Towards understanding of magnetic interactions within a series of tetrathiafulvalene–π conjugated-verdazyl diradical cation system: a density functional theory study

Abstract

The intramolecular magnetic exchange coupling constants (J) for a series of tetrathiafulvalene (TTF) and verdazyl diradical cations connected by a range of pi conjugated linkers have been investigated by means of methodology based on unrestricted density functional theory. The magnetic interaction between radicals is transmitted via pi-electron conjugation for all considered compounds. The calculation of J yields strong or medium ferromagnetic coupling interactions (in the range of 56 and 300 K) for diradical cations connected by linkers with an even number of carbon atoms that are able to provide a spin polarization pathway, while antiferromagnetic coupling is predicted when linkers with an odd number of carbon atoms are employed. The topological analysis of spin density distributions have been used to reveal the effects of the spin polarization on both linkers and spin carriers. The absence of heteroatoms that impede the spin polarization pathway, and the existence of a unique spin polarization path instead of several possible competitive routes are factors which contribute to large positive J values favoring ferromagnetic interactions between the two terminal pi-radicals. The magnitude of J depends strongly on the planarity of the molecular structure of the diradical cation since a more effective orbital overlap between the two pi-systems can be achieved. Hence, the dependence of J on the torsion angle (theta) of each spin carrier has been analyzed. In this respect, our findings show that this geometrical distortion reduces largely the calculated J values for ferromagnetic couplings, leading to weak antiferromagnetic interactions for a torsion angle of 90 degrees .

Quantum chemical approaches for controlling and evaluating intramolecular magnetic interactions in organic diradicals

“…As the torsion angle increases, spin polarization through p2p interaction in the p-system gradually diminishes (monotonic decrease in the J) and the interaction between singly occupied p2orbital at the radical center and bonding r2orbital at the coupler increases, which destabilize the triplet state when compared with the singlet state (spin crossover) beyond a certain point of torsional distortion. [44,45] In our case, these characteristic behaviors involved the magnetic interaction of the OVERN-m-benzene-OVERN (DA7), OVERN-m-benzene-OVERC (DA3 and DA4), and OVERN-p-benzene-OVERC (DA6), which had notable dihedral distortion Figure 16. Spin density distributions in T and BS states of diradicals (16)(17)(18)(19)(20)(21) calculated at B3LYP/6-31111G(d,p) level.…”

“…The dihedral angles between radicals and coupler hinder their effective intramolecular magnetic interaction through p-p exchange coupling, resulting in decreased J. [45] Spin crossover even occurs from the ferromagnetic ground Coupler length is another important factor determining the strength of the magnetic interaction between radicals. The strength of the magnetic coupling between radicals generally decreases as coupler length increases.…”

“…The rule of spin alternation in the UHF or UDFT levels predicts the magnetic ground states without fail, although counter examples against the former regarding the SOMO interactions were reported. [23] In addition, it was known that intramolecular magnetic interactions between spin sources through a coupler can be influenced by several other factors: the geometrical connectivity, [23,[29][30][31][32][33][34] coupler chain length, [40][41][42][43] aromatic nature of a coupler, [32] dihedral angle, [44,45] and so on.…”

Photo-physical properties of donor-acceptor-radical triad based on functionalized tetrathiafulvalene and nitronyl nitroxide radical

“…The design and elaboration of such "TTF-radical" dyads are very challenging and to date only few researchers took this synthetic route: Sugawata et al [23]- [26] developed the functionalization of the TTF core with nitrosyl or nitronyl nitroxide spin carriers while Yamaguchi et al [27] and Datta et al [28], [29] studied such donor-radical dyads from a computational point of view. Additionally, Pilkington et al [30], [31] used the verdazyl moiety as stable radical while Veciana et al exploited the perchlorotriphenylmethyl radicals [32]- [36] to design their dyads.…”

Diradicals acting through diamagnetic phenylene vinylene bridges: Raman spectroscopy as a probe to characterize spin delocalization

“…Magnetism in these organic systems is attained by connecting "stable and persistent" radical units (i.e., chemical groups with one unpaired electron) to adequate molecular bridges, so promoting a magnetic coupling between the radical units via through-bond super-exchange interaction. [1][2][3][4][5][6] The most promising linkers are π −conjugated bridges whose ability to promote electronic π −conjugation favors strong magnetic couplings or inter-spin interaction transmission. At the intramolecular level, there are two relevant mechanisms that make super-exchange magnetic coupling possible: [7][8][9] (i) spin delocalization, in which the bridge a) Author to whom correspondence should be addressed.…”