COMMUNICATIONS TO THE EDITOR
378 Finally, the comment' on the triplet quenching of 1,3,5-hexatriene neglected the fact that we had noted similar reservations about active olefins used as quenchers.6 We also noted5 that 1,3,5-hexatriene failed to quench 1,3,4,7-tetraphenylisobenzofuranpreannihilative fluorescence emission.69'0 The ion radicals of the latter should be as reactive toward the triene as the phenanthrene anion radical. This is, therefore, still uncontested evidence for triplet quenching in the phenanthrene system and lack of triplet quenching in the isobenzofuran system.
potentiometry and cyclic voltammetry. In marked contrast to results in DMF, there was no luminescence in benzonitrile until both cations and anions were generated. We see no reason why the heterogeneous generation of triplets should be so solvent dependent. In DMF we have detected solvent decomposition products formed by reaction with radical cations, even though the potential never reached the normal oxidative decomposition of the solvent. Thus, results obtained in DMF for oxidations which occur much above the sce potential are to be viewed with caution. We have no disagreement with the postulation of (10) A. Zweig, A. K. Hoffmann, D. L. Maricle, and A. H. Maurer. fluorescence arising from homogeneous TTA. We have Chem. Comm., 106 (1967). no comment on the assignment of the broad emission CENTRAL RESEARCH DIVISION ARNOLD ZWEIO from phenanthrene as its phosphorescence, although we AMERICANCYANAMID COMPANY DONALD L. MARICLE note that Ar- and Ar+ are likely to be efficient difSTAMFORD, CONNECTICUT 06904 fusion-controlled quenchers of 3Ar*. However, we feel RECEIVED AUGUST21, 1967 that the observations of Zweig, et a1.,2 cannot be explained on the basis of direct formation of 3Ar* at metal electrodes via reduction of Ar+ or oxidation of Ar-. Marcus' suggested recently that, on theoretical On Preannihilation Electrochemiluminescence grounds, the electrochemical formation of excited states at metal electrodes is unlikely. and the Heterogeneous Electrochemical We shall show with thermodynamic arguments that, Formation of Excited States at electrode potentials sufficient to form aAr* from Ar+ or Ar-, 3Ar* would necessarily be quenched via Sir: There have been two recent communications diselectron exchange with the electrode. Consider the cussing the mechanism of preannihilation electrofollowing chemiluminescence (ECL). The first paper,' which aAr* = 'Ar light AF* = -ET(eV) described a study of rubrene in dimethylformamide (DMF), suggested that a portion of the observed Ar e- = Ar'&d(V us. sce) fluorescent emission "may be due to direct generation (V us. sce) Ar+ e- = Ar of the triplet in a heterogeneous electron-transfer step," via electron transfer directly to or from the appropriate We may now calculate couples for 3Ar* radical ion of the hydrocarbon, followed by tripletAr+ e- = 3Ar* 3&ox = 'Eox - ET (V us. sce) triplet annihilation (TTA). The second paper2reported observation of luminescence assigned to phenanthrene 3Ar* e- = ArET (V us. sce) a&red = phosphorescence upon electrooxidation of the phenanthrene radical anion in DMF solution and implied Values for representative hydrocarbons are given in that the same generation of triplets occurred. We shall Table I. discuss some of our experimental results as well as some Standard potentials for couples involving 3Ar* define simple thermodynamic arguments which make the potential ranges over which 3Ar* would be electrohypothesis of direct electrogeneration of triplets active. Since ET is always positive, is always less dubious. positive than 'Eox while *&red is more positive than We consider first the case of rubrene in DMFa3 Our '&red. At potentials above 3Ar* would be oxidized studies in this solvent,* although not as extensive as those of the above authors,' indicated that the pre(1) D. L. Maricle and A. H. Maurer, J . Ana. Chem. soc., 89, 188 annihilation light is about 1% of that generated upon (1967). cation-anion annihilati~n.~We were concerned about (2) A. Zweig, D. L. Maricle, J. 8. Brinen, and A. H.Maurer, ibid., 89,473 (1967). these results because we had previously6 studied 9,lO(3) D.M. Hercules, R. C. Lansbury, and D. K. Roe, ibid., 88, 4578 diphenylanthracene and 9,lO-dimethylanthracene in (1966). acetonitrile, where the respective cations and anions are (4) R.E.Visoo, E. A. Chandross, and J. W. Longworth, unpublished results. stable, and had failed to observe any preannihilation (5) R. E. Visco and E. A. Chandross, J . Am. Chem. soc., 86, 5350 ECL. We have now studied rubrene extensively at (1964); E.A. Chandross, J. W. Longworth, and R. E. Visco, ibid., Pt electrodes in benzonitrile.6 This system has the 87,3259 (1965). advantage over DMF that both the rubrene cation (6) R. E. Visco, E. A. Chandross, and R. Sonner, to be published. and anion are very stable, as determined by chrono(7) R. A. Marcus, J . Chem. Phys., 43, 2664 (1966).
