Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K31: Biomagnetic Chemical Sensing (QIS3)Focus
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Sponsoring Units: DCP Chair: Michael Berman, Air Force Office of Science Research Room: BCEC 203 |
Wednesday, March 6, 2019 8:00AM - 8:36AM |
K31.00001: Quantum Biochemical Compasses Invited Speaker: Christiane Timmel The fact that many animals are able to use the Earth's magnetic field to navigate and orient has been firmly established since the 1970s. The hypothesis that this capability might be driven by a quantum mechanical process involving a pair of photoinduced, highly spin polarized radicals, was originally proposed in the same decade. But only with the discovery of cryptochromes, a family of blue light photoreceptor proteins ubiquitous in Nature, did this radical pair hypothesis take centre stage in the discussion of animal magnetosensitivity and is now, arguably, the most likely mechanism to drive this fascinating process. |
(Author Not Attending)
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K31.00002: Observations of Coherence in Bacterial Reaction Centers Using Two-Dimensional Electronic Spectroscopy Veronica Policht, Andrew Niedringhaus, Jennifer P Ogilvie Bacterial reaction centers (BRC) are photosynthetic proteins which perform electronic energy and charge transfer at ultrafast timescales (fs - ps) with near unity quantum efficiency in wild type proteins. Several recent observations of coherences in BRCs and other photosynthetic proteins have proposed that coherent superposition states are partly responsible for the impressive functionality of these systems. In attempting to answer this question, conflicting origin assignments of the coherences have been proposed. In order to develop our understanding of coherences we have performed Two-dimensional electronic spectroscopy on BRCs from purple bacteria Rh. capsulatus and its predominant pigment Bacteriochlorophyll a (BChla). We are able to identify strong intramolecular vibrational contributions in both systems as well as signs of vibronic coherence in the BRC. We additionally resolve the weak upper-excitonic peak of the strongly coupled special pair in the BRC, made visible due to resonance of vibrational modes with excitonic energy gaps. We simulate the signatures of vibronic coherence using a reduced BRC model. These results should inform future efforts to model electronic structure of the BRC. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K31.00003: Optical Tweezers: from Biophysics to Chemical Physics. Maria Kamenetska, Jacob Black, Alexander Parobek, Ziad Ganim Optical Tweezers have resulted in transformative mechanistic understanding of biological processes by allowing unambiguous and reproducible measurements on single molecules. Using this technique, we can trap and isolate a single molecule away from surface heterogeneities and manipulate it with nanometer resolution and piconewton control. The application of this powerful technique to solution-phase chemistry and materials characterization has been hampered by the inability to achieve stable trapping in organic solvents. Here, I present our recent advances in extending optical tweezers to experiments on single molecules in a variety of solvents. Furthermore, I report on our preliminary measurements of optically-induced forces and demonstrate that force-detected absorption spectroscopy allows for nanoscale mapping of absorption profiles. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K31.00004: The interplay between magnetism and chemical binding Sudipto Chakrabarti Single molecule spintronics aim to identify new spin transport effects near the limit of electronic component’s miniaturization. While the focus in this field is on magneto-transport properties, not much is known about the effect of magnetism on the structure of molecular conductors. Here, we reveal a new phenomenon: the direction of applied magnetic field can affect the properties of a metal-molecule chemical bond. Specifically, we show that magnetic field direction affects the formation of metal-single molecule-metal junctions and the stability of the metal-molecule bond. Our findings reveal the interplay between magnetism and chemical binding at the level of a single chemical bond. