The fluorescence properties of 4-aminophthalimide derivatives are influenced by the side-chain attached to the paramagnetic ring or its diamagnetic precursor. PET (photo-induced electron transfer)-induced fluorescence enhancement in the presence of Fe3+ ions was observed only in the case of diamagnetic basic side chain derivatives, since transient metal ions are present in biological systems, only double sensors lacking the above sensitivity are considered as potential reactive oxygen probes.