Investigation of the Electronic and Magnetic Properties of Electron Exchange: Exchange Coupled Donor-Bridge-Acceptor Biradicals and Novel Magnetic Behaviors of Bis(pyridyl) Cobalt Dioxolene Valence Tautomers

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Title: Investigation of the Electronic and Magnetic Properties of Electron Exchange: Exchange Coupled Donor-Bridge-Acceptor Biradicals and Novel Magnetic Behaviors of Bis(pyridyl) Cobalt Dioxolene Valence Tautomers
Author: Schmidt, Robert David
Advisors: Dr. Martin L. Kirk, Committee Member
Dr. David A. Shultz, Committee Chair
Dr. Reza A. Ghiladi, Committee Member
Dr. Christopher B. Gorman, Committee Member
Dr. Paul A. Maggard, Committee Member
Abstract: Expansion into the emerging field of molecular spintronics demands a complete understanding of electron exchange interactions within molecular systems and electrode-molecule junctions. To this end, several complexes with unique mechanisms of electron correlation have been prepared and characterized, demonstrating useful trends and establishing a theoretical basis on which to build future molecular systems. The series of DBA (Donor-Bridge-Acceptor) Semiquinone-Bridge-Nitronylnitroxide metal complexes is extended to include the thiophene-bridged molecule (SQ-T-NN). Progress on the synthesis of this system is described, and the final biradical complex is partially characterized through electron paramagnetic resonance and electronic absorption spectroscopy. The results of these experiments are compared to those from the SQNN and SQ-Ph-NN systems prepared by previous group members, falling in line with the expected coupling behavior. Novel synthetic methodologies developed as a general synthetic method for the preparation of this and other similar systems are also described. The thermal- and photo-induced valence tautomerism of an isostructural series of Co(dioxolene)2(4-X-py)2 complexes is described. The thermal valence tautomerism (ls-CoIII(SQ)(Cat) ↔ hs-CoII(SQ)(SQ)) is only observed for complexes with a specific substituent size, where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample is held at 10 K in a SQUID magnetometer and irradiated with white light the hs-CoII tautomer is formed, presenting near indefinite kinetic stability below ca. 40 K. When the light source is removed, and the sample slowly heated, the hs-CoII tautomer persists until ca. 90 K, representing a marked improvement over similar examples. Heating and cooling the sample under constant irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (ΔT = ca. 25 K). Through an in-depth analysis of the magnetic behaviors, electronic absorption spectra, and crystal packing, a reasonable mechanism of action is presented relying on intermolecular interactions for stability. This mechanism is shown to account for the high cooperativity and hysteresis in the thermal conversion, metastability of the photogenerated species at temperatures below ca. 90 K and dependence on substituent size and ability to form hydrogen bonds rather than electronic nature. The electronic structure and origin of ferromagnetic exchange coupling for two new valence tautomeric complexes is described where the dioxolene ligands are based on our Semiquinone-Bridge-Nitronylnitroxide radical ligands (SQNN and SQ-Ph-NN). Near-IR electronic absorption spectroscopy reveals a low energy optical band that has been assigned as a transition involving bonding and antibonding linear combinations of delocalized dioxolene (SQ/Cat) valence frontier molecular orbitals. The ferromagnetic exchange interaction where dioxolene = SQNN is so strong that only the high-spin quartet state (S = 3/2) is thermally populated at temperatures up to 300 K. The temperature-dependent magnetic susceptibility data where dioxolene = SQ-Ph-NN reveals that an excited state spin doublet (S = 1/2) is populated at higher temperatures, indicating that the phenylene spacer attenuates the magnitude of the magnetic exchange. The valence delocalization within the dioxolene dyad promotes ferromagnetic alignment of two localized NN radicals separated by over 2.2 nm. This ferromagnetic exchange is demonstrated to result from a spin-dependent delocalization process, the first such example to possess purely organic spin carriers. A detailed theoretical and spectroscopic analysis of this process is described. The long correlation length (2.2 nm) of the itinerant electron that mediates this coupling indicates that high-spin π-delocalized organic molecules could find applications as nanoscale spin-polarized electron injectors and molecular wires.
Date: 2010-05-04
Degree: PhD
Discipline: Chemistry

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