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Organic electrochromic polymers hold great potential for integration into low-power flexible electrochromic displays (F-ECDs) due to their wide range of colors and simple processing. However, challenges such as inefficient charge transfer and degradation upon device integration hinder their practical applications. Herein, we report an innovative, general approach that utilizes template-induced supramolecular nanostructuring to engineer established electrochromic polymers, enhancing their performance and durability. We modified a well-known, albeit underperforming in F-ECDs, poly-thiophene polymer (ECP Orange; PT) by incorporating a pyrene appendage, resulting in a copolymer ( PTPy ) capable of undergoing large-scale assembly in the presence of multi-walled carbon nanotubes (MWCNTs), driven by the establishment of π-π interactions between the pyrene and the MWCNTs ( PTPy/MWCNTs ). F-ECDs based on these hybrids, produced by spray coating, exhibit improved color switching speeds ( t 90 OX = 3.6 s, t 90 RED = 0.3 s) compared to those of the PT polymer ( t 90 OX = 53.2 s, t 90 RED = 2.5 s). Additionally, PTPy/MWCNTs F-ECDs demonstrate longer cyclability (half-life based on Δ E , Δ E 50% = 17.6k cycles) compared to PT (Δ E 50% = 278 cycles), also when blended with MWCNTs (Δ E 50% = 282 cycles). This work highlights the pivotal role of engineered supramolecular nanostructuring in boosting the performance of organic electrochromic materials, making them suitable for F-ECD scalable commercial applications., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

New β-amino-substituted porphyrin derivatives bearing carboxy groups were synthesized and their performance as sensitizers in dye-sensitized solar cells (DSSC) was evaluated. The new compounds were obtained in good yields (63-74%) through nucleophilic aromatic substitution reactions with 3-sulfanyl- and 4-sulfanylbenzoic acids. Although the electrochemical studies indicated suitable HOMO and LUMO energy levels for use in DSSC, the devices fabricated with these compounds revealed a low power conversion efficiency (PCE) that is primarily due to the low open-circuit voltage (V oc ) and short-circuit current density (J sc ) values., Competing Interests: The authors declare no conflicts of interest.

The kinetics, thermodynamics, and degradation of malvidin mono- and diglucosides were studied following a holistic approach by extending to the basic medium. In acidic conditions, the reversible kinetics of the flavylium cation toward the equilibrium is controlled by the hydration and cis - trans isomerization steps, while in the basic medium, the OH - nucleophilic addition to the anionic quinoidal bases is the slowest step. There is a pH range (transition pHs), between the acidic and basic paradigms, that includes physiological pH (7.4), where degradation reactions occur faster, preventing the system from reaching the equilibrium. The transition pH of the diglucoside is narrower, and in contrast with the monoglucoside, there is no evidence for the formation of colored oligomers among the degradation products. Noteworthy, OH - addition in position 4 to form B4 2- , a kinetic product that decreases the overall equilibration rate, was observed only for the diglucoside.

A set of acenaphthylene dyes with arylethynyl π-bridges was tested for dye-sensitized solar cells (DSSCs). Crucial steps for the extension of the conjugated system from the acenaphylene core involved Sonogashira coupling reactions. Phenyl, thiophene, benzotriazole, and thieno-[3,2- b ]thiophene moieties were employed to extend the conjugation of the π-bridges. The systems were characterized by cyclic voltammetry and by UV-vis absorption and emission. The spectroscopic characterization showed that the last three bridges resulted in red-shifted absorption and emission spectra relative to the parent phenyl-bridged compound, in accordance with TD-DFT calculations. The phenylethynyl derivative 6a achieved a conversion efficiency of 2.51% with V oc , J sc , and FF values of 0.365 V, 13.32 mA/cm 2 , and 0.52, respectively. The efficiency of this compound improved to 3.15% with the addition of CDCA (10 mM), representing the best efficiency result in this study. The overall conversion efficiency of the other aryl derivatives 6b - d proved to be significantly inferior (14-40%) to that of 6a due to a significant decrease of J sc ., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

1,8-Naphthalimides (NIs) represent a class of organic dyes with interesting optical properties that has been extensively explored in the last decades in lighting devices, chemosensors, optical probes or medicinal chemistry. However, despite their remarkable potential, reports on organometallic dyes bearing NIs are scarce and virtually inexistent regarding palladium(II) complexes. Herein, we report the synthesis of NIs bearing phosphine and amine chelating moieties and the characterization of their optical properties both as single molecules and when complexed on Pd(II) ions. It is shown that the introduction of phosphine moieties in the naphthalimide core results in a marked increase in non-radiative processes, leading to a significant reduction of the emission efficiency and lifetime of these dyes, compared to amine-bearing counterparts. The complexation to Pd(II) sequesters the electronic contribution of chelating moieties, with complexes assuming an optical behavior similar to that of unsubstituted 1,8-naphthalimide. The complexation significantly increases the acidity of chelating secondary amines, giving rise to an unexpected intramolecular reaction that results in the formation of a novel 1,8-naphthalimide dye bearing a cyclic phosphorylamide moiety. The new dye exhibits good emission quantum yield, long fluorescence lifetime and sensitivity to basic media, evidencing potential for application in optical imaging and sensing scenarios., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The tailored design of a light-triggered supramolecular cascade results in an artificial machinery that assimilates the transduction of photons into chemical communication and the final release of a neurotransmitter. This is reminiscent of key steps in the natural vision process.

