Synthesis, absorption and fluorescence of hydrazone colorants based on pyrrolinone esters

Since the azo-hydrazone tautomerism was discovered, it has been the subject of much research and much heated debate. Only modern spectroscopic methods led to clear experimental evidences. Therefore, the first part of the review describes the spectroscopic methods (UV/VIS, IR, mass, NMR and X-ray photoelectron spectra and crystal structure analysis) associated with a more detailed evaluation of the certainty of the results so obtained. It is shown that nuclear magnetic resonance (δH, δ13C, δ15N, δ14N, J15NH) is a generally applicable tool and it leads to the most reliable results. In the second part, a survey of the tautomerism of p-hydroxyazo-benzenes, phenylazonaphthols and the azocoupling products of pyrazolones and s-diketones is given. The effects of substituents, anellation and H-bonding are summarized. The various assumptions, indications and evidences of tautomeric structures and solvent effects are discussed critically.

Über die Azo‐Hydrazon‐Tautomerie

Intramolecular charge transfer (ICT) chromophores 2-(2-(4-methoxy-2-nitrophenyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione (CD1) and 2-(2-(4-methyl-2-nitrophenyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione (CD2) have been synthesized and used for the first time as chemosensors with reversible “on–off” sensing capabilities for biologically and environmentally significant Co2+, with detection limits of 3 μm to 7 μm. The new metal ion sensors that contain hydrazones of β-diketones have been synthesized and characterized by FT-IR, 1H and 13C NMR spectroscopy, scanning electron microscopy (SEM) and single crystal X-ray diffraction studies. The FT-IR and NMR spectral data clearly show effective intramolecular hydrogen bonding in all the synthesized substituted hydrazones. SEM was employed to investigate their morphology. A single crystal X-ray diffraction study confirms the exact structures of CD1 and CD2. A packing diagram explains the strong intramolecular hydrogen bonding in the CD1 and CD2 molecules. The absorption spectra are all similar, irrespective of the substituent and solvent. By comparison, the fluorescence is strongly dependent on the electronic character of the substituent. The sensors show excellent selectivity and sensitivity, with fluorescence enhancement in response to Co2+ over other cations in ethanol aqueous solution. Combined experimental and theoretical studies were conducted on their molecular structures using density functional methods (B3LYP) invoking the 6-31G basis set. The optimized geometric bond lengths and bond angles obtained by the DFT method show good agreement with the experimental values. The energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been predicted.

Facile, cost effective synthesis and DFT-based studies of substituted aryl hydrazones of β-diketones: a new selective fluorescent chemosensor for Co2+

The chemistry of making color in art

Binary systems with induced smectic phases due to EDA complex formation

Studies on UV/Vis Absorption Spectra of Azo Dyes. VI. The Effect of Substituents on the Color of Derivatives of 1-Phenylazo-2-naphthols



The absorption spectrum of 1-(4′-diethylaminophenylazo)-2-naphthol has been examined, and it has been found that the absorption band at 510 nm corresponds to the azo form and the shoulder at 610 nm corresponds to the hydrazone tautomeric form. Substituents in 3-position of 2-naphthol which have a free proton (e.g. OH, COOH, CONHR) stabilize the hydrazone tautomeric form by an intramolecular hydrogen bonding between substituent and hydrazone oxygen. Substituents which have not a free proton (e.g. OCH3, COOCH3, CONR1R2) have little influence on the absorption spectrum.



The relative intensities of the two bands of the tautomeric forms are dependent on: 



— the electron attracting or electron releasing effect of the substituent R in CONHR




— the hydrogen bonding acidity of the substituents in 2′-position if R in CONHR is a phenylring




— the hydrogen bonding acidity of the solvent.






The effect of substituents in the phenylazo subsystem is also discussed.

Untersuchungen zum UV/Vis‐Spektralverhalten von Azofarbstoffen. VI. Zum Einfluß von Substituenten auf die Farbe von Derivaten des 1‐Phenylazo‐2‐naphthols

H. Mustroph

Papers

Untersuchungen zum UV/Vis‐Spektralverhalten von Azofarbstoffen. VI. Zum Einfluß von Substituenten auf die Farbe von Derivaten des 1‐Phenylazo‐2‐naphthols