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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
Published on: August 22, 2018
pH-Dependent cis --> trans isomerization rates for azobenzene dyes in aqueous solution
Nicholas J Dunn1, William H Humphries, Adam R Offenbacher
1Department of Chemistry and Biochemistry, Ohio Northern University, 525 S. Main Street, Ada, Ohio 45810, USA.
The Journal of Physical Chemistry. A
|July 22, 2009
Summary
Azobenzene molecular switches isomerize faster in water than nonpolar solvents. Their thermal relaxation rates are pH-dependent, enabling potential pH control over switching times for azobenzene applications.
Area of Science:
- Photochemistry
- Physical Chemistry
- Materials Science
Background:
- Azobenzenes are molecular switches utilizing cis <--> trans photoisomerization.
- Switch stability relies on thermal relaxation rates, influenced by solvent.
- Limited kinetic data exists for azobenzenes in aqueous environments.
Purpose of the Study:
- To measure thermal cis --> trans isomerization rates of substituted azobenzenes in water.
- To investigate the influence of pH on azobenzene relaxation kinetics.
- To determine acid ionization constants and compare them with spectral data.
Main Methods:
- Nanosecond UV laser flash photolysis-transient absorption spectroscopy.
- Kinetic analysis of absorption transients.
- Systematic variation of pH from 4 to 11.
Main Results:
- Observed relaxation rate constants range from 10(5) to 10(1) s(-1).
- Isomerization rates in water are significantly faster than in nonpolar solvents.
- Relaxation rates exhibit systematic, predictable pH dependence due to dye ionization.
Conclusions:
- Azobenzene isomerization kinetics are strongly influenced by pH in aqueous solutions.
- Ionized forms of azobenzenes isomerize more rapidly.
- These findings may facilitate pH-controlled azobenzene molecular switching.
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