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3-Deoxyanthocyanidins and their O-β-D-glucosides are natural pigments abundant in black sorghum. O-glycosidation can perturb the acid-base properties of the chromophore and lower its electron density with a large impact on the distribution of colored and colorless forms in aqueous solution. In this work, the influence of O-glycosidation on color is systematically studied from a series of 3-deoxyanthocyanin analogs. The pH- and light-dependent reversible reactions of 7-β-D-glucopyranosyloxy-4'-hydroxyflavylium (P3) and 4'-β-D-glucopyranosyloxy-7-hydroxyflavylium (P5) were completely characterized in mildly acidic solution and compared with the parent aglycone 4',7-dihydroxyflavylium ion and the O-methylethers of P3 and P5. Except P5, the chalcone forms of the pigments exhibit a high cis-trans isomerization barrier that allows a pseudo-equilibrium involving all species except the trans-chalcone. At equilibrium, only the flavylium cation and trans-chalcone are observed. With all pigments, the colored flavylium ion can be generated by irradiation of the trans-chalcone (photochromism). Glycosidation of C7-OH accelerates hydration and strongly slows down cis-trans isomerization with the pH dependence of the apparent isomerization rate constant shifting from a bell-shaped curve to a sigmoid. The color of P5 is much more stable than that of its regioisomer P3 in near-neutral conditions. [ABSTRACT FROM AUTHOR]

The effect of methyl, hydroxyl, and chloride substituents in position 3 of the 3′,4′,7-trihydroxyflavylium core structure was studied. The stability, relative energy of each of chemical species (thermodynamics), and their rates of interconversion (kinetics) are very dependent on these substituents. By comparing the mole fraction distribution at equilibrium of the three multistate systems with the parent 3′,4′,7-trihydroxyflavylium, introduction of a methyl substituent in position 3 increases the mole fraction of hemiketal at the expense of the trans-chalcone and increases the hydration rate very significantly; a hydroxyl substituent in position 3 gives rise to a degradation process, as observed in anthocyanidins. In the case of 3-chloro-3′,4′,7-trihydroxyflavylium, a dramatic increase of the flavylium cation acidity was observed and a photochromic system can be operated upon irradiation of the respective trans-chalcone in 1 m HCl. According to the photochromic response of 3,3′,4′,7-tetrahydroxyflavylium and 3′,4′,7-trihydroxyflavylium, some requirements for a good photochromic performance are discussed. [ABSTRACT FROM AUTHOR]

A photochromic system based on dracoflavylium, a natural pigment extracted from Dragon's blood, a resin appearing in the injury parts of the tree Dracaena draco, is studied in water. The photochromism arises from the irradiation of the trans-chalcone, giving a mixture of flavylium cation/quinoidal base as a photoproduct via cis-chalcone and hemiketal. The performance of the photochromic system can be improved in the presence of (2-hydroxypropyl)-β-cyclodextrin. A mathematical model to account for the details of the kinetics and thermodynamics of the system was deduced. The model is general for all the host–guest systems involving the flavylium network of chemical reactions with 1 : 1 stoichiometric association. [ABSTRACT FROM AUTHOR]

Abstract: The network of chemical reactions involving anthocyanins and related compounds has been historically considered as the chemical transformations taking place when the flavylium cation (which is generally stable only in very acidic solutions) disappears upon a pH jump to higher pH values (direct pH jump). In this work profit is taken from the appearance of flavylium cation from equilibrated solutions at moderately acid pH values, (reverse pH jumps) as well as upon irradiation of trans-chalcone in the cases where photochemistry is available. Reverse pH jumps, flash photolysis and reaction quantum yields unequivocally show that at sufficiently acidic pH values the tautomerization becomes the rate determining step of flavylium cation formation. This is a lucky situation since gives kinetic information about tautomerization, the most difficult process to access in the anthocyanins and related compounds network of chemical reactions. The thermodynamic and kinetic details of flavylium base photochromic systems are described. Mathematical expressions for direct and reverse pH jumps, stopped flow and flash photolysis are presented as well as the procedure to obtain all of the rate and equilibrium constants of the system. [Copyright &y& Elsevier]

