Photoswitchable chemical sensing based on the colorimetric pH response of ring-opened naphthopyrans

Photoswitchable chemical sensor is a special variety of sensors that could be turned ON and OFF upon light modulation. Previous photoswitchable ion-selective sensors have been demonstrated on spiropyrans toward Cl^-, Na⁺, K⁺, and Ca²⁺. Herein, we demonstrate the first photoswitchable pH sensor based on naphthopyrans, a known family of photoswitchable compounds with better photostability than spiropyran. The naphthopyrans undergo ring-opening reactions under 365 nm UV irradiation. The ring-opened forms are demonstrated here to be pH sensitive due to the presence of piperidine and morpholine moieties. Protonation of the ring-opened forms shifted the absorption peaks from ca. 530 nm (deprotonated) to ca. 624 nm (protonated) leading to a vivid color change. These compounds were evaluated in organic solvents, plasticized poly(vinyl chloride) (PVC-DOS) films, and polyurethane hydrogels. In the hydrogels, quantitative pH sensing was achieved in the dynamic range from pH 3 to 7, and similarly, the electrodes containing the naphthopyran also exhibited Nernstian response in the acidic range. UV light stimulation on the electrode membrane also revealed that there was a basicity change between the ring-opened and ring-closed naphthopyran forms. However, judging from the small change of electrode potential (ca. 17 mV), the difference was much smaller compared with previous reported spiropyrans (ca. 350 mV). Based on these results, we believed the proposed compounds and strategy could form the basis of a new variety of photoswitchable and photoactivatable chemical sensors.