz radiation is surprisingly similar between these two structurally similar proteins. Specifically, at the lowest frequencies probed, the cytoplasmic loop regions of the proteins are highly active; and at the higher terahertz frequencies studied, the extracellular loop regions of the protein systems become vibrationally activated. In the case of bacteriorhodopsin, the calculated terahertz spectra are compared with the experimental terahertz signature. This work illustrates the importance of terahertz spectroscopy to identify vibrational degrees of freedom which correlate to known conformational changes in these proteins.
Project description:2D metal organic frameworks (MOFs) with layered structure and much exposed atoms on the surface are expected to be promising electrode materials for hybrid supercapacitors. However, the insulating character strongly hinders their further applications. Herein, we propose a novel MOF//MOF strategy to enhance 2D MOF's conductivity, by which two kinds of 2D MOFs with specific functions are concurrently incorporated into one homogeneous layered MOF with enhanced conductivity and electrochemical performance. The synthesized Ni//Cu MOF shows a triple high capacitance of 1,424 Fg-1 and excellent rate capability compared with the pristine Ni MOF. A hybrid supercapacitor is thereof fabricated, which can provide a maximum energy density of 57 Wh kg-1 and a maximum power density of 48,000 W kg-1. These results not only demonstrate that our strategy can effectively boost the conductivity and redox activity but also pave new routes to synthesize new MOFs for various applications.
Project description:Collective dynamics are considered to be one of the major properties of soft materials, including biological macromolecules. We present coherent neutron scattering studies of the low-frequency vibrations, the so-called boson peak, in fully deuterated green fluorescent protein (GFP). Our analysis revealed unexpectedly low coherence of the atomic motions in GFP. This result implies a low amount of in-phase collective motion of the secondary structural units contributing to the boson peak vibrations and fast conformational fluctuations on the picosecond timescale. These observations are in contrast to earlier studies of polymers and glass-forming systems, and suggest that random or out-of-phase motions of the ?-strands contribute greater than two-thirds of the intensity to the low-frequency vibrational spectra of GFP.