Spin-flip State Research Articles

Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)-Er(III) Assembly.

Pseudo-octahedral CrIIIN6 chromophores hold a unique appeal for low-energy sensitization of NIR lanthanide luminescence due to their exceptionally long-lived spin-flip excited states. This allure persists despite the obstacles and complexities involved in integrating both elements into a metallosupramolecular assembly. In this work, we have designed a structurally optimized heteroleptic CrIII building block capable of binding rare earths. Following a complex-as-ligand synthetic strategy, two heterometallic supramolecular assemblies, in which three peripherical CrIII sensitizers coordinated through a molecular wire to a central ErIII or YIII, have been prepared. Upon excitation of the CrIII spin-flip states, the downshifted Er(4I13/2 → 4I15/2) emission at 1550 nm was induced through intramolecular energy transfer. Time-resolved experiments at room temperature reveal a CrIII → ErIII energy transfer of 62-73% efficiencies with rate constants of about 8.5 × 105 s-1 despite the long donor-acceptor distance (circa 14 Å). This efficient directional intermetallic energy transfer can be rationalized using the Dexter formalism, which is promoted by a rigid linear electron-rich alkyne bridge that acts as a molecular wire connecting the CrIII and ErIII ions.

On the Prediction of Spectroscopic Fingerprints of Co2+ Complexes Relevant for the ZIF Nucleation Process.

The nucleation process of zeolitic imidazolate frameworks (ZIFs) is to date not completely understood. Recently, it has been found that, during the formation of Co-ZIF-67, after mixing imidazole-type ligands with octahedral precursors containing oxygen-coordinated ligands, a metal-organic pool with a diversity of transition metal complexes (TMCs) is formed showing fingerprints of octahedral and tetrahedral Co2+ complexes with both types of ligands [Filez, M. Cell Rep. Phys. Sci. 2021, 2, 100680]. In order to further unravel this process, we aim to characterize the d-d transitions of the TMCs and focus on their number, intensity, and position, which change during the process and can thus serve as a fingerprint for the formed species. It was previously shown that the number of ligands and symmetry has a detrimental influence on the ground state properties of Co2+ TMCs. Herein, we investigate how far excited state properties of TMCs relevant during nanoporous formation processes can be predicted by time-dependent density functional theory (TDDFT) and ligand field density functional theory (LFDFT). As TMCs are known to be challenging systems with possibly degenerate ground states and double excitations, we first investigate the performance of both techniques on first-row octahedral aqua-complexes. With this knowledge, we then focus on tetrahedral Co2+ complexes with aqua and imidazole-type ligands in order to investigate in how far we can propose a spectroscopic fingerprint that allows us to follow the Co2+ complexes during the formation of Co-ZIF-67. The results of TDDFT and LFDFT are qualitatively in agreement and provide complementary information. We found that various features can be used to distinguish between the species. However, as LFDFT is not suited for TMCs possessing the extended imidazole-type ligands and double and spin-flip states are not included in TDDFT, both techniques need to be complemented with more advanced methods to obtain complete insight into the d-d excitations of TMCs with imidazole ligands. Therefore, we particularly explored ab initio ligand field theory, which is capable of describing double excitations and is, in contrast to LFDFT, suitable for TMCs with extended ligands.

Electronic Structure and Excited-State Dynamics of the NIR-II Emissive Molybdenum(III) Analogue to the Molecular Ruby.

