Photo-Electroswitchable Arylaminoazobenzenes

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University of Alabama Libraries

Azobenzenes consist of two phenyl rings linked by an azo unit (N=N), existing in E or Z isomeric forms. Their ability to reversibly transform between 'extended' thermodynamically favored E and higher energy 'contracted' Z isomeric forms upon photo-stimulation make them useful molecular 'flexors.' E→Z switching is rapidly achieved using light, which has popularized the use of azobenzenes in a variety of chemical systems to gain nano mechanical photoswitchable characteristics. In most cases, Z→E isomerization occurs slowly thermally or, when possible by photoisomerization, though slower than photo E→Z conversion and typically incomplete.To address the Z→E isomerization limits (slow or incomplete), we have developed using electron removal as a new azobenzene switching mechanism for amino-substituted azobenzenes and investigated the prospect of switching multiple azo linkages with a single electron loss event. Blackstock and coworkers have covalently attached a redox aryl amine to the azobenzene moiety allowing for rapid, catalytic, and compete Z→E isomerization upon oxidation. The oxidized redox auxiliary dramatically reduces the Z→E isomerization energy barrier by factors of at least 105. Once initiated, the aryl amine radical cation is chemically stable and persistent enough to exchange electrons with a neutral Z isomer amine, generating an efficient electron-transfer chain reaction for Z→E isomerization. The synthesis, photo- and thermoisomerization, and lifetime effects of linking multiple azobenzenes to a single arylamine redox center are investigated for four tertiary amine derivatives: 4-methoxy-4'-(N,N-dianisyl)-aminoazobenzene (11), N,N-bis(azobenzene)-p-anisidine (20), N,N-bis(2,2',6,6'-tetrafluoroazobenzene)-p-anisidine (21), and N,N,N-tris(azobenzene)amine (29). Blue light irradiation of these azobenzene systems yields an equilibrium of Z-enriched isomers as a photostationary state (PSS). Dynamic UV-vis and NMR spectroscopy are used to measure PSS compositions and thermal dynamics of these mixtures. Ortho-fluorination is employed to increase Z isomer lifetime from hours (20) to weeks (21), resulting in an extended switching time domain for the dual-flexor system. Electron loss from a single arylamine efficiently catalyzes the Z→E isomerization of up to three connected azobenzene units, resulting in rapid, large geometry changes for these conglomerate structures. Stimulated, reversible flexing is thus demonstrated using electronic excitation and electron transfer. Incorporating a photosensitizer (methylene blue) allows for a dual photo, photo-electron transfer Z,E switching mechanism, which can be easily cycled with light. Red light excites methylene blue, which in turn oxidizes the redox amine to achieve rapid, complete Z→E conversion. Thus, blue and red light irradiation in tandem is shown to generate an E→Z→E switching cycle for three of the systems (11, 20, and 29).

Electronic Thesis or Dissertation
Azobenzene, Photo-Electro Switching, Stilbene