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Date of Award

2-2012

Access Type

Campus Access

Document type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

First Advisor

Paul M. Lahti

Second Advisor

Dhandapani Venkataraman

Third Advisor

Michael J. Knapp

Subject Categories

Materials Chemistry | Organic Chemistry

Abstract

Synthesis and magneto-structural characterization of hydrogen bonded organic nitronylnitroxide and verdazyl radicals were done. 35diMeO-4OHPhNN displayed 1D antiferromagnetic interactions attributed to the chain contacts between radical NO groups. Benzimidazole-based verdazyl radicals 2BImverd, and 5BImverd did not give diffraction quality crystals and were only characterized by EPR. The analogues 2BImisoverd and 5BImisoverd both gave x-ray diffraction quality single crystals that displayed formation of hydrogen bonded chains through the imidazole moieties. The magnetic susceptibility results indicated the presence of weak 1D AFM interactions for both radicals. The weakness of interactions was attributed to bulky isopropyl groups pushing the molecules apart and decreasing spin orbital overlap.

A series of organic radical solid solutions (alloys) were made using BImNN and its fluorinated analogue F4BImNN. (F4BImNN)x (BImNN) (1-x) with x < 0.8 gave orthorhombic unit cells, while x > 0.88 gave monoclinic unit cells. (F4BImNN) (x) (BImNN)(1-x) (x = 0.1, 0.17, 0.25, 0.5, 0.75, 0.83, 0.9) displayed ferromagnetic interactions with J/k = (+)14-22 K, mainly controlled by hydrogen-bonded assembly of the radicals. Magnetic analysis over 0.4-300 K showed ordering behavior for all of these materials. The ordering temperatures of the orthorhombic samples increased linearly as (1-x) increased from 0.25 to 1.00. The variation was attributed to increased inter-chain distance as more F4BImNN was added into the orthorhombic lattice. The monoclinic samples were not part of the same trend, which was attributed to a change in the inter-chain arrangement. This was the very first study giving such complete magnetostructural detail linking ordering behavior to specific crystallographic features and intermolecular contacts.

The magnetic behavior of F4BImNN was investigated at increased external pressures. The crystallographic c -axis (along which hydrogen bonds form) was compressed by 3% at 10 kbar and by 4% at 17.8 kbar. The overall lattice volume contracted by 12% from ambient pressure to 17.8 kbar. The magnetic susceptibility measured over 1.8-300 K showed an increase in ferromagnetic exchange interactions as pressure increased. The increase in exchange strength was attributed to pressure-increased overlap of spin orbitals in the hydrogen-bonded chains, which favored 1D ferromagnetic interaction.

Electron paramagnetic resonance experiments on a single crystal of F4BImNN were also performed. The variation in g -value as a function of the crystal's position with respect to the applied magnetic field was investigated. The angular dependence of g was more pronounced at temperatures below 30 K.

Pyrrole-based nitronylnitroxide radicals mNNPP, N-PN and 35NNPP were studied. mNNPP displayed formation of 1D chains with weak intra-chain FM interactions, N-PN gave 1D AFM interactions, and 35NNPP showed intramolecular FM and intermolecular AFM interactions.

DOI

https://doi.org/10.7275/5686721

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