Single Molecule Magnetism in Linear Fe(I) Complexes with Aufbau and non-Aufbau ground-state

With the ongoing efforts on synthesizing mono-nuclear single-ion magnets (SIMs) with promising applications in high-density data storage and spintronics devices, the linear Fe(I) complexes emerge as the enticing candidates possessing large unquenched angular momentum. Herein, we have studied five experimentally synthesized linear Fe(I) complexes to uncover the origin of single-molecule magnetic behavior of these complexes. To begin with, we benchmarked our methodology on the experimentally and theoretically well-studied complex, [Fe{C(SiMe3)}3]−1] (1) (SiMe3 = trimethylsilyl) which is characterized with large spin-reversal barrier of 226 cm−1 [Nat. Chem. 2013, 5, 577–581]. Further, the two Fe(I) complexes, i.e., [Fe(cyIDep)2]+1 (2) ((cyIDep= 1,3-bis(20,60-diethylphenyl)-4,5-(CH2)4-imidazol-2-ylidene) and [Fe(sIDep)2]+1] (3) (sIDep = 1,3-bis(20,60-diethylphenyl)-imidazolin-2-ylidene) are studied that do not possess SIM behavior under ac or dc magnetic fields, however, they are reported to exhibit large opposite axial zero field splitting (-62.4 and +34.0 cm−1 respectively) from ab initio calculations. Employing state-of-the-art ab initio calculations, we have unwrapped the origin of this contrasting observation between experiment and theory by probing their magnetic relaxation pathways and the pattern of d-orbitals splitting. Additionally, the two experimentally synthesized Fe(I) complexes, i.e., [(η6-C6H6)FeAr*-3,5-Pri 2] (4) (Ar*-3,5-Pri 2 = C6H-2,6-(C6H2-2,4,6-Pri 3)2-3,5-Pri 2) and [(CAAC)2Fe]+1 (5) (CAAC = cyclic (alkyl)(amino)carbene) are investigated for SIM behavior, since there is no report on their magnetic properties. To this end, complex 4 presents itself as the potential candidate for SIM

Auxiliary Atomic Relay Center Facilitates Enhanced Magnetic Couplings in Blatter’s Radical

The recent accomplishments in obtaining the strong ferromagnetic exchange interactions in organic diradicals have made the field quite fascinating and even more promising towards its technological applications. In this context, herein we report a unique combination of remarkably strong ferromagnetic exchange interactions coupled with the molecular rigidity utilizing super-stable Blatter’s radical as a spin source. The planar analogues of the parent Blatter’s radical obtained by annulation with a chalcogen coupled to nitronyl nitroxide (NN) are investigated using density functional theory (DFT) along with the wave function based multi-configurational self-consistent field (MCSCF) methods e.g. CASSCF/NEVPT2. The calculations reveal phenomenal modulation in exchange couplings upon annulation such that remarkably strong ferromagnetic interactions are realized especially for a certain class of the Blatter - nitronyl nitroxide diradicals. The modulation of spin spin interactions is rationalised by variation in spin density distribution and molecular torsional angles. We demonstrate that annulation in OMMs opens an additional coupling pathway via auxiliary X-atom acting as atomic relay center which strongly manipulates the magnitude of exchange couplings.</div

How Plausible Is Getting Ferromagnetic Interactions by Coupling Blatter’s Radical via Its Fused Benzene Ring?

In an effort to obtain superior magnetic properties, all the possible isomers of di-Blatter diradical coupled through its fused benzene ring are investigated employing numerous density and wave function-based methods. It reveals that the energetically stable and also experimentally reported diradicals are anti-ferromagnetic in nature due to dominant coexisting exchange interactions between the strongly localized micro-magnetic radical centers. However, due to strong steric hindrance in certain cases, the exchange interaction switches from anti-ferromagnetic to weak ferromagnetic interactions. Moreover, we propose the modified version of spin alternation rule, called here as zonal spin alternation rule, which can be applied successfully to predict exchange interactions in such diradicals.<br /

First-Principles Investigations of Magnetic Anisotropy and Spin-Crossover Behavior of Fe(III)-TBP Complexes