Cytochrome b 5 reductase (C b 5 R) is a flavoprotein that participates in the reduction of multiple biological redox partners. Co-localization of this protein with nitric oxide sources has been observed in neurons. In addition, the generation of superoxide anion radical by C b 5 R has been observed. A search for specific inhibitors of C b 5 R to understand the role of this protein in these new functions has been initiated. Previous studies have shown the ability of different flavonoids to inhibit C b 5 R. Anthocyanins are a subgroup of flavonoids responsible for most red and blue colors found in flowers and fruits. Although usually represented by the flavylium cation form, these species are only stable at rather acidic pH values (pH ≤ 1). At higher pH values, the flavylium cation is involved in a dynamic reaction network comprising different neutral species with the potential ability to inhibit the activities of C b 5 R. This study aims to provide insights into the molecular mechanism of interaction between flavonoids and C b 5 R using flavylium salts as dynamic inhibitors. The outcome of this study might lead to the design of improved specific enzyme inhibitors in the future.

Primary hyperoxalurias (PHs) are a group of inherited alterations of the hepatic glyoxylate metabolism that result in an excess of oxalate production by the oxidation of glyoxylate by the human lactate dehydrogenase A enzyme (hLDHA). The selective liver inhibition of this enzyme is one of the therapeutic strategies followed in the treatment of this disease. Even though several efforts have been recently performed using gene silencing by the RNA interference approach, small-molecule inhibitors that selectively reach hepatocytes are preferred since they present the advantages of a lower production cost and better pharmacological properties. In that sense, the design, synthesis, and physicochemical characterization by NMR, FTIR, DLS and TEM of two nanocarriers based on chitosan conjugates (1, non-redox-sensitive; 2, redox-sensitive) have been performed to (i) achieve the selective transport of hLDHA inhibitors into hepatocytes and (ii) their disruption once they reach the hepatocytes cytosol. Polymer 2 self-assembled into micelles in water and showed high drug loadings (19.8-24.5 %) and encapsulation efficiencies (31.9-40.8%) for the hLDHA inhibitors (I-III) tested. The non-redox-sensitive micelle 1 remained stable under different glutathione (GSH) concentrations (10 μM and 10 mM), and just a residual release of the inhibitor encapsulated was observed (less than 10 %). On the other hand, micelle 2 was sufficiently stable under in vitro physiological conditions (10 μM, GSH) but it quickly disassembled under the simulated reducing conditions present inside hepatocytes (10 mM GSH), achieving a 60 % release of the hLDHA inhibitor encapsulated after 24 h, confirming the responsiveness of the developed carrier to the high levels of intracellular GSH., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Author(s). Published by Elsevier B.V. All rights reserved.)

The thermodynamics and kinetics of compound 7-diethylamino-4'-dimethylaminoflavylium were studied in water : ethanol (1 : 1) and water in the presence of SDS and CTAB micelles. The blue flavylium cation is in equilibrium with the pink protonated flavylium cation defined by p K AH 2+ /AH + and the yellow trans -chalcone, defined by p K AH + /Ct . The difference between these two p K s gives the pH domain of the flavylium cation, Δp K = 1.95 in CTAB, Δp K = 5.6 in water/ethanol (1 : 1) and Δp K = 8.5 in SDS micelles. On the other hand, the pH domain of the trans -chalcone is limited by p K AH + /Ct and p K Ct/Ct - . It is lower in SDS micelles Δp K = 2.7, increases in ethanol/water (1 : 1) Δp K = 5.1 and is maximum in CTAB micelles, Δp K = 6.8. All these effects can be explained by the electric charge present at the micellar surface. Relative energy level diagrams that allow for the explanation of the driving forces for any pH stimuli or light absorption were constructed from the calculated equilibrium constants. Irradiation of the trans -chalcone at 466 nm leads to the formation of the flavylium cation. In water : ethanol (1 : 1), the photochemistry is residual with Φ < 0.00002, while in SDS micelles at pH = 7 light increases the rate of the spontaneous conversion of trans -chalcone to the flavylium cation, with quantum yield Φ = 0.002; photochromism from trans -chalcone to give the flavylium cation with the same quantum yield is also observed in CTAB micelles.

