Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “triplet state”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

A fluorescent-protein spin qubit

Quantum bits (qubits) are two-level quantum systems that support initialization, readout and coherent control1. Optically addressable spin qubits form the foundation of an emerging generation of nanoscale sensors. The engineering of these qubits has mainly focused on solid-state systems. However, fluorescent proteins, rather than exogenous fluorescent probes, have become the gold standard for in vivo microscopy because of their genetic encodability. Although fluorescent proteins possess a metastable triplet state, they have not been investigated as qubits. Here we realize an optically addressable spin qubit in enhanced yellow fluorescent protein. A near-infrared laser pulse enables triggered readout of the triplet state with up to 20% spin contrast. Using coherent microwave control of the enhanced-yellow-fluorescent-protein spin at liquid-nitrogen temperatures, we measure a (16 ± 2) μs coherence time under Carr–Purcell–Meiboom–Gill decoupling. We express the qubit in mammalian cells, maintaining contrast and coherent control despite the complex intracellular environment. Finally, we demonstrate optically detected magnetic resonance in bacterial cells at room temperature with contrast up to 8%. Our results introduce fluorescent proteins as a powerful qubit platform that paves the way for applications in the life sciences, such as nanoscale field sensing and spin-based imaging modalities.

Feder, Jacob S. [Univ. of Chicago, IL (United Stat↗
DOE OSTI2023

Tetrafluorenofulvalene as a sterically frustrated open-shell alkene

Electronic and steric effects are known to greatly influence the structure, characteristics and reactivity of organic compounds. A typical π bond is weakened by oxidation (corresponding to the removal of electrons from bonding orbitals), by reduction (through addition of electrons to antibonding orbitals) and by unpairing of the bonding electrons, such as in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules do not hold. Through the synthesis, experimental characterization and computational analysis of its charged species spanning seven redox states, the central alkene bond in TFF is shown to become substantially stronger in the tri- and tetraanion, generated by chemical reduction. Furthermore, although its triplet state contains a weaker alkene bond than the singlet, in the quintet state its bond order increases substantially, yielding a flatter structure. This behaviour originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin pairing.

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

Low-Threshold Visible-to-Ultraviolet Solid-State Triplet-Fusion Upconversion

Visible-to-ultraviolet (UV) triplet-triplet annihilation (TTA) upconversion is an energy-relevant photon-management strategy that converts visible photons into UV photons capable of driving high-energy photochemical processes. Practical implementation requires solid-state thin films, which often require high excitation power. Here, we demonstrate the first solid-state visible-to-UV TTA upconversion thin-film device, using 1,4-bis((tricyclopentylsilyl)ethynyl)naphthalene (TCPS-NAP) as the annihilator and tris(2-phenylpyridine)iridium(III) (Ir(ppy) 3 ) as the sensitizer on silver. Under incoherent 455 nm excitation, the device exhibits blue-to-UV upconversion with a threshold of 4.8 mW/cm 2 . Using surface plasmons in the planar silver film, we achieve green-to-UV upconversion with a 1.11 eV anti-Stokes shift and 6.9 mW/cm 2 threshold, a 12.2 × threshold enhancement over far-field 532 nm excitation. In solution, TCPS-NAP shows excimer-dominated anti-Stokes emission rather than UV upconversion. These results establish a platform for harvesting low-intensity visible light to drive UV processes while demonstrating that solution-phase screening does not predict solid-state upconversion performance.

Luminescence↗
DOE OSTI2020

Spin-Triplet Pairing State Evidenced by Half-Quantum Flux in a Noncentrosymmetric Superconductor

