J. Phys. Chem. 1981, 85,3079-3082
3079
Photoperoxidation of Unsaturated Organic Molecules. 21. Sensitizer Yields of O2 'Ag B. Stevens,' K. L. Marsh, and J. A. Barltropt Department of Chem&try, Universtty of South Fbrhja, Tampa, Florhja 33620 (Received: January 20, 198 1; In Final Form: M y 8, 708 1)
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The simultaneous excitation of reactive and unreactive O2'A, sensitizers in the same solution provides access to their relative yields of O2 'A, production; these are normalized to the coronene-sensitized yield of 0 2 'A, estimated from the coronene-sensitized photoperoxidation of 9,lO-dimethylanthracene.Oxygen quenching of the singlet states of anthracene, 9,10-dimethylanthracene, 1,3-diphenylisobenzofuran,and rubrene produces O2 'A, with an efficiency of 240%, whereas quenching of the triplet states of coronene and anthracene yields O2 'Ag with 100% efficiency. The yield of O2 'A, from benzophenone triplet states is reduced from 90% in oxygen-saturated benzene solution to 50% in air-saturated solution, which is attributed to incomplete oxygen quenching of this triplet state; the extrapolated yield of O2 '$ from benzophenone at infinite oxygen concentration is 100%. There is no evidence from these measurements that 0 2 lA, is not produced by oxygen quenching of the triplet states examined, and it is shown that reencounter amplification of the encounter quenching probability CY 5 l/s can accommodate rate constants for triplet energy transfer to oxygen up to -40% of the diffusion-limitedvalue. Introduction The overall quantum yield of photoperoxidation may be expressed as the product of the quantum yield of O2 'A,.formation (yA)and the fraction ( 4 ~of) 0 2 'Ag formed which reacts with acceptor A: Y A =~ YA@A (1)
yield of O2 'A, formation becomes YA= YA(s1) + YA(T1) = { ~ Y B+ (6 + 4K[O211/(1 + K[021) (VI If processes 5-7 are solely responsible for O2 IA, consumption 0 2 'A + A A02 (5) -+
To the extent that the sensitizer triplet state (TI) is produced with unit efficiency by oxygen quenching of the corresponding single state (SI)in hydrocarbon solvents,l this quenching must involve one or both of the spin-allowed exothermic processes 1 and 2 if the S1-T, energy separation exceeds the O:Ag excitation energy of -8000 cm-l: Si + 0 2 32 TI + 0 2 'A (1)
-+
T1 O2 32
(2) The yield of O2 lA, produced directly from S1 may therefore be expressed as rA(S1) = 6K[021/(1 + K[021) (11)
where 6 = k , / ( k l + k2), and K = (kl+ k 2 ) ~ is s independently available as the Stern-Volmer constant for oxygen quenching of S1 with lifetime rS. If yIs denotes the S1 T1intersystem crossing efficiency in the absence of dissolved oxygen, the total yield of triplet-state formation is given by YT = (71s + K[O21)/(1 + K[021) (111) O2'$ is also produced by oxygen quenching of the triplet state (process 3) if the triplet-state energy exceeds 8000 cm-', which, on the basis of Franck-Condon considerat i o q 2 is expected to compete favorably with process 4: T1 + 02321 So + 02'A (3)
-
-
- so +
0 2 321
(4)
The yield of O2 'A, from S1via T1 is therefore expressed as YA(T1) = EYT (IV) where e = k 3 / ( k 3+ k4),and, at concentations of dissolved oxygen sufficient to quench TI effectively, the quantum On leave from Oxford University, England. 0022-3654/81/2085-3079$0 1.25/0
A
+ 0 2 32
(6)
'A
0 2
321
(7)
-+
0 2
-
then 4Aassumes the form $A = PO[AI/P(PO + [AI) where P = k,/k5 2 Po = k,/(k5 + kf3) and eq I, with eq V and VI, becomes
(VI)
Evidence has recently acc~mulated"~ in support of both processes 1and 2 for the oxygen quenching of certain T,T* singlet states; thus Wu and Trozzolo5find that 6 E x 1.5 for several aromatic hydrocarbon sensitizers in hexane, indicating that kl = k2 if e = 1.0. This partitioning of singlet quenching products has been interpreted as a reencounter of the correlated products (T, O2'A) of process 1to form the products of process 2 in the sequence
+
+
s1
+
Y 0,3c
+
(T,
T'
+
0, 'A
(la)
t 0,'~)
which requires that the reactive reencounter probability + k2). The quantitative aspects
p = k2/(k1 k2) = 0.5 (k,x
(1) R. Potashnik, C. R. Goldschmidt, and M.Ottolenghi, Chem. Phys. Lett., 9, 424 (1971). (2)K. Kawaoka, A.U. Khan, and D. R. Kearns, J. Chem. Phvs., - . 46,. 1942 (1967). (3) B. Stevens and J. A. Ors, J. Phys. Chem., 80, 2164 (1976). Brauer, Mol. Photochem., 7, 441 (1976). (4)H.Wagener and H.-D. (5)K. C. Wu and A. M. Trozzolo, J. Phys. Chem., 83, 2823, 3180 (1979).
