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Experimental and Theoretical Characterization of Ultrafast Water-Soluble Photochromic Photoacids
Cody R Aldaz1, Theodore E Wiley1, Nicholas A Miller1
1Department of Chemistry, University of Michigan, 930 North University Avenue Ann Arbor, Michigan 48109-1055, United States.
The Journal of Physical Chemistry. B
|April 19, 2021
Summary
Photoacid isomerization triggers rapid deprotonation, with the cis form proving more acidic. Current quantum chemical methods struggle to accurately predict this acidity change, necessitating further research.
Area of Science:
- Photochemistry
- Physical Chemistry
- Computational Chemistry
Background:
- Merocyanine/spiropyran photoacids are molecules that change properties upon light exposure.
- Understanding their photochemical mechanisms and acid-base properties is crucial for developing new materials.
- Previous studies have explored the behavior of these photoacids, but a complete mechanistic understanding remains elusive.
Purpose of the Study:
- To elucidate the photochemical mechanism of two specific merocyanine/spiropyran photoacids: phenylhydroxy-MCH and indazole-MCH.
- To investigate the relationship between photoacid isomerization and deprotonation.
- To benchmark the accuracy of quantum chemical methods in predicting changes in acidity (ΔpKa) upon isomerization.
Main Methods:
- Combined UV-visible transient absorption spectroscopy with quantum mechanical simulations.
- Investigated two photoacids: phenylhydroxy-MCH and indazole-MCH.
- Employed nine levels of theory, including continuum solvent models and explicit water, for calculations.
Main Results:
- Observed that trans-to-cis isomerization of the photoacids leads to deprotonation on a picosecond timescale.
- Confirmed that the cis form of phenylhydroxy-MCH is a stronger acid than its trans form.
- Found that current computational methods are insufficient to accurately predict the ΔpKa upon isomerization for these photoacids.
Conclusions:
- The photochemical mechanism involves rapid isomerization followed by deprotonation.
- The cis isomer exhibits enhanced acidity compared to the trans isomer.
- Further theoretical development is required to accurately model the acidity of cis photoacids and understand the underlying physical principles.
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