7308
J . Phys. Chem. 1991, 95,7308-7312
Slnglet Oxygen Generatlon by Triplet Charge-Transfer Complexes Alexander P. Darmanyan,**tJames W. Arbogast, and Christopher S.Foote* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569 (Received: February 19, 1991)
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Quantum yields of singlet oxygen generation by complexes of triplet pchloranil and various benzene derivatives with partial or complete charge transfer were measured by timaresolved phosphorescence. The probability of IO2generation (SJ demases in the series triplet p-chloranil 1 nonpolar triplet exciplex > triplet ion-radical pair (TIRP). For the TIRPs with anisole, durene, hexamethylbenzene, and 1,2,3-trimethoxybenzene,SA= 0.05-0.20. The quantum yields of pchloranil semiquinone radical production were obtained by nanosecond laser photolysis. The H-atom abstraction reaction proceeds by a two-step mechanism of electron transfer followed by proton transfer inside thermalized TIRPs; the contribution of "fast" H-atom transfer is approximately 10% in the cases of durene and hexamethylbenzene and negligible with the other donors.
Introduction There have been numerous studies of IO2generation caused by quenching of excited singlet and triplet states of various organic compounds by molecular oxygen.'-' However, determination of the probability of IO2 formation (SA) from quenching of other transient species has not been thoroughly studied. The quantum yield of IO2generation (aA) by pyrene excimers is 0.47 in airsaturated benzene.' This low value was explained by the small energy splitting between the excited singlet and triplet states of the excimers. The @A from oxygen quenching of the metal-toligand charge-transfer triplet excited state of tris(bipyridine)ruthenium( 11) complex is 0.83.5 Chattopadhyay et al. established in this work that the mechanism by which the intramolecular triplet ion-radical pair (TIRP) is quenched is primarily energy transfer. Quenching of an intermolecular TIRP formed by quenching of triplet 2,6-diphenyl- 1,4-benzoquinone by triphenylamine has been thoroughly studied by Darmanyan: who found that the probability of IO2 generation is very low. For example, SA(the fraction of TIRP giving IO2on reaction with '02)is 0.14 in toluene. The authors concluded that quenching of the TIRP by molecular oxygen occurs mainly by enhancement of intersystem crossing to the ground state of the ion pair. However, Levin et al.' recently measured the quenching rate constant by molecular oxygen of different intermolecular TIRPs and concluded that energy transfer from the TIRP to '02or lower energy aromatic triplet acceptors is forbidden. The authors suggested that the excited singlet state of the ion-radical pair l(A'-.-D'+) is slightly lower in energy than the triplet state (A*--D'+) and that oxygen enhances the rate of intersystem crossing from triplet to singlet. The resulting '(A'-.-D'+) state decays rapidly by a spin-allowed back electron transfer. The present paper deals with the mechanism of quenching by complexes of triplet p-chloranil and benzene derivatives with partial or complete charge transfer by molecular oxygen and with the mechanism of the hydrogen-atom-transfer reaction within these complexes.
'-
Experimental Section The spectral and kinetic characteristics of transient intermediates were measured with a laser photolysis apparatus with a time resolution of 5 2 0 ns. The solutions were excited in a l-cm quartz cell by a Quanta-Ray Nd:YAG laser (10-20 mJ/7-ns pulse) at 355 nm. The decay kinetics were measured with a IP-28 photomultiplier tube and an digitizing/averaging system consisting of a LeCroy 9410 waveform recorder coupled with a Macintosh llci computer using Labview software. Kinetic curves were averaged over 20-50 laser pulses when necessary. The decay kinetics of the IO2luminescence at 1-27pm were measured by means of an IR laser fluorimeter with time resolution , ~ conductivity experiments of 51 ps described e l s e ~ h e r e . ~Pulsed 'On leave from the Institute of Chemical Physics, Academy of Sciences, USSR, Moscow, USSR.
