| Nitronyl nitroxide（NN）radicals are a family of stable radicals under ambient conditions with its single electron delocalizes on the five atoms of O-N-C-N-O.The influence of c coupler bridges in the biradical molecules allows them to exhibit interesting magnetic properties in the solid state.Spirobifluorene compounds are often used in the design of various organic optoelectronic and magnetic materials due to their special spiro conjugation effect,and this special spiro-conjugation effect and two independent orthogonalπsystems will modulate the two-radical intramolecular coupling magnetism.This dissertation mainly focuses on the mediated nitronyl nitroxide biradicals,the research work mainly includes the following two parts:In part one,to investigate the mechanism of this spiro conjugation magnetic behavior,we designed and synthesized three diradicals 22’SBF-NN,44’SBFNN and 27SBF-NN.They are bridged by spirobifluorene and nitronyl nitroxide 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=-5.86K 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 and VT-EPR gives a very weak coupling constant of 2Jcalc/k B=0.06 K,with ferromagnetic（FM）interaction as the proof,which is consistent with the spin-polarized prediction.This there is indeed an extremely weak FM interaction in the 44′position diradical.We performed a geometrically optimized structural analysis of the 44’SBF-NN with a dihedral angle of nearly 45° in the NN part and the adjacent benzene ring plane,such a large torsion angle hampers the spin density from leaving the domain,leading to a greater tendency to interact through space weak FM coupling.Combined with the analysis of molecular orbital calculation results,the anomalous intramolecular AF coupling mechanism of 22’SBF-NN is further explained.In the second part,although neutral heavier group-14 radicals are becoming well-established,a few important challenges remain unfulfilled in this area.The notable progress in the stabilization of main group radicals by[R?-?R]（E=Si,Ge,and Sn）species no neutral silicon and germanium biradicals have been isolated so far due to synthetic challenges.Herein,we report the results based on spiro-conjugation 9,9’-Spirobi[9H-9-silafluorene]and spirogermanium nitronyl nitroxide diradicals 1 and 2.As different central spiro-atoms form the structure in the overall performance has a great difference,where the main group of carbon for example,with the increase in atomic number,spiro atoms form the spiro-conjugation effect in the weakening.Therefore,the same decreasing trend is expected to be obtained when discussing the spirofluorene di-radical coupling with carbon,silicon and germanium as the central spiro-atoms,respectively.In order to investigate the difference of spiro-conjugated compounds with different spiro-atom centers mediating the magnetic coupling of nitronyl nitroxide diradicals,the diradicals with carbon,silicon and germanium as the central spiro-effects were designed separately,and the three radical coupling trends and modes of action were discussed using VT-EPR and DFT simulation calculations.Figure[43]table[12]reference[99]... |
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