In conclusion, we report a new merocyanine photoacid with improved solubility and the highest pK a dark value with respect to reported examples. 4, 6-7, 10 These improved properties enabled the highest visible-light-induced pH jump of 3.5 units (pH 7.3 to 3.8) with good stability over 10 cycles. Visible-lightinduced pH switching was extended well into the basic pH range for the first time with significant reversible pH drops from pH 8.3 to 5.2 that could be repeated over 10 cycles with minimal loss in function. These switching cycles of considerable magnitude and access to pH switching from basic to acidic environments make merocyanine 1 a useful tool to control chemical and biological activity with visible light. Application of merocyanine 1 will significantly broaden the scope of pH sensitive systems that can now be externally (and non-invasively) controlled with visible light.