The design, synthesis and photophysical properties of two novel 1,8-naphthalimide fluorescent pH sensors based on PET and ICT

The changes in the photophysical properties of two novel, yellow-green emitting pH chemosensors namely a 1,8-naphthalimide dye and its condensed heterocyclic derivative, as a function of pH were investigated in water/ethanol. The 1,8-naphthalimide dye displayed on–off pH sensing properties owing to photoinduced electron transfer, while its condensed heterocyclic derivative revealed off-on-off pH sensor activity derived from simultaneous photoinduced electron transfer and internal charge transfer. In alkaline media the fluorescence of the 1,8-naphthailimide dye was quenched owing to photoinduced electron transfer being disabled in acidic media. The pKa of the 1,8-naphthailimide dye was 5.49, which defines the dye as a highly efficient “off–on” switch. For the heterocyclic 1,8-naphthalimide derivative two fluorescence quenching processes at different pH were observed, one with a pKa value of 6.08 and a second with a pKa value of 3.95. These results indicate that the heterocyclic derivative would be able to act as an efficient “off–on–off” fluorescent pH switch.