J. phys. Chem. 1983, 87, 1765-1768
does decrease with increasing naphthalene concentration. At the same time there is no significant change in the rate constant k,, and the yield of delayed fluorescence is constant within 10%. These results are consistent with the existence of a high-energy (-3.6-eV) triplet charge-transfer complex with a lifetime of -200 ps.34
Conclusions This work has demonstrated that the triplet-triplet annihilation reaction of magnesium octaethylporphyrin produces ions and delayed fluorescence. It, therefore, provides confirmation of the suggestion made in the pioneering work on this subject'* that both dismutative redox products and excited singlets could result from the triplet-triplet annihilation reaction. A detailed analysis of ion production in methanol demonstrates that, within the precision of our data, ions are not produced by the reaction of triplet with the ground-state porphyrin. This contrasts the result of Ballard and Mauzerall,6 who give evidence for the production of ions by the triplet-ground-state route albeit for a different porphyrin (zinc octaethylporphyrin), different solvents, and different measurement technique. It is difficult to predict whether or not such reaction! can occur since they depend upon AEredox, E T , and uncertain (34) 0.5 M naphthalene reducee the ion yield by approximatelya factor of 2,ao that, if the quenching rate constant is diffusion controlled, Le., -1.0 X l O l o M-' s-l, the lifetime of the state quenched is about 200 pa.
1765
estimates of the entropic c ~ r r e c t i o n , ~ ,Studies ~ , ~ ~ ~of~ ~ electrogenerated chemilumine~cence~~~' have provided direct evidence that cations and anions of a given porphyrin (or chlorophyll) can react to produce the triplet. If AEredox is large enough to produce the triplet, it is less likely that the triplet-ground-state reaction could produce ions. The existence of a high-energy triplet charge-transfer complex is supported by an experiment utilizing selective naphthalene quenching. Although high concentrations of naphthalene are required to intercept the short-lived complex, the rate constants for triplet-triplet annihilation and for delayed fluorescence are not affected. Acknowledgment. We thank Drs. Norman Sutin and Carol Creutz for the use of the neodymium laser flash photolysis apparatus and the MPF-4 fluorescence spectrophotometer. The many helpful discussions which we have had with Dr. Jack Fajer are also greatly appreciated. This work was supported by the Division of Chemical Sciences, US.Department of Energy, Washington, DC, under Contract No. DE-AC02-76CH00016. Registry No. P, 20910-35-4;P', 34606-97-8;P-, 84927-46-8. (35) Waaielewski, M. R.; Smith, R. L.; Koatka, A. G. J.Am. Chem. SOC. 1980,102,6293. (36) Weller, A.; Zachariasse, K. J. Chem. Phys. 1967,46, 4984. (37) Faulkner, L. R.; Bard, A. J. "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10,p 1.
Photoperoxidation of Unsaturated Organic Molecules. 23. Dependence of the Pyrene-Sensitized 0, 'Ae Yield on Pyrene Concentration K.
L. Marsh and B. Stevens'
apartment of Chemism, University of South Floride, Tampa, Fioride 33620 (Received:Ju/y 26, 1982)
A competitive sensitization technique has been used to determine the pyrene-sensitized yield yAsof O2lag in benzene solution as a function of pyrene concentration. The reduction in yASwith increase in pyrene concentration is consistent with monomer and excimer yields of 0,'$ of hi5f 0.15and 0.4, f 0.08 in air-saturated solutions. The lower excimer-sensitized yield of 0, '$ is attributed to an excimer singlet-triplet energy separation of less than 7900 cm-l necessary for the production of O2lAg by oxygen quenching of the excimer singlet state, and to the smaller fraction of excimers quenched by oxygen.
Introduction Evidence has recently accumulated14 in support of both processes 1and 2 for the oxygen quenching of T,T* singlet M(SJ
-
+ 0, 38
-
M(TJ
+ 0, I A
M(Tl) + O232
(1)
M(TJ
+
(1) B. Stevens and J. A. Ora, J. Phys. Chem., 80, 2164 (1976). (2) H. Wagener and H.-D. Brauer, Mol. Photochem., 7, 441 (1976). (3) K. C. Wu and A. M. Trozzolo, J. Phys. Chem., 83, 2823, 3180 (1979). (4) B. Stevens, K. L. Marsh,and J. A. Barltrop, J. Phys. Chem., 86, 3079 (1981).
