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The Electrochemiluminescence of the
Figure 1. Cyclic voltammogram with light emission as a function of potential. Scan rate = 200 mT’/sec; [diphenylanthracene] = 3.0 X 10-3 M in DMF; solution was exhaustively degassed with prepurified helium; platinum electrode.
dation process indicating a negligible amount of electrogenerated anion radical is consumed by the subsequent processes. These solutions were deoxygenated with prepurified helium by bubbling for periods of 15-30 min. It is interesting to note that a solution which was maintained a t the reduction potential of diphenylanthracene, - 1.9 Q (vs.sce), exhibited a constant light emission when stirred vigorously with a stream of prepurified helium. Furthermore, when the potential is sufficiently negative so that the dianion of the hydrocarbon is the product, no light is observed. Although other workers have previously suggested that direct formation of excited states may be possible at an ele~trode,~‘ the following experiments demonstrate conclusively that a reaction must occur in the “bulk” (ie., diffusion layer) of solution. Similar conclusions have been made theoretically for such a process a t a metal electrode.’ I n cyclic voltammetry experiments (scan rate = 200 mQ/sec potential range between 0 and -2.0 Q) emission is observed as the reduction peak occurs and the light decays only slowly during the reoxidation portion of the cycle as the electrode proceeds to more positive potentials (see Figure 1) One concludes that some of the product of the charge-transfer reaction has diffused away from the electrode and has engaged in a homogeneous chemical reaction in the bulk of the solution since the rate constant is much too long for emission from an excited state (tIl2of decay > 20 sec). A second type of experiment which corroborates this conclusion was a chronoamperometric study in which the potentiostat was turned off after 15 sec of electrolysis. The solution was then bubbled vigorously with prepurified helium after standing for 15-30 sec. Emission accompanies the bubbling by helium as the diffusion layer is destroyed. The nature of the reaction following electron transfer has been studied qualitatively. These results also indicate that the light emitted arises from a bulk chemical reaction. On changing solvents from dimethylform-
with the fluorescence spectrum of the neutral hydrocarbon.4 Unlike the previously reported electrochemiluminescent systems6which require stepping to positive potentials after the reduction process has taken place in order to effect iight emission, this study reports low intensity light emission (approximately 3 orders of magnitude less than annihilation intensities) which occurs upon direct reduction of the hydrocarbon and which is directly related to the rate of diffusion of the hydrocarbon to the electrode. Chronoamperometric experiments with potential reversale and cyclic voltammetry show that the electrode process is a one-electron reversible reaction. Chronocoulometric experiments with a double potential step show linear slopes for Q plotted os. ?i for the reduction process and a linear slope vs. [(t)’l2- (2 - 7)’9 for the reoxi-
(1) (a) All electrochemical experiments were carried out with a three-electrode potentiostat. (b) D. D. DeFord, presented before the Analytical Division, 133rd National Meeting of the American Chemical Society, San Francisco, Calif., 1958; (c) W. M. Schwarr and I. Shain, Anal. Chem., 35, 1770 (1963). (2) (a) K. S. V. Santhanam and A. Bard, J . Amer. Chern. SOC.,88, 2669 (1966); (b) R. E. Sioda, J . Phys. Chem., 72, 2322 (1968). (3) Emission was measured using an EM1 6256s photomultiplier tube operat,ed at 1700 V. (4) Spectra were obtained using a Jarrell-Ash 0.25-m monochromator. (5) (a) A. Zweig, A. K. Hoffman, D. L. Maricle, and A. H. Maurer, Chem. Commun., 106 (1967); (b) A. Zweig, A. K. Hoffman, D. L. Maricle, and A. H. Maurer, J . Amer. Chem. Soc., 90, 261 (1968); (c) D. L. Maricle and A. Maurer, ibid., 89, 188 (1967); (d) A. Zweig, D. L. Maricle, J. S. Brinen, and A. H. Maurer, ibid., 89, 473 (1967); (e) E. A. Chandross and R. E. Visco, J. Phys. Chem., 72, 378 (1968); (f) A. Zweig and D. L. Maricle, ibid., 72, 377 (1968); (9) R. E. Visco and E. A. Chandross, J . Amer. Chem. SOC.,86, 5350 (1964); (h) E. A. Chandross, J. W. Longworth, and R. E. Visco, ibid.. 87, 3269 (1965). (6) W. M. Schwarz and I. Shain, J . Phys. Chem., 69, 30 (1965). (7) R. A. Marcus, J . Chem. Phys., 43, 2654 (1965).
