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Publisher Copyright: © 2022 American Chemical Society The kinetics and thermodynamics of the pH-dependent reversible and irreversible processes leading to color fading of pelargonidin-3-O-glucoside, peonidin-3-O-glucoside, malvidin-3-O-glucoside, and cyanidin-3-O-glucoside dyes in aqueous solutions are reported. Following the addition of base to the flavylium cation, the quinoidal bases disappear by three distinct steps: (i) in an acidic medium by a biexponential process, in which the faster step is controlled by the hydration reaction and the slower one by cis–trans isomerization; the degradation process occurs essentially from the anionic quinoidal base; (ii) in a basic medium (pH > 9.5), in which the disappearance of the anionic bases is monoexponential, with the rate proportional to the hydroxyl concentration (hydroxyl attack), leading to anionic chalcones (cis and trans) at equilibrium─the slower degradation step occurs from the di- and trianionic chalcones; and (iii) in the pH region circa 7.7 < pH < 9.5, in which hydration and hydroxyl attacks are much slower than anionic quinoidal base degradation (which is the rate-controlling step) and the equilibrium cannot be attained. authorsversion published

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

Anthocyanins co-pigmentation models with application on 1:1 complexes were revisited, and their limitations were critically commented. The flavylium multistate of species is dramatically simplified to a single acid-base equilibrium between flavylium cation and its conjugated base CB, equal to the sum of quinoidal base, hemiketal, and

Funding Information: This work was supported by the Associate Laboratory for Green Chemistry‐LAQV which is financed by national funds from FCT/MCTES (UIDB/50006/2020). FCT/MCTES is also acknowledged for supporting the National Portuguese NMR Network (ROTEIRO/0031/2013‐PINFRA/22161/2016, cofinanced by FEDER through COMPETE 2020, POCI, PORL, and FCT through PIDDAC) and for grants PTDC/QUI‐QFI/30951/2017 and 2020.07313.BD (A.S.). China Scholarship Council (S.Y.) is acknowledged. 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 (K1:1 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 kobs=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. authorsversion published

While identified by the respective flavylium cation, anthocyanins are much more than this molecule. The flavylium cation (generally appearing only at very acidic pH values) is one of the molecules of a complex sequence of pH dependent molecular species reversibly interconnected by different chemical reactions. These species include the red flavylium cation, purple quinoidal base and blue or bluish anionic quinoidal bases. At the common pH of the vacuoles of simpler anthocyanins, the red flavylium cation is present only at very acidic pH values and at moderately acidic pHs there is no significant colour of the purple quinoidal base. Moreover, the blue or bluish anionic quinoidal base appearing around neutral pH values is not stable. Intermolecular (copigmentation) and intramolecular (in acylated anthocyanins) interactions increase the colour hue and yield bathochromic shifts in the absorption bands, permitting to extend the pH domain of the flavylium cation and increase the mole fraction of the quinoidal bases. Metal complexation is another strategy. In particular, the Al3+ cation plays an essential role in the blue colour of hydrangea. The most sophisticated structures are however the metaloanthocyanins, such as the one that gives the blue colour of commelina communis, constituted of six anthocyanins, six flavanones and two metals. In this work we discuss how physical chemical tools are indispensable to account for the chemical behaviour of these complex systems. The experimental procedures and the equations needed to calculate all equilibrium constants of anthocyanins and the consequent pH dependent mole fraction distributions in the absence or presence of copigments are described in detail. Reverse pH jumps monitored by stopped flow have been shown to be an indispensable tool to calculate these parameters.

Herein, a series of 3-O-glucosilated pyranoanthocyanin derivatives were synthesized following a bio-mimetic approach. The compounds were studied by means of UV–Vis spectroscopy in order to establish their chemical equilibria and determine the stability of the colored species over the pH range. The resistance of pyranoanthocyanins to hydration grants this family of compounds, closely related to anthocyanins, increased color stability. Finally, the synthetized compounds were successfully applied as light harvesters in DSSCs and relations between dye structure and cell efficiency were found. The influence of different linker units (carboxylate, catechol) and electron-donor groups (methoxy, hydroxyl, methyl), as well as the presence of sugar moieties have impact in the various parameters related to DSSC efficiency. An overall efficiency of 1.57% was obtained for the best performing compound, methylpyranocyanidin-3-O-glucoside. The impact of using different additives was also studied and the results compared with other families.

