Colorants

Tb⁴⁺-doped sodium zirconate samples, Na₂Zr_1−xTb_xO₃, were synthesized as novel environmentally friendly inorganic yellow pigments by a conventional solid-state reaction method. Their crystal structures, optical properties, and colors were characterized. A single-phase form was obtained for the samples in the x range of x ≤ 0.18, while impurity phases were detected for the sample with x = 0.20. All samples showed strong optical absorption in the blue light region, due to the charge transfer transition between O²⁻ and Tb⁴⁺. As a result, the sample color became yellow, which is the complementary color of blue, and the color became more vivid with increasing Tb⁴⁺ content in the single-phase region. Among the samples, Na₂Zr₀.₈₂Tb₀.₁₈O₃ was the optimal composition, with the highest yellowness (b* = +67.2) and pure yellow hue (h° = 90.1). Although the b* value was lower than commercial yellow pigments such as BiVO₄ and ZrSiO₄:Pr, this sample had a purer yellow hue. Since Na₂Zr_0.82Tb_0.18O₃ is composed of non-toxic elements, it could be a new environmentally friendly inorganic yellow pigment. Full article

Naturally occurring and synthetic ancient pigments have a history of use spanning thousands of years. Curiously, some of their newly discovered properties make them excellent candidates for semiconductors, anticounterfeiting agents and so much more. In this paper, we will review their ancient roots in art and modern emergence as 21st century workhorses. You can never judge a pigment by its color alone! Full article

This article compares spectroscopic properties of the series of dipyrromethene dyes, namely their complexes of boron (III), zinc(II) and cadmium(II) with the halogenated ligands of the same structure. Absorption and emission spectra, lifetimes of long-lived emission and quantum yields of luminescence were studied as the functions of molecular structure of dipyrromethene complexes. The role of the position and nature of a substituent in a ligand, polarity of a solvent and temperature of media were also investigated. The studies demonstrate that replacing the central atom boron(III) by zinc(II) decreases the fluorescence quantum yield, indicating the increased role of non-radiative processes in excitation energy deactivations such as intersystem crossings. In addition, according to the heavy atom effect, the efficiency of intersystem crossings in halogen-substituted zinc(II) and cadmium(II) dipyrromethene complexes is higher than in the corresponding boron fluoride dipyrromethenes (BODIPY), which leads to increase in phosphorescence at low temperatures (frozen solutions). The obtained results make it possible to carry out further investigations of potential sensory properties that are required for systematic use of halogenated dipyrromethene complexes for the creation of modern optical oxygen sensors and singlet oxygen photosensitizers for photodynamic therapy or photocatalytic oxidative reactions. Full article

The efficiency of chemoinformatics methods based on a fragment approach for the analysis of relationships between the chemical structure of textile dyes and colour fastness of the dyeings have been shown by examining a large set of properties, including the light fastness of acid dyes on wool and polyamide fibres, the sensitivity of acid dyes on wool to oxygen bleaching, the wash fastness of acid dyes on wool, the adsorption of direct dyes on cotton, and the photodegradation of azo dyes in solution. An analysis of the developed regression models depicted the contribution of ten substructural molecular fragments for each indicator of the colour fastness properties of acid and direct azo dyes on textile materials. The similarity of several individual multi-atomic fragments for acid and direct azo dyes was found for wool, polyamide, and cotton fibres, which indicates the coinciding mechanisms of the physicochemical processes that accompany the destruction of dyes while testing the light fastness and sensitivity of the dyeings to oxygen bleaching, as well as their adsorption/desorption with the wash fastness and dyeability of wool and cotton. Full article

Rhodopsin, the first visual pigment identified in the animal retina, was shown to be a photosensitive membrane protein containing covalently bound retinal in the 11-cis configuration, as a chromophore. Upon photoexcitation the chromophore isomerizes in femtoseconds to all-trans, which drives the protein into the active state. Soon thereafter, another geometric isomer—9-cis retinal—was also shown to stably incorporate into the binding pocket, generating a slightly blue-shifted photosensitive protein. This pigment, coined isorhodopsin, was less photosensitive, but could also reach the active state. However, 9-cis retinal was not detected as a chromophore in any of the many animal visual pigments studied, and isorhodopsin was passed over as an exotic and little-relevant rhodopsin analog. Consequently, few in-depth studies of its photochemistry and activation mechanism have been performed. In this review, we aim to illustrate that it is unfortunate that isorhodopsin has received little attention in the visual research and literature. Elementary differences in photoexcitation of rhodopsin and isorhodopsin have already been reported. Further in-depth studies of the photochemical properties and pathways of isorhodopsin would be quite enlightening for the initial steps in vision, as well as being beneficial for biotechnological applications of retinal proteins. Full article

