Halochromic Properties Research Articles

Synthesis and photophysical properties of pyridyl- and quinolinyl-substituted 4-(4-aminophenyl)quinazolines

4-(4-Aminophenyl)-2-azinylquinazolines bearing pyridyl or quinolinyl residue in the position 2 have been designed and synthesized. The basic photophysical properties of obtained compounds and their 2-phenylquinazoline counterpart have been investigated in both solution and the solid state. Quinazoline chromophores exhibited positive emission solvatochromism, and the collected data were analyzed using the Lippert-Mataga equation, as well as Kosower and Dimroth/Reichardt scales. Moreover, halochromic properties upon addition of trifluoroacetic acid have been examined; it was shown that chromophores display bathochromically shifted emission band in acidic media due to the protonation at 2-azynylquinazoline fragment. The luminescence response towards metal cations has also been analyzed for the synthesized heterocycles. The obtained data represent an important step for further structure modification to improve sensitivity and selectivity to analyts.

Alizarin-functionalized organic-inorganic silane coatings for the development of wearable textile sensors

HypothesisThe broad detection properties of alizarin, not only concerning pH variations but also temperature, glucose and health-like relevant cations alterations, make it a molecule of great scientific interest, particularly for developing multifunctional wearable sensors. ExperimentHerein, the alizarin red S dyestuff is bonded with trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane, as a sol–gel precursor, to functionalize cotton fabrics. The chemical and structural properties of both plain and silane-functionalized dyestuffs are investigated in solution and solid-state by several chemical-physical characterization techniques. FindingsThe hybrid dyestuff characterization reveals the epoxy ring-opening of the silica precursor, leading to covalent linkages to the sulfonic group of alizarin, which retains its structure during the sol–gel reaction. The silane-functionalized halochromic dyestuff shows similar halochromic behaviour as its pristine solution in the investigated pH range, thus demonstrating a color shift from yellow to red due to the protonation/deprotonation reversible mechanism of the chromophore. The reversibility and repeatability of pH-sensing properties of treated cotton fabrics are confirmed by diffuse reflectance and CIELAB color space characterizations. Cotton fabric functionalized with alizarin-containing sol–gel coating shows excellent durability of halochromic properties, thus emerging as a versatile platform for stimuli-responsive materials.

Photostability and Halochromic Properties of Near‐Infrared Absorbing Anionic Heptamethine Cyanine Dyes

AbstractWe synthesized near‐infrared‐absorbing anionic heptamethine cyanine (HMC) dyes and investigated their photostability and halochromic properties in detail. Upon changing the methyl groups of the anionic HMC dye to phenyl or perfluorophenyl groups, the maximum absorption wavelengths of the dye solutions were red‐shifted and dye photostability in solution under white LED irradiation at 25 °C was improved. The anionic HMC dye featuring methyl groups presented halochromic properties in solution, and the solution color changed from green to yellow in the presence of acids. This is the first reported anionic HMC dye with halochromic properties. Furthermore, we produced a stimuli‐responsive filter paper adsorbed with anionic HMC dyes that changed color from green to yellow upon exposure to HCl gas and from yellow to green upon subsequent exposure to NH3 gas.

Synthesis and photophysical properties of pyridyl- and quinolinyl-substituted bis(arylthienyl)pyridines

