Spectroscopic and Theoretical Study of the “Azo”-Dye E124 in Condensate Phase: Evidence of a Dominant Hydrazo Form

Abstract

Spectroscopic techniques, including Raman, IR, UV/vis, and NMR were used to characterize the samples of the azo dye Ponceau 4R (also known as E124, New Coccine; Cochineal Red; C.I. no. 16255; Food Red No. 102), which is 1,3-naphthalenedisulfonic acid, 7-hydroxy-8-[(4-sulfo-1-naphthalenyl) azo] trisodium salt in aqueous solution and solid state. In addition, first principle calculations were carried out for the azo (OH) and hydrazo (NH) tautomers in order to assist in the assignment of the experimental data. The two intense bands observed in the UV/vis spectrum, centered at 332 and 507 nm, can be compared to the calculated values at 296 and 474 nm for azo and 315 and 500 nm for hydrazo isomer, with the latter in closer agreement to the experiment. The Raman spectrum is quite sensitive to tautomeric equilibrium; in solid state and aqueous solution, three bands were observed around 1574, 1515, and 1364 cm(-1), assigned to mixed modes including deltaNH + betaCH + nuCC, deltaNH + nuC horizontal lineO + nuC horizontal lineN + betaCH and nuCC vibrations, respectively. These assignments are predicted only for the NH species centered at 1606, 1554, and 1375 cm(-1). The calculated Raman spectrum for the azo (OH) tautomer showed two strong bands at 1468 (nuN = N + deltaOH) and 1324 cm(-1) (nuCC + nuC-N), which were not obtained experimentally. The (13)C NMR spectrum showed a very characteristic peak at 192 ppm assigned to the carbon bound to oxygen in the naphthol ring; the predicted values were 165 ppm for OH and 187 for NH isomer, supporting once again the predominance of NH species in solution. Therefore, all of the experimental and theoretical results strongly suggest the food dye Ponceau 4R or E124 has a major contribution of the hydrazo structure instead of the azo form as the most abundant in condensate phase.

Raman investigations on marker pen inks

“…The presence of Ponceau4R also in the second spot, which has a different Retard factor, may suggest that the new bands detected could be assigned to a tautomeric form of the same Ponceau4R, because Raman spectrum is quite sensitive to tautomeric equilibrium. It has to be mentioned that a recent theoretical and spectroscopic study of Almeida et al on Ponceau4R assigns the standard spectrum reported in Fig. (spectrum B) to the hydrazo tautomer structure and claims that this is the predominant tautomeric form in solution, as opposed to the azo structure, at odds with the traditional description for such species.…”

Theoretical Studies of Molecular Structures, Infrared Spectra, NBO and NLO Properties of Some Novel 5-arylazo-6-hydroxy-4-phenyl-3-cyano-2-pyridone Dyes

“…The predicted O-H· · ·N (azo) and N-H· · ·O (hydrazone) distances for the compounds (1) and (2), respectively, are 1.968 and 1.976 Å for theazo tautomer, and 1.982 Å and 1.977 Å for the hydrazone tautomer (with B3LYP/6-31G(d,p)). These distances are significantly smaller than the summation of the Van der Waals radii (2.6 Å), just confirming the presence of a very strong hydrogen interaction in these compounds [6,37]. The N 2 ···O distance is approximately 2.60 Å for (1) and (2) compounds, which is smaller than those intramolecular hydrogen bond, N − · ··O=C, in molecules that do not present electronic configuration (the N· · ·O distance varies from 2.996 to 3.210 Å [38]).…”

Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections

“…Ferreira et al40 used a series of spectroscopic techniques to elucidate the enol–keto tautomeric equilibrium of a similar azo dye, Sudan I, in gas and condensed phases. Almeida et al37 explored another similar food dye, Ponceau 4R, in condensate phase and suggested the hydrazo structure as the most abundant form. Unfortunately, the detailed photophysical and photochemical mechanism of these novel azobenzene variants, which is useful for understanding the excited‐state properties of many azo dyes, is unknown until now.…”