Building future fault-tolerant quantum computers may rely on a basic but so far elusive building-block-type of materials, superconductors with spin-triplet pairing, which were considered to be very rare. Now physicists demonstrate that they can be found in materials with low symmetry, different from most common crystalline materials. The new finding focuses on a particular type of crystals, the structure of which does not have inversion symmetry. For more than a decade, this material class (noncentrosymmetric superconductor) has been inferred to be a potential rich mine of spin-triplet superconductors. It is conceived that the spin-triplet pairing is promised by the broken inversion symmetry, however key experimental evidences remain lacking. Xu et al. applied a newly developed method to examine a prototype noncentrosymmetric superconductor, α-BiPd. Their experiment reveals the presence of the highly unusual half-integer quantization of magnetic flux in polycrystalline rings of α-BiPd, which is a smoking-gun evidence for spin-triplet pairing. This new finding ignites hopes that spin-triplet pairing may be a common presence among the dozens of known noncentrosymmetric superconductors as theoretically expected, which can now be unambiguously identified by the unique phenomenon of half-quantum magnetic flux.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Electronic couplings for singlet fission: Orbital choice and extrapolation to the complete basis set limit

Abstract For the search for promising singlet fission candidates, the calculation of the effective electronic coupling, which is required to estimate the singlet fission rate between the initially excited state (S 0 S 1 ) and the multiexcitonic state ( 1 TT, two triplets on neighboring molecules, coupled into a singlet), should be sufficiently reliable and fast enough to explore the configuration space. We propose here to modify the calculation of the effective electronic coupling using a nonorthogonal configuration interaction approach by: (a) using only one set of orbitals, optimized for the triplet state of the molecules, to describe all molecular electronic states, and (b) only taking the leading configurations into consideration. Furthermore, we also studied the basis set convergence of the electronic coupling, and we found, by comparison to the complete basis set limit obtained using the cc‐pV n Z series of basis sets, that both the aug‐cc‐pVDZ and 6–311++G** basis sets are a good compromise between accuracy and computational feasibility. The proposed approach enables future work on larger clusters of molecules than dimers.

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

Coherent photoexcitation of entangled triplet pair states

The functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S 1 → 1 TT). Here, the dark state 1 TT is typically generated from bright S 1 after optical excitation; however, the mechanistic details are hotly debated. Here we report a 1 TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S 1 . Using <10-fs transient absorption spectroscopy and pumping sub-resonantly, 1 TT is directly generated from the ground state. Applying this method to a range of pentacene dimers and thin films of various aggregation types, we determine the critical material properties that enable this forbidden pathway. Through a strikingly simple technique, this result opens the door for new mechanistic insights into 1 TT and other dark states in organic materials.

14 SOLAR ENERGY↗
DOE OSTI2021

Excited-State Dynamics and Nonlinear Optical Properties of Hyperpolarizable Chromophores Based on Conjugated Bis(terpyridyl)Ru(II) and Palladium and Platinum Porphyrinic Components: Impact of Heavy Metals upon Supermolecular Electro-Optic Properties

A new series of strongly coupled oscillators based upon (porphinato)Pd, (porphinato)Pt, and bis(terpyridyl)-ruthenium(II) building blocks is described. These RuPPd, RuPPt, RuPPdRu, and RuPPtRu chromophores feature bis- (terpyridyl)Ru(II) moieties connected to the (porphinato)metal unit via an ethyne linker that bridges the 4!-terpyridyl and porphyrin macrocycle meso-carbon positions. Pump–probe transient optical data demonstrate sub-picosecond excited singlet-to-triplet-state relaxation. The relaxed lowest-energy triplet (T 1 ) excited states of these chromophores feature absorption manifolds that span the 800–1200 nm spectral region, microsecond triplet-state lifetimes, and large absorptive extinction coefficients [ε(T 1 → T n ) > 4 × 10 4 M –1 cm –1 ]. Dynamic hyperpolarizability (β λ ) values were determined from hyper-Rayleigh light scattering (HRS) measurements carried out at several incident irradiation wavelengths over the 800–1500 nm spectral region. Relative to benchmark RuPZn and RuPZnRu chromophores which showed large βHRS values over the 1200–1600 nm range, RuPPd, RuPPt, RuPPdRu, and RuPPtRu displayed large βHRS values over the 850–1200 nm region. Generalized Thomas–Kuhn sum (TKS) rules and experimental hyperpolarizability values were utilized to determine excited state-to-excited state transition dipole terms from experimental electronic absorption data and thus assessed frequency-dependent β λ values, including two- and three-level contributions for both β zzz and β xzx tensor components to the RuPPd, RuPPt, RuPPdRu, and RuPPtRu hyperpolarizability spectra. Furthermore, these analyses qualitatively rationalize how the βzzz and βxzx tensor elements influence the observed irradiation wavelength-dependent hyperpolarizability magnitudes. The TKS analysis suggests that supermolecules related to RuPPd, RuPPt, RuPPdRu, and RuPPtRu will likely feature intricate dependences of experimentally determined βHRS values as a function of irradiation wavelength that derive from substantial singlet–triplet mixing, and complex interactions among multiple different β tensor components that modulate the long wavelength regime of the nonlinear optical response.

