Synthetic flavylium cations have been extensively employed as surrogates for unravelling the pH-dependent chemistry of naturally occurring anthocyanin plant pigments, as well as for the development of strategies for the stabilization of their color in practical applications. Although a wide variety of synthetic flavylium cations were previously found to exhibit detectable phosphorescence in a rigid trifluoroacetic acid (TFA)-acidified isopropanol glass at 77 K, the question remained as to the possibility of significant formation of triplet states in room temperature fluid solution. Of the 14 different flavylium cations investigated here, only four sensitized the formation of significant yields of singlet oxygen in TFA-acidified acetonitrile, while the other ten produced no detectable singlet oxygen. Correlation of the results with the chemical structures and the corresponding photophysical properties of the lowest energy excited singlet state of the compounds suggests that the formation of excited triplet states of natural anthocyanins is probably negligible in vivo in plants.

Sodium-ion batteries (SIBs) are emerging as promising energy storage devices due to the widespread availability of low-cost sodium and their electrochemical mechanism, which bears similarities to lithium-ion batteries (LIBs). The promising potential of Ni-rich layered oxides, coupled with the sense of d & eacute;j & agrave; vu from the advancements seen in LIBs, strongly suggests the practical application of these compounds in SIBs. For the first time, the recent advancements in cathode materials for SIBs are highlighted, focusing on Ni-rich layered transition metal oxides such as NaNixCoyMnzO2 (Na-NCM), NaNixCoyAlzO2 (Na-NCA), Na-NixFeyMnzO2 (Na-NFM) and NaNixCoyMnz[TM]1-x-y-zO2 (Na-NCM[TM], TM = other transition metal) with x >= 0.60 (x + y + z = 1). These materials offer practical synthesis methods, impressive specific capacity, and environmental friendliness. However, challenges remain, including energy density and cycle life. Strategies to engineer high-energy-density SIBs are being pursued, notably developing Ni-rich layered oxide cathode materials. In conclusion, an outlook that assesses the strengths and limitations of this field is pointed out, providing valuable insights to steer future research efforts toward enhancing Ni-rich cathodes, thereby paving the way for further advancements. This Highlight explores advancements in Ni-rich cathode materials for sodium-ion batteries, which offer practical synthesis methods, high specific capacity, and environmental benefits while addressing energy density and cycle life challenges.

Ni-rich Co-poor layered oxides exhibit great promise as battery cathode materials, offering high energy density and working voltage. However, their commercial viability is hindered by suboptimal structural stability, cycling performance, and thermal safety. Recently, various strategies have been used to address the chemical/mechanical stability issues of Ni-rich Co-low cathode materials, aiming at sustainable and low-cost energy storage devices. This review seeks to explore comprehensively, for the first time, the recent strides in designing Super Ni-rich Co-poor cathode materials. The specific focus lies on LiNixCoyMnzO2 (NCM), LiNixCoyAlzO2 (NCA), and LiNixCoyMnzAl1-x-y-zO2 (NCMA) with x ≥ 0.85. Our discussion encompasses emerging trends in the development of these materials. Additionally, we outline effective strategies for the deliberate design of Super Ni-rich Co-poor layered oxide cathodes. To conclude, we provide an outlook, evaluating the strengths and limitations of this field and offering insights to guide future directions for the enhancement of Super Ni-rich cathodes, paving the way for further advancements.

Ni-rich Co-poor layered oxides exhibit great promise as battery cathode materials, offering high energy density and working voltage. However, their commercial viability is hindered by suboptimal structural stability, cycling performance, and thermal safety. Recently, various strategies have been used to address the chemical/mechanical stability issues of Ni-rich Co-low cathode materials, aiming at sustainable and low-cost energy storage devices. This review seeks to explore comprehensively, for the first time, the recent strides in designing Super Ni-rich Co-poor cathode materials. The specific focus lies on LiNixCoyMnzO2 (NCM), LiNixCoyAlzO2 (NCA), and LiNixCoyMnzAl1-x-y-zO2 (NCMA) with x ≥ 0.85. Our discussion encompasses emerging trends in the development of these materials. Additionally, we outline effective strategies for the deliberate design of Super Ni-rich Co-poor layered oxide cathodes. To conclude, we provide an outlook, evaluating the strengths and limitations of this field and offering insights to guide future directions for the enhancement of Super Ni-rich cathodes, paving the way for further advancements.

