Comparative studies of excited state intramolecular proton transfer (ESIPT) and azo-hydrazone tautomerism in naphthalene-based fluorescent acid azo dyes by computational study

We found that benzothiazole is responsible for enhancing fastness properties of dyes. On comparing the total electronic energies of naphthol tautomers (−1425.16 eV) and its analogs (−2146.59 eV), we found that benzothiazolyl series is more stable than the naphthol series. Frontier Molecular Orbitals (FMOs) also show flow of charge transfer from the donor to the acceptor in benzothiazole-containing compounds, whereas it is absent in plane naphthol series. Among the benzothiazolyl isomers, the hydrazone form is found to be more stable and responsible for fluorescence possession. Highest Occupied Molecular Orbital (HOMO)–Lowest Occupied Molecular Orbital (LUMO) energy band gap also indicates the same. Electrophilicity index and hyper-hardness values of both the series were found to be positive which directly ratifies photostability and reactivity. Benzothiazolyl series was found to be more stable, hence light fastness, enhanced.

Theoretical investigation on the mechanism of the OH-initiated degradation process of reactive red 2 azo dye

The dual descriptor (Δf) data of azo form (a, RR2) and hydrazone form (b, HRR2) of RR2 dianion. For Δf> 0 (green), the site is favorable for nucleophilic attack, for Δf< 0 (blue), the site is favorable for electrophilic attack. Key bond lengths in Å.

N-Acylamino-pyridine-2,6-dione based heterocyclic dyes and their oxidative ring-degradation under alkaline conditions

Three new N-acylamino-pyridine-2,6-dione based heterocyclic dyes in the same hydrazone form and their oxidative ring-degradation under alkaline conditions have been described, together with a two-step ring-opening and subsequent oxidative cleavage mechanism.

Azo-hydrazone tautomerism of aryl azo pyridone dyes

In the last three or four decades disperse dyes derived from pyridones (in particular azo pyridone dyes) have gained in importance, and are widely used in various fields. These compounds have excellent coloration properties, and are suitable for the dyeing of polyester fabrics. Basic features of these dyes are simplicity of their synthesis by diazotation and azo coupling. They generally have high molar extinction coefficient with medium to high light and wet fastness. The absorption maxima of these dyes show their visible absorption wavelength ranging from yellow to orange, which can be attributed to poorly delocalized electrons in the pyridone ring. However, there are several dyes with deep colors such as red or violet. Pyridone dyes with alkyl and aryl groups in ortho position to azo group show 2-pyridone/2-hydroxypyridine tautomerism, while those containing OH and NHR groups conjugated with the azo group show azo-hydrazone tautomerism. Determining azo-hydrazone tautomerism could be therefore interesting, since the tautomers have different physico-chemical properties and most importantly different coloration. The literature on azo-hydrazone tautomerism, determination of equilibrium position, and investigation of substituent and solvent influence on tautomerism has been summarized in the presented review. The general conclusion is that the equilibrium between two tautomers is influenced by the structure of the compounds and by the solvents used. The tautomeric behavior patterns of the arylazo pyridone dyes in the reviewed literature has been studied using various instrumental techniques, including FT-IR, UV-vis, and NMR spectroscopy. The quantum chemical calculations related to the azo-hydrazon tautomerism have also been included. A large number of pyridone dyes exist in hydrazone form in solid state, while in solvents there is a mixture of tautomers. In addition, the X-ray single-crystal diffraction data analysis of some commercial pyridone dyes has been discussed concluding that they all crystallize in the hydrazone form.

Synthesis and Tautomeric Structure of 2-arylazo-4H-imidazo[2,1-b][1,3,4]thiadiazines

Two series of the title compounds were prepared via reaction of N-aryl 2-oxohydrazonoyl halides with 1-amino-4-phenylimidazoline-2-thione. Their tautomeric structure was elucidated by spectral analysis, and the correlation of their acid dissociation constants with the Hammett equation, to be as the hydrazone form.

15N, 13C, and 1H NMR Spectra of Azo and Hydrazo Compounds Derived from 1,3,3-Trimethyl-2-methylidene-2,3-dihydroindole (Fischer Base)

The 15N, 13C, and 1H NMR spectra were measured for azo and hydrazo compounds derived from 1,3,3-trimethyl-2-methylidene-2,3-dihydroindole (Fischer base), which is a passive component with a terminal methylidene group. Products prepared by coupling in hydrochloric acid exist in the corresponding hydrazone form as the E-isomers. Neutralization gives a mixture of two isomeric azo compounds which differ in the arrangement at the C(2)=C(10) double bond. This mixture was alkylated with methyl iodide to obtain the =N-N(CH3)- hydrazone derivatives. The geometric isomers were resolved based on the NOESY approach and the stereospecific behaviour of the 2J(15N,13C) coupling constants was studied for the 15N-labelled compounds.