0 1981 American Chemical Society
3080
Stevens et al.
The Journal of Physical Chemistry, Vol. 85,No. 21, 1981
TABLE I: 0, ' A , Yields from Sensitizer Triplet States in Benzene ref
14 13 12 this work this work
See ref 19.
103[0,], M
1.9 9.0 1.9(?)U
1.9 9.0 Relative values.
anthracene
0,co
€(anthracene)
0, lag counter
Hg,313 nrn
0.6Bb 0.7 1.0b
0.12b 0.4 0.56b
1.0 1.0
0.54 0.90
0.17 0.57 0.56 1.0 1.0
DTBFCconsumption D P B F ~consumption p-carotene triplet formation DPBF, DMAe consumption DPBF, DMA consumption
electron N, laser Hg,365 nm Hg,365 nm
Di-tert-butylfuran.
1,3-Diphenylisobenzofuran. e 9,lO-Dimethylanthracene.
of this interpretation depend however on the magnitude of e or the relative significance of processes 3 and 4. Support for process 3 as the major triplet quenching process was provided7by the analysis of azulene inhibition data which indicates that k3 + k4 is 1 order of magnitude less than kl + k2 consistent with k4 = Q and a spin-statistical factor of l / g for process 3, Subsequently the classical work of Porter's group* showed directly that the rate constant for oxygen quenching of aromatic hydrocarbon triplet states has an upper limit of kD 9 for triplet states in the energy range 10000-14 000 cm- if the diffusion-controlledrate constant kD = kl + k,; this is consistent with the value e = 1. Recently reported values of kT = k3 + k4 > kD/9 for the n,a* triplet states of N-methylindole: ~ a n t h o n e ,c~a r b a ~ o l e ,acetone,lo ~ and N-methylthioacridone" have been interpreted in terms of a contribution from process 4 which, with a spin-statistical factor of permits an upper limit to kT of k D ( I / g + '/3) = 4 k ~ / 9 . More direct evidence for process 4 has been provided by rep~rted'~-'~ values of E = k3/(k3+ k4) < 1estimated from measurements of ya. For those sensitizers with high intersystem crossing efficiencies (yrs * 1.0) and corresponding short singlet-state lifetimes (K[O,] = 0), exemplified by benzophenone, e is directly available as yA(eq V); however, the evaluation o f t for other sensitizers (e.g., anthracene) requires independent estimates of yIs and K together with an evaluation of 6 from measurements of yA([02]).Reported values of e for benzophenone and anthracene summarized in Table I appear to vary with the conditions of excitation and/or the O2 '$ counting technique employed. This contribution examines further the evidence for process 4 based on measurements of relative quantum yields of 0, lAg production by the simultaneous UY excitation of two sensitizers, one of which also acts as O2l$ acceptor, in air- and oxygen-saturated solutions. These are converted to absolute yields (yA)by reference to absolute yields for 9,10-dimethylanthracene and coronene, to assess the relative contributions of processes 3 and 4, and of processes 1 and 2 where applicable.