The thermodynamic and kinetic behavior of 4-(2,4-dihydroxybenzylidene)-6-hydroxy-1,2,3,4-tetrahydroxanthylium chloride was studied in detail. Similarly to anthocyanins, this compound generates a multistate of species that are reversibly interconverted by pH stimuli. The multistate system was studied by NMR and UV–visible (conventional spectroscopy and stopped-flow techniques). A theoretical approach adapted from the one reported for anthocyanins and related compounds was used to rationalize the complex multistate. In particular, the thermodynamic diagram where all the species are positioned was extended to the ionized species. Cis and trans isomers of flavylium cation, quinoidal bases, and ionized chalcone at extremely high pH values were identified. The multistate is dominated in the range 5

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. publishersversion published

UIDB/50006/2020 UIDP/50006/2020 PTDC/QUI-OUT/29013/2017 SFRH/BD/143309/2019 SFRH/BD/146549/2019 MEX/Ref. 288188 IF/00225/2015 The multistate of species originated by the compound 5-hydroxy-4′-dimethylaminoflavylium was studied in the presence of sodium dodecyl sulfate micelles 0.1 M. A series of pH jumps obtained by addition of base to the equilibrated solutions of the flavylium cation results in a metastable equilibrium between the flavylium cation and the quinoidal base with pKa = 7.6. In neutral to moderately basic medium, only at 60 °C and after 5 days a new distribution involving the flavylium cation and trans-chalcones is achieved. On the other hand, the anionic trans-chalcone can be formed in few minutes in very basic medium at room temperature and from this, another meta-stable state constituted by protonated, neutral and anionic trans-chalcones, pKCt+/Ct = 2.6 and pKCt/Ct- = 10.7 is reached. The equilibrium is apparently attained between flavylium cation, neutral and anionic trans-chalcones respectively for pKAH+/Ct = 4.1 and pKCt/Ct- = 10.7. Irradiation of the neutral (stable or metastable) and protonated (meta-stable) trans-chalcones yields to the appearance of the quinoidal base or the flavylium cation in an extended pH region 1 < pH < 10. The peculiar properties of this flavylium based photochromic system are explained by means of an energy level diagram where cis-chalcone and hemiketal lay above the quinoidal base. authorsversion published

UIDB/50006/2020 PTDC/QUI-OUT/29013/2017 MEX/Ref. 288188 SFRH/BD/143309/2019 IF/00225/2015 IF/01355/2014 CEECIND/00466/2017 REQUIMTE/EEC2018/PTDC/QUI-OUT/29925/2017 The aggregation of malvidin 3-O-(6-O-p-coumaroyl) glucoside flavylium cation (OeninCoum) in water is reported. Intra and intermolecular interactions of OeninCoum were characterized by UV–vis absorption, 1H NMR and circular dichroism (CD). Theoretical calculations were also performed to describe the intramolecular interactions. At higher temperature (>60 °C) the intermolecular aggregates are disrupted but CD signal attributed to an intramolecular interaction remains. Moreover, the kinetic and thermodynamic data for Oenin and OeninCoum at 50 °C and 25 °C shows that the increase of temperature stabilizes the quinoidal base of this later. authorsversion published