Pyranoflavylium cations are synthetic analogues of pyranoanthocyanins, the much more color-stable compounds that are formed spontaneously from grape anthocyanins during the maturation of red wines. In the present work, our studies of the photophysical properties of pyranoanthocyanin analogues are extended to include nine pyranoflavylium cations substituted with one or two bromo and/or iodo heavy atoms. The room temperature fluorescence, 77 K fluorescence and phosphorescence, triplet formation in solution, and sensitized singlet oxygen formation, with excited state acidity suppressed by the addition of trifluoroacetic acid, are compared to those of similar pyranoflavylium cations that do not contain a heavy atom. Heavy atom effects on the photophysics of the S1 state of pyranoflavylium cations are found to be relatively small, which is attributed to the nodal properties of the orbitals involved, which prevent effective mixing of the spin-orbit coupling on the heavy atoms into the excited singlet state, S1, of the pyranoflavylium chromophore. Heavy atom effects on the phosphorescence of these heavy atom-substituted pyranoflavylium cations at 77 K are somewhat larger, consistent with a spin-orbit coupling-induced increase in the radiative rate constant for phosphorescence, as are the triplet-sensitized singlet oxygen formation quantum yields in fluid solution.

The use of organic fillers such as recycled polyethylene terephthalate (PET) represents an interesting environmentally friendly alternative for reducing the costs of production of flexible polyurethane foams. By using an appropriate siloxane-based SiOC surfactant rather than a conventional SiC surfactant, flexible polyurethane foams containing significant quantities of micronized recycled PET filler can be prepared without compromising the relevant physical properties required of mattress-quality polyurethane foams by ABNT standards, such as density, resilience, comfort and fatigue. In the present work, it is demonstrated that, by employing micronized PET fluorescently labelled with a fluorescein chromophore, the three-dimensional distribution of the PET filler throughout the foam block can be visualized. Differences in the distribution of the fluorescent PET are found in foams produced with a conventional SiC surfactant compared to those produced with the SiOC surfactant, consistent with the differences in physical properties of the foams. Finally, examination of the cellular structure of the foam points to predominant localization of the fluorescence in the tetrahedral Plateau borders of the foam in the resultant PET-containing polyurethane foams due to the heat generated by the polymerization reaction and the capillary and/or Marangoni flows that accompany the expansion and thinning of the foam cells during the polymerization.

Hydroxypyranoflavylium (HPF) cations are synthetic analogs possessing the same basic chromophore as the pyranoanthocyanins that form during the maturation of red wine. HPF cations absorb strongly in the visible spectral region, and most are fluorescent, triplet‐sensitize singlet oxygen formation in solution and are strong photooxidants, properties that are desirable in a sensitizer for photodynamic therapy (PDT). The results of this study demonstrate that several simple HPF dyes can indeed function as PDT sensitizers. Of the eight HPF cations investigated in this work, four were phototoxic to a human cervical adenocarcinoma cell line (HeLa) at the 1–10 μmol dm−3 level, while only one of the eight compounds showed noticeable cytotoxicity in the dark. Neither a Type I nor a Type II mechanism can adequately rationalize the differences in phototoxicity of the compounds. Colocalization experiments with the most phototoxic compound demonstrated the affinity of the dye for both the mitochondria and lysosomes of HeLa cells. The fact that relatively modest structural differences, e.g., the exchange of an electron‐donating substituent for an electron‐withdrawing substituent, can cause profound differences in the phototoxicity, together with the relatively facile synthesis of substituted HPF cations, makes them interesting candidates for further evaluation as PDT sensitizers.

Fluorescence spectra and lifetimes were determined for 16 synthetic flavylium cation analogues of anthocyanin plant pigments in dry acetonitrile acidified with trifluoroacetic acid (TFA). Phosphorescence was also observed from the lowest excited triplet state for all of the flavylium cations at 77 K in a rigid TFA-acidified isopropanol glass. The fluorescence quantum yields and lifetimes depend in a systematic manner on the nature and position of the substituents on the flavylium chromophore and three specific substitution patterns associated with significant decreases in the fluorescence quantum yield were identified. A 4′-bromo or 4′-iodo substituent in the B-ring of the flavylium cation produced a small but normal heavy-atom effect, reducing the fluorescence quantum yield and the phosphorescence lifetime relative to analogues without the halogen atom. In contrast, three flavylium cations with a 3′-bromo substituent exhibited an “inverse” heavy atom effect, i.e., an increase in the fluorescence quantum yield rather than a decrease, which was rationalized on the basis of the nodal properties of the natural transition orbitals (NTOs) involved in the S0→S1 radiative transition.

