Origin of the metastable stability in flavylium multistate systems

2-Hydroxychalcone-β-Cyclodextrin Conjugate with pH-Modulated Photoresponsive Binding Properties

Stimuli-responsive supramolecular receptors are important building blocks for the construction of self-assembled functional materials. We report the design and synthesis of a pH- and light-responsive 2-hydroxychalcone-β-cyclodextrin conjugate (1-Ct) and its characterization by spectroscopic and computational methods. 1-Ct follows the typical reaction network of trans-chalcone-flavylium photoswitches. Upon light irradiation, 1-Ct can be photochemically converted into the cis-chalcone/hemiketal forms (1-Cc/1-B) under neutral pH conditions or to the flavylium cation (1-AH⁺) at acidic pH values. This stimuli-responsive β-cyclodextrin host, 1-Ct, was found to form stronger intramolecular self-inclusion complexes (K_intra = 14) than 1-AH⁺ (K_intra = 3) and weaker than 1-Cc/1-B (overall K_intra = 179), allowing control over their stability and binding properties by combinations of pH and light stimuli.

Natural and Synthetic Flavylium-Based Dyes: The Chemistry Behind the Color

Flavylium compounds are a well-known family of pigments because they are prevalent in the plant kingdom, contributing to colors over a wide range from shades of yellow-red to blue in fruits, flowers, leaves, and other plant parts. Flavylium compounds include a large variety of natural compound classes, namely, anthocyanins, 3-deoxyanthocyanidins, auronidins, and their respective aglycones as well as anthocyanin-derived pigments (e.g., pyranoanthocyanins, anthocyanin-flavan-3-ol dimers). During the past few decades, there has been increasing interest among chemists in synthesizing different flavylium compounds that mimic natural structures but with different substitution patterns that present a variety of spectroscopic characteristics in view of their applications in different industrial fields. This Review provides an overview of the chemistry of flavylium-based compounds, in particular, the synthetic and enzymatic approaches and mechanisms reported in the literature for obtaining different classes of pigments, their physical-chemical properties in relation to their pH-dependent equilibria network, and their chemical and enzymatic degradation. The development of flavylium-based systems is also described throughout this Review for emergent applications to explore some of the physical-chemical properties of the multistate of species generated by these compounds.

Exploring the pH-dependent kinetics, thermodynamics and photochemistry of a flavylium-based pseudorotaxane

Flavylium-based molecular switches are attractive molecular components to devise stimuli-responsive host-guest systems such as rotaxanes and pseudorotaxanes. These compounds display a pH-dependent reaction network of several species that reversibly interconvert within different time scales. Therefore, to explore and take profit of exceptional stimuli-responsive properties of these systems, detailed kinetic and thermodynamic characterizations are often required. In this work, we present the results of such characterization for a new flavylium compound decorated with a trimethylalkylammonium substituent designed to form a pseudorotaxane with cucurbit[7]uril (CB7). The formation of the pseudorotaxane was characterized in detail, and the thermodynamic and kinetic aspects of the flavylium interconversion reactions in the assembly were investigated and compared with the free molecular switch.

Multiswitchable photoacid-hydroxyflavylium-polyelectrolyte nano-assemblies

Light- and pH-responsive nano-assemblies with switchable size and structure are formed by the association of a photoacid, anthocyanidin, and a linear polyelectrolyte in aqueous solution. Specifically, anionic disulfonated naphthol derivatives, neutral hydroxyflavylium, and cationic poly(allylamine) are used as building blocks for the ternary electrostatic self-assembly, forming well-defined supramolecular assemblies with tunable sizes of 50 to 500 nm. Due to the network of possible chemical reactions for the anthocyanidin and the excited-state dissociation of the photoacid upon irradiation, different ways to alter the ternary system through external triggering are accessible. The structure and trigger effects can be controlled through the component ratios of the samples. Dynamic and static light scattering (DLS, SLS) and ζ-potential measurements were applied to study the size and the stability of the particles, and information on the molecular structure was gained by UV-vis spectroscopy. Isothermal titration calorimetry (ITC) provided information on the thermodynamics and interaction forces in the supramolecular assembly formation.