Intermolecular copigmentation of malvidin-3- O -glucoside with caffeine was studied using a holistic procedure that includes the extension to basic pH values. In moderately basic solutions (7.5 < pH < 9.5) and independently of the copigment presence, there is a pH region where degradation of the quinoidal base and anionic quinoidal bases is faster than hydration and OH - nucleophilic addition, preventing the system from reaching the equilibrium. Intermolecular copigmentation with caffeine reduces significantly the degradation rate of quinoidal bases. In a more basic medium, the equilibrium is reached and degradation occurs from the anionic chalcones. In this case, the addition of caffeine also reduces the degradation rate in the interval 10 < pH < 11.5., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The concept of molecular metamorphosis is described. A molecule (flavylium cation) generates a sequence of other different molecules by means of external stimuli. The reversibility of the system allows for the flavylium cation to be recovered by other external stimuli, completing one cycle. Differently from supramolecular chemistry, molecular metamorphosis is not a bottom-up approach. All events occur at the bottom. The procedures to characterize the kinetics and thermodynamics of the cycles are summarized. They are based on direct pH jumps (addition of a base to the flavylium cation) and reverse pH jumps (addition of an acid to equilibrated solutions at higher pH values). Stopped flow is an indispensable tool to characterize these systems. The following metamorphic cycles will be described to illustrate the concept: (i) introducing the flavanone in the metamorphic system and illustrating the concept of a timer at the molecular level; (ii) response of the flavylium-based metamorphosis to light inputs and the write-lock-read-unlock-erase molecular system; (iii) a one-way cycle of direct-reverse pH jumps; (iv) interconversion of the flavylium cation with 2,2'-spirobis[chromene] derivatives; (v) 6,8 A-ring substituent rearrangements., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The past decade has seen growing interest in marine natural pigments for biotechnological applications. One of the most abundant classes of biological pigments is the tetrapyrroles, which are prized targets due their photodynamic properties; porphyrins are the best known examples of this group. Many animal porphyrinoids and other tetrapyrroles are produced through heme metabolic pathways, the best known of which are the bile pigments biliverdin and bilirubin. Eulalia is a marine Polychaeta characterized by its bright green coloration resulting from a remarkably wide range of greenish and yellowish tetrapyrroles, some of which have promising photodynamic properties. The present study combined metabolomics based on HPLC-DAD with RNA-seq transcriptomics to investigate the molecular pathways of porphyrinoid metabolism by comparing the worm's proboscis and epidermis, which display distinct pigmentation patterns. The results showed that pigments are endogenous and seemingly heme-derived. The worm possesses homologs in both organs for genes encoding enzymes involved in heme metabolism such as ALAD, FECH, UROS, and PPOX. However, the findings also indicate that variants of the canonical enzymes of the heme biosynthesis pathway can be species- and organ-specific. These differences between molecular networks contribute to explain not only the differential pigmentation patterns between organs, but also the worm's variety of novel endogenous tetrapyrrolic compounds.

A set of 3-ethynylaryl coumarin dyes with mono, bithiophenes and the fused variant, thieno [3,2- b ] thiophene, as well as an alkylated benzotriazole unit were prepared and tested for dye-sensitized solar cells (DSSCs). For comparison purposes, the variation of the substitution pattern at the coumarin unit was analyzed with the natural product 6,7-dihydroxycoumarin (Esculetin) as well as 5,7-dihydroxycomarin in the case of the bithiophene dye. Crucial steps for extension of the conjugated system involved Sonogashira reaction yielding highly fluorescent molecules. Spectroscopic characterization showed that the extension of conjugation via the alkynyl bridge resulted in a strong red-shift of absorption and emission spectra (in solution) of approximately 73-79 nm and 52-89 nm, respectively, relative to 6,7-dimethoxy-4-methylcoumarin (λ abs = 341 nm and λ em = 410 nm). Theoretical density functional theory (DFT) calculations show that the Lowest Unoccupied Molecular Orbital (LUMO) is mostly centered in the cyanoacrylic anchor unit, corroborating the high intramolecular charge transfer (ICT) character of the electronic transition. Photovoltaic performance evaluation reveals that the thieno [3,2- b ] thiophene unit present in dye 8 leads to the best sensitizer of the set, with a conversion efficiency (η = 2.00%), best V OC (367 mV) and second best J sc (9.28 mA·cm -2 ), surpassed only by dye 9b (J sc = 10.19 mA·cm -2 ). This high photocurrent value can be attributed to increased donor ability of the 5,7-dimethoxy unit when compared to the 6,7 equivalent ( 9b ).

Anthocyanins are the basis of the color of angiosperms, 3-deoxyanthocyanins and sphagnorubin play the same role in mosses and ferns, and auronidins are responsible for the color in liverworts. In this study, the color system of cyanidin-3- O -glucoside (kuromanin) as a representative compound of simpler anthocyanins was fully characterized by stopped flow. This type of anthocyanin cannot confer significant color to plants without intra- or intermolecular interactions, complexation with metals or supramolecular structures as in Commelina communis. The anthocyanin's color system was compared with those of 3-deoxyanthocyanins and riccionidin A, the aglycone of auronidins. The three systems follow the same sequence of chemical reactions, but the respective thermodynamics and kinetics are dramatically different.

