The six most common 3-glucoside anthocyanins, pelargonidin-3-glucoside, peonidin-3-glucoside, delphinidin-3-glucoside, malvidin-3-glucoside, cyanidin-3-glucoside and petunidin-3-glucoside were studied in great detail by NMR, UV-vis absorption and stopped flow. For each anthocyanin, the thermodynamic and kinetic constants of the network of chemical reactions were calculated at different anthocyanin concentration, from 6 x 10(-6) M up to 8 x 10(-4) M; an increasing of the flavylium cation acidity constant to give quinoidal base and a decreasing of the flavylium cation hydration constant to give hemiketal were observed by increasing the anthocyanin concentration. These effects are attributed to the self-aggregation of the flavylium cation and quinoidal base, which is stronger in the last case. The UV-vis and H-1 NMR spectral variations resulting from the increasing of the anthocyanin concentration were discussed in terms of two aggregation models; monomer-dimer and isodesmic, the last one considering the formation of higher order aggregates possessing the same aggregation constant of the dimer. The self-aggregation constant of flavylium cation at pH = 1.0, calculated by both models increases by increasing the number of methoxy (-OCH3) or hydroxy (-OH) substituents following the order: myrtillin (2 -OH), oenin (2 -OCH3), 3-OGI-petunidin (1 -OH, 1 -OCH3), kuromanin (1 -OH), 3-OGI-peonidin (1 -OCH3) and callistephin (none). Evidence for flavylium aggregates possessing a shape between J and H was achieved, as well as for the formation of higher order aggregates. (C) 2012 Elsevier Ltd. All rights reserved.

The compounds 3'-butoxy-7-methoxyflavylium and 3'-butoxy-7-hydroxyflavylium were synthesized and the respective equilibrium and rate constants determined by two complementary techniques, pH jumps and flash photolysis. An experimental strategy based on these two techniques allowed calculation of all the equilibrium and rate constants of the system carried out for the first time in flavylium compounds lacking of the high cis-trans isomerization barrier. Irradiation of the trans-chalcone gives rise to the formation of the cis-chalcone still during the lifetime of the flash, which disappears through two parallel reactions: (i) one leading to the recovery of the trans-chalcone and the other, (ii) forming flavylium cation via hemiketal. This last reaction is globally dependent on pH and at less acidic pH the system reverts back to the trans-chalcone. The highest yield of colour production upon the flash takes place in the pH range 2-3.5. (c) 2012 Elsevier B.V. All rights reserved.

The introduction of an ester group in the flavylium core allowed the reversible conversion between two different flavylium compounds each one exhibiting its own reaction network. An unidirectional switching cycle between 7-diethylamino-2-(4-(methoxy-carbonyl)phenyl)-1-benzopyrylium and 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium was achieved by means of alternate acid and base stimuli. Addition of base to a methanolic solution of the ester derivative gives rise to the trans-chalcone of the parent carboxylic acid, which upon acidification of the solution forms the respective flavylium cation. This species esterifies under very acidic conditions to restore the original methyl ester derivative. The chemical reaction networks of both compounds were fully characterized from their thermodynamic and kinetic aspects, by a series of pH jumps followed by UV-vis absorption and emission spectroscopy, stopped flow and H-down arrow NMR. The crystal structure of the trans-chalcone of the ester derivative was unveiled showing a supramolecular structure involving hydrogen bonding.

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 approximate to 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 H-1 and C-13 NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 (Ct(-)) reacts in aqueous solutions (lambda(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 x 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 degrees 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 x 10(-2) s(-1) in 1-dodecanol at 5 degrees C.

A phase-change thermochromic system was designed through the reversible transformation of the 4-substituted flavylium dye 4-(2-carboxyphenyl)-7-diethylamino-4'-dimethyl-amino-1-benzopyrylium into its leuco form, in the presence of a developer (ethyldiisopropylamine) and a suitable solvent (e.g., acetonitrile, n-pentadecanonitrile). The leuco form of the flavylium-based dye is a spirolactone species whose ring opens at low temperature (below the solvent melting point) to form the blue flavylium cation. Decarboxylation of the lactone to give 4-phenyl-7-diethylamino-4'-dimethylamino-1-benzopyrylium was observed upon irradiation of the system with UV light, erasing the thermochromic effect.

We report a spectroscopic study of the network of reactions of a flavylium dye encapsulated in the one-dimensional channels of zeolite L. The positively charged 7,4'-dihydroxyflavylium (AH(+)) is easily incorporated and remains stable in zeolite L channels. Once encapsulated, the flavylium exhibits a red shift in the excitation spectrum comparative to aqueous solutions. Moreover, contrary to the observed behavior in water, no excited state proton transfer takes place in the loaded crystals, corroborating the encapsulation of AH(+). The trans-chalcone (Ct) form from the same flavylium network could also be encapsulated inside the zeolite L, using toluene with 20% triethylamine as solvent and K(+) as counter ion of the negative framework of the zeolite. The encapsulation of Ct is confirmed by changes on the excitation spectrum and by a blue shift in the emission. The encapsulated Ct was shown to generate AH(+) when the Ct-loaded crystals were suspended in water, which proves that isomerization, tautomerization and dehydration reactions take place inside the zeolite L.