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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Alkali-silica reaction products and cracks: X-ray micro-tomography-based analysis of their spatial-temporal evolution at a mesoscale

In this study, we propose a laboratory-scale methodology, based on X-ray micro-tomography and caesium (Cs) as a contrast agent, to advance the understanding of cracking due to alkali-silica reaction (ASR) in concrete. The methodology allows achieving a completely non-destructive and time-lapse characterization of the spatial-temporal patterns of both the cracks and the ASR products. While Cs addition slightly accelerated the ASR kinetics, the crack patterns, with and without Cs addition, were statistically equivalent. Cracks with ASR products appeared first in the aggregates, close to the interface with the cement paste. They propagated afterwards towards the aggregates interior. Some products were then extruded for several mm into air voids and cracks in the cement paste. This process suggests that, in the early stage, the ASR products may be a low-viscosity gel that can flow away from the source aggregate and may settle later elsewhere as a rigid phase, upon calcium uptake.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Heat accelerates degradation of β -lactoglobulin fibrils at neutral pH

Fibrous aggregates of β-lactoglobulin display superior mechanical and interfacial properties compared to the native protein. These properties directly link to the protein morphology and structure. When incorporated into food matrices, during processing protein fibrils are exposed to pH shifts and high temperature conditions, which accelerate their degradation. In the present study, neutralized β-lactoglobulin fibrils were heated at 100 °C and 121 °C for various times to assess their degradation. Fibril morphology, structure, and viscosity in solution were examined by microscopy, scattering, spectroscopy, and rheology. Atomic force microscopy showed the contour length of the protein fibrils decreased gradually with heating at 100 °C and 121 °C, with greater decreases at 121 °C. Increased fibril diameters (~15–25 nm) were observed at 121 °C for 5–15 min heating and were disrupted upon further heating. Small-angle x-ray scattering indicated an increase in fibril radius with heating at pH 7 followed by a decrease at prolonged heating, whereas fibril length decreased continuously with heating. Thioflavin T fluorescence, circular dichroism and Fourier transform infrared spectroscopy confirmed the conversion of β-sheet to random coils as fibrils were degraded during thermal treatment at pH 7. Surface hydrophobicity of fibrils decreased with increase in heating temperature and time, coinciding with an increase in the content of non-aggregated proteins. Viscosity of fibril solutions increased when fibrils were heated at 100 °C, whereas at 121 °C their viscosity first increased and then decreased. Furthermore, these findings imply heating at 100 °C and 121 °C facilitates degradation and depolymerisation of β-lactoglobulin fibrils with aggregation as an intermediate step.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Changing chemistry of particulate manganese in the near- and far-field hydrothermal plumes from 15°S East Pacific Rise and its influence on metal scavenging

Dissolved Mn(II) in the hydrothermal plume is known to be microbially oxidized to form Mn(III/IV) oxides, and the Mn oxides scavenge other trace elements in seawater. In the GEOTRACES GP16 cruise, dissolved Mn (dMn) and particulate Mn (pMn) were found to be transported over 4000 km westwards from the Southern Eastern Pacific Rise. Previous studies in this plume showed different removal rates of dMn and pMn as well as pMn size distribution between the near-field (<80 km from the ridge axis) and far-field (>80 km) plumes. In order to understand Mn cycling in these plumes, spatial distribution, oxidation states, and mineral structures of Mn in small size fraction (SSF; 0.8–51 μm) and large size fraction (LSF; >51 μm) particles from the near-field and far-field plumes were examined using micro X-ray fluorescence spectrometry (μ-XRF), X-ray absorption near-edge structure spectroscopy (XANES), chemical species mapping, and extended X-ray absorption fine-structure spectroscopy (EXAFS). In the near-field plume, pMn in the SSF is dominated by oxidized Mn with Mn(III) fractions of ~30%. They are a mixture of δ-MnO 2 and triclinic birnessites that is known to be formed as a result of autocatalytic Mn(II) oxidation at the surface of freshly-formed δ-MnO2, suggesting that both microbial and autocatalytic Mn oxidation occur in the near-field plume. The LSF pMn in the near-field plume is also oxidized and often found in large aggregates several hundreds of μm in size. These aggregates settle out in the near-field and during transport, and are not found in the far-field plume. In the far-field plume where Mn oxides are not newly formed, pMn in the SSF is oxidized, but their Mn(III) fractions are smaller than in the near-field pMn. Unlike the SSF, the far-field plume LSF pMn is dominated by reduced Mn, implying very slow aggregation of pMn in the far-field plume. The different characteristics of pMn between the near-field and far-field plumes affect its scavenging of other trace elements. In the near-field plume, Co, Mo, 231 Pa are associated with pMn, but not in the far-field plume. 231 Pa is adsorbed to pFe rather than pMn in the far-field plume, and Pb is adsorbed to pFe in the entire plume. The result shows that freshly-formed Mn oxides in the near-field plume have higher scavenging capacity than the far-field plume pMn. Our findings suggest that the mineralogical age of Mn oxides may be an important parameter that controls the scavenging of many other trace elements and isotopes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Evaluating the Rheo-electric Performance of Aqueous Suspensions of Oxidized Carbon Black

