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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

New O+CO2 Scattering Cross Sections and Their Impact on Photochemical Atmospheric Escape of Oxygen from Mars

Dissociative recombination of O2+ and CO2+ molecular ions in the upper atmosphere of Mars is a major source of suprathermal (hot) oxygen atoms. A significant fraction of hot O atoms are sufficiently fast to overcome Mars' gravitational potential and escape to space. This mechanism is known as photochemical escape (PE) and found to be one of the major escape mechanisms presently active on Mars. The Mars Atmosphere and Volatile Evolution Mission (MAVEN) can constrain the PE rate from measurements of electron temperature and density and ion temperature and density. Escape probabilities of hot O can then be constructed from in-situ neutral densities and escaping O flux loss due to collisions with atmospheric background gases. Due to CO2 abundance in the upper atmosphere, O+CO2 scattering is the most important sink of the escaping hot O flux that strongly affects estimates of total atomic oxygen escape rate obtained from MAVEN data, as outlined above. New O+CO2 elastic and inelastic cross sections were constructed from first principles for collision energies from 0.01-5 eV. O(3P)+CO2 collisions were described using new 4D electronic potential energy surfaces (PESs) for restricted geometries. Three lowest triplet states of CO2 were considered for symmetric, bending, and asymmetric vibrational modes. The cross sections were determined from close-coupling calculations performed on individual surfaces using vibrating rotor approximations for CO2 molecule. Differential and momentum transfer cross sections, of use in energy transfer atmospheric models, were also constructed. The obtained cross sections are in excellent agreement with published results. Our new elastic cross sections are significantly smaller than assumed in earlier atmospheric escape models, resulting in larger photochemical escape rate of O from Mars, as estimated from a 1D escape model. Our results agree well with estimated hot O flux (about 9 x 10(sup 25) s(sup -1)) derived from altitude profiles of 130.4 nm O emission measured up to ~4,000 km by the Imaging Ultraviolet Spectrograph on MAVEN orbiter.

Atmospheric↗
DOE OSTI2023

Correction to “Ruthenium Dye Excitations and Relaxations in Natural Sunlight”

An error in computing the optical rate coefficients for dye RuP was discovered in Table S4 of the Supporting Information Section S8 Model S? for this article, published in J. Phys. Chem. A, 2021, 125, 4365-4372, 10.1021/acs.jpca.1c02386. The optical rate coefficients include absorption, stimulated emission, and ground-state bleach associated with the ground state and the singlet and triplet excited states as well as for the excited-state absorption between the excited states and implicit higher-energy excited states. The optical rate coefficients for continuous solar irradiance were computed using eq 3 of the main text in which the product of the individual signal components and the solar spectrum are integrated and multiplied by the fundamental optical rate coefficient. We determined the incorrect signal components were used for RuP in the published Supporting Information Table S4. The corrected Table S4 column for RuP is presented with the reported values from the original paper in the corrected Supporting Information. New simulations have been performed using the corrected values, and the results were used to update Figures 5 and 6 and also Figure S9 in the corrected Supporting Information. The published conclusions are unchanged. The populations of RuP excited states, Figure 5, are only marginally smaller, which should be expected with minor changes to the optical rate coefficients. There is not a significant change in the results for the number of radiative and nonradiative interactions per dye per second (the optical transitions and nonradiative transitions increased, respectively, by approximately 2% and 10%, Figure 6A). The occurrence of excited-state absorption (ESA) and excited-state emission (ESE) events, Figure 6C, both decreased from the original simulation results by 20% and 33%. The most notable difference is exhibited in Figure 6B; the fraction of population moving from the lowest-energy singlet state (i.e., intersystem crossing or ultrafast nonradiative relaxation) and nonradiative relaxation from the triplet state have decreased from ~0.45 to ~0.4 and increased from ~0.35 to ~0.4, respectively..

