Phutuchemistry ondPhorobiology. 1969. Vol. 10, pp, 441-444. Pergamon Press. Printed in Great Britain

RESEARCH NOTE

CHEMILUMINESCENCE FROM THE ENDOPEROXIDE OF 1,4-DIMETHOXY-9,lO-DIPHENYLANTHRACENE

THERESE WILSON Converse Memorial Laboratory, Marvard University, Cambridge, Mass. 02 138, U.S.A.

(Received 20 May 1969; in revisedform 16 June 1969)

INTEREST in the chemiluminescent decomposition of polyacene endoperoxides [ 11 derives from the early work of Dufraisse and his co-workers[2], who identified the products as molecular oxygen and the original polyacene. This apparent overall reversibility of the photooxygenation process, a typical singlet oxygen reaction, can reach 98% for the endoperoxide (11) of 1,4-dimethoxy-9,10-dipheny1anthracene (I) which is the most chemiluminescent [3] of the anthracene derivatives.

Recently, however, Rigandy [4a, b] and Baldwin ef a1.[4c] independently showed that under acid conditions I1 can undergo rearrangement, with or without ring cleavage, leading to the aldehyde-ester I11 arid to other products such as the o-quinone IV. We

I11 IV

shall present two lines of evidence indicating that it is this or a similar type of acid- catalyzed rearrangement of 11 that leads to the chemiluminescence, not reaction (2).

( 1) The luminescence depends critically on the acidity of the solution. Solutions of I1 in pyridine undergo first-order decomposition (k = 1.08 x sec-' at 25", E , = 19 kcal) yielding near-quantitative amounts of oxygen and I, with emission of a very weak and brief luminescence. When the peroxide is prepared in toluene, xylene or benzene?,

tBy irradiating at 405 nm and bubbling 0, through lo-* M solutions of I , at-20" in toluene or xylene, or

$More than 2 hr at SO". at 10" in benzene.

44 1

442 T. WILSON

its solutions in those solvents produce intense, long lasting luminescence on heating$, but only variable, often low yields of I and 0,. The main other reaction product is the aldehyde 111 (crude product nmr, i.r., and U.V. agreeing with the data from [4]). Addi- tion of small quantities of bases such as DABCO or pyridine quench the light emission, restoring at the same time the high yields of 0, and I. Washing a toluene solution of the peroxide with aqueous NaOH also inhibits the luminescence, whereas dilute acetic acid brings about an immediate increase in intensity.

The spectrum of the luminescence in toluene was recorded: it matches the fluores- cence of I (peak at 495 nm) indicating that the emitter is I in its singlet excited state. However, this fact has little mechanistic significance as this is clearly a sensitized luminescence. Addition of I to the peroxide solution indeed increases the light output. For a pure solution of I1 with no anthracene I present at the start the intensity of light rises very slowly, presumably because of the slow build up of I through the alternate reaction (2).

If the peroxide is prepared in pyridine, the pyridine then evaporated under vacuum and replaced by toluene, no chemiluminescence is observed upon heating. Addition of dilute acetic acid does not reestablish the luminescence except for a very brief low intensity flash, unless a fluorescer is present ( I , rubrene, perylene, or to a lesser extent 9,lO-diphenylanthracene). No anthracene I is reformed with acid; the only product detected is 111.

Even in the presence of a fluorescer, with or without acetic acid, the luminescence intensity of fresh solutions of peroxide in toluene does not follow a first order decay. Plots of the intensity versus time are bell-shaped curves. The reaction evidently involves several steps. Because of the nature of the product aldehyde 111, it is con- ceivable that one metastable intermediate governing the light emission could be the dioxetane V [4c], formed during the acid catalyzed decomposition of the peroxide 11.

In the absence of added acid, the formation of V would be catalyzed by the traces of acid formed during the preparation of the peroxide in toluene[4]. We suggest that the concerted cleavage and rearrangement of the I ,2-peroxide V yields the aldehyde I11 in an excited state because of orbital symmetry conservation [5].

v j 111" (3)

III* +fluorescer F (or I) -+ I11 + IF* (or l1*) (4)

IF* (or l1*) ---f F (or I)+ hu ( 5 )

The electronic state of I I I t is left unspecified, although the singlet excited state seems more likely?. It would not fluoresce but transfer its energy to the fluorescer by ( 5 ) ; this is compatible with the low quantum yield observed (see below).

Chemiluminescence from endoperoxide 443

The quantum yield of the reaction was measured using a radioactive light standard [6]. It is of the order of low4 hv per peroxide molecule, which is low enough to make any assignment of the light emitting process to a specific sequence of steps somewhat speculative. Besides, an alternate, non-luminescent path needs to be included to take into account the fact that although dilute acetic acid enhances the luminescence, in high concentrations it also shortens its duration and reduces the total light yield.

