Two-Photon-Induced Photoisomerization of an Azo Dye

The cis − trans photoisomerization of an azo dye was induced by two-photon excitation using a titanium:sapphire femtosecond laser, and was monitored by 1 H 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.