Singlet Spin Configuration Research Articles

Radical triads, not pairs, may explain effects of hypomagnetic fields on neurogenesis.

Adult hippocampal neurogenesis and hippocampus-dependent cognition in mice have been found to be adversely affected by hypomagnetic field exposure. The effect concurred with a reduction of reactive oxygen species in the absence of the geomagnetic field. A recent theoretical study suggests a mechanistic interpretation of this phenomenon in the framework of the Radical Pair Mechanism. According to this model, a flavin-superoxide radical pair, born in the singlet spin configuration, undergoes magnetic field-dependent spin dynamics such that the pair's recombination is enhanced as the applied magnetic field is reduced. This model has two ostensible weaknesses: a) the assumption of a singlet initial state is irreconcilable with known reaction pathways generating such radical pairs, and b) the model neglects the swift spin relaxation of free superoxide, which abolishes any magnetic sensitivity in geomagnetic/hypomagnetic fields. We here suggest that a model based on a radical triad and the assumption of a secondary radical scavenging reaction can, in principle, explain the phenomenon without unnatural assumptions, thus providing a coherent explanation of hypomagnetic field effects in biology.

Moiré-Trapped Interlayer Trions in a Charge-Tunable WSe2/MoSe2 Heterobilayer

Transition-metal dichalcogenide heterobilayers offer attractive opportunities to realize lattices of interacting bosons with several degrees of freedom. Such heterobilayers can feature moiré patterns that modulate their electronic band structure, leading to spatial confinement of single interlayer excitons (IXs) that act as quantum emitters with C3 symmetry. However, the narrow emission linewidths of the quantum emitters contrast with a broad ensemble IX emission observed in nominally identical heterobilayers, opening a debate regarding the origin of IX emission. Here we report the continuous evolution from a few trapped IXs to an ensemble of IXs with both triplet- and singlet-spin configurations in a gate-tunable 2H−MoSe2/WSe2 heterobilayer. We observe signatures of dipolar interactions in the IX ensemble regime which, when combined with magneto-optical spectroscopy, reveal that the narrow quantum-dot-like and broad ensemble emission originate from IXs trapped in moiré potentials with the same atomic registry. Finally, electron doping leads to the formation of three different species of localized negative trions with contrasting spin-valley configurations, among which we observe both intervalley and intravalley IX trions with spin-triplet optical transitions. Our results identify the origin of IX emission in MoSe2/WSe2 heterobilayers and highlight the important role of exciton-exciton interactions and Fermi-level control in these highly tunable quantum materials.Received 18 January 2021Revised 14 May 2021Accepted 9 June 2021DOI:https://doi.org/10.1103/PhysRevX.11.031033Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasExcitonsLuminescenceNanophotonicsPhysical Systems2-dimensional systemsQuantum dotsTransition metal dichalcogenidesCondensed Matter, Materials & Applied Physics

Molecular and electronic structure, spectroscopic and electrochemical properties of Copper(II) complexes: Experimental and DFT studies

Mononuclear copper(II) Schiff base complexes [Cu (L1)2] and [Cu (L2)2] (HL1 = 4-((E)-phenyldiazenyl)-2-((E)-(propylimino) methyl) phenol and HL2 = salicylidenepropylamine) were synthesized and characterized by elemental analysis and spectroscopic techniques. X-ray diffraction analysis showed that, in the solid state, the complex [Cu(L1)2] is centro-symmetric and the copper(II) ion has square-planar coordination geometry. It was found that the crystal packing of the compound is stabilized by π···π interactions between the phenyl and metal chelate rings. Molecular and electronic structures and electronic transitions of [Cu(L1)2], [Cu(L2)2] and free ligands were studied by calculations at DFT/B3LYP level of theory. According to the DFT calculations, the coordination geometry of the copper(II) in [Cu(L1)2] is deformed in the gas phase. The free gaseous [Cu(L2)2] complex was obtained as almost centro-symmetric square planar geometry. DFT calculations indicated that, by reduction of the complexes, the electron is added to the metal dx2-y2 orbital and copper ions in [Cu(L1)2]‾ and [Cu(L1)2]‾ show highly distorted tetrahedral coordination geometry. By one-electron oxidation of the complexes, d9 copper(II)-radical ligand complexes are formed in gaseous state. Moreover, gas phase 3[Cu(L1)2]+ and 3[Cu(L2)2]+ complexes are more stable than the corresponding geometries with singlet spin configuration. The measured UV–Vis spectrum of [Cu(L1)2] is better described by TDDFT calculation results for the optimized compound, rather than the centro-symmetric square-planar complex. Redox peak separations (ΔE = Ea-Ec) for [Cu(L1)2] and [Cu(L2)2] were measured by cyclic voltammetry at the scan rate of 0.1 V-1, to be 340 mV and 240 mV, respectively.

Zero Quantum Coherence in a Series of Covalent Spin-Correlated Radical Pairs

Photoinitiated subnanosecond electron transfer within covalently linked electron donor-acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet ms = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D-A systems. While transitions between the singlet and triplet ms = 0 spin states are formally forbidden (Δms = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet-triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron-nuclear hyperfine interactions.

