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Updated: Jul 5, 2026
09:33
Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
Published on: February 7, 2022
Real-Time Vibrational Spectroscopy Reveals an Inversion Transition State in the Photoisomerization of
Sena Hashimoto1, Izumi Iwakura1, Tsubasa Tanaka1
1Department of Applied Chemistry, Faculty of Chemistry and Life Science, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan.
The Journal of Physical Chemistry Letters
|July 3, 2026
Summary
Azo photoisomerization primarily occurs via N-inversion, not N=N rotation. Ultrafast spectroscopy reveals a transient N=N bond order increase, supporting inversion as the dominant pathway.
Area of Science:
- Physical Chemistry
- Photochemistry
- Spectroscopy
Background:
- The mechanism of azo photoisomerization, specifically whether N═N rotation or N-inversion dominates, is a long-standing debate in photochemistry.
- Understanding the reaction pathway is crucial for controlling isomerization dynamics in molecular systems.
Purpose of the Study:
- To investigate the dominant productive pathway in azo photoisomerization using ultrafast spectroscopy.
- To provide direct vibrational evidence for the transient changes in the N═N bond during photoisomerization of trans-2-(phenylazo)imidazole (t-PAI).
Main Methods:
- Employed visible 5 femtosecond (fs) pump-probe spectroscopy to monitor the N═N stretching mode (νN═N) during photoisomerization.
- Utilized spectrogram analysis to track the evolution of the N═N vibrational frequency.
- Correlated experimental findings with theoretical predictions from TD-DFT and multireference perturbation theory.
Main Results:
- Observed an initial upshift in νN═N from 1450 to 1850 cm-1, followed by a downshift, indicating a transient increase and subsequent decrease in N═N bond order.
- The observed vibrational dynamics are consistent with nitrogen atom rehybridization near a quasi-linear inversion geometry.
- Theoretical calculations supported the inversion pathway, predicting an initial increase in N═N bond order, contradicting predictions for the rotation pathway.
Conclusions:
- The study provides the first direct vibrational evidence supporting the N-inversion pathway as the dominant mechanism in azo photoisomerization.
- The transient increase in N═N bond order is a key indicator of the inversion coordinate's role.
- These findings resolve the debate regarding the productive pathway in azo photoisomerization.
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