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Updated: Jul 5, 2026

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.
Abstract:
Whether the productive pathway in azo photoisomerization is N═N rotation or N-inversion remains debated. We use visible 5 fs pump-probe spectroscopy to track the N═N stretching mode (νN═N) during photoisomerization of trans-2-(phenylazo)imidazole (t-PAI) after n-π* excitation. Spectrogram analysis reveals an upshift-then-downshift trajectory of νN═N, with an initial upshift from 1450 to 1850 cm-1 followed by a downshift. This indicates a transient increase followed by a decrease in N═N bond order, consistent with rehybridization of one azo nitrogen atom toward sp-like character near the quasi-linear inversion geometry. This agrees with TD-DFT and multireference perturbation theory predictions for inversion and is incompatible with those for rotation, which predict the opposite sequence: an initial downshift followed by an upshift toward the cis-like geometry. These results provide the first direct vibrational evidence for a transient increase in N═N bond order during azo photoisomerization, supporting the inversion coordinate as the dominant productive pathway.
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