Two series of V-shaped 6,7-dihydro-5H-cyclopenta[c]pyridine chromophores bearing two 5-arylthiophen-2-yl fragments as well as pyridyl or quinolinyl residue have been designed and synthesized. The basic photophysical properties of the compounds have been investigated both in solutions and the solid state. 3,4-Bis(5-(4-diethylaminophenyl)thiophen-2-yl)-1-(quinolin-2-yl)-6,7-dihydro-5H-cyclopenta[c]pyridine 2a demonstrated positive emission solvatochromism, and the collected data were analyzed by using Lippert-Mataga equation, as well as Kosower and Dimroth/Reichardt scales. The ability to detect metal cations has been analyzed for the obtained heterocycles. It is found that addition of 1.5 eq of Cu2+ to diphenylamino-substituted chromophore 1b lead to totally luminescence quenching and solution of chromophore 1b turns dark yellow that can be seen with the naked eye. Additionally, halochromic properties have been examined, it was shown that diethylamino derivatives 1a and 2a demonstrated blue shifts in both absorption and emission spectra upon the treatment with TFA. Also, we checked emission behavior of chromophore 1a in MeCN–water mixtures with different water volume fractions. Finally, DFT calculations have been performed and according data, the replacement of pyridyl residue with quinolinyl one in studied heterocyclic ensembles leads to decrease in the value of energetic gap.

Halochromic polybutylene adipate terephthalate (PBAT) for acid base sensor

A novel pH sensory material of biodegradable halochromic film based on polybutylene co-adipate terephthalate (PBAT) that can change color at acidic and base exposure is presented. The effect of starch and polyethylene glycol (PEG) addition on the efficacy of halochromic pH films was assessed. The solution casting method was used to fabricate the halochromic film, and the incorporation of PBAT, starch, PEG, and dye was confirmed using Fourier transform infrared (FTIR). The halochromic film sensitivity to color changes was revealed by Ultraviolet–Visible Spectroscopy (UV–Vis), and the mechanical performance was determined by tensile strength, elongation at break and young’s modulus. The halochromic film displays a clear color response from yellow in an acidic solution to purple in the base solution. The transition from yellow to purple color can be clearly seen at pH 6, between 420 and 450 nm and 550–600 nm absorbance peak. The addition of starch enabled the halochromic properties, and PEG improved the homogeneity of dye spreading onto the film. On top of that, PEG increased the flexibility of the PBAT-pH sensor, suggesting a wide potential for use in many applications such as food packaging as well as biomedical and environmental industrial applications. Real testing of food applications using lemon and fish were investigated where the film revealed a clear color change on 2nd day of exposure to fish, indicating that the halochromic film was efficient to sense pH changes due to bacterial activities in fish decomposition. However, the color changes under the acidic condition were less relevant.

Identification of azaphilone derivatives of Monascus colorants from Talaromyces amestolkiae and their halochromic properties

Currently, the ability to produce several kinds of water-soluble red natural colorants makes the genus Talaromyces particularly important to the dye industry, which can be an alternative to the use of harmful synthetic colorants. In this study, colored compounds produced by Talaromyces amestolkiae were extracted, characterized chemically and the color stability of the fermented broth without any extraction procedure was further evaluated over pH variation. Five azaphilones compounds were detected by Ultrahigh Performance Liquid Chromatography-Mass Spectrometry system, all being complexes of the fatty acid amino-hexanedioic acid and azaphilone Monascus colorants. The color of the fermented broth was stable at a wide range of pH (3–9). Furthermore, T. amestolkiae colorants precipitated through hydrolysis of key chemical groups at extremely acidic (pH 1) and lose red color in extremely basic (pH 13) medium, showing negative halochromism. Nevertheless, these findings enhance the industrial relevance of azaphilone colorants produced by biotechnological process.

Development of a Nitrazine Yellow-glycidyl methacrylate coating onto cotton fabric through thermal-induced radical polymerization reactions: a simple approach towards wearable pH sensors applications

In this paper, the bifunctional monomer glycidyl methacrylate (GMA) was employed in a simple two-step synthetic strategy with the aim of developing a Nitrazine Yellow-based pH-meter wearable sensor. The catalyzed epoxy ring-opening of GMA allowed the formation of a covalent bond with the sulfonate groups of the halochromic dyestuff, while potassium persulfate thermally initiated the radical grafting of the functionalized NY onto cellulose. Both reactions have been confirmed by Nuclear Magnetic Resonance Spectroscopy and by Infrared Spectroscopy, respectively. Furthermore, the morphology of the treated cotton samples was studied by means of Scanning Electron Microscopy. The pH-halochromic behaviour of the functionalized NY molecule was studied both in solution and onto the fabrics, by means of UV–Vis Spectroscopy, UV–Vis reflectance measurements and CIELAB colour changes. All experimental findings confirmed that the immobilized NY dyestuff still pertains consistent and typical halochromic properties in the pH range 6.0–7.0. The novel functional fabric developed in this work paves the way to non-disposable and reversible wearable pH sensors in smart textiles.

