The effects of spacer length on the fluorescence quantum yields of the benzofurazan compounds bearing a donor-acceptor system.

We studied the effects of spacer length on the fluorescence quantum yields (Phi) of photoinduced electron transfer (PET) reagents, using nitrobenzoxadiazole (NBD) derivatives that have the -NMe2 moiety and NBD-NH- fluorophore as electron donor (D) and electron acceptor (A), respectively. The Phi values were reduced as the spacer length became shorter (n < or = 4; n is the number of methylene units of the spacer) and the fluorescence recovered by suppression of the PET process. It is necessary for the useful PET reagents to link D and A with a short spacer to obtain a difference in the Phi values between fluorescent "off-state" and "on-state".