Organic azo dyes, which are widely used in industrial, health and cosmetic fields, pose genotoxic risks due to their chemical structures; however, the molecular details of the undesirable effects of these dyes on DNA have been poorly or insufficiently clarified. In this computational molecular docking study, the DNA binding modes and binding affinities of 14 azo dyes, previously determined to show DNA clastogenicity, were characterized using 2 different double-stranded DNA (dsDNA) conformations (an intact dsDNA and dsDNA with an intercalation gap). In this study, it was determined that 10 out of the 14 genotoxic azo dyes were strong dsDNA minor groove binders, while the remaining ones formed tight binding complexes with dsDNA through intercalation or threading intercalation modes. The azo, nitro, hydroxyl, ammonium, sulfonate, naphthalene, methoxyphenyl, bromine, nitrophenyl, imidazole, amino-phenylethanol and chloro-nitrophenyl groups were found to play primary role in the most favorable binding conformations of these dyes on dsDNA with an affinity ranging from −6.35 kcal/mol to −9.42 kcal/mol. It was determined that dsDNA sequences containing GT dinucleotides are frequently preferred in binding by these dyes, and that rings and polar groups are important features for tight binding with dsDNA. It was concluded that these dyes may be banned, or non-genotoxic congeners should be manufactured with appropriate molecular optimization for the genetic health of the human population and for future generations. Full article

Four triphenylamines with extended π-systems were synthesized. Cyclic voltammetry (CV) measurements showed that they gave radical cations, which are stable in solution. Radical cations obtained upon one electron chemical oxidation showed strong absorption in the near-infrared region. The radical cations of the naphthalene-substituted derivatives show a maximum absorption wavelength above 1000 nm and are classified as NIR-II dyes. Molecular design rules of novel near-infrared absorbing dyes are described. Full article

Daylight fluorescent artists’ colors have been well established as fugitive. Upon exposure to light, these vibrant colors can fade and exhibit color shifts. Artwork containing these fluorescent colorants presents complex challenges for art conservators faced with conserving these inherently problematic materials. This paper examined nine fluorescent colorants obtained from Kremer Pigmente, referred to the previous literature and research, and attempted to quantify the visual and photographic observations of fading and color changes. It provides additional information that could be useful in considering conservation documentation and treatment. Fiber optic spectroscopy using ultraviolet and visible light sources was used to measure the spectral shifts of the colorants before and after exposure to light. The fluorescent colors exhibited alterations in intensity coupled with primary peak shifts in the spectrum corresponding to the optical fading and color shifts. Multimodal imaging was executed to analyze the pigments in different regions of the spectrum before and after aging, which has not been documented before with these fluorescent colorants. Imaging in various regions of the spectrum indicated differences in absorption and reflectance between the pigments as captured by a modified camera. The results were compared to recently published research including the identification of the dyes present in the Kremer line of pigments. Multimodal imaging and fiber optic spectroscopy provided valuable information for future documentation and conservation of artworks containing these colorants. Specifically, these non-invasive techniques provide a method to document and identify the spectral changes between the aged and unaged pigment, graph and predict the direction of overall color change, and provide useful data for establishing future conservation treatment protocols. Full article

Photodynamic therapy (PDT) is an established treatment modality for a range of superficial and localized cancers. There has been tremendous interest in the development of advanced photosensitizers that exhibit superior photophysical properties, high tumor selectivity, and improved pharmacokinetics. Glucose is one of the well-studied targeting moieties that can deliver various therapeutic agents to cancer cells selectively via the Warburg effect. However, the use of glucosylated photosensitizers for targeted PDT has remained little studied and to the best of our knowledge, the PDT effect of the positional isomers of these conjugates has never been compared. We report herein the preparation and photophysical properties of the C1α and C1β anomers of a glucosylated boron dipyrromethene-based photosensitizer. The cellular uptake and photocytotoxicity of both anomers were also studied and compared using A549 human lung carcinoma cells and HEK293 human embryonic kidney cells. Interestingly, the cellular uptake of the C1α anomer was approximately 2-fold higher than that of the C1β anomer regardless of the cell type and incubation time. The uptake pathway of both anomers was also studied. It was found that they were internalized through energy-dependent receptor/protein-mediated endocytosis rather than the well-known glucose transporters and sodium-driven glucose symporters. Full article