UID/QUI/50006/2019 PTDC/QEQ-QFI/1971/2014 PD/BD/135087/2017 SFRH/BD/136556/2018 SFRH/BD/143309/2019 IF/00225/2015 DL57/2016 Program Contract (HC). UID/CTM/50025/2019 PTDC/CTM-ENE/5125/2014 CNPq 444061/2018-5 Universal grant 408181/2016-3 The pyranoanthocyanins present in red wine display great potential as photosensitizers in bio-inspired Dye-Sensitized Solar Cells (DSSCs). Following a biomimetic approach, a series of amino-π-bridge-pyranoanthocyanin derivatives were employed as dye sensitizers in DSSCs. The dimethylamine group was selected to take advantage of its electron-donor character and the possibility of ‘dual-mode anchoring’ ([sbnd]OH vs. dimethylamino) to titanium dioxide. The increase in π-conjugation via insertion of C[dbnd]C bonds affected molecule flexibility, electron-donor ability and the pH-dependent equilibria of the pyranoanthocyanin derivatives. The current vs. potential properties of photoanodes using these dyes pointed to essential features of the relationship between power conversion efficiency and dye structure. These included the influences of the dimethylamine group, of π-conjugation and of substitution in ring B on the adsorption of the dyes to TiO2 and on the overall performance of the DSSCs prepared from them with and without added acid. An overall efficiency of 2.55% was obtained for the best performing compound, 4-(dimethylamino)-cinnamyl-pyranocyanidin-3-O-glucoside (JO3), which consolidates the importance of this family of compounds as potential dye-sensitizers for DSSC applications. authorsversion published

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 1H 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.

UID/QUI/50006/2019 PTDC/LLT-EGL/30984/2017 IF/00225/2015 IF/01355/2014 REQUIMTE/EEC2018/PTDC/QUI-OUT/29925/2017 PTDC/QUI-OUT/29925/2017 PD/BD/105895/2014 The molecular structure of the medieval watercolor known as folium has finally been solved in the 21st century. The interdisciplinary approach taken was the key to producing extracts that had been prepared following medieval instructions, and shows the blue/purple chromophore as the major dye in Chrozophora tinctoria fruits (shell). A multi-analytical characterization of its structure was made using HPLC-DAD-MS, GC-MS, NMR (1H, 13C, COSY, HSQC, HMBC, INADEQUATE), and computational studies. The results demonstrate that the blue compound corresponds to 6'-hydroxy-4,4'-dimethoxy-1,1'-dimethyl-5'-{[3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yl] oxy}-[3,3'-bipyridine]-2,2',5,6(1H,1'H)-tetraone, a hermidin derivative, which we named chrozophoridin. Experimental data and computational modeling studies show that this mono-glycosylated dimer is represented by two stable isomers (atropisomers). This is an indispensable piece of knowledge for the characterization of this medieval dye in works of art such as medieval manuscript illuminations and for testing its stability and contributes to the preservation of our cultural heritage. publishersversion published

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.

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'

In this work, two bipyridinium di-cations based on [(allyl)2bpy]2+ and [(C10)2bpy]2+ combined with halogenate (I or Br), bistriflimide ([NTf2]) and dicyanamide [DCA] anions have been prepared and electrochemical characterized. These salts were studied using a conventional electrolyte (TBAP in acetonitrile) and alternative electrolyte based DES (LiTfO:6EG) for 3- and 2- electrodes configuration devices. In the 2-configuration device, [(allyl)2bpy][DCA]2 and [(C10)2bpy][DCA]2 showed different radical cation colorations particularly green and blue colors with different recovery times to original ones. In the case of [(C10)2bpy]I2 a blue and a red-violet colorations attributed to the electrogenerated radical cation and the dimer respectively was obtained in agreement with previous literature. In parallel, bipyridinium based DES by suitable combination between [(C10)2bpy]X2 and [(allyl)2bpy]X2 (X = Br, NTf2, DCA) with several hydrogen bond donors in 2-electrode configuration devices have been also studied. DES based on [(allyl)2bpy][NTf2]2:PEG200 and [(allyl)2bpy][DCA]2:PEG200 presented different coloration grades from pale brown to green taking longer times to recover the original color. The selected DES as alternative electrolytes or based bipyridinium derivatives can be promissory materials for efficient electrochromic devices.

The interaction of malvidin-3-O-glucoside (1) 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 1 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 (A) equilibrium, the AH+ form being more stabilized than A (pKa1 = 4.4 ± 0.1) compared with 1 in the absence of the lignosulfonate (pKa1 = 3.9 ± 0.1). Furthermore, comparing the hydration (kh = 0.028 s–1) and dehydration (k–h = 40 M–1 s–1) processes of 1 in the presence of the lignosulfonate with the processes of 1 (kh = 0.12 s–1 and k–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.

