Isomerization,\nProtonation, and Hydrolysis Properties\nof Naphthalimide-Containing Spiropyran in Aqueous Media

Synthesis of spiropyrans exhibiting ring-opening/closing\nisomerization\ndriven by external stimuli is a challenging subject for the development\nof molecular sensors. A napthalimide-containing spiropyran (<b>1</b>) promotes rapid isomerization between the spirocyclic (SP)\nform and ring-opened merocyanine (MC) form by the change in solvent\npolarity even under the dark condition at room temperature. In this\nwork, the effect of water on the isomerization behavior of <b>1</b> was studied. The addition of water caused an MC–water H-bonding\ninteraction and shifted the resonance MC structure to the zwitterionic\nform with a lower ground-state energy. The stabilized MC form promoted\nspontaneous SP → MC isomerization and increased the equilibrium\nMC concentration. The effect of pH on the behavior of <b>1</b> was also studied. In acidic–neutral media, protonation/deprotonation\nof the naphthalimide moiety led to rapid and reversible changes in\nthe absorption spectra. In contrast, strongly basic media (pH >\n12)\npromoted irreversible base-catalyzed hydrolysis of the alkene moiety.