With the ongoing efforts to obtain mononuclear 3d-transition metal complexes that manifest slow relaxation of magnetization and hence, can behave as single molecule magnets (SMMs), we have modelled 14 novel Fe(III) complexes out of which nine behave as potential SMMs. These complexes possess large zero-field splitting (ZFS)parameter D in the range of -40 to -60 cm-1. The first-principles investigation of the ground-spin state applying density functional theory (DFT) and wave-function basedmulti-configurations methods e.g. SA-CASSCF/NEVPT2 are found to be quite consistent except for few delicate cases with near degenerate spin-states. In such cases, thehybrid B3LYP functional is found to be biased towards high-spin (HS) state. Altering the percentage of exact exchange admixed in B3LYP functional leads to intermediate spin(IS) ground state consistent with the multireference calculations. The origin of large zero field splitting (ZFS) in the Fe(III)-based trigonal bipyramidal (TBP) complexesis investigated and the D-values are further tuned by varying the axial ligands with group XV elements (N, P and As) and equatorial halide ligands from F, Cl, Br and I. Furthermore, a number of complexes are identified with very small Gibbs free energy values indicating the possible spin-crossover phenomenon between the bi-stable spin-states.</div

Quantum Interference and Spin Filtering Effects in Photo-responsive Endoperoxide Based Single Molecular Device

The development of stimuli responsive systems that can switch between two distinct spin states under the application of an external stimuli has always remained an illusory challenge. Here, we report a stimuli-based spin filter by utilizing photo-responsive endoperoxide (EPO) based single molecule device. The photo-irradiation on EPO triggers the homolytic cleavage of the peroxide O-O bond generating diradical intermediate centered on two O-atoms which facilitates high spin filtering efficiency when placed between gold electrodes. The broken conjugated scenario due to peroxide bridge of EPO hinders the propagation of de-Broglie waves across the molecular skeleton. While the diradical intermediate of EPO yields high conductance for one of the spin configuration. The transmission characteristics of various photoproducts along the photochemical reaction pathway of EPO are also investigated using density functional theory in combination with non-equilibrium Green’s function (NEGF-DFT) technique. We demonstrate the key role played by Quantum Interference (QI) effects in dramatic modulation of conductance arising due to different degree of conjugation along the reaction pathway of EPO.</div

Tuning the Magnetic Properties of Diamagnetic Di-Blatter’s Zwitterion to Antiferro- and Ferromagnetic Diradical

In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), the molecule embraced with two Blatter’s moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from zwitterionic ground state to diradicalized state. With larger coupler, remarkably strong ferromagnetic interactions are realized based on DFT and WFT based CASSCF/NEVPT2 methods. An analysis based on extent of spin contamination, CASSCF orbitals occupation numbers, HOMO-LUMO and SOMOs energy gap is demonstrated that marks the transition of ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on TPHA skeleton with diradicaloid state and in some cases, even triplet ground state.</div

Harnessing Colossal Magnetic Anisotropy in Sandwiched 3d^2-Metallocenes

Single-molecule magnet (SMM) based quantum technology is gaining attentions in recent years with growing focus on achieving higher barrier of magnetization reversal. Metallocenes owing unique sandwiched-structure, assure themselves as plausible molecular systems for development of novel SMMs. Here in this work, we have explicitly investigated metallocenes of first row transition metal elements, along with their one electron oxidized (cationic) and reduced (anionic) analogues, for their magnetic anisotropies (D) adopting multi-reference ab initio calculations. Herein, we report the unprecedented high D values for 3d^2 systems among all the 3d-metallocenes

High-Spin Blatter’s Triradicals

Robust organic triradicals with high-spin quartet ground-state provide promising applications in molecular magnets, spintronics, etc. In this context, a triradical based on Blatter’s radical has been synthesized recently possessing two doublet-quartet energy gaps with 70% occupation of quartet ground state at room temperature. The traditional broken-symmetry (BS)-DFT computed energy gaps are reported to be somewhat overestimated in comparison to the experimentally observed values. In this work, we have employed different ab initio methods on this prototypical system to obtain more accurate doublet-quartet energy gaps for this triradical. The spin constraint broken symmetry (CBS)-DFT method has been used to reduce the overestimation of energy gaps from BS-DFT. To address the issues of spin-contamination and multi-reference nature of low-spin states affecting the DFT methods, we have computed the energy gaps using appropriately state-averaged CASSCF and NEVPT2 computations. Using a series of active spaces, our calculations are shown to provide quite accurate values in concordance with the experimentally observed results. Further, we have proposed and modeled another three triradicals based on Blatter’s radical which are of interest for experimental synthesis and characterization. Our computations show that all these triradicals also have quartet ground state with similar energy difference between the excited doublet states