were carried out in a special cell containing two platinum electrodes ( R , = 200 fl, V = 200 V). The absorption spectra of solutions were recorded by using a Beckman Model 25 spectrophotometer. Oxygen was removed from solutions by Ar purging. In quantum yield experiments, the solutions were bubbled with a mixture of 02/N2containing 60% O2or with pure O2for 20 min. Since the solubility of oxygen in the solvents investigated is similar,I0 the concentration of O2was assumed to be 1.8 X lO-' and 9.0 X lO-' M in air- and 02-saturated solutions, respectively. All investigations were carried out with a p-chloranil concentration of 3 X lo4 M (ODJsS= 0.3),except for experiments involving TIRPs, where thepchloranil concentration was 8 X 10-4 M (OD3sS 0.8). In this case,the small absorbance of the electron donor-acceptor (EDA) complexes at the excitation wavelength can be neglected. The other quenchers do not absorb at 355 nm. When not specifically mentioned, the error in the values of quantum yields, decay and quenching rate constants, equilibrium constants, and extinction coeficients of EDA complexes does not exceed *lo%. The values of @Aob were averaged over five independent experiments and were independent of laser pulse energy up to 20 mJ/pulse. p-Chloranil (CA), benzophenone, acridine, anthracene, and 1,2,3-trimethoxybenzene (TMB) (Aldrich) and durene (DH) and hexamethylbenzene (HMB) (Eastman) were purified by recrystallization from ethanol or hexane when necessary. Anisole (AH) (Eastman) and benzene, toluene, and chloroform (Fisher Scientific) were purified by distillation. All experiments were conducted at room temperature.
Results ChbraNl Photophysics. Photoexcitation of chloranil (CA) in Ar-saturated chloroform, benzene, and toluene resulted in for= 520 nm mation of triplet-triplet absorption spectra with A, and decay rate constants koT. Both spectra and decay rates are in good agreement with those published previously.'J1 The observed decay rate constant of CA triplet in air-saturated solvents (kOkT)was also obtained, and the values of quenching rate con(1) Fwtc, C. S.In Free Radiculs in Biology; Ryor, W. A., Ed.; Academic Press: New York, 1976; pp 85-133. (2) Gorman. A. A,; Rodgers, M. A. J . J . Am. Chcm. Soc. 1989, 111. 5557-5560. (3) Brauer, H. D.; Am, A.; Drews, W.; Gabriel, R.; Ghaeni, S.;Schmidt, R. J . Phorochcm. 1984, 25,475-488. (4) Marsh, K. L.; Stevens, B. J . Phys. Chcm. 1983,87, 1765-1768. (5) Chattopadhyay, S.K.; Kumar, C. V.; Das, P. K. J . Phorochem. 1984, 24. 1-9 - _. (6) Darmanyan, A. P. Khim. Fiz. 1988, 7, 13-20. (7) Levin, P. P.; Pluzhnikov,P. F.;Kuzmin, V. A. Chcm. Phys. Lrrr. 1988, 152,409-4 1 3. (8) Ogilby, P. R.; Foote, C. S.J . Am. Chcm. Soc. 1982,104,2069-2070. (9) Darmanyan, A. P.; Tatikolov. A. S.J . Phorochem. 1986,32. 157-163. ( I O ) Landolt-Bbrnstein, Zahlenwerre und Functionen; Springer-Verlag: Berlin, 1967. (1 1) Kawai, K.; Shirota, Y.; Tsubomura, H.; Mikawa, H. Bull. Chcm. Soc. Jpn. 1972, 45, 77-8 1 .
_.
0022-3654/91/2095-7308S02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7309
Singlet Oxygen Generation TABLE I: Pbotopbysid roper tie^ Of Tripkt p-Chlonnil E1IzaX, V
solvent chloroform benzene
vs SCE 2.3"
toluene
1.98"
lo-%:, S-l
M-? s-I
1.5
2.2
1.2
0.48 0.29
10.0
**&
104kT(02),
air 0.66 0.60 0.12
60% 0 2 0.72 0.68 0.21
100% 01
SA
0.73 0.70 0.26
0.75 0.73 0.35
%AH'
0.35 0.17 0.1 1
In acetonitrile.30
stants by molecular oxygen (kqT(02))were evaluated (Table I) from eq 1.
