377
COMMUNICATIONS TO THE EDITOR prepared the hydrates (n-C4Hg)4NOH3 1 H ~ 0 and (n-C4Hg)&F 18H20 and they note that “no salts of ions other than tetra-n-butylammonium and tetraisoamylammonium have been found that form high hydrates.” Wen and Saitoa also found that the concentration dependence of the partial molal volume of ( T L - C ~ H ~ ) exhibited ~ K B ~ peculiar behavior. Similarly, we found that the tetramethyl, -ethyl, and -propyl bromides, chlorides, and iodides did not exhibit freezing under pressure. The solubility of some of the iodides, however, does decrease a t high pressure, as evidenced by B slower increase in resistance and the precipitation of white crystals, but this slow precipitation of a fine powder contrasts so sharply with the nearinstantaneous solidification throughout the (n-C4H9)4NBr solution into a semiclear mass, that it seems unlikely that the latter phenomenon is also the result of a pressure-induced decrease in solubility rather than hydrate formation. The crystal structure of the higher hydrates of n-tetrabutylammonium salts is believed to be a polyhedral clathrate-type age.^^^ The apparent stabilization of the present hydrate substance by pressure is quite extraordinary for usually the application of pressure destroys the coulombic hydration atmosphere of normal ions.6~~The implication of the present finding is that, unlike the Frank-Wen cluster-like total coulombic hydration atmosphere of “normal” ionslS the hydrophobic hydration atmosphere of the (n-C4H9)4N+ion is more dense than the liquid water a t some great distance from the ion. That is to say, the present, results constitute further evidence that the local water structure near hydrophobic ions, molecules, or segments of molecules is fundamentally different from the local water structure near “normal” ions and charge sites. (3) W. Y . Wen and 5. Saito, J . Phys. Chem., 68, 2639 (1964). (4) R. K. M c M u l h and G. A. Jeffrey, J. Phys.9 31, 1231 (1959); 42, 2726 (1965). (5) R, K, McMulJ,an, M, Bonamico, and G. A. Jeffrey, &id., 39, 3295 (1963). (6) R . A. Horne, Nature, 200, 418 (1963). (7) R. A. Horne, rldv. High Pressure Res., in press. (8) R. A. Horne and J. D. Birkett, Electrochim. Acta, 12, 1153 (1967).
ARTHURD. LITTLE,INC. 02140 CAMBRIDGE, MASSACHUSETTS RECEIVED AUGUST7, 1967
R. A. HORNE R. P. YOUNG
On the Mechanism of Preannihilative Electrochemiluminescence
Sir: In the following communication,’ Chandross and Visco have proposed that thermodynamic arguments make the hypothesis of direct electrogeneration of
triplets dubious. We have suggested this process as a possible source of some emission under some circumstances.2 Although we are aware of no definite evidence of such heterogeneous generation of excited states, we do not feel that it can be ruled out purely on the basis of their arguments. The failure’ of Chandross and Visco to observe emission from rubrene in benzonitrile until both the stable cation and anion were generated is in conflict with the results of Chang, Lytle, Roe, and Hercules13 who found emission while oxidizing the rubrene anion in the same solvent at potentials as far negative as -0.2 V vs. sce. This is more than 1 V short of the R+-R half-wave potential and cannot possibly be accounted for by Nernstian generation of R f followed by an ionannihilation p r o ~ e s s . ~ The assignment of cation decomposition products as being responsible for preannihilative emission’ is untenable with regard to the cited works,2v8 since the investigations were made with sequences of potentials such that cations were not produced in the test solution until after the preannihilative emission was measured.e As to whether our assignment of the phenanthrene phosphorescence emission is “questionable,” Weller and Zachariasse? have found such emission for chrysene under not dissimilar conditions and also note that, in the presence of paramagnetic species such as ion radicals, the radiative lifetimes of triplet states may be decreased many orders of magnitudes7 The main item in the paper,’ “the thermodynamic arguments which make the hypothesis of direct electrogeneration of triplet dubious,” is also worthy of comment. The thermodynamics described are a simple extension of the considerations made by Wellefl and other^.^ However, the relative rates of emission and electrode reaction are unknown. It is evident that, because of the paramagnetic environment, the rate of emission has become quite rapid.7 Because the corntransfer at an peting rate Of trip1et has not been experimentally measured, nothing can be said with certainty about the possibility of triplet quenching via heterogeneous electron transfer.
(1) E. A. Chandross and R. E. Visco, J . Phys. Chem., 7 2 , 378 (1968). (2) D. L. Maricle and A. H. Maurer, J. A m . Chem. Soc., 89, 188 (1967). (3) J. Chang, E’. E. Lytle, D. K. Roe, and D. M. Hercules, Symposium on Electrochemical Processes and the Energy States of Electrons, Schloss Elmau, April 23-29, 1967. CITCE Abstracts, p 71. (4) D. K. Roe, private communication. (6) A. Zweig, D. L. Maricle, J. 9. Brinen, and A. H. Maurer, {bid., 89, 473 (1967). (6) D. L. Maricle, A. Zweig, A. Maurer, and J. Brinen, CITCE Symposium 1967, Elman, Germany; Electrochem. Acta, in press. (7) A. Weller and K. Zachariasse, J . Chem. Phva., 46, 4984 (1967). (8) H. Leonhardt and A. Weller, Be?. Bunsenges. Physik. Chem., 67. 791 (1963). (9) J. Feitelson and N. Shaklay, J . Phys. Chem., 7 1 , 2682 (1967).
Volume 7.2, Number 1 January 1068
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* a t 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).
+
+ +
+ +
The Journal of Physical Chemistry
+
