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Updated: Jun 8, 2025
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Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
Published on: February 7, 2022
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A cautionary tale of basic azo photoswitching in dichloromethane finally explained
Coral Hillel1, Sara Rough1, Christopher J Barrett1,2
1Department of Physics and Astronomy, York University, Toronto, ON, Canada.
Communications Chemistry
|November 2, 2024
Summary
Chlorinated solvents like DCM cause unreliable azobenzene photoswitch isomerization results due to solvent photodecomposition. UV light triggers protonation, accelerating thermal isomerization and altering mechanisms, explaining experimental warnings.
Area of Science:
- Photochemistry
- Organic Chemistry
- Physical Chemistry
Background:
- Azobenzene photoswitches are studied for isomerization in polar aprotic solvents.
- Chlorinated hydrocarbons, such as dichloromethane (DCM), are commonly used.
- Inconsistent results plague azobenzene isomerization studies in these solvents.
Purpose of the Study:
- Investigate the root cause of spurious results in azobenzene thermal isomerization.
- Understand the role of chlorinated solvents under UV irradiation.
- Clarify the mechanism of thermal isomerization in DCM.
Main Methods:
- UV irradiation of azopyridine photoswitches in DCM.
- Protonation studies of the pyridine moiety.
- Density Functional Theory (DFT) calculations.
- Analysis of thermal isomerization rates.
Main Results:
- UV irradiation of azopyridine in DCM causes solvent photodecomposition and protonation.
- Protonation significantly accelerates the thermal isomerization rate.
- The typical singlet-triplet rotation mechanism is abolished by protonation.
- Spurious results are attributed to photolytically generated protons.
Conclusions:
- Chlorinated solvents are unsuitable for UV-triggered azobenzene isomerization studies.
- Photodecomposition of DCM generates protons that alter isomerization pathways.
- Understanding this phenomenon explains experimental inconsistencies and warnings.
- Potential applications of photolytically generated protons are suggested.
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