A reversible donor-acceptor fluorescence redox switch connected by a rigid but redox-adjustable spacer that can be chemically turned ON and OFF through the ring opening and ring closing of a heterobicyclic moiety is demonstrated. A coumarin-based oxazabicyclic derivative was efficiently synthesized as an example for illustration. While the rigid ring-closed oxazabicycle emits moderate fluorescence in toluene, the sodium borohydride induced ring opening of the heterobicyclic moiety results in a distinct decrease in fluorescence. The resulting nonfluorescence ring-opened form can be reverted to the original fluorescence ring-closed form via DDQ or H 2O 2 oxidation. ? Georg Thieme Verlag Stuttgart.