Photoactive chromium(III) complexes saw a conceptual breakthrough with the discovery of the prototypical molecular ruby mer-[Cr(ddpd)2]3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine), which shows intense long-lived near-infrared (NIR) phosphorescence from metal-centered spin-flip states. In contrast to the numerous studies on chromium(III) photophysics, only 10 luminescent molybdenum(III) complexes have been reported so far. Here, we present the synthesis and characterization of mer-MoX3(ddpd) (1, X = Cl; 2, X = Br) and cisfac-[Mo(ddpd)2]3+ (cisfac-[3]3+), an isomeric heavy homologue of the prototypical molecular ruby. For cisfac-[3]3+, we found strong zero-field splitting using magnetic susceptibility measurements and electron paramagnetic resonance spectroscopy. Electronic spectra covering the spin-forbidden transitions show that the spin-flip states in mer-1, mer-2, and cisfac-[3]3+ are much lower in energy than those in comparable chromium(III) compounds. While all three complexes show weak spin-flip phosphorescence in NIR-II, the emission of cisfac-[3]3+ peaking at 1550 nm is particularly low in energy. Femtosecond transient absorption spectroscopy reveals a short excited-state lifetime of 1.4 ns, 6 orders of magnitude shorter than that of mer-[Cr(ddpd)2]3+. Using density functional theory and ab initio multireference calculations, we break down the reasons for this disparity and derive principles for the design of future stable photoactive molybdenum(III) complexes.

Charge-Transfer and Spin-Flip States: Thriving as Complements.

Transition metal complexes with photoactive charge-transfer excited states are pervasive throughout the literature. In particular, [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine), with its metal-to-ligand charge-transfer emission, has been established as a key complex. Meanwhile, interest in so-called spin-flip metal-centered states has risen dramatically after the molecular ruby [Cr(ddpd)2 ]3+ (ddpd=N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridine-2,6-diamine) led to design principles to access strong, long-lived emission from photostable chromium(III) complexes. This Review contrasts the properties of emissive charge-transfer and spin-flip states by using [Ru(bpy)3 ]2+ and [Cr(ddpd)2 ]3+ as prototypical examples. We discuss the relevant excited states, the tunability of their energy and lifetimes, and their response to external stimuli. Finally, we identify strengths and weaknesses of charge-transfer and spin-flip states in applications such as photocatalysis and circularly polarized luminescence.

Towards Luminescent Vanadium(II) Complexes with Slow Magnetic Relaxation and Quantum Coherence.

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V2+ complexes [V(ddpd)2 ]2+ with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm-1 allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T1 of around 300 μs and a phase memory time TM of around 1 μs at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)2 ]3+ are emissive in the near-IR-II region, the [V(ddpd)2 ]2+ complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states.

Low-Energy M1 States in Deformed Nuclei: Spin Scissors or Spin-Flip?

The low-energy $$M$$ 1 states in deformed $${}^{164}$$ Dy and spherical $${}^{58}$$ Ni are explored in the framework of fully self-consistent Quasiparticle Random-Phase Approximation (QRPA) with various Skyrme forces. The main attention is paid to orbital and spin $$M$$ 1 excitations. The obtained results are compared with the prediction of the low-energy spin-scissors $$M$$ 1 resonance suggested within Wigner Function Moments (WFM) approach. A possible relation of this resonance to low-energy spin-flip excitations is analyzed. In connection with recent WFM studies, we consider evolution of the low-energy spin-flip states in $${}^{164}$$ Dy with deformation (from the equilibrium value to the spherical limit). The effect of tensor forces is briefly discussed. It is shown that two groups of $$1^{+}$$ states observed at 2.4–4 MeV in $${}^{164}$$ Dy are rather explained by fragmentation of the orbital $$M$$ 1 strength than by the occurrence of the collective spin-scissors resonance. In general, our calculations do not confirm the existence of this resonance.

Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis

The photoredox activity of well-known RuII complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. CrIII polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a CrIII complex is very similar to that for a RuII polypyridine, and thereby validate the concept of estimating the redox potential of d3 spin-flip excited states in analogous manner as for the MLCT states of d6 compounds. Building on this insight, we use our CrIII complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2.