Natural dyes and pigments offer incomparable diversity of structures and functionalities, making them an excellent source of inspiration for the design and development of synthetic chromophores with a myriad of emerging properties. Formed during maturation of red wines, pyranoanthocyanins are electron-deficient cationic pyranoflavylium dyes with broad absorption in the visible spectral region and pronounced chemical and photostability. Herein, we survey the optical and electrochemical properties of synthetic pyranoflavylium dyes functionalized with different electron-donating and electron-withdrawing groups, which vary their reduction potentials over a range of about 400 mV. Despite their highly electron-deficient cores, the exploration of pyranoflavyliums as photosensitizers has been limited to the "classical" n-type dye-sensitized solar cells (DSSCs) where they act as electron donors. In light of their electrochemical and spectroscopic properties, however, these biomimetic synthetic dyes should prove to be immensely beneficial as chromophores in p-type DSSCs, where their ability to act as photooxidants, along with their pronounced photostability, can benefit key advances in solar-energy science and engineering.

Pyranoflavylium cations are synthetic analogues of pyranoanthocyanin, colored pigments, formed from grape anthocyanins during the maturation of red wines. Studies of a series of monosubstituted pyranoflavylium cations, ranging from methoxy (PF+-OMe) to cyano (PF+-CN), have shown that they display fluorescence and form triplet states that sensitize singlet oxygen formation in acidified acetonitrile. In alcohol-water mixtures, they behave as photoacids, undergoing adiabatic excited state proton transfer (ESPT) to water on a picosecond timescale, as confirmed in this report by femtosecond pump-probe spectroscopy. In contrast, the corresponding dimethylamino substituted pyranoflavylium cation (PF+-NMe2) is virtually non-fluorescent under the same conditions and exhibits a long-wavelength absorption band that has been attributed to a charge-transfer (CT) transition. Indeed, pump-probe spectroscopy of PF+-NMe2 in acidified acetonitrile shows ultrafast (<1 ps) formation of a CT state that decays back to the ground state with a 12–13 ps lifetime. In acidified methanol, the initial Franck-Condon CT state (ca. 3 ps lifetime) converts to a 13 ps lifetime CT state analogous to that in acetonitrile. In 50:50 ethanol:water and 30:70 methanol:water mixtures, PF+-NMe2 exhibits a short-lived (3–8 ps) initial CT state, an intermediate lifetime (30 ps) CT state and a much longer lived (130 ps) species attributed to a twisted intramolecular CT state. Thus, in addition to demonstrating that the photophysics of PF+-NMe2 is dominated by CT rather than ESPT, pump-probe spectroscopy provides details of the solvent-dependent dynamics of the CT decay pathways.

Anthocyanins are particularly noteworthy natural plant pigments that are responsible for the red, purple and blue color of many fruits, vegetables and flowers. Dietary anthocyanins have potential health benefits, but their color loss due to pH-dependent chemistry limits widespread application as coloring agents in food and consumer products. The ability to predict the color of synthetic analogues of natural pigments is crucial to the rational development of new bioinspired dyes and pigments with tailored colors and properties with lower potential for environmental and health concerns than those of conventional synthetic pigments. In this work, we employ the ab initio second-order algebraic diagrammatic construction, ADC(2), level of theory to calculate the absorption spectra of 26 synthetic flavylium cation analogues of anthocyanins. In general, the theoretical absorption spectra compare favorably with the corresponding experimental absorption spectra in an aqueous solution. The excitation energies, the charge transfer character, and the oscillator strengths of the electronic transitions provide insight into the effects of the type and position of the substituents on the excited singlet and triplet states of these compounds. The results show that the theoretical framework employed constitutes a reliable tool for obtaining a deeper understanding of the properties of bioinspired synthetic analogues of anthocyanin pigments.