- kT)/[021 (1) The IO2quantum yield (aA&) from pchloranil was determined k302) = (k:b
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by comparison to an optically-matched reference (st) solution of benzophenone in air-saturated benzene. Benzophenone has been studied by various method^;'^-'^ we used an average value of @A8t = 0.30. To determine aA*,the amplitude of IO2 luminescence (Io) was measured immediately after the laser pulse, and eq 216
was used to correct for changes in solvent and sensitizer where n is the refractive index, k, is the IO2 radiative rate constant in the solvent investigated, and k,"' is the radiative rate constant in C6H6. In this work we used k,"/k, (toluene) = 0.9716 and kP/k,(CHC13) = 1.32.'' Gorman et al. have recently reaffirmed the necessity of correcting for the change in IO2 radiative lifetime on changing solvent.'* The values for @A""' were obtained under various concentrations of O2 and are presented in Table I. The quantum yield of IO2 generation under conditions of total quenching of the CA triplet by molecular oxygen is given in eq 3: @A
= @A*
kT + k:(O2)[021 k,T(02)lo21
(3)
However, we used the approximation method below which does
not depend on the accuracy of measurement of the rate constants k l and kT(02). Since the quantum yield of CA triplet is 1 .O (see ref 19 an%references therein) and @A = SA,eq 3 can be rewritten as follows: (4) If is plotted versus 1 / [ 0 2 ] , the ordinate intercept is l/Sk This method was used for determination of SAvalues for triplet CA in three solvents (Table I) and SAof the TIRPs (see below). From the data in Table I, SAfor CA triplet in toluene is approximately half that in chloroform and benzene. To test whether this decrease is connected with a specific interaction between CA and toluene, we examined the behavior of benzophenone in toluene. Benzophenone is a weaker electron acceptor than p-chloranil and has a lowest n,r* triplet state in both solvents. Benzophenone was found to have practically the same SAvalue (0.31) in toluene (Figure 1) and benzeneI4 (0.35). (12) Gorman, A. A.; Hamblett, 1. J . Phorochem. 1984, 25, 115-1 16. (13) Gorman, A. A.; Hamblett, 1.; Lambert, C.; Prescott, A. L.; Rodgers, M. A. J.; Spcnce, H. M. J. Am. Chem. Soc. 1987,109,3091-3097. (14) Redmond, R. W.; Braslavsky. S. E. Chem. Phys. Leu. 1988, 148, 523-529. (IS) McLean, A. J.; McGarve , D.J.; Truscott, T. G.; Lambert, C. R.; Land, E. J. J. Chem. Soc., f a r a d y Trans. 1990,86, 3075-3080. (16) Darmanyan, A. P.Khlm. N z . 1987.6, 1192-1 198. (17) Schmidt, R.; Afshari, E. J . Phys. Chem. 1990, 91, 4377-4378. ( 1 8) Gorman, A. A.; Krasnovsky, A. A.; Rodgers, M. A. J. J. Phys. Chem. 1991, 95, 598-601. (19) Kobashi, H.; Funabashi, M.;Kondo, T.; Morita, T.; Okada, T.; Mataga, N.Bull. Chem. Soc. Jpn. 1984, 57, 3557-3565.
0 ',,,,(.,,,I,,,,,,,,,'II1.,I.I.'....,IIIIIIIII,II~IIIIII, 0 1w ZOO 300 400 500
600
u" Figure 1. Dependence of IO2quantum yield production by benzophenone on concentration of O2in toluene. l/O*,
TABLE II: Parameters of the EDA Complexes in Beweme substrate AH' DHb HMB'
K, M-I
TMB~ a Anisole. benzene.
Durene.
0.063 0.44 1.05 0.65
, ,A
nm 450 470 510 450
Hexamethylbenzene.
