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The cis − trans photoisomerization of an azo dye was induced by two-photon excitation using a titanium:sapphire femtosecond laser, and was monitored by 1H NMR spectroscopy. The NMR signals of the two isomers were well separated, allowing the unambiguous identification of the photoproducts. No photoproducts other than the cis isomer were observed. The operation of a two-photon mechanism was confirmed by measurement of the dependence of the initial reaction rate on the laser power. The approach of the system to the photostationary state was investigated, and the rate constants for photoisomerization, which comprises both photoexcitation and excited-state isomerization processes, were found to be similar for both two-photon and one-photon excitation using typical excitation conditions.

1H NMR spectroscopy coupled with in situ laser irradiation has been used together with density functional theory (DFT) computation to examine the structures of the photoisomers of a series of sulfonated reactive azo dyes. Assignment of 1H NMR spectra acquired at the photostationary state has allowed, for the first time, NMR characterisation of unstable cis isomers of commercially relevant water-soluble azo dyes. Structural features of the two isomeric forms predicted by DFT calculations are clearly reflected in the experimental NMR data. The trans-cis photoisomerisation process could be unambiguously identified in each case, based on the large chemical shift change observed for resonances associated with aromatic protons adjacent to the azo linkage.

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