The most important results obtained during the first century of pinacyanol (PIN) application in fundamental and applied research are summarized. The main usages of PIN during the century are described. The structural, protolytic, and thermodynamic properties of the dye in different systems (aqueous and aqueous–organic solutions, premicellar and micellar surfactant systems, and salt-containing solutions) have been analyzed. Quantitative characteristics of intermolecular interactions are considered. Self-association and dissimilar association of PIN with the participation of other dyes, organic ions and molecules, and metal complex compounds under 5 × 10⁻⁷–8 × 10⁻⁵ mol/L concentrations of PIN are discussed. The analyzed data indicate that at present, as before, the scientific interest in this dye is very high. Full article

Mica-titania pearlescent pigments (MTs) were previously coated with organic molecules to obtain combination pigments (CPs) for achieving certain improvements or functionalities. Anthocyanins (ACNs) are molecules that can be extracted from natural resources and exhibit color changes via pH modifications of the enclosing medium. The purpose of the study was to produce a new series of CPs by depositing ACNs on MTs at different pH values, to observe the changes in color, and to associate these changes to thermogravimetrically determined deposition efficiencies in light of spectral differences. The extraction and deposition methods were based on aqueous chemistry and were straightforward. The ACN deposition generally increased with increasing pH and correlated with the consistency between the charges of the MT surfaces and the dominant ACN species at a specific pH value. The fluorescence of the CPs was inversely correlated with the deposition quantities invoking the possibility of a quenching effect. Full article

Red bell pepper extract rich in carotenoids was (RBPE) encapsulated with four different encapsulating agents: calcium caseinate (ECC), bovine gelatin (EBG), whey proteins isolate (EWPI), and concentrate (EWPC), aiming to investigate the most effective material to coat and enable the water dispersibility of pigments. Formulations were obtained by the oil in water (O/W) emulsification technique, followed by freeze-drying. Samples were analyzed by encapsulation efficiency, high-performance liquid chromatography (HPLC), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), atomic force microscopy (AFM), thermogravimetric analysis (TGA), dispersion stability, and CIELab. Nanoformulations showed a carotenoid encapsulation efficiency of 54.0% (ECC), 57.6% (EWPI), 56.6 % (EWPC), 64.0 % (EBG). Recovered carotenoid profiles from nanoformulations showed similarity to the RBPE, indicating the efficiency of the encapsulation process. Average particle sizes of approximately 109 nm (ECC), 71 nm (EWPI), 64 nm (EWPC), and 173 nm (EBG) were obtained. AFM revealed that all formulations exhibited spherical forms and a heterogeneous distribution profile. Regarding TGA, formulations presented similar thermal behaviors to and lower decomposition speeds than RBPE, suggesting improved thermal stability. Powder formulations were easily dispersed in water (8 mg/mL) and presented intense color and stability to sedimentation for 48 h. Results indicated that all formulations and the chosen technique efficiently increased carotenoid dispersibility in water, indicating their potential to be applied as natural food pigments. Full article

The surface of polypropylene (PP) was modified with fluorine gas at 25 °C and 10–380 Torr for 1 h. The surface roughness of the fluorinated PP samples was approximately two times larger than that (5 nm) of the untreated sample. The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy showed that the bonds (e.g., -C-C- and -CH_x) derived from raw PP decreased and were converted into fluorinated bonds (e.g., -CF_x) after surface fluorination. These fluorinated bonds showed higher electronegativity according to the zeta potential results. Fluorinated PP could be stained with the methylene blue basic dye because of the increased surface roughness and the negatively charged surface. Full article

Melanin and melanoma tumors are two fields of increasing interest in biomedical research. Melanins are ubiquitous biopigments with adaptive value and multiple functions, and occur in the malignant melanoma. Although several chemical structures have been proposed for eumelanin, molecular modeling and orbitals indicate that a planar or spiral benzoquinone-porphycene polymer would be the model that better explains the broad-band light and ultrasound absorption, electric conductivity, and graphite-like organization shown by X-ray crystallography and electron microscopy. Lysosomes and melanosomes are selectively labeled by vital probes, and melanin also binds to metal cations, colorants, and drugs, with important consequences in pharmacology, pathology, and melanoma therapy. In addition to traditional and recent oncologic treatments, photodynamic, photothermal, and ultrasound protocols represent novel modalities for melanoma therapy. Since eumelanin is practically the ideal photothermal and ultrasound sensitizer, the vibrational decay from photo-excited electrons after NIR irradiation, or the electrochemical production of ROS and radicals after ultrasound absorption, induce an efficient heating or oxidative response, resulting in the damage and death of tumor cells. This allows repetitive treatments due to the remaining melanin contained in tumoral melanophages. Given that evolution and prognosis of the advanced melanoma is still a concern, new biophysical procedures based on melanin properties can now be developed and applied. Full article

