Exploration of Alkyne-bridged Multi[(Porphinato)metal] Oligomers for Charge Transport Applications and Spin-Spin Exchange Coupling Properties Using Synthetic, Spectroscopic, Potentiometric, and Magnetic Resonance Methods
As silicon-based microelectronics approaches its fundamental physical limit, molecular electronics is emerging as a promising candidate for future ultra-dense electronic devices with individual molecules as active device components. The emerging of molecular spintronics, which exploits the spin-dependent charge transport through organic materials, further demonstrate the promising future of molecular electronics. This dissertation describes the charge transport and spin-spin electronic coupling properties of an extraordinary class of molecular wires, alkyne-bridged porphyrin arrays. Chapter one provides a general background of molecular electronics and molecular wires, as well as the basics of electron paramagnetic resonance (EPR). Chapter two describes utilizing highly conjugated (porphinato)metal-based oligomers (PMn structures) as molecular wire components of nanotransfer printed (nTP) molecular junctions; electrical characterization of these “bulk” nTP devices highlights device resistances that depend on PMn wire length. This study demonstrates the ability to fabricate “bulk” and scalable electronic devices in which function derives from the electronic properties of discrete single molecules, and underscores how a critical device function—wire resistance—may be straightforwardly engineered by PMn molecular composition. Chapter three describe the electronic exchange coupling between two unpaired spin on Cu(II) ions in meso-meso alkyne-bridged multi[copper(II) porphyrin] (mmPCu2). Spin and conformational dynamics in symmetric mmPCu2 have been studied in toluene solution at variable temperature using EPR spectroscopy. Comparison of the dimer EPR spectra to those of Cu porphyrin monomers shows clear evidence of an isotropic exchange interaction (Javg) in these biradicaloid structures, manifested by a significant line broadening in the dimer spectra. Comparison of ethyne and butadiyne alkyne bridges reveals a remarkable sensitivity to orbital interactions between the spacer and the metal, which is reflected in measurements of Javg as a function of temperature. The results suggest that orbital symmetry relationships may be more important than previously recognized in the design of optimized molecular spintronic devices. Chapter four reports a study of β-β linked bis[(porphinato)copper(II)] complexes (ββPCu2), which exhibit very different electronic structures compared to their mm linked analogs. By using electron paramagnetic resonance (EPR) spectroscopy, this study exhibits that a wide range (3 orders of magnitude) of the average electronic spin-spin exchange coupling can be achieved by varying the length of bridges and points of connections between the porphyrin rings. The pathways for mmPCu2 and ββPCu2 complexes were also investigated, with the ββPCu2 complexes exhibiting a dominant σ-type pathway and the mmPCu2 complexes showing a dominant π-type pathway.
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