Novel Fluorescent pH Sensors Based on Intramolecular Hydrogen Bonding Ability of Naphthalimide

4-Piperidine-naphthalimide derivatives containing potential acceptors for hydrogen bonding were synthesized, and their fluorescence properties were examined. Some of the compounds with a 2-imino-oxalidin (thiazolidin) side chain at the imide moiety exhibit a strong fluorescence quench and some red shift in weakly acidic conditions, caused by the formation of an intramolecular hydrogen bond. Their pK(a) values were estimated to be about 6.4-7.5. The results showed that these compounds could serve as novel fluorescent pH sensors for further application. [structure: see text]