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8-Phenyl-5,7-dihydroxyflavylium and 8-methyl-5,7-dihydroxyflavylium were synthesized to observe the 6,8-rearrangement. 8-Phenyl and 8-methyl residues were introduced by Suzuki–Miyaura reaction of 8-iodochrysin, then reduced by LiAlH4 to give the corresponding 3-deoxyanthocyanidins. At pH 1.0 the stable form is the 8-substituted flavylium cation in both compounds. At higher pH values the quinoidal bases are the stable species and some evidence for the 6,8-rearrangement was obtained, but the respective spectral variations are very small. This was overcome by using a modified cyclodextrin (captisol), which favors the formation of the trans-chalcone at the expense of the quinoidal bases. The trans-chalcone was isolated and dissolved in CD3OD/DCl (pD < 1) to give a mixture of the two flavylium isomers. This was confirmed by ESI-MS analysis (recorded in positive ion mode) of the two isomers after separation by HPLC, which gave the same peak ([M]+ m/z 253). The 6-isomer slowly reverts to the most stable 8-isomer. The 6,8-rearrangement was also observed after irradiation of the trans-chalcone (in the presence of captisol) at pH 5. Acidification of this photostationary state gave a mixture of both flavylium cations. The UV/Vis absorption of the flavylium cation (6-isomer) was blueshifted in comparison to the 8-isomer.

Symmetrical and nonsymmetrical oxobis(bipyridinium) cations combined with iodide and bistriflimide anions were prepared. In the case of the bistriflimide-based bis(bipyridinium) salts, two room-temperature ionic liquids (RTILs) with a combination of linkers and terminal ethers, [(C3O)bpy(C4O)bpy(C3O)][NTf2]4 and [(C10)bpy(C4O)bpy(C3O)][NTf2]4, were obtained, with glass transition temperatures (between −6.9 and 6.0 °C) higher than those reported previously for salts of dication bipyridinium scaffolds (−17.5 to −30.0 °C). The prepared salt containing [(C10)bpy(C4O)bpy(C10)][NTf2] can form an ionic liquid crystal structure. The most promising salts were tested as efficient and reversible multicolored electrochromic devices through cyclic voltammetry studies and detailed characterizations of their electrochromic performances. In general, tetracations based on oxobis(bipyridinium) salts show higher chromatic contrast and lower coloration efficiency than similar dication salts. Exchange of the anion from iodide to [NTf2] increases the transition times and coloration efficiencies for all the prepared di- and tetracation salts. The electrochromic device stability of [(C3O)bpy(C4O)bpyC3O]I4 was tested by on/off cycling; this device loses around 23 % of the initial color after 8640 redox cycles and shows very good cycling stability compared with previously reported diiodide salts.

Electrochromic symmetric and non-symmetric disubstituted oxo-bipyridinium ionic liquids are developed in high purities and yields. Detailed spectroscopic (NMR, FTIR and elemental analysis), thermal (DSC for melting point and glass transition temperatures, Tg determination) and electrochemical (cyclic and square wave voltammetry for determination of redox potential and evaluation of reversibility behaviour) analyses are performed. The most promising electrochromic oxo-bipyridinium ionic liquids in combination with transparent electrolytes are tested as reversible electrochromic devices. More efficient and sustainable electrochromic devices inspired by bipyridinium ionic liquids can be conceived for industrial applications such as electrochromic car rear-view mirrors and smart windows.

By using hyper-Rayleigh scattering experiments and quantum-chemical calculations, we demonstrate that nonlinear optics can be used to probe unequivocally, within a non-destructive process, the multiple electronic states that are activated upon pH- and light-triggered transformations of the 4a-hydroxyflavylium ion. These results open new perspectives in the design of molecular-scale high-density optical memory. Remember this: Hyper-Raleigh scattering measurements provide evidence for large variations of the quadratic nonlinear optical responses to pH-triggered transformations of the 4a-hydroxyflavylium ion. By exploiting the capability of nonlinear optics to probe the electronic states without triggering uncontrolled photoconversions, the network of interconverting processes in this system might be a basis for the development of high-density multiple-storage optical memory.

Novel mono and dialkylbipyridinium (viologens) cations combined with iodide, bromide, or bis(trifluoromethanesulfonyl)imide [NTf2] as anions were developed. Selective alkylation synthetic methodologies were optimized in order to obtain the desired salts in moderate to high yields and higher purities. All prepared mono- and dialkylbipyridinium salts were completely characterized by (1)H, (13)C, and (19)F NMR spectroscopy, Fourier-transform IR spectroscopy, and elemental analysis (in the case of NTf2 salts). Melting points, glass transition temperatures by differential scanning calorimetry (DSC) studies, and decomposition temperatures were also checked for different prepared organic salts. Viscosities at specific temperatures and activation energies were determined by rheological studies (including viscosity dependence with temperature in heating and cooling processes). Electrochemical studies based on cyclic voltammetry (CV), differential pulsed voltammetry (DPV), and square-wave voltammetry (SWV) were performed in order to determine the redox potential as well as evaluate reversibility behavior of the novel bipyridinium salts. As proof of concept, we developed a reversible liquid electrochromic device in the form of a U-tube system, the most promising dialkylbipyridinium-NTf2 ionic liquid being used as the electrochromic material and the room-temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide [EMIM][NTf2], as a stable and efficient electrolyte.