A different way to put triplets in play

Photophysics Most molecules adopt a singlet spin configuration: All their electrons are arranged in pairs. Unpaired triplet states engage in a variety of useful reactions but are hard to produce. Quantum mechanics dictates that photo-excitation from singlet to triplet states is inefficient. Instead

Direct observation of triplet energy transfer from semiconductor nanocrystals.

Triplet excitons are pervasive in both organic and inorganic semiconductors but generally remain confined to the material in which they originate. We demonstrated by transient absorption spectroscopy that cadmium selenide semiconductor nanoparticles, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surface-anchored polyaromatic carboxylic acid acceptors, extending the excited-state lifetime by six orders of magnitude. Net triplet energy transfer also occurs from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in an aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to the bulk solution implies that such materials are generally effective surrogates for molecular triplets. The nanoparticles could thereby potentially sensitize a range of chemical transformations that are relevant for fields as diverse as optoelectronics, solar energy conversion, and photobiology.

Theoretical investigation of polymer chain stability in the metal coordinated azorubine and cyclam complex

Theoretical investigations have been performed for unit systems with Ni(II) and Zn(II) coordination between azorubine and 1,4,8,11-tetraazacyclotetradecane (cyclam) complexes using the conventional DFT and the DFT-based tight binding (DFTB) methods. Two different geometries (short and long) and spin states (singlet and triplet) of the model system built by two mesylate groups and the cyclam ring together with Ni(II) and Zn(II) ions were energetically characterized. For the Ni(II) coordination complex the triplet geometry is preferred, but one could not exclude also the presence of the singlet spin configuration due to the huge energy barrier defined by the intersystem crossing. The intersystem crossing geometry of the singlet–triplet transition was studied in details and the corresponding spin–orbit couplings were discussed. For the Zn(II) coordination complex only the singlet state was found. Polymer chain build up from four unit systems presents irregular forms with strong coordination bonds between units.

Supercurrent through a multilevel quantum dot close to singlet-triplet degeneracy

We investigate two serially aligned quantum dots in the molecular regime of large tunnel couplings $t$. A Zeeman field $B$ is used to tune the energy difference of singlet and triplet spin configurations. Attaching this geometry to BCS source and drain leads with gap $\ensuremath{\Delta}$ and phase difference $\ensuremath{\varphi}$ gives rise to an equilibrium supercurrent $J$. To compute $J$ in the presence of Coulomb interactions $U$ between the dot electrons, we employ the functional renormalization group (FRG). For $B\ensuremath{\approx}t$, where the singlet and (one out of a) triplet spin states are equal in energy, the current exhibits characteristics of a 0-$\ensuremath{\pi}$ transition similar to a single impurity. Its magnitude in the $\ensuremath{\pi}$ phase, however, jumps discontinuously at $B=t$, being smaller on the triplet side. By exploiting the flexibility of the FRG, we demonstrate that this effect is generic and calculate $J$ for realistic experimental parameters $\ensuremath{\Delta}$, $U$, and gate voltages $\ensuremath{\epsilon}$. To obtain a more thorough understanding of the discontinuity, we analytically treat the limit $\ensuremath{\Delta}=\ensuremath{\infty}$, where one can access the exact many-particle states. Finally, carrying out perturbation theory in the dot-lead couplings substantiates the intuitive picture that Cooper-pair tunneling is favored by a singlet spin configuration while inhibited by a triplet one.

A computational investigation of the Ni-doped Si n( n=1−8) clusters by a density functional method

The NiSi n ( n=1−8) clusters are investigated computationally using a Density Functional approach. Geometry optimizations of the NiSi n ( n=1−8) units are carried out under the constraint of well-defined symmetries at the B3LYP level employing a pseudopotential method in conjunction with Los Alamos double zeta basis sets. Consequently, the resulting total energies, natural populations, fragmentation energies, and equilibrium geometries of the NiSi n ( n=1−8) clusters are presented and discussed. Theoretical results show that the Ni atom in the small NiSi n clusters absorbs on the surface site, and that the Ni atom in the NiSi 8 cluster prefers being encapsulated into the Si frame. The most stable NiSi n ( n=1, 2) and NiSi n ( n=3−8) clusters correspond to the triplet and singlet spin configurations, respectively. The natural population of the Ni atom in the most stable NiSi n ( n=1−8) clusters varies from positive to negative. The relative stabilities are discussed; the calculated results reveal that the NiSi 8 cluster is the most stable structure.

Hartree-Fock investigations of the structure of Sb4 clusters.

We have investigated the geometry as well as the equilibrium bond lengths for a number of possible structures of the Sb 4 cluster by the Hartree-Fock procedure. The tetrahedral structure is shown to exhibit higher stability than several planar structures studied. No stability could be found for a square in the singlet spin configuration, in keeping with considerations based on the Jahn-Teller theorem