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 radical ions with various donors and accepters by using high speed potential pulses at a microelectrode. The reaction pathways were identified by examining the reaction orders of the limiting reagents and by comparison of the temporal emission pulses with simulated reaction schemes. For reactions with a free energy value more negative than that required to directly form the excited state singlet (3.06 eV), first-order behavior was observed for DPA radical ions indicating direct singlet formation (S-route). For the remaining systems the reaction order was second order in DPA radical ions, indicative of excited singlet formation via triplet-triplet annihilation (T-route). Despite evidence of triplet quenching by the unreacted radical ions, the efficiency of photon production at high concentrations of DPA by the T-route (0.012) approached that of the S-route, and is much higher than previously reported. For DPA(.+) and naphthyl phenyl ketone a mixed reaction order was found. The free energy of this reaction is deficient by 0.05 eV, and in this case, direct singlet formation is able to compete with the more energetically favorable triplet pathway. The high efficiency experimentally found for the T-route suggests that it may be an alternate scheme to employ in solid-state display devices.