The equilibrium between different tautomers that can be colored or colorless is an important feature for rhodamine dyes. Presently, this phenomenon is mostly discussed for rhodamine B. Herein, we studied the tautomerism and acid–base dissociation (HR⁺ ⇄ R + H⁺) of a set of rhodamines in organic media. Form R is an equilibrium mixture of the colored zwitterion R^± and colorless lactone R⁰. Absorption spectra in 90 mass% aqueous acetone reflects the correlation between the dyes structure and the equilibrium constant, K_T = [R⁰]/[R^±]. Increase in the pK_a value on transferring from water to organic solvents confirms the highly polar character of the R^± tautomer. To reveal the role of the solvent nature, the tautomerism of an asymmetrical rhodamine, 2-(12-(diethyliminio)-2,3,5,6,7,12-hexahydro-1H-chromeno[2,3-f]pyrido[3,2,1-ij]quinolin-9-yl)benzoate, was examined in 14 media. This chain–ring tautomerism is an intramolecular acid–base reaction; the central carbon atom acts as a Lewis acid. The interaction with other Lewis acids, Li⁺, Ca²⁺, Mg²⁺, and La³⁺, results in rupture of lactone cycle. In polar solvents, lactones undergo photocleavage resulting in formation of highly fluorescent R^±, whereas the blue fluorescence and abnormally high Stokes shift in low-polar media may be explained either by another photoreaction or by spiroconjugation and charge transfer in the exited state. Full article

A systematic review and bibliometric study were undertaken utilizing the Science Citation Index Expanded database from 1990 to 2020 to obtain insights into the trajectory of pigment coloration research. The study focused on the publication performance in terms of the yearly production and citations, as well as mainstream journals, categories of the Web of Sciences, leading nations, well-known institutions, and research trends. The survey found that the yearly output of scholarly publications on pigment coloration research climbed gradually throughout the first quarter of the study period and then increased substantially in the latter five years. The present research emphasis and future trends were examined after summarizing the paper title and abstract analyses, author keyword analysis, and the most regularly used keywords derived from words in KeyWords Plus. To expand its reach in numerous application areas, pigment coloration research will continue to focus on improving pigment qualities. Full article

Flavylium ions (6a–6e) were synthesized using Aldol condensation to compare the spectroscopic properties caused by the different numbers and locations of hydroxy groups on the flavylium cations (AH⁺). Without the addition of acid, increasing the concentration of flavylium ions to 10 mM in ethanol led to the following observation. The flavylium ions with the least number of OH groups (6a and 6b) showed a change in peak at higher concentrations, whereas 6c and 6d did not show the same degree of change in their ¹H-NMR signals. This suggests an equilibrium that shifts the isomers B, C_E, and C_z more towards the flavylium ion with more OH groups even at low concentrations. One possibility for the cause of this shift is that the flavylium ions become more stable through aggregation. In addition to the NMR results, the UV measurements confirmed that isomers with fewer OH groups showed a more dramatic shift towards the flavylium ion at higher concentrations. Using modeling data at DFT//B3LYP/6311**G(d) level, the self-association was investigated to show that the hydrogen bonding of OH groups is the main player but cannot stabilize entirely without the presence of the chloride ion in the complex. Full article

The interaction between single- or double-charged anions of bromocresol purple (BP) and cyanine cations (quinaldine blue, QB, or quinaldine red, QR) at concentrations of dyes 5.0·10⁻⁷–4.0·10⁻⁵ mol/L has been investigated by vis-spectroscopy. The thermodynamic constants of dissimilar associations (K_as) have been studied. Comparison of the values of lg K_as shows that QB⁻ associates of BP⁻ are more stable (6.61 ± 0.07) than QR associates (4.84 ± 0.06); a similar phenomenon is observed for associates of the BP²⁻ anion. Semi-empirical calculations (PM3 method) are in agreement with the vis-spectroscopy data and indicate that the association of dye into an associate is possible. The standard enthalpies of formation of associates (Δ_fH^o) and energy diagrams have been determined. The Δ_fH^o data indicate that the formation of an associate between dye ions is an energetically favourable process. The gain in energy significantly exceeds the systematic error of semi-empirical calculations and increases from 157 kJ/mol (associate ”BP⁻ + QB⁺”) to 729 kJ/mol (associate “BP²⁻ + QR⁺”). The most probable structures of dissimilar associates are presented. The study of the dissimilar association develops the concept of intermolecular interactions in solutions. Full article