Highlights: [•] Photostationary state of the flavylium network of chemical reactions is analyzed. [•] A energy diagram is proposed to visualize the photostationary state. [•] The model compound 4′,7-dihydroxyflavylium was used to test the model. [ABSTRACT FROM AUTHOR]

Abstract: Flavylium is the basic structure of anthocyanins, an important family of natural dyes responsible for the color in many flowers and fruits. While anthocyanins in vitro are involved in a pH dependent equilibrium between a flavylium cation (acidic species) and a hemiketal (as major basic species), some synthetic flavylium compounds present the trans-chalcone as the major basic species. In this case irradiation of the trans-chalcone leads to appearance of colored products, either a flavylium cation and/or a quinoidal base, via cis-chalcone and hemiketal. Taking profit from the complexity of the network it is possible to design photochromic systems, exhibiting a thermal back reaction, or models for optical memories, when the back reaction is prevented. The flavylium photochromism was also observed in ionic liquids, gels, micelles and reversed micelles opening up possible applications. In this work an overview of the photochromism of synthetic flavylium compounds is presented, from the first observations of the light effect to recent developments. [Copyright &y& Elsevier]

Differently from water, efficient photochromism with a strong colour contrast has been observed for the multistate compound 7,4′-dihydroxyflavylium in the presence of cetyl trimethylammonium bromide (CTAB) micelles. Two states are responsible for the photochromism: trans-chalcone (inside the micelle) in the dark, and flavylium cation, AH+, (in bulk water) upon irradiation. The kinetics of the system was characterized by flash photolysis and pH jumps. Evidence that the photochemical process leading to AH+ occurs in the micelle was achieved. The best colour contrast is obtained at pH = 1.5, from a solution practically colourless in the dark, to an intense yellow upon irradiation (quantum yield 0.4). The system is completely reversible with a lifetime of 38 min at room temperature, and exhibits a reasonable stability. A kinetic model capable of fitting the data from thermal entrance of the compound into the micelle, its ejection to bulk water upon irradiation and quantum yields of the photochemical process is proposed. [ABSTRACT FROM AUTHOR]

The multistate system of 4′,7-dihydroxy-3-methoxyflavylium is constituted by a multiequilibrium involving trans-chalcone, cis-chalcone, hemiketal, flavylium cation and quinoidal base. This system possesses two independently addressable inter-connected photochromic systems based on the cis–trans isomerization and ring opening-closure of the hemiketal. [ABSTRACT FROM AUTHOR]

Describes a photochromic system of methoxyflavylium perchlorate which is capable of write-lock-read-unlock-erase cycle. Reversible photoisomerization of photochromatic compounds; Use of temperature and/or pH stimulus for locking or unlocking the written information; Characterization of the system under experimental conditions.

• Photochromic system in water based on 5OH4′NMe2 in the presence of SDS micelles. • Photochemical response leading to a good colour contrast in an extended range of pH. • Photochromism based on stable and long-life meta-stable states. The multistate of species originated by the compound 5-hydroxy-4′-dimethylaminoflavylium was studied in the presence of sodium dodecyl sulfate micelles 0.1 M. A series of pH jumps obtained by addition of base to the equilibrated solutions of the flavylium cation results in a metastable equilibrium between the flavylium cation and the quinoidal base with p K a = 7.6. In neutral to moderately basic medium, only at 60 °C and after 5 days a new distribution involving the flavylium cation and trans -chalcones is achieved. On the other hand, the anionic trans -chalcone can be formed in few minutes in very basic medium at room temperature and from this, another meta-stable state constituted by protonated, neutral and anionic trans -chalcones, p K Ct+/Ct = 2.6 and p K Ct/Ct- = 10.7 is reached. The equilibrium is apparently attained between flavylium cation, neutral and anionic trans -chalcones respectively for p K AH+/Ct = 4.1 and p K Ct/Ct- = 10.7. Irradiation of the neutral (stable or metastable) and protonated (meta-stable) trans -chalcones yields to the appearance of the quinoidal base or the flavylium cation in an extended pH region 1 < pH < 10. The peculiar properties of this flavylium based photochromic system are explained by means of an energy level diagram where cis -chalcone and hemiketal lay above the quinoidal base. [ABSTRACT FROM AUTHOR]

