Design and Synthesis of Molecular Wires for Gold Mechanically Controlled Break Junctions (MCBJs)

Abstract

This dissertation presents a systematic investigation of structure–property relationships governing charge transport in single-molecule junctions and extends these principles to the design of advanced π-conjugated materials. The central objective is to understand how molecular electronic structure, linker chemistry, and molecule–electrode interface geometry can be rationally controlled to tune conductance and optoelectronic properties. Chapter 2 focuses on verdazyl radicals as stable open-shell systems for molecular electronics. A series of thiomethyl-functionalized verdazyl derivatives was designed and synthesized to evaluate how substituent effects and functionalization patterns influence single-molecule conductance in gold mechanically controlled break junctions (MCBJs). This work establishes structure–transport relationships in radical systems and highlights their potential for enhanced conductance and spin-dependent transport. Chapter 3 investigates oligophenylene vinylene (OPV3) derivatives as conjugated molecular wires. Two key challenges are addressed: overcoming the anticorrelation between renormalization energy and ionization energy through balanced substituent design and exploring alternative linker groups beyond sulfur-based systems. Furthermore, OPV3 derivatives incorporating dihydrobenzo–thiophene (BT), isocyanide, and amine linkers were synthesized and studied to evaluate how backbone modification and linker identity affect charge transport in single-molecule junctions. Chapter 4 examines the role of linker group denticity and spatial arrangement in molecular conductance using a series of N-heterohexacene derivatives. By systematically varying the number and placement of thioether linkers while maintaining a constant conjugated backbone, this work demonstrates that molecular geometry, particularly the orientation of linker groups relative to the π-system plays a dominant role in determining conductance, challenging the assumption that increased denticity necessarily enhances transport. Chapter 5 extends these design principles to highly conjugated macrocyclic systems through the synthesis of a zinc phthalocyanine (ZnPc) derivative incorporating pyrene-fused pyrazaacene (PPA) building blocks. The resulting expanded π-conjugated macrocycle, combined with solubilizing substituents, addresses limitations associated with aggregation and poor processability in conventional phthalocyanines, while enabling tunable optical and electronic properties. Overall, this work demonstrates that charge transport in molecular systems is governed by an interplay of intrinsic electronic structure, linker chemistry, and interfacial geometry. By integrating synthetic design with single-molecule measurements and materials development, this dissertation provides key insights and design principles for the development of next-generation molecular electronic and optoelectronic devices.