Multi-stimuli-responsive polymers enabled by bio-inspired dynamic equilibria of flavylium chemistry

As part of a complex equilibria network with other chemical species, flavyliums, the chromophoric component of anthocyanins, hold great potential for use in functional polymers. This study presents the successful syntheses of polymers containing two distinct flavylium-structures, generated via post-modification of a parent polymer synthesised using reversible addition-fragmentation chain transfer (RAFT) polymerisation. The selective modification of acetophenone moieties enabled precise tuning of the polymers' properties, which are strongly influenced by the markedly different chemical characteristics of flavyliums and the other species in equilibria with them. The synthesised flavylium-containing polymers exhibit multi-stimuli responsiveness to variations in solvent, pH, light, and temperature, thereby introducing intricacy and viable functionality to the polymer system. The surface activity and critical aggregation concentrations (CAC) of the synthesised polymers were studied using profile analysis tensiometry (PAT), revealing distinct aggregation and self-assembly behaviours. Fractal-like aggregates formed by the flavylium-containing polymers were investigated using cryogenic electron microscopy (Cryo-EM) and small-angle X-ray scattering (SAXS). This research bridges the colourful dynamic equilibria of flavylium chemistry with polymer chemistry, paving the pathway for further investigations into flavylium-polymer interactions and the development of tuneable material properties of responsive polymers.

Synergistic regulation of metal-organic cage architectures via temperature- and solvent-driven atropisomerism

Regulating multistimulus responses in artificial systems remains a challenge in smart material development. We present a versatile chemical switching system that precisely controls the self-assembly of metal-organic cages via temperature and solvent changes. The key component, cyclo[2](1,3-(4,6-dimethyl)benzene) (4-pyridine)[6](1,3-(4,6-dimethyl)benzene) (CP2), was generated as three atropisomers (1, 2, and 3) with Cs, C1, and C2v symmetries. Thermally, metastable isomers (1 and 2) convert into the stable isomer (3), which reacts with Pd2+ to form specific molecular cages. Depending on the solvent, either rectangular M2L2 cages (5' and 5) form in 1,4-dioxane or hexagonal M3L3 cages (6) in 1,1',2,2'-tetrachloroethane. The solvent dictates the cage type and enables reversible transformation between cages 5 and 6. Additionally, cage 5', formed from metastable isomer 1, can switch to other cage types (i.e., 5 or 6) depending on temperature and solvent conditions. This multipathway system offers a precise strategy for controlling self-assembly in smart materials.

Correspondence on "Visible-Light-Switchable Chalcone-Flavylium Photochromic Systems in Aqueous Media"

In a recent Communication in this journal, Bandyopadhyay and co-workers proposed a rational design of chalcone photoswitches that are claimed to convert into the respective flavylium cations upon irradiation with blue, green, and red light in aqueous media. Here we argue that the reported improvements on the optical/photochemical properties of the new chalcone photoswitches are not substantiated and that the proposed design and data interpretation rely on assumptions that have yet to be validated. We also identify a series of control experiments that can be performed to verify the hypotheses and better support the conclusions presented by the authors.

Light and pH responsive catanionic vesicles based on a chalcone/flavylium photoswitch for smart drug delivery: From molecular design to the controlled release of doxorubicin

Spatially and temporally localized delivery is a promising strategy to circumvent adverse effects of traditional drug therapy such as drug toxicity and prolonged treatments. Stimuli-responsive colloidal nanocarriers can be crucial to attain such goals. Here, we develop a delivery system based on dual light and pH responsive vesicles having a cationic bis-quat gemini surfactant, 12-2-12, and a negatively charged amphiphilic chalcone, C4SCh. The premise is to exploit the chalcone/flavylium interconversion to elicit a morphological change of the vesicles leading to the controlled release of an encapsulated drug. First, the phase behavior of the catanionic system is studied and the desirable composition yielding stable unilamellar vesicles identified and selected for further studies. The solutions containing vesicles (Dh ≈ 200 nm, ζ-potential ≈ 80 mV) are in-depth characterized by light microscopy, cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS) and surface tension measurements. Upon subjecting the vesicles to UV irradiation (λ = 365 nm) at near neutral pH (≈ 6.0), no morphological effects are observed, yet when irradiation is coupled with pH = 3.0, the majority of the vesicles are disrupted into bilayer fragments. The anticancer drug doxorubicin (DOX) is successfully entrapped in the non-irradiated vesicles, yielding an encapsulation efficiency of ≈ 25% and a loading capacity of ≈ 3%. The release profile of the drug-loaded vesicles is then studied in vitro in four conditions: i) no stimuli (pH = 6.0); ii) irradiation, pH = 6.0; iii) no irradiation and adjusted pH = 3.0; iv) irradiation and adjusted pH = 3.0 Crucially, irradiation at pH = 3.0 leads to a sustained release of DOX to ca. 80% (within 4 h), whereas cases i) and ii) lead to only ≈ 25 % release and case iii) to 50% release but precipitation of the vesicles. Thus, our initial hypothesis is confirmed: we present a proof of concept delivery system where light and pH act as inputs of an AND logic gate mechanism for the controlled release of a relevant biomedical drug (output). This may prove useful if the irradiated nanocarriers meet acidified physiological environments such as tumors sites, endosomes or lysosomes.