i
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CON
excitation
Experimental Section The simultaneous excitation of two sensitizers, one of which, S, does not react with or quench O2lA whereas the other, A, also acts as an O2 IAg acceptor,\eads to the (6) B. Stevens and R. D. Small,Jr., Chem. Phys. Lett., 61 233 (1979). (7)B. E.Algar and B. Stevens, J . Phys. Chem., 74,3029 (1970). ( 8 ) L.K. Patterson, G. Porter, and M. R. Topp, Chem. Phys. Lett., 7 , 612 (1970);0.L.J. Gijzeman, F. Kaufman, and G. Porter, J.Chem. Soc.,
Faraday Trans. 2,69,709(1973). (9)A. Garner and F. Wilkinson, Chem. Phys. Lett., 45,432 (1977). (IO) T. Wilson and A. M. Halpem,J. Am. Chem. Soc, 102,7279(1980). (11)A. Safarzadeh-Amiri,D.A. Condirston, R. E. Verrall, and R. P. Steer, Chem. Phys. Lett., 77,99 (1981). (12) A. Garner and F. Wilkinson, "Singlet Oxygen", B. Ranby and J. F. Rabek, Eds., Wiley, New York, 1978,p 48. (13) A. A. Gorman, G.Lovering, and M. A. J. Rodgers, J. Am. Chem. Soc., 100, 4527 (1978). (14)A. A. Gorman, I. R. Gould, and I. Hamblett, Tetrahedron Lett., 21, 1087 (1980).
photoperoxidation of A described by the quantum yield expression y i o z = 4A{yiODS/OD+ r%ODA/OD) (VIII) where yzo, is defined in terms of light absorption by both S (optical density ODS) and A (optical density OW = OD - ODS); yf and 7: are the quantum yields of O2 lAg formation by sensitizers S and A, respectively. If yio, denotes the photoperoxidation quantum yield in the absence of S at the same concentration of A, then eq VI11 rearranges to = d020D/Y~OzODA - 1= ( ~ i / r i ) { ( o D / o D -~ )1) (IX) which affords direct access to y i / y i from measurements of relative quantum yields of reaction ria / y i o as a function of the initial optical density OD of the sofution with ODA constant. Equations VI11 and IX are valid to the extent that energy transfer between S and A is not significant in either singlet or triplet manifolds. Under the conditions of measurement, triplet energy transfer is minimized by the use of dissolved-oxygen concentrations at least 2 orders of magnitude greater than that of the sensitizer of lowest triplet energy. However, the necessary use of relatively high concentrations of benzophenone as sensitizer (S) introduces the possibilitylk of singlet energy transfer from A to S (process 8) lA S A IS* (8) with the result that a fraction 4s = Ks[SI/(l + K[021 + &[SI) of light absorbed by A results in the excitation of 'S*; here Ks = kgrAand K is the Stern-Volmer constant for oxygen quenching of IA* with lifetime TA. Under these conditions equation VI11 becomes f(YA02)
+
-
+
~ J A M O D ~ / O+D[rIdJs + y%(l- 4s)10DA/OD1 which rearranges to the convenient form P(rA0,) = yi0,0D/r8020DA- (1- dJS) = ( ~ ~ / T ~ ) { ( O D / O-D(1 A) 4s)) (X) 9,lO-Dimethylanthracene (DMA), 1,3-diphenylisobenzofuran (DPBF), and rubrene were used as acceptors A. Quantum yields of their photoperoxidation were obtained from the initial time dependence of the optical density of solutions at the actinic wavelength (365 nm) filtered from a 100-W dc mercury arc (Oriel) as previously described,16using Aberchrome 540 actinometry.17a The extent of O2'$ quenching by (or reaction with)sensitizers S was estimated from their effect on the rate of self-sensitized rubrene photoperoxidation at 546 nm where ODS ~ 8 0= ,
(15) (a) The perspicacity of a referee in this connection is gratefully acknowledged; (b) with k3 = 1.8 X IOD M-' s-l (ref 14). (16)(a) B.Stevens and B. E. Algar, J. Chem. Phys., 72,3468 (1968); (b) J. Saltiel, H. C. Curtis, L. Metta, J. W. Miley, J. Winterle, and M. Wrighton, J. Am. Chem. Soc., 92,410 (1970). (17)(a) H.G. Heller, Chem. Ind. (London),193 (1978); (b) B.Stevens and K. L. Marsh, unpublished.
