1711
PHOTOPEROXIDATION OF UNSATURATED ORGANIC MOLECULES concentrations,6 but this effect is improbable at lower concentrations and does not affect the rest of the interpret,at'ion. The spectrum of Figure 1, attributed to C6HaC&+, bears little resemblance to one obtained by pulse radiolysis of C6H6CH3in CCh by Cooper and tho ma^.^ Their spectrum, observed over 400-600 nm only, may contain a contribution from the absorption of pure pulse radiolyzed CC1,. This is more nearly evident for their spectrum of benzene which is remarkably similar to that for pure CC14. Also the spectrum of Figure 1 does not resemble that for CeHe, adsorbed on silica-alumina and uv irradiated, which was attributed'O to C6H7+and C6H6+. On the other hand, the band with A,, 1020 nm agrees with spectra observed with C6H6CH3in both Cc14l1 and C4H&14 matrices, and C6H6gives a similar
spectrum. Bands for these compounds are enhanced by addition of electron scavengers and diminished by addition of olefins or other hole traps. Finally, under optical excitation a t -1000 nm of the assumed cations of C6H6or C6HSCH3 in 3MP with tetramethyl-p-phenylenediamine (TMPD) also present, the -1000-nm bands are removed and the T M P D + bands are increased.12 The cationic character of the -1000-nm absorbers is clear, and their assignment to C6H6+and C6HsCH3+ seems most likely. (9) R. Cooper and J. K. Thomas, Advances in Chemistry Series, No. 82, American Chemical Society, Washington, D. C., 1968, p 351. (10) V. A. Barachevskii and A. N. Terenin, Opt. Spectry. (USSR), 17, 161 (1964). (11) T. Shida and W. H. Hamill, J . Chem. Phgs., 44, 2375 (1966). (12) J. B. Gallivan and W. H. Hamill, (bid., 44, 2378 (1966).
The Photoperoxidation of Unsaturated Organic Molecules.
IV. The Photosensitized Reaction by B. Stevens' and B. E. Algar Department of Chemistry, Shefield University, Shefield, England
(Received November 8, 1968)
The photoperoxidation of 9,lO-dimethylanthracene and of 9,lO-dimethyl-1,a-benzanthracene sensitized by azulene, anthanthrene, and perylene have been investigated as a function of dissolved oxygen concentration in benzene at 25'. A quantitative analysis of the experimental data, in terms of singlet oxygen participation, provides triplet-state formation efficienciesfor the sensitizers anthanthreneand perylene and is consistent with the assignment of the sensitizer triplet state as the sole precursor of the singlet oxygen molecule. The absence of substrate peroxidation in the presence of azulene, excited in its long-wave absorption band, is attributed to an extremely rapid internal conversion of the lowest excited singlet state of this molecule which effectively prevents intersystem crossing to the triplet manifold.
Introduction The photoperoxidation of an unsaturated hydrocarbon M in the presence of a light-absorbing sensitizer S and dissolved oxygen may be represented by the overall process
nil
the excited singlet IS* or triplet state of the sensitizera in the spin-allowed processes 5 and 7 of the following detailed scheme
+ o23 M O ~
where the peroxide or hydroperoxide &tozis usually the sole product, and, except in thc autoperoxidation (S=M) of certain aromatic hydrocarbons, the sensitizer remains chemically unchanged. Recent evidence2a has provided strong support for the suggestion2b that the reactive intermediate is an electronically excited singlet state 'OZ*('Ag) of molecular oxygen produced by energy transfer from either
IS*
s + hv +'S* IS* +s + h Y F
(1)
'S* 43s*
(2)
IS* +s 302 -+ 3S*
(3)
+
+ 302
(4)
(1) Department of Chemistry, University of South Florida, Tampa, Fla. 33620. (2) (a) Cf. C. 8. Foote, Acct. Chem. Res., 1, 104 (1968); (b) H. Kautsky and H. de Bruijn, Naturwissenschaften, 19, 1043 (1931). (3) T.Wilson, J . Amer. Chem. Soc., 88, 2898 (1966).
Volume 73, Number 6 June 1969
B. STEVENS AND B. E. ALGAR
1712
lS*
+ 30z+3S* + lo2*
(5)
s
3s*+
(6)
+ 30z+S + lo2* 3S + 302 +S + 30z
(71
%*
?ti
+ ioz*
---f
lo2*-+
(8)
n402
(9)
302
(10)
in which the asterisk denotes electronic excitation. Under conditions where oxygen quenching of the sensitizer fluorescence is significant, i.e.
ka
kZ
I n view of the failure to detect fluorescence from the lowest excited singlet state of azulene' the latter alternative expressed as shown in the following equation (7) M. Beer and H. C. Longuet-Higgins, J . Chem. Phys., 23, 1390 (1955); G. Viswanath and M. Kasha, ibid., 24,574 (1956).
Volume 73, Number 6 June 1969
B. STEVENS A N D B. E. ALGAR
1714 Table I : Rate Parameters from Photosensitized Peroxidation Yields in Benzene a t 25 f 2' ---Anthanthrenen------DMA~
YIsPI(a
+PY
$hf a
3 . 3 x 10-4
10-4 0.73 f 0.05 0.25 f 0.04 0.18 f 0.04 0.02 i 0.09 0.09 f 0.09 0.52 f 0.06 0.12 f 0.02
YIC6
Sensitizer S. 'Substrate M.
7
Provides 71s with P
Measured independently.4
DMBA~
DMA~
3 . 3 x 10-4
[MI, mol 1.-1 YFC
Peryleneu-------
_ I _ _ _
DMBA~
10-4 0.89 f. 0.05 0.06 f 0.06 0.06 f 0.06 0.05 f 0.11 0.05 i 0.11 0.52 f 0.06 0.12 f 0.02
>> a.
e
(1 - YF
- TIS).
' Property of substrate.
Table 11: Rate Constants from Photosensitized Peroxidation Yields in Benzene at 25 f 2'
1.90 =I=0.30 4.5 i1.4 2.3 f2.3 3.55 iz 0.60 3350 i 700
6.6 f 1.5 0.5 f2.3
a
Sensitizer S.
Substrate M.
TIC =
1-
YF
-
TIS =
1
+
+
YIsa/(a
+ P) - 0
within the quoted uncertainty limits, or a