Design and in vivo evaluation of alginate-based pH-sensing electrospun wound dressing containing anthocyanins

In this study, a pH-sensing electrospun mat containing purple cabbage (PC) (Brassica oleracea) anthocyanins was developed for wound dressing application. The pH-sensing electrospun mat was prepared from sodium alginate (Alg) and poly (vinyl alcohol) (PVA) via electrospinning. PC anthocyanins were embedded into the electrospun mat with the addition of PC extract into the Alg/PVA electrospun solution. The characterization studies were conducted by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), water contact angle (WCA) and thickness measurements. The halochromic behavior of the pH-sensing Alg/PVA/PC electrospun mat was elucidated via UV–Vis spectrophotometric analyses and color measurements obtained from quantitative color data. In vivo performance of the pH-sensing mat was evaluated by animal studies. The electrospun mat was successfully used to monitor pH changes of an open wound. In addition, wound healing efficacy of the electrospun mat was compared with a commercial wound dressing. The results revealed that a wound dressing benefiting from halochromic properties of the anthocyanins besides wound healing efficacy of Alg was successfully developed. The pH-sensing Alg/PVA/PC electrospun mat has a usage potential for wound dressing applications especially for monitoring wound status on real-time during healing period.

Synthesis of near-infrared absorbing and fluorescent bis(pyrrol-2-yl)squaraines and their halochromic properties

1,2-Squaraines (partially conjugated neutral structure) showed blue-shifted λmax compared to the corresponding 1,3-squaraines (fully conjugated zwitterionic structure) and the mono-protonated thiophene-fused 1,3-squaraine showed λmax at 1007 nm.

C4-aldehyde of guaiazulene: synthesis and derivatisation.

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

β-Trioxopyrrocorphins: pyrrocorphins of graded aromaticity.

Octaethyltrioxopyrrocorphins unexpectedly show macrocycle-aromatic properties, even though they contain the macrocyclic π-system of the non-aromatic pyrrocorphins (hexahydroporphyrins). Two of the four possible triketone regioisomers were first reported in 1969 by one-pot oxidation of octaethylporphyrin but remained essentially unexplored since. We detail here the targeted preparation of the remaining two triketone isomers and the optical and NMR spectroscopic properties of all isomers. All four regioisomers possess unique electronic properties, including broadly varying degrees of diatropicity that were experimentally determined using 1H NMR spectroscopy and computationally verified. Structural patterns modulating the aromaticity were recognized. These differences highlight the regioisomerically differentiated influences of the three β-oxo-functionalities. We also present the solid state structure of the two most common isomers (in their free base form or as zinc complexes), allowing further conclusions to be made about the resonance structures present in these triketones. Remarkably, also, the halochromic properties of the triketones differ sharply from those of regular (hydro)porphyrins, providing further support for the proposed 16-membered, 18 π-electron aromatic ring-current. The work conceptually expands the understanding of tris-modified hydroporphyrinoid analogues and the factors that enable and control porphyrinoid aromaticity.