Pillar[5]arene based water-soluble [3]pseudorotaxane with enhanced fluorescence emission for cell imaging and both type I and II photodynamic cancer therapy

Water-soluble [3]pseudorotaxane with enhanced fluorescence emission was successfully constructed and applied in cell imaging and photodynamic cancer therapy.

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.

Photomodulation of ultrastable host–guest complexes in water and their application in light-controlled steroid release

Host–guest complexation of dithienylethene photoswitches with cucurbit[8]uril leads to photoresponsive binding pairs with picomolar affinity in water.

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.

Light- and pH-regulated Water-soluble Pseudorotaxanes Comprising a Cucurbit[7]uril and a Flavylium-based Axle

A linear double pyridinium-terminated thread comprising a central chalcone moiety is shown to provide two independent binding sites with similar affinity for cucurbit[7]uril (CB7) macrocycles in water as judged from NMR, UV-Visible and fluorescence spectroscopies. Association results in [2] and [3]pseudorotaxanes, which are both pH and photosensitive. Switching from the neutral chalcone to the cationic flavylium form upon irradiation at 365 nm under acidic conditions provided an enhanced CB7 association (K_1:1 increases from 1.2×10⁵  M^-1 to 1.5×10⁸  M^-1 ), limiting spontaneous on-thread cucurbituril shuttling. This co-conformational change in the [2]pseudorotaxane is reversible in the dark with k_obs =4.1×10^-4  s^-1 . Threading the flavylium moiety into CB7 leads to a dramatic increase in the fluorescence quantum yield, from 0.29 in the free axle to 0.97 in the [2]pseudorotaxane and 1.0 in the [3]pseudorotaxane.

Synergistic regulation of nonbinary molecular switches by protonation and light

We report a molecular switching ensemble whose states may be regulated in synergistic fashion by both protonation and photoirradiation. This allows hierarchical control in both a kinetic and thermodynamic sense. These pseudorotaxane-based molecular devices exploit the so-called Texas-sized molecular box (cyclo[2]-(2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene); 1⁴⁺, studied as its tetrakis-PF₆ ^- salt) as the wheel component. Anions of azobenzene-4,4'-dicarboxylic acid (2H⁺•2) or 4,4'-stilbenedicarboxylic acid (2H⁺•3) serve as the threading rod elements. The various forms of 2 and 3 (neutral, monoprotonated, and diprotonated) interact differently with 1⁴⁺, as do the photoinduced cis or trans forms of these classic photoactive guests. The net result is a multimodal molecular switch that can be regulated in synergistic fashion through protonation/deprotonation and photoirradiation. The degree of guest protonation is the dominating control factor, with light acting as a secondary regulatory stimulus. The present dual input strategy provides a complement to more traditional orthogonal stimulus-based approaches to molecular switching and allows for the creation of nonbinary stimulus-responsive functional materials.

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.

Toward Light-Controlled Supramolecular Peptide Dimerization

The selective photodeprotection of the NVoc-modified FGG tripeptide yields the transformation of its 1:1 receptor-ligand complex with cucurbit[8]uril into a homoternary FGG₂@CB8 assembly. The resulting light-induced dimerization of the model peptide provides a tool for the implementation of stimuli-responsive supramolecular chemistry in biologically relevant contexts.