Porphyrins and derivatives form one of the most abundant classes of biochromes. They result from the breakdown of heme and have crucial physiological functions. Bilins are well-known representatives of this group that, besides significant antioxidant and anti-mutagenic properties, are also photosensitizers for photodynamic therapies. Recently, we demonstrated that the Polychaeta Eulalia viridis , common in the Portuguese rocky intertidal, holds a high variety of novel greenish and yellowish porphyrinoid pigments, stored as granules in the chromocytes of several organs. On the follow-up of this study, we chemically characterized pigment extracts from the worm's skin and proboscis using HPLC and evaluated their light and dark toxicity in vivo and ex vivo using Daphnia and mussel gill tissue as models, respectively. The findings showed that the skin and proboscis have distinct patterns of hydrophilic or even amphiphilic porphyrinoids, with some substances in common. The combination of the two bioassays demonstrated that the extracts from the skin exert higher dark toxicity, whereas those from the proboscis rapidly exert light toxicity, then becoming exhausted. One particular yellow pigment that is highly abundant in the proboscis shows highly promising properties as a natural photosensitizer, revealing that porphyrinoids from marine invertebrates are important sources of these high-prized bioproducts.

The kinetics and thermodynamics (in acidic solutions) of the five chemical species reversibly interconnected by external stimuli (a multistate), such as pH and light, generated by the liverworts colorant riccionidin A were investigated. The degradation products of the multistate formed after 10 days at neutral pH were identified. The behaviour of riccionidin A multistate was compared with previous results reported for the equivalent systems based on 3-deoxyanthocyanidins (found in mosses and ferns) and anthocyanins (ubiquitous in angiosperms). The five chemical species have mutatis mutandis similar structures in the three multistates. The most dramatic difference is the extremely slow interconversion rate between flavylium cation and trans-chalcone in riccionidin A and related compounds multistates (tens of days) when compared with deoxyanthocyanins (a few days) and anthocyanins (several hours), at room temperature. The mole fraction distribution of the five species that constitute the multistate as a function of pH is also different in the three families of compounds. Some considerations regarding the chemical evolution of the three systems are given., Competing Interests: Declaration of competing interest There are no conflicts of interest., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

In the present work, five 2-styryl-1-benzopyrylium salts and their relative self-assembly processes towards TiO 2 nanocrystalline layers were evaluated as photosensitizers in dye-sensitized solar cells (DSSCs). Integration of these 2-styryl-1-benzopyrylium salts with the semiconductor allow for the performance of highly specific functions suitable for smart applications in material science. Spectroscopic and photoelectrochemical measurements conducted on these five bio-inspired dyes, in solution and upon adsorption onto titanium dioxide films, allowed detailed discussion of the anchoring ability of the different donor groups decorating the 2-styryl-1-benzopyrylium core and have demonstrated their ability as photosensitizers. Our results suggest that the introduction of a dimethylamino group in position 4' of the 2-styryl-1-benzopyrylium skeleton can alter the conjugation of the molecule leading to larger absorption in the visible region and a stronger electron injection of the dye into the conduction band of TiO 2 . Moreover, our experimental data have been supported by theoretical calculations with the aim to study the energy of the excited states of the five compounds. In this specific case, the simulations reported contributed to better describe the properties of the compounds used and to help create the necessary basis for the design of new and targeted bio-inspired molecules.

The comparison of the ground-state reactivity of anthocyanins and aurone model compounds (i.e. with and without the furano bridge) has shown that the kinetic paradigm does not depend on the bridge but only on the hydroxyl substituent pattern, independently of the presence of the bridge: (i) bell shaped kinetics for those with two hydroxyl substituents in position 4' and 7, and (ii) four distinct kinetic steps for the mono substituted compounds with a hydroxyl in position 4'. The excited state proton transfer (ESPT) properties of these compounds were also investigated using steady-state and time-resolved spectroscopic techniques. It was found that the ESPT efficiency is significantly higher for the bridged compounds. Interestingly, pH-dependent steady-state fluorescence emission experiments show that in 4',7-dihydroxyfuranoflavylium the hydroxyl group in position 7 is the more acidic one in the excited state, while 1 H NMR titration curves indicate a higher acidity constant in the ground state for the proton at the hydroxyl group in position 4'. Differently, the fluorescence emission spectrum of the quinoidal base deprotonated at position 7 is only observed upon excitation of the flavylium cation while the one from the base deprotonated at 4' is observed upon direct excitation.

True green pigments in the animal kingdom are scarce and are almost invariably porphyrinoids. Endogenous porphyrins resulting from the breakdown of haem are usually known as "bile pigments". The pigmentation of intertidal Polychaeta has long gained attention due to its variety and vivid patterning that often seems incompatible with camouflage, as it occurs with Eulalia viridis, one of the few truly green Polychaeta. The present study combined UV and bright-field microscopy with HPLC to address the presence and distribution of pigments in several organs. The results showed two major types of porphyrin-like pigments, yellowish and greenish in colour, that are chiefly stored as intraplasmatic granules. Whereas the proboscis holds yellow pigments, the skin harbours both types in highly specialised cells. In their turn, oocytes and intestine have mostly green pigments. Despite some inter-individual variation, the pigments tend to be stable after prolonged storage at -20 °C, which has important implications for future studies. The results show that, in a foraging predator of the intertidal where melanins are circumscribed to lining the nervous system, porphyrinoid pigments have a key role in protection against UV light, in sensing and even as chemical defence against foulants and predators, which represents a remarkable adaptive feature.

