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 investigated in the presence of alkali metal cations (Na+ or K+ for 13- or 16-membered azocrowns, respectively). Two types of recognition behaviour have been observed. The first type consists in a positive shift of the redox potential (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 recognition results were obtained using SW voltammetry with the peak potential for the complex up to 0.32 V (for the azoxycrown derivative) more positive of the original azocrown compound. For azo- and azoxycrowns exhibiting ‘two-wave’ behaviour, the binding enhancement (ratio of binding constants for reduced and neutral species) was 102–105 M−1. For other azocrown compounds with single peak recognition behaviour the binding constants for reduced species were estimated to be 104–105 M−1. In general, K+ binds to 16-membered neutral azocrowns more strongly than Na+ to 13-membered ones, whilst binding to the anion-radical has the opposite trend.