Photochemistry and Pholobiology Vol. 43. No. 6. pp. 661 - 662. 1Y86 Printed in Great Britain

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RESEARCH NOTE

SPECTRAL OBSERVATION OF SINGLET MOLECULAR OXYGEN FROM AROMATIC ENDOPEROXIDES IN

SOLUTION THBRESE WILSON'*, AHSAN U. KHAN' and MUKUND M . MEHROTRA'

'The Biological Laboratories and 'Department of Chemistry, Harvard University. Cambridge, MA 02138, USA

(Received 6 January 1986; accepted 7 January 1986)

Abstract-The characteristic near-infrared emission band of Oz ('A8) at 1.28 pm has been recorded from carbon tetrachloride solutions of the 1 ,4-endoperoxides of 1,4-dimethyInaphthalene and 1,4-dimethoxy- 9.10-diphenylanthracene undergoing thermal decomposition.

INTRODUCTION

Oxygen in the 'A, state reacts with naphthalenes and anthracenes to produce endoperoxides which, upon heating, regenerate molecular oxygen and the hydro- carbon (Moureu et a[. , 1926; Dufraisse et a l . , 1939; Rigaudy et al . , 1971; Wasserman and Larsen, 1972). Chemical reactivity studies have shown that in the case of some of these transannular peroxides a large fraction if not all the product oxygen appears to be in the singlet excited state (Wasserman et al . , 1972; Wilson, 1969; Turro et al . , 1981). The uniqueness of such a reversible photochemical process invites con- firmation by emission spectroscopy. The develop- ment of a polymer bound methyl-substituted naph- thalene endoperoxide as a reusable source of singlet oxygen is a recent example of the practical interest of this reaction (Saito, 1985).

Two aromatic compounds, 1,4-dimethyl- naphthalene 1 and 1,4-dimethoxy-9,10-diphenyl- anthracene 2, were selected because of the lability of their corresponding 1,4-endoperoxides in a conve- nient temperature range and the reported high yields of singlet oxygen from their thermolysis (Turro et al., 1981, and references therein).

MATERIALS AND METHODS

The peroxide of 1 was prepared in methylene chloride (0.06 M ) by methylene blue sensitized photooxidation at 0°C with a 200 W tungsten lamp and continuous bubbling of Oz through the solution (Wasserman and Larsen, 1972). Progress of the reaction was followed by tlc (90% conver- sion after 4 days). After solvent removal, the endoperoxide was chromatographed over neutral alumina and recrystal- lized, all in the 4°C cold room.

Compound 2 was photooxidized in pyridine (0.01 M ) by bubbling O2 while irradiating either at 405 nm (for - 2 h) or with the total output from a 500 W projector bulb passed through a UV cut-off filter C.S.0-52 (- 20 min) until disappearance of the intense fluorescence of 2. Tempera- ture was maintained at -10°C. Pyridine was chosen as solvent for the photooxidation of 2 in order to minimize the possible subsequent acid-catalyzed rearrangement of the endoperoxide (Wilson, 1969; Lundeen and Adelman, 1970;

*To whom correspondence should be addressed.

the visible chemiluminescence emitted by this endoperoxide in some conditions is actually a consequence of this rearrangement).

The emission spectra were recorded with a near-infrared spectrometer, previously described by Khan (1981), from solutions of the endoperoxides in carbon tetrachloride. Carbon tetrachloride was chosen because of the long lifetime of '0, in this solvent (Monroe, 1985).

RESULTS AND DISCUSSION

Figure 1 shows the courses of the photoperoxida- tion of 2 and of the thermal decomposition of the

1.oc

W 0 z a

0.5C

z a

Ph OMc Ph O M m>y& Ph OM* ph O W

1 400

Ph OMe

z i + h Y

F% OM*

B

400 5 1

WAVELENGTH, nm Figure 1. Absorption spectra of solutions of 2 during the course of the photoperoxidation (A) and of the product endoperoxide during its thermolysis at 52°C (B); the first order rate constant for decomposition was 2.4 x IO-'s-'. The spectra were scanned at the time in minutes indicated on the figures. In A, successive aliquots of the pyridine solution of 2 were taken out and diluted 1 : 100 in cyclohex- ane for spectra. In B, the final solution of endoperoxide shown in A was allowed to decompose in the cuvette maintained at 52°C and spectra recorded at the times

indicated.

66 1

662 THBRBSE WILSON et al

resulting peroxide (Scheme 1 where A is the aroma- tic) as monitored by absorption spectroscopy. In this example, 2 was reformed in ca. 90% yield.