The Journal of Physical Chemistry, Vol. 85,
Photoperoxidation of Unsaturated Organic Molecules
TABLE 11: Rate Constants ( k , ) for Fluorescence Quenching by Benzophenone in Benzene at 25 "C
ESl,
cm
-1
k,,
chrysene
pyrene
27700
26900
1.1 X 10" 8 X
M-I s-l
lo9
DMA
No. 21,
1981 3081
5r
DPBF rubrene
-25000 22100 18900
4 X lo9
a 9,lO-Dimethylanthracene. b 1,3-Diphenylisobenzofuran. From eq X. M in air-saturated benzene at 25 "C. text). e [O,] = 1.9 x
TABLE IV: 0,
1.02i 0.06 0.52 * 0.03
1.1 f 0.1 1.4 i 0.2
1.02 0.54 0.96 0.93 1.12 0.88 0.74 0.29 0.86f 0.56f 0.82 0.34
Reference 16. ' Value for ethanol, A. R. Horrocks and a B. Stevens and B. E. Algar, J. Phys. Chem., 72,2582 (1968). F. Wilkinson, Proc. R . SOC.London, Ser. A , 306,257 (1968). Taken as 1 - YF in benzene where fluorescence quantum yield 7~ = 0.26,J. B. Birks and D. J. Dyson, Proc. R. SOC.London, Ser. A , 275,135(1963). e Reference 6. f Computed as TA-' = 1 + 1.8 x 10-3/[0,];see text. ps,lSbwhich is considerablyshorter than the experimental value of 37 = 8 ps16bin pure benzene. This could be a consequence of diffusion-limited quenching of triplet benzophenone by a solvent impurity present at a concentration of 3 x lo4 mol%. The data for anthracene and benzophenone are in significant disagreement with the absolute yields reported by Rodgers et d.13for oxygen-saturated benzene solutions and with the relative yields found for air-saturated benzene solutions by Gorman et al.14 The former authors acknowledge the dependence of their absolute yields on triplet-triplet extinction coefficients, but it is noted that inclusion of 024 quenching by benzophenone (with a rate constant of 1.8 X lo7 M-l s-l) in the computation of G(024)increases 7: to 0.8, which is comparable with their value for anthracene. The latter authors accommodate benzophenone quenching both of O2 lAg and of the benzophenone triplet state in estimating y A from relative rates of sensitized di-tert-butylfuran (DTBF) consumption; moreover, their neglect of process 1 for anthracene would not account,for the low value of yi(benzophenone) obtained in this way (Table I). This low relative yield may however reflect an increase in the anthracene-sensitized consumption of DTBF by direct photochemical reaction as in the case of furan,l'lbwhich must involve the anthracene singlet state since no significant quenching of the triplet state by DTBF is 0b~erved.l~ Since t = 1for anthracene, coronene, and benzophenone and 6 t 1.5 for the other compounds examined, we
+
-
conclude that there is no direct evidence for process 4 from measurement of 7Ain these systems. Indirect evidence afforded by adoption of kT = k3 + k4 > kD/9 as a criterion of process 4 is based on the erroneous assumption18bthat kT = a!kDwhere the encounter quenching probability a! has a spin-statistical upper limit of for process 3. The recognition that unsuccessful quenching partners can undergo reactive reencounter with probability p leads to the appropriate e x p r e s ~ i o n l ~ ~ * ~ ~ kT
= k3 = kDP/Po = ffkD/(1- Po
ffPo)
for the time-independent quenching constant where p o = p ( a = l), the total reencounter probability with or without reaction, is estimated to be 0.8 for DPBF/02 correlated pairs in benzene6 and 0.74 for heterocoerdianthrone/02 pairs in the same solvent.21 In this case with a! I l/g, kT/kD 5 0.3-0.4,which, in view of the direct experimental inaccessibility of FZD, accommodates values of kT reported to date for triplet states of energy greater than 8000 cm-' without invoking a contribution from process 4 with the possible exception of thioketones.'l Acknowledgment. We are grateful to the National Science Foundation for its continued support under Grant CHE-78-01578. (20) B. Stevens, J. Phys. Chem., submitted. (21) W. Drews, R. Schmidt, and H.-D. Brauer, J. Photochern.,6,391
(1976/77).