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

Triplet Excitons in Pentacene Are Intrinsically Difficult to Dissociate via Charge Transfer

Singlet fission (SF) has the potential to bypass the Shockley–Queisser limit for solar cell efficiency through the production of two electron–hole pairs per photon. However, in polycrystalline pentacene this goal is hindered by slow charge transfer from triplets (=10 7 s –1 ) after SF. In this paper we demonstrate that slow charge transfer is an intrinsic property of triplet states in this material and most likely not connected with the triplet pair states that may result from singlet fission. We compare two perylene diimide/pentacene charge transfer systems that differ only by triplet generation mechanism: SF versus intersystem crossing (ISC), sensitized by using a soluble lead phthalocyanine derivative. We use time-resolved microwave conductivity (TRMC) to measure the charge yield in each system and transient absorption (TA) to follow the triplet population dynamics. These experiments are described by a single global kinetic model, with most of its parameters fixed via control experiments. While we observe modest differences in the charge-transfer rate constants between each sample, 4–10× as a function of triplet generation mechanism, all samples remain far below the predicted diffusion-limited rate constant of 10 7 –10 8 s –1 .

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

Spin–Orbit versus Hyperfine Coupling-Mediated Intersystem Crossing in a Radical Pair

While spin–orbit coupling (SOC) is typically the dominant interaction that couples singlet and triplet states within individual chromophores, hyperfine coupling (HFC) becomes important in multichromophoric systems, particularly in relation to the radical pair mechanism. Here, we use TD-DFT to calculate the spin–orbit coupling and hyperfine coupling between the first singlet and triplet charge transfer states of the radical pair 2 Pyrene– and 2 N,N-dimethylaniline + . We show that, as the intermolecular donor–acceptor distance grows, SOC decays to zero (as one would expect) because singlet and triplet states are characterized by identical orbitals in space, while the HFC remains comparatively constant. Here, the switching region occurs around 4 Å, beyond which HFC dominates over SOC as far as defining the rate of intersystem crossing (ISC).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Direct Exciton Harvesting from a Bound Triplet Pair

Abstract Singlet fission is commonly defined as the generation of two triplet excitons from a single absorbed photon. However, ambiguities within this definition arise due to the complexity of the various double triplet states that exist in SF chromophores and the corresponding interconversion processes. To clarify this process, singlet fission is frequently depicted as sequential two‐step conversion in which a singlet exciton decays into a bound triplet‐pair biexciton state that dissociates into two “free” triplet excitons. However, this model discounts the potential for direct harvesting from the coupled biexciton state. Here, it is demonstrated that individual triplet excitons can be extracted directly from a bound triplet pair. It is demonstrated that due to the requirement for geminate triplet–triplet annihilation in intramolecular singlet fission compounds, unique spectral and kinetic signatures can be used to quantify triplet‐pair harvesting yields. An internal quantum efficiency for triplet exciton transfer from the triplet pair of >50%, limited only by the solubility of the compounds is achieved. The harvesting process is not dependent on the net multiplicity of the triplet‐pair state, suggesting that an explicit, independent dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.

