In the dark the protonated merocyanine form (MCH) is in equilibrium with the deprotonated merocyanine form (MC) and the ring-closed spiropyran form (SP) (Figure ). The pK a dark value was determined by measuring the UV-vis absorption spectra of equilibrated samples at pH 5.1-9.6 (Figure ,iii see S4. Increasing the length of the side chain (R 3 ) by just one methylene group led to increased solubility in line with previously reported compound (R 1 = H, R 2 = H, R 3 = (CH 2 ) 4 SO 3 -). We were able to prepare 3 mM solutions of merocyanine 1 which is around one order of magnitude higher than the solubility of propyl sulfonate analogue 6 (c = 0.4 mM). As expected, the increased solubility of 1 resulted in improved pH switching properties. A 3 mM solution of 1 in 20 mM KCl was adjusted to pH 7.3 by adding minimal amounts of aqueous potassium hydroxide solution (1.0 M). Exposure to visible light at λ = 450 nm resulted in a drop to pH 3.8, equivalent to a 3.5 unit reversible pH jump (Fig. ). This switching process was repeated over 10 cycles with only minimal loss in the pH jump (98% recovery/cycle, Fig. ). Adjusting the initial pH to 8.3 yielded a light-induced pH jump of 3.2 units to pH 5.2 (Fig. ). By comparison, the corresponding switching of a previously reported merocyanine (R 1 = MeO, R 2 = H, R 3 = (CH 2 ) 3 NMe 3 + ) at an initial pH of 7.5 resulted in a pH drop of only 1.6 units whereas an initial pH of 8.8 gave a drop of just 0.1 units.