Isomerization, Protonation, and Hydrolysis Properties of Naphthalimide-Containing Spiropyran in Aqueous Media

Synthesis of spiropyrans exhibiting ring-opening/closing isomerization driven by external stimuli is a challenging subject for the development of molecular sensors. A napthalimide-containing spiropyran (<b>1</b>) promotes rapid isomerization between the spirocyclic (SP) form and ring-opened merocyanine (MC) form by the change in solvent polarity even under the dark condition at room temperature. In this work, the effect of water on the isomerization behavior of <b>1</b> was studied. The addition of water caused an MC-water H-bonding interaction and shifted the resonance MC structure to the zwitterionic form with a lower ground-state energy. The stabilized MC form promoted spontaneous SP → MC isomerization and increased the equilibrium MC concentration. The effect of pH on the behavior of <b>1</b> was also studied. In acidic-neutral media, protonation/deprotonation of the naphthalimide moiety led to rapid and reversible changes in the absorption spectra. In contrast, strongly basic media (pH > 12) promoted irreversible base-catalyzed hydrolysis of the alkene moiety.