Spin Multiplicity Research Articles

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

Cover Feature: Combining Open‐Shell Verdazyl Environment and Co(II) Spin‐Crossover: Spinmerism in Cobalt Oxoverdazyl Compound (ChemPhysChem 9/2022)

The Cover Feature illustrates the coexistence of local spin states on a cobalt(II) ion in the field produced by two radical verdazyl-based ligands. Entanglement combines different local spin states to generate a given total spin multiplicity, a spinmerism phenomenon. More information can be found in the Research Article by Vincent Robert and co-workers.

Long-Lived Charge-Transfer State in Spiro Compact Electron Donor-Acceptor Dyads Based on Pyromellitimide-Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization.

We observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).

Long‐Lived Charge‐Transfer State in Spiro Compact Electron Donor–Acceptor Dyads Based on Pyromellitimide‐Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization

AbstractWe observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).

He atom in a quantum dot: Structural, entanglement, and information-theoretical measures

Energy eigenvalues of ground and singly excited $1sns$ ($^{1,3}S$) ($n=2\ensuremath{-}5$, being the principal quantum number) states of a He atom in a quantum dot have been investigated in detail by incorporating explicitly correlated Hylleraas-type wave functions in the framework of the Ritz variational method. The quantum dot environment is simulated by considering the influence of the finite oscillator (FO) potential. We have examined the behavior of different energy components contributing to the total energy. In this regard, the Hund's spin multiplicity rule for $1sns$ ($^{1,3}S$) ($n=2\ensuremath{-}5$) states of a He atom has been examined in depth in terms of observed unusual ordering of the electron repulsion and total energy. The energy contribution due to the total correlation (in the presence of both radial and angular correlation) effect, the radial correlation limit and angular correlation limit of a He atom under different strengths of the FO potential have been studied. As a quantitative replication of our results, we have introduced and verified the Hellmann-Feynman theorem and virial theorem for both a He atom and its first ionization threshold, i.e., ${\mathrm{He}}^{+}$ ($1s,\phantom{\rule{0.16em}{0ex}}^{2}S$) ion under the influence of the FO potential. The expectation value of different radial quantities ${r}_{1}, {r}_{1}^{2}, {r}_{12},\phantom{\rule{0.16em}{0ex}}{r}_{12}^{2}, {r}_{<}=min({r}_{1},{r}_{2}),\phantom{\rule{0.16em}{0ex}}{r}_{>}=max({r}_{1},{r}_{2})$, angular quantities, e.g., interparticle angles ${\ensuremath{\theta}}_{1}, {\ensuremath{\theta}}_{12}$, one-electron delta function $\ensuremath{\delta}({\stackrel{P\vec}{r}}_{1})$, and two-electron delta function $\ensuremath{\delta}({\stackrel{P\vec}{r}}_{12})$ have been determined for different strengths of the FO potential. The effect of the FO potential on the quantum similarity index and dissimilarity between the ${\mathrm{He}}^{+}$ ($1s,\phantom{\rule{0.16em}{0ex}}^{2}S$) and He ($1{s}^{2},^{1}S$ and $1s2s,^{1,3}S$) as well as between He ($1s2s,^{1}S$) and He ($1s2s,^{3}S$) has been examined. The von Neumann, linear, and Shannon information entropy for the ground state He atom have been computed to ascertain the characteristic features of the electronic entanglement and the charge distribution under the FO potential.

Cross-Linked Polyphosphazene Nanospheres Boosting Long-Lived Organic Room-Temperature Phosphorescence.

Long-lived organic room-temperature phosphorescence (RTP) has sparked intense explorations, owing to the outstanding optical performance and exceptional applications. Because triplet excitons in organic RTP experience multifarious relaxation processes resulting from their high sensitivity, spin multiplicity, inevitable nonradiative decay, and external quenchers, boosting RTP performance by the modulated triplet-exciton behavior is challenging. Herein, we report that cross-linked polyphosphazene nanospheres can effectively promote long-lived organic RTP. Through molecular engineering, multiple carbonyl groups (C═O), heteroatoms (N and P), and heavy atoms (Cl) are introduced into the polyphosphazene nanospheres, largely strengthening the spin-orbit coupling constant by recalibrating the electronic configurations between singlet (Sn) and triplet (Tn) excitons. In order to further suppress nonradiative decay and avoid quenching under ambient conditions, polyphosphazene nanospheres are encapsulated with poly(vinyl alcohol) matrix, thus synchronously prompting phosphorescence lifetime (173 ms longer), phosphorescence efficiency (∼12-fold higher), afterglow duration time (more than 20 s), and afterglow absolute luminance (∼19-fold higher) as compared with the 2,3,6,7,10,11-hexahydroxytriphenylene precursor. By measuring the emission intensity of the phosphorescence, an effective probe based on the nanospheres is developed for visible, quantitative, and expeditious detection of volatile organic compounds. More significantly, the obtained films show high selectivity and robustness for anisole detection (7.1 × 10-4 mol L-1). This work not only demonstrates a way toward boosting the efficiency of RTP materials but also provides a new avenue to apply RTP materials in feasible detection applications.

