Open AccessWater-Soluble Colorimetric Amino[bis(ethanesulfonate)] Azobenzene pH Indicators: A UV–Vis Absorption, DFT, and 1H–15N NMR Spectroscopy Study
Author(s) -
Maria A. Cardona,
Damjan Makuc,
Konrad Szaciłowski,
Janez Plavec,
David C. Magri
Publication year - 2017
Publication title -
acs omega
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.779
H-Index - 40
ISSN - 2470-1343
DOI - 10.1021/acsomega.7b00887
Subject(s) - protonation , azobenzene , chemistry , proton nmr , intramolecular force , metal ions in aqueous solution , photochemistry , crystallography , metal , inorganic chemistry , nuclear chemistry , ion , stereochemistry , molecule , organic chemistry
Water-soluble azobenzene derivatives containing amino[ bis (ethanesulfonate)] groups are demonstrated as colorful pH indicators in water and on filter paper. Vibrant color changes were observed from yellow/orange to pink between pH 1 and 4, which are attributed to an intramolecular charge-transfer mechanism. The p K a s of the indicators range from 2.1 to 2.6. 1 H/ 1 H- 15 N NMR studies in deuterium oxide reveal that the protonation of the azobenzene pH indicators occurs predominantly at the β-azo nitrogen atom, in agreement with the density functional theory calculations. Excellent selectivity for protons was confirmed in water over common biologically relevant metal ions. Studies in methanol, however, indicate that the pH indicator with a methoxy group ortho to the amino[ bis (ethanesulfonate)] group facilitates the selective coordination of Cu 2+ with a binding constant pβ Cu2+ of 4.6 ± 0.1. The indicators complement the existing library of azobenzene indicator dyes and may be useful for measuring the environmental pH at higher proton concentrations.