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr(bpmp)2]3+ (bpmp = 2,6-bis(2-pyridylmethyl)pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D2O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr(bpmp)2]3+ and its facile reduction to [Cr(bpmp)2]2+ result in a high excited state redox potential. The ligand's methylene bridge acts as a Brønsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru(α-diimine)3]2+ charge transfer complexes, which are commonly employed in optical sensing and photo(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Microscopic analysis of low-energy spin and orbital magnetic dipole excitations in deformed nuclei

A low-energy magnetic dipole $(M1)$ spin-scissors resonance (SSR) located just below the ordinary orbital scissors resonance (OSR) was recently predicted in deformed nuclei within the Wigner function moments (WFM) approach. We analyze this prediction using fully self-consistent Skyrme quasiparticle random phase approximation (QRPA) method. Skyrme forces SkM*, SVbas, and SG2 are implemented to explore SSR and OSR in $^{160,162,164}\mathrm{Dy}$ and $^{232}\mathrm{Th}$. Accuracy of the method is justified by a good description of $M1$ spin-flip giant resonance. The calculations show that isotopes $^{160,162,164}\mathrm{Dy}$ indeed have at 1.5--2.4 MeV (below OSR) ${I}^{\ensuremath{\pi}}K={1}^{+}1$ states with a large $M1$ spin strength ($K$ is the projection of the total nuclear moment to the symmetry $z$ axis). These states are almost fully exhausted by $pp[411\ensuremath{\uparrow},411\ensuremath{\downarrow}]$ and $nn[521\ensuremath{\uparrow},521\ensuremath{\downarrow}]$ spin-flip configurations corresponding to $pp[2{d}_{3/2},2{d}_{5/2}]$ and $nn[2{f}_{5/2},2{f}_{7/2}]$ structures in the spherical limit. So the predicted SSR is actually reduced to low-orbital ($l=2,3$) spin-flip states. Following our analysis and in contradiction with WFM spin-scissors picture, deformation is not the principle origin of the low-energy spin $M1$ states but only a factor affecting their features. The spin and orbital strengths are generally mixed and exhibit interference: weakly destructive in SSR range and strongly constructive in OSR range. In $^{232}\mathrm{Th}$, the $M1$ spin strength is very small. Two groups of ${I}^{\ensuremath{\pi}}={1}^{+}$ states observed experimentally at 2.4--4 MeV in $^{160,162,164}\mathrm{Dy}$ and at 2--4 MeV in $^{232}\mathrm{Th}$ are mainly explained by fragmentation of the orbital strength. Distributions of nuclear currents in QRPA states partly correspond to the isovector orbital-scissors flow but not to the spin-scissors one.

Origin of the hysteresis of magnetoconductance in a supramolecular spin-valve based on aTbPc2single-molecule magnet

We present a time-dependent microscopic model for Coulomb blockade transport through an experimentally realized supramolecular spin-valve device driven by an oscillating magnetic field, in which the $4f$-electron magnetic states of an array of $\mathrm{Tb}{\mathrm{Pc}}_{2}$ single-molecule magnets (SMMs) were observed to modulate a sequential tunneling current through an underlying substrate nanoconstriction. Our model elucidates the dynamical mechanism at the origin of the observed hysteresis loops of the magnetoconductance, a signature of the SMM-modulated spin-valve effect, in terms of a phonon-assisted multi-spin-reversal cascade relaxation process, which mediates the switching of the device between the two conductive all-parallel spin configurations of the SMM array. Moreover, our proposed model can explain the zero-bias giant magnetoresistive transport gap measured in this device, solely within the incoherent transport regime, consistently with the experimental observations, as opposed to previous interpretations invoking Fano-resonance conductance suppression within a coherent ballistic transport regime. Finally, according to the proposed Coulomb blockade scenario, the SMM-mediated giant magnetoresistance effect is predicted to increase with the number of SMMs aligned on the nanoconstriction surface, on account of the increased number of intermediate nonconducting spin-flip states intervening in the phonon-assisted multi-spin-reversal cascade relaxation process necessary to switch between the two conducting all-parallel SMM spin configurations.

