(E)-1-(4-Meth­oxy­anthracen-1-yl)-2-phenyl­diazene

The title compound, C21H16N2O, has an E-conformation about the diazene N=N bond. It is reasonably planar with the phenyl ring being inclined to the mean plane of the anthracene moiety [planar to within 0.077 (3) Å] by 6.43 (10)°. The crystal structure is stabilized by C—H⋯π and weak π–π inter­actions [centroid–centroid distances of 3.7192 (16) and 3.8382 (15) Å], leading to the formation of two-dimensional networks stacking along [001] and lying parallel to (110)

Tautomerism of 4,4′-dihydroxy-1,1′-naphthaldazine studied by experimental and theoretical methods

BACKGROUND: The title compound belongs to the class of bis-azomethine pigments. On the basis of comparative studies on similar structures, insight into the complex excited state dynamics of such compounds has been gained. It has been shown, for example, that only compounds that possess hydroxyl groups are fluorescent, and that the possibility for cis-trans isomerisation and/or bending motions of the central bis-azomethine fragment allows for different non-radiative decay pathways. RESULTS: The compound, 4,4'-dihydroxy-1,1'-naphthaldazine (1) was synthesized and characterized by means of spectroscopic and quantum chemical methods. The tautomerism of 1 was studied in details by steady state UV-Vis spectroscopy and time resolved flash photolysis. The composite shape of the absorption bands was computationally resolved into individual subbands. Thus, the molar fraction of each component and the corresponding tautomeric constants were estimated from the temperature dependent spectra in ethanol. CONCLUSIONS: According to the spectroscopic data the prevalent tautomer is the diol form, which is in agreement with the theoretical (HF and DFT) predictions. The experimental data show, however, that all three tautomers coexist in solution even at room temperature. Relevant theoretical results were obtained after taking into account the solvent effect by the so-called supermolecule-PCM approach. The TD-DFT B3LYP/6-31 G** calculated excitation energies confirm the assignment of the individual bands obtained from the derivative spectroscopy

Dynamics of Excited State Proton Transfer in Nitro Substituted 10-Hydroxybenzo[h]quinolines

Barrierless proton transfer in HBQ and a barrier upon nitro substitution.</p

Isomerization and aggregation of 2-(2-(2-hydroxy-4-nitrophenyl)hydrazono)-1-phenylbutane-1,3-dione: recent evidences from theory and experiment

The title compound potentially can exist as four isomers in solution. Recently Lycka has proposed a protocol for distinguishing two of them based on 15N NMR. This approach has been confirmed theoretically, in the current study, and further developed into a logical scheme that allows the existence of each of the isomers to be proven in solution. The experimental data, obtained by NMR and UV–Vis spectroscopy, have shown that the studied compound exists as a mixture of two solvent stabilized isomers in diluted solutions of dimethyl sulfoxide. However, the concentration effect also plays a substantial role allowing formation of linear aggregates as the X-ray analysis and theoretical calculations show

A concept for stimulated proton transfer in 1-(phenyldiazenyl)naphthalen-2-ols

A series of aryl azo derivatives of naphthols (1–3) were studied by means of UV–Vis and NMR spectroscopy in different solvents as well as by quantum chemical calculations and X-ray analysis. Previous studies have shown that Sudan I (1) exists as a tautomeric mixture. The effect of the solvents is minimized by the existing intramolecular hydrogen bond. Therefore, the influence on the tautomeric state in structurally modified 1 has been investigated. Structure 2 contains an additional OH- group, which deprotonates easily and affects the position of the tautomeric equilibrium by changing the electronic properties of the substituent. The implementation of a sidearm in 3 creates a condition for competition between the nitrogen from the azo group and from the piperidine unit for the tautomeric proton. In this case the use of acid as a stimulus for controlling the tautomeric process was achieved

Controlled tautomerism – switching caused by an ‘underground’ anionic effect

In a previous communication, we demonstrated a conceptual idea for a tautomeric switching system based on implementation of a flexible piperidine unit in 4- (phenyldiazenyl)naphthalen-1-ol (1). The results showed that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. However, the effect of the counter ion in the process of protonation was never considered. The crystallographic analysis of protonated cyano and nitro derivatives of 4-(phenyldiazenyl)-2-(piperidin-1- ylmethyl)naphthalen-1-ol have shown an interesting and unexpected feature: the counter ion is captured in the process of protonation and the shift in the position of the tautomeric equilibrium is achieved through a bridged complex formation. To the best of our knowledge this is a rare example when controlled shift in the position of tautomeric equilibrium is achieved through anion complexation. The results from the solid state analyses are confirmed by NMR spectroscopy in solution and by quantum- chemical calculations

Tautomerism in 1-phenylazo-4-naphthols: experimental results vs quantum-chemical predictions

The reliability in the description of the tautomerism of 1-phenylazo-4-naphthol by using of HF and MP2 ab initio levels of theory and DFT methods with variety of pure GGA (OLYP), hybrid (B3LYP and B3PW91), long range corrected (LC-BLYP) and double-hybrid (B2PLYP and mPW2PLYP) functionals with large number of basis sets was estimated. In this evaluation three criteria were used: reproduction of the bond lengths in the structures of the individual tautomers, description of the non-planarity of the enol tautomer and prediction of the position of the tautomeric equilibrium (∆G value) at 298 K. The results show that in substantial number of cases HF reasonably covers all requirements. The tested pure (OLYP) and hybrid functionals (B3LYP and B3PW91) fail in the prediction of the position of the equilibrium independent on the basis set. The situation is slightly better at the long range corrected functional (LC- BLYP), which give predominance of the enol tautomer at 6-31+G** and D95++**. The double hybrid functionals give very good description with D95++** basis set, but at substantial computational costs

Tautomerism in azo dyes: Border cases of azo and hydrazo tautomers as possible NMR reference compounds

In order to meet the need for NMR reference compounds in the study of tautomeric azo dyes, two series of azo dyes, derived from 3-methyl-1-phenyl-4- (phenyldiazenyl)-1H-pyrazol-5-amine and 5-methyl-2-phenyl-4-(2- phenylhydrazono)-2,4-dihydro-3H-pyrazol-3-one, have been studied by using molecular spectroscopy (UV–Vis and NMR) and quantum-chemical calculations (M06- 2X/TZVP) in solution. The results clearly indicate that 3-methyl-1-phenyl-4- (phenyldiazenyl)-1H-pyrazol-5-amines are present in pure azo tautomeric form, while 5-methyl-2-phenyl-4-(2-phenylhydrazono)-2,4-dihydro-3H-pyrazol-3-ones exist as hydrazone tautomers. The tautomeric state is not substantially influenced by nitro group substitution in the phenyl ring. Consequently, the studied compounds can be used as model tautomers in the NMR evaluation of the tautomeric state in various azo dyes in solution. According to the crystallographic data (obtained by us or available in the literature) the conclusions about the tautomerism of the studied compounds in solution are valid in solid state as well