Fabric dyeing with colorimetric pH‐responsive colours

AbstractThis paper explores the possibility of dyeing textile fabrics with halochromic dyes. Five different halochromic dyes were tested: bromocresol purple, m‐cresol purple, chlorophenol red, curcumin and bromothymol blue. Preliminary dyeing was carried out on wool, polyacrylonitrile, polyester, polyamide 6.6, cotton and cellulose acetate at 100°C for 40 minutes to evaluate affinity. All the dyes showed good affinity to wool and polyamide 6.6. Cotton required a mordant phase before dyeing. Washing and light fastness were evaluated for the dyed fabrics, as well as colour changes at different pH values. Some fibres displayed good dye fastness and halochromic properties. Bromothymol blue was selected to dye polyamide 6.6 and cotton fabrics. The dyed fabrics displayed good halochromic properties across a pH range of 4 to 10. Cotton fabrics exhibited an immediate colour change; polyamide 6.6 changed colour after several minutes. An industrial dyeing process was performed on natural cellulosic fibres. The results confirmed the possibility of scaling up the process with regard to textile applications of halochromic dyes.

Synthesis and Photoluminescence Behavior of π-Conjugated 3-Substituted Isoquinoline Derivatives

In this work, a series of π-conjugated isoquinoline derivatives, bearing diverse aryl groups at the 3-position, were synthesized in a facile and straightforward manner. A Pdcatalyzed protocol, the Suzuki-Miyaura coupling of 3-bromoisoquinoline and different aryl boronic acids, was applied, and the structures of the resulting target compounds were determined by 1H-NMR, 13C-NMR, and HRMS. Furthermore, the photophysical properties of these compounds were investigated. Distinct halochromic properties were revealed for them, which exhibited UV emissions in ethanol but visible emissions in an acidic solution.

From cyclohexanone to photosensitive polyesters: Synthetic pathway, basic characterization, and photo-/halochromic properties

A bifunctional photosensitive monomer, 2,6-bis-(4-hydroxybenzylidene)cyclohexanone (DHCH) was successfully synthesized by the condensation reaction of cyclohexanone with p-hydroxybenzaldehyde in the presence of hydrochloric acid, as a catalyst. Based on DHCH, a series of new photosensitive polyesters were prepared by a polycondensation reaction, in solution, using two phosphonic dichlorides, namely, phenylphosphonic dichloride and phenyl dichlorophosphate, or two aromatic dichlorides, namely, terephthaloyl chloride and terephthaloyl-bis-(4-oxybenzoylchloride). The synthesis of monomer and polymers was structurally confirmed by FT-IR and NMR spectroscopies. The thermal behavior of polyesters was investigated by DSC measurements and polarized optical microscopy observations, which evidenced thermotropic liquid crystalline properties for some of the studied polymers. The photoresponsive properties of the prepared polyesters were extensively investigated in CHCl3 and DMSO solutions, under irradiation with UV light (254 and 365 nm) at different time intervals and after successive titrations with acid/base solutions. Under UV light irradiation, these compounds undergo photocrosslinking reactions and their photoluminescence properties were strongly enhanced. Deprotonation (under the basic conditions) of the polymers led to the emergence of a new absorption band (with maximum at 508 nm), accompanied by a photochromic behavior. This color change, from colorless to orange, observed upon titration with dilute NaOH solution, was completely recovered to its original colorless after the addition of an equivalent amount of dilute HCl solution. The reversible color changes (halochromic properties) observed upon protonation/deprotonation prove that these polymers have potential applications in chemical sensors.

Butterfly‐like Tetraazaacenequinodimethane Derivatives: Synthesis, Structure and Halochromic Properties

Two novel molecules TAP and TAH with pronounced reversible halochromic properties have been synthesized and fully characterized. Their butterfly-like structures have been confirmed through single-crystal X-ray diffraction. Their UV-Vis absorption after protonation dramatically red-shifted with naked-eye-visible color change in a very dilute concentration of 10-5 M. Note that the original color of the solution can be recovered after the neutralization with a base.

