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

Abstract

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

Nonreductive homolytic scission of endoperoxide bond for activation of artemisinin: A parallel mechanism to Heterolytic cleavage

“…For the heterolytic cleavage, the additional electron resides on one of the oxygen atoms and the other oxygen atom becomes the radical center constrained density functional theory (CDFT). [43][44][45][46] Becke's three-parameter and Lee-Yang-Parr's exchangecorrelation hybrid functional (B3LYP) [47] with the 6-311G** double-zeta basis set with polarization functions are employed in the quantum chemistry code ORCA 4.0. [48] All the calculations were performed within the unrestricted Kohn-Sham formalism treating the radical species as open-shell molecules within the DFT framework.…”

Anti-ohmic Nanoconductors: Myth, Reality and Promise

“…To further validate the OSS character, spin-squared value <S 2 > in BS state and radicaloid character (y), which are used as the descriptors of diradical character of a molecule 36,37 are presented in Figure 1b and Figure 1d. The diagnostic value of <S 2 > in BS determinant should be 1.0.…”

The influence of the radicaloid character of polyaromatic hydrocarbon couplers on magnetic exchange interactions

“…52,53 Understanding the correlation between the magnetic coupling and the radicaloid nature of the spacer will provide an additional window to modulate the intramolecular magnetic exchange interactions. 54,55 In the present work, we aim to find the relation between the magnetic exchange coupling constants of localized spins and the radicaloid character (y) of an open-shell singlet (OSS) coupler mediating this magnetic exchange. To do so, decacene is chosen as the prototypical coupler having high radicaloid character owing to its OSS nature and promoting long-range interactions as explicitly investigated in our previous study.…”

“…To do so, decacene is chosen as the prototypical coupler having high radicaloid character owing to its OSS nature and promoting long-range interactions as explicitly investigated in our previous study. 55 The couplers (Fig. 1(b 0 )-(e 0 )) are then modeled from this base decacene spacer by step-wise addition of benzenoid rings protruding from decacene's central benzene rings.…”

“…52,53 Understanding the correlation between the magnetic coupling and the radicaloid nature of the spacer will provide an additional window to modulate the intramolecular magnetic exchange interactions. 54,55…”