Here, the macroscopic properties of carbon black suspensions are primarily determined by the agglomerate microstructure built of primary aggregates. Conferring colloidal stability in aqueous carbon black suspensions should thus have a drastic impact on their viscosity and conductivity. Carbon black was treated with strong acids following a wet oxidation procedure. An analysis of the resulting particle surface chemistry and electrophoretic mobility was performed in evaluating colloidal stability. Changes in suspension microstructure due to oxidation were observed using small-angle X-ray scattering. Utilizing rheo-electric measurements, the evolution of the viscosity and conductivity of the carbon black suspensions as a function of shear rate and carbon content was thoroughly studied. The carboxyl groups installed on the carbon black surface through oxidation increased the surface charge density and enhanced repulsive interactions. Electrostatic stability inhibited the formation of the large-scale agglomerates in favor of the stable primary aggregates in suspension. While shear thinning, suspension conductivities were found to be weakly dependent on the shear intensity regardless of the carbon content. Most importantly, aqueous carbon black suspensions formulated from electrostatically repulsive primary aggregates displayed a smaller rise in conductivity with carbon content compared to those formulated from attractive agglomerates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Dispersion of high oxygen permeability ionomers in water-propanol solutions

There is a growing interest in high oxygen permeability ionomers (HOPI) for better-performing proton exchange membrane fuel cell (PEMFC) catalyst layers. In this study, we used small-angle x-ray scattering (SAXS) to analyze dispersions of two types of perfluorinated sulfonic acid (PFSA) ionomers and a HOPI in water and propanol solvents, at concentrations between 1 wt% and 15 wt%. HOPI dispersions exhibited the interference peak typical of ionomers; by separately fitting the peak and higher- q regions with several models, we demonstrated that HOPI aggregates have a closer average spacing than conventional PFSA ionomers, and smaller aggregate size when assuming a cylindrical geometry. By measuring conventional PFSA ionomers with two side chain lengths and several equivalent weights, we showed that these side chain parameters alone did not explain the differences in aggregate spacing or size. In addition, the pattern was present whether samples were prepared in a 50/50 water/propanol or a more propanol-rich mixture. As such, the structural differences of HOPI could be due to its different backbone monomer. A better understanding of the influence of the HOPI chemistry on dispersion properties could help explain the ink structure and drying characteristics of HOPI observed in prior work. Future study of ionomer-catalyst mixtures, such as measuring ionomer adsorption to carbon, would help elucidate the role of ionomer from ink formulation to the cast electrode.

Aquivion↗
DOE OSTI2021

Molecular dynamics studies of lattice defect effects on tritium diffusion in zirconium

Tritium diffusion in α-Zr containing point defects such as vacancies or self-interstitial atoms (SIAs) is simulated using molecular dynamics. Point defects rapidly aggregate to form extended defects, such as 3D nanoclusters and Frank loops. The geometry of extended defects is affected by the presence of tritium. At low temperature and in the absence of tritium, vacancies aggregate to form stacking fault pyramids. Addition of tritium at these temperatures promotes aggregation of vacancies to form 3D nanoclusters, within which the tritium concentration can be sufficiently high to suggest that these defects may serve as nucleation sites for hydride precipitation. Trapping of tritium in vacancy nanocluster reduces the calculated bulk diffusivity by an amount proportional to the vacancy concentration. At high temperature, vacancy clusters change shape to form planar basal dislocation loops, which bind tritium less strongly, leading to a sharp reduction in the fraction of trapped tritium and a corresponding increase in tritium diffusivity at high temperature. In contrast, SIAs increase tritium diffusion through α-Zr. Analysis of atomic trajectories shows that tritium does not interact directly with SIAs. In conclusion, diffusion enhancement is instead related to expansion of the lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Occupant-driven end use load models for demand response and flexibility service participation of residential grid-interactive buildings