Combining Open-Shell Verdazyl Environment and Co(II) Spin-Crossover: Spinmerism in Cobalt Oxoverdazyl Compound.

The spin states of a Co(II) oxoverdazyl compound are investigated by means of wavefunction-based calculations. Within a ca. 233 K energy window, the ground state and excited states display a structure-sensitive admixture of low-spin SM =1/2 in a dominant high-spin SM =3/2 Co(II) ion as indicated by the localized molecular orbitals. The puzzling spin zoology that results from the coupling between open-shell radical ligands and a spin-crossover metal ion gives rise to this unusual scenario, which extends the views in molecular magnetism. In agreement with experimental observation, the low-energy spectroscopy is very sensitive to deformations of the coordination sphere, and a growing admixture of Co(II) low-spin is evidenced from the calculations. In analogy with mesomerism that accounts for charge delocalization, entanglement combines different local spin states to generate a given total spin multiplicity, a spinmerism phenomenon.

An orbital-based representation for accurate quantum machine learning.

We introduce an electronic structure based representation for quantum machine learning (QML) of electronic properties throughout chemical compound space. The representation is constructed using computationally inexpensive ab initio calculations and explicitly accounts for changes in the electronic structure. We demonstrate the accuracy and flexibility of resulting QML models when applied to property labels, such as total potential energy, HOMO and LUMO energies, ionization potential, and electron affinity, using as datasets for training and testing entries from the QM7b, QM7b-T, QM9, and LIBE libraries. For the latter, we also demonstrate the ability of this approach to account for molecular species of different charge and spin multiplicity, resulting in QML models that infer total potential energies based on geometry, charge, and spin as input.

Long-Lived Triplet Charge Separated State and Thermally Activated Delayed Fluorescence in a Compact Orthogonal Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyad.

A long-lived triplet charge separated state (3CS state lifetime: 0.56 μs) was observed in a compact electron donor-acceptor dyad with electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C-N bond (AQ-PTZ). The 1CS state energy (2.0 eV in cyclohexane) is lower than those of the 3AQ (2.7 eV) or the 3PTZ state (2.6 eV). By oxidation of the PTZ unit, thus increasing of the 1CS state energy (2.7 eV in cyclohexane), thermally activated delayed fluorescence (TADF) was observed [τ = 17.7 ns (99.9%)/1.5 μs (0.1%)]. Time-resolved electron paramagnetic resonance (TREPR) spectra confirm the electron spin multiplicity of the 3CS state, and the zero-field-splitting (ZFS) parameters |D| and |E| are 48.2 mT and 11.2 mT, respectively. These results are useful for design of compact electron donor-acceptor dyads to access the long-lived 3CS state and study the TADF mechanism.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes.

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained. Quantum effects, including tunneling, zero-point vibrational energy, and reaction path interference, can be accounted for. In the limit of an adiabatic unimolecular reaction proceeding on a single electronic state, NAST reduces to the traditional transition state theory. Because NAST requires molecular properties at only a few points on potential energy surfaces, it can be applied to large molecular systems, used with accurate high-level electronic structure methods, and employed to study slow nonadiabatic processes. The essential NAST input data include the nuclear Hessian at the reactant minimum, as well as the nuclear Hessians, energy gradients, and spin-orbit coupling at the minimum energy crossing point (MECP) between two states. The additional computational tools included in the NAST package can be used to extract the required input data from the output files of electronic structure packages, calculate the effective Hessian at the MECP, and fit the reaction coordinate for more advanced NAST calculations. We describe the theory, its implementation, and three examples of application to different molecular systems.