© Copyright 2018 American Chemical Society. The design, synthesis and photoluminescence characterization of sulfide cluster-based zeolitic materials with the general formula Na8[SiAlO4]6SxCl2-2x is presented in this report. The employed experimental conditions, which included the thermal treatment of sodium Linde type A (Na-LTA) zeolites in the presence of Na2SO4 and NaCl, yielded highly photoluminescent sulfur-zeolites with remarkably large Stokes shifts. An emission band at 650 nm for samples with low sulfur contents is observed when excited at 380 nm, which is fully consistent with S2- clusters as light-emitting centers. These orange emitting materials display external quantum efficiencies (EQEs) reaching 53% with peculiar high temperature photoluminescence stability up to 450 °C. For higher S/Cl molar ratios, a gradual conversion to near-infrared (NIR) light-emitting centers is observed with a maximum at 780 nm. This novel NIR luminescent species with EQEs of about 10% are attributed to polysulfide clusters (tentatively S42- species). Given the high EQE values, large Stokes shifts, and thermal stability, the materials presented in this report can find applications in light down-conversion systems or as phosphors in lighting devices. ispartof: JOURNAL OF PHYSICAL CHEMISTRY C vol:122 issue:26 pages:14761-14770 status: published

All equilibrium and rate constants of heavenly blue anthocyanin (HBA 1) as well as the derivatives with two (HBA 2) or none (HBA 3) acylated units were determined. The three acylated units of the sugar in position 3 of the peonidin chromophore of HBA 1 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 (kh) and increases 8.8-fold the dehydration reaction (k–h), when compared with the peonidin chromophore HBA 3. The conjugation of these two rates lowers 308-fold the hydration equilibrium constant (Kh), corresponding to a raise of the energy level of the hemiketal by 14.2 kJ mol–1. Conversely, the pKa of the quinoidal base in HBA 1 is only slightly stabilized in comparison with that of HBA 2 and HBA 3. The energy level of hemiketal increases with the number of acylated units, but the ...

The fact that many copigments such as cinnamic acids exhibit an acid-base equilibrium at moderately acidic pH values have not been taken in consideration in the copigmentation models and respective equations. In this work, the copigmentation constants of malvidin-3-O-glucoside and sinapic acid (pKa = 4.5) is reported. The flavylium cation (AH+) in moderately acid medium (pH ≤ 6) can be considered a monoprotic acid in equilibrium with its conjugated base (CB) equal to the sum of the concentrations of quinoidal base (A), hemiketal (B), cis and trans chalcones (Cc and Ct), with equilibrium constant K'a. Based on this behavior a new mathematical expression to account for the copigmentation of AH+ and CB was deduced. The system was also studied by a series of reverse pH jumps monitored by stopped flow and a previously reported procedure was extended to consider the acid-base equilibrium of the copigment. The copigmentation constant of the flavylium cation with the acidic form of the copigment, CP, is 243 M−1, and no copigmentation with the basic form of the copigment, CP(−), was observed. This was explained by the very small superposition of these two species in the pH domain. The copigmentation constants of CB with the acidic and basic forms of the copigment, calculated from the UV–Vis measurements are respectively KCBCP = 53 M−1 and KCBCP(−) = 78 M−1. The reverse pH jumps monitored by stopped flow allowed the calculation of the copigmentation constants of the acidic form of the copigment (CP) with quinoidal base (A), hemiketal (B) and cis-chalcone (Cc), respectively KACP = 531 M−1, KBCP = 17, KCcCP = 41 M−1 as well as with the basic form of the copigment KACP(−) = 196 M−1, KBCP(−) = 79, KCcCP(−) = 94 M−1. The reverse pH jumps followed by a standard spectrophotometer allows for determination of KCtCP = 55 M−1 and KCcCP(−) = 80 M−1 and KCBCP = 54 M−1 and KCBCP(−)=89 M−1. The estimated error for all copigmentation constants is circa 10%.

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