Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium

2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1-benzopyrylium (styrylflavylium) cations establish in aqueous solution a series of equilibria defining chemical reaction networks responsive to several stimuli (pH, light, redox potential). Control over the mole fraction distribution of species by applying the appropiate stimuli defines a horizontal approach to supramolecular chemistry, in agreement with the customary bottom-up approach toward complex systems. In this work, we designed an asymmetric styrylchalcone able to cyclize in two different ways, producing two isomeric styrylflavylium cations whose chemical reaction networks are thus interconnected. The chemical reaction networks of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH⁺) and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium (AH⁺_iso) comprise the usual species observed in flavylium-derived networks, in this case, the styryl derivatives of quinoidal bases, hemiketals, and chalcones. The thermodynamics and kinetics of the crossed networks were characterized by the use of UV–vis absorption and NMR spectroscopy as well as time-resolved pH jumps followed by stopped-flow. The two styrylflavylium cations are connected (isomerize) through two alternative intermediates, the asymmetric <i>trans</i>-styrylchalcone (Ct) and a spiropyran-type intermediate (SP). At pH = 1, AH⁺ slowly evolves (<i>k</i>_obs ≈ 10^–5 s^–1) to a mixture containing 62% AH⁺_iso through the Ct intermediate, while at pH = 5, the SP intermediate is involved. The observed rate constants for the conversion of the styrylflavylim cations into equilibrium mixtures containing essentially Ct follow a pH-dependent bell-shaped curve in both networks. While at pH = 1 in the dark, AH⁺ evolves to an equilibrium mixture containing predominantly AH⁺_iso, irradiation at λ > 435 nm induces the opposite conversion

A Multistate Molecular Switch Based on the 6,8-Rearrangement in Bromo-apigeninidin Operated with pH and Host–Guest Inputs

The equilibrium between 6- and 8-bromo-apigeninidin is quantitatively displaced toward the formation of the former in the presence of cucurbit[7]­uril because of the selective recognition of the 6-bromo isomer by the host. This phenomenon permits us to conceive a unidirectional multistate switch addressed with host–guest inputs and enables the reversible activation and deactivation of the 6-/8-bromo-apigeninidin dynamic molecular multistate through coupled host–guest and pH inputs

Rationalizing the Color in Heavenly Blue Anthocyanin: A Complete Kinetic and Thermodynamic Study

All equilibrium and rate constants of heavenly blue anthocyanin (HBA <b>1</b>) as well as the derivatives with two (HBA <b>2</b>) or none (HBA <b>3</b>) acylated units were determined. The three acylated units of the sugar in position 3 of the peonidin chromophore of HBA <b>1</b> are essential to confer the peculiar stability of its purple and blue colors. The sugars generate an efficient protective environment around position 2 (and 4) of the flavylium cation, through an intramolecular sandwich-type stacking that retards 35-fold the hydration reaction (<i>k</i>_h) and increases 8.8-fold the dehydration reaction (<i>k</i>_–h), when compared with the peonidin chromophore HBA <b>3</b>. The conjugation of these two rates lowers 308-fold the hydration equilibrium constant (<i>K</i>_h), corresponding to a raise of the energy level of the hemiketal by 14.2 kJ mol^–1. Conversely, the p<i>K</i>_a of the quinoidal base in HBA <b>1</b> is only slightly stabilized in comparison with that of HBA <b>2</b> and HBA <b>3</b>. The energy level of hemiketal increases with the number of acylated units, but the inversion of energies between hemiketal and quinoidal base takes place exclusively for HBA <b>1</b> (three acylated units), permitting in moderately acidic solutions the stabilization of the purple quinoidal base. Identical inversion of energy was observed for the corresponding ionized species, allowing the stabilization of the blue ionized quinoidal base in slightly basic solutions. At pH values higher than 8, the hydroxyl groups of the hydroxycinnamic acid units start to deprotonate disrupting the intramolecular sandwich-type stacking and the more or less slow degradation of the anthocyanin is observed

Formation of Photoluminescent Lead Bromide Nanoparticles on Aluminoborosilicate Glass

A multicomponent aluminoborosilicate photoluminescent glass was synthesized by introducing Pb­(II) and NaBr in its composition. The room-temperature photoluminescence is due to the existence of 4 nm nanocrystals, shown using TEM imaging and XRD analysis, which are assigned to PbBr₂ nanocrystals. The glasses display a broad emission band with a peak at 2.85 eV by exciting at 3.35 eV, with an anisotropy equal to 0.19 at room temperature. At 77 K, the emission intensity increases 1 order of magnitude and a vibronic structure appears, indicating an electron–phonon coupling with the glass matrix. Time-resolved luminescence measurements of these nanocrystals reveal mixed-order kinetics, with second-order recombination of self-trapped electron centers and a first-order temperature-dependent nonradiative rate constant connected with pathways due to confinement of self-trapped centers

Emptying the β‑Cyclodextrin Cavity by Light: Photochemical Removal of the <i>trans</i>-Chalcone of 4′,7-Dihydroxyflavylium

