a School of Chemistry, UNSW Sydney, Sydney, New South Wales 2052, Australia. jbeves@unsw.edu.au b Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg 412 96, Sweden. Basic-to-acidic reversible pH switching with a merocyanine photoacid Laura Wimberger a Joakim Andréasson, b and Jonathon E. Beves * a The application of merocyanine photoacids has been previously largely limited to neutral and acidic pH values. Here we introduce a new merocyanine photoacid with superior pH switching qualities. By increasing the pK a in the dark (pK a dark ) and the solubility we increased the reversible visible light induced pH jump to 3.5 units. Moreover, it is the first demonstration of a merocyanine photoacid able to generate a significant pH drop from a basic (pH 8.3) to an acidic (pH 5.2) environment. 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. 1) 1 sparked interest in a wide-spread range of applications in supramolecular 2 and materials chemistry. 3 Recently developed merocyanines demonstrated increased pH jumps ranging from 2.5 (R 1 = H, R 2 = H, R 3 = (CH 2 ) 4 SO 3 – 4 and R 1 = H, R 2 = polymer, R 3 = (CH 2 ) 3 SO 3 – 5 ) and 3.0 pH units (R 1 = MeO, R 2 = H, R 3 = (CH 2 ) 3 SO 3 – ) 6 to the highest reported value of 3.2 pH units (R 1 = MeO, R 2 = H, R 3 = (CH 2 ) 3 NMe 3 + ). 7 The pH ranges these examples 4, 6-7 can be used for is limited to near neutral or acidic environments. 8 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. Merocyanine photoacid 1 was synthesized over three-steps (for full characterisation and data, see Supporting Information S2). The proton dissociation behaviour of 1 was studied in the dark and under visible light irradiation (λ = 455 nm) by established methods. 4, 7 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 2ai). 9 The pK a dark value was determined by measuring the UV-vis absorption spectra of equilibrated samples at pH 5.1–9.6 (Figure 2aii,iii see S4.3 in Supporting Information). This parameter accounts for the coupled equilibria between MCH, MC and SP. The corresponding pK a value under irradiation, pK a hν , is determined by the equilibrium between the metastable cis-MCH and SP form (Fig 2bi) 4, 7 and was calculated from the absorption spectra of equilibrated samples under visible light irradiation (λ = 455 nm) over a pH range from 2.2 to 6.6 (Fig. 2bii, iii). Fig. 1 Overview over pKa dark and pKa hν values of reported 4, 6-7, 10 merocyanine photoacids and derivative 1 with respective substitution patterns (right).