| Abstract
| - Fluorogenic probes for redox metabolism are shown to exhibit two-photon absorption using femtosecondpulses from a Ti/sapphire laser. The probes consist of a coumarin or naphthalene core and a ketone−alcoholfunctional group. The probe design is based on the changes in fluorescence properties and two-photon crosssection values when the ketone derivative is reduced to the corresponding alcohol. The resulting contrastratio of the fluorescence differs significantly from that obtained by one-photon excitation. This phenomenonwas demonstrated with all three switches examined herein and represents an attractive approach to modulationof emission properties of molecular switches. The practical applicability of the two-photon-excitation redoxswitch was demonstrated in an enzyme-catalyzed transformation. The nonfluorescent probe 1 is efficientlyconverted by AKR1C3, a human 3α-hydroxysteroid dehydrogenase, to the fluorescent derivative 2. Usingtwo-photon excitation (775 nm), we achieved a large contrast ratio between the fluorescence in the presenceand the absence of the enzyme of ∼300.
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