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Published on: July 17, 2015
Hydroxystilbene isomer-specific photoisomerization versus proton transfer
Frederick D Lewis1, Elizabeth M Crompton
1Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA. lewis@chem.northwestern.edu
Journal of the American Chemical Society
|April 3, 2003
Summary
3-Hydroxystilbene acts as a photoacid in water, unlike its isomer 4-hydroxystilbene. This difference in behavior is due to variations in their excited-state C=C torsional barriers, impacting proton transfer capabilities.
Area of Science:
- Photochemistry
- Physical Organic Chemistry
- Spectroscopy
Background:
- Hydroxystilbenes are organic compounds with potential applications in various fields.
- Understanding their acid-base properties in different states is crucial for predicting their reactivity.
- Isomeric differences can lead to distinct photophysical and photochemical behaviors.
Purpose of the Study:
- To investigate the ground- and excited-state acidities of 3- and 4-hydroxystilbene in aqueous solution.
- To elucidate the reasons behind the differential photoacidic behavior of these isomers.
- To determine accurate acidity values and understand the underlying photophysical mechanisms.
Main Methods:
- Förster equation for approximate pKa* determination.
- Singular value decomposition with self-modeling for precise pKa* calculation.
- Comparative analysis of excited-state C=C torsional barriers.
Main Results:
- Approximate excited-state acidity (pKa*) values were similar for both isomers.
- 3-Hydroxystilbene exhibited photoacidic behavior in water, while 4-hydroxystilbene did not.
- Singular value decomposition provided more accurate pKa* values for 3-hydroxystilbene.
Conclusions:
- The differing photoacidic properties are attributed to variations in excited-state C=C torsional barriers.
- 3-Hydroxystilbene's high barrier and long singlet lifetime facilitate proton transfer.
- 4-Hydroxystilbene's low barrier hinders proton transfer compared to C=C torsion.
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