Evaluating quantum alchemy of atoms with thermodynamic cycles: Beyond ground electronic states.

Due to the sheer size of chemical and materials space, high-throughput computational screening thereof will require the development of new computational methods that are accurate, efficient, and transferable. These methods need to be applicable to electron configurations beyond ground states. To this end, we have systematically studied the applicability of quantum alchemy predictions using a Taylor series expansion on quantum mechanics (QM) calculations for single atoms with different electronic structures arising from different net charges and electron spin multiplicities. We first compare QM method accuracy to experimental quantities, including first and second ionization energies, electron affinities, and spin multiplet energy gaps, for a baseline understanding of QM reference data. Next, we investigate the intrinsic accuracy of "manual" quantum alchemy. This method uses QM calculations involving nuclear charge perturbations of one atom's basis set to model another. We then discuss the reliability of quantum alchemy based on Taylor series approximations at different orders of truncation. Overall, we find that the errors from finite basis set treatments in quantum alchemy are significantly reduced when thermodynamic cycles are employed, which highlights a route to improve quantum alchemy in explorations of chemical space. This work establishes important technical aspects that impact the accuracy of quantum alchemy predictions using a Taylor series and provides a foundation for further quantum alchemy studies.

Catalytic properties of cyclo-carbon clusters: An investigation on o[formula omitted] activation and CO oxidation

The carbon allotrope-based catalysts attracted considerable attention for oxidation–reduction reactions due to their low-cost and environmentally friendly properties. Systematic investigations are required to explore the catalytic properties and corresponding mechanisms of the new discovered carbon allotropes such as cyclocarbon clusters (cyclo-Cn). Here, using first-principles calculations, we study the stability and electronic properties of cycloCn. Regarding the spin multiplicity and geometry of cycloCn the electronic properties of n-odd and n-even cycloCn were found to be significantly different. Also, based on the EleyRideal (ER) and LangmuirHinshelwood (LH) mechanisms, we focus on the CO oxidation over sp-hybridized molecular carbon allotropes (cyclo-Cn) including recently synthesized cycloC18 by Kaiser, K. et al. (Science2019 365(6459), 1299–1301). In particular, thirteen cycloCn (with n=12-24) are studied. It was found that these carbon clusters may be useful for CO oxidation. We also elucidate the effects of NaCl(100) substrate on the catalytic performance of two cycloCn (n=17, 18) where we found that their optimized structures and the corresponding energy barriers are not very different than that of isolated molecular allotropes. The results show that cycloCn clusters may be a metal-free catalyst for CO oxidation. However, the stability of the cycloCn is a major concern under the acidic or basic conditions required for oxygen electrocatalysis that other groups must consider for further in situ calculations and characterizations.

Reaction mechanisms of the initial steps for the oxidation of (000[formula omitted]) C and (0001) Si faces of SiC with OH radicals

The reaction mechanisms of OH radicals on the C and Si faces of SiC were analyzed using density functional theory calculations to understand the difference in reactivity. For the first OH radical, it was shown that the O–H bond dissociation occurs on both faces, but at different spin states, i.e., the reaction proceeds at a lower spin multiplicity on the Si face. Then, the generated surface O atom causes dissociation of a C–Si bond to form a C–O–Si structure on the C face, whereas it binds to two surface Si atoms to form a Si–O–Si structure on the Si face. These differences are explained by the atomic charge and spin density of the surface atoms. The second OH radical generates a CO molecule on the C face, and a second Si–O–Si structure is formed on the Si face. These results shed light on the reasons why the grinding technique using a tribocatalytic abrasive, which is considered to be promoted by oxidation of the surface by oxidants such as the OH radical, is inefficient on the Si face of SiC.

Origin of Perovskite Multiferroicity and Magnetoelectric-Multiferroic Effects—The Role of Electronic Spin in Spontaneous Polarization of Crystals