+
M(So) + 02 'A
M(S& + 0 2 '8
(3) (4)
responsible for triplet-state quenching by oxygen, the overall quantum yield of 0 2 lAg production is given by
(2)
states M(SJ of sensitizers in which the singlet-triplet energy separation AEsT exceeds the excitation energy of O2 '$ at 7900 cm-l. If processes 3 and 4 are together
+ 02 'E
T A = (€TIS + (e
+ 6)K[0211/(1 + K[021)
(1)
where e = k 3 / ( k 3+ k4); 6 = kl/(kl + k,); yIs denotes the triplet-state yield (in the absence of oxygen) and K is the Stern-Volmer constant for oxygen quenching of M(Sl). Wu and Trozzolo3 find that 6 + e 1.5 for several aromatic hydrocarbon sensitizers in hexane indicating that kl k2 if process 3 is solely responsible for triplet-state quenching ( e = 1). Similar findings have been reported4 for different sensitizers in air- and oxygen-saturated benzene solutions under which conditions it was possible
-
0 1983 American Chemical Society
-
1766
The Journal of Physical Chemistry, Vol. 87,
-
No. 10, 1983
to demonstrate that kl k2 for anthracene (6 = 0.46) and that k4 7900 cm-l; i.e., the excimer does not sensitize the production of O2 directly if its triplet state is dissociated. In support of this interpretation yAfor naphthalene: in which the singlet excimer-triplet monomer energy separationlo is 8900 cm-', is 1.1 f 0.1 in the concentration range 0-0.04 M and is reported5 to increase at higher naphthalene concentrations in methanol, although the value for KM(-1.5 M-l in hexane at this temperature'l) is very much lower than for pyrene. Process 11 is kinetically indistinguishable from process 13 if the triplet excimer D(TJ and monomer M(TJ pro-
-
-
'$
D(SJ
+ 02 ' 2
-
D(TJ
+ O2 l A
(13)
duce O2 'Ag exclusively, but requires an excimer triplet energy less than 23000 - 7900 = 15100 cm-' corresponding to a binding energy (relative to M(T,)) of >1800 cm-l. This species has not been observed directly in solution, although van Voorst et a1.12 have assigned a structureless phosphorescence band at 13800 cm-' in crystalline pyrene to a triplet (defect) pyrene excimer with a binding energy of 2000 cm-l. If this is present in solution and process 13 is operative, it must be concluded that the parameter yAD < 1reflects the failure of D(TJ to generate O2'$ in which case rasshould approach zero as process 14 competes with M(TJ + M(SJ D(T1) (14) process 3 at high concentrations of pyrene. The conclusion that eM 0.5 or k , = k2 has been interpreted4 in terms of an initial quenching process 1 with a 50% probability of reencounter quenching of O2 lA, by M(T,) produced simultaneously. In this case the oxygen quenching rate constant provided by the data tabulated is k l = 2.5 x 1O'O M-' s-l, which is significantly larger than the excimer quenching rate constant k,, + klz k12 = 1.8 x 1O'O M-' s-l due either to a lower excimer diffusion +
-
-
J. Phys. Chem. 1983, 87, 1768-1776
1788
coefficient or the different nature of quenching processes 1 and 12.
Acknowledgment. We are grateful to the National Science Foundation for its continued support of this work under Grant No. CH78-01578 and to the University of South Florida for a Graduate Council Fellowship award to K.L.M. Appendix Oxygen Quenching of Excimer Fluorescence. Molecular and excimer relaxation are generally coupled through processes 7 and 10, the overall time dependence of excited species following &function excitation being given by the equation -d In [ M W dt
7D0/7D
- 1 = KD[02](1- YMD)-'
(A2)
with YMD
= kio/(ka + k, + kio)
The Stern-Volmer constant KDdefined by eq V is available from relative quantum yields 7FM and 7 F D of molecular and excimer fluorescence given by the photostationary expression yFD/yFM= (k,/k,)MMI/(~, + k9 + kl0 + kll[O2l + k,2[021) whence, with [MI constant
+ D(Sl)l +k1[021 kz[O2l)[M(Si)I [M(Si)I + [D(Si)l (ka + k g + kii[O21 + ki2[Ozl)[D(Si)I [M(Si)I + [D(Si)l
(k5
+
An exponential decay is observed under the limiting conditions: (a) [M(Sl)] >> [D(S,)], established by the absence of an excimer component in the emission spectrum, when the observed relaxation time of M(Sl) is given by 1/7M
= k5
+ k6 + (kl + k2)[021 - 1 = K[02]
(-41)
where TMo
= TM([o2] = 0)
(b) [M(Sl)]