The concept of photosensitization had its origins in the serendipitous discovery in the 19th century that dye molecules could enhance the spectral range of photographic emulsions. In the latter half of the 20th century, photosensitization reached maturity as a mechanistic photochemical tool for the identification and quantification of the role of triplet states as intermediates in organic photoreactions in solution. The photophysical properties desirable in a photosensitizer and the kinetic aspects of the diffusional encounter of sensitizer and energy acceptor are outlined, together with the role of spin selection rules in photosensitization processes, with emphasis on the photosensitized formation of singlet oxygen, the key photophysical step in Type II photosensitization.

The ubiquitous presence of nitro-derivatives of polycyclic aromatic hydrocarbons like pyrene in the environment is a source of preoccupation given the fact that many of them have been shown to be toxic, mutagenic and/or carcinogenic. An understanding of their photophysics and photochemistry can provide insight into the potential for their sunlight-induced photodegradation in the environment. In the present work, ab initio quantum chemical methods (MP2/def2-TZVP and ADC(2)/def2-TZVP) were employed to calculate the geometries of the three mononitro pyrenes, three of the dinitropyrenes and a trinitropyrene in the ground state and in the lowest excited singlet and triplet states. Absorption spectra predicted from the vertical excitation energies and oscillator strengths of the first 10 excited singlet states (ADC(2)/def2-TZVP/COSMO acetonitrile) compare favorably with the experimental spectra in acetonitrile and adiabatic triplet energies with values derived from phosphorescence spectra. Except for 2-nitropyrene, which was predicted to be planar in the ground (S-0) and lowest excited singlet (S-1) and triplet (T-1) states, the nitro groups of the other compounds were not in the plane of the ring in S-0 or T-1, but one of the nitro groups was predicted to become coplanar with the ring in the optimized geometry of S-1. The theoretical results are discussed in the context of their overall consistency with the experimentally observed photophysical properties of these compounds.

The two hydroxyl groups of the 4′,7-dihydroxyfurano-3,2′-flavylium cation (1), a synthetic analog of the aurone pigments of plants, have been shown to have different relative acidities in the ground state (S0) and the lowest excited singlet state (S1). In the ground state, the 4′-OH group is slightly more acidic, while in the excited state, the molecule is strongly photoacidic and deprotonation occurs preferentially from the 7-OH group. In order to compare the relative acidities of these two OH groups via quantum chemical methodology, a common reference state was employed in which an explicit water molecule was hydrogen-bonded to each of the OH groups of 1. The relative acidities of the two OH groups were then inferred from the differential change in energy along the coordinate for proton transfer to the explicit water molecule via time-dependent density functional calculations (B3-LYP with Grimme’s D3 dispersion correction; TZVP basis set; and PCM to simulate an aqueous environment). The calculated acidity changes confirm the experimentally observed inversion in the relative acidities between S0 and S1. The enhanced photoacidity of S1 was also mirrored in the natural transition orbitals and the decrease in the negative change on the oxygen atoms of the OH groups. Employing a common reference state with an explicit water as the proton acceptor should thus serve as a convenient strategy for exploring the relative ground- and excited-state acidities of the OH groups of natural or synthetic dyes, especially when the values are not readily accessible through experiment.

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-pi-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' (-OH vs. dimethylamino) to titanium dioxide. The increase in pi-conjugation via insertion of C=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 pi-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.

There is increasing interest in using natural colorants like anthocyanins in cosmetics, food and pharmaceuticals as replacements for synthetic colorants. During the maturation of red wines, the anthocyanin pigments contained in grapes are transformed via reaction with copigments and metabolic products into pyranoanthocyanins, responsible in part for the final color of the wine. In order to understand structural effects on the absorption spectra of pyranoanthocyanins, the calculated excited state energies and spectroscopic states of a series of substitued pyranoflavylium cation analogs of pyranoanthocyanins have been compared to experimental spectroscopic data for these compounds. The vertical excitation energies, calculated by using the ADC(2) approach, gave excellent agreement with the experimental UV-Vis spectra and the nature of the lowest excited state correlates with the observed photophysical behavior in solution. The present results thus provide a basis for the design of new pyranoflavylium chromophores with the desired colors and photophysics, as well as for understanding the analogous properties of natural pyrano-anthocyanin pigments in red wine.

