Every process occurring in an aqueous environment will be sensitive to pH, whether it is enzyme functionality, controlling chemical reactivity, or the ocean's capacity to store carbon dioxide. By using light to induce proton dissociation, pH can be modified externally and non-invasively with high precision in space and time. Spiropyran/merocyanine photoswitches are reversible metastable photoacids that can significantly decrease the pH of an aqueous solution when irradiated with visible light, with subsequent recovery to the initial pH occurring thermally. The first reported merocyanine photoacid able to generate a substantial reversible pH jump of 1.5 units (R 1 = H, R 2 = H, R 3 = (CH 2 ) 3 SO 3 -, Fig. ) ) and 3.0 pH units (R 1 = MeO, R 2 = H, R 3 = (CH 2 ) 3 SO 3 -) to the highest reported value of 3.2 pH units (R 1 = MeO, R 2 = H, R 3 = (CH 2 ) 3 NMe 3 + ). The pH ranges these examples 4, 6-7 can be used for is limited to near neutral or acidic environments. Herein we report a new merocyanine derivative 1 with a butyl sulfonate group (R 3 ) and substituents R 1 = MeO and R 2 = H capable of generating an unsurpassed pH jump of 3.5 units. Moreover, it is the first demonstration of a pH jump covering a significant part of the basic region.