A symmetric bis(flavylium) constituted by two 7-hydroxyflavylium moieties linked by a methylviologen bridge was synthesized. The thermodynamic and kinetics of the network of chemical reactions involving bis(flavylium) and the model compound 7-hydroxy-4'-methylflavylium was completely characterized by means of direct and reverse pH jumps (stopped flow) and flash photolysis. Both compounds follow the usual pH-dependent network of chemical reactions of flavylium derivatives. The equilibrium species of the model compound are the flavylium cation (acidic species) and the trans-chalcone (basic species) with an apparent pK'(a)=2.85. In the case of the bis(flavylium) it was possible to characterize by (1)H NMR spectroscopy three species with different degrees of isomerization: all flavylium, flavylium-trans-chalcone, and all trans-chalcone. Representation of the time-dependent mole fraction distribution of these three forms after a pH jump from equilibrated solutions of all-flavylium cation (lower pH values) to higher pH values, shows that formation of trans-chalcone is not completely stochastic (two independent isomerizations), the isomerization of one flavylium showing a small influence on the isomerization of the other. The radical of the methyl viologen bridge is formed upon reduction of the bis(trans-chalcone) with dithionite. The system is reversible after addition of an oxidant in spite of the occurrence of some decomposition.

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.

A phase-change thermochromic system was designed through the reversible transformation of the 4-substituted flavylium dye 4-(2-carboxyphenyl)-7-diethylamino-4'-dimethylamino-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.

The chemistry and photochemistry of the compound 4'-iodo-8-methoxyflavylium tetrafluoroborate was studied in solvent mixtures of water and ethanol. The trans-chalcone form (Ct) is always the most stable species at equilibrium. In particular, for the first time, Ct and not the flavylium cation is reported as the most stable species in mixtures containing water at pH 1.0. The rate of Ct formation as a function of the water content exhibits a bell-shaped dependence, showing a maximum at approximately 50 % water. These trends are explained by a slower cis-trans isomerization in ethanol relative to that in water due to the expected stabilization in polar solvents of the zwitterionic intermediate; on the other hand, the decrease in the concentration of cis-chalcone (Cc) in the pseudoequilibrium with increasing water content, gives rise to a decrease in the rate of Ct formation (k(obs)=k(real)[Cc]). The hydration reaction was found to be much more efficient when water is present in low concentrations. This result has important consequences for the interpretation of the coloring phenomena caused by anthocyanins in plants, as well as for applications of flavylium compounds in the field of optical memories.

Two multistate networks based on 4′-acetamidoflavylium and 4′-aminoflavylium, possessing six- and sevenfold multistates, respectively, and that can be reversibly interchanged by inputs of light, pH jumps and heat, are characterized. The two networks are connected irreversibly, the 4′-acetamidoflavylium being transformable into the 4′-aminoflavylium by means of a heat input in extremely acidic solutions. The possibility of performing several different write-read-erase cycles, as models for optical memories operating by different algorithms, is discussed. An upper level of operation of these photochromic devices in which the algorithms are used as inputs to switch between the several memory cycles is proposed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

In this paper we report the encapsulation of the compound 4′,7-dihydroxyflavylium in a water-permeable crosslinked poly(2-hydroxyethyl methacrylate) matrix. The water permeability of the polymer and its transparency allow the use of pH and light stimuli to attain different states on the network defined by the various chemical reactions (multistate/multifunctional system). Unlike in water, in the polymer the compound 4′,7-dihydroxyflavylium encapsulated exhibits a thermal barrier between the cis- and trans-chalcones, a necessary requirement to define a write–read–erase cycle.

We have investigated the network of reactions observed for the photochromic 4'-hydroxy-6-nitroflavylium compound in aqueous solutions upon pH changes (including pH jump and stopped flow experiments) and light excitation. The changes observed in the NMR and UV/Vis spectra allowed identification of ten different forms in which this compound can be transformed depending on the experimental conditions. Equilibrium and kinetic constants have been determined. Compared with other members of the flavylium family, 4'-hydroxy-6-nitroflavylium is characterized by a large cis--trans isomerization barrier, and a very efficient hydration reaction. These peculiar features allow writing, reading, storing and erasing photonic information on 4'-hydroxy-6-nitroflavylium by a novel cyclic process that involves the following steps: write-lock/read/unlock/enable-erase/erase.