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The Journal of Physical Chemistry
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379
COMMUNICATIONS TO THE EDITOR Table I" Naphthalene
E: (1Ar*) ET(aAr*) l&redd
'Eok %ox
'&red
3.8 +2.6 -2.6 +1.5 -1.1 0.0
Anthracene
Phenanthracene
Rubrene
3.2 4-1.8 -2.0 4-1.1
3.5 +2.7 -2.5 +1.5 -1.2 +0.2
2.3 +1.2O -1.4 +l.O1 -0.2 -0.2
-0.7 -0.2
ET is the energy difference between aAr* and 1Ar and is positive. AF = nF&, but we discuss only cases where n = 1 and & (V) is equal directly to -AF (eV). The entropy term due to degeneracy of the triplet is small and is neglected. Ea (eV) is the energy of the 0-0 fluorescence band. The rubrene value is estimated from the absorption spectrum. Estimated, based on 10,250 cm-1 (1.27 eV) found for tetracene by S. P. McGlynn, M. R. Padhye, and M. Kasha, J . Chem. Phys., 23, 593 (1955). Except for rubrene,1*6G. J. Hoijtink, Rec. Trav. Cheim., 74, 1525 (1955). e Except for rubrene,1j6 E. S. Pysh and N. C. Yang, J . Am. Chem. Soc., 86, 2124 (1963).
'
to Ar+. However, at potentials below *Sox, Ar+ will be reduced to 'Ar. Thus, in the range between 'Sox and 'Ar* will be quenched by the electrode via electron transfer reactions. A parallel argument for the conversion of 3Ar*to 1Ar via Ar- demonstrates that the triplet will be quenched at electrode potentials between '&red and '&red. For the hydrocarbons cited in the table, it is obvious that 'Gox is generally lower than '&red. Therefore, 'Ar* at the electrode must be quenched over the potential range from to '&red. This is a general conclusion for hydrocarbons smaller than tetracene, because the separation between 'Sox and '&red is usually a few tenths of a volt greater than Es,while ET > '/2Es. If ET < 1/2Es,'Eox will not lie below '&red and 'AI-* would be stable over some narrow potential range. Then, however, TTA to give 'Ar* would be impossible. These arguments also lead to the conclusion that the recently discusseds heterogeneous generation of 'Ar* is impossible. Further, it will not be possible to determine oxidation and/or reduction potentials for externally generated excited states. The only assumption involved in this argument is that the standard rate constants for electrochemical reactions involving aAr* are comparable to those for the similar reactions of 'Ar. These rates would then be sufficiently fast that a nonequilibrium, steady-state concentration of 'Ar* could not be maintained. The standard rate constants for charge transfer to or from aromatic hydrocarbons are among the largest electrochemical rate constants k n o ~ n . ~ J O Preannihilation ECL of rubrene in DMF, starting from either Ar+ or Ar-, was observed' beyond certain critical potentiads and continued up to the potentials for the usual ECL. This is inconsistent with the thermodynamic arguments presented here. Even if triplets could be formed at Pt electrodes over some
narrow range, the intensity of the preannihilation light arising from TTA should decrease with increasing electrode polarization. The phenanthrene case2 is similar. Triplet phenanthrene could not be formed under the conditions cited2 and, if Ar- or solvent decomposition had not occurred, the only electrochemical process possible i s oxidation of Ar-. The luminescence might be phenanthrene phosphorescence, but an explanation other than the electrooxidation of Ar- to aAr* is required. The necessity of invoking other reactions makes the spectrum assignment questionable. The arguments presented2 involving homogeneous quenchers are inconclusive. It is necessary to have a triplet quencher which is not only electroinactive, as hexatriene is, but is also inert to the foreign radicals which we feel are responsible for the homogeneous luminescent reaction. 1,3,5-Hexatriene does not meet this requirement because of its high reactivity. In summary, we have demonstrated that aAr*cannot be formed by electron transfer at metal electrodes and that other explanations are required for preannihilation ECL. We believe that decomposition products are responsible in some way for the unusual observations in DMF, results which we have not found when the system is stable. In response to the rebuttal to be published with this paper, we urge the reader to examine the pertinent references, We reiterate that the single kinetic assumption involved in the thermodynamic arguments is a reasonable one and, further, that after consideration of what is known about the electrochemistry of aromatic hydrocarbons, there is no reason to suppose that charge transfer between a triplet and an electrode should be anything but fast. Thus, any preannihilation luminescence observed must be a consequence of reactions involving impurities or decomposition products, rather than heterogeneous generation of excited states. (8) A. Zweig, A. K. Hoffmann, D. L. Maricle, and A. H. Maurer, Chem. Comnt., 106 (1967). (9) M. E. Peover and B. 8. White, J . Electronanal. Chem., 13, 93 (1967). (10) P. Malachesky, T. Miller, T. Layhoff, and R. Adams, Proceedings of the Symposium on Exchange Reactions at Brookhaven National Laboratory, Upton, N. Y . , 1966, p 157. See also P. Malachesky, Ph.D. Thesis, University of Kansas, 1966.
BELLTELEPHONE LABORATORIES MURRAY HILL,NEWJERSEY 07971
EDWINA. CHANDROSS ROBERT E. VISCO
RECEIVED AUGUST29, 1967
Comments on the Paper "Solubilization of a Water-Insoluble Dye as a Method for Determining Micellar Molecular Weights" by Hans Schott
Sir: Schott' concluded from some measurements of the solubility of an organic dye in aqueous solutions of a Volume 7k?,Number 1 January 1968
