Electrochemical recognition properties of 13- and 16-membered azo- and azoxycrowns in solution

The voltammetric reduction of 13- and 16-membered azo- and azoxycrowns is i nvestigated in the presence of alkali metal cations (Na+ or K+ for 13- or 1 6-membered azocrowns, respectively). Two types of recognition behaviour hav e been observed. The first type consists in a positive shift of the redox p otential (100-150 mV) upon addition of metal cation (single peak behaviour) . The second type consists in the appearance of two separated redox peaks ( 150-320 mV more positive, two-wave situation). Better electrochemical recog nition results were obtained using SW voltammetry with the peak potential f or the complex up to 0.32 V (for the azoxycrown derivative) more positive o f the original azocrown compound. For azo- and azoxycrowns exhibiting 'two- wave' behaviour, the binding enhancement (ratio of binding constants for re duced and neutral species) was 10(2)-10(5) M (-1). For other azocrown compo unds with single peak recognition behaviour the binding constants for reduc ed species were estimated to be 10(4)-10(5) M-1. In general, K+ binds to 16 -membered neutral azocrowns more strongly than Na+ to 13-membered ones, whi lst binding to the anion-radical has the opposite trend. (C) 2001 Elsevier Science B.V. All rights reserved.