title: Phase-dependent photochromism of a lactone-stabilized chromene from a flavylium reaction network.
abstract: 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 AH(+) is formed at very acidic pH values through hemiketal B and cis-chalcone Cc, with global acidity constants of pK'(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.
authors.affiliation: REQUIMTE, CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. yoann.leydet@dq.fct.unl.pt
authors.forename: Yoann
authors.initials: Y
authors.lastname: Leydet
authors.forename: Raquel
authors.initials: R
authors.lastname: Gavara
authors.forename: Luís
authors.initials: L
authors.lastname: Cunha-Silva
authors.forename: A Jorge
authors.initials: AJ
authors.lastname: Parola
authors.forename: Fernando
authors.initials: F
authors.lastname: Pina