Switching azonaphthols containing a side chain with limited flexibility. Part 1. Synthesis and tautomeric properties

Abstract

A series of azo dyes, possessing amide fragments with restricted flexibility tethered to 4-(phenyldiazenyl)naphthalen-1-ol, was obtained from 1-hydroxy-2-naphthoic acid by subsequent conversion to amides and diazo coupling. It was shown that the position of the tautomeric equilibrium in solution strongly depends on the solvent in both UV and NMR concentration scale. The compounds exist as pure enol forms in chloroform and hydrocarbons, while in polar solvents (acetone, acetonitrile, alcohols) a tautomeric mixture is observed. According to the quantum-chemical calculations the aggregation of the keto tautomer is the possible reason for this shift in the position of the tautomeric equilibrium. To support the theoretical predictions, it was found that from acetone the keto form crystallizes as a dimer with hydrogen bonding between N 1 eH in the one molecule and amide C]O in the other forming a three-dimensional structure. The importance of the side-chain nitrogen atom on the dimer formation was confirmed by solution and solid state study of 4-(phenyldiazenyl)-2-acetylnaphthalen-1-ol. The results indicate that the new azo-dyes obtained could be suitable candidates for switching and sensing applications in non-polar solvents.

Tautomerism: Introduction, History, and Recent Developments in Experimental and Theoretical Methods

“…Unfortunately, the efficiency of the switching system crucially depends on tautomerism in the parent skeleton: it works in the case of the azonaphthols 13 and 14 [14] and the heterocyle 15 [15], but does not in azophenol 16 or azoanthranol 17, where the tautomeric equilibrium is strongly shifted a priori. The replacement of the side arm, as it is in 18 [16] and 19 [17], does not stabilize the enol tautomer because of keto dimer formation (18) or double hydrogen bonding stabilization of the keto tautomer (19). …”

Tautomers and UV-Induced Photoisomerization of a Strongly Intramolecularly H-Bonded Aromatic Azo-Dye: 1-(Cyclopropyl)diazo-2-naphthol

“…Several theoretical and experimental studies on the azo-hydrazone tautomeric equilibrium can be found in the literature. − However, due to the dependence of the tautomeric forms on the surrounding chemical environment, their structural and spectroscopic characterizations are not trivial. Indeed, many issues concerning the relative stability of these species and related with the intramolecular proton transfer that interconvert them into each other remain unclear.…”

Tautomerism as primary signaling mechanism in metal sensing: the case of amide group

“…Obviously, the formation of the seven-membered hydrogen-bonding ring in 6E cannot compete with the flexibility of the system in the case of the intermolecular association. Compared to another tautomeric C=O containing ionophore, recently developed [13], it seems that the existence of a carbonyl group leads in some cases to stabilization of the keto tautomer through formation of associates.…”