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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
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 OSTI2026

In Crystallo Synthesis of a Triplet Silver Nitrene

Reactive silver nitrenes are proposed intermediates in Ag-catalyzed C–H amination and olefin aziridination reactions. Synthesis and characterization of reactive Ag nitrenes has proven elusive due to fleeting lifetimes and argentophilic aggregation of Ag(I) precursors in solution. Here, we report the templated in crystallo synthesis and characterization of a mononuclear Ag nitrene supported by a tris(pyrazolyl)borate ligand. Photolytic N 2 elimination from the corresponding aryl azide complex yields a structurally defined Ag nitrene with a triplet ground state, as confirmed by crystallography, magnetic measurements, and quantum chemical calculations. This species displays both stoichiometric and catalytic nitrene transfer reactivity, enabling carbazole and indole formation from organic azides. These results leverage in crystallo templating to isolate a mononuclear triplet nitrene of Ag and provide a platform to experimentally elucidate ligand-controlled selectivity in Ag-catalyzed nitrene transfer catalysis.

Azides↗
DOE OSTI2023

Spin–vibronic coherence drives singlet–triplet conversion

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. Further, we find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

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

Spin-triplet pairing induced by near-neighbor attraction in the extended Hubbard model for cuprate chain

Abstract In quantum materials, the electronic interaction and the electron-phonon coupling are, in general, two essential ingredients, the combined impact of which may drive exotic phases. Recently, an anomalously strong electron-electron attraction, likely mediated by phonons, has been proposed in one-dimensional copper-oxide chain Ba 2− x Sr x CuO 3+ δ . Yet, it is unclear how this strong near-neighbor attraction V influences the superconductivity pairing in the system. Here we perform accurate many-body calculations to study the extended Hubbard model with on-site Coulomb repulsion U > 0 and near-neighbor attraction V < 0 that could well describe the cuprate chain and likely other similar transition-metal materials with both strong correlations and lattice effects. We find a rich quantum phase diagram containing an intriguing Tomonaga-Luttinger liquid phase — besides the spin density wave and various phase separation phases — that can host dominant spin-triplet pairing correlations and divergent superconductive susceptibility. Upon doping, the spin-triplet superconducting regime can be further broadened, offering a feasible mechanism to realize p -wave superconductivity in realistic cuprate chains.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Pseudospin-triplet pairing in iron-chalcogenide superconductors

Abstract Understanding the pairing symmetry is a crucial theoretical aspect in the study of unconventional superconductivity for interpreting experimental results. Here we study superconductivity of electron systems with both spin and pseudospin-1/2 degrees of freedom. By solving linearized gap equations, we derive a weak coupling criterion for the even-parity spin-singlet pseudospin-triplet pairing. It can generally mix with the on-site s -wave pairing since both of them belong to the same symmetry representation ( A 1 g ) and their mixture could naturally give rise to anisotropic intra-band pairing gap functions with or without nodes. This may directly explain why some of the iron-chalcogenide superconductors are fully gapped (e.g. FeSe thin film) and some have nodes (e.g. LaFePO and LiFeP). We also find that the anisotropy of gap functions can be enhanced when the principal rotation symmetry is spontaneously broken in the normal state such as nematicity, and the energetic stabilization of pseudospin-triplet pairings indicates the coexistence of nematicity and superconductivity. This could be potentially applied to bulk FeSe, where gap anisotropy has been experimentally observed.

Physics↗
DOE OSTI2020

Ligand-triplet migration in iridium( iii ) cyclometalates featuring π-conjugated isocyanide ligands

The manipulation of the triplet excited state manifold leads to large differences in the photophysical properties within a given class of metal–organic chromophores. By the appropriate choice of ancillary ligand, large changes can be made both to the order and nature of the lowest excited states and therefore to the resulting photophysical properties. Herein, a series of four bis-2-phenylpyridine (ppy) cyclometalated Ir(III) compounds bearing two arylisocyanide ligands were synthesized and photophysically characterized to understand the effects of using ancillary ligands featuring systematic changes in π-conjugation. By varying the arylisocyanide ligands, the photoluminescence quantum yield ranged from 5% to 49% and the excited state lifetime ranged between 24 μs and 2 ms. These variations in photophysical response are consistent with lowering the triplet ligand-centered ( 3 LC) state of the arylisocyanide ligand as the π system was extended, confirmed by 77 K photoluminescence emission spectra and ultrafast transient absorption experiments. Here, the latter analysis gleaned detailed insight into the importance of the interplay of the 3 LC state of the phenylpyridine and arylisocyanide ligands in these polychromophic Ir(III) molecules.

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

Benchmarking the semi-stochastic CC( P;Q ) approach for singlet–triplet gaps in biradicals

Here, we recently proposed a semi-stochastic approach to converging high-level coupled-cluster (CC) energetics, such as those obtained in the CC calculations with singles, doubles, and triples (CCSDT), in which the deterministic CC(P;Q) framework is merged with the stochastic configuration interaction Quantum Monte Carlo propagations [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017)]. In this work, we investigate the ability of the semi-stochastic CC(P;Q) methodology to recover the CCSDT energies of the lowest singlet and triplet states and the corresponding singlet–triplet gaps of biradical systems using methylene, (HFH) − , cyclobutadiene, cyclopentadienyl cation, and trimethylenemethane as examples.

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

Triplet-pair spin signatures from macroscopically aligned heteroacenes in an oriented single crystal

The photo-driven process of singlet fission generates coupled triplet pairs (TT) with fundamentally intriguing and potentially useful properties. The quintet 5 TT 0 sublevel is particularly interesting for quantum information because it is highly entangled, is addressable with microwave pulses, and could be detected using optical techniques. Previous theoretical work on a model Hamiltonian and nonadiabatic transition theory, called the JDE model, has determined that this sublevel can be selectively populated if certain conditions are met. Among the most challenging, the molecules within the dimer undergoing singlet fission must have their principal magnetic axes parallel to one another and to an applied Zeeman field. Here, we present time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy of a single crystal sample of a tetracenethiophene compound featuring arrays of dimers aligned in this manner, which were mounted so that the orientation of the field relative to the molecular axes could be controlled. The observed spin sublevel populations in the paired TT and unpaired (T+T) triplets are consistent with predictions from the JDE model, including preferential 5 TT 0 formation at z ‖ B 0 , with one caveat—two 5 TT spin sublevels have little to no population. This may be due to crossings between the 5 TT and 3 TT manifolds in the field range investigated by TR-EPR, consistent with the intertriplet exchange energy determined by monitoring photoluminescence at varying magnetic fields.

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

Enhancement and reentrance of spin triplet superconductivity in UTe 2 under pressure

Spin triplet superconductivity in the Kondo lattice UTe 2 appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe 2 . As the superconductivity and magnetism exist on two opposite sides of the first-order phase boundary, our results in this work indicate other microscopic mechanisms may be playing a role in stabilizing spin triplet superconductivity in addition to spin fluctuations associated with magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Spin-Triplet Excitonic Insulator: The Case of Semihydrogenated Graphene

While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a semihydrogenated graphene (known as graphone), however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. Here, the critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in the semihydrogenated graphene, which offers yet another test by experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1971

On the He(plus) triplet line intensities

The theoretical calculations of helium triplet line strengths, including collisional enhancement, are compared to astronomical observations. Both are plotted on an I(10830)/I(5876) vs I(5876)/I(4471) plane. It appears that the theory of helium triplet line strengths agrees with present observations, and that the question of an additional depopulation mechanism for the 2 3S population is probably predicted correctly within 30%.

Daltabuit, E.↗
NASA NTRS1972

On the He/plus/ triplet line intensities.

Check of the adequacy of theoretical calculations of helium-triplet line strengths, including collisional enhancement, by comparing theoretical prediction with astronomical observations. It is shown that the observations of Peimbert and Torres-Peimbert (1971) and of Peimbert and Costero (1969) fit the theoretical predictions rather well, placing planetary nebulae and H II regions in separate density and temperature ranges. It appears that the theory of helium-triplet line strengths agrees with present observations.

Daltabuit, E.↗
NASA NTRS1978

A theoretical and observational analysis of He I triplet lines in class 1 Seyfert galaxy spectra

The equations of statistical equilibrium are solved for 11 levels of the helium atom that are most relevant to the production and radiative transfer of He I triplet lines. Solutions are obtained for several densities and temperatures over a run of optical depth in the 10,830-A line, and the results for emission-line strengths are plotted. Spectrophotometric scans and pertinent line strengths are presented for the class 1 Seyfert galaxies 3C 120, NGC 7469, and Mrk 618, as well as for Mrk 335, which exhibits a 'more normal' He I 5876/H-beta ratio. A comparison of the theoretically determined line intensities with the observational results verifies that the He I triplet line intensities are significantly enhanced by electron collisional excitation from 2(3)P and 2(3)S. It is suggested that the gas which gives rise to the broad He I lines in 3C 120, Mrk 618, and NGC 7469 is characterized by a number density of about 5 billion per cu cm, a temperature of approximately 15,000 K, and an optical depth of 100 to 120 in the 10,830-A line.

Feldman, F. R.↗
NASA NTRS1984

Physical conditions in the cool parts of prominences. II - The Mg triplet lines

The statistical equilibrium of Mg under prominence conditions is considered. It is shown that the principal lines of the triplet system can serve as an equivalent to the Na D lines for spectral diagnostic purposes. Like the Na D lines, these Mg lines are relatively easy to measure in prominences and exhibit small optical depths. It is demonstrated that potential complications due to possible radiative transfer effects in the Mg resonance transition (3 1S0 - 3 1P1) have little consequence for the triplet populations. The results are used to repeat the spectral analysis of Landman (1983) using the Mg multiplets in place of the Na ones. The results of the two analyses are in general agreement. The finding of a low degree of H ionization in the cool prominence regions and a correspondingly high gas pressure is confirmed.

Landman, D. A.↗
NASA NTRS1993

Observations of the Ca II infrared triplet in chromospherically active single and binary stars

Spectroscopic observations of the Ca II infrared triplet (8498, 8542, 8662 A) have been obtained for 45 stars which are known or suspected to be chromospherically active. The sample includes both single and binary stars of spectral types from F2 to M5 spanning luminosity classes III, IV, and V. Several different types of activity diagnostics were measured, and their relative merits are discussed. Dependence of chromospheric emission upon rotation period, luminosity, temperature, and duplicity are analyzed. Synchronous binaries show a slight trend of increased emission with decreasing period while the asynchronous binaries show abnormally high activity levels for their rotation periods. Several stars exhibit rotationally modulated emission which is anticorrelated with the stellar brightness. Finally, estimates of chromospheric energy losses are presented with the result that the total loss in the infrared triplet is about twice that of the H and K lines.

Dempsey, Robert C.↗
NASA NTRS1995

DNA CTG triplet repeats involved in dynamic mutations of neurologically related gene sequences form stable duplexes

DNA triplet repeats, 5'-d(CTG)n and 5'-d(CAG)n, are present in genes which have been implicated in several neurodegenerative disorders. To investigate possible stable structures formed by these repeating sequences, we have examined d(CTG)n, d(CAG)n and d(CTG).d(CAG)n (n = 2 and 3) using NMR and UV optical spectroscopy. These studies reveal that single stranded (CTG)n (n > 2) forms stable, antiparallel helical duplexes, while the single stranded (CAG)n requires at least three repeating units to form a duplex. NMR and UV melting experiments show that the Tm increases in the order of [(CAG)3]2 < [(CTG)3]2 << (CAG)3.(CTG)3. The (CTG)3 duplex is stable and exhibits similar NMR spectra in solutions containing 0.1-4 M NaCl and at a pH range from 4.6 to 8.8. The (CTG)3 duplex, which contains multiple-T.T mismatches, displays many NMR spectral characteristics similar to those of B-form DNA. However, unique NOE and 1H-31P coupling patterns associated with the repetitive T.T mismatches in the CTG repeats are discerned. These results, in conjunction with recent in vitro studies suggest that longer CTG repeats may form hairpin structures, which can potentially cause interruption in replication, leading to dynamic expansion or deletion of triplet repeats.

NASA Discipline Exobiology↗