M-l cm-l
2.9 3.1
2.2 0.8
1,2,3-Trimethoxy-
Chloranil H-Abstraction. Kobashi et al. showed that the triplet state of CA abstracts a hydrogen atom very efficiently from solvent, forming the long-lived semiquinone radical (CAW), which has maximum absorption a t 435 nm.I9 It is necessary to correct for this process in determining the yields of other reactions of 'CA. The CAH' absorption (ADCAH,)was measured immediately after the laser flash, and from it the concentration and the quantum yield of CAW formation (@cAH.). The triplet-triplet absorption of acridine in Ar-saturated benzene was used as a standard, with @sTst= 0.84,14 Am.xT-T = 440 nm, and cm.xT-T = 2.43 X IO-' M-' cm-I.20 The extinction coefficient of CAH' in 1,4-dioxane2', cCAH.(435) = 7.7 X IO3 M-'cm-l, was used. Since the shape of the absorption spectrum and ,A, are practically the same in all solvents, we assume that cCAH. is also the same in all solvents investigated. The *CAH. values were obtained from eq 5 and are listed in Table I. The formation of CAH' radicals was not observed in 02-saturated solvents.
Ground-State Electron Donor-Acceptor Complexes. Groundstate electron donor-acceptor complexes (EDA, charge-transfer complexes) with CA for all studied electron donors were observed as a broad structureless visible absorption. The absorption spectra (20) Bensasson. R.; Land, E.J. Trans. Faraday Soc. 1971,67,1904-1915. (21) Wong, S. K.; Fabes, L.; Green, W. J.; Wan, J. K. S. J . Chem. Soc., Faraday Trans. I 1972.68, 221 1-2217.
7310 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
Darmanyan et al.
TABLE III: Photopbyrical Parameters of the Trlpkt I o ~ ~ R a d i cPairs d EI~O', v ETP: I P IO^), ~ 10-7:p: IO+~~(O~): donor AH DH HMB TMB
vs SCE 1.76" 1.59*
1.52" 1.42'
cm-l M-l 15700 0.1 14400 2.9 13800 5.9 13000 5.1
M-9 s-I 0.53 11.0 200 4.4
sI .3 2.3 2.1 2.3
M-I s-I 1.6 0.73 0.77 4.3
*A*
air 60%02 100%02 SA 2 X l o ' M) cannot be used either, because the anthracene reabsorbs the laser radiation very strongly. Earlier work6 showed that 8-carotene can be successfully used as a quencher because it has a low-energy triplet state and direct excitation does not lead to pulation of its triplet. Quenching of the long-lived TIRP (k$ 5.0 X IO6 s-I) by p-carotene takes place with a rate constant of approximately 3 X lo9 M-I s-I. It was also observed that an increase in quencher concentration led to an increase in triplet-triplet absorption of @-carotene,confirming that &carotene quenches TIRP by an energy-transfer mechanism. Thus, the suggested reversal in energy of the singlet and triplet excited ion-radical pairs' is improbable, at least for the systems studied in this work. conclusions We have shown that an increase of the degree of electron m in the quantum transfer in the triplet exciplexes leads to a d yield of IO2generation in the series triplet carbonyl 1 nonpolar triplet exciplexes > triplet ion-radical pair. An increase in the energy-transferquenching rate constant by molecular oxygen takes place with decreasing energy of the triplet ion-radical pair. Simultaneously, according to the energy-gap law, an increase in the quenching rate constant takes place via enhancement of intersystem crossing to give the ground state of the ion-radical pair. The H-atom abstraction by triplet pchloranil from the benzene derivatives studied occurs through a two-step mechanism: electron transfer forms the thermalized triplet ion-radical pair, followed by slow proton transfer within the radical pair. A small contribution (- 10% of 'acAH.) of "fast" H-atom abstraction, with proton transfer in the unrelaxed triplet ion-radical pair or activationless fast H-atom transfer (with the collision complexes in a specific configuration) was observed only for durene and hexamethylbenzene.
Acknowledgment. Supported by NSF Grant CHE-89-11916. ReglShy NO. CA, 118-75-2; TMB, 634-36-6; DH, 95-93-2; HMB, 87-85-4; AH, 100-66-3; 02,7782-44-1.
