Functional Nanotubes from Self-Assembled Bis-Urea Macrocycles

University Of South Carolina At Columbia, Columbia SC

Investigators

Abstract

Nature employs self-assembly approaches to control the size and shape of biological structures for performing specific functions. Examples include protein channels and enzymes. A protein channel is formed by a protein that folds into a hollow tubular structure and spans across a membrane to allow passage of small molecules or ions from one side of a membrane to the other side through the channel. An enzyme is a protein that folds into a 3-dimensional structure to create a pocket of specific size and shape for binding and converting target molecules into desired chemical products. Inspired by Nature, Professor Linda Shimizu of the University of South Carolina makes small donut-shaped molecules that assemble in high fidelity into straw like structures called nanotubes. These nanotubes have tiny 1-dimensional channels of less than a nanometer (about 1/1000 times of the width of a strand of hair) in diameter. The Shimizu group conducts research to understand how gases and small molecules are organized and move through these tiny channels. Her research group also uses these nanotubes to restrict how molecules in these channels are oriented with respect to one another, which alters how these trapped guests can react. Fundamental knowledge to be gained from this research could have important implications in separation technologies, flow reactors and the development of new synthetic methods. The broad research approach provides a challenging and interdisciplinary environment to train a diverse group of graduate students as well as provides first research experiences for undergraduates and high school students. In addition, this award supports a chemistry outreach program that broadly promotes public interest in science by bringing chemists into South Carolina K-12 classrooms of high minority enrollment middle schools and high schools. These school visits are intended to showcase the scientific method, highlight cutting edge research, and foster interest in chemistry and in the natural sciences. Work in Professor Linda Shimizu's laboratory, supported by the Macromolecular, Supramolecular and Nanochemistry Program at the National Science Foundation, utilizes the controlled assembly of bis-urea macrocycles to afford functional tubular assemblies to probe fundamental questions. This research investigates structure-property relationship of macrocycles and linear analogues that contain triphenylamines, benzophenones, and other organic triplet sensitizers. After UV-irradiation, these assemblies form radicals within the walls of the nanotubes. The quantity, persistence and properties of these radicals are related to their structure and organization. In this project, the Shimizu group 1) studies the assemblies to understand the origin of the radical stability; 2) probes diffusion within the nanochannels; 3) evaluates how radicals in the channel walls interact with guests encapsulated within the channels; 4) evaluates the assemblies of triplet sensitizers in oxygenated solutions to determine their efficiency at forming reactive oxygen species; and 5) studies the ability of the nanoreactors to facilitate photooxidations, photocycloadditions and photopolymerization reactions in solution. The knowledge acquired should lead to new predictive tools to determine when persistent radicals will form as well as afford a better understanding of the effects of confinement on reactivity. These results should lead to an increase in structural diversity of organic radicals, which are of interest as molecular magnets, as probes in biological systems, for polarizing agents and as radical initiators for polymerization. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.