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Experimental evaluation of the stabilization of the COT orbitals by 4f orbitals in COT 2 Ce using a Hubbard model

Significant orbital mixing is rare in lanthanide complexes because of the limited radial extent of the 4f orbitals, which results in a generally small stabilization due to 4f orbital interactions. Nevertheless, even a small amount of additional stabilization could enhance lanthanide separations. One lanthanide complex in which orbital mixing has been extensively studied both experimentally and computationally is cerocene, COT 2 Ce, where COT is cyclooctatetraene. This compound has a singlet ground state with a low-lying, triplet excited state. Previous fluorescence studies on trimethylsilyl-substituted cerocenes indicate the triplet state is 0.4 eV higher in energy than the singlet state. In addition, computational studies predict that the triplet is 0.3 to 1 eV higher in energy than the singlet. The synthesis of highly pure COT 2 Ce by Walter and Andersen allowed its physical properties to be accurately measured. Using these measurements, we evaluate the stabilization of the 4f orbitals using two, independent approaches. A Hubbard model is used to evaluate the stabilization of the ground state due to orbital mixing. This stabilization, which is also the singlet–triplet gap, is –0.29 eV using this model. Furthermore, this gap was also from the temperature independent paramagnetism of COT 2 Ce, which yielded a value of –0.32 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Photochemical Au(I)–Au(I) Bond Formation: A Battle between Intersystem Crossing and Internal Conversion

The transition metal complex Au(CN) 2 – has provided experimental evidence of photoinduced bond formation between Au(I) atoms in solution. However, the underlying photochemical driving force for this bond formation reaction remains unclear. In this study, we investigate the ultrafast Au–Au bonding process in the [Au(CN) 2 ] 2 2– dimer using nonadiabatic dynamics simulations that incorporate intersystem crossing and internal conversion pathways. Reaction pathways and transitions among photochemically accessible singlet and triplet excited states are analyzed. Computational results indicate that intersystem crossing is the primary driving force in the early stages of ultrafast photochemical dynamics, while internal conversion among triplet states plays a critical role after the system stabilizes in a higher-lying triplet state. Furthermore, this work provides a mechanistic perspective on modulating photochemical reactions by tuning the relative strengths of spin–orbit coupling and nonadiabatic coupling.

Computational chemistry↗
DOE OSTI2025

Highly Conjugated Porphyrin Arrays Enable Optical Resolution of Ferromagnetic and Antiferromagnetic Aligned States of the Triplet Exciton and an Incorporated Stable Radical

Well-defined photogenerated molecular spin systems have potential utility in spintronics and quantum information science (QIS). Because molecular magnetic, optical, and electronic properties can be controlled by design, diverse spin systems can be prepared at modest temperatures. Photogenerated molecular spin systems often involve states prepared from the interaction of excitons and charges. Resolving the nature of electron spin alignment in photogenerated spin states described by the coupling of a triplet exciton and a stable radical commonly relies on EPR spectroscopy. Here, we describe ethyne-bridged (porphinato)metal ( PM n ) oligomers that incorporate a macrocycle-bound Cu(II) radical center. Upon photoexcitation of such PM n arrays, a singdoublet ( 2 S 1 ) state is formed; ultrafast internal conversion (IC) then produces a tripdoublet ( 2 T 1 ) state, which undergoes intersystem crossing (ISC) to produce a tripquartet ( 4 T 1 ) state, before relaxation to the ground state ( 2 S 0 ). These highly conjugated Cu(II) radical-containing PMn arrays enable direct observation of copper porphyrin 2 T 1 → 4 T 1 ISC dynamics from the biexponential decay of the near-infrared (NIR) 2,4 T 1 → 2,4 T n transient absorption manifold. Multireference n-electron valence perturbation theory (NEVPT2) computations illuminate how PM n electronic structure controls the relaxation dynamics of these long-lived (>10 ns) electronically excited multiplet states. These studies show that highly conjugated and polarizable porphyrin arrays incorporating stable spin centers provide rare π-delocalized systems where the ferromagnetic and antiferromagnetic alignment between a triplet exciton and a stable radical are both spectrally resolved and addressable using transient optical spectroscopy at wavelengths exceeding 1 μm, providing new opportunities to QIS.