The interaction in aqueous solution of adenosine 5′-triphosphate (ATP) with a series of open-chain polyamines linked at one or both ends to anthrylmethyl or naphthylmethyl fragments was followed by potentiometric titration, 1H-, 13C-, and 31P-NMR spectroscopy, and by steady-state fluorescence measurements. The results revealed greater stabilities for the compounds containing one anthracene moiety than for those with one naphthalene moiety, the stabilities of the compounds with both ends N-substituted with naphthylmethyl groups being close to those containing just one anthrylmethyl unit. The 1H-NMR spectra showed that in all systems, there is involvement of π-π stacking interactions in the stabilization of the adduct species. The competitive effect of the anions afforded by the supporting electrolyte was checked in some of the studied systems working at two different ionic strenghts (0.15M and 1.0M NaCl). The joint analysis of the spectrofluorimetric titrations and pH-metric species-distribution curves showed that for all the ATPreceptor systems, a quenching of the fluorescence occurred upon protonation of the adenine N(1)atom. Steady-state fluorescence and time-correlated single-photon-counting analysis of a system made up of ATP and a bis-chromophoric polyamine receptor containing anthracene and naphthalene fluorophores established that the energy-transfer process between the naphthalene and anthracene moieties is still operative despite the presence of ATP.

Independently of the natural or synthetic origin, flavylium derivatives follow the same network of chemical reactions. Actually, the flavylium cation is stable only at low pH values. Increasing the pH gives rise to the formation of several species: quinoidal bases, hemiketal, cis- and trans-chalcones, and their deprotonated forms. A deep knowledge of the thermodynamics and kinetics of these species is an essential tool to practical applications of these compounds, in particular, in the domain of food chemistry. In this work the network of chemical reactions involving flavylium derivatives is presented, and the respective thermodynamics and kinetics are discussed in detail, including the mathematical expressions and a step-by-step procedure to calculate all of the rate and equilibrium constants of the system. Examples of systems possessing a high or low cis-trans isomerization barrier are shown. Recent practical applications of anthocyanins and related compounds illustrate the potentialities of the flavylium-based family of compounds.

The structural transformations of flavylium ion in aqueous solutions caused by pH jumps and photoexcitation have been investigated. At pH < 1, the stable form is the colored cationic species (AH+). By increasing pH, the concentration of AH+ decreases and, at pH = 5, this form is no longer present. The species obtained immediately after a pH jump undergo transformation processes with pH-dependent rate constants. At pH = 5.2 and 20 °C, the final product is the uncolored trans-chalcone (Ct). This form can be transformed by light excitation into the cis-chalcone (Cc) isomer, which is in equilibrium with the hemiacetal form (B2). This mixture is relatively inert due to the existence of a kinetic barrier that slows down the back thermal isomerization of Cc to the stable Ct form. Such a back reaction to Ct can be totally prevented if the irradiated solution is submitted to a pH jump to pH = 1, which transforms the photoproducts into the stable AH+ species. In basic solution, two more species were detected, namely the anionic forms Cc– and Ct– of the cis and trans chalcone. Ct– is a stable, not photosensitive and luminescent species, whereas Cc– is not stable, being converted into Ct– in the dark. The photochemical and pH-induced transformations of the flavylium cation in the pH range 0–11 can be taken as a basis to design write–lock–read–unlock–erase cycles for an optical molecular-level memory with multiple readout capacity.

Reaction of Re2{μ-O2CC(CH3)3}4Cl2 with [(CO)5M–CN]Na (M = Cr, Mo, W) leads to tetranuclear complexes of formula Re2{μ-O2CC(CH3)3}4[–NC–M(CO)5]2 (M = Cr, Mo, W). These complexes were characterized by 1H-, 13C-, and 95Mo-NMR, IR and Raman spectroscopy, elemental analysis and examined by cyclic voltammetry. The applied methods show the donor capabilities of the [(CO)5M–CN]− ligands which shift electron density towards the Re centers weakening the Re–Re quadruple bond. The Re–Re bond lengths and the ν(Re–Re) force constants are estimated based on the FT-IR and Raman examinations. Photochemical examinations and TG/MS experiments have also been conducted. The latter method shows that the product complexes decompose around 100 °C, by first loosing their carbonyl substituents; as do the Cr, Mo, W precursor compounds. The dirhenium tetrapivalate unit decomposes only at higher temperatures in a distinct second step.

The structural transformations and the photochromic properties of the 7-hydroxyflavylium ion have been investigated by means of the pH jump technique and continuous and pulsed light excitation. The various forms of flavylium compounds, trans- and cis-chalcone (Ct and Cc), quinoidal base A, and hemiacetal form B, are related by interconversions like communicating vessels containing a fluid. Since the amount of colored species formed upon irradiation depends on the pH of the solution, pH can be viewed as a “tap” that modulates the color intensity generated by light excitation.

