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Molecular crystal assembly of organic radicals and biradicals

Handan Akpinar, University of Massachusetts Amherst


Magnetostructural investigations were carried out on pyrene-1-yl (Pyr) bearing nitronylnitroxide (NN) and iminoylnitroxide (IN) radicals. PyrNN gives two allotropes: one has spin-paired dyads with ΔE = J/k ≈ -178 K, and the other is only half spin-paired with ΔE = J/k ≈ -102 K and the other half paramagnetic. PyrIN also gives two allotropes, an anti conformation that is spin paired in the crystal lattice with ΔE = J/k = -410 K, and a syn conformation that is disordered and paramagnetic. PyrNN also was discovered to co-crystallize with C6F6 in 2:1 ratio to give chains of radical networks linked into networks exhibiting low dimensional 1-D or 2-D antiferromagnetic exchange behavior. Furthermore, PyrNN was discovered to form a 2:1:2 co-crystal with octafluoronaphthalene (OFN) and entrapped solvent dichloromethane (DCM), in which the radical is ``shepherded'' into forming chains of radical-radical contacts on the peripheries of (PyrNN-OFN-PyrNN)n pi-stacks, giving weak, low dimensional inter-radical antiferromagnetic (AFM) exchange interactions. Anthraquinone-substituted nitronylnitroxide radical (AntQNN) was synthesized and found to form two crystal polymorphs. Magnetostructural investigations carried on these indicated that both have antiferromagnetic (AFM) exchange behavior attributed to chain-type inter-radical contacts: one with J1D/k ≈ -3 K, and one with J1D/k ≈-17 K. Five different anthracene nitroxide-type biradicals were synthesized: 27AntdNN, 27AntdIN, 26AntdNN, 26AntdIN, and 9Br27AntdNN. Room temperature solution state, and frozen solution state electron spin resonance (ESR) studies were carried on all of these biradicals. Crystallographic packing information was successfully obtained for 27AntdIN, 26AntdNN, 26AntdIN, and 9Br27AntdNN. Magnetic susceptibility measurements were carried on 27AntdNN, 27AntdIN, 26AntdIN and 9Br27AntdNN. The NN derivatives showed both ferromagnetic (presumed intramolecular) and antiferromagnetic exchange interactions in the solid state. Four different anthraquinone nitroxide-type biradicals were synthesized: 27AntQdNN, 27AntQdIN, 26AntQdNN, 26AntdIN. ESR studies were carried on these biradicals, and showed that 27AntQdNN is not a stable organic radical. While ESR spectra confirmed that 27AntQdNN, 27AntQdIN are biradicals, ESR spectra with isolated monoradical behavior were obtained for 26AntQdNN, 26AntQdIN. Iodine substituted meta-phenylene nitroxide biradicals, IPhdNN, IPhNNIN, and IPhdIN were synthesized. Room temperature and frozen solution ESR studies showed triplet states with strong intramolecular spin interaction. Magnetic behavior and crystallography for IPhdIN (which incorporates DCM), showed halogen bonding between molecules that assists formation of chains between radical sites.^

Subject Area

Chemistry|Organic chemistry|Materials science

Recommended Citation

Akpinar, Handan, "Molecular crystal assembly of organic radicals and biradicals" (2013). Doctoral Dissertations Available from Proquest. AAI3603047.