| For several years, workers in the area of electrogenerated chemiluminescence (ECL) have labored to explain the mechanisms of light production by high-energy electron-transfer reactions. Through a variety of experiments, which have been reviewed extensively, a set of schemes has been developed. In general, an electron transfer reaction that is sufficiently energetic to produce the emitting state of a product is expected to do so directly. This "S-route" to emission in such "energy-sufficient" systems is therefore quite straightforward. On the other hand, many systems are known in which the electron transfer is insufficiently energetic to leave a product in the observed emitting state. These "energy-deficient" cases are usually regarded as proceeding to emission by producing triplet intermediates in electron transfer, then generating the emitters by triplet-triplet annihilation (T route).; The principal kinetic tool for studying ECL, i.e. the sequential step technique for producing light transients, has repeatedly given results that are not consistent with this picture. Thus, there is strong evidence that the kinetic features of most systems are more complicated than is generally believed.; In this experimental method, one generates the oxidant and reductant from precursors at a small electrode in quiescent solution. In the first step (of duration t(,f)), the first reagent is produced and diffuses into the solution. At t = t(,f), the potential of the electrode is switched to a value where the second reagent is produced. It then diffuses into the solution and reacts with the first reagent at a total rate (N, moles/sec) that declines with time as the first species is consumed. The reaction rate can be predicted from the laws of diffusion. A light transient results from the time dependence of the redox reaction rate N and the kinetics of light producing processes.; For an S-route system, the linkage between the total light intensity (I, einsteins/sec) and N is very simple, viz. I = (phi)(,f) (phi)(,s)N where (phi)(,s) is the probability that a redox event will produce an emitter and (phi)(,f) is the quantum yield for emission from that species. Usually, the emitter is a singlet and the emission is fluorescence. The product (phi)(,f)(phi)(,s) should be independent of time because (phi)(,s) is a branching ratio for an elementary process, and the excited state lifetime is generally too short to be quenched by components in the system having time-dependent concentrations. Thus the intensity ought to be proportional to N, and the transient should follow a readily predictable shape. Such behavior has almost never been seen among energy-sufficient systems, where it ought to be routine.; The development of a model S-route system is crucial for verification of our current understanding of ECL systems. T-route systems are more complicated kinetically and theoretically, and closed form solutions are more difficult, if not impossible, to obtain. S-route systems can be treated exactly and decay curve analysis is much more easily understood.; In the absence of a paradigm showing predictable behavior with respect to all experimental variables, one cannot be sure whether the deviations from nonideality manifest defects in the experimental approach, significant misconceptions about the fundamental chemistry, or both.; It is the goal of the present work to establish the tris(2,2'-bipyridine)ruthenium(II) and 5,6,11,12-tetraphenylnapthacene systems as reasonably well-behaved model S-route systems in N,N-dimethylformamide at low temperatures (-25(DEGREES) to -40(DEGREES)). Factors influencing the mechanisms of light production in ECL systems, such as adsorption onto the electrode, solvent system, temperature, ion instability, etc., have been explored in this study for the above and other energy-sufficient systems. |