Quenching of the fluorescence of pyrene by AuCl4- ion was investigated in aqueous micellar solutions of hexadecyltrimethylammonium chloride (CTAC) and 3-(N,N-dimethylmyristylammonio)propanesulfonate (SB3-14) in the presence of 0.010 mol L--(1) HCl. Because AuCl4- is excluded from anionic sodium dodecyl sulfate (SDS) micelles due to electrostatic repulsion effects, no quenching of pyrene fluorescence by AuCl4- was observed in micellar SDS. Absorption spectral shifts show that AuCl4- binds strongly to both CTAC and SB3-14 micelles and time-resolved fluorescence results indicate that the quenching of the fluorescence of micelle-solubilized pyrene by AuCl4- is dynamic in nature. Fits of the pyrene fluorescence decays with the Tachiya model for micelle quenching in the limit of slow exchange of quenchers between the aqueous phase and micelles provided estimates of the micelle aggregation numbers that are slightly smaller than those determined with other quenchers, and intrarnicellar quenching rates 3-4 times higher than those observed with known collisional quenchers. The strong interaction of AuCl4- with SB3-14 and CTAC led to the stabilization of gold nanoparticles, highlighting the use of zwitterionic surfactants for the synthesis of stable and biocompatible gold nanoparticles. (C) 2020 Elsevier B.V. All rights reserved.

The color of mature red wines is due in large part to the chemical transformation of grape anthocyanins into pyranoanthocyanins. Given the difficulties of isolation and purification of pyranoanthocyanins from wines, experimental investigations have focused on pyranoflavylium cations, synthetic analogs that contain the basic chromophoric moiety of pyranoanthocyanins. Quantum chemical methodologies have been extensively employed to predict the physical, spectroscopic and photophysical properties of anthocyanins and, more recently, pyranoflavylium cations. In the present work, we employ TD-DFT with the B3-LYP functional and the def2-TZVP basis set, combined with estimation of solvation free energies via COSMO, to estimate a priori the pK a values of the ground state and the lowest excited singlet and triplet states of a series of seven substituted hydroxypyranoflavylium cations. For the ground state and the first excited singlet state, the quantum chemical results compare favorably to experimental values for most of these compounds. Although there are currently no experimental data for the lowest excited triplet state, the acidity is predicted to be more similar to that of the ground state than to the excited singlet state, as is generally found experimentally for photoacids.

Flavylium cations are synthetic analogues of anthocyanins, the natural plant pigments that are responsible for the majority of the red, blue, and purple colors of flowers, fruits, and leaves. Unlike anthocyanins, the properties and reactivity of flavylium cations can be manipulated by the nature and position of substituents on the flavylium cation chromophore. Currently, the most promising strategies for stabilizing the color of anthocyanins and flavylium cations appear to be to intercalate and/or adsorb them on solid surfaces and/or in confined spaces. We report here that hybrid pigments with improved thermal stability, fluorescence, and attractive colors are produced by the cation-exchange-mediated adsorption of flavylium cations (FL) on two synthetic clays, the mica-montmorillonite SYn-1, and the laponite SYnL-1. Compared to the FL/SYn-1 hybrid pigments, the FL/SYnL-1 pigments exhibited improved thermal stability as judged by color retention, better preferential adsorption of the cationic form of FL1 at neutral to mildly basic pH (pH 7-8), and lower susceptibility to color changes at pH 10. Although both clays adsorb the cationic form on their external surfaces, SYnL-1 gave more evidence of adsorption in the interlayer regions of the clay. This interlayer adsorption appears to be the contributing factor to the better properties of the FL/SYnL-1 hybrid pigments, pointing to this clay to be a promising inorganic matrix for the development of brightly colored, thermally more stable hybrid pigments based on cationic analogues of natural plant pigments.

Aqueous micellar solutions of sulfobetaine surfactants provide a simple physical–chemical system for investigating the origin and consequences of the specific interactions of anions with a model zwitterionic interface. Studies of ground-state reaction kinetics in micellar solutions of these surfactants provide a window into some of the more intriguing aspects of both anion and cation interactions with a zwitterionic micelle surface. Recent molecular dynamics simulations paint a more physically reasonable picture of the zwitterionic micelle–water interface than that usually depicted in the literature and should contribute to our future understanding of the factors that contribute to specific anion binding.