Classifications, properties, recent synthesis and applications of azo dyes.

Azo dyes account for 60–70% of all dyes known to date. Understanding the factors that affect the direction of keto–enol tautomerism in azo dyes through spectral measurements is crucial to their potential applications. This review encompasses the most important spectroscopic studies of different azo dyes categorized by their structures within the last few years (2010–2014). It is concluded that the stability of the keto and enol forms largely arises from the ability to establish intra-molecular and inter-molecular hydrogen bonding, respectively. There are many factors that affect the keto or enol form, for example, polar solvent, high temperature, neutral pH and electron withdrawing substituents favor the keto form through intramolecular hydrogen bonding, whereas, nonpolar solvent, low temperature, high pH and electron donating groups favor the enol form through intermolecular hydrogen bonding. Encapsulation inside nanocaged microheterogenous systems creates a rigid environment that stabilizes the enol form mostly, while in the solid state, most of the tautomeric equilibrium lies in favor of the keto form. Understanding of keto–enol tautomerism in azo dyes helps to probe solvation dynamics, to tune the pKa values in chemical sensing, and to explain proton transfer in the excited state.

Spectroscopic studies of keto–enol tautomeric equilibrium of azo dyes

H-Bonding and charging mediated aggregation and emission for fluorescence turn-on detection of hydrazine hydrate.

Fabrication of semisynthetic collagenic materials for mere/synergistic adsorption: A model approach of determining dye allocation by systematic characterization and optimization.

In this present work we have reported the synthesis and structural characterisations of a N2O2 donor Schiff base chemosensor with an azo arm (H2L). Various spectroscopic tools like single crystal X-ray, NMR, UV-vis, FTIR, ESI-mass spectrometry etc. have been deployed to develop the present work. In recent years a number of azo derived chemosensors have been reported by different research groups. This is first time we are reporting the design and properties of an azo derived chemosensor (H2L) for the detection of aluminium ions in semi aqueous medium. It has been found that it selectively senses Al3+ ions in semi aqueous solution. Here, the sensing process is mainly based on a chelation enhanced fluorescence process (CHEF). It has very high selectivity over other metal ions and anions. A detailed literature survey has been carried out and compared with this work. It has an appreciably low detection limit i.e. 6.93 nM. 1H NMR titration was carried out to support the plausible complexation process. 1 : 1 stoichiometry binding between the chemosensor and Al3+ ions has been confirmed from Job's plot. An inhibition molecular logic gate has been constructed using chemosensor (H2L), where Al3+ and EDTA act as inputs and fluorescence emission is the output. The structural and electronic parameters of the chemosensor (H2L) and complex [AL(L)]NO3 have been studied in detail using theoretical tools like DFT and TDDFT.

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A robust fluorescent chemosensor for aluminium ion detection based on a Schiff base ligand with an azo arm and application in a molecular logic gate

The azo-hydrazone tautomerism of two pyridine-2,6-dione based Disperse Yellow dyes has been achieved by pH control and metal-ion complexation, respectively, which is evidenced by UV-visible spectra using pH-titration, 1H NMR and X-ray single-crystal diffraction techniques for two dyes and one neutral dinuclear dye–metal complex. pH-titration experiments under strong and weak acidic conditions (HCl and HOAc) as well as strong and weak alkaline conditions (NaOH and ammonia) demonstrate that there is an equilibrium between the azo (HL1-A and HL2-A) and hydrazone (HL1-H and HL2-H) tautomers for two dyes in solution but the hydrazone form is dominant under conventional conditions. The hydrazone proton is also observed in the 1H NMR spectra of HL1-H and HL2-H which can be verified by the hydrogen–deuterium exchange and the presence of cooperative six-membered intramolecular hydrogen rings involving the hydrazone proton in their X-ray single-crystal structures. Moreover, the azo-hydrazone tautomerism is evidenced by the formation of a novel neutral dinuclear dye–metal complex Cu2(L2-A)4, where all the ligands are in the azo form and two types of coordination modes are present for four L2-A ligands. Namely, the side two ligands serve as the bidentate capping ligands, while the middle ones act as the quadridentate bridging ligands linking adjacent CuII centers in a reverse fashion.

Azo-hydrazone tautomerism observed from UV-vis spectra by pH control and metal-ion complexation for two heterocyclic disperse yellow dyes

Two pairs of 1 : 2 neutral trans mononuclear transition-metal (M = NiII and CuII) complexes of pyridine-2,4-dione and quinoline-2,4-dione based heterocyclic dyes have been structurally and spectrally characterized and compared herein. X-ray single-crystal diffraction analyses of four complexes, namely trans-[Ni(La)2(DMF)2] (1), trans-[Cu(La)2(DMF)2] (2), trans-[Ni(Lb)2(DMF)2] (3) and trans-[Cu(Lb)2(DMF)2] (4), reveal that they have the same trans configuration between the bidentate chelating dianionic ligands and two axially coordinated DMF molecules. Furthermore, a transformation from the hydrazone to azo configuration has been observed for both bidentate chelating ligands La− and Lb− after metal-ion complexation. More importantly, the simultaneous DSC/TG-MS-FTIR method has been used to explore the thermal stability of four neutral metal-complex dyes 1–4, where the two axially coordinated DMF molecules in NiII and CuII complexes exhibit distinguishable decomposition behavior because of their different M–O bond lengths originating from the Jahn–Teller distortions.

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Two pairs of 1 : 2 nickel(II) and copper(II) metal-complex dyes showing the same trans configuration and azo–hydrazone transformation but different thermal properties

The invention discloses a benzo isothiazole azo pyrazolone disperse dye The benzo isothiazole azo pyrazolone disperse dye is a compound having the structure shown in a formula (I) in the specification Conjugate migration of hydrogen connected with a heteroatom in the benzo isothiazole azo pyrazolone heterocyclic ring disperse dyes is performed among benzo isothiazole ring, azo and pyrazolone to display bright-colored yellow, red and blue; and moreover, the dye is large in hue span and rich in colors; and therefore, the benzo isothiazole azo pyrazolone disperse dye can be used for preparing a fiber identification coloring agent The invention discloses a preparation method of the benzo isothiazole azo pyrazolone disperse dye

Benzo isothiazole azo pyrazolone disperse dye as well as preparation method and use thereof

The invention discloses a benzoisothiazoleazopyridone disperse dye, which is a compound I with the structure as shown in the specification. The benzoisothiazoleazopyridone disperse dye and hydrogen connected with hetero atoms can generate conjugated migration among a benzoisothiazole ring, azo and a pyridine ring under a certain pH value, so that yellow color, red color and blue color can be displayed, the color-phase span is large and the colors are various. Therefore, the benzoisothiazoleazopyridone disperse dye disclosed by the invention can be used for preparing a fiber recognition reagent. The invention further discloses the preparation method of the benzoisothiazoleazopyridone disperse dye.

Yin-Ge Wang

Papers

Azo-hydrazone tautomerism observed from UV-vis spectra by pH control and metal-ion complexation for two heterocyclic disperse yellow dyes

Two pairs of 1 : 2 nickel(II) and copper(II) metal-complex dyes showing the same trans configuration and azo–hydrazone transformation but different thermal properties

Benzo isothiazole azo pyrazolone disperse dye as well as preparation method and use thereof

Benzoisothiazoleazopyridone disperse dye, preparation method thereof and use of benzoisothiazoleazopyridone disperse dye as fiber recognition reagent