New Procedure To Calculate All Equilibrium Constants in Flavylium Compounds: Application to the Copigmentation of Anthocyanins

A new experimental procedure to calculate all equilibrium constants of the multistate of species of anthocyanins and related compounds, including those in basic medium, is reported. The procedure is based on a series of pH jumps monitored by stopped flow from an extended pH range of solutions at pseudo-equilibrium (when there is no significant formation of trans-chalcones) or at equilibrium to pH = 1.0. The experimental procedure is described for the anthocyanin model compound 4'-hydroxyflavylium, which exhibits a peculiar behavior in moderately acidic medium, because the quinoidal base, hemiketal, and cis-chalcone have similar mole fractions at pseudo-equilibrium, permitting good discrimination among these species. The experimental procedure can be extended to the copigmentation phenomenon and allow the calculation of the 1:1 copigmentation constants of the flavylium cation, quinoidal base, hemiketal, and cis- and trans-chalcones (this last from the equilibrium) and their respective ionized forms. The method was applied to calculate the copigmentation constants of the model compound 4'-hydroxyflavylium as well as malvidin-3-glucoside with caffeine. In the last compound, the strongest interaction takes place with the quinoidal base (K = 303 M^-1) and flavylium cation (K = 134 M^-1) and, to a lesser extent, with the ionized quinoidal base (K = 43 M^-1) and cis-chalcone (K = 17 M^-1). The caffeine interaction with the hemiketal and the other ionized species is negligible.

Impacts of hydroxylation on the photophysics of chalcones: insights into the relation between the chemical composition and the electronic structure

A combined theoretical/experimental study of the photoreactivity of two flavylium-derived chalcones, 2,4,4'-trihydroxychalcone and 2,4'-dihydroxychalcone, at the multiconfigurational wavefunction level of theory (CASSCF//CASPT2) in vacuo and in an implicit solvent (water, treated as a polarisable continuum) and by means of linear absorption spectroscopy is presented. The photosensitivity of flavium salts is expressed in the ability of their chalcone form to undergo a cis-trans isomerisation which has found application in logical networks. Despite a considerable amount of experimental data documenting the dependence of the isomerisation on solvent, pH and temperature, the knowledge of how chalcones process energy under various conditions at the molecular level is still scarce. On the example of 2,4,4'-trihydroxychalcone we unravel the complex excited state deactivation mechanism in vacuo involving ultrafast decay through conical intersections, formation of twisted intramolecular charge transfer species, intramolecular proton transfer and inter system crossings. Furthermore, we rationalise the observed discrepancies in the linear absorption spectra of 2,4,4'-trihydroxychalcone and 2,4'-dihydroxychalcone, thereby establishing a link between the functionalisation pattern and the observed spectral properties.

Effect of β-Cyclodextrin on the Multistate Species Distribution of 3-Methoxy-4',7-dihydroxyflavylium. Discrimination of the Two Hemiketal Enantiomers

The effect of β-cyclodextrin on the mole fraction distribution of the multistate species of the anthocyanin model compound, 3-methoxy-4',7-dihydroxyflavylium, was studied by NMR, stopped flow, circular dichroism, and UV-visible absorption spectroscopy. The formation of inclusion complexes with hemiketal and trans-chalcone, possessing transition dipole moments in a parallel orientation to the cyclodextrin n-fold axis, was unequivocally proved by means of the positive-induced circular dichroism signal. The discrimination of the two hemiketal enantiomers was achieved by the splitting of ¹H NMR peaks in the presence β-cyclodextrin. The spectroscopic data shows that the β-cyclodextrin has only moderate enantioselectivity, slightly favoring one of the optical isomers. The observed binding affinity of β-cyclodextrin for the 3-methoxy-4',7-dihydroxyflavylium multistate species increases in the order flavylium cation < quinoidal base < hemiketal < trans-chalcone < cis-chalcone. As a result of this selectivity and the dynamic nature of the network, the equilibrium is displaced toward the formation of the chalcone species.