Tuning the Spin Coupling Interactions in the Nitroxide‐Based Bisphenol‐Like Diradicals

Song, Mingming; Song, Xinyu; Bu, Yuxiang

doi:10.1002/cphc.201700731

Cited by 10 publications

(7 citation statements)

References 65 publications

(155 reference statements)

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“…The conformationally restricted di-radicals are specifically opted here to minimize the influence of torsional angles on magnetic coupling constant values. [23][24][25][26] The nitroxy radicals are connected at m-positions of the polyacene to have a ferromagnetic coupling between these radical centers. The spacer length is uniformly increased by adding a unit of fused benzene ring with 1 corresponding to diradical system with benzene coupler and 10 corresponding to diradical system with decacene coupler.…”

Section: Introductionmentioning

confidence: 99%

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

Kaur

Ali²

2020

Preprint

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The polyacene tend to transition from closed-shell to open-shell singlet systems with increase in the number of linearly fused benzene rings thus, enriching it's electronic as well as magnetic properties. Here, we have probed the role of such a polyacene couplers in mediating the magnetic exchange coupling between nitroxy radicals. For this, magnetic exchange coupling constants (2J) are computed employing broken-symmetry approach within density functional theory (DFT) as well as symmetry-adopted wave function based multi-configuration approach. Within DFT approach, apart from most commonly used exchange-correlation functional (generalized gradient approximation (GGA), meta-GGA, and hybrid) constrained spin density (CDFT) and on-site Coulomb correlation corrected GGA+U functionals have also been used to evaluate the exchange coupling constants in the diradicals. All DFT based methods approximate an exponential increase in the coupling constant values with increase in the length of the coupler. This is indeed an unexpected observation with no experimental report available to such an outcome by DFT calculations. Therefore, the electronic structures for these diradical systems with the increase in number of benzene rings were looked upon thoroughly. The properties like increase in number of degenerate or quasi-degenerate molecular orbitals (MOs), the lowering in energy of low-lying excited states and the development of intrinsic radical character of the coupler add to the complexity in evaluation of 2J values for such diradical systems with DFT approach. Consequently, wave function based multi-reference approach (CAASCF/NEVPT2) has been adopted. The active space has been limited to π-orbitals which are found to play an important role in the magnetic exchange interactions. With increase in the coupler size, the active space even with only π obitals would increase enormously which could not be taken into account even in the CASSCF study. The limited active space calculations also indicate quite strong exchange interaction, thus in principle, supporting the DFT observations of long-range magnetic exchange interactions but not an exponential increase in 2J with length of couplers.

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Section: Introductionmentioning

confidence: 99%

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

Kaur

Ali²

2020

Preprint

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“…The conformationally restricted di-radicals are specifically opted here to minimize the influence of torsional angles on magnetic coupling constant values. [31][32][33][34] The nitroxy radicals are connected at m-positions of the polyacene to have a ferromagnetic coupling between these radical centers. The spacer length is uniformly increased by adding a unit of fused benzene ring with 1 corresponding to diradical system with benzene coupler and 10 corresponding to diradical system with decacene coupler.…”

mentioning

confidence: 99%

First principle investigations of long‐range magnetic exchange interactions via polyacene couplers

