Torsionally responsive C3-symmetric azo dyes: azo-hydrazone tautomerism, conformational switching, and application for chemical sensing.

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.