A tripodal coumarin derivative shows complex photoreactivity, changing from intra- to intermolecular photodimerization with increasing concentration. At high concentration, the compound undergoes efficient photopolymerization, resulting in the formation of polymeric submicron-sized particles. The size of these particles can be precisely increased through photoirradiation, without affecting their polydispersity.

The discovery of stimuli-responsive high affinity host-guest pairs with potential applications under biologically relevant conditions is a challenging goal. This work reports a high-affinity 1:1 complex formed between cucurbit[8]uril and a water-soluble photochromic diarylethene derivative. It was found that, by confining the open isomer within the cavity of the receptor, a redshift in the absorption spectrum and an enhancement of the photocyclization quantum yield from Φ=0.04 to Φ=0.32 were induced. This improvement in the photochemical performance enables quantitative photocyclization with visible light that, together with the near-infrared light-induced ring-opening reaction and the 100-fold selectivity for the closed isomer, confirms this as an outstanding light-responsive affinity pair., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The multistate of chemical species generated by 4'-hydroxy-3,2'-furanoflavylium is similar to that of anthocyanins and related compounds. This furanoflavylium multistate system was fully characterized by UV-visible and NMR spectroscopy, allowing determination of the respective equilibrium and rate constants. In contrast to the multistate generated by flavylium cations derived from anthocyanins and related compounds, the furanoflavylium multistate is characterized by much slower hydration and tautomerization (pyran ring opening-closing). In addition, the cis-trans isomerization of the chalcones of this system (2'-hydroxyaurones) is extremely slow when compared with anthocyanins. The observed similar order of magnitude for tautomerization and isomerization rate constants leads to peculiar kinetics from the flavylium cation (pH = 1) to the stable trans -chalcone (higher pH values). The hemiketal appears and disappears during the first stages of the kinetics, which gives the intermediate cis -chalcone (pseudo-equilibrium). This last species disappears in a much slower process, as fully characterized by 1 H NMR, to give the final trans -chalcone., Competing Interests: The authors declare no competing financial interest.

As in supramolecular chemistry, complexity could also be achieved through a bottom-up approach. Anthocyanins and related compounds such as the compound ( E )-6-(dimethylamino)-4-(4-(dimethylamino)-2-hydroxybenzylidene)-1,2,3,4-tetrahydroxanthylium chloride ( 1 ), here reported, exhibit this type of complexity. The thermodynamics and kinetics of the complex multistate of species of compound 1 were studied by conventional and stopped-flow UV-visible spectrophotometry as well as by NMR. The system follows the same multistate of species of anthocyanins, except for the presence at moderately basic pH values of a species possessing a spiro carbon. The introduction of two dimethylamino substituents in positions 4' and 7, modulates deeply the thermodynamic and kinetics of the system. A beautiful pH-dependent palette of colors is obtained, including a blue flavylium cation at unusually high pH values. The protonation of the dimethylamino substituents is the key aspect for explaining the details of the spiro opening kinetics. The system was fully characterized by representing the mole fraction distribution and the relative energy level diagram of all multistate species as a function of pH., Competing Interests: The authors declare no competing financial interest.

A convenient method to synthesize A-type proanthocyanidin analogues from flavylium salts and π-nucleophiles has been developed. It was found that the thermodynamic stability of the starting flavylium salt, assessed by the measurement of the apparent acidity constant ( K' a ), was the key parameter to design effective one-pot reactions between flavylium salts and nucleophiles such as phloroglucinol and (+)-catechin. When flavylium salts have a p K' a value of 1.7 or lower, the synthesis of the corresponding 2,8-dioxabyciclo[3.3.1]nonane derivative was properly achieved.

With the purpose of modifying organic fluorescent dyes based on the coumarin scaffold, and developing and evaluating a route to its incorporation into a polymeric backbone, a study was conducted on the co-polymerization of 3-vinylcoumarins with styrene and methyl acrylate using 2,2-azobis(isobutyronitrile) (AIBN) as the radical initiator. The structural and photophysical characterization proved the incorporation of the coumarin monomers into the polymeric chain and further showed a decrease in the fluorescence quantum yields in the co-oligomers., Competing Interests: The authors declare no conflict of interest.

A fluorescent bistable pseudorotaxane was devised and shown to display pH-gated photoresponsive ring shuttling. At pH > 7.5 shuttling does not take place after light stimulation while at pH ≈ 5 the macrocycle is quantitatively translocated.

A four component host-guest system comprising β-cyclodextrin (β-CD), cucurbit[7]uril (CB7) and two chalcone guests was demonstrated to undergo pH-triggered self-sorting in aqueous solution. This phenomenon arises from the contrasting complexation induced pK a shifts displayed by these guests after complexation with CB7.

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 cis-trans 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.

Incorporation of flavylium-derived chalcones in the cetyltrimethylammonium bromide-templated synthesis of mesoporous silica particles with no subsequent removal of the micellar phase leads to high luminescence (0.3 < ϕ f < 0.5) and strong color-contrast photochromic pigments finely tunable over a large pH range (1 < pH < 11)., Competing Interests: The authors declare no competing financial interest.

