Torsionally Responsive C₃-Symmetric Azo Dyes: Azo−Hydrazone Tautomerism, Conformational Switching, and Application for Chemical Sensing

Abstract

An efficient triple azo coupling reaction between anilines and phloroglucinol furnished a series of C(3)-symmetric molecules 7-9 supporting multiple conjugation pathways that converge at the molecular core. A combination of (1)H/(13)C NMR spectroscopy, X-ray crystallography, and density functional theory computational studies provided a coherent picture of the [n,pi]-conjugated molecular core, which is best described as the tris(hydrazone) [rather than tris(azo)] tautomer stabilized by resonance-assisted hydrogen bonding. For a homologous series of compounds, an increase in the torsional angles between the planar molecular core and the peripheral aryl groups results in a systematic blue shift in the low-energy electronic transitions (7, 523 nm; 8, 505 nm; 9, 445 nm in CHCl(3)) that qualitatively correlates with the shrinkage of effective conjugation through structural distortion. Similar spectral shifts could also be induced by amine substrates that interact with the intramolecular hydrogen-bonding network to trigger bond-twisting motions. Specifically, a brief exposure of a thin film of 7 to vapor samples of butyl-, hexyl-, diethyl-, and diisopropylamine resulted in a rapid and reversible color change from pink to dark-orange. Under similar conditions, however, triethylamine did not elicit any detectable color change, despite the fact that it has a significantly higher vapor pressure than n-hexylamine. These findings implicate that the hydrogen-bonding donor ability is a key requirement for the binding-induced conformational switching, which allows for direct naked-eye detection of volatile amines under ambient conditions.

Aryl and heteroaryl azo dyes derived from 6, 8‐dichloro‐4‐hydroxyquinolin‐2 (1H)‐one: synthesis, characterisation, solvatochromism and spectroscopic properties

“…According to results published in the literature, the restricted rotation around the C=N double bonds in the hydrazone form would lead to two distinctive geometric isomers (E and Z isomers), as shown in Scheme . In order to examine the existence of hydrazone isomerism, we also measured the 1 H NMR of 6, 8‐dichloro‐3‐(3, 4‐dimethoxyphenylazo)‐4‐hydroxyquinolin‐2(1 H )‐one 5 .…”

NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State

“…Both δ( 15 N) and 1 J N-H in the two tautomers are characteristically different and can be employed to study the equilibria [-N(1)=N-(δ N-1 = 119.4 ppm) -N(1)H-N= (δ N-1 = −42.6 ppm); δ N-2 = 219.8 ppm; 1 J N-1,NH = 96 Hz] [6] but model values are needed of the certain tautomers 66a,b (therefore reference compounds have to be included that exhibit no temperature dependence of the implemented NMR parameters [6,129], or the pure tautomers are computed theoretically) [130,131]. In addition, δ( 13 C) of a characteristically different aromatic ring carbon atom was used [74], the β-isotope effect [132] was employed, and the chemical exchange of the N-H protons was followed by nuclear Overhauser effect spectroscopy by NOESY experiments adopting various mixing times [133].…”

DETERMINATION OF THE DISSOCIATION CONSTANTS OF SOME p-SUBSTITUTED AROMATIC HYDRAZONES

“…On the other hand, tautomerism and isomerism phenomena for these compounds are of particular chemical and theoretical interest. The hydrazo tautomer exists in neutral solutions whereas the azo tautomer exists in strong acidic or alkaline solutions [11].…”