A simple and efficient anionic chromogenic chemosensor based on 2,4-dinitrodiphenylamine in dimethyl sulfoxide and in dimethyl sulfoxide-water mixtures

Solutions of 2,4-dinitrodiphenylamine (1) in dimethylsulfoxide (DMSO) are colorless but upon deprotonation they become red. Addition of various anionic species (HSO4-, H2PO4-, NO3-, CN-, CH3COO-, F-, Cl-, Br-, and I-) to solutions of 1 revealed that only CN-, F-, CH3COO-, and H2PO4- led to the appearance of the red color in solution. The presence of increasing amounts of water in solutions containing 1 made it progressively selective toward CN- and the system with the addition of 4.3% (v/v) of water was highly selective for CN- among all anions studied. The experimental data collected indicated that proton transfer from I to the anion occurs, and a model was used to explain the experimental results, which considers two 1:anion stoichiometries, 1: 1 and 1:2. For the latter, the data suggest that the anion forms firstly a hydrogen-bonded complex with a second anion equivalent necessary for the abstraction of the proton, with the formation of a [HA(2)](-) complex. The study performed here demonstrates the important role of the environment of the anion and I for the efficiency of the chromogenic chemosensor. Besides the different affinities of each anion for water, the solvation of both the anion and 1 is responsible for reducing the interaction between these species. In small amounts, water or hydrogen-bonded DMSO-water complexes are able to stabilize the conjugated base of I through hydrogen bonding, making I more acidic, which explains the change from 1:1 and 1:2 toward 1:1:1:anion stoichiometry upon addition of water. In addition, water is able to solvate the anion and also 1, which hinders the formation of 1:1 hydrogen-bonded 1:anion complexes prior to the abstraction of the proton. (C) 2009 Elsevier B.V. All rights reserved.