We demonstrate that the electrical 'switching" behavior of single molecules connected between two electrode contacts can be controlled by altering their structure and electrochemical characteristics. The electrical properties of gold vertical bar molecule vertical bar gold single molecule junctions incorporating HS(CH2)(6)-X-(CH2)(6)SH, where X = viologen (4,4'-bipyridinium) or pyrrolotetrathiafulvalene, are determined using a scanning tunneling microscopy based technique. The switching behavior, controlled through a tuneable electrochemical gate, changes from an on-off response (viologen) to an off-on-off response (pyrrolotetrathiafulvalene) on changing the central redox group. In contrast, the electrical properties of junctions incorporating redox-inactive HS(CH2)(6)-1,4-C6H4-(CH2)(6)SH do not alter significantly as a function of applied potential.