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Kirk, Martin L.

Publications and source records attributed to Kirk, Martin L..

14 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Determining the Effects of Zero-Field Splitting and Magnetic Exchange in Dimeric Europium(II) Complexes

Related BAP [BAP = bis(acyl)phosphide] and Acac (Acac = β-diketonate) molecules perform as robust supports for both lanthanide and actinide metals. Here, a molecular bimetallic Eu 2+ complex was successfully targeted and isolated by employing sodium bis(mesitoyl)phosphide [Na( mes BAP)] in a salt metathesis with EuI 2 , producing [Eu( mes BAP) 2 (et 2 o)] 2 (et 2 o = metal-coordinated diethyl ether). The corresponding Acac-Eu 2+ complex was targeted using mes Acac– (1,3-dimesityl-1,3-propanedione), generating [Eu( mes Acac) 2 (et 2 o)] 2 . Both complexes were characterized by single-crystal X-ray diffraction, UV–vis, IR, and NMR spectroscopies, and variable-temperature magnetic susceptibility. [Eu( mes BAP) 2 (et 2 o)] 2 was persistent under anaerobic, anhydrous conditions, whereas the analogous [Eu( mes Acac) 2 (et 2 o)] 2 showed evidence of decomposition under identical conditions. Finally, variable-temperature magnetic susceptibility and magnetization studies of [Eu( mes BAP) 2 (et 2 o)] 2 and [Eu( mes Acac) 2 (et 2 o)] 2 were performed, resulting in similar magnetic exchange coupling values of J ex = –0.018 and –0.023 cm –1 and axial zero-field-splitting D values of –0.38 and –0.51 cm –1 , respectively.

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

Photoinduced Magnetic Exchange-Jump Promotes Ground State Biradical Electron Spin Polarization

Photoinduced electron spin polarization (ESP) is reported in the electronic ground states of three Pt(II) complexes that are comprised of two S = ½ nitronyl nitroxide (NN) radicals attached through different length para-phenylethynyl bridges to the 3,6 positions of a catecholate (CAT, donor) and 4,4’-di-tert-butyl-2,2’-bipyridine (bpy, acceptor). Complexes 1-3 have from 17 to 41 bonds separating NN radicals, and display cw-EPR spectra consistent with |J NN-NN | >> |a N |, |J NN-NN | ≥ |a N |, and |J NN-NN | < |a N |, respectively, where J NN-NN is the magnetic exchange coupling between NN radicals in the electronic ground state, and a N is the isotropic 14 N hyperfine coupling constant. Light-induced transient EPR spectra characterized as enhanced ground-state absorption were observed for all three complexes using 532 nm pulsed laser excitation into the ligand-to-ligand charge transfer (LL’CT) band of the (CAT)Pt(bpy) chromophore. The magnitude of the observed ESP increases in the order 1 < 2 < 3 and is inversely correlated with the magnitude of ground-state J NN-NN . In addition to the experimental observation net absorptive polarization in 1-3, light excitation also produces multiplet polarization in 2. Since the weak dipolar coupling leads to strong spectral overlap of the absorptive and emissive components, the multiplet polarization is not observed in 1 and 3 and is very weak in 2. Here, the ability to spin polarize multiple radical spins with a single photon is anticipated to advance new photoinduced multi qubit/qudit ESP protocols for quantum information science applications.

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

Optical Generation and Manipulation of Spin Qubits for Molecular Quantum Information Science (DE-SC0020199 Final Report)

Final Report containing published results and major accomplishments. Major accomplishments during this grant include elucidating the complex mechanisms of photoinduced ground-state electron spin polarization (PIESP) for these complexes. Several detailed studies relating molecular structure and PIESP are detailed. In addition, we have demonstrated polarization of two coupled ground-state spins with a single photon.