Kaur¹,

Ali²

2021

Int J of Quantum Chemistry

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The electronic and magnetic properties of polyacenes become quite fascinating as the number of linearly conjugated benzene rings increases. Higher-order conjugated polyacenes develop radicaloid characters due to the transition of electronic structures from closed-shell to the open-shell system. Here we have investigated the role of such polyacenes as the magnetic coupler when placed between the two spinsources based on nitroxy radicals. To do so, the magnetic exchange interactions (2J) are computed employing broken-symmetry (BS) approach within the density functional theory (DFT). In this approach, various genre of exchange-correlation (XC) functionals such as generalized gradient approximation (GGA), meta-GGA, hybrid functional, constrained spin density (i.e. CDFT) and on-site Coulomb correlation corrected GGA + U functionals are adopted. All DFT based calculations estimate an exponential increase in 2J values with the length of the couplers. This observation has been understood in terms of increase in number of near-degenerate or quasidegenerate molecular orbitals (MOs), reduction of HOMO-LUMO energy gap and descend of low-lying excited states as the number of fused benzene rings are increased. K E Y W O R D SDFT, intramolecular exchange interactions, long-range interactions, organic diradicals, polyacene | INTRODUCTIONMolecular magnets based on the stable organic radicals have attracted enormous attention due to their potential applications in spintronics, [1] magnetic logic devices, [2] and quantum computers. [3] The non-magnetic spacer between the radical centers control the nature and strength of intramolecular magnetic exchange interactions. [4][5][6] The seamless π-conjugations between the radical centers and spacer play a vital role in magnetic couplings. [7][8][9] Strong magnetic exchange interactions are reported mostly for couplers with smaller lengths. [10] However, the fascinating long-range couplers desired [11,12] for magnetic and spintronic applications are relatively less explored. On the top of it accomplishment of long range strong magnetic exchange interactions in molecular system is another challenge. The exchange interactions through the larger molecular spacers were investigated by Matsuda et al. for various π-conjugated couplers and established a correspondence between electrical conductance and the magnetic exchange interaction. [13] Both the molecular conductance and the exchange interactions decay exponentially with an increase in distance between radical centers. They observed that the increased π-conjugations slow down the decay process. [14,15] The strength of the magnetic exchange coupling constants (2J), mediated through π-conjugated molecular spacers was found to decay exponentially with the length of the couplers. [13] Essentially, the conductance and magnetic coupling (2J) between two spin sources are related through the molecular bridge properties. [16-18] However, in recent years interest has increased in designing and finding molecular candidates that would show an increase in

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“…The nature of magnetic exchange interactions and consequently the ground spin state can be visually predicted from the empirical spin alternation rule. This rule works perfectly well for the conjugated diradicals wherein the exchange takes place through the bond . However, the deviation from the spin alternation rule has been reported for sterically hindered cases where the large dihedral angle between the radical moieties breaks the electronic conjugation between π orbitals and facilitates the direct exchange between the radical centers. , Such cases usually favor the parallel orientation of electrons residing in the p z orbitals of radical centers following the Hund’s rule, thereby apparently breaking the spin alternation rule and yielding ferromagnetic interactions.…”

Section: Resultsmentioning

confidence: 80%

“…This rule works perfectly well for the conjugated diradicals wherein the exchange takes place through bond. 45 However, the deviation from the spin alternation rule has been reported for sterically hindered cases where the large dihedral angle between the radical moieties breaks the electronic conjugation between π-orbitals and facilitates the direct exchange between the radical centres. 27,46 Such cases usually favors the parallel orientation of electrons residing in the p z -orbitals of radical centres following the Hund's Rule.…”

Section: Zonal Spin Alternation Rulementioning

confidence: 99%

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

Khurana¹,

Bajaj²,

Ali³

2020

J. Phys. Chem. A

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With ongoing efforts to synthesize super-stable Blatter's diradicals having strong ferromagnetic exchange interactions, all the ten possible isomers of di-Blatter diradical coupled through the fused benzene rings are investigated. A variety of electronic structure theory such as broken-symmetry methods in density functional theory (DFT), spin-constraint DFT (CDFT), and wave function-based multi-configurational methods e.g. CASSCF/NEVPT2 are applied to compute the magnetic exchange interactions. Surprisingly, anti-ferromagnetic interactions are revealed for all the stable isomers of di-Blatter diradicals. Indeed it commensurates with the experimental observations for the only available synthesized isomer. However, the other nine isomeric diradicals in the series are yet to synthesize. Despite a good match between theory and experiment, the anti-ferromagnetic exchange interactions could not be explained based on the spin alternation rule due to unique spin-distributions in the triazinyl ring. Thus, we propose the zonal spin-alternation rule which explains the observed ground spin-state for the conjugated di-Blatter diradicals quite accurately. Further, the fractional spin-moment localization on the N-atoms activates multiple exchange pathways and the dominating exchange interactions render anti-ferromagnetic interactions in the conjugated isomers. The study further reveals that due to strong steric hindrance in certain coupled isomers, the exchange interaction switches from anti-ferromagnetic to weak ferromagnetic interactions with the cost of stabilization energy of the radicals. Thus it questions the possibility of synthesizing ferromagnetic di-Blatter diradicals.

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Tuning the Spin Coupling Interactions in the Nitroxide‐Based Bisphenol‐Like Diradicals

Cited by 10 publications

References 65 publications

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

First principle investigations of long‐range magnetic exchange interactions via polyacene couplers

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

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