Spirobifluorene Mediating the Spin–Spin Coupling of Nitronyl Nitroxide Diradicals

Abstract

To investigate the mechanism of this spiro conjugation magnetic behavior, we designed and synthesized three diradicals�22′SBF-NN, 44′SBF-NN, and 27SBF-NN. They are bridged by spirobifluorene and nitronyl nitroxide (NN) diradicals as the spin centers. Notably, by SQUID and electron paramagnetic resonance (EPR) zero-field splitting data analyses, the 22′SBF-NN and 27SBF-NN diradicals exhibit intramolecular, distinctly antiferromagnetic (AF) coupling, with 2J (22′SBF-NN) /k B = −5.86 K and 2J (27SBF-NN) /k B = −24.6 K, respectively. The AF of 22′SBF-NN is opposite to that predicted by the spin density alternation rule based on Hund's rule. Diradical intramolecular conjugation coupling bridged by spiro-carbon conjugation is discussed, in which the 22′SBF-NN is smaller than that of 27SBF-NN, corresponding to the room-temperature EPR characterization. This spiro conjugation is weaker than the traditional planar conjugation and generally leads to a weaker spin−spin coupling in the helical biradical molecule. The EPR spectrum of the 44′SBF-NN diradical shows a deformed nine-line curve, indicating intramolecular exchange coupling. The density functional theory calculation gives a very weak coupling constant of 2J calc / k B = 0.06 K, with ferromagnetic (FM) interaction as the proof, which is consistent with the spin-polarized prediction. Further analysis of magnetic susceptibility χ m and VT-EPR data shows that there is indeed an extremely weak FM interaction in the 44′ position diradical. We find the bridge, which is a 44′ substituted SBF structure, blocks the conjugation and contains a larger twist in steric hindrance, which also hampers sufficient spin density delocalization, resulting in a much weaker spin coupling interaction. Combined with the analysis of molecular orbital calculation results, the anomalous intramolecular AF coupling mechanism of 22′SBF-NN is further explained.

N-Heterocyclic Carbene-Derived 1,3,5-Trimethylenebenzene: On-Surface Synthesis and Electronic Structure

“…For realizing their applications, the synthesis of di- and polyradicals as well as the understanding and control of their spin–spin coupling are important prerequisites. Over the past few decades, a number of diradicals have been reported, and the spin–spin coupling within diradicals (ferromagnetic or antiferromagnetic) and its correlation with the chemical structure have been explored. − For example, it has been shown that stable diradicals with strong ferromagnetic coupling can be prepared by incorporating two stable organic radicals, such as Blatter radical and nitronyl nitroxide radical, using a m -phenylene bridge unit. The spin coupling strength can be modulated through substitution, π-conjugation extension, and heteroatom doping . Compared with diradicals, the synthesis of triradicals is more challenging, and the spin–exchange interactions are more complicated because of the increased unpaired electrons, especially for triradicals with 3-fold symmetry .…”

Homoconjugation Mediated Spin-Spin Coupling in Triptycene Nitronyl Nitroxide Diradicals

“…However, predicting intramolecular magnetic exchange interaction for the σ-frame diradical systems [38,39] remains challenging. Examples of conjugated systems containing saturated atoms bridging diradicals have been reported [23,34,[40][41][42][43][44]. Izuoka et al [34] studied the A and B structures with neutral thianthrene diradicals in Scheme 1 consistent However, predicting intramolecular magnetic exchange interaction for the σ-frame diradical systems [38,39] remains challenging.…”

“…Izuoka et al [34] studied the A and B structures with neutral thianthrene diradicals in Scheme 1 consistent However, predicting intramolecular magnetic exchange interaction for the σ-frame diradical systems [38,39] remains challenging. Examples of conjugated systems containing saturated atoms bridging diradicals have been reported [23,34,[40][41][42][43][44]. Izuoka et al [34] studied the A and B structures with neutral thianthrene diradicals in Scheme 1 consistent with the spin polarization of the connecting unit.…”

“…The first spirocyclic conjugated diradical was synthesized by Frank et al [41]. Subsequently, Yue et al [42] investigated three spirobifluorene diradical isomers and further demonstrated that magnetic coupling interaction occurred through spiro conjugation. In 1942, Bartlett et al [45] firstly synthesized aromatic hydrocarbon triptycene (TP), composed of three benzene rings joined by two sp 3 hybrid carbons.…”

Synthesis and Characterization of a One-Dimensional Malleable Spin-Crossover Polymer Complex Modified by Methoxy Polyethylene Glycol

“…Superconductivity quantum interference device (SQUID) measurements revealed the spin magnetic properties in the range of temperature from 10 K to 300 K of the spin crossover performance of SCO materials functionalized by the MPEG chain, and differential scanning calorimetry (DSC) measurements also provided strong evidence for reversible spin transitions of the Fe (II) polymer complex; meanwhile, the crystal structures of the new SCO complex was characterized by using X-ray diffraction (XRD). In addition, DFT Gaussian computations was used in SCO polymer complexes for the first time and was reported to reveal the internal mechanism or the relationship between the molecular chain structure and the spin magnetic properties [ 31 ]. Because of the inappropriate of DFT computations for extremely big or long flexible polymer complexes due to the too many possible conformations, we made an approximation of different four-position substituted [Fe(1,2,4-triazole) 3 ] 2+ derivatives with different length of repeat units in SCO molecules and solved these computational problems above.…”