As demand response becomes increasingly used as a tool to support improved grid flexibility, it is important to consider that there are many potential types of energy end uses that may be used to support such flexibility. Residential appliances, often accounting for 30 % or more of residential energy use, are a currently untapped source of demand flexibility, particularly when aggregated together across homes. To date there has been very limited analysis of residential appliances for use as grid-interactive loads. As such, this research uses disaggregated energy end use data for 564 households, to model the electricity demand flexibility potential of the use of residential dishwashers, clothes washers, clothes dryers, ovens, and ranges (oven + stovetop) on both weekdays and weekends. This includes both at the building level, as well as aggregated to the grid level, specifically the Midcontinent Independent System Operator (MISO) region. This study was divided into two parts. Part 1 focuses on determining appliance-level loads, and Part 2, which involves aggregation to the grid. Findings suggest that among the studied appliances, clothes dryers provide the greatest demand reduction potential for most times of the day, followed by dishwashers and clothes washers. The maximum potential reduction for clothes dryers is found to be approximately at 11:00 a.m. and this potential sustains throughout most of the daytime period. When considering the willingness of households to participate, based on a survey of households in the Midwest region, clothes dryers still have the most potential for demand reduction. The availability of appliances for load modulation on weekdays and weekends indicates similar load reduction potential for all appliances. Overall, the results of this study suggest that there is an opportunity for shifting appliance usage to optimize grid efficiency and enhance demand response strategies.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

A case study on parallel HDF5 dataset concatenation for high energy physics data analysis

In High Energy Physics (HEP), experimentalists generate large volumes of data that, when analyzed, helps us better understand the fundamental particles and their interactions. This data is often captured in many files of small size, creating a data management challenge for scientists. In order to better facilitate data management, transfer, and analysis on large scale platforms, it is advantageous to aggregate data further into a smaller number of larger files. However, this translation process can consume significant time and resources, and if performed incorrectly the resulting aggregated files can be inefficient for highly parallel access during analysis on large scale platforms. In this paper, we present our case study on parallel I/O strategies and HDF5 features for reducing data aggregation time, making effective use of compression, and ensuring efficient access to the resulting data during analysis at scale. We focus on NOvA detector data in this case study, a large-scale HEP experiment generating many terabytes of data. Here, the lessons learned from our case study inform the handling of similar datasets, thus expanding community knowledge related to this common data management task.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Formulation and solution approach for calibrating activity-based travel demand model-system via microsimulation

This study addresses the problem of calibrating utility-maximizing nested logit activity-based travel demand model-systems. After estimation, it is common practice to use aggregate measurements to calibrate the estimated model-system’s parameters prior to their application in transportation planning, policy making, and operations. However, calibration of activity-based model-systems has received much less attention. Existing calibration approaches are myopic heuristics in the sense that they do not consider the fundamental inter-dependencies among choice-models and do not have a systematic way to adjust model parameters. Also, other purely simulation-based approaches do not perform well in large-scale applications. In this study, we focus on utility-maximizing nested logit activity-based model-systems and calibrating aggregate statistics such as activity shares, mode shares, time-dependent & mode-specific OD flows, and time-dependent & mode-specific sensor counts. We formulate the calibration problem as a simulation-based optimization problem and propose a stochastic gradient-based solution procedure to solve it. The solution procedure relies on microsimulation to calculate expectations of the aggregate statistics of interest to the calibration problem. Additionally, we derive approximate analytical expressions for the gradient of the objective function —that are evaluated through microsimulation on mini-batches of the population. The proposed solution procedure is sensitive to the fundamental structure of the activity-based model-system and is non-myopic in considering the dependencies across its model components. The formulated optimization problem is non-convex, highly nonlinear, and potentially has multiple-minima. Lastly, we show —through a real-world application— that the proposed solution procedure outperforms other state-of-the-art purely simulation-based optimization approaches in terms of computational efficiency, stability, and convergence. We also compare various gradient-based solution algorithms to determine the best algorithm to update the parameters. This work has the potential to facilitate wider and easier application of activity-based model-systems.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Calcium-organic matter fouling in nanofiltration: Synchrotron-based X-ray fluorescence and absorption near-edge structure spectroscopy for speciation