Cucurbit[7]uril inclusion complexes with guests bearing dimethylamino groups show the expected upward p K a shifts, whereas their diethylamino counterparts display a decrease in p K a due to the preferential stabilization of the unprotonated form. These results identify the diethylamino group as the substituent of choice to avoid receptor-assisted protonation of guest molecules and present new evidence for the role of the hydrophobic effect as a driving force in cucurbituril complexation., Competing Interests: The authors declare no competing financial interest.

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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The introduction of two amine substituents in 4' and 7 positions, leads to the formation of a blue flavylium cation, 7-(N,N'-diethylamino)-2-(9-julolidine)-1-benzopyrilium, which is extremely stable across a wide acidic pH range. The kinetic and thermodynamic constants of the multistate system have been calculated by studying the relaxation kinetics after equilibrium perturbation by addition of base (direct pH jumps) or acid (reverse pH jumps). Except for the cis-chalcone, which is an elusive species, the relative energy levels of the other species could be calculated and a global energy level diagram constructed. The diagram explains that the stability of the diamino compound is due to the high energy level of the hemiketal species, which is difficult to access in acidic medium.

Two room temperature ionic liquids (ILs) bearing coumarin and p-hydroxycinnamic acid moieties are synthesized, and their photochemistry is studied in solution and neat conditions. Irradiation at absorption maxima leads to photochemical transformations and results in changes of their rheological properties which are evaluated by rotational rheometry. Samples of ionic liquids are also studied by Hyper-Rayleigh scattering, and the effect of their photochemistry on ionic nanoaggregation is discussed.

A multistate molecular dyad containing flavylium and viologen units was synthesized and the pH dependent thermodynamics of the network completely characterized by a variety of spectroscopic techniques such as NMR, UV-vis and stopped-flow. The flavylium cation is only stable at acidic pH values. Above pH ≈ 5 the hydration of the flavylium leads to the formation of the hemiketal followed by ring-opening tautomerization to give the cis-chalcone. Finally, this last species isomerizes to give the trans-chalcone. For the present system only the flavylium cation and the trans-chalcone species could be detected as being thermodynamically stable. The hemiketal and the cis-chalcone are kinetic intermediates with negligible concentrations at the equilibrium. All stable species of the network were found to form 1 : 1 and 2 : 1 host : guest complexes with cucurbit[7]uril (CB7) with association constants in the ranges 10(5)-10(8) M(-1) and 10(3)-10(4) M(-1), respectively. The 1 : 1 complexes were particularly interesting to devise pH responsive bistable pseudorotaxanes: at basic pH values (≈12) the flavylium cation interconverts into the deprotonated trans-chalcone in a few minutes and under these conditions the CB7 wheel was found to be located around the viologen unit. A decrease in pH to values around 1 regenerates the flavylium cation in seconds and the macrocycle is translocated to the middle of the axle. On the other hand, if the pH is decreased to 6, the deprotonated trans-chalcone is neutralized to give a metastable species that evolves to the thermodynamically stable flavylium cation in ca. 20 hours. By taking advantage of the pH-dependent kinetics of the trans-chalcone/flavylium interconversion, spatiotemporal control of the molecular organization in pseudorotaxane systems can be achieved.

The equilibrium forms of malvidin-3-O-glucoside trimer present in grape skins were studied in aqueous solution at different pH values through UV-Visible spectroscopy. It was observed that the reactivity of this compound is strongly dominated by acid-base chemistry (pKa1=3.61±0.03; pKa2=6.83±0.06), with the reaction sequence hydration-tautomerization-isomerization accounting less than 10% of the overall reactivity. The decrease of hydration of this flavylium cation derivative when compared to the original anthocyanins results from the formation of a cluster of the pigment with a high-energy of solvation that inhibits the access of water molecules to the flavylium cation core preventing by this way the hydration reactions. Overall of these results raise the hypothesis that polymerization may be a natural stabilization mechanism for the red color of anthocyanins., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

A new polyazamacrocycle (L) containing two 2,2'-bipyridino (bpy) units, where the heteroaromatic nitrogen atoms point outwards from the macrocyclic cavity, was synthesized and characterized by elemental analysis, ESI-MS, (1)H and (13)C NMR, FTIR and TGA. Five protonation constants involving aliphatic nitrogens with log K in the range 9.39-3.07 were determined by potentiometry and NMR and a sixth protonation (log K = 2.2) involving a bipyridine moiety could be detected by UV-Vis and NMR titrations. The interaction of L with the cyanometallate anions [Pt(CN)4](2-) and [Co(CN)6](3-) was studied by potentiometry yielding respectively log K values in the ranges 4.0-6.4 and 5.2-10.5, covering protonation degrees from 1 to 5. Studies on L with [Ru(CN)6](4-) led to early precipitation preventing potentiometry, but crystals of [H4L][Ru(CN)6]·10H2O and also of [H2L](ClO4)2·4H2O suitable for X-ray diffraction could be obtained and the crystalline structures are described. The interaction of L with Zn(2+) was investigated by potentiometry and UV-Vis showing the formation of mono- and dinuclear complexes involving the bipyridines. The macrocycle was used as a bridging ligand for the successful synthesis of a new dinuclear [(Ru(bpy)2)2(L)](PF6)4·4HPF6 complex that was fully characterized. The protonation (log K in the range 9.9-1.6) and coordination features of this compound towards Cu(2+) and Zn(2+) were explored by UV-Vis absorption and emission spectroscopies.