A supporting argument in favor of a metastable intermediate can be derived from the following experiment, with a solution of peroxide with added anthracene I as fluorescer. If a period of low temperature decomposition with low luminescence is followed by quick heating, an intense burst of light is emitted before the intensity realigns itself on a 'normal' course for that temperature. This could result from the rapid luminescent decomposition of an intermediate which had been produced and accumulated during the cool period.

(2) An energy pooling process involving singlet oxygen along the lines of Khan and Kasha's hypothesis[7] is not the cause of the luminescence, despite the fact that the fraction of the reaction occurring by (2) presumably yields singlet oxygen. In pyridine, where reaction (2) proceeds cleanly, the '0, formed can indeed be used to peroxidize rubrene or 1,3-diphenylisobenzofuran, although with lower yields than Wasserman obtained with the peroxide of 9,lO-diphenylanthracene [S]. In toluene, the presence of a 1000 fold excess of tetramethylethylene, which should be expected to capture '0, efficiently, is without effect on the luminescence, and the same seems to be the case for p-carotene [9] which does not quench the luminescence more than the fluores- cence of the anthracene I. DABCO is an efficient quencher of both the luminescence and '0, [ 101, but pyridine also quenches the luminescence without quenching '0, and therefore it is likely that both act simply as bases in this system. Moreover, experiments with externally generated '0, bubbled into a heated xylene solution of I fail to produce the immediate fluorescence of I. Therefore an overall mechanism,

which applies for violanthrone [ 1 11 and rubrene [ 121, is not operant here. Free-radicals are not involved in the luminescent path either, as the inhibitor

2,6-di-r-butyl phenol does not reduce the intensity. It thus appears that the luminescence of I1 is the result of a compromise between

the acid-catalyzed rearrangement of the peroxide yielding the energy-rich precursor, and the 'reversible' path (2) producing the fluorescer (and singlet oxygen). Here production of singlet oxygen or chemiluminescence can be triggered on or off by controlling the acidity of the solution.

Acknowledgements- It is a pleasure to thank Professor P. D. Bartlett for helpful discussions and Mr. G . D. Mendenhall for valuable suggestions.

~~ ~

tThe triplet state of 111 would be too low to excite I , perylene or DPA, besides being a spin-forbidden process. This is supported by the observation that 9.10-dibromoanthracene is not a better activator in this system, although the spin restriction would be expected to be less stringent because of the heavy atom effect. Excitation of the singlet states of these sensitizers by triplet-triplet annihilation is not likely in aerated solutions.

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444 T. WILSON

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(Edited by W. A. Noyes, Jr., G. S. Hammond and J. N. Pitts, Jr.). Vol. 6, p. 108. Wiley, New York (1968). 2. (a) C. Moureu, C. Dufraisse and P. M. .Dean, Compt. Rend. 182,1584 (1926). (b) A. Etienne, In Trait&

de Chimie Organique, (Edited by V. Grignard), Vol. 17, Fasc. 2, p. 1299. Paris (1949). 3. (a) C. Dufraisse and L. Velluz, Bull. SOC. Chim. France 9, 171 (1942). (b) C. Dufraisse, J. Rigaudy,

J. J. Basselier and Nguyen Kim Cuong, Compr. Rend. 260,503 1 (1965). 4. (a) J. Rigandy, PureAppl. Chem. 16,169 (1968). (b)J. Rigandy, C. Deletang, D. Sparfel and N. K. Cuong,

Compt. Rend. 267, 1714 (1968). (c) J. E. Baldwin, H. H. Basson, and H. Krauss, Jr.. Chem. Comm. 1968, 984.

5. See (a) F. McCapra, Chem. Comm. 1968, 155. (b) M. M. Rauhut, Accounts Chem. Res. 2,80 (1969). . (c) K. R. Kopecky and C. Mumford, Can. J . Chem. 47,709 (1969).

6. J. W. Hastings and G. Weber, Phorochem. Phorobiol. 4, 1049 (1965). 7. A.U.KhanandM.Kasha,J.Am.Chem.Soc.88,1574(1966). 8. H. H. Wasseman and J. R. Scheffer, J. Am. Chem. SOC. 89,3073 (1967). 9. C. S. Foote and R. W. Denny,J. Am. Chem. SOC. 90,6233 (1968). 10. C. OuannCs and T. Wilson, J. Am. Chem. SOC. 90,6527 (1968). 1 1. E. A. Ogryzlo and A. E. Pearson, J. Phys. Chem. 72,29 13 (1 968). 12. T. Wilson, J. Am. Chem. SOC. 91,2387 (1969).