Calcium (Ca)-enhanced organic matter (OM) fouling of nanofiltration (NF) membranes leads to reduced flux during desalination and requires frequent cleaning. Fouling mechanisms are not fully understood, which limits the development of targeted fouling control methods. This study employed synchrotron-based X-ray fluorescence (XRF) and X-ray absorption near-edge structure (XANES) spectroscopy to quantify the spatial distribution and mass of Ca deposition as well as changes in the Ca coordination environment characteristic of specific fouling mechanisms, respectively. Bench-scale filtration experiments were performed using feed solutions containing Ca and ten different types of organic matter (OM), as well as the common scalants, calcium carbonate (CaCO 3 ) and calcium sulfate (CaSO 4 ). Osmotic backwash (OB) was performed at regular intervals for fouling control. Ca-OM aggregation resulted in greater flux decline and lower flux recovery during OB than Ca conditioning of membranes followed by filtration of feed solution with OM. Linear combination fitting (LCF) of XANES absorption spectra from fouled membranes indicated that Ca-OM aggregation preferentially occurred for OM types that exhibited both high carboxylic group and negative charge density. Consequently, these OM types exhibited greater deposition of Ca and TOC on the membrane surface when compared to other OM types. For the coexistence of scalants and OM, Ca speciation within the fouling layer was characteristic of both Ca bound to the membrane (i.e. potential bridging, charge screening) as well as Ca-OM aggregation and deposition mechanisms, while a range of crystal polymorphs were observed to occur simultaneously. XRF and XANES represent powerful tools for the elucidation of NF fouling mechanisms by quantification of Ca deposition as well as Ca speciation. Fouling control methods should target OM types with high carboxyl group density and negative charge to neutralize or eliminate interactions with Ca.

42 ENGINEERING↗
DOE OSTI2022

Bassanite Grows Along Distinct Coexisting Pathways and Provides a Low Energy Interface for Gypsum Nucleation

Calcium sulfate is a naturally abundant and industrially important mineral; understanding its formation mechanism contributes to reconstructing geochemical record and controlling its scaling in industry. Formation of gypsum is shown to follow a multistage-processes via bassanite, but the evolution of bassanite and the mechanism by which it is replaced by gypsum are poorly understood. Here we outline a three-stage process: (i) growth of bassanite nanoparticles into nanorods or rosettes, (ii) extension of bassanite rosettes by addition of fundamental growth units and aggregation of bassanite nanorods into aggregates, and (iii) heterogeneous nucleation of gypsum on the tips of bassanite rosettes and aggregates. The findings demonstrate that bassanite grows via distinct coexisting pathways and provides a low energy interface for heterogeneous nucleation of gypsum, thus providing proofs that more than one growth pathway can operate simultaneously in a crystallization system and that the precursors in multistage crystallization processes can imprint their pattern of precipitation onto the final phase.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Preprocessing Affords 3D Crystalline Poly(3-hexylthiophene) Structure

The aggregation and crystallization of poly(3-hexylthiophene- 2,5-diyl) (P3HT), a representative active layer material used for organic field-effect transistor (OFET) applications, are influenced by the solution pretreatment and deposition process. This study explores vibration-assisted convective deposition for the fabrication of OFETs in comparison to spin coating, blade coating, and convective deposition without vibration. The ultraviolet–visible spectroscopic analysis demonstrates that convective deposition, especially assisted with vibration, leads to a greater degree of intrachain interactions, longer conjugation length, and enhanced polymer backbone planarization. When the P3HT solution is preprocessed via sonication and aging, the P3HT films exhibit J-like aggregation, and (h11) peaks can be observed through grazing-incidence wide-angle X-ray scattering, suggesting an ordered 3D crystalline structure. OFETs based on such films exhibit high mobilities (up to 0.14 cm 2 V –1 s –1 ). The results point to the sensitivity of P3HT charge transport behavior to the intramolecular interactions and backbone planarity and further deepen our understanding of the relationship between processing, aggregates, molecular ordering, and resultant device properties.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Self-Assembly of Silica Nanoparticles at Water–Hydrocarbon Interfaces: Insights from In Operando Small-Angle X-ray Scattering Measurements and Molecular Dynamics Simulations

