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 2 J ( 22′SBF-NN ) / k B = −5.86 K and 2 J ( 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 2 J 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.

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