Novel blue emitting tetra- and pentamethylpiperidin-4-yloxy-1,8-naphthalimides as photoinduced electron transfer based sensors for transition metal ions and protons

Two novel blue emitting 1,8-naphthalimides (1 and 2), containing 2,2,6,6-tetramethylpiperidin-4-yloxy moieties, and newly synthesized under phase transfer catalysis conditions 4-(1,2,2,6,6-pentamethylpiperidin-4-yloxy)-1,8-naphthalimide 7, were configured as “fluorophore–spacer–receptor” systems. Basic photophysical characteristics of the novel fluorescent brighteners were investigated in both DMF and water/DMF (4:1, v/v) solution. Their ability to detect cations was evaluated in the presence of transition metal ions (Cu2+, Pb2+, Zn2+, Ni2+, Co2+) and protons. The presence of metal ions and protons was found to disallow a photoinduced electron transfer leading to an enhancement in the fluorophore emission intensity. The results clearly showed a good sensor activity towards transition metal ions and protons, especially of compound 7, containing tertiary amine receptor. This indicates the potential of the novel compounds as highly efficient “off–on” pH switchers and fluorescent detectors for metal ions.