Solid state electrochemically generated luminescence based on serial frozen concentration gradients of Ru-III/II and Ru-II/I couples in a molten ruthenium 2,2'-bipyridine complex

The attachment of two polyethylene glycol tails (n = 7, MW = 350) to ruthenium tris(bipyridine) via ester links on the 4,4'-positions of one of the bipyridine ligands yields a highly viscous (eta approximate to 10(7) cP) molten salt (abbreviated [Ru(bpy)(2)(bpy(CO(2)MePEG350)(2))](ClO4)(2)) that glasses at ca. -5 degrees C. At room temperature, the ionic conductivity of the melt is sufficiently high that application of 2.4 V across the fingers of a Pt interdigitated electrode array (IDA) coated with the melt leads to the electrolytic development of serial concentration gradient microstructures of Ru-III/II and Ru-II/I states. At the intersection of the two concentration gradients, in the interior of the coating, reaction between the Ru-III and Ru-I states leads to ECL emission with an efficiency of 0.2% photons/electron. Cooling a concentration gradient-containing film to -20 degrees C under voltage bias, so as to preserve the gradient microstructure, yields a film that has an emission efficiency of 0.1%, a current and light emission response that rapidly changes with the applied voltage bias, and a diode-like current-voltage profile with a ca. 100 rectification ratio.