In this semi-review paper, we show that the multiferroic properties of perovskite ABO3 crystals with B(dn), n > 0, centers are fully controlled by the influence of the electronic spin on the local dipolar instability that triggers the spontaneous polarization of the crystal. Contrary to the widespread statements, the multiferroicity of these crystals does not emerge due to the addition of unpaired electrons (carrying magnetic moments) to the spontaneously polarizing crystal; the spin states themselves are an important part of the local electronic structure that determines the very possibility of the spontaneous polarization. This conclusion emerges from vibronic theory, in which the ferroelectricity is due to the cooperative interaction of the local dipolar distortions induced by the pseudo-Jahn-Teller effect (PJTE). The latter requires sufficiently strong vibronic coupling between ground and excited electronic states with opposite parity but the same spin multiplicity. The detailed electronic structure of the octahedral [B(dn)O6] center in the molecular orbital presentation shows how this requirement plays into the dependence of the possible perovskite magnetic, ferroelectric, and multiferroic properties on the number of d electrons, provided the criterion of the PJTE is obeyed. Revealed in detail, the role of the electronic spin in all these properties and their combination opens novel possibilities for their manipulation by means of external perturbations and exploration. In particular, it is shown that by employing the well-known spin-crossover phenomenon, a series of novel effects become possible, including magnetic-ferroelectric (multiferroic) crossover with electric-multiferroic, magnetic-ferroelectric, and magneto-electric effects, some of which have already been observed experimentally.

Role of magnetization on catalytic pathways of non-oxidative methane activation on neutral iron carbide clusters.

Methane has emerged as a promising fuel due to its abundance and clean combustion properties. It is also a raw material for various value-added chemicals. However, the conversion of methane to other chemicals such as olefins, aromatics, and hydrocarbons is a difficult task. In recent years, ionic iron carbide clusters have been explored as potential catalysts for efficient direct methane conversion. Herein, we have investigated the gas-phase methane conversion process on various neutral iron carbide clusters with different Fe:C ratios using density functional theory. Reaction pathways were studied on mononuclear and trinuclear iron carbide clusters in the three lowest energy spin multiplicity channels. Three descriptors - methane binding energy, the effective energy barrier for C-H bond activation, and the effective energy required for methyl radical evolution - were chosen to identify the best catalyst among the clusters considered. Isomers of Fe3C6 (Fe3C6-iso) and Fe3C9 (Fe3C9-iso) are recognized as being the most promising catalysts among all the clusters considered here because they require the least methyl radical evolution energy, a step that is crucial in methane conversion to higher hydrocarbon but also requires the most energy.

Structural Symmetry and Spin Multiplicity of Sumanene Derivative Radical Molecules

Structural Symmetry and Spin Multiplicity of Sumanene Derivative Radical Molecules

Oxidation State Localized Orbitals: A Method for Assigning Oxidation States Using Optimally Fragment-Localized Orbitals and a Fragment Orbital Localization Index.

Oxidation states represent the ionic distribution of charge in a molecule and are significant in tracking redox reactions and understanding chemical bonding. While effective algorithms already exist based on formal Lewis structures as well as using localized orbitals, they exhibit differences in challenging cases where effects such as redox noninnocence are at play. Given a density functional theory (DFT) calculation with chosen total charge and spin multiplicity, this work reports a new approach to obtaining fragment-localized orbitals that is termed oxidation state localized orbitals (OSLO), together with an algorithm for assigning the oxidation state using the OSLOs and an associated fragment orbital localization index (FOLI). Evaluating the FOLI requires fragment populations, and for this purpose a new version of the intrinsic atomic orbital (IAO) scheme is introduced in which the IAOs are evaluated using a reference minimal basis formed from on-the-fly superposition of atomic density (IAO-AutoSAD) calculations in the target basis set and at the target level of theory. The OSLO algorithm is applied to a range of challenging cases including high valent metal oxide complexes, redox noninnocent NO and dithiolate transition metal complexes, a range of carbene-containing TM complexes, and other examples including the potentially inverted ligand field in [Cu(CF3)4]-. Across this range of cases, OSLO produces generally satisfactory results. Furthermore, in borderline cases, the OSLOs and associated FOLI values provide direct evidence of the emergence of covalent interactions between fragments that nicely complements existing approaches.

II-VI core/shell quantum dots and doping with transition metal ions as a means of tuning the magnetoelectronic properties of CdS/ZnS core/shell QDs: A DFT study