Spin-isospin response in finite nuclei from an extended Skyrme interaction

The magnetic dipole (M1) and the Gamow-Teller (GT) excitations of finite nuclei have been studied in a fully self-consistent Hartree-Fock (HF) plus random phase approximation (RPA) approach by using a Skyrme energy density functional with spin and spin-isospin densities. To this end, we adopt the extended SLy5st interaction which includes spin-density dependent terms and stabilize nuclear matter with respect to spin instabilities. The effect of the spin-density dependent terms is examined in both the mean field and the spin-flip excited state calculations. The numerical results show that those terms give appreciable repulsive contributions to the M1 and GT response functions of finite nuclei.

Excited Cr impurity states in Al2O3from constraint density functional theory

The excited states, ${}^{4}{T}_{2g}$ and ${}^{2}{E}_{g}$, of a Cr impurity in Al${}_{2}$O${}_{3}$ were treated by constraint density functional theory by imposing a density matrix constraint (constraint field) to control the electron occupation numbers of the $d$ orbitals. The calculated excitation energies, directly calculated from the self-consistent total energies of the ${}^{4}{A}_{2g}$ ground states and the various excited states, correctly reproduce the experimental ordering. In addition, we find that there is no stationary solution for the excited ${}^{4}{T}_{2g}$ state corresponding to the crystal-field transition state in the usual Kohn-Sham equation, i.e., with no constraint field. By contrast, the excited ${}^{2}{E}_{g}$ state of the spin-flip transition state is a (meta-) stable stationary solution, and may be responsible for the long radiative decay lifetime observed in experiments on ruby.

Magnetic excitations inL-edge resonant inelastic x-ray scattering from cuprate compounds

We study the magnetic excitation spectra in L-edge resonant inelastic x-ray scattering (RIXS) from undoped cuprates. We analyze the second-order dipole allowed process that the strong perturbation works through the intermediate state in which the spin degree of freedom is lost at the core-hole site. Within the approximation neglecting the perturbation on the neighboring sites, we derive the spin-flip final state in the scattering channel with changing the polarization, which leads to the RIXS spectra expressed as the dynamical structure factor of the transverse spin components. We assume a spherical form of the spin-conserving final state in the channel without changing the polarization, which leads to the RIXS spectra expressed as the 'exchange'-type multi-spin correlation function. Evaluating numerically the transition amplitudes to these final states on a finite-size cluster, we obtain a sizable amount of the transition amplitude to the spin-conserving final state in comparison with that to the spin-flip final state. We treat the itinerant magnetic excitations in the final state by means of the 1/S-expansion method. Evaluating the higher-order correction with 1/S, we find that the peak arising from the one-magnon excitation is reduced with its weight, and the continuous spectra arising from the three-magnon excitations come out. The interaction between two magnons is treated by summing up the ladder diagrams. On the basis of these results, we analyze the L_3-edge RIXS spectra in Sr_2CuO_2Cl_2 in comparison with the experiment. It is shown that the three-magnon excitations as well as the two-magnon excitations give rise to the intensity in the high energy side of the one-magnon peak, making the spectral shape asymmetric with wide width, in good agreement with the experiment.

Spin-flip effect on transport properties of a Mn3 molecule

Electron transport through a single-molecule magnet [NEt4]3[Mn3Zn2(salox)3O(N3)6Cl2] is investigated by spin-polarized density functional theory combined with the Keldysh nonequilibrium Green’s function technique. Our study demonstrates that spin-filtering effect and negative differential resistance exist in the ground state of this molecule. When the magnetic state of the molecule is changed from its ground state to the spin-flip state, substantial changes are induced not only in energy levels of the molecule, but also in the coupling of molecular states with eigenstates of Ag(100) nano-electrodes, which lead to the disappearance of spin-filtering effect and negative differential resistance.

Observation of Nonconventional Spin Waves in Composite-Fermion Ferromagnets

We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.

Nitrogen vacancy and oxygen impurity in AlN: spintronic quantum dots

Point defects with non-zero spin are prototypical spintronic quantum dots. Here two anion-site defects in AlN are studied intensively in terms of their spin and related properties. The charged states of the nitrogen vacancy and substitutional oxygen impurity, in both ground and spin-flip states, are analyzed. The theoretical analysis includes optical absorption and emission, diffuse excited states, spin densities and local mode force constants. The relevance to spintronic quantum dots in semiconductors is discussed.