Halochromic properties and antimicrobial potential of crude extracts from five species of ornamental plants

Introduction: the colours of flowers are a result of secondary metabolites that have long been used in the medical and textile industries, and those that are halochromic are used in colour display because they change color according to pH changes, but many species are yet to be studied in detail. Objective: to explore the halochromic properties and the antimicrobial potentials of the crude extracts of several ornamental plants. Methods: we used aqueous and organic solvents to extract pigments from petals of five fascinating flowers planted around International Institute of Tropical Agriculture station, Cotonou, Benin: Allamanda blanchetii, Cascabela thevetia, Eichhornia crassipes, Ixora casei and Thunbergia erecta, followed by an investigation into their halochromic properties. Antibacterial potentials of the extracts were tested on important rice pathogens: Xanthomonas oryzae pv. oryzae, and Pantoea agglomerans, which are gram-negative bacteria; and on Bacillus subtilis, a gram-positive bacterium. Results: The crude extracts of T. erecta and A. blanchetii have good halochromic properties within pH 2 – 12, exhibiting distinct colours. The chromophores of the C. thevetia, E. crassipes, and I. casei are not halochromic as the colours of the crude extracts remain the same at the pH range except pH 12 which is similar for the five extracts. Crude extracts of T. erecta inhibited growth of P. agglomerans without development of resistance, whereas the bacteria developed resistance against Penicillin after 18 hrs of incubation. T. erecta and A. blanchetii were able to inhibit growth of X. oryzae and both inhibited B. subtilis. Conclusion: Pigments from both T. erecta and A. blanchetii are good pH indicators; however, T. erecta is a better antibacterial agent than A. blanchetii because it has broad-spectrum activities against bacteria.

Azulenes with aryl substituents bearing pentafluorosulfanyl groups: synthesis, spectroscopic and halochromic properties

Azulenes with SF5-containing substituents gave significantly different spectroscopic responses to protonation depending on the regioisomer in question.

Synthesis, Photophysical, Electrochemical, and Halochromic Properties of peri-Naphthoindigo.

A facile synthesis of peri-naphthoindigo (PNI) was reported for the first time from simple precursor. Installation of a chromophore at the peri-position of naphthalene is very unique in terms of synthetic challenges and properties. PNI exists in monoenol form, undergoes halochromism in acidic medium, and displays a wide and strong absorption band (ε = 33390 M-1cm-1) with maxima at 632 nm (chloroform). The dye undergoes oxidation and reduction at +0.30 and -0.58 V (vs Fc/Fc+), respectively, in chloroform.

Structural Modulation of Chromic Response: Effects of Binding-Site Blocking in a Conjugated Calix[4]pyrrole Chromophore.

Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5‐di‐tert‐butyl‐4‐oxocyclohexadien‐2,5‐yl)porphyrinogen (OxP) by structural modification. N‐Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole‐type binding sites of OxP and serial variation of the chromic properties of the products, double N‐benzylated Bz2OxP and tetra N‐benzylated Bz4OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz2OxP, and Bz4OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding‐site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators.

Chirality Sensing, Halochromism and Solvatochromism in a Conjugated Calix[4]Pyrrole

The conjugated calix[4]pyrrole tetrakis(3,5-di-t-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) is a highly coloured macrocycle with excellent potential as a sensing element in various applications. The first of these is chiral sensing where guest binding at the macrocyclic NH groups allows for estimation of enantiomeric excesses in non-racemic mixtures of chiral analytes. This feature is permitted by the relatively weak binding of the guest resulting in its rapid exchange so that the molecule essentially acts as a probe of average solution chirality. Chirality sensing in OxP is complicated by tautomeric processes when an acid analyte is used. This has allowed us also to develop an acidity sensor for use in non-polar media. More recently, we have designed a series of OxP type molecules that undergo modulation of halochromic (sensitivity to acid) properties and the development of colour scales based on the blocking of OxP binding sites and involving allied protic tautomerization of the OxP chromophore. We will discuss the sensing properties of OxP in relation to chirality, tautomeric processes and the complicating factor of solvatochromism.