A + hv + 'A* - 3A*

These spectra unambiguously confirm the genera- tion of singlet oxygen from the thermal decomposi- tion of the endoperoxides. The peak luminescence intensity at 1.28 pm emitted by these convenient sources of singlet oxygen is therefore a specific probe which can be used for studies of the reactivity and quenching of '02 in organic solvents.

3A* + 3 0 2 + A + lo2 A + ' 0 2 + A02

A02 + A + ' 0 2

Scheme 1.

Samples of the dry solid endoperoxides were dissolved in preheated (ca. 50°C) carbon tetrachlor- ide in a cuvette placed directly in front of the entrance slit of the near-infrared spectrometer. The emission spectra recorded from both endoperoxides (Fig. 2 A and B) show the sharp peak at 1.28 pm associated with the (0,O) transition of O2 'A, '2;. In the case of the more intense emission from 1, the (0,l) transition at 1.58 pm is also clearly visible. The main reason for the weaker emission from 2 is its very fast rate of back reaction with '02 ( k , = 1.4 X lo8 M-'s - l compared to k , = 1.2 X lo4 M-' s-' for 1; see Monroe, 1985).

z cn 0 v,

M O O 1400 1700 1100 1400 1700

WAVELENGTH, nrn

Figure 2. Emission spectra from the endoperoxides of 1 (A) and 2 (B) in carbon tetrachloride at 50°C. Scan rate: 600 nm in 180 s. In B the thermal emission from the heated solvent (giving an apparent maximum at ca. 1600 nm due to the drop in detector sensitivity) is seen in addition to the '02 peak (Kanofsky, 1984). Figure 2B shows two scans, one taken immediately after addition of the peroxide, the other

after its complete decomposition.

Acknowledgements-This work was supported by the National Science Foundation and the National Foundation for Cancer Research.

REFERENCES Dufraisse, C., L. Velluz and L. Velluz (1939) L'union

labile de I'oxygkne au carbone. Un peroxyde spontane- ment dissociable ?i froid. C.R. Acad. Sci. 208,1822-1824.

Kanofsky, J . R. (1984) Near-infrared emission in the catalase-hydrogen peroxide system: a reevaluation. J . Am. Chem. Soc. 106, 4277-4278.

Khan, A. U. (1981) Direct spectral evidence of the generation of singlet molecular oxygen ('Ag) in the reaction of potassium superoxide with water. J . Am. Chem. Soc. 103, 6516-6517.

Lundeen, G. W. and A. H. Adelman (1970) Chemi- luminescence of decomposition of 1,4-peroxy-l,4- dimethoxy-9,lO-diphenylanthracene. J . Am. Chem. SOC. 92, 39143919.

Monroe, B. M. (1985) Singlet oxygen in solution: life- times and reaction rate constants. In Singlet O,, (Edited by A. A. Frimer), Vol. 1 , pp. 177-224. CRC Press. Boca Raton, Florida.

Moureu, C., C. Dufraisse and P. M. Dean (1926) Un peroxyde organique dissociable: le peroxyde de rubrene. C.R. Acad. Sci. 1982, 15861585.

Rigaudy, J., J . Guillaume and D. Maurette (1971) Effet des substituants sur la photooxygtnation des derives aromatiques. Bull. Soc. Chem. Fr. 144152.

Saito, I., R. Nagata and T. Matsuura (1985) Methyl- substituted poly (vinylnaphthalene) as a reversible singlet oxygen carrier. J . Am. Chem. Soc. 107, 6329-6334.

Turro, N. J., M.-F. Chow and J . Rigaudy (1981) Mechanism of thermolysis of endoperoxides of aromatic compounds. Activation parameters, magnetic field, and magnetic isotope effects. J . Am. Chem. Soc. 103, 7218- 7224.

Wasserman, H. H., J. R. Scheffer and J . L. Cooper (1972) Singlet oxygen react ions with 9, lO- diphenylanthracene peroxide. J . Am. Chem. Soc. 94,

Wasserman, H. H. and D. L. Larsen (1972) Formation of 1,4-endoperoxides from the dye-sensitized photo- oxygenation of alkyl-naphthalenes, J . C.S. Chem. Comm. 253-254.

Wilson, T. (1969) Chemiluminescence from the en- doperoxide of 1,4-dimethyoxy-9,1O-diphenylanthracene. Photochem. Photobiol. 10, 441-444.

4991-4996.

Nofe added in proof. Since this note was submitted, a recent paper came to our attention which also reports the spectrum of '02 from the thermal decomposition of the endoperoxide of 1. See Chou, P.-T. and H. Frei (1985). Sensitization of O2 'E.S++ 'Ag emission in solution, and observation of O2 'Ag-+3C; chemiluminescence upon decomposition of 1.4- dimethylnaphthalene endoperoxide. Chem. Phys. Leu. 122, 87-92.