14 SOLAR ENERGY↗
DOE OSTI2021

Anomalous normal fluid response in a chiral superconductor UTe2

Abstract Chiral superconductors have been proposed as one pathway to realize Majorana normal fluid at its boundary. However, the long-sought 2D and 3D chiral superconductors with edge and surface Majorana normal fluid are yet to be conclusively found. Here, we report evidence for a chiral spin-triplet pairing state of UTe 2 with surface normal fluid response. The microwave surface impedance of the UTe 2 crystal was measured and converted to complex conductivity, which is sensitive to both normal and superfluid responses. The anomalous residual normal fluid conductivity supports the presence of a significant normal fluid response. The superfluid conductivity follows the temperature behavior predicted for an axial spin-triplet state, which is further narrowed down to a chiral spin-triplet state with evidence of broken time-reversal symmetry. Further analysis excludes trivial origins for the observed normal fluid response. Our findings suggest that UTe 2 can be a new platform to study exotic topological excitations in higher dimension.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Triplet Resonating Valence Bond State and Superconductivity in Hund’s Metals

A central idea in strongly correlated systems is that doping a Mott insulator leads to a superconductor by transforming the resonating valence bonds (RVBs) into spin-singlet Cooper pairs. Here, we argue that a spin-triplet RVB (tRVB) state, driven by spatially, or orbitally anisotropic ferromagnetic interactions can provide the parent state for triplet superconductivity. We apply this idea to the iron-based superconductors, arguing that strong on site Hund’s interactions develop intra-atomic tRVBs between the $t_{2_g}$ orbitals. On doping, the presence of two iron atoms per unit cell allows these interorbital triplets to coherently delocalize onto the Fermi surface, forming a fully gapped triplet superconductor. In this work, this mechanism gives rise to a unique staggered structure of on site pair correlations, detectable as an alternating π phase shift in a scanning Josephson tunneling microscope.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

On‐Surface Synthesis and Determination of the Open‐Shell Singlet Ground State of Tridecacene**

Abstract The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open‐shell character has so far been missing. Here, we present the on‐surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many‐body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet‐triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.

Chemistry↗
DOE OSTI2023

Haldane topological spin-1 chains in a planar metal-organic framework

Haldane topological materials contain unique antiferromagnetic chains with symmetry-protected energy gaps. Such materials have potential applications in spintronics and future quantum computers. Haldane topological solids typically consist of spin-1 chains embedded in extended three-dimensional (3D) crystal structures. Here, we demonstrate that [Ni(μ-4,4'-bipyridine)(μ-oxalate)] n (NiBO) instead adopts a two-dimensional (2D) metal-organic framework (MOF) structure of Ni 2+ spin-1 chains weakly linked by 4,4'-bipyridine. NiBO exhibits Haldane topological properties with a gap between the singlet ground state and the triplet excited state. The latter is split by weak axial and rhombic anisotropies. Several experimental probes, including single-crystal X-ray diffraction, variable-temperature powder neutron diffraction (VT-PND), VT inelastic neutron scattering (VT-INS), DC susceptibility and specific heat measurements, high-field electron spin resonance, and unbiased quantum Monte Carlo simulations, provide a detailed, comprehensive characterization of NiBO. Vibrational (also known as phonon) properties of NiBO have been probed by INS and density-functional theory (DFT) calculations, indicating the absence of phonons near magnetic excitations in NiBO, suppressing spin-phonon coupling. The work here demonstrates that NiBO is indeed a rare 2D-MOF Haldane topological material.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Long-Lived Excited State in a Solubilized Graphene Nanoribbon

Graphene nanoribbons have excellent light-absorbing properties, but often exhibit short excited-state lifetimes that prevent their applications in photocatalysis. Here, we report a long-lived charge-transfer triplet excited state in a well solubilized, chlorinated graphene nanoribbon (Cl-GNR) with edges modified by bipyrimidine (bpm) moieties. The photophysical behavior of Cl-GNR was observed and characterized by steady-state UV-vis absorption and emission spectroscopy, transient absorption spectroscopy on the ps-ms timescale, and density functional theory (DFT) calculations. In this work, both the Cl-GNR and its monomeric subunit, chlorinated graphene quantum dot (Cl-GQD), were synthesized using bottom-up techniques to produce the H- analogs of the compounds followed by edge-chlorination to achieve soluble products. The absorption spectra of Cl-GQD and Cl-GNR appear in the UV-vis range with lowest-energy peaks at 375 and 600 nm, respectively. The excitons in Cl-GNR were found to exhibit charge-transfer character with the bpm edges serving as electron acceptors. DFT calculations indicate that the excitons are relatively localized, spreading over at most two monomeric units of the GNR. Transient absorption spectroscopy shows that singlet excited states of Cl-GQD and Cl-GNR undergo intersystem crossing with ~300 ps lifetime to form triplet states that last for 15.7 μs (Cl-GQD) and 106 μs (Cl-GNR). These properties, combined with the ability of the bpm sites to coordinate transition metals, make Cl-GNRs promising light-harvesting motifs for photocatalytic applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Highly Coupled Heterobicycle‐Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C 60

