Electrogenerated chemiluminescence. VII. Influence of an external magnetic field on luminescence intensity

The effect of an external magnetic field on the intensity of electrogenerated chemiluminescence (ecl) was determined for nine systems involving anthracene, 9,lO-diphenylanthracene (DPA), rubrene, 1,3,6,%tetraphenylpyrene (TPP), and fluoranthene as emitting species in N,N-dimethylformamide (DMF) solutions. Enhancements in emission intensity up to 27% with increasing field strength were noted for the energy-deficient oxidations of anthracene, DPA, rubrene, and TPP anion radicals by Wurster’s Blue cation, for the energy-deficient oxidation of fluoranthene anion radical by the cation radical of 10-methylphenothiazine, and for the energy-deficient reduction of the rubrene cation radical by the anion radical derived fromp-benzoquinone. No field effect was seen on luminescence arising from the apparently energy-sufficient mutual annihilation of the anion and cation radicals derived from DPA. The field enhanced luminescence from the reaction between the rubrene anion and cation radicals, but it exerted no effect on the intensity of emission from the TPP anion-cation radical annihilation. These results have been interpreted as reflecting a dual mechanism for chemiluminescent electron-transfer processes which divides on or about the line of energy sufficiency. In particular it is suggested that the field effect accompanying luminescence from energy-deficient systems arises from an inhibition by the field of the rate of quenching of emitter triplets by radical ions. Thus, the results are consistent with the triplet mechanism for luminescence from energydeficient systems. This interpretation also indicates that energy-sufficient systems yield luminescence without required triplet intermediates. For the two marginal systems involving rubrene and TPP alone, it is suggested that the rubrene anion-cation annihilation gives rise to luminescence predominantly via the triplet mechanism, while the TPP anion-cation reaction may be essentially energy sufficient, and that most luminescence from this process arises by direct population of the emitting state. arly in 1969, we first reported our observation E that an external magnetic field exerts unusual effects on the intensity of electrogenerated chemiluminescence (ecl) from two widely studied systems. Specifically, an enhancement of luminescence intensity was seen with increasing field strength for the light arising from the oxidations of the anthracene and 9,lOdiphenylanthracene (DPA) anion radicals by the Wurster’s Blue cation (the cation radical of N,N,N’ ,N’tetramethyl-p-phenylenediamine, WB) in N,N-dimethylformamide (DMF). In an interesting contrast, the intensity of light accompanying the mutual annihilation of DPA anion and cation radicals was unaffected (1) L. R. Faulkner and A. J. Bard. J. Amer. Chem. SOC., 91, 209 (1969). by the field. The two reactions involving the Wurster’s Blue cation as an oxidant are energy deficient in the sense that the energy contained in a single emitted photon is greater than the energy available from a single electron-transfer event. The third system is apparently energy sufficient. Thus, the contrast in the field effect on luminescence from these two kinds of systems was taken as evidence for long-suspected mechanistic differences between them. *-* Specifically, the results suggested that there exists at least one elementary reaction in the mechanism for luminescence from the energy-deficient systems whose rate is influenced by the (2) K. S. V. Santhanam and A. J. Bard, ibid., 87, 139 (1965). (3) A. Weller and K. Zachariasse, J. G e m . Phys., 46, 4984 (1967). (4) G. J. Hoytink, Discuss. Furaduy SOC., No. 45, 14 (1968).