An optical memory device with multiple storage in two different memory levels and nondestructive readout capacity requires the properties exhibited by the 4′-hydroxyflavylium ion, which can exist in several forms (multistate) that can be interconverted by more than one type of external stimulus (multifunctional), as depicted on the right. Its intricate network of light- and/or pH-induced transformations form a basis for simple logic operations.

The excited state properties of the chalcone isomers of malvidin 3,5-diglucoside (malvin) in acidic aqueous solution (0 < pH < 4) were investigated using steady-state and time-resolved fluorescence spectroscopy. The two chalcone isomers of malvin were first isolated by high-performance liquid chromatography and then characterized by UV/visible absorption and fluorescence spectroscopy. The results were supported by molecular orbital calculations. The rate constants for photodeprotonation (k1= 1 × 109 s−l) and protonation (k−1= 1.3 × 1010 L mol−1 s−l) were determined, both from the multiexponential fluorescence decays and the fluorescence intensities measured at the emission wavelengths of the neutral and ionized chalcone forms. The results here obtained are relevant for the understanding of the photoreactivity of anthocyanins in acidic medium.

Chemosensors have attracted interest in many different scientific fields, such as environmental chemistry, medicine, and the processing and storage of information. These molecular-scale devices have the advantage of working on the same spatial scale as the chemical structures that are responsible for macroscopic behaviour observed in the environment or those associated with health problems. Moreover, they allow the construction of molecular-scale devices for information storage. In this review, we describe a family of chemosensors based on a polyamine receptor and a fluorescent signalling unit. Polyamine receptors are water-soluble ambidentate receptors; they are able to coordinate either metal ions, when sufficient deprotonated amino groups are available, or anionic species, when there are sufficient protonated amino groups. On the other hand, the use of fluorescent signalling units confers the advantage of an immediate visual response/signal.

Coordination/detachment of a pendent functionality in the Zn(II) complex with a macrocyclic ligand L gives rise to on/off switching of exciplex emission, defining an elementary molecular machine whose movements are driven by both pH and light.

The Co(sep)/sup 3 +/ complex (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)eicosane) in aqueous solution forms ion pairs with the I/sup -/ and Br/sup -/ anions, which exhibit a relatively intense charge-transfer absorption. In deoxygenated aqueous solution no net reaction is obtained upon continuous light excitation in the ion-pair charge-transfer bands, but formation of transient species is observed in flash photolysis experiments. For the Co(sep)/sup 3 +/-I/sup -/ system, I/sub 2//sup -/ ions are formed that decay in the 50-..mu..s time scale to give I/sub 3//sup -/. The latter species disappears in a time scale of seconds, leading the system back to the preexcitation conditions. For the Co(sep)/sup 3 +/-Br/sup -/ system, only formation of the Br/sub 2//sup -/ transient is observed, followed by regeneration of the preexcitation conditions in the 20-..mu..s time scale. In order to elucidate the kinetic aspects of the transient formation and disappearance, pulse radiolysis experiments on the Co(sep)/sup 3 +/-I/sup -/ and Co(sep)/sup 3 +/-Br/sup -/ systems have been carried out. The rate constants of the reactions of Co(sep)/sup 2 +/ with I/sub 2//sup -/, I/sub 3//sup -/, and Br/sub 2//sup -/ have been measured, and a complete picture of the redox processes that follow flash photolysismore » and pulse radiolysis excitations has been obtained. 5 figures.« less

Mecanisme. Rendement quantique, fonction du pH et de la teneur en oxalate. Comparaison avec le comportement photochimique de la paire-ions Co(en) 3+ -oxalate

The Co(EDTA)− complex in aqueous solution gives rise to a specific interaction with I− ions as evidenced by a new, relatively intense band formed at 290–300 nm. This specific interaction is attributed to the formation of an ion-pair between Co(EDTA)− and I−, even though they are like charged ions. Irradiation of this ion-pair in air- equilibrated solutions with 313 nm light, causes the reduction of the Co(EDTA)− to Co(EDTA)2− and the oxidation of the I− ion to I3−. The results obtained are interpreted on the basis of a mechanism in which Co(EDTA)2− and I· are the primary photoproducts. The I· radical is then scavenged by I− to yield I2−, which subsequently disproportionates to I3− and I− and reoxidizes Co(EDTA)2− to Co(EDTA)−. At the beginning of the photoreaction, the I2− decay is equally distributed on the two reaction pathways. It was possible to determine a value of 0.2 ± 0.05 for the photoreaction quantum yield and an efficiency of the primary photochemical step almost unitary. A schematic representation of the energetics of the overall reaction is reported.