Abstract Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole‐fused porphyrin dimers and their mono‐ and bis−zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady‐state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free‐base and zinc porphyrin) reveals the occurrence of efficient singlet‐singlet energy transfer (>95 % efficiency and rate constant >10 12 s −1 ) within the dyad. Further, donor‐acceptor conjugates were formed by metal‐ligand axial coordination of phenyl imidazole functionalized C 60 and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge‐separated state was in the 30–40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π‐spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

Chemistry↗
DOE OSTI2020

Visible-Light-Driven Triplet Sensitization of Polycyclic Aromatic Hydrocarbons Using Thionated Perinones

Metal-free chromophores that efficiently generate triplet excited states represent enticing alternatives with respect to transition metal-containing photosensitizers, such as those featuring metal-to-ligand charge transfer excited states. Yet, such molecular constructs have remained underexplored due to the unclear relationship(s) between molecular structure and efficient/rapid intersystem crossing. In this regard, we present a series of three thionated perinone chromophores serving as a newly conceived class of heavy metal-free triplet photosensitizers. We demonstrate that thionation of the lone C=O substituent in each highly fluorescent perinone imparts red-shifted absorbance bands that maintain intense extinction coefficients across the visible spectrum, as well as unusually efficient triplet excited state formation as inferred from the measured singlet O 2 quantum yields at 1270 nm (Φ Δ = 0.78–1.0). Electronic structure calculations revealed the emergence of a low energy S 1 (n → π*) excited state in the proximity of a slightly higher energy S 2 (π → π*) excited state. The distinct character in each of the two lowest-lying singlet state manifolds resulted in the energetic inversion of the corresponding triplet excited states due to differences in electron exchange interactions. Rapid S 1 → T 1 intersystem crossing was thereby facilitated in this manner through spin–orbit coupling as predicted by the El Sayed rules. The lifetimes of the resultant triplet excited states persisted into the microsecond time regime, as measured by transient absorbance spectroscopy, enabling effective bimolecular triplet sensitization of some common polycyclic aromatic hydrocarbons. The synthetically facile interchange of a single O atom to an S atom in the investigated perinones resulted in marked changes to their photophysical properties, namely, conversion of dominant singlet state fluorescence in the former to long-lived triplet excited states in the latter. The combined results suggest a general strategy for accessing long-lived triplet excited states in organic chromophores featuring a lone C=O moiety residing within its structure, valuable for the design of metal-free triplet photosensitizers.

14 SOLAR ENERGY↗
DOE OSTI2021

General Design Rules for Bimetallic Platinum(II) Complexes

A series of platinum(II) bimetallic complexes were studied to investigate the effects of ligands on both the geometric and electronic structure. Modulating the Pt-Pt distance through the bridging ligand architecture was found to dictate the nature of the lowest energy electronic transitions, localized in one-half of the molecule or delocalized across the entire molecule. By reducing the separation between the platinum atoms, the lowest energy electronic transitions will be dominated by the metal-metal-to-ligand charge transfer transition. Conversely, by increasing the distance between the platinum atoms, the lowest electronic transition will be largely localized metal-to-ligand charge transfer or ligand centered in nature. Additionally, the cyclometalating ligands were observed to have a noticeable stabilizing effect on the triplet excited states as the conjugation increased, arising from geometric reorientation and increased electron delocalization of the ligands. Furthermore, such stabilization of the triplet state energy has been shown to alter the excited state potential energy landscape as well as the excited state trajectory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Utilizing Essential Symmetry Breaking in Auxiliary-Field Quantum Monte Carlo: Application to the Spin Gaps of the C 36 Fullerene and an Iron Porphyrin Model Complex