High resolution study of isovector negative parity states in theO16(He3,t)F16reaction at 140 MeV/nucleon

The isovector transitions from the ground state (g.s.) of $^{16}\mathrm{O}$ to the negative parity states in $^{16}\mathrm{F}$, i.e., the ${J}^{\ensuremath{\pi}}={0}^{\ensuremath{-}}$ g.s., the 0.193 MeV, ${1}^{\ensuremath{-}}$ state, the 0.424 MeV, ${2}^{\ensuremath{-}}$ state, the 0.721 MeV, ${3}^{\ensuremath{-}}$ state, and the ${4}^{\ensuremath{-}}$ ``stretched'' state at 6.372 MeV, were studied by using a high resolution $^{16}\mathrm{O}(^{3}\mathrm{He},t)^{16}\mathrm{F}$ reaction at 140 MeV/nucleon. With the help of high energy resolution, these states were, for the first time, clearly resolved in a charge exchange reaction at an intermediate energy, which favorably excites spin-flip states. Angular distributions of the reaction cross sections were measured in the laboratory frame from ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$ to ${14}^{\ifmmode^\circ\else\textdegree\fi{}}$. Parameters of phenomenological effective interactions were derived so as to reproduce these angular distributions in distorted wave Born approximation (DWBA) calculations. The angular distribution of the ${0}^{\ensuremath{-}}$ state could be reproduced well at ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}l{10}^{\ifmmode^\circ\else\textdegree\fi{}}$. The empirical values, however, are larger by a factor of 2--2.5 in the larger angle region, where the contribution of the so-called ``condensed pion field'' is expected. The high resolution also enabled the decay widths of these states to be measured.

Gamma-ray spectroscopy of hypernuclei: Recent results and future plans

A series of hypernuclear γ-ray spectroscopy experiments using a germanium detector array, Hyperball, have accumulated precise data on various p-shell Λ hypernuclei. Recently, 12ΛC and 11ΛB were studied at KEK using the (π+,K+γ) reaction, and a transition from a directly-populated spin-flip state, 16ΛO(2−), was observed in BNL E930 data. It is discussed whether the spin-dependent ΛN interaction parameters determined from previous experiments can consistently explain other γ spectroscopy data. At the J-PARC facility, further hypernuclear γ-ray spectroscopy experiments are planned, particularly to measure the g factor of a Λ in a nucleus from a spin-flip B(M1) value and to investigate the ΛN interaction more in detail.

Observation of the 7 MeV excited spin-flip and non-spin-flip partners in 16 ΛO by γ -ray spectroscopy

We have observed three λ-ray transitions in Λ 16 O from both 7MeV excited spin-flip and non-spin-flip partners (2−, 1 2 − ) to the ground-state doublet (1 1 − , 0−) via the 16O(K −, π−) reaction. The 7MeV excitation energies of the spin-doublet members (2−, 1 2 − ) were determined to be 6784 ± 4 ± 4 keV and 6562 ± 1 ± 2 keV, respectively, and thus the spacing was obtained to be 222 ± 4 ± 5 keV. This is the first observation of the spin-flip state directly populated by the (K −, π−) reaction. Moreover, such directly populated spin-flip and non-spin-flip partners were resolved for the first time.

Pressure-driven magnetic moment collapse in the ground state of MnO

The zero temperature Mott transition region in antiferromagnetic, spin MnO is probed using the correlated band theory LSDA+U method. The first transition encountered is an insulator–insulator volume collapse within the rocksalt structure that is characterized by an unexpected Hund's rule violating ‘spin-flip’ moment collapse. This spin-flip to takes fullest advantage of the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy (which increases with decreasing volume) while retaining a sizable amount of the magnetic exchange energy. While transition pressures vary with the interaction strength, the spin-flip state is robust over a range of interaction strengths and for both B1 and B8 structures.