A new trimeric flavylium derivative pigment was detected for the first time in a young Port wine and structurally characterised using both API-LC ESI MS/MS and NMR techniques. Its structure comprises a malvidin-3-glucoside molecule (terminal unit) linked to two units of flavans (extension units) involving two A-type linkages (C4→C8 and C7-O-C2 ether linkages). The energies of the minimized geometries for all four conformations of this compound were calculated and comparing the structures of all isomers, it was verified that the binding mode (α or β) between the first and second anthocyanin unit does not have a key role in the compound stability. However, for isomers 1 and 2 where the interaction between the second and third anthocyanin units occurs by a "β" mode in opposition to the "α" mode verified in isomers 3 and 4, revealed a higher stability. The presence of these compounds in red wines is thought to result from their extraction from grape skin during fermentation, as the presence of these trimeric pigments was detected in a grape skin extract by API-LC ESI MS/MS spectrometry., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The ground and excited state (in the singlet state, S1) acid–base equilibria, together with the photophysical properties of the two main constituents of brazilwood, brazilin and brazilein, have been investigated in aqueous solutions in the pH range: −1 < pH < 10. Brazilin is the colorless reduced form of brazilein where three ground and three excited state species (B(red)H(n), with n = 2–4 representing the protonated hydroxyl groups) are observed with two corresponding acidity constants: pKa1 = 6.6 and pKa2 = 9.4 (pKa1* = 4.7 and pKa2* = 9.9, obtained from the Förster cycle). In the case of brazilein, three ground species (pKa1 = 6.5 and pKa2 = 9.5) and four excited state species were identified (again from the Förster cycle: pKa1* = 3.9 and pKa2* = 9.8). The colorless species (brazilin) presents a high fluorescence quantum yield (F = 0.33) and competitive radiative channel (kF = 1.3 × 10(9) s(–1)) over radiationless processes (kNR = 2.6 × 10(9) s(–1)). In contrast to this behavior, brazilein displays a F value 2 orders of magnitude lower and a dominance of the radiationless decay pathways, which is suggested to be linked to an excited state proton transfer leading to a quinoidal-like structure. This is further supported by time-resolved data (obtained in a ps time domain). The overall data indicates that brazilin is more prone to degradation than brazilein, mainly due to the high efficiency of the radiationless decay channel (likely through internal conversion), which confers a stabilizing inherent characteristic to the latter. In the case of brazilein, the efficiency of the radiationless channel is linked to an excited state intramolecular proton transfer resulting from an excited state equilibrium involving neutral and zwitterionic tautomeric species of this compound. Furthermore, a theoretical study has been performed with the determination of the optimized ground-state and excited molecular geometries for the two compounds together with the prediction of the lowest vertical one-electron excitation energy and the relevant molecular orbital contours and charge densities changes using density functional theory calculations. These were found to corroborate differences in acidity in the ground and excited states.

Keeping it in the family: A new family of 5-styrylcoumarins exhibit E-Z isomerization around the C α C β bond with large extinction coefficients, medium-lived excited states, and moderate fluorescence quantum yields. The alteration of these photophysical properties is also corroborated by computational studies., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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, (1)H and (13)C 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 formation of the same final chromene derivative (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 trans-chalcone singlet excited state (1)Ct* (LE), which evolves into a twisted state (1)P*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of cis-chalcone (Cc) through a simple isomerization process. The resulting cis-chalcone evolves into the chromene B through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H2O/CH3OH (1/1, v/v), the P* state becomes unstable and evolves into a new state attributed to the tautomer (1)Q*. This state directly evolves into 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.

Seven flavylium salt dyes were employed for the first time as sensitizers for dye-sensitized solar cells (DSSCs). The theoretical and experimental wavelengths of the maximum absorbances, the HOMO and LUMO energy levels, the coefficients, the oscillator strengths and the dipole moments are calculated for these synthetic dyes. The introduction of a donor group in the flavylium molecular structure was investigated. Photophysical and photoelectrochemical measurements showed that some of these synthetic analogues of anthocyanins are very promising for DSSC applications. The best performance was obtained by a DSSC based on the novel compound 7-(N,N-diethylamino)-3',4'-dihydroxyflavylium which produced a 2.15% solar energy-to-electricity conversion efficiency, under AM 1.5 irradiation (100 mW cm(-2)) with a short-circuit current density (J(sc)) of 12.0 mA cm(-2), a fill factor of 0.5 and an open-circuit voltage (V(oc)) of 0.355 V; its incident photocurrent efficiency of 51% at the peak of the visible absorption band of the dye is remarkable. Our results demonstrated that the substitution of a hydroxylic group with a diethylamine unit in position 7 of ring A of the flavylium backbone expanded the π-conjugation in the dye and thus resulted in a higher absorption in the visible region and is advantageous for effective electron injection from the dye into the conduction band of TiO2.

