Phase‐Dependent Photochromism of a Lactone‐Stabilized Chromene from a Flavylium Reaction Network

New trans ‐2‐hydroxychalcones bearing a carboxylate group at position 2′ ( Ct − ) were synthesized (compounds 2 and 3 ). These compounds lead to a network of chemical reactions depending on pH value, light, and solvent. In water, when the pH value is lowered, the ionized trans ‐chalcone is protonated and the flavylium cation A H + is formed at very acidic pH values through hemiketal B and cis ‐chalcone Cc , with global acidity constants of p K ′ a ≤−1 and ≈0.1, respectively, for 2 and 3 . The electron‐acceptor character of the carboxylic substituent not only increases the observed acidity of the flavylium cation, but also decreases the rate of the ring‐opening/‐closing from a subsecond timescale to hours relative to model compound 1 (without carboxylate). The photochemistry of the network was studied in detail by means of continuous irradiation, monitored by UV/Vis absorption and 1 H and 13 C NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 ( Ct − ) reacts in aqueous solutions ( λ irr =313 nm) to form B − and Cc − , with a global quantum yield of 0.15, and fully reverts back to Ct − with a rate constant of k =6.7×10 −5  s −1 . The flavylium cation is no longer formed in methanol, and irradiation of Ct − leads to the formation of B − and the new lactone‐trapped chromene species La . The formation of La takes place through a sequence of three photochemical steps: photoisomerization of Ct − , photo‐ring‐closing reaction of Cc − , and photolactonization of B − . Only the cis / trans isomerization and ring‐closing reactions are thermally reversible on a timescale of seconds and hours, respectively. A photochromic system was achieved in rigid matrices of methanol (at 77 K) and 1‐dodecanol (5 °C) by irradiating lactone La to give a red ortho ‐quinone allide through a photo‐ring‐opening reaction; the color disappears with a rate constant of k =1.25×10 −2  s −1 in 1‐dodecanol at 5 °C.