Mesomeric Tuning of Merocyanine Photoacids

We synthesized a library of methoxy-substituted merocyanine photoacids to systematically investigate how mesomeric effects influence their hydrolytic stability and photoacid properties. Our results demonstrate that multiple methoxy groups directly conjugated to the ene-iminium core exert a cumulative electron-donating effect, with the bis-substituted derivative 4 exhibiting a nearly three-fold longer half-life than either of the mono-substituted analogues 2 or 3 (3.5 vs. 1.3 days), which in turn are nearly three times more hydrolytically stable than unsubstituted compound 1 (1.3 vs. 0.5 days). Compound 4 displays an acidity constant close to neutrality (pKa^GS = 7.1), which decreases by more than 3 pK units (pKa^MS = 3.9) upon 500 nm light irradiation, as determined by UV-Vis spectrophotometric titrations under dark conditions and in the photostationary state, respectively. These properties enabled the preparation of visible light-switchable buffer solutions featuring basic-to-acidic pH jumps of 3.2 units that can be reversibly triggered over at least two consecutive days with minimal loss, highlighting the potential of MCH 4 in solar energy harvesting applications.