PH-dependent cis → trans isomerization rates for azobenzene dyes in aqueous solution

Nicholas J. Dunn, William H. Humphries IV, Adam R. Offenbacher, Travis L. King, Jeffrey A. Gray

Research output: Contribution to journalArticlepeer-review

44 Scopus citations


Azobenzenes can function as molecular switches driven by their unusual cis ↔ trans photoisomerization properties. The stability of an azobenzene-based switch depends on its rate of thermal relaxation, which is known to depend on the solvent environment, but few kinetic studies in aqueous media have been reported. We use nanosecond UV laser flash photolysis-transient absorption spectroscopy to measure thermal cis → trans isomerization rates for mono-and disubstituted p-aminoazobenzenes and p-hydroxyazobenzenes in water at 23 °C over the pH range of 4 to 11. Observed absorption transients are fit to first-order relaxation rate constants between 105 and 10 1 s-1, which is generally much faster than in nonpolar solvents, and the relaxation rates vary systematically and predictably with pH as the equilibrium shifts to ionized forms of the dyes that isomerize much more rapidly. Acid ionization constants for these dyes determined from our kinetic mechanism are compared with the pH dependence of their equilibrium UV-vis spectra. New kinetics results may enable pH control of azobenzene-based molecular switching times.

Original languageEnglish (US)
Pages (from-to)13144-13151
Number of pages8
JournalJournal of Physical Chemistry A
Issue number47
StatePublished - Dec 1 2009
Externally publishedYes


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