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978-3-8439-3785-6, Reihe Physikalische Chemie
Johanna F. Nothacker
Magnetometric Investigation of the Spin Properties in Poly(alkylthiophene)
204 Seiten, Dissertation Universität Köln (2018), Softcover, A5
In this work, the magnetic properties of a series of poly(alkylthiophene)s (PAT)s featuring different chain topologies and morphologies are investigated.
First, the investigated materials are tailored by a systematic integration of ill-coupled bithiophene (defect) units. By means of UV-vis, the effect of the engineered conjugation length LC on the optical band gap is revealed. Differential scanning calorimetry (DSC) methods are subsequently employed to accurately distinguish between amorphous and semi-crystalline samples. Moreover, diverse morphologies featuring a constant number of topological defects are prepared by thermal treatment, providing a measure to deconvolute the influence of topology and morphology in the magnetic properties of PATs. To facilitate such endeavor, PATs are classified into amorphous and semi-crystalline.
Secondly, field- and temperature-dependent SQUID magnetization experiments demonstrate that the magnetic susceptibility of all samples is composed of up to four different contributions. A relatively constant diamagnetic susceptibility contribution for all samples is identified, and many of the samples show a complex temperature-dependent susceptibility similar to the Curie-Weiss behavior of antiferromagnetic solids. The density of spins contributing to this phenomenon is approximated. For amorphous samples. It is found that the antiferromagnetic coupling weakens with increasing LC, to the point of vanishing for an amorphous sample with large LC (low density of defects).
Finally, an ab initio π-based four-level molecular magnetism model is employed to simulate the temperature-dependent magnetization curves. Originally developed by Van Vleck in 1932, the approach provides access to the susceptibility caused by spins in triplet states.
In summary, amorphous poly(alkylthiophene) samples show temperature-dependent magnetic response, arising from a density of spins proportional to the density of topological chain defects. For semi-crystalline systems an antiferromagnetic-like behavior, experimental evidence for temperature-independent, field-dependent, positive susceptibility is shown. Altogether this work provides evidence for a new class of tailored magnetic polymers with potential applications in magnetic storage devices and emerging organic-based technologies.