We present three distinct examples where phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) can be reliably performed with a single-determinant trial wave function with essential symmetry breaking. Essential symmetry breaking was first introduced by Lee and Head-Gordon [ Phys. Chem. Chem. Phys. 2019, 21, 4763-4778, 10.1039/C8CP07613H]. We utilized essential complex and time-reversal symmetry breaking with ph-AFQMC to compute the triplet–singlet energy gap in the TS12 set. We found statistically better performance of ph-AFQMC with complex-restricted orbitals than with spin-unrestricted orbitals. We then showed the utilization of essential spin symmetry breaking when computing the singlet–triplet gap of a known biradicaloid, C 36 . ph-AFQMC with spin-unrestricted Hartree–Fock (ph-AFQMC+UHF) fails catastrophically even with spin-projection and predicts no biradicaloid character. With approximate Brueckner orbitals obtained from regularized orbital-optimized second-order Møller–Plesset perturbation theory (κ-OOMP2), ph-AFQMC quantitatively captures strong biradicaloid character of C 36 . Lastly, we applied ph-AFQMC to the computation of the quintet–triplet gap in a model iron porphyrin complex where brute-force methods with a small active space fail to capture the triplet ground state. We show unambiguously that neither triplet nor quintet is strongly correlated using UHF, κ-OOMP2, and coupled-cluster with singles and doubles (CCSD) performed on UHF and κ-OOMP2 orbitals. There is no essential symmetry breaking in this problem. By virtue of this, we were able to perform UHF+ph-AFQMC reliably with a cc-pVTZ basis set and predicted a triplet ground state for this model geometry. The largest ph-AFQMC in this work correlated 186 electrons in 956 orbitals. Our work highlights the utility, scalability, and accuracy of ph-AFQMC with a single-determinant trial wave function with essential symmetry breaking for systems mainly dominated by dynamical correlation with little static correlation.

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

Directly Fused Porphyrin‐Tetracyanopentacenequinone Conjugates: Role of the Cross‐Conjugated, Powerful Electron Acceptor in Promoting Highly Efficient Charge Separation

Tetracyanopentacenequinone, a powerful electron acceptor, is fused directly to the porphyrin π-system to create a new class of donor-acceptor conjugates. Owing to the direct fusion and electron-deficient property of tetracyanopentacenequinone, strong intramolecular charge transfer both in the ground and excited states was witnessed. As a control, porphyrin fused with pentacenequinone was also investigated. Upon complete spectral and electrochemical characterization, the excited state properties were initially probed by time-dependent DFT studies, and the occurrence of electron transfer from different excited states was established. Free-energy calculations revealed higher exothermic electron transfer (>600 mV) than the control pentacenequinone-porphyrin systems. Pump-probe studies covering broad spatial and temporal regions revealed efficient excited state charge separation. This was unlike the control pentacenequinone-porphyrin system, where slow charge separation was witnessed only in the case of the zinc derivatives but not the free-base ones, followed by the populating of the triplet excited state. The lifetimes of the charge-separated states ranged between 30–500 ps depending on the solvent and metal ion in the porphyrin cavity. Nanosecond transient absorption studies established the charge recombination path to populate the triplet state of porphyrin or directly to the ground state as a function of solvent polarity and the nature of the conjugate. Furthermore, the significance of cross-conjugated tetracyanopentacenequinone fused directly to the porphyrin π system in promoting highly exothermic and efficient charge separation, irrespective of its cross conjugation, is borne out from this study.

Cross conjugation↗
NASA NTRS1995

The Low-Lying Electronic States of LiB

The spectroscopic constants for the triplet and singlet states of LiB below about 30 000/ cm are determined using an internally contracted multireference configuration interaction approach in conjunction with [6s 5p 3d 2f] atomic natural orbital basis sets. The ground state is (sup 3)Pi as found in previous work. No excited triplet states are found to be ideal for characterizing the ground state; the (1)(sup 3)Sigma(sup -) state has a transition energy that is too small for many experimental approaches and the (2)(sup 3)Pi and (3)(sup 3)Pi states have bond lengths that are significantly longer than the ground state, resulting in transition intensities that are spread out over many vibrational levels of the ground state.

Ricca, Alessandra↗