Incorporating Tailored Hydrogen Bonding Interactions in Organic Optoelectronic Thin Films - Pursuing Universality of Form and Function

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Carbon-based (organic) materials hold enormous potential for use in everyday applications including displays, lighting, solar cells, transistors, and sensors. In these contexts, it is often the three-dimensional (3-D) arrangement of the molecules in thin (10-1,000 nanometer) films that ultimately affects efficiency, stability, and overall utility. This research borrows an approach used by nature, called self-assembly, to encourage organic materials to adopt profitable arrangements in thin films largely independent of their size and shape. Central to the universal approach of self-assembly are hydrogen bonding interactions that are also responsible for "sticking" water molecules or the DNA bases together. In the broadest sense, the research contributes to a fundamental understanding of how to control the 3-dimensional structure of organic matter at the nano-/mesoscale to accelerate development of next-generation organic-based electronic materials. Broader impacts with respect to education and training come from graduate and undergraduate students being exposed to multidisciplinary science that includes synthetic and physical organic chemistry, spectroscopy, computation, and materials processing/characterization. Professors Castellano and Xue take advantage of the University of Florida (UF) Bridge to Doctorate Fellowship program, a collaboration between the Graduate School at UF and the Florida-Georgia Louis Stokes Alliance for Minority Participation (FGLSAMP), to help recruit female and minority students contribute to research and receive mentorship to encourage degree completion. Through the team's participation in U-FUTuRES, physical science content is delivered to Florida science teachers (grades 4?8).The prevailing interactions between pi-conjugated organic molecules in thin films do not yield large energy differences among competing solid-state packing arrangements, leading to kinetically-driven spatial structures that are susceptible to minor changes to molecular structure and processing. The structure-property studies in this research expose the thermodynamic and kinetic drivers of hydrogen bond (H-bond) guided thin film assembly to allow the charge mobility and optical characteristics to be independently addressed through rational design and synthesis. This project specifically examines the interplay of kinetics and thermodynamics in film formation through variation of programmed intermolecular interactions and deposition conditions. The research also introduces a bioinspired structural scan to relate H-bonding configuration to optoelectronic properties and examines alternative molecular designs involving repositioned H-bonding units within the pi-framework to improve assembly orientational control in the pi-stacking direction and with respect to the substrate.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.