This paper examines the alterations in the properties of II-VI Quantum Dots (QDs) when these are coated with a shell made of another material of the same family and investigates the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs. The core/shell QDs have been constructed by building the shell over the bare core QD and it is found that this construction of a shell over the bare QD can bring about dramatic changes in its optical properties. On changing the shell by varying either the cation or the anion, substantial variations are brought about in the band gap and electrophilicity. The trend of Fermi energies is more negative for core/shell QDs than for the QDs without a shell, and the value is almost the same for core/shell QDs with the same core. Swapping of the core and the shell materials brings greater stability in the case of shells of the wider band gap materials. Binding energy data demonstrates that the CdS/ZnS, CdSe/ZnSe, CdSe/CdS core/shell systems are more stable than ZnS/CdS, ZnSe/CdSe, CdS/CdSe core/shell systems, respectively. An augmentation in the properties is found on doping the QD with transition metal ions. The binding energies are found to be functions of the kind of dopant as well as the spin multiplicity and account for the stability of one spin state over the other at a specific site of the QD. The most fascinating property that plays a decisive role in the extant work is the introduction of magnetism in core/shell QDs as a result of the entry of unpaired electrons within the CdS/ZnS QDs on doping with transition metal ions. The deviation of the observed magnetic moments from the expected values increases as the dopant is varied from Mn2+ to Fe2+ to Co2+ to Ni2+ to Cu2+. Hirshfeld charge analysis shows that the doped ion accepts negative charge from the sulfide ions in the core, with the smallest charge transfer seen in the case of Hg2+ ions. As we move from Mn2+ to Hg2+, the trend followed for the Hirshfeld charges indicates that the overall charge on the core is lower and that on the shell is higher for all the doped cases in comparison to the undoped CdS/ZnS core/shell QD. The band gap values reveal that the Fe2+ doped CdS/ZnS core/shell structures have the smallest band gaps. Hence, we expect that this paper will help researchers to develop a strategy to produce QDs of the anticipated properties for various applications, and transition metal ions can be successfully employed for modification of various magnetoelectronic properties of the host semiconductor for future applications in nanotechnology.

Reactivity and Fe-complexation investigation by computational simulation studies on phenyltetrazole derivatives as mild steel corrosion inhibitors in aqueous acidic medium

B3LYP, B3PW91, CAM-B3LYP, HCTH, ωB97XD, and M06-2X are six hybrid correlation-exchange and meta-GGA functionals, with 6-31G(d,p) basis set for the H, C, N, O and Cl atoms and the LANL2DZ basis set for the Fe atom. Utilized to determine the most favorable to be applied with phenyltetrazole derivatives, namely: 5-phenyl-1H-tetrazole (Ph-T), 5-p-tolyl-1H-tetrazole (Me-Ph-T), 5-(4-methoxyphenyl)-1H-tetrazole (Me-O-Ph-T), 5-(4-chlorophenyl)-1H-tetrazole (Me-O-Ph-T), and 5-(4-chlorophenyl) (Cl-Ph-T). Aside from that, the quintet complexes were the most stable of the spin multiplicities studied. Furthermore, three forms of interactions between Fe and phenyltetrazole compounds are identified using Fukui functions, including Fe-Me/Me-O/Cl, Fe-ϕ, and Fe-N among eleven complexes. The meta-GGA functionals are ideally suited to predict metal–ligand interaction in the complexes investigated, which contain the transition metal iron with unsaturated valences. While the bond energies and enthalpies estimated for the two Fe- Me-O-Ph-T and Fe- Cl-Ph-T complexes are the lowest and almost identical, the two inhibitors investigated act similarly. Furthermore, chemisorption is promoted by the flatness of the Cl-Ph-T and Me-O-Ph-T inhibitors. Moreover, the adsorption energy of the Cl-Ph-T and Me-O-Ph-T inhibitors.(-185.661 and −186.005 kcal/mol, respectively), suggests that they are the most stable and strongest adsorption systems demonstrated by MDS. The discovered dynamic descriptors were in excellent accord with the quantum study's findings.

Synthesis and Isolation of a Kinetically Stabilized Crystalline Triangulene.

The synthesis and isolation of hydrocarbons with a triplet ground state in crystalline forms have been sought in materials science. Triangulene is one of the most famous triplet-ground-state benzenoid hydrocarbons. Its unique electronic structure and highly symmetric structure have prompted many scientists to synthesize and isolate triangulene and its derivatives, but all attempts so far to isolate them as crystals have been unsuccessful. Herein we report the synthesis and isolation of a kinetically stabilized crystalline triangulene for the first time. The key to success is the introduction of bulky substituents onto the reactive zigzag edges. Its highly symmetric structure was confirmed by X-ray crystallography, and its fundamental properties, including the triplet ground state, were revealed. The achievement here will open the door for the synthesis and isolation of other hydrocarbons with higher spin multiplicity.