To achieve tunable controls on the interactions of siliceous materials in subsurface environments bearing water and hydrocarbons, it is essential to determine the influence of hydrocarbon–water interfaces on the self-assembly of the silica particles. The hydrophilic silica particle have a tendency to aggregate on the water front of the interface. Self-assembly of silica nanoparticles proceeds via the migration of these nanoparticles to the water–hydrocarbon interface followed by aggregation at the interface. Fractal-like morphologies of assembled silica nanoparticles at water–hydrocarbon interfaces are observed. Rapid assembly of the hydrophilic silica nanoparticles at water–hydrocarbon interfaces corresponds to an overall reduction in the surface tension of water–toluene and water–heptane systems. These studies demonstrate the silica aggregation that is the precursor of silica polymerization, or nucleation and growth is influenced by the presence of hydrocarbons in subsurface geologic environments. These insights were derived from in operando ultrasmall and small-angle X-ray scattering (USAXS/SAXS) measurements, cryo-scanning electron microscopy (Cryo-SEM) imaging, and classical molecular dynamics simulations. As a result, these studies are intended to inform current and future efforts aimed at tuning silica reactivity in subsurface geologic environments to enhance permeability, the design and use of silica-based proppants to enhance fractures, and the development of effective strategies to control silica-based scaling behavior in subsurface reservoirs.

42 ENGINEERING↗
DOE OSTI2025

How Thermodynamic, Electronic, and Steric Factors Influence Mesitylcopper Oligomers

Mesitylcopper (CuMes) is a highly versatile organocopper reagent used in both organic and inorganic syntheses. It has previously been shown that CuMes exists as a tetrameric or pentameric cyclic oligomer [CuMes] n (n = 4, 5), both in solution and in the solid state. The bonding arrangement between the [CuMes] units has qualitatively been described as localized three-center twoelectron (3c-2e) bonds. However, the electronic, structural, and thermodynamic forces driving this aggregation are still not well understood. For this reason, we employed density functional theory (DFT) calculations to study mesitylcopper as a monomeric [CuMes] unit and [CuMes]n oligomers with n = 2 to n = 7. We found that there is a strong electronic driving force for aggregation caused by strong mixing between the Cu’s d orbitals and Mes’s π orbitals in oligomers larger than the dimer. This mixing is only optimized in oligomers with n ≥ 3, where the mesityl group is no longer bonded to a single copper center but instead becomes a bridging ligand. Beyond the trimer, steric and entropic factors become relevant for determining the relative stabilities of the different aggregates, with midsized oligomers (n = 4−5) having the optimal balance between the electronic Cu−C bonding character, Cu···Cu attractive forces, entropy, reduced internal ring strain, and reduced steric interactions between the mesityl groups.

Copper↗
DOE OSTI2026

X‑ray Coherent Diffractive Imaging of Large Helium Nanodroplets Doped with Small Molecules

We report the first X-ray coherent diffractive imaging experiment on molecule-doped helium nanodroplets. It complements previous work, where we reported single-shot X-ray coherent diffractive imaging studies of Xe dopant clusters formed in 4He and 3He droplets. These noble gas clusters were used to visualize the impact of rotational excitation of the droplets on the spatial distribution of atomic dopants within the droplets, and to study the differences and connections between quantum and classical droplet rotational motion. Here, we expand our studies to the molecular dopants CF4, CHF3, CH3CN, and SF6, imaged with 1.5 keV photons. We find multiple Bragg spots in the diffraction patterns of molecule-doped droplets with radii of approximately 600 nm, which provide evidence that molecules form elongated clusters with preferential alignment along the angular momentum axis of the 4He droplets, in agreement with our previous results on the aggregation of Xe clusters on quantum vortices. Real-space reconstructions of molecular dopant cluster density profiles are obtained for droplets with smaller radii of approximately 300 nm. The diffuse images suggest the formation of low-density, potentially porous, molecular clusters upon aggregation at T = 0.4 K in 4He droplets. In the normal fluid 3He droplets, molecules aggregate into loose clusters on the droplets' equator, similar to previous observations for Xe atoms. Time-of-flight mass spectra reveal that the doped helium nanodroplet moieties fragment extensively into constituent atomic ions, producing only a small fraction of molecular fragment ions. The findings are discussed in the context of previously proposed schemes to use He droplets as potential tamper materials for ultrafast X-ray imaging experiments.