Diphenylan thracene Radical Anion
by Michael D. Malbin and Harry B. Mark, Jr. Department of Chemistry, The UnCersity of Michigan, Ann Arbor, Michigan (Received February 18, 1969)
The one-electron electrolytic reduction1 of diphenylanthracene2 or rubrene at a platinum or mercury stationary electrode immersed in a diinethylformamide solution (containing tetrabutylammonium perchlorate) was found to give rise to a concomitant emission of light.* The luminescence which occurs was identical
a
I
*.5
I
0.0
I I I -50 -1.00 -1.50 VOLTS ( vs. SCE)
The Journal of Physical Chemistry
I
-200
2787
NOTES
FIRST CYCLE
-A \
SECOND. CYCLE
oxygen reduction in nonaqueous media.Q Oxygen adsorbed on the platinum electrode accounts for the abnormally high oxygen concentration observed on the first cycle. This suggests that the superoxide ion may be involved in the process. The reactions of aromatic hydrocarbon radical anions with molecular oxygen, superoxide anion, and peroxide are currently under study in hopes of relating the above process with other luminescent systems which have been reported.1° Although aromatic hydrocarbon radicals10Band anionic species’l have been shown to produce luminescence on reaction with oxygen, this constitutes the first evidence for such a reaction of aromatic hydrocarbon radical anions. Acknowledgment. The authors wish to acknowledge the support of the Petroleum Research Fund through Grant No. PRF 2880-A3,5.
-.50
-1.00 -1.50 VOLTS (E. SCE)
-2.00
Figure 2. Cyclic voltammogram with light emission as a function of potential. [Diphenylanthracene] = 3.0 X M in DMF; scan rate = 200 mV/sec. Solution was bubbled for 30 min with nitrogen gas containing 1% 02. The first cycle shows t h e irreversible peak due t o adsorbed material on t h e platinum electrode. All subsequent cycles (10) gave traces identical with those shown for cycle 2.
amine t o dimethylacetamide* the luminescence was still observed with approximately the same intensity under identical conditions. These experiments indicate that the chemical reaction is not proton abstraction from the solvent. Also, trace known amounts of water and phenol were added to the system and it was found that in both cases light emission intensity decreased with increasing proton-donor concentration. Attempts were made to remove completely all traces of molecular oxygen from the system by use of a more sophisticated bubbling techniquelsd but as shown previously,sd they were found to be unsuccessful although the oxygen levels were well below the limits of electrochemical detectability. It was found that when bubbling solutions with nitrogen mixtures containing small known amounts of oxygen (1 to 5%), the intensity of the emitted light was enhanced as the oxygen content increased and were considerably greater when compared to those which had been exhaustively deoxygenated by helium. It was also found that in each case the first cycle of a continuous cyclic voltammetry study was accompanied by a more intense emission than any succeeding cycle and usually the electrochemistry showed a very small irreversible peak a t -0.85 V (see Figure 2). This peak corresponds to that observed for
(8) Some controversy exists in the electrochemistry of aromatic hydrocarbons as to whether a proton abstraction reaction can occur to a significant extent in dimethylformamide. See (a) M. Peover, “Electrochemistry of Aromatic Hydrocarbons and Related Substances” in “Electroanalytical Chemistry,” Vol. 11, A. Bard, Ed., Marcel Delrker, Inc., New York, N. Y.,1967, p 28, and references cited therein. For evidence that dimethylformamide is labile in the presence of a strong base and/or electron donor, see (b) H. Brederick, F. Effenberger, and R. Gleiter, Angew. Chem. Int. Ed., 4, 951 (1965), and (c) J. C. Powers, R. Weidner, and T. G. Parsons, Tetrahedron Lett., 1713 (1965). For a more current study of the reactions of aromatic hydrocarbon radical anions with proton donors, see (d) J. Janata and H. B. Mark, Jr., J. Phys. Chem., 72, 3616 (1968). The use of dimethylacetamide negates this possibility. (9) (a) D.L. Maricle and W.G. Hodgson, Anal. Chem., 37, 1562 (1965); (b) A. D.Goolsby and D. T. Sawyer, ibid., 40, 83 (1968). (10) (a) R. E’. Vassil’ev and A. A. Vichutinski, Nature, 194, 1276 (1962); (b) J. Stauff, Photochem. Photobiol., 4, 1199 (1965); (c) J. Stauff, Angew. Chem. Int. Ed., 7, 477 (1968); (d) E.J. Bowen, “Organic Photochemistry,’’ International Symposium, Strasbourg, 1964,Butterworth and Co. Ltd., London, p 473, (11) K. D. Legg and D. M. Hercules, J . Amer. Chem. Soc., 91, 1902 (1969).
Molecular Orbital Theory of Electron DonorAcceptor Complexes. 111. The Relationship
of State Energies and Stabilization Energies to the Charge-Transfer Transition Energy
by R. L. Flurry, Jr., and Peter Politzer Department of Chemistry, Louisiana State University in New Orleans, New Orleans, Louisiana 70122 (Received Febrtkary 17,1969)
I n the earlier papers of this series (parts I and 11),1 there has been presented a linear combination of molecular orbitals approach to the quantum-mechanical description of donor-acceptor complexes. An important feature of this treatment is the explicit inclusion of the (1) Part I: R. L. Flurry, Jr., J . Phys. Chem., 69, 1927 (1965); part 11: R. L. Flurry, Jr., {bid., 73, 2111 (1959). Volume 79,Number 8 August 1960