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K31.00005: Infrared spectroscopic nano-imaging of molecular coupling and dynamics in poly-tetrafluoroethylene Sven A Doenges, Bernd Metzger, Jun Nishida, Eric A Muller, Markus B Raschke In organic materials, local morphology and environment determine material properties through intra- and intermolecular coupling. Vibrational spectroscopy can be used as a sensitive probe to variations in molecular structure, coupling, and dynamics. However, conventional far-field spectroscopic imaging spatially averages over the molecular ensemble, obscuring the effects of the local chemical environment. Here, we use broadband and ultrafast scattering scanning near-field optical microscopy (s-SNOM) to image nanoscale heterogeneity in the technologically important polymer of polytetrafluoroethylene (PTFE). Probing symmetric and antisymmetric coupled C-F stretch vibrations, from their variations and correlations in vibrational solvatochromism we reveal spatial sub-ensembles and their arrangements from nanometer to micron length scale of ordered and disordered domains in as cast, stretched, and rubbed PTFE films. The work demonstrates the use of vibrational marker resonances as intrinsic labels and molecular rulers as sensitive probes of their local structural environment associated with materials functional properties. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K31.00006: Scanned Probe Microscopy Studies of MoS2 Catalysis on Insulating Substrates Steven Arias, Tan Dao, Shawna Hollen Molybdenum Disulfide (MoS2) is one of the most commonly used commercial hydrodesulfurization (HDS) catalysts, but a microscopic understanding of the active sites and reaction mechanisms has been difficult to develop. Understanding these reactions on an atomic scale provide guidance to catalyst design. Improving HDS catalyst design will also rely on our knowledge of the catalyst on industrially relevant, insulating substrates. We will present data from scanning tunneling microscopy and non-contact atomic force microscopy on exfoliated MoS2 flakes on a silicon dioxide substrate. To determine active sites for HDS, we introduce thiophene molecules that adsorb to the MoS2. This system provides the first opportunities for atomic scale catalysis studies on industrially-relevant substrates. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K31.00007: Investigating Tunneling Controlled Reactions with Ring Polymer Molecular Dynamics Xinyang Li, Pengfei Huo Here we present an ab-initio on-the-fly rate constant computation on the decay of methylhydroxycarbene with Ring polymer molecular dynamics (RPMD). Experiments show that hydroxycarbenes can decay rapidly through a high but thin barrier instead of a lower but broader barrier at 11 K. We investigate the tunneling of the transferring proton with RPMD which can accurately describe nuclear quantum effects. Quantum free energy profiles and transmission coefficients are computed in order to construct the full rate constants. We compute RPMD reaction rates at various temperatures which agree with previous experiments and theoretical studies. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K31.00008: Self-assembling Ordered Arrays of Virus-like Particles Mediated by Linkers Nicholas Brunk, Masaki Uchida, Trevor Douglas, Vikram Jadhao Virus-like particles (VLPs), themselves self-assembled from protein subunits, can be exploited to generate hierarchical functional materials for applications in catalysis and photonics. We present an integrated experimental and computational method to understand and control higher-order VLP assembly into three dimensional, ordered arrays that is applicable to a variety of VLP-linker systems. Specifically, we study the assembly of bacteriophage P22 VLPs mediated by oppositely-charged, macromolecular dendrimers. The integrated approach demonstrates VLPs self-assemble into ordered arrays in the presence of dendrimers as the ionic strength is lowered below a threshold value. This threshold may be tuned by genetically engineering the VLP surface charge. At threshold, the common lattice structure exhibits the same long-range order and a similar configuration of bridging dendrimers, regardless of the VLP surface charge. The experimentally-validated model identifies key electrostatic and kinetic mechanisms, predicting dendrimer concentration as a control parameter for modulating assembly. The integrated approach opens new design and control strategies to fabricate active hierarchical materials. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K31.00009: Scaling relations quantify the hierarchical self-assembly of capillary-densified nanofiber arrays into shape-tunable architectures Ashley Kaiser, Itai Y Stein, Kehang Cui, Brian L Wardle Capillary-mediated densification is a facile and versatile method to create high-density, hierarchical structures from nanofiber (NF) arrays, such as aligned carbon nanotubes, whose exceptional intrinsic properties motivate their use as shape-tunable materials. Here, scaling relations are presented that accurately predict the morphology of capillary-densified NF arrays exhibiting multiple spatial scales, including long-range cellular networks formed from bulk-scale arrays, and solid, micron-scale pins formed via the densification of patterned arrays within the critical pattern size separating cell vs. pin formation. Both experiments and models show that the effective elastic modulus of the densifying NF arrays governs the resulting geometries, including the cell width and area, cell and pin wall thickness, and the NF volume fraction within the densified walls, which increase monotonically with array height. Further structural tunability, including the densification of mm- to cm-tall NF arrays, is possible by altering the NF-substrate adhesion strength. Collectively, these results could enable the broad use of capillary densification to predictably pattern hierarchical NF arrays for applications in optoelectronics, composite reinforcement, and advanced thermomechanical devices. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K31.00010: Optical properties of free base tetrasulfonatofenil porphyrin (H2TPPS4), and tetrapyridyl porphyrin (H2TPyP) with ruthenium group Ehsan Zolghadr, Keshav Sharma, Jefferson Marcio Sanches Lopes, Renato Neiva Sampaio, Alzir Azevedo Batista, Amando Siuiti Ito, Newton M Barbosa Neto, Paulo T Araujo Recently we proposed new insight to the absorption and fluorescence (FL) of free base tetrapyridyl porphyrin (H2TPyP). In this project, we extend our study to the free base tetrasulfonatofenil porphyrin (H2TPPS4), and to supramolecular structure composed by H2TPyP and four ruthenium groups ([RuCl(terpy)(PPh3)2]PF6). Despite the fact that literature has reported the transitions observed in porphyrins’ absorption and fluorescence spectra, a more accurate interpretation has been evasive and still needed for H2TPPS4 and [RuCl(terpy)(PPh3)2]PF6 systems. We found that these systems follow our recent findings for H2TPyP where each of the absorption Q-bands consist of two quasi-degenerated bands namely Qx1, Qx2 and Qy1, Qy2, respectively. We also explore the FL and its polarization degree (polarization components) via the Stokes spectroscopy method. Using the Stokes method, we found that the FL polarization degree depends on the concentration of the molecules in the solvent. In order to elucidate the polarization mechanisms, further investigations are being conducted via polarization-resolved, temperature-dependent and lifetime spectroscopy experiments. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K31.00011: Spin Crossover Predictions in Transition Metal Complexes using the Density Corrected DFT LUIS SORIANO, Alberto Vela The phenomenon of spin-crossover (SCO) in transition metal complexes is of great importance in the development of magnetic materials whose properties are used in visualization, memory, and electrical devices, to mention a few. This phenomenon has been amply studied with Density Functional Theory (DFT) using a broad variety of exchange-correlation (XC) functionals with disappointing outcomes. Such functionals seem incapable of predicting reliably the energy differences between the high- and low-spin configurations. Recently, an approach called HF-DFT, which consists of evaluating the energy of a selected XC functional with the Hartree-Fock (HF) density, has been applied successfully to several problems, including SCO in some iron complexes with small ligands, yielding results that are close to coupled cluster (CC) and Diffusion Monte Carlo calculations, at a much lower computational cost. In this work we show that HF-DFT, using DFT optimized geometries, also offers an excellent alternative to describe SCO in manganocenes with ligands in the cyclopentadienyl rings going from hydrogen to tert-butyl. The results are in excellent agreement with available CCSD calculations. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K31.00012: Resonance Energy Transfer on a Metallic Thin Film: Characteristic Distance and the Origin of Plasmon Enhancement Liang-Yan Hsu, Jhih-Sheng Wu Resonance energy transfer (RET) around metallic structures has received considerable attention during the past few years. In this study, we analyze the mechanisms of RET on a metallic thin film in the framework of macroscopic quantum electrodynamics and investigate the distance dependence of RET enhancements. Our theoretical analysis shows that the mechanisms of RET can be separated into mirror dipoles, surface plasmons, and retardation. Besides, we find the characteristic distance of RET coupled with surface plasmon polaritons and it can be modified by varying the thickness of thin film, indicating that RET can be significantly enhanced at a short range. |
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