A chromogenic macrocycle exhibiting cation-selective and anion-controlled color change: an approach to understanding structure-color relationships.

[reaction: see text] A N-azo-coupled macrocycle (1) with a NO(2)S(2) donor set was synthesized as a chromoionophore and shown to exhibit Hg(2+) selectivity; anion control of the color of the Hg(2+) complex was observed. As a first approach, the nature of the color generation process was probed by determining the crystal structures of the two different colored species obtained with perchlorate and iodide anions. The results can be explained in terms of a "Push-n-Pull Process", which serves to illustrate how the coordinating ability of the anion controls the color change through formation of endo- or exo-metal complexes. The use of "simple" salts to induce color-switching of the above complex species was also reported.