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 trans-styrylchalcone (Ct) and a spiropyran-type intermediate (SP). At pH = 1, AH(+) slowly evolves (k(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.

The photoreaction of indigo and two other derivatives in its reduced (leuco) form was investigated by absorption and fluorescence (steady-state and time-resolved) techniques. The fluorescence quantum yield (φ(F)) dependence with the UV irradiation time was found to increase up to a value of φ(F) ≈ 0.2-0.3 (after 16 min) for indigo and φ(F) = 0.2 (at ~150 min) for its derivative 4,4'-dibutoxy-7,7'-dimethoxy-5,5'-dinitroindigo (DBMNI). With a model compound, where rotation around the central C-C bond is blocked, the φ(F) value was found constant with the UV irradiation time. Time-resolved fluorescence revealed that initially the decays are fitted with a biexponential law (with 0.12 and 2.17 ns), ending with an almost monoexponential decay (~2.17 ns). Quantum yields for the isomerization photoreaction (φ(R)) were also obtained for indigo and DBMNI with values of 0.9 and 0.007, respectively. The results are rationalized in terms of a photoisomerization (conversion) reaction occurring in the first excited singlet state of trans to cis forms of leuco indigo., (© 2012 American Chemical Society)

Flavylium compounds are versatile molecules that comprise anthocyanins, the ubiquitous colorants used by Nature to confer colour to most flowers and fruits. They have found a wide range of applications in human technology, from the millenary colour paints described by the Roman architect Vitruvius, to their use as food additives, combining colour and antioxidant effects, and even as light absorbers in solar cells aiming at a greener solar energy conversion. Their rich complexity derives in part from their ability to switch between a variety of species (flavylium cations, neutral quinoidal bases, hemiketals and chalcones, and negatively charged phenolates) by means of external stimuli, such as pH, temperature and light. This critical review describes (i) the historical advancements in the understanding of the equilibria of their chemical reaction networks; (ii) their thermodynamics and kinetics; (iii) the mechanisms underlying their colour development, such as co-pigmentation and host-guest interactions; (iv) the photophysics and photochemistry that lead to photochromism; and (v) applications in solar cells, models for optical memories, photochromic soft materials such as ionic liquids and gels, and their properties in solid state materials (274 references).

The introduction of an ester group in the flavylium core allowed the reversible conversion between two different flavylium compounds each one exhibiting its own reaction network. An unidirectional switching cycle between 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium and 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium was achieved by means of alternate acid and base stimuli. Addition of base to a methanolic solution of the ester derivative gives rise to the trans-chalcone of the parent carboxylic acid, which upon acidification of the solution forms the respective flavylium cation. This species esterifies under very acidic conditions to restore the original methyl ester derivative. The chemical reaction networks of both compounds were fully characterized from their thermodynamic and kinetic aspects, by a series of pH jumps followed by UV-vis absorption and emission spectroscopy, stopped flow and (1)H NMR. The crystal structure of the trans-chalcone of the ester derivative was unveiled showing a supramolecular structure involving hydrogen bonding.

A variety of 2-hydroxy aldehydes on reaction with 3-butenoic acid afford in a one-pot reaction the corresponding 3-vinylcoumarins. As expected, extension of the delocalized π-electron system accomplished by Heck coupling reactions with aryl halides results in an increased fluorescence of the compounds whose applicability is yet to be established., (© 2011 American Chemical Society)

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., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In the present work, the proton-transfer reactions of the methylpyranomalvidin-3-O-glucoside pigment in water with different pH values was studied by NMR and UV-visible spectroscopies. The results showed four equilibrium forms: the methylpyranomalvidin-3-O-glucoside cation, the neutral quinoidal base, the respective anionic quinoidal base, and a dianionic base unprotonated at the methyl group. According to the NMR data, it seems that for methylpyranomalvidin-3-O-glucoside besides the acid-base equilibrium between the pyranoflavylium cation and the neutral quinoidal base, a new species is formed at pD 4.88-6.10. This is corroborated by the appearance of a new set of signals in the NMR spectrum that may be assigned to the formation of hemiketal/cis-chalcone species to a small extent. The two ionization constants (pK(a1) and pK(a2)) obtained by both methods (NMR and UV-visible) for methylpyranomalvidin-3-O-glucoside are in agreement (pK(a1) = 5.17 ± 0.03; pK(a2) = 8.85 ± 0.08; and pK(a1) = 4.57 ± 0.07; pK(a2) = 8.23 ± 0.04 obtained by NMR and UV-visible spectroscopies, respectively). Moreover, the fully dianionic unprotonated form (at the methyl group) of the methylpyranomalvidin-3-O-glucoside is converted slowly into a new structure that displays a yellow color at basic pH. On the basis of the results obtained through LC-MS and NMR, the proposed structure was found to correspond to the flavonol syringetin-3-glucoside.