Feinberg, AlexandraJ↗
DOE OSTI2020

Self-Assembled Solute Networks in Crowded Electrolyte Solutions and Nanoconfinement of Charged Redoxmer Molecules

Redoxmers are electrochemically active organic molecules storing charge and energy in electrolyte fluids circulating through redox flow batteries (RFBs). Such molecules typically have solvent-repelling cores and solvent-attracting pendant groups introduced to increase solubility in liquid electrolytes. These two features can facilitate nanoscale aggregation of the redoxmer molecules in crowded solutions. In some cases, this aggregation leads to the emergence of continuous networks of solute molecules in contact, and the solution becomes microscopically heterogeneous. In this study, we used small-angle X-ray scattering (SAXS) and molecular dynamics modeling to demonstrate formation of such networks and examine structural factors controlling this self-assembly. We also show that salt ions become excluded from these solute aggregates into small pockets of electrolytes, where these ions strongly associate. This confinement by exclusion is also likely to occur to charged redoxmer molecules in a “sea” of neutral precursors coexisting in the same solution. Here, we demonstrate that the decay lifetime of the confined charged molecules in such solutions can increase several fold compared to dilute solutions. We attribute this behavior to a “microreactor effect” on reverse reactions of the confined species during their decomposition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Hexagonal Lattices of HIV Capsid Proteins Explored by Simulations Based on a Thermodynamically Consistent Model

HIV capsid proteins (CAs) may self-assemble into a variety of shapes under in vivo and in vitro conditions. Here, we employed simulations based on a residue-level coarse-grained (CG) model with full conformational flexibility to investigate hexagonal lattices, which are the underlying structural pattern for CA aggregations. Facilitated by enhanced sampling simulations to rigorously calculate CA dimerization and polymerization affinities, we calibrated our model to reproduce the experimentally measured affinities. Here using the calibrated model, we performed unbiased simulations on several large systems consisting of 1512 CA subunits, allowing reversible binding and unbinding of the CAs in a thermodynamically consistent manner. In one simulation, a preassembled hexagonal CA sheet developed spontaneous curvatures reminiscent of those observed in experiments, and the edges of the sheet exhibited local curvatures larger than those of the interior. In other simulations starting with randomly distributed CAs at different concentrations, existing CA assemblies grew by binding free capsomeres to the edges and by merging with other assemblies. At high CA concentrations, rapid establishment of predominant aggregates was followed by much slower adjustments toward more regular hexagonal lattices, with increasing numbers of intact CA hexamers and pentamers being formed. Our approach of adapting a general CG model to specific systems by using experimental binding data represents a practical and effective strategy for simulating and elucidating intricate protein aggregations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Vibronic Coupling and Exciton Chirality: Electronic and Structural Rearrangement between Helical to Zero Momentum Molecular Exciton States

Helical porphyrin aggregates are attractive macromolecular nanostructures for biomimetic light-harvesting and energy transfer in solar energy technology due to their large light absorbing cross sections and promising energy conservation benefits from chiral-induced spin selectivity. However, with these soft materials, it is imperative that we understand how the nuclear degrees of freedom impact the excitonic energy landscape and dynamics, particularly pertaining to how it influences the chiral angular momentum of the excitons. To this end, we have measured time-resolved depolarization ratios using femtosecond stimulated Raman spectroscopy to uncover the vibronic coupling guided by molecular vibrations between excitons of different angular momentum in helical tetra(sulfonatophenyl)porphyrin aggregates. We find that while transient absorption anisotropy reveals rapid (1 ps lifetime) exciton rotation from helical to achiral states in the Q-band, time-resolved vibrational depolarization ratios evolve on remarkably faster time scales (<500 fs lifetime) for the totally symmetric 1530 cm –1 vibration and the nontotally symmetric 1540 cm –1 vibration. The time-resolved depolarization ratios of the 1540 cm –1 vibration, in particular, show strong evidence of vibronic coupling and nonadiabatic exciton behavior that mediates the nonradiative transition between chiral and achiral excitons. Furthermore, our study shows that for rational design of helical molecular aggregates for exciton transport, the vibronic coupling between excitons of different angular momenta driven by molecular vibrations must be considered especially in the case where chiral-induced spin selectivity is desired.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