The interaction between the network of chemical reactions of the compound 4′,7-dihydroxyflavylium and β-cyclodextrin was studied by means of pH jumps, followed by UV–vis absorption, flash photolysis, stopped flow, and NMR. The <i>trans</i>-chalcone is the network species exhibiting the strongest interaction with the host. In moderately acidic medium, 95% of the <i>trans</i>-chalcone, 2.5 × 10^–5 M, in the presence of β-cyclodextrin, 9 × 10^–3 M, is expected to fill the host cavity (association constant 2.2 × 10³ M^–1). In contrast, flavylium cation does not interact (association constant ≈ 0). Irradiation of the <i>trans</i>-chalcone in the presence of β-cyclodextrin 9 mM leads to the flavylium cation appearance. Light is thus capable of removing the <i>trans</i>-chalcone from the β-cyclodextrin, leaving the cavity empty. The system is reversible and <i>trans</i>-chalcone goes back to the initial state upon switching off the light due to the thermodynamic favorable conversion of flavylium cation to <i>trans</i>-chalcone in the presence of β-cyclodextrin

Photoluminescent Nanocrystals in a Multicomponent Aluminoborosilicate Glass

In this study, stable and nonexpensive aluminoborosilicate glasses with different photoluminescence colors were synthesized by doping with Pb­(II), Ba­(II) and sodium halides. While glasses with NaF and NaCl exhibit no (or very low) luminescence, glasses doped with NaBr and NaI display room-temperature photoluminescence at 435 and 530 nm, respectively. The observed room-temperature photoluminescence is attributed to nanocrystals whose presence is revealed by transmission electron microscopy. The crystalline nature of the particles, which are pointed out as barium-lead halides, is also revealed by anisotropy measurements for Br and I doped samples. Time-resolved luminescence measurements show a second-order kinetics component combined with a first-order nonradiative rate constant. The photoluminescence properties here described are important for the future design of new optical materials or devices based on lead halide nanocrystals

Impact of Lignosulfonates on the Thermodynamic and Kinetic Parameters of Malvidin-3‑<i>O</i>‑glucoside in Aqueous Solutions

The interaction of malvidin-3-<i>O</i>-glucoside (<b>1</b>) and a lignosulfonate was studied by UV–visible spectroscopy, and the results obtained showed the formation of a complex between the negatively charged lignosulfonate and the flavylium cation form (AH⁺) of this anthocyanin at pH 1. The thermodynamic and kinetic parameters of <b>1</b> in the presence of a lignosulfonate were determined by UV–visible spectroscopy and stopped-flow techniques. The main differences were observed in the flavylium cation (AH⁺)/quinoidal base (<b>A</b>) equilibrium, the AH^<b>+</b> form being more stabilized than <b>A</b> (p<i>K</i>_a1 = 4.4 ± 0.1) compared with <b>1</b> in the absence of the lignosulfonate (p<i>K</i>_a1 = 3.9 ± 0.1). Furthermore, comparing the hydration (<i>k</i>_h = 0.028 s^–1) and dehydration (<i>k</i>_–h = 40 M^–1 s^–1) processes of <b>1</b> in the presence of the lignosulfonate with the processes of <b>1</b> (<i>k</i>_h = 0.12 s^–1 and <i>k</i>_–h = 35 M^–1 s^–1) show that the hydration process is slower while the dehydration process is practically unaffected in the presence of the lignosulfonate

Additional file 1 of Iron-gall inks: a review of their degradation mechanisms and conservation treatments

Additional file 1. Cysteine-based interventions to protect metals from corrosion

Impact of Water on the Cis–Trans Photoisomerization of Hydroxychalcones

The photochromism of a 2-hydroxychalcone has been studied in CH₃CN and H₂O/CH₃OH (1/1, v/v), as well as in analogous deuterated solvents using steady-state (UV–vis absorption, ¹H and ¹³C NMR) and time-resolved (ultrafast transient absorption and nanosecond flow flash photolysis) spectroscopies. Whereas the irradiation of <i>trans</i>-chalcone (<b>Ct</b>) under neutral pH conditions leads to the formation of the same final chromene derivative (<b>B</b>) in both media, two distinct photochemical mechanisms are proposed in agreement with thermodynamic and kinetic properties of the chemical reaction network at the ground state. Following light excitation, the first steps are identical in acetonitrile and aqueous solution: the Franck–Condon excited state rapidly populates the <i>trans</i>-chalcone singlet excited state ¹<b>Ct</b>* (LE), which evolves into a twisted state ¹<b>P</b>*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of <i>cis</i>-chalcone (<b>Cc</b>) through a simple isomerization process. The resulting <i>cis</i>-chalcone evolves into the chromene <b>B</b> through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H₂O/CH₃OH (1/1, v/v), the <b>P</b>* state becomes unstable and evolves into a new state attributed to the tautomer ¹<b>Q</b>*. This state directly evolves into <b>B</b> in one photochemical step through a consecutive ultrafast tautomerization process followed by electrocyclization. This last case represents a new hypothesis in the photochromism of 2-hydroxychalcone derivatives

2,2′-Spirobis[chromene] Derivatives Chemistry and Their Relation with the Multistate System of Anthocyanins

The chemistry of 2,2′-spirobis­[chromene] derivatives is intimately related to the one of anthocyanins and similar compounds. The 2,2′-spirobis­[chromene] species plays a central role in the network of chemical reactions connecting two different flavylium-based multistate systems. In the present work, a new asymmetric 2,2′-spirobis­[chromene] intermediate possessing a constrained propylenic bridge between carbons 3 and 3′ was isolated and its role as a pivot in the anthocyanins-type multistate of chemical reactions was investigated by the conjugation of absorption spectroscopy, stopped-flow, NMR, and X-ray crystallography. It was confirmed that the propylenic bridge is essential to stabilize the spirobis­[chromene] species. Furthermore, under acidic conditions, two <i>cis</i>–<i>trans</i> styrylflavylium isomers were identified, which could be interconverted directly into one another with light. This is the first report of styrylflavylium cations with photoisomerization on the styryl moiety