FREE-ENERGY CONTROL OF REACTION PATHWAYS IN ELECTROGENERATED CHEMILUMINESCENCE

Electrogenerated chemiluminescence (ECL) from 9,10-diphenylanthracene (DPA) in acetonitrile has been examined following reaction of its radi cal ions with various donors and accepters by using high speed potenti al pulses at a microelectrode. The reaction pathways were identified b y examining the reaction orders of the limiting reagents and by compar ison of the temporal emission pulses with simulated reaction schemes. For reactions with a free energy value more negative than that require d to directly form the excited state singlet (3.06 eV), first-order be havior was observed for DPA radical ions indicating direct singlet for mation (S-route). For the remaining systems the reaction order was sec ond order in DPA radical ions, indicative of excited singlet formation via triplet-triplet annihilation (T-route). Despite evidence of tripl et quenching by the unreacted radical ions, the efficiency of photon p roduction at high concentrations of DPA by the T-route (0.012) approac hed that of the S-route, and is much higher than previously reported. For DPA(.+) and naphthyl phenyl ketone a mixed reaction order was foun d. The free energy of this reaction is deficient by 0.05 eV, and in th is case, direct singlet formation is able to compete with the more ene rgetically favorable triplet pathway. The high efficiency experimental ly found for the T-route suggests that it may be an alternate scheme t o employ in solid-state display devices.