The pH and mercury ion stimuli-responsive supramolecular assemblies of cucurbit[7]uril and Hoechst 33342

In the present work, we have investigated the effect of pH and mercury ions on the host-guest complex formed between cucurbit[7]uril (Q[7]) and Hoechst 33342 (H33342). ¹H NMR, UV-vis and fluorescence spectroscopy revealed that acid/base stimulation could change the binding stoichiometry between Q[7] and H33342. The results suggest that two complexation equilibria (1:1 and 2:1) may exist between H33342 and Q[7] at pH 2.0 and 4.5, respectively. However, a 1:1 host-guest complex was formed between H33342 and Q[7] at pH 7.0 and 10.0. Q[7] shows differential affinities for the protonated and neutral forms of H33342 dye. Moreover, the switching between H33342∙2H⁺@2Q[7](1:2) at pH 4.5 and H33342∙H⁺@Q[7](1:1) at pH 7.0 was reversible. Furthermore, as a metal stimulus, Hg²⁺ ions could push (i) Q[7] from the piperazine ring to the benzimidazole position with a 1:1 guest-host ratio and (ii) a second Q[7] onto the ethyl position with a 1:2 guest-host stoichiometry. This stimulus response system will have potential applications in the field of molecular switch design.

Photoresponsive Binding Dynamics in High-Affinity Cucurbit[8]uril-Dithienylethene Host-Guest Complexes

The use of external stimuli to control the binding kinetics in supramolecular systems is of critical importance for the development of advanced molecular machines and devices. In this work, a study focused on the kinetics of a water-soluble host-guest system based on cucurbit[8]uril and two dithienylethene (DTE) photoswitches is reported. It is shown that for the DTE guest comprising two anionic sulfonate side arms appended to pyridinium moieties, the formation/dissociation of the pseudorotaxane structures is slowed down by more than 100000-fold with respect to its bipyridinium analogue. The decrease in ingression rate leads to the emergence of a competitive metastable product with the open DTE isomer that has an important influence in the overall binding kinetics. Moreover, the host-guest dissociation kinetics is demonstrated to be approximately 100-fold slower for the closed DTE isomer (t_1/2 =107 h vs. t_1/2 =1.2 h for the open isomer) allowing control over the dissociation rate with light.

Flavylium Dye as pH-Tunable Fluorescent and CD Probe for Double-Stranded DNA and RNA

The interaction of 4′-(N,N-dimethylamino)-6-hydroxyflavylium cation with double stranded (ds-) DNA/RNA was studied by UV/Vis spectrophotometry, circular dichroism (CD), and also steady-state and time-resolved emission spectroscopies at neutral and weakly acidic conditions. At pH 5, the studied molecule, in its flavylium cationic form, showed considerable binding affinities (5 < logKs < 6) for all ds-DNA/RNA, contrary to chalcones forms (dominant at pH 7), which did not show binding to polynucleotides. Flavylium cation intercalated into ds-DNAs at variance to dominant groove aggregation within ds-RNA, which was reported by RNA-specific bisignate induced CD spectrum (ICD) bands. The intrinsically negligible fluorescence of flavylium was strongly increased upon the addition of DNA or RNA, whereby both the fluorescence intensity and emission lifetimes of complexes differed considerably: the strongest emission increase was observed for AU-RNA (detection limit estimated to 10 nM) followed by AT-DNAs and the much weaker effect of GC-DNAs. Both fluorescence sensitivity on the ds-DNA/RNA secondary structure and sequence-selective ICD bands make the flavylium–chalcones system an intriguing pH-switchable new probe for distinguishing between various polynucleotide sequences.

A pseudorotaxane formed from a cucurbit[7]uril wheel and a bioinspired molecular axle with pH, light and redox-responsive properties

A pH-, light- and redox-responsive flavylium-bipyridinium molecular dyad (bioinspired in natural anthocyanins) was synthesized and employed to devise a pseudorotaxane with the macrocycle cucurbit[7]uril (CB7) in aqueous solution. The inclusion complex was characterized by UV-Vis absorption, fluorescence emission, NMR and electrochemical techniques which demonstrate formation of a stable binary complex between the dyad and CB7 both under acidic and neutral conditions. It is noteworthy that the flavylium-bipyridinium tricationic dyad is only stable in highly acidic media, undergoing a reversible hydration reaction at slightly acidic or neutral pH to give a trans-chalcone-bipyridinium dication. 1H NMR experiments showed that in this last species the CB7 binds to the bipyridinium unit while in the tricationic species the macrocycle is positioned between the flavylium and the bipyridinium moieties. The different location of the CB7 wheel in the two dyad states allows control of the shuttling movement using light and pH stimuli that trigger the interconversion between these two species.

Light-induced cargo release from a cucurbit[8]uril host by means of a sequential logic operation

A combination of a chalcone/flavylium photoswitch with a supramolecular host–guest complex that can be used to phototrigger the logically-controlled and selective release of cargo was devised.