Rational design strategies based on practical fluorescence modulation mechanisms would enable us to rapidly develop novel fluorescence probes for target molecules. Here, we present a practical and general principle for modulating the fluorescence properties of fluorescein. We hypothesized that (a) the fluorescein molecule can be divided into two moieties, i.e., the xanthene moiety as a fluorophore and the benzene moiety as a fluorescence-controlling moiety, even though there is no obvious linker structure between them, and (b) the fluorescence properties can be modulated via a photoinduced electron transfer (PeT) process from the excited fluorophore to a reducible benzene moiety (donor-excited PeT; d-PeT). To evaluate the relationship between the reduction potential of the benzene moiety and the fluorescence properties, we designed and synthesized various derivatives in which the reduction potential of the benzene moiety was fine tuned by introducing electron-withdrawing groups onto the benzene moiety. Our results clearly show that the fluorescence properties of fluorescein derivatives were indeed finely modulated depending upon the reduction potential of the benzene moiety. This information provides a basis for a practical strategy for rational design of novel functional fluorescence probes.