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

Advancing Our Understanding of Pyranopterin-Dithiolene Contributions to Moco Enzyme Catalysis

The pyranopterin dithiolene ligand is remarkable in terms of its geometric and electronic structure and is uniquely found in mononuclear molybdenum and tungsten enzymes. The pyranopterin dithiolene is found coordinated to the metal ion, deeply buried within the protein, and non-covalently attached to the protein via an extensive hydrogen bonding network that is enzyme-specific. However, the function of pyranopterin dithiolene in enzymatic catalysis has been difficult to determine. This focused account aims to provide an overview of what has been learned from the study of pyranopterin dithiolene model complexes of molybdenum and how these results relate to the enzyme systems. This work begins with a summary of what is known about the pyranopterin dithiolene ligand in the enzymes. We then introduce the development of inorganic small molecule complexes that model aspects of a coordinated pyranopterin dithiolene and discuss the results of detailed physical studies of the models by electronic absorption, resonance Raman, X-ray absorption and NMR spectroscopies, cyclic voltammetry, X-ray crystallography, and chemical reactivity.

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

Origin of Ferromagnetic Exchange Coupling in Donor–Acceptor Biradical Analogues of Charge-Separated Excited States

A new donor–acceptor biradical complex, Tp Cum,Me Zn(SQ-VD) (Tp Cum,Me Zn + = zinc(II) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate complex cation; SQ = orthosemiquinone; VD = oxoverdazyl), which is a ground-state analogue of a charge-separated excited state, has been synthesized and structurally characterized. Here, the magnetic exchange interaction between the S = 1/2 SQ and the S = 1/2 VD within the SQ-VD biradical ligand is observed to be ferromagnetic, with $J_{SQ-VD}$ = +77 cm –1 (H = -2$J_{SQ-VD}\bar{S}_{SQ}·\bar{S}_{VD}$) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in Tp Cum,Me Zn(SQ-VD) can be compared with that of the related donor–acceptor biradical complex Tp Cum,Me Zn(SQ-NN) (NN = nitronyl nitroxide, S = 1/2), where $J_{SQ-NN}$ ≅ +550 cm –1 . This represents a dramatic reduction in the biradical exchange by a factor of ~7, despite the isolobal nature of the VD and NN acceptor radical SOMOs. Computations assessing the magnitude of the exchange were performed using a broken-symmetry density functional theory (DFT) approach. These computations are in good agreement with those computed at the CASSCF NEVPT2 level, which also reveals an S = 1 triplet ground state as observed in the magnetic susceptibility measurements. A combination of electronic absorption spectroscopy and CASSCF computations has been used to elucidate the electronic origin of the large difference in the magnitude of the biradical exchange coupling between Tp Cum,Me Zn(SQ-VD) and Tp Cum,Me Zn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQ SOMO → NNLUMO charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet (S = 1) ground state in Tp Cum,Me Zn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VD LUMO and NN LUMO ) with the SOMO of the SQ radical (SQ SOMO ) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between Tp Cum,Me Zn(SQ-VD) and Tp Cum,Me Zn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the Tp Cum,Me Zn(SQ-VD) ground state, and the intrinsically weaker mixing between the VD LUMO and the SQ SOMO compared to that observed for Tp Cum,Me Zn(SQ-NN), where this orbital mixing is significant. The results of this comparative study contribute to a greater understanding of biradical exchange interactions, which are important to our understanding of excited-state singlet–triplet energy gaps, electron delocalization, and the generation of electron spin polarization in both the ground and excited states of (bpy)Pt(CAT-radical) complexes.

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

Single-Photon-Induced Electron Spin Polarization of Two Exchange-Coupled Stable Radicals

Transient electron paramagnetic resonance spectroscopy has been used to probe photoinduced electron spin polarization of a stable exchange-coupled organic biradical in a Pt(II) complex comprising 4,4'-di-tert-butyl-2,2'-bipyridine (bpy) and 3,6-bis(ethynyl-para-phenyl-nitronyl nitroxide)-o-catecholate (CAT(o-C$\equiv$C-Ph-NN) 2 ). Photoexcitation results in four unpaired spins in excited states of this complex, with spins being localized on each of the two radicals, CAT •+ and bpy •– . The four spins are all exchange-coupled in these excited states, and an off-diagonal matrix element in the CAT •+ -NN exchange allows for exchange-enhanced intersystem crossing to the 3 T 1a state, which possesses (bpy •– )Pt(CAT •+ ) chromophoric triplet character. Further, fast mixing between this 3 T 1a state and other thermally accessible excited LL'CT state(s) followed by fast relaxation provides spin polarization of the exchange-coupled NN radicals in the 3 S 0 ground state of the complex. Our results demonstrate that well-defined quantum states of a ground-state biradical can be initialized with single-photon excitation and have the potential for further spin manipulation directed toward quantum information science applications.

