Date of Award

Fall 2021

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Linda S. Shimizu

Abstract

Triphenylamines (TPAs) are known to form persistent organic radicals either by chemical, electrochemical or photoinduced oxidation. Typically, fully para-substituted TPAs form stable radical cations while the radical cations in partially substituted systems quickly degrade. Herein, we study the effects of solid-state organization on a series of urea tethered halogenated TPAs 1 (X = H, Cl, Br, I) and compare their radical cation formation and persistence after UV-irradiation. These halogenated urea tethered TPAs were examined by single-crystal X-ray diffraction where their assembly was guided by threecentered urea hydrogen bonding interaction. As expected, all compounds form photogenerated radical cations in solution, but the structures quickly degrade. In contrast, supramolecular assembly enhanced the stability and persistence of the radicals, which appear to undergo charge recombination without degradation. Greater quantities of these radical cations are observed for 1Br and 1H. Theoretical calculations on single molecules and hydrogen-bonded dimers with time-dependent density functional theory (TD-DFT) suggest that the charge transfer (CT) is feasible upon UV-irradiation, favoring an intramolecular process for 1Br versus an intermolecular CT process in 1I. In addition, electrical charge coupling leads to faster charge recombination in 1I versus 1Br. Further modification of the para positions with iodine leads to needle shape crystals that are organized by hydrogen and halogen bonding. We have investigated the relationship of CT and radical formation with mobility and conductivity measurements as conductive assembled TPA have particular importance as hole transporters in organic solar cells. Chapter 1 provides an overview of urea tethered TPA linear and macrocyclic derivatives prepared in the Shimizu group and compared with literature examples. Chapters 2 and 3 examine how the supramolecular assembly of linear TPA frameworks controls the resulting charge/electron transfer of materials. Finally, Chapter 4 focuses on assembled fluorinated TPA bis-urea macrocycles. The goal is to use 19F NMR to follow transient radical cation and anion species formed as the sample is UV irradiated. TD-DFT was employed to investigate the spin density distribution in this macrocycle and predict if the position of the fluorine is reasonable to report on the TPA radical cations.

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