Polyacene Spacers in Intramolecular Magnetic Coupling

Abstract

We predict the intramolecular magnetic exchange coupling constant (J) for eleven nitronyl nitroxide diradicals (NN) with different linear and angular polyacene couplers from broken-symmetry density functional treatment. For the linear acene couplers, J initially decreases with increase in the number of fused rings. But from anthracene coupler onward, the J value increases with the number of benzenoid rings due to an increasing diradical character of the coupler moiety. The J value for the diradical with a fused bent coupler is always found to be smaller than that for a diradical with a linear coupler of the same size. The nuclear independent chemical shift (NICS) is calculated, and it is observed that the average of the NICS values per benzenoid ring in the diradical is less than that in the normal polyacene molecule. An empirical formula for the magnetic exchange coupling constant of a NN diradical with an aromatic spacer is obtained by combining the Wiberg bond order (BO), the angle of twist (phi) of the monoradical (NN) plane from the plane of the coupler, and the NICS values. A comparison of the formula with the computed values reveals that, from tetracene onward, the diradical nature of the linear acene couplers becomes prominent thereby leading to an increase in the ferromagnetic coupling constant. Isotropic hyperfine coupling constants are calculated by using a polarized continuum model for the diradicals in different solvents and in vacuum.

Physical analysis of the through-ligand long-distance magnetic coupling: spin-polarization versus Anderson mechanism

“…The decrease of J as a function of the conjugated-bridge length has been theoretically evidenced, using UDFT calculations in nytronil nitroxide diradicals [12] and dinitrenes [13]. In recent experimental and theoretical studies [14,15] performed on dinuclear copper(II) complexes connected through oligoacenebis(oxamate) or oligo-paraphenylenediamine bridging ligands, Unrestricted Density Fonctional Theory (UDFT) calculations support the occurrence of a spin polarization mechanism for the exchange interaction between the two distant unpaired electrons.…”

Intriguing diaza effects on magnetic coupling characteristics in diaza‐benzo[k]tetraphene‐bridged nitroxide diradicals

“…The ideal BS state is an equal mixture of the singlet and the triplet, exhibiting a spin‐state contamination, however. With the use of spin‐projected methods, the influence of the redundant spin contamination from the energy of the BS state can be eliminated . Based on this, the expression of J was developed by Yamaguchi and co‐workers to be as: J = ( E BS − E T )/(< S 2 > T − < S 2 > BS ), where E BS , E T , < S 2 > BS , and < S 2 > T refer to the energies and average spin square values of the BS and T states, respectively.…”

(Azulene‐1,3‐diyl)‐bis(nitronyl nitroxide) and (Azulene‐1,3‐diyl)‐bis(iminonitroxide) and Their Copper Complexes

“…These ferromagnetic interactions are also consistentw ith their theoretical calculations. [17,19] The intramolecular exchange interactions of the nitroxide diradicals with Az spacers (J obs /k B =+10 Kf or NN 2 Az and j J intra / k B j < 1K for IN 2 Az) are inclinedt ob es maller than those with m-phenylene spacers [J/k B =+32, [15] + 20, [16] and + 7K [17] for NN 2 (m-C 6 H 4 )a nd J/k B =+25 [17] and + 20 K [18] for IN 2 (m-C 6 Figure 2), whereas small torsion angles( 5a nd 108)a re observed for IN 2 Az because of the absence of such steric repulsion. Hence, unexpectedly,t he most planar diradical, IN 2 Az, has the smallest exchange interaction (j J intra /k B j < 1K).…”