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

Magnetism Studies of Bis(acyl)phosphide-Supported Eu 3+ and Eu 2+ Complexes

A series of bis(acyl)phosphide-supported Eu complexes were synthesized (bis(acyl)phosphide = BAP). Here, in this study, BAP ligands proved to be excellent ligands for the synthesis of both Eu 3+ and Eu 2+ molecular complexes. Sodium bis(mesitoyl)phosphide (Na( mes BAP)) and sodium bis(2,4,6-triisopropylbenzoyl)phosphide (Na( tripp BAP)) were employed as ligand precursors for the synthesis of the Eu 3+ complexes Eu(bis(mesitoyl)phosphide) 3 (thf) 2 (Eu( mes BAP) 3 (thf) 2 ) and Eu(bis(2,4,6-triisopropylbenzoyl)phosphide) 3 (Eu( tripp BAP) 3 ), as well as the Eu 2+ complex, Eu(bis(2,4,6-triisopropylbenzoyl)phosphide) 2 (dme) 2 (Eu( tripp BAP) 2 (dme) 2 ) (thf = tetrahydrofuran, dme = 1,2-dimethoxyethane). All complexes were characterized using a combination of UV–vis–NIR–IR and NMR spectroscopies, and single-crystal X-ray diffraction (SC-XRD). The magnetic properties of these three monomeric Eu complexes were investigated by variable-temperature magnetic susceptibility. The magnetic data are typical for these ions, with Eu( tripp BAP) 2 (dme) 2 displaying Curie-type behavior. Both Eu( tripp BAP) 3 and Eu( mes BAP) 3 (thf) 2 possess similar 7 F 0 - 7 F 1 spin–orbit energy gaps and a similar zero-field splitting of the 7 F 1 state.

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

Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States

Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT- p -Me 2 PhMe 2 -NN) (bpy = 5,5'-di- tert -butyl-2,2'-bipyridine, CAT = 3- tert -butylcatecholate, p -Ph = para -phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet 2 S 1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a 2 T 1 (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction ( J SQ-NN ), which determines the 2 T 1 – 4 T 1 energy gap. Ground state ESP is dependent on the magnitude of J SQ-NN , and we postulate that this results from differences in 2 T 1 and 4 T 1 state mixing. Mechanisms that lead to the rapid transfer of the excited state ESP to the ground state are discussed. Although subnanosecond 2 T 1 state lifetimes are measured optically in solution, the ground state ESP decays very slowly at 20 K and is observable for more than a millisecond.

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

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

The sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground doublet states ( 2 S o /D o ) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronylnitroxide, NN) covalently attached to a donor acceptor chromophore via a meta-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4’-di-tert-butyl-2,2’-bipyridine, M = Pd II (1-Pd) or Pt II (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light pro-duces an initial exchange-coupled, 3-spin (bpy •- , CAT +• = semiquinone (SQ) and NN • ), charge-separated doublet 2 S 1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2 T 1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. Furthermore, it is postulated that zero-field splitting induced transitions between the chromophoric 2 T 1 and 4 T 1 states (1-Pd and 1-Pt) and spin-orbit induced transitions between 2 T 1 and NN-based quartet states (1-Pt) ac-count for the differences in polarization.

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

Exploiting Chemistry and Molecular Systems for Quantum Information Science

The power of chemistry to prepare new compositions of matter has driven the quest for new approaches to solve problems having global societal impact, such as renewable energy, healthcare, and information science. In the latter case, the intrinsic quantum nature of molecules offers intriguing new possibilities to advance the emerging field of Quantum Information Science (QIS). In this Perspective, we discuss how chemical systems and reactions can impact quantum computing, communication, and sensing. Hierarchical molecular design and synthesis, from small molecules to supramolecular assemblies, combined with new spectroscopic probes of quantum coherence and theoretical modeling of complex systems, offer a broad range of possibilities to realize practical QIS applications.

quantum computing, Spectrocopy, computational chem↗