Metastablestates regarding the network of chemical reactions involvingflavylium compounds were investigated as well as the role they mayplay in models for optical memories capable of write–read–erase.A necessary requirement to achieve metastable states in flavyliumsystems is the existence of a high cis–transisomerization barrier, as in 4′-hydroxyflavylium describedthrough this paper. In an optical memory, the metastable state couldbe the signal to be detected upon the write step. In that case theautoerase is prevented by the metastable state. Conversely, the metastablestate may be the initial state and prevents the auto and unwantedwrite step. The compound 4′-hydroxyflavylium offers the possibilityof achieving both of these two situations, depending on the sequenceof the pH stimuli prior to light absorption. In this work the pH dependentdistribution of the flavylium species of the network in the presenceof β-cyclodextrin was calculated. Improvement of the performanceof the photochromic system in the presence of β-cyclodextrinwas observed. [ABSTRACT FROM AUTHOR]

The network of chemical reactions of the compound 4′-(2-hydroxyethoxy)-7-hydroxyflavylium and its photochromic properties were studied in the absence and presence of β -cyclodextrin. The system was characterized by 1 H NMR, UV–Vis, continuous irradiation and circular dichroism. In the presence of β -cyclodextrin 4.8 mM the p K ′ a is reduced to 2.2. The association constants with β -cyclodextrin, follow the order trans -chalcone > quinoidal base > flavylium cation , respectively 1.7 × 10 3 M −1 ; 5.6 × 10 2 M −1 ; ≈0. The rate of inter-conversion of the network species increases in the presence of β -cyclodextrin circa 2.6 times. The appearance of an induced circular dichroism signal of trans -chalcone inside the β -cyclodextrin is described for the first time. It gives information about the position of the guest inside the host and allows the calculation of the respective association constant. [ABSTRACT FROM AUTHOR]

Highlights: [•] The so-called styrylflavylium compounds are promising alternatives to anthocyanins and related compounds. [•] They follow the same network of reactions of anthocyanins including hydration, tautomerization and cis–trans isomerization. [•] The absorption spectra of styrylflavyliums are red shifted in comparison with the flavylium analogues. [•] A switching photochromic system from yellow to green is obtained in the presence of CTAB micelles. [Copyright &y& Elsevier]

Thephotochromism of a 2-hydroxychalcone has been studied in CH3CN and H2O/CH3OH (1/1, v/v), as wellas in analogous deuterated solvents using steady-state (UV–visabsorption, 1H and 13C NMR) and time-resolved(ultrafast transient absorption and nanosecond flow flash photolysis)spectroscopies. Whereas the irradiation of trans-chalcone(Ct) under neutral pH conditions leads to the formationof the same final chromene derivative (B) in both media,two distinct photochemical mechanisms are proposed in agreement withthermodynamic and kinetic properties of the chemical reaction networkat the ground state. Following light excitation, the first steps areidentical in acetonitrile and aqueous solution: the Franck–Condonexcited state rapidly populates the trans-chalconesinglet excited state 1Ct* (LE), which evolvesinto a twisted state 1P*. This excited stateis directly responsible for the photochemistry in acetonitrile inthe nanosecond time scale (16 ns) leading to the formation of cis-chalcone (Cc) through a simple isomerizationprocess. The resulting cis-chalcone evolves intothe chromene Bthrough a tautomerization process in theground state (τ = 10 ms). Unlike in acetonitrile, in H2O/CH3OH (1/1, v/v), the P* state becomesunstable and evolves into a new state attributed to the tautomer 1Q*. This state directly evolves into Bin one photochemical step through a consecutive ultrafast tautomerizationprocess followed by electrocyclization. This last case representsa new hypothesis in the photochromism of 2-hydroxychalcone derivatives. [ABSTRACT FROM AUTHOR]

Two oaklins guaiacylcatechinpyrylium (GCP) and syringylcatechinpyrylium(SCP) and a model compound deoxypeonidin (DOP) were synthesized, andthe rate and equilibrium constants of the respective pH dependentnetwork of chemical reactions were calculated. In contrast to anthocyanins,the three compounds possess a small cis–transisomerization barrier and hence the rate of the trans-chalcone formation follows a bell-shaped curve as a function of pH.The three compounds exhibit photochromism obtained by irradiationof the trans-chalcone, which, depending on pH, leadsto the colored species flavylium cation and quinoidal base. The flashphotolysis together with pH jumps followed by UV–vis absorptionand stopped flow is a very useful tool to achieve the rate and equilibriumconstants of the network of chemical reactions followed by these molecules.Oaklin compounds which are formed in wine aged in oak barrels presentphysical-chemical properties more similar to simpler deoxyanthocyanidinsrather than anthocyanins and may play a significant role in colorchanges observed in wine aging. Given their higher stability, theymay be regarded as potential food colorants. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Flavylium compounds suffer in aqueous solution the nucleophilic addition of water at moderately acidic pH values (hydration reaction). The hydration is possible in two positions, namely at position 2, forming hemiketal B2, and at position 4, forming hemiketal B4. B2 can subsequently evolve to give the cis- and trans-chalcone species. At the present work the network of chemical reactions involving the naphthoflavylium compound in aqueous solution was studied by means pH jumps, stopped flow, continuous irradiation and flash photolysis. The equilibrium and rate constants of the system were calculated through a mathematical model. The species B4 has a kinetic effect similar to the one observed for the quinoidal base (for flavylium dyes bearing acidic groups), i.e. B4 is a kinetic product retarding the rate of equilibration. Flash photolysis experiments in comparison with reverse pH jumps results show that the appearance of the flavylium ion is faster in the photochemical-induced process than in the thermal one, suggesting an additional photochemical pathway (besides photoisomerization) after the excitation of the trans-chalcone. [Copyright &y& Elsevier]

The syntheses, thermodynamic and kinetic properties of a series of 2-styryl-1-benzopyrylium compounds are reported. This family of compounds was found to follow the same pH- and light-dependent network of chemical reactions previously described for flavylium (2-phenyl-1-benzopyrylim) compounds. However, 2-styryl-1-benzopyrylium compounds exhibit absorption spectra substantially red shifted when compared with flavylium analogues (up to 90nm). In particular, a photochromic system switching from yellow to light blue based on derivatives of natural anthocyanins is for the first time documented. [Copyright &y& Elsevier]

The similarity between 2,6-Bis(2-hydroxyarylidene)cyclohexanones and 2′-hydroxychalcones in generating respectively styrylflavylium and flavylium cations was explored. The complex pH and light dependent multistate of species of the asymmetric of 2,6-Bis(2-hydroxyarylidene)cyclohexanone bearing a diethyl amine in position 7 of the flavylium cation was studied by pH jumps, monitored by UV–vis spectrophotometry, stopped flow, and NMR. The possible structural isomers corresponding to the migration of the amine substituent at the benzopyrylium to ring B of the flavylium cation via chalcone, or spiro species were not detected. However, cis and trans isomers from the pendent arm take place and a photochromic system involving flavylium cation switching from red to blue was characterized. In acidic medium the flavylium cation and the hemiketal protonated at the amine substituent are the equilibrium species. In moderately acidic solutions the spiro is the most stable species and in very basic medium the ionized trans -chalcone. The spiro and the ionized chalcone are key compounds to define the multistate of chemical species of compound 1. The spiro is stable in moderately acidic medium and gives cis -flavylium and ionized chalcone respectively in acidic and basic medium. The chalcone is stable only at very basic medium, and gives trans -flvaylium cation in acidic medium. Image 1 • 2,6-Bis(arylidene)cyclohexanones possesses 17 species that are reversibly interconnected by pH and light external stimuli. • There are for each species cis-trans and structural isomers, except for the trans-trans and cis-cis chalcones and the spiro. While the structural isomers may communicate through these 3 common species, as observed in other similar systems, no evidence for this behavior was achieved. • The cis-trans isomers lead to a photochromic system. [ABSTRACT FROM AUTHOR]