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Molecular Engineering of Synthetic Amino Acids for the Mechanistic Study of Dipole Effects on Intramolecular Charge Transfer

  • Author(s): Clinton, Jillian Marie
  • Advisor(s): Vullev, Valentine I
  • et al.
Abstract

Photon conversion efficiency is primarily impaired by interfacial charge recombination. Natural light harvesting systems address this challenge by incorporating molecular electrets. Protein alpha helices are the best examples of molecular electrets since the ordered amide and hydrogen bonds naturally generate a co-directionally ordered permanent electric dipole.

Although the local electric fields generated by the protein's dipole provides a means for “steering” electron transduction, they cannot be directly incorporated as electronic material because 1) electron transfer is mediated via tunneling, 2) they contain large band gaps and 3) are conformationally sensitive.

Therefore, a significant part of my studies focused on developing a library of non-native, aromatic beta-amino acid residues composed of anthranilic acid derivatives. The aromatic moiety is designed to provide an electronic framework that supports a hole hopping mechanism resulting in a system that captures the benefits of natural systems. Furthermore, altering the attached substituents results in a range of reduction potentials over 1 volt.

Electrochemical examination revealed the structure-function relationship between the position and strength of the electron donating substituent. These findings provided the criteria for generating a long-lived radical cation to be 1) the reduction potential must be under 1.5 V vs. S.C.E and 2) the electron spin density must not extend over the C-terminal amide.

From these findings, I development a set of ad hoc hole-transfer molecular electrets. Based on a fluorinated aminoanthranilamide templet, each residue contained a both an electron withdrawing and donating group. The spin-density-distribution and electrochemical analysis revealed that the fluorine atom induced a positive shift in the reduction potentials without destabilizing the oxidized residue. Additionally, the regio-selective nucleophilic aromatic substitution of the starting material provides a straightforward synthesis.

Overall, the most significant contributions from my doctoral research are: (1) the design and development of a synthetic amino acid library with a wide range of reduction potentials; (2) the determination of the effects of location and electron donating strength on the stability of anthranilamide radical cations; and (3) the implantation of nucleophilic aromatic substitution to synthesize residues for hole hopping.

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