ORGN 79 |
| Azobenzene derivatives have unusual photo-isomerization properties that may lead to novel molecular switching applications. Photo-initiated cis-trans isomerization rates are known to depend on solvent environment, but kinetic studies in aqueous media have not been previously reported. We use nanosecond UV laser flash photolysis-transient absorption spectroscopy to measure relaxation rates for mono- and di-substituted p-aminoazobenzenes and p-hydroxyazobenzenes in water over the pH range of 11 to 4. Our observed absorption transients fit to simple first-order relaxation rate constants between 101 and 105 s-1, which are generally much larger values than in non-polar solvents, and the nearly linear relationship between rate constant and pH for neutral and acidic solutions suggests that the isomerization mechanism involves an acid-catalyzed pathway. Structural calculations to predict relative energy barriers for neutral and ionic transition states are in progress. Isomerization kinetics for hydroxyazobenzenes in high-pH solution may also involve an ionic pathway but the mechanism is apparently more complex. |
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Physical Organic Chemistry, Combinatorial and Process Chemistry, New Reactions and Methodology, Peptides, Proteins and Amino Acids
8:00 PM-10:00 PM, Sunday, April 6, 2008 Morial Convention Center -- Hall A, Poster
Division of Organic Chemistry |