| Summary: | Photoresponsive molecular devices can be a valuable tool to promote chemical changes in response to multiple signals, such as photons and pH, to deliver drugs or to detect physiological conditions in vivo. For example, <i>trans</i>-chalcones (<b>Ct</b>) from 4′-hydroxyflavylium (<b>F1</b>) and 7-hydroxyflavylium (<b>F2</b>) can undergo <i>cis-trans</i> isomerization by photoreaction into many different structures. The isomerization takes place at a slow rate in response to pH change; however, it can be done in seconds by photoreaction. In the investigation, as confirming the previous reports, 3-(2-hydroxy-phenyl)-1-(4-hydroxy-phenyl)-propenone, the <i>trans</i>-chalcone (<b>Ct<sub>F1</sub></b>) from <b>F1,</b> produces flavylium ions in pH = 1–4.5. Then, we further discovered that the flavylium quickly releases protons to yield the corresponding quinoidal base (<b>A</b>) in a solution of pH = 5.2 during irradiation with 350 nm. Meanwhile, the photolysis of 3-(2,4-dihydroxy-phenyl)-1-phenyl-propenone, the <i>trans</i>-chalcone (<b>Ct<sub>F2</sub></b>) from <b>F2</b> at pH = 5.6, induces photoacid behavior by losing a proton from the <i>trans</i>-chalcone to generate <b>Ct<sup>2</sup></b><sup>−</sup>. The different outcomes of these nearly colorless chalcones under similar pH conditions and with the same photochemical conditions can be useful when yielding colored <b>AH<sup>+</sup></b>, <b>A</b>, or <b>Ct<sup>2</sup></b><sup>−</sup> in a mildly acidic pH environment with temporal and spatial control using photochemical means.
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