Chemistry↗
DOE OSTI2024

Homomolecular Triplet–Triplet Annihilation in Metalloporphyrin Photosensitizers

Metalloporphyrins are ubiquitous in their applications as triplet photosensitizers, particularly for promoting sensitized photochemical upconversion processes. In this study, bimolecular excited state triplet–triplet quenching kinetics, termed homomolecular triplet–triplet annihilation (HTTA), exhibited by the traditional triplet photosensitizers–zinc(II) tetraphenylporphyrin (ZnTPP), palladium(II) octaethylporphyrin (PdOEP), platinum(II) octaethylporphyrin (PtOEP), and platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP)–were revealed using conventional transient absorption spectroscopy. Nickel(II) tetraphenylporphyrin was used as a control sample as it is known to be rapidly quenched intramolecularly through ligand-field state deactivation and, therefore, cannot result in triplet–triplet annihilation (TTA). The single wavelength transients associated with the metalloporphyrin triplet excited state decay–measured as a function of incident laser pulse energy in toluene–were well modeled using parallel first- and second-order kinetics, consistent with HTTA being operable. The combined transient kinetic data enabled the determination of the first-order rate constants (k T ) for excited triplet decay in ZnTPP (4.0 × 10 3 s –1 ), PdOEP (3.6 × 10 3 s –1 ), PtOEP (1.2 × 10 4 s –1 ), and PtTPBP (2.1 × 10 4 s –1 ) as well as the second-order rate constant (k TT ) for HTTA in ZnTPP (5.5 × 10 9 M –1 s –1 ), PdOEP (1.1 × 10 10 M –1 s –1 ), PtOEP (7.1 × 10 9 M –1 s –1 ), and PtTPBP (1.6 × 10 10 M –1 s –1 ). In most instances, triplet excited state extinction coefficients are either reported for the first time or have been revised using ultrafast transient absorption spectroscopy and singlet depletion: ZnTPP (78,000 M –1 cm –1 ) at 470 nm, PdOEP (67,000 M –1 cm –1 ) at 430 nm, PtOEP (51,000 M –1 cm –1 ) at 418 nm, and PtTPBP (100,000 M –1 cm –1 ) at 460 nm. Furthermore, the combined experimental results establish competitive time scales for homo- and heteromolecular TTA rate constants, implying the significance of considering HTTA processes in future research endeavors harnessing TTA photochemistry using common metalloporphyrin photosensitizers.

14 SOLAR ENERGY↗
DOE OSTI2023

A Nonorthogonal Configuration Interaction Approach to Singlet Fission in Perylenediimide Compounds

Perylenediimide molecules constitute a family of chromophores that undergo singlet fission, a process in which an excited singlet state converts into lower energy triplets on two neighboring molecules, potentially increasing the efficiency of organic solar cells. Here, the nonorthogonal configuration interaction method is applied to study the effect of the different crystal packing of various perylenediimide derivatives on the relative energies of the singlet and triplet states, the intermolecular electronic couplings, and the relative rates for singlet fission. The analysis of the wave functions and electronic couplings reveals that charge transfer states play an important role in the singlet fission mechanism. Dimer conformations where the PDI molecules are at large displacements along the long axis and short on the short axis are posed as the most favorable for singlet fission. The role of the substituent at the imide group has been inspected concluding that, although it has no effect in the energies, for some conformations it significantly influences the electronic couplings, and therefore, replacing this substituent with hydrogen may introduce artifacts in the computational modeling of the PDI molecules.

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

Cool carriers: triplet diffusion dominates upconversion yield

Perovskites have gained popularity both as the active material in photovoltaics and as bulk triplet sensitizers for solid-state triplet–triplet annihilation upconversion (TTA-UC). Prior to widespread implementation into commercial photovoltaics, an in-depth understanding of the environmental influences on device performance is required. To this point, the temperature-dependent structure–function properties of TTA-UC within methylammonium formamidinium lead triiodide (MAFA)/rubrene UC devices are explored. A strong temperature dependence of the underlying UC dynamics is observed, where the maximum UC efficiency is achieved at 170 K, reflecting the competition between triplet diffusion length, diffusion rate, and triplet–triplet encounter events. Furthermore, a combination of spectroscopic and structural methods and theoretical modelling illustrates that despite the significantly increased carrier lifetime of the perovskite at low temperatures, the TTA-UC dynamics are not governed by the underlying sensitizer properties but rather limited by the underlying triplet diffusion.

Sullivan, Colette M.↗