Conjugated Metal–Organic Macrocycles as Solution-Processable Semiconductors

Abstract

There has been recent widespread interest in porous self-assembling crystalline materialsfor a wide variety of applications, but a striking lack of solution processability can greatly hinder attempts to scale these materials commercially. Tremendous research progress has been made with two-dimensional metal–organic frameworks (2D MOFs) which are desirable for their crystallinity, porosity, and conductivity which opens the door to applications in electrocatalysis, chemical sensing, and energy storage and conversion. MOFs are extended materials and typically lack solution processability meaning they cannot be dissolved in organic solvents without destroying the framework overall. This problem has generated renewed interest in macrocycles and cages which are dimensionally reduced molecular units that can self-assemble into unique materials, demonstrating similar or improved properties. This work describes the synthesis of a new class of conjugated metal–organic macrocycles with remarkable tunability, including access to solution processable materials, device fabrication, and studies on crystal growth and morphology. Chapter 1 provides an overview of shape-persistent macrocycles and progress into theiraccessible morphologies and applications. Emphasis is placed on their ability to self-assemble into supramolecular arrays, and the influence of hydrogen-bonding on self-assembly and crystallinity. Metal–organic macrocycles based on Pd and Pt are discussed first, followed by organic macrocycles and varying applications depending on what functional groups comprise the ring. Chapter 2 discusses the synthesis and characterization of new Cu-based metal–organicmacrocycles which mimic 2D MOF pores but offer enhanced solution processability and ease of field-effect transistor (FET) device fabrication. The strategy involves truncating HHTP (2,3,6,7,10,11-hexahydroxytriphenylene), a common tritopic MOF ligand to a ditopic alternative H4TOTP (TOTPn– = 2,3,5,6-tetraoxidotriphenylene) and coordinating to square planar copper centers to make a family of macrocycles with tunable carbon side chains. Extending chains to eighteen carbons enables solubility in common organic solvents and fabrication of FETs. Chapter 3 details the expansion of the side chain library with a focus on gaining deeperinsight into the crystal packing and crystal morphology. Side chains are carefully chosen to increase steric bulk, polarity, or hydrogen-bonding. Steric bulk is found to disrupt the π-π stacking, polar side-chains can be installed and maintain hexagonal packing, and hydrogen- bonding improves order, dramatically increasing crystallite size which allows characterization by optical microscopy and scanning electron microscopy (SEM) for the first time. Appendix A includes an introduction to field-effect transistor architecture and measurement. Preliminary atomic force microscopy results are shown, where slower drying methods from higher boiling solvents provide greater alignment